Nitric oxide synthase inhibition restores vasopressor effects of norepinephrine in ovine hyperdynamic sepsis

To investigate the hypothesis that nitric oxide synthase (NOS) inhibition restores the vasopressor response to norepinephrine (NE) in ovine hyperdynamic sepsis, eight sheep were chronically instrumented. In the non-septic portion of the study, NE was titrated to achieve an increase in mean arterial pressure (MAP) by 15 mm Hg ("small dose"). Small-dose NE was repeated 1 h after administration of the NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME; bolus 5 mg/kg, followed by 1 mg.kg-1.h-1). After 3 days of recovery, sepsis was induced by a continuous endotoxin infusion (Salmonella typhosa, 10 ng.kg-1.h-1). Three animals died during this period (data excluded). After 24 h, small-dose NE was given. If MAP increased less than 15 mm Hg, the NE dose was increased to achieve the targeted MAP change ("large dose"). Finally, both doses of NE were given after L-NAME administration. To increase MAP by 15 mm Hg in nonseptic animals, the rate of NE infusion was 0.18 +/- 0.03 microgram.kg-1.min-1 (small dose). During L-NAME infusion, this NE dose increased MAP by 32 +/- 8 mm Hg. In septic animals, small-dose NE increased MAP by only 9 +/- 2 mm Hg (P < 0.05 versus nonseptic state). To increase MAP by 15 mm Hg, the NE dose had to be increased to 0.34 +/- 0.06 microgram.kg-1.min-1 (large dose). During L-NAME infusion, NE administration increased MAP by 16 +/- 2 mm Hg and 28 +/- 4 mm Hg (small and large dose, respectively). Thus, L-NAME restored the vasopressor response to NE in sepsis, and increased the vasopressor response to NE in a similar fashion in healthy and septic sheep.     

The effects of a converting enzyme inhibitor (captopril) and angiotensin II on fetal renal function

1. Renal function was studied in chronically catheterized fetal sheep (119-128 days gestation), before and during treatment of the ewe with the angiotensin converting enzyme (ACE) inhibitor, captopril, which crosses the placenta and blocks the fetal renin angiotensin system. 2. An i.v. dose of 15 mg (about 319 micrograms kg-1) of captopril to salt-replete ewes followed by an infusion to the ewe of 6 mg h-1 (about 128 micrograms kg-1 h-1) caused a fall in fetal arterial pressure (P < 0.01), and a rise in fetal renal blood flow (RBF) from 67.9 +/- 5.6 to 84.9 +/- 8.3 ml min-1 (mean +/- s.e. mean) (P < 0.05). Renal vascular resistance and glomerular filtration rate (GFR) fell (P < 0.01); fetal urine flow (P < 0.01); fetal urine flow (P < 0.01) and sodium excretion declined (P < 0.05). 3. Ewes were treated for the next 2 days with 15 mg captopril twice daily. On the 4th day, 15 mg was given to the ewe and fetal renal function studied for 2 h during the infusion of captopril (6 mg h-1) to the ewe. Of the 9 surviving fetuses, 3 were anuric and 3 had low urine flow rates. When 6 micrograms kg-1 h-1 of angiotensin II was infused directly into the fetus RBF fell from 69 +/- 10.1 ml min-1 to 31 +/- 13.9 ml min-1, GFR rose (P < 0.05) and urine flow (P < 0.01) and sodium excretion increased in all fetuses. 4. It is concluded that the small fall in fetal arterial pressure partly contributed to the fall in fetal GFR but in addition, efferent arteriolar tone fell so that the filtration pressure fell further. Thus maintenance of fetal renal function depends on the integrity of the fetal renin angiotensin system. These findings explain why use of ACE inhibitors in human pregnancy is associated with neonatal anuria.     

Vasodepressor actions of angiotensin-(1-7) unmasked during combined treatment with lisinopril and losartan

Blockade of angiotensin II (Ang II) function during 8 days of oral therapy with lisinopril (20 mg/kg) and losartan (10 mg/kg) normalized the arterial pressure (112+/-3/70+/-3 mm Hg) and raised the plasma concentrations of the vasodilator peptide angiotensin-(1-7) [Ang-(1-7)] of 21 male spontaneously hypertensive rats (SHR). Treated animals were then given a 15-minute infusion of either mouse immunoglobulin G1 or a specific monoclonal Ang-(1-7) antibody while their blood pressure and heart rate were recorded continuously in the awake state. The concentrations of Ang II and Ang-(1-7) in arterial blood were determined by radioimmunoassay. Infusion of the Ang-(1-7) antibody caused significant elevations in mean arterial pressure that were sustained for the duration of the infusion and were accompanied by transient bradycardia. Although the hemodynamic effects produced by infusion of the Ang-(1-7) antibody had no effect on plasma levels of Ang II, they caused a twofold rise in the plasma concentrations of Ang-(1-7). A pressor response of similar magnitude and characteristics was obtained in a separate group of SHR treated with the combination of lisinopril and losartan for 8 days during an infusion of [Sar1-Thr8]Ang II. The pressor response induced by the administration of this competitive, non-subtype-selective Ang II receptor blocker was not modified by pretreatment of the rats with an angiotensin type-2 (AT2) receptor blocker (PD123319). Plasma concentrations of Ang II and Ang-(1-7) were not changed by the administration of [Sar1-Thr8]Ang II either in the absence or in the presence of PD123319 pretreatment. These results are the first to indicate an important contribution of Ang-(1-7) in mediating the vasodilator effects caused by combined inhibition of angiotensin-converting enzyme and AT1 receptors. The comparable results obtained by administration of [Sar1-Thr8]Ang II suggest that the vasodepressor effects of Ang-(1-7) during the combined treatment is modulated by a non-AT1/AT2 angiotensin subtype receptor.     

