ANP enhances bradycardic reflexes in normotensive but not spontaneously hypertensive rats

Baroreflex control of heart rate in spontaneously hypertensive rats (SHR) is defective, largely because of a poor vagal contribution to the reflex. We have demonstrated previously that atrial natriuretic peptide (ANP) enhances reflex bradycardia in normotensive rats through an action on nonarterial vagal afferent pathways. In the present study, we investigated whether ANP could reverse the baroreflex abnormality in SHR. Heart rate reflexes were activated by three different methods in conscious, instrumented SHR and Wistar-Kyoto rats (WKY) in the presence of intravenous infusions of vehicle (saline) or rat ANP (150 ng/kg per minute). Heart rate responses were measured by (1) the steady-state changes in blood pressure after alternating slow infusions (over approximately 15 to 30 seconds) of a pressor (methoxamine) and depressor (nitroprusside) drug (stimulating predominantly arterial baroreceptors), (2) the ramp method of rapid infusion of methoxamine (over < 10 seconds; stimulating arterial and cardiopulmonary baroreceptors), and (3) the von Bezold-Jarisch method of activating chemically sensitive cardiac receptors through serotonin injections. ANP enhanced the heart rate range of the arterial baroreflex (steady-state method) by 13 +/- 3% in WKY but had no significant effect on the sensitivity or any other parameter of the steady-state baroreflex. When a very rapid rise in blood pressure was elicited by the ramp method in WKY, ANP significantly enhanced baroreflex bradycardia (sensitivity increased by 29 +/- 9%, P < .05). ANP also enhanced the bradycardia of the von Bezold-Jarisch reflex (by 33 +/- 16%, P < .05) in WKY. By contrast, ANP did not influence baroreceptor or chemoreceptor heart rate reflex responses in SHR. We conclude that in normotensive rats, ANP facilitates cardiopulmonary bradycardic reflexes. The lack of effect of ANP in SHR may be related to an underlying structural or genetic alteration in their cardiac sensors, perhaps associated with cardiac hypertrophy, that prevents the ANP-induced activation of cardiac sensory afferents, resulting in cardioinhibition.     

Platelet lipoxygenase in spontaneously hypertensive rats

We have previously reported that the nonselective lipoxygenase inhibitor phenidone is a potent hypotensive agent in the spontaneously hypertensive rat (SHR). In the present study, we examined the relationship between production of platelet 12-hydroxyeicosatetraenoic acid (12-HETE) and intra-arterial blood pressure in SHR and Wistar-Kyoto rats (WKY) using both a cross-sectional analysis and an acute pharmacological intervention. Basal generation rate of 12-HETE by platelets collected from the SHR was approximately 3.7-fold higher than in the WKY (0.86 +/- 0.24 versus 0.23 +/- 0.05 nmol/mL per 10 minutes, respectively; P < .01). Systolic arterial pressure was positively related to platelet 12-HETE formation rate when the entire rat population was considered (r = .70, P < .001). The specific 12-lipoxygenase inhibitor cinnamyl-3,4-dihydroxycyanocinnamate induced lowering of both arterial blood pressure and platelet 12-lipoxygenase activity in SHR. At 15 mg/kg, cinnamyl-3,4-dihydroxycyanocinnamate elicited a marked hypotensive effect in SHR but not in WKY. This reduction in arterial pressure was accompanied by an approximate 70% inhibition in platelet 12-HETE production rate. The return of high blood pressure to basal levels was associated with a significant rise in the production of platelet 12-HETE toward control values (baseline, 0.97 +/- 0.33 nmol/mL per 10 minutes; nadir of blood pressure, 0.19 +/- 0.03; resumption of basal pressure, 0.42 +/- 0.14). In contrast, captopril (15 mg/kg) induced a quantitatively similar decrease in blood pressure but had no effect on platelet 12-HETE generation rate. Thus, hypertension in SHR is linked to increased production rate of platelet 12-HETE. Acute blood pressure reduction attained during lipoxygenase inhibition but not by angiotensin converting enzyme inhibition leads to a concomitant reduction in the production of platelet 12-HETE. We speculate that since rat arterial tissue produces 12-HETE, increased 12-lipoxygenase activity in SHR may contribute to the maintenance of elevated arterial pressure in this strain.     