Contribution of humoral systems to the recovery of blood pressure following severe haemorrhage

1. Profound haemorrhage activates a number of pressor mechanisms, including the release of catecholamines, angiotensin II and arginine-vasopressin, which contribute to the subsequent cardiovascular recovery. Using specific single or combined blockade with prazosin, losartan and Manning compound (AVPX), the aim of this study was to evaluate the involvement of the three pressor systems in blood pressure recovery following severe haemorrhage (20 ml kg-1). 2. Haemorrhage of conscious, unrestrained rats resulted in a significant initial decrease in blood pressure of approximately 60 mmHg, and heart rate of approximately 70 bpm. Then, blood pressure tended to return to the control level within 10 min. The total cardiovascular recovery corresponded to increments of 52 +/- 5 mmHg (81% of the acute fall) for systolic blood pressure, and of 92 +/- 22 bpm (124%) for heart rate at 60 min post-bleeding. Significant falls in haematocrit (-10.5 +/- 1.2%, P < 0.01), in plasma concentrations of proteins (-10.3 +/- 0.9 g l-1, P < 0.01) and haemoglobin (-2.58 +/- 0.72 g 100 ml-1, P < 0.05) were observed at 60 min post-bleeding. 3. Pretreatment with one or two specific antagonists did not exaggerate the initial fall in blood pressure. The initial bradycardia was weakened only by combined blockade with losartan and AVPX. 4. The blood pressure recovery from a haemorrhage was delayed by approximately 25 min by the inhibition of vasopressin activity. The systolic blood pressure recovery in control animals (81% of the acute fall) was blunted by losartan (55% of the acute fall), prazosin (49%), combined losartan and AVPX (36%), prazosin and AVPX (36%), and also by prazosin plus losartan (13%). The diastolic blood pressure recovery was blunted only in the groups where the activity of angiotensin II was inhibited by losartan. 5. In conclusion, we have shown that neither catecholamines, angiotensin II nor vasopressin, although activated, individually compensate the acute hypotensive response to haemorrhage. The contribution of vasopressin to the blood pressure recovery post-bleeding is transient and is rapidly replaced by the pressor activity of the catecholamines and angiotensin II. The full systolic blood pressure recovery from severe haemorrhage requires the combined activity of these two pressor systems, while the diastolic blood pressure recovery seems to be only dependent upon angiotensin II activity.     

The effect of stevioside on glucose metabolism in rat

This study was conducted to determine the effect of stevioside (SVS) on glucose metabolism. The experiments were performed in male Wistar rats treated with SVS either by intravenous infusion or feeding. SVS infusion (150 mg/mL) was carried out in doses of 0.67, 1.00, and 1.33 mL.kg-1 body weight.h-1. The plasma glucose level significantly increased both during and after SVS infusion, whereas it was not affected by SVS feeding (13.3 mL.kg-1 body weight). The glucose turnover rate (GTR) of [14C(U)]glucose and [3(-3)H]glucose was not significantly different between control and SVS infusion animals. Percent glucose carbon recycling and glucose clearance were reduced from 28.7 +/- 1.3 to 23.0 +/- 1.6% (p < 0.05) and from 6.46 +/- 0.34 to 4.99 +/- 0.20 mL.min-1.kg-1 body weight (p < 0.01), respectively. The plasma insulin level did not change, whereas the plasma glucose level significantly increased from 120.3 +/- 5.9 to 176.8 +/- 10.8 mg% (p < 0.01) during SVS infusion. Animals pretreated with angiotensin II and arginine vasopressin showed no significant effect, while animals pretreated with prazosin had an attenuated hyperglycemic effect of SVS infusion. Pretreatment with indomethacin or N omega-nitro-L-arginine methyl ester (L-NAME) alleviated the plasma glucose level during the second period of SVS infusion. Pretreatment with the combination infusion of indomethacin and L-NAME reduced the plasma glucose level from 117.0 +/- 1.8 to 109.0 +/- 1.7 mg% (p < 0.001), and normalized the plasma glucose level in the second period of SVS infusion. Insulin infusion inhibited the hyperglycemic effect of SVS infusion. The present results show that the elevation of the plasma glucose level during SVS infusion is not due to the reduction of the insulin level. It is probably the effect of SVS on glucose transport across the cell. Insulin response to a high plasma glucose level is suppressed during SVS infusion. Several interactions among norepinephrine, prostaglandin, and nitric oxide are involved in modulating the hyperglycemia during SVS infusion.     

Prolonged systemic and regional haemodynamic effects of intracerebroventricular angiotensin II in conscious sheep

1. The haemodynamic mechanisms by which infusion of angiotensin II (AngII), either into the lateral cerebral ventricles (i.c.v.) or intravenously (i.v.), increased arterial pressure were studied in conscious sheep. 2. Sheep were previously fitted with flow probes for measurement of cardiac output and coronary, mesenteric, renal and iliac blood flows. 3. Intracerebroventricular AngII (10 nmol/h for 1 h) increased arterial pressure by 11 +/- 4 mmHg (P < 0.001) due to vasoconstriction, predominantly in the mesentric vasculature. These effects developed over 30 min and took 2 h to return to control. Following the infusion renal conductance increased continuously for 3 h, resulting in a parallel increase in renal blood flow (to 75 +/- 18 mL/min above control, P < 0.001). 4. Intracerebroventricular AngII increased plasma vasopressin from 0.8 +/- 0.3 to 7.2 +/- 1.8 pg/mL (P, 0.01), and reduced plasma renin concentration from 0.9 +/- 0.3 to < 0.4 nmol/L/h. 5. The pressor effect of i.v. AngII (5, 10, 25, 50 nmol/h) also depended on peripheral vasoconstriction, but the pattern of responses was different. The greatest degree of vasoconstriction occurred in the renal, followed by the mesentric and iliac vascular beds; these effects were rapid in onset and offset. 6. In conclusion, the pressor responses to both i.c.v. and i.v. angiotensin depended on peripheral vasoconstriction, but there were contrasting regional haemodynamic changes. ICV AngII caused a prolonged pressor response, mainly due to mesentric vasoconstriction possibly partly due to vasopressin release, and following the infusion there was a pronounced, long-lasting renal vasodilatation. In contrast, i.v. AngII caused vasoconstriction preferentially in the renal vascular bed and its effects were short lasting.     

Contribution of nitric oxide to the acute antihypertensive effect of blockers of AT1 angiotensin receptors in spontaneously hypertensive rats

The acute vasodepressor effect of AT1 angiotensin receptor blockers losartan and CL329167 was compared in spontaneously hypertensive rats (SHR) pretreated and not pretreated with NG-monomethyl-L-arginine (LNMMA; 15 mg/kg i.v. bolus plus infusion at 10 mg/kg/h), an inhibitor of nitric oxide (NO) synthesis. The antihypertensive effect of losartan (30 mg/kg, i.v.) in SHR pretreated with LNMMA (-13 +/- 4 mmHg) was greatly diminished (P < 0.01) relative to the antihypertensive effect of losartan in SHR not pretreated with LNMMA (-44 +/- 8 mmHg). Similarly, the antihypertensive effect of CL329167 (5 mg/kg, i.v.) in SHR pretreated with LNMMA (-12 +/- 3 mmHg) was surpassed (P < 0.01) by the antihypertensive effect in SHR not pretreated with LNMMA. (-41 +/- 4 mmHg). However, pretreatment of SHR with LNMMA did not minimize the vasodepressor effect of prazosin, isoproterenol or sodium nitroprusside. The impairment in vasodepressor responsiveness to losartan in rats pretreated with LNMMA was not demonstrable in rats concurrently receiving sodium nitroprusside to correct for the loss of endogenous NO, or atrial natriuretic peptide which also increases vascular cGMP. These data suggest that a mechanism mediated by NO and/or cGMP is necessary for the full expression of the acute antihypertensive effect of AT1 angiotensin receptor blockers in SHR.     

A comparison of the effects of doxazosin and terazosin on the spontaneous sympathetic drive to the bladder and related organs in anaesthetized cats

The effects of i.v. infusion of the alpha1-adrenoceptor antagonists doxazosin and terazosin (2 mg kg-1 h-1) on spontaneous hypogastric, renal and inferior cardiac nerve activity, spontaneous bladder contractions, blood pressure, heart rate and femoral arterial flow were investigated separately in alpha-chloralose-anaesthetized cats. Both drugs caused a reduction in hypogastric nerve activity associated with no overt changes in spontaneous bladder contractions. Doxazosin was more potent than terazosin, in that there was a significant reduction in hypogastric nerve activity after 20 min (0.67 mg kg-1) of infusion, while for terazosin this occurred after 40 min (1.33 mg kg-1). Both drugs also caused significant falls in blood pressure of 34 +/- 3 mm Hg and 33 +/- 4 mm Hg after 60 min. This was associated with no change in heart rate for doxazosin while terazosin caused an initial and significant increase in heart rate of 20 +/- 3 beats min-1 by 5 min, declining by 30 min to 1 +/- 5 beats min-1. This terazosin-induced tachycardia was associated with a significant increase in cardiac nerve activity of 128 +/- 22%. Both drugs caused increases in renal nerve activity however only for doxazosin was this increase significant. Femoral arterial conductance was also increased by both drugs, however, for doxazosin this increase was immediate and larger over the infusion period. These results demonstrate that alpha1-adrenoceptor antagonists can reduce sympathetic drive to the bladder and related organs.     