Hypercapnic cerebral blood flow in spontaneously hypertensive rats

OBJECTIVE: Hypercapnic cerebral vasodilation appears to be endothelium-dependent, as it involves nitric oxide and prostaglandins. Since chronic hypertension has been associated with impaired endothelial function, we designed a study to find out whether hypercapnic cerebral blood flow and its nitric oxide- and prostaglandin-sensitive component is reduced in spontaneously hypertensive rats (SHR) compared with normotensive controls. METHODS: Cerebral blood flow was measured in enflurane-anesthetized SHR (n=53), Wistar-Kyoto (WKY, n=20) and Sprague-Dawley (n=50) rats using the hydrogen clearance method. Cerebral blood flow was measured during eucapnia and hypercapnia; it was also assessed after administering either nonisoform-selective or isoform-selective neuronal nitric oxide synthase inhibitors and during inhibition of prostaglandin production. RESULTS: Hypercapnic cerebral blood flow did not differ among the strains. Nitric oxide synthase inhibition with intracortical N(G)-monomethyl-L-arginine reduced hypercapnic cerebral blood flow in SHR by 23+/-4% and in Sprague-Dawley rats by 23+/-7% without affecting eucapnic flow. Intraperitoneal administration of the inhibitor of neuronal nitric oxide synthase, 7-nitroindazole, reduced eucapnic flow by 18+/-5% in SHR and 27+/-5% in WKY rats, and hypercapnic flow by 48+/-3 and by 51+/-6%, respectively. Indomethacin produced a similar decrease in hypercapnic flow in Sprague-Dawley rats and SHR (49+/-5 and 62+/-4%, respectively). CONCLUSION: Hypercapnic cerebral blood flow was not impaired in SHR. The contribution of nitric oxide- and prostaglandin-dependent vasodilation appeared to be intact Our results are consistent with the hypothesis that neuronal rather than endothelial production of nitric oxide may be responsible for maintaining hypercapnic cerebral vasodilation in SHR.     

Peripheral nerve vascular changes in spontaneously hypertensive rats

Circulation to the brain is affected by hypertension. Hypertension-dependent cerebrovascular changes were documented primarily in brain pial arteries, whereas no information is so far available concerning changes of peripheral nerve vascularization in hypertension. This study was designed to assess the occurrence of structural changes of interfascicular and intrafascicular arteries supplying peripheral nerves (the so called vasa nervorum) in spontaneously hypertensive rats (SHR). The investigation was performed in 8-month-old SHR, by using standard microanatomical techniques associated with quantitative image analysis. In SHR a significant increase of systolic pressure values accompanied by thickening of the arterial wall, narrowing of the lumen and increase of the wall-to-lumen ratio were observed in comparison with age-matched normotensive Wistar-Kyoto rats. Hypertension-related structural changes involved primarily interfascicular arteries and to a lesser extent intrafascicular arteries. These findings indicate that similarly as documented for cerebral arteries, the vascular supply to peripheral nerves is impaired in hypertension. Structural changes of interfascicular and intrafascicular arteries of SHR could lead to ischemia of peripheral nerves. Further work is in progress to evaluate the functional relevance of hypertensive changes to peripheral nerve vasculature.     