Mechanisms of angiotensin II chronotropic effect in anaesthetized dogs

1. The chronotropic effect of angiotensin II (5 micrograms in 1 ml of Tyrode solution), injected directly into the sinus node artery of 24 anaesthestized and vagotomized dogs pretreated with a beta-adrenoceptor antagonist, was evaluated before and after the administration of: (a) an angiotensin II AT1 receptor antagonist (losartan, 50 micrograms kg-1 min-1 infused i.v. for 120 min), (b) an alpha-adrenoceptor antagonist (prazosin, 1 mg kg-1 i.v. bolus injected), (c) a Ca2+ channel blocker (nifedipine 50, 100 and 200 micrograms kg-1 i.v. bolus injected) and (d) a protein kinase inhibitor (staurosporine, 800 nM infused via the sinus node artery at 0.6 ml min-1 for 15 min). 2. Losartan and staurosporine by themselves had no effect on basal systemic arterial pressure and heart rate, whereas prazosin and nifedipine caused significant diminutions of both parameters. 3. Angiotensin II induced significant increases in heart rate, the mean augmentations being 29 +/- 2 beats min-1. Losartan, nifedipine and staurosporine significantly decreased the chronotropic effect of angiotensin II, the mean respective diminutions being 65 +/- 8, 40 +/- 9 and 64 +/- 10%, whereas prazosin had no effect. 4. This work has demonstrated that angiotensin II exerts in vivo a significant positive chronotropic effect that is mediated via AT1 receptors located in the region of the sinoatrial node. This effect is independent of the adrenergic system. It is decreased by the inhibition of the production of protein kinases, most probably of protein kinase C, and by the blockade of the voltage-sensitive L-type Ca2+ channels. Other studies are obviously needed to ascertain the role of angiotensin II in the control of heart rate and/or the genesis of arrhythmias.     

Angiotensin subtype 1 blockade selectively potentiates adenosine subtype 2-mediated vasodilation

A previous report demonstrated that infusion of adenosine into the forearm increased local vascular production of angiotensin II. We hypothesize that this increase in angiotensin II could attenuate the vasodilator response to adenosine subtype 2 (A2) receptor activation. The depressor and regional hemodynamic responses to the A2-selective adenosine agonist DPMA were measured in the presence and absence of angiotensin subtype 1 (AT1) receptor blockade (losartan, 10 mg/kg IV) in anesthetized rats. Losartan pretreatment (without versus with losartan) significantly potentiated DPMA-induced reductions in renal (-13 +/- 2% versus -22 +/- 4%, P < .05) and mesenteric (-11 +/- 2% versus -23 +/- 4%, P < .05) vascular resistances, resulting in a greater depressor response (-7 +/- 2 versus -18 +/- 3 mm Hg, P < .05). The decrease in hindquarter vascular resistance was not affected. To test the specificity of this interaction, we also evaluated nitroglycerin and nifedipine. Pretreatment with losartan had no effect on the responses to nitroglycerin, whereas the responses to nifedipine either were not affected or were attenuated (percent change in mesenteric vascular resistance: without losartan pretreatment, -30 +/- 1%; with losartan pretreatment, -24 +/- 2%, P < .05). To determine whether the decrease in arterial pressure after losartan pretreatment contributed to the potentiation of the DPMA-mediated effects, we infused nitroglycerin to lower mean arterial pressure comparably to losartan treatment. None of the hemodynamic responses to subsequent DPMA administration were affected. These data suggest that endogenous levels of angiotensin II, whether released locally or systemically, selectively attenuate the A2-mediated reductions in renal and mesenteric vascular resistances.     

Cortisol differentially regulates pituitary-adrenal function in the sheep fetus after disconnection of the hypothalamus and pituitary

We have investigated the effects of a 5 day infusion of cortisol into fetal sheep, in which the hypothalamus and pituitary were surgically disconnected (HPD), on fetal pituitary-adrenal function. Fetal HPD and vascular catheterization were carried out at between 104 and 124 days gestation. Cortisol was administered (3.5 mg 24 h-1) for 120 h between 134 and 140 days (HPD + F group; n = 5) and saline was administered during the same gestational age range to HPD (HPD group; n = 12) and intact fetal sheep (Intact group; n = 6). Cortisol infusion into the HPD fetal sheep did not suppress the mRNA levels for Proopiomelanocortin (POMC) in the fetal anterior pituitary at 139/140 days gestation (POMC mRNA: 18S rRNA: Intact 0.40 +/- 0.05; HPD 0.56 +/- 0.07; HPD + F 0.49 +/- 0.07). Similarly, there was no significant effect of either HPD or cortisol infusion on the plasma concentrations of immunoreactive (ir) ACTH or ACTH(1-39). The adrenal: fetal body weight ratio was significantly higher, however, in the HPD + F (88.4 +/- 8.7 mg kg-1) and Intact groups (84.1 +/- 5.6 mg kg-1) when compared with the HPD fetal sheep (63.7 +/- 5.4 mg kg-1). The ratio of total IGF-II mRNA: 18S rRNA was similar in the adrenals of the Intact (0.48 +/- 0.09), HPD (0.78 +/- 0.09) and HPD + F (0.71 +/- 0.11) groups. The ratios of CYPIIA1, 3 beta-HSD and CYP21A1 mRNA: 18S rRNA were significantly lower in adrenals from the HPD group when compared to those in the Intact group and were not restored to normal by cortisol infusion. We have therefore demonstrated that cortisol does not act directly at the fetal pituitary to suppress POMC synthesis or ACTH secretion in late gestation. Cortisol does, however, stimulate fetal adrenal growth after HPD in the absence of any effects on adrenal IGF-II or steroidogenic enzyme mRNA levels. The data provide evidence that an intact hypothalamic-pituitary axis and cortisol each play an important role in the stimulation of adrenal growth and steroidogenesis which occurs during the last 10-15 days of gestation in the sheep.     

Losartan attenuates modest but not strong renal vasoconstriction induced by nitric oxide inhibition

Previous studies showed variable success of angiotensin II (ANG II) antagonists to oppose systemic and renal vasoconstriction during long-term nitric oxide synthase (NOS) inhibition. We explored in short-term experiments whether the systemic and renal vasodilatory response to angiotensin II type 1 (AT1)-receptor blockade depends on the extent of NOS blockade. In the first series of experiments, anesthetized rats underwent clearance studies during continuous monitoring of mean arterial pressure (MAP), renal blood flow (RBF, flow probe), and renal vascular resistance (RVR). Compared with control animals, low-dose infusion of the NOS-inhibitor nitro-L-arginine (NLA) increased MAP and RVR, decreased glomerular filtration rate, RBF, and sodium excretion, and had no effect on plasma and kidney ANG II content. High-dose NLA induced stronger effects, did not affect plasma ANG II, and reduced kidney ANG II to approximately 60%. In the second series of experiments, we studied the effect of low- and high-dose NLA on autoregulation of RBF. NLA induced a dose-dependent increase in MAP and decrease in RBF but left autoregulation intact. The AT1-receptor antagonist losartan restored MAP and RBF during low-dose NLA but had no depressor or renal vasodilating effect during high-dose NLA. In summary, short-term NOS blockade causes a dose-dependent pressor and renal vasoconstrictor response, without affecting renal autoregulation, and AT1-receptor blockade restores systemic pressor and renal vasoconstrictive effects of mild NOS inhibition but fails to exert vasorelaxation during strong NOS blockade. Both levels of NOS inhibition did not importantly alter intrarenal ANG II levels. Apparently the functional role of endogenous ANG II as determinant of vascular tone is diminished during strong NOS inhibition.     

Differential regional hemodynamic effects of corticotropin in conscious sheep

The regional hemodynamic effects of 5 days of intravenous infusion of corticotropin (ACTH) (5 micrograms/kg per day) were examined in conscious sheep (n = 8). Mean arterial pressure increased from 81 +/- 2 to 93 +/- 3 mm Hg (P < .001) on day 2 of ACTH and remained at this level during the infusion. Cardiac output increased from 5.13 +/- 0.19 to 6.06 +/- 0.33 L/min (P < .01) because of an increase in stroke volume from 65 +/- 4 to 79 +/- 8 mL per beat (P < .01); heart rate remained unchanged. ACTH did not alter total peripheral conductance but had differential effects on regional conductances. Mesenteric conductance fell from 5.8 +/- 0.2 to a minimum of 4.9 +/- 0.3 (mL/min)/mm Hg (P < .05), and renal conductance increased from 3.5 +/- 0.3 to 4.6 +/- 0.3 (mL/min)/mm Hg (P < .001). There was a small increase in iliac conductance (P < .05) and no change in coronary conductance. Mesenteric and iliac conductances fell progressively over 24 to 48 hours, whereas renal conductance increased rapidly after 3 hours of ACTH, reaching a maximum after 6 hours. Renal blood flow was increased during ACTH infusion from 278 +/- 18 to 403 +/- 23 mL/min (P < .001); mesenteric blood flow was unchanged; there was a small increase in iliac blood flow (P < .01); and coronary blood flow increased (P < .05), paralleling the change in cardiac output.(ABSTRACT TRUNCATED AT 250 WORDS)     

Increased pressor function of central vasopressinergic system in hypertensive renin transgenic rats

OBJECTIVE: Renin transgenic hypertensive rats [TGR(mRen2)27] have increased contents of angiotensin II and arginine vasopressin (AVP) in the cardiovascular brain regions. The aim of the present study was to evaluate the effects of centrally released AVP on the regulation of baseline blood pressure in TGR(mRen2)27 rats and to determine the interaction between AVP and angiotensin II in the central control of blood pressure in this model of hypertension. DESIGN: Three basic series of experiments were performed on 20 TGR(mRen2)27 and 20 Hannover Sprague-Dawley conscious rats, chronically instrumented with lateral cerebral ventricle (LCV) cannulae and femoral artery catheters. In series 1, blood pressure and heart rate were recorded during an LCV infusion of artificial cerebrospinal fluid before and after LCV administration of angiotensin II. In series 2, the effects of an LCV administration of angiotensin 11 (100 ng) on mean arterial pressure and the heart rate were determined during LCV infusion of a selective AVP receptor (V1) antagonist [1-(1-mercapto-4-methylcyclohexaneacetic acid)-8-arginine vasopressin (MeCAAVP) and d(CH2)5[Tyr(Me)2,Ala-NH2(9)]AVP] or a selective angiotensin II type 1 (AT1) receptor antagonist (losartan) or both. In series 3, mean arterial pressure and the heart rate were determined after an LCV injection of either AVP (10 ng) or AVP together with angiotensin II. RESULTS: The LCV infusions of antagonists to V1 and AT1 receptors caused significant comparable decreases in baseline MAP in TGR(mRen2)27 but not in Sprague-Dawley rats. Angiotensin II elicited significant pressor responses, both in TGR(mRen2)27 and in Sprague-Dawley rats. Blockade of V1 receptors significantly reduced the duration and the maximum amplitude of the central pressor response to angiotensin II in TGR(mRen2)27 rats, whereas in Sprague-Dawley rats the maximum pressor effect was not significantly altered. In both strains, the pressor response to angiotensin II was abolished by blockade of AT1 receptors. CONCLUSIONS: The results indicate that the elevated blood pressure in TGR(mRen2)27 rats is partly caused by increased function of the brain angiotensinergic AT1 and vasopressinergic V1 systems. Centrally released AVP is involved in mediation of the pressor effect exerted by centrally applied angiotensin II in TGR(mRen2)27 rats.     