Resistance to endotoxin shock in spontaneously hypertensive rats

Septic shock involves systemic vasodilation mediated by proinflammatory cytokines. In essential hypertension, vascular and immune dysfunctions are closely associated. The response of hypertensive animals compared with normotensive controls to endotoxin (lipopolysaccharide; LPS) challenge is not known. Age-matched (12 weeks) normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) were exposed to intravenous injection of 10 mg/kg LPS. Survival rate at 24 hours was markedly higher in SHR than in WKY (12 of 15 and 3 of 15, respectively; P<0.01). Survival of LPS-injected SHR was not related to their hypertension because hydralazine-treated SHR with normalized pressure had similar survival rates, and WKY made hypertensive by clipping of one renal artery showed fatality similar to that of normotensive WKY. Continuous arterial pressure and sequential plasma levels of interleukin-6 (IL-6) and tumor necrosis factor (TNF) were measured in LPS-treated SHR and WKY. Both the duration of the delayed hypotensive phase and the systemic release of IL-6 were much lower in SHR than WKY, whereas both acute hypotension and plasma TNF peak were equivalent. We further explored in vitro the inflammatory response and showed that LPS-activated whole blood from SHR produced less TNF and IL-6 than WKY LPS-activated whole blood. Our results indicate that SHR have a greater ability to resist endotoxic shock than WKY. This is not related to their hypertension but is associated with an attenuated inflammatory response to LPS.     

The effects of intrathecal clonidine in spontaneously hypertensive rats

The relationship between intrathecal clonidine and hypertension was investigated in SHR rat and WKY rat. After the animals were given clonidine 15 micrograms into the lumbar intrathecal space, blood pressure, heart rate and respiratory rate were recorded for 60 minutes under nembutal anesthesia. Percent change of mean blood pressure was significantly larger in SHR rat than in WKY rat at 50 and 60 minutes after clonidine injection. No difference was observed in percent change of heart rate and respiratory rate. The results suggest the possibility that lumbar intrathecal clonidine injection produces greater hypotension in hypertensive subjects than in normotensive subjects.     

Susceptibility to neonatal streptozotocin-induced diabetes in spontaneously hypertensive rats

We studied the difference in the susceptibility to neonatal streptozotocin (STZ) diabetes between spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY). Two-day-old female SHR and WKY were injected intraperitoneally with 75.0 mg/kg of STZ or vehicle for control. Hyperglycemia developed in both strains at 4 days of age, but SHR were more hyperglycemic. Overt hyperglycemia developed in SHR with aging after a partial recovery from initial hyperglycemia at 10 days of age, whereas WKY did not develop significant hyperglycemia except shortly after STZ treatment. Percentage of insulin-positive B cells in total islet cells and pancreatic immunoreactive insulin (IRI) content were measured at 4 days, 10 days, 4 weeks, and 12 weeks of age. B cells per islet and pancreatic IRI content were significantly reduced in STZ-treated groups as compared with control in both SHR and WKY at 4 days of age, but later they increased significantly with aging in both strains. However, the reduction in pancreatic IRI content relative to control was significantly greater in SHR than in WKY from 4 days (-94.5 +/- 3.5%, -84.1 +/- 4.8%; p < 0.01) to 12 weeks (-97.1 +/- 2.1%, -28.0 +/- 2.5%; p < 0.05), and the reduction in B cells per islet was also greater in SHR at 4 weeks of age. These results indicated that the initial destruction of pancreatic B cells induced by STZ was greater, and the following regeneration was less in SHR than in WKY. The association of the susceptibility to neonatal STZ diabetes with the development of genetic hypertension in SHR remained to be elucidated.     

Losartan inhibits experimental venous thrombosis in spontaneously hypertensive rats

The potential antithrombotic action of losartan, the AT1 receptor antagonist, in an experimental model of venous thrombosis in spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY) was tested. The involvement of nitric oxide and prostacyclin in this effect was also studied. Venous stasis was induced by ligation of the vena cava. Losartan, after administration of a single, hypotensive dose (10 mg/kg, p.o.), significantly reduced the thrombus weight in SHR but not in WKY. The antithrombotic activity of losartan in SHR was abolished by NG-nitro L-arginine methyl ester (L-NAME) (30 mg/kg s.c.) but not by indomethacin (2.5 mg/kg s.c.). No changes in primary hemostasis, platelet aggregation, coagulation parameters such as activated partial thromboplastin time, prothrombin time, euglobulin clot lysis time, and fibrinogen level, either in SHR or in WKY rats, were found. Our results indicate the NO-dependent mechanism in the antithrombotic effect of losartan on venous thrombosis in SHR.     