In vivo quantification of endotoxin-induced nitric oxide production in pigs from Na15NO3-infusion

1. In this investigation the NO production rate is quantified in the pig during normotensive endotoxin-induced shock with increased cardiac output and during subsequent treatment with the NO synthase inhibitor N omega-monomethy-L-arginine (L-NMMA). NO production rate was derived from the plasma isotope-enrichment of 15N-labelled nitrate (15NO3-). 2. Three groups of animals (control, n = 5; endotoxin, n = 6; endotoxin + L-NMMA, n = 6) were anaesthetized and instrumented for the measurement of systemic and pulmonary haemodynamics. Each animal received a primed-continuous infusion of stable, non-radioactively labelled Na15 NO3 (bolus 30 mg, infusion rate 2.1 mg h-1). Arterial blood samples were taken 5, 10, 15, 30, 60 and 90 min later and every 90 minutes until the end of the experiment. 3. Continuous i.v. infusion of endotoxin was incrementally adjusted until mean pulmonary artery pressure (PAP) reached 50 mmHg and subsequently titrated to keep mean PAP approximately 35 mmHg. Hydroxyethylstarch was administered as required to maintain mean arterial pressure (MAP) > 60 mmHg. Six hours after the start of the endotoxin continuous i.v. L-NMMA (1 mg kg-1 h-1) was administered to the endotoxin + L-NMMA group. Haemodynamic data were measured before as well as 9 h after the start of the endotoxin. 4. After conversion of NO3- to nitro-trimethoxybenzene and gas chromatography-mass spectrometry analysis the total NO3- pool, basal NO3- production rate and the increase per unit time in NO3- production rate were calculated from the time-course of the 15NO3- plasma isotope-enrichment. A two compartment model was assumed for the NO3- kinetics, one being an active pool in which newly generated NO3- appears and from which it is eliminated, the other being an inactive volume of distribution in which only passive exchange takes place with the active compartment. 5. Although MAP did not change during endotoxin infusion alone, cardiac output (CO) increased by 42 +/- 40% (P < 0.05 versus baseline) by the end of the experiment due to a significant (P < 0.05 versus baseline) fall in systemic vascular resistance (SVR) to 65 +/- 25% of the baseline value. L-NMMA given with endotoxin did not change MAP, and both CO and SVR were maintained close to the pre-shock levels. 6. Baseline plasma NO3- concentrations were 43 +/- 13 and 40 +/- 10 mumol l-1 in the control and endotoxin animals, respectively, and did not differ at the end of the experiment (39 +/- 8 and 44 +/- 15 mumol l-1, respectively). The mean NO3- pool and basal NO3- production rate were 1155 +/- 294 mumol and 140 +/- 32 mumol h-1, respectively, without any intergroup difference. Endotoxin significantly increased NO3- production rate (23 +/- 10 mumol h-2, P < 0.05 versus control (6 +/- 7 mumol h-2) and endotoxin + L-NMMA groups). L-NMMA given with endotoxin (-1 +/- 2 mumol h-2, P < 0.05 versus control and endotoxin groups) had no effect. 7. Analysis of the time course of the 15NO3- plasma isotope enrichment during primed-continuous infusion of Na15NO3 allowed us to quantify the endotoxin-induced increase in NO3- production rate independently of total NO3- plasma concentrations. Low-dose L-NMMA blunted the increase in NO3- production rate while maintaining basal NO3- formation.     

Comparative effects of chronic angiotensin-converting enzyme inhibition and angiotensin II type 1 receptor blockade on cardiac remodeling after myocardial infarction in the rat

BACKGROUND: After myocardial infarction, the noninfarcted left ventricle develops reactive hypertrophy associated with a depressed coronary flow reserve, myocardial interstitial fibrosis, and reduced capillary density. The present study investigated the comparative cardiac effects of chronic angiotensin-converting enzyme (ACE) inhibition and selective angiotensin II type 1 receptor (AT1) blockade in the rat model of myocardial infarction and failure. METHODS AND RESULTS: Seven days after coronary ligation (MI), rats were randomized to enalapril (n = 8; 500 micrograms.kg-1.d-1), losartan (n = 9; 3 mg.kg-1.d-1), or placebo (n = 8) and treated for 6 weeks. Sham-operated rats (n = 10) served as controls. Coronary blood flow was measured with radiolabeled microspheres during baseline and maximal coronary dilation induced by dipyridamole (2 mg.kg-1.min-1 over 10 minutes). Right and left ventricular (LV) weight was increased in infarcted rats compared with sham-operated animals and enalapril- and losartan-treated MI rats. Minimal LV and right ventricular coronary vascular resistance was increased in MI rats but normalized with enalapril and losartan (LV:sham, 8.9; MI-placebo, 12.7; MI-enalapril, 9.2; MI-losartan, 8.8 mm Hg.mL-1.min-1.g-1, all P < .05 versus MI-placebo). Interstitial fibrosis determined from perfusion-fixed hearts was increased in infarcted rats but reduced by both enalapril and losartan. Myocardial capillary density improved with enalapril and losartan. In separate groups treated as above, plasma and tissue ACE activity was determined and demonstrated significantly higher ACE activity in noninfarcted LV tissue of MI-placebo rats compared with sham (0.64 vs 0.27 nmol.mg protein-1.min-1, P < .05). Enalapril and losartan reduced LV ACE activity (0.39 and 0.29 nmol.mg protein-1.min-1, P < .05 versus MI-placebo). CONCLUSIONS: The present study demonstrates that both chronic ACE inhibition and AT1 receptor blockade (1) reduces cardiac hypertrophy, (2) restores minimal coronary vascular resistance in postinfarction reactive hypertrophy, and (3) attenuates the development of myocardial interstitial fibrosis in the noninfarcted LV. These results suggest that inhibition of generation of angiotensin II and AT1 receptor blockade are equally effective in preventing important features of ventricular remodeling after myocardial infarction.     

Lack of involvement of bradykinin in the vascular sympathoinhibitory effects of angiotensin converting enzyme inhibitors in spontaneously hypertensive rats

1. The aim of this study was to investigate the contribution of endogenous bradykinin to the vascular sympathoinhibitory effects exerted by angiotensin I converting enzyme inhibitors (ACEIs) in the spontaneously hypertensive rat (SHR). 2. Adult SHRs were treated daily for 8 days with either perindopril (3 mg kg-1), or a selective angiotensin II AT1 receptor antagonist, losartan (10 mg kg-1) both given orally--these two doses being equipotent in inhibiting angiotensin I (AI)-induced vascular responses--or distilled water (controls). After pithing, the animals were instrumented for determination of blood pressure, heart rate, cardiac output, regional (renal, mesenteric, hindlimb) blood flows (pulsed Doppler technique) and corresponding vascular resistances. Afterwards, half of the animals of each group were given the selective bradykinin B2 receptor antagonist, icatibant, used in a dose (10 micrograms kg-1, i.v.) that achieved B2 receptor blockade, the other half received saline (10 microliters kg-1, i.v.). Haemodynamic responses to increasing frequencies of spinal cord stimulation were then measured. 3. Pressor and vasoconstrictor responses to AI were significantly and similarly reduced in both perindopril- and losartan-treated groups. Perindopril and losartan both decreased to a similar extent the pressor and vasoconstrictor responses to electrical stimulation of the spinal cord. 4. In the dose used, icatibant did not affect any of the investigated haemodynamic parameters in any of the experimental groups. Furthermore, icatibant did not affect the stimulation frequency-response curves in the control animals and did not modify the vascular sympathoinhibitory effects exerted by perindopril and by losartan. 5 Taken together, these results demonstrate that endogenous bradykinin does not, through B2 receptor activation, contribute to the vascular sympathoinhibitory effects of ACEIs in SHRs.     

The role of angiotensin AT1 receptors in the diuretic, natriuretic, kaliuretic and blood pressure responses induced by angiotensin II activation of the median preoptic nucleus in conscious rats

We determined the effects of two classical angiotensin II (ANG II) antagonists, [Sar1, Ala8]-ANG II and [Sar1, Thr8]-ANG II, and losartan (a nonpeptide and selective antagonist for the AT1 angiotensin receptors) on diuresis, natriuresis, kaliuresis and arterial blood pressure induced by ANG II administration into the median preoptic nucleus (MnPO) of male Holtzman rats weighing 250-300 g. Urine was collected in rats submitted to a water load (5% body weight) 1 h later. The volume of the drug solutions injected was 0.5 microliters over 10-15 s. Pre-treatment with [Sar1, Ala8]-ANG II (12 rats) and [Sar1, Thr8]-ANG II (9 rats), at the dose of 60 ng reduced (13.7 +/- 1.0 vs 11.0 +/0 1.0 and 10.7 +/0 1.2, respectively), whereas losartan (14 rats) at the dose of 160 ng totally blocked (13.7 +/- 1.0 vs 7.6 +/- 1.5) the urine excretion induced by injection o 12 ng of ANG II (14 rats). [Sar1, Ala8]-ANG II impaired Na+ excretion (193 +/- 16 vs 120 +/- 19), whereas [Sar1, Thr8]-ANG II and losartan block Na+ excretion (193 +/- 16 vs 77 +/- 15 and 100 +/- 12, respectively) induced by ANG II. Similar effects induced by ANG II on K+ excretion were observed with [Sar1, Ala8]-ANG II, [Sar1, Thr8]- ANG II, and losartan pretreatment (133 +/- 18 vs 108 +/- 11, 80 +/- 12, and 82 +/- 15, respectively). The same doses as above of [Sar1, Ala8]-ANG II (8 rats), [Sar1, Thr8]-ANG II (8 rats), and losartan (9 rats) blocked the increase in the arterial blood pressure induced by 12 ng of ANG II (12 rats) (32 +/- 4 vs 4 +/- 2, 3.5 +/- 1, and 2 +/- 1, respectively. The results indicate that the AT1 receptor subtype participates in the increases of diuresis, natriuresis, kaliuresis and arterial blood pressure induced by the administration of ANG II into the MnPO.     