Urinary dopamine and sodium excretion in spontaneously hypertensive rats

We measured urinary dopamine in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) before (days 0-6) and during high-salt diet, in the absence (days 6-10) and presence (days 10-14) of added L-dopa (2 mg/kg/day by gavage). Urinary excretion of sodium (UNaV) increased 20-fold during intake of chow containing 8% NaCl in both strains. Systolic blood pressure (SBP) of SHR increased slightly (9 +/- 4 mmHg; p < 0.05) on the high-salt diet, whereas SBP did not change in WKY. Urinary dopamine excretion was not different between strains in the basal state, and was as great or greater in SHR than WKY during high-salt intake with and without added L-dopa. SBP was unaffected by L-dopa administration and UNaV did not increase or differ between strains despite higher urinary dopamine excretion in SHR. We conclude that renal dopamine formation in vivo is not diminished in SHR, compared with WKY, on normal or high-salt diets, and that elevation of renal dopamine formation secondary to L-dopa administration is not associated with reductions in SBP or altered UNaV in these rats.     

Alpha-adrenergic reactivity of the microcirculation in conscious spontaneously hypertensive rats

The goal of this study was to determine the functional distribution of alpha 1- and alpha 2-adrenoceptors in the striated muscle microcirculation. Experiments were performed in intact conscious spontaneously hypertensive rats (SHR) that were provided with a dorsal microcirculatory chamber to allow microvascular diameter measurements. Administration of selective alpha 1- and alpha 2-agonists, phenylephrine and azepexole, respectively, induced different patterns of microvascular constriction. alpha 1-Adrenoceptor stimulation showed a preferential constriction of large arteries and venules. The entire arteriolar microvasculature was sensitive to alpha 2-adrenoceptor stimulation, whereas the venular vessels did not respond to azepexole. The selective alpha 1- and alpha 2-antagonists prazosin and yohimbine showed patterns of vasodilator activity comparable to those of the corresponding agonists. The specificity of the drug-induced effects was verified by comparing their effects with those of graded hemorrhage, a non-pharmacological method for blood pressure lowering. In the range of blood pressure decreases comparable to that obtained by alpha-adrenoceptor antagonists, graded hemorrhage did not influence microvascular diameters. These results show a differential functional distribution of alpha 1- and alpha 2-adrenoceptors along the microvascular tree in striated muscle of conscious SHR.     

Noradrenergic hyperinnervation in the heart of stroke-prone spontaneously hypertensive rats

1. Noradrenergic (NA) nerve fibre distribution was investigated in the epicardium and myocardium of the heart in stroke-prone spontaneously hypertensive rats (SHRSP) and was compared to that in normotensive Wistar-Kyoto (WKY) rats. Fluorescent NA nerve fibres in the left and right epicardium of both strains aged 10, 30, 60, 90 and 180 days, and in the myocardium of left and right ventricles and the ventricular septum of both strains aged 30, 90 and 180 days were examined by the glyoxylic acid method. The distribution densities of NA nerve fibres were measured by quantitative image analysis. 2. The distribution pattern of NA nerve fibres in the epicardium of both strains showed a constant meshwork pattern throughout the entire examination period. 3. In the myocardium, NA nerve fibres were distributed irregularly between myocytes of both strains in all ages examined. 4. The densities of NA nerve fibres in the epicardium of SHRSP were significantly higher (P < 0.01 and 0.05; Student's t-test, 6 d.f.) than those of WKY at all ages examined except left epicardium at 90 days of age. 5. The densities in the right myocardium in 30 and 90 day old SHRSP were significantly higher (P < 0.05; Student's t-test, 6 d.f.) than those of WKY. 6. NA hyperinnervation in the epicardium and the myocardium of SHRSP may be assumed to be caused by the hyperfunction of the stellate ganglia which innervate the heart and may give rise to hypertrophy of the heart in SHRSP by a trophic effect of NA nerve fibre.     

The development of chronic kidney failure in spontaneously hypertensive rats

Normotensive Wistar and spontaneously hypertensiverten Okamoto rats with relatively low blood pressure were compared with regard to the development of chronic renal failure after subtotal renal ablation. Structural and biochemical studies revealed that Okamoto rats had more intensive glomerular sclerosis and higher degrees of chronic renal failure than Wistar rats.     

Antihypertensive and vasodilator actions of antioxidants in spontaneously hypertensive rats

This study was designed to determine whether the antioxidants ascorbic acid, aminotriazole, and glutathione acutely reduce blood pressure (BP) by endothelium-independent or -dependent vasorelaxation in spontaneously hypertensive rats. Blood pressure of male Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) was measured before and 4 h after administration of antioxidants. Thoracic aortic rings with and without endothelium were suspended in organ chambers for isometric tension recordings. Each of the antioxidants, administered in vivo, significantly decreased blood pressure in SHR but had no significant effect on BP in WKY rats. The endothelium-dependent impaired relaxation of SHR aortic rings to acetylcholine (ACh) was improved by prior in vivo administration of each antioxidant. ACh-induced relaxations of aortic rings from WKY was not affected by prior antioxidant treatment. Addition of each antioxidant directly to the organ chamber containing SHR or WKY aortas produced dose- and endothelium-dependent relaxations. Moreover, antioxidant pretreatment of SHR aortic rings significantly potentiated ACh-induced relaxations in these aortas, suggesting that this effect was endothelium dependent. Relaxations induced by the antioxidants alone or by ACh in the presence of antioxidants were inhibited by addition of either xanthine plus xanthine oxidase or nitro-L-arginine. These findings suggest that either excess production of oxidants or a deficiency of antioxidant systems may contribute to the high blood pressure and the endothelium-dependent impairment of vascular relaxation in SHR.     

Tissue-specific regulation of angiotensinogen gene expression in spontaneously hypertensive rats

Angiotensinogen is expressed in many tissues besides the liver. Recent studies have suggested that abnormalities in the regulation of angiotensinogen gene expression may be involved in the development of hypertension. However, little information is available concerning the functional significance of tissue angiotensinogen. In this study, we measured plasma angiotensinogen concentration by radioimmunoassay and examined the expression of tissue angiotensinogen by Northern blot analysis in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Although plasma angiotensinogen concentration in SHR was comparable to that in WKY at 6 weeks of age, it was increased significantly at 14 weeks of age in SHR and became higher than that in WKY. The levels of hepatic angiotensinogen mRNA were similar in SHR and WKY, and the levels of aortic, adrenal, and renal angiotensinogen mRNAs were lower in SHR than in WKY at both 6 and 14 weeks of age. Brain angiotensinogen expression in SHR was higher than in WKY at 6 weeks of age and was comparable to that in WKY at 14 weeks of age. On the other hand, cardiac and fat angiotensinogen mRNA levels were significantly increased at 14 weeks of age in SHR. These results demonstrate that the expression of tissue angiotensinogen is regulated differently in SHR and WKY and indicate that the development of hypertension is accompanied at least temporally with increases in plasma angiotensinogen concentration as well as cardiac and adipogenic angiotensinogen mRNA in SHR.     

Platelet thrombus formation in microvessels of young spontaneously hypertensive rats

We have previously shown an increase in platelet-to-endothelial cell adhesion in microvessels of spontaneously hypertensive rats (SHR) during the established stage of hypertension (12 weeks). The objective of the current study was to determine if the platelet-to-endothelial cell interaction would be altered in the early developmental phase of hypertension. Male weanling (3 weeks old) SHRs (n=6) and age matched normotensive Wistar-Kyoto (WKY) rats (n=6) were used to study platelet thrombus formation. Intravascular fluorescein isothiocyanate tagged to bovine serum albumin was activated with 450-490 nm light to induce thrombus formation in microvessels. Plasma concentrations of von Willebrand factor (vWF), fibrinogen and fibronectin (FN) were measured in rats during both early (3 week) and established stages of hypertension development. Thrombus initiation time in both arterioles (847+/-85 sec) and venules (222+/-40 sec) of young SHRs was significantly shorter (p<0.05) than in arterioles (1270+/-88 sec) and venules (630+/-72 sec) of age matched WKY rats respectively. After thrombus appearance, however, overall time for vessel occlusion in arterioles (2590+/-90 sec) and venules (935+/-131 sec) of SHRs was not different compared to that in arterioles (2650+/-191 sec) and venules (1240+/-93 sec) of age matched WKY rats. The plasma concentration of FN was increased (p<0.05) in both the young (0.9+/-0.1 mg/ml) and mature (1.1+/-0.2 mg/ml) hypertensive rats (n=5) compared to that in young (0.6+/-0.03 mg/ml) and mature (0.5+/-0.1 mg/ml) WKY rats (n=5), while fibrinogen content (3.6 +/-0.3 mg/ml) was elevated (p<0.05) only in mature SHRs (n=5) compared to that (2.7+/-0.02 mg/ml) in age matched WKY rats (n=5). The plasma concentration of vWF was similar to that of controls in either age group of hypertensive animals. These results suggest that changes in platelet-to-endothelial cell interactions occur in the early phase of genetic hypertension development in rats, and appears to result from alteration of plasma concentration of adhesion proteins.     

Hemodynamic effects of amlodipine and benazeprilat in spontaneously hypertensive rats

We wished to assess the hemodynamic effects of administration of the combination of the calcium channel blocking agent amlodipine and the angiotensin-converting enzyme (ACE) inhibitor benazeprilat in conscious spontaneously hypertensive rats (SHR). In SHR previously instrumented for measurement of mean arterial blood pressure (MAP) and heart rate (HR), intravenous (i.v.) injection of amlodipine (0.25-4 mg/kg) produced dose-dependent decreases in blood pressure (BP). Administration of benazeprilat (0.1-10 mg/kg i.v.) decreased arterial MAP, and benazeprilat (10 mg/kg) effectively blocked the effects of exogenously administered angiotensin I (AI). In animals surgically prepared for measurement of BP, HR, and hindquarter, renal, and mesenteric blood flows, administration (i.v.) of the combination of amlodipine (0.5 mg/kg) with benazeprilat (10 mg/kg) evoked a decrease in BP that was greater than that elicited by monotherapy. The tachycardic response observed after administration of the combination was no different from that observed after monotherapy with amlodipine. Simultaneous administration of amlodipine and benazeprilat produced reductions in vascular resistance in the hindquarter, renal and mesenteric beds that were greater than the responses evoked by injection of either agent. The major finding of these studies was that dual therapy with amlodipine and benazeprilat produced an additive hypotensive effect in conscious SHR. Regional vasodilation accompanied the large degree of hypotension evoked by the combination.     

Coronary hemodynamics in aging spontaneously hypertensive and normotensive Wistar-Kyoto rats

Conventionally, the hamstring:quadriceps strength ratio is calculated by dividing the maximal knee flexor (hamstring) moment by the maximal knee extensor (quadriceps) moment measured at identical angular velocity and contraction mode. The agonist-antagonist strength relationship for knee extension and flexion may, however, be better described by the more functional ratios of eccentric hamstring to concentric