Maintenance of baseline angiotensin II potentiates insulin hypertension in rats

Chronic insulin infusion in rats increases mean arterial pressure (MAP) by a mechanism dependent on angiotensin II (Ang II). However, the fact that plasma renin activity (PRA) decreases with insulin infusion suggests that Ang II sensitivity is increased and that the parallel reduction in Ang II may partly counteract any hypertensive action of insulin. This study tested that hypothesis by clamping Ang II at baseline levels during chronic insulin infusion. Sprague-Dawley rats were instrumented with artery and vein catheters, and MAP was measured 24 hours per day. In seven angiotensin clamped rats (AC rats), renin-angiotensin II system activity was clamped at normal levels throughout the study by continuous intravenous infusion of the angiotensin-converting enzyme inhibitor benazepril at 5 mg/kg per day (which decreased MAP by 18+/-2 mm Hg) together with intravenous Ang II at 5 ng/kg per minute. Control MAP in AC rats after clamping averaged 99+/-1 mm Hg, which was not different from the 101+/-2 mm Hg measured before clamping Ang II levels. Control MAP in the 8 vehicle-infused rats averaged 105+/-2 mm Hg. A 7-day infusion of insulin (1.5 mU/kg per minute IV) plus glucose (20 mg/kg per minute IV) increased MAP in both groups of rats; however, the increase in MAP was significantly greater in AC rats (12+/-1 versus 5+/-1 mm Hg). This enhanced hypertensive response to insulin in AC rats was associated with a greater increase in renal vascular resistance (153+/-10% versus 119+/-6% of control) and a significant increase in renal formation of thromboxane (149+/-11% of control). Thus, decreased Ang II during insulin infusion limits the renal vasoconstrictor and hypertensive actions of insulin, and this may be caused, at least in part, by attenuation of renal thromboxane production.     

Mediation of endothelin-1-induced inhibition of platelet aggregation via the ETB receptor

1. The effects of FR139317 (ETA antagonist) or PD145065 (non-selective ETA/ETB antagonist) on endothelin-1 (ET-1)-induced changes in blood pressure and inhibition of ex vivo platelet aggregation were investigated in the anaesthetized rabbit. 2. ET-1 (1 nmol kg-1, i.a. bolus) caused a sustained increase in mean arterial pressure (MAP) (peak increase 47 +/- 5 mmHg, n = 8). Intravenous infusion of FR139317 at 0.2 (n = 4) or 0.6 mg kg-1 min-1 (n = 4) inhibited the ET-1 pressor response by 83 or 89%, respectively. Infusion of PD145065 at 0.2 (n = 4) or 0.6 mg kg-1 min-1 (n = 4) inhibited the ET-1-induced increase in MAP by 79 or 75%, respectively. 3. The transient depressor response (-16 +/- 3 mmHg) which preceded the rise in blood pressure induced by ET-1 (1 nmol kg-1, i.a., n = 8) was enhanced by an intravenous infusion of FR139317 (0.6 mg kg-1 min-1) to -35 +/- 5 mmHg (P < 0.05, n = 4). This enhancement was abolished by indomethacin (5 mg kg-1, i.v.) pretreatment (-17 +/- 1 mmHg, n = 4). PD145065 (0.2 mg kg-1 min-1, i.v.) attenuated the ET-1-induced fall in blood pressure to -9 +/- 1 mmHg (n = 4), while a higher dose of this antagonist (0.6 mg kg-1 min-1, i.v.) completely abolished the ET-1-mediated depressor response. 4. ET-1 (1 nmol kg-1, n = 8) inhibited ex vivo platelet aggregation by 96% at 5 min after injection of the peptide. FR139317 (0.2 or 0.6 mg kg-1 min-1, i.v.) or PD145065 (0.2mg kg-1 min-1, i.v.) did not affect the inhibition of ex vivo platelet aggregation in response to ET-1. In contrast, intravenous infusion of PD145065 (0.6 mg kg-1 min-1) abolished the anti-aggregatory effects of ET-1.5. Thus, FR139317 inhibits the pressor, but not the depressor actions of ET-1 and has no effect on the ET-l-induced inhibition of ex vivo platelet aggregation. In contrast, PD145065 antagonizes the pressor and depressor responses to ET-1 and abolishes the anti-aggregatory effects of the peptide.6. These results strongly suggest that ET-1-induced vasoconstriction in the anaesthetized rabbit is primarily mediated via the ETA receptor while the depressor and antiaggregatory actions of ET-1 are due to activation of the ETB receptor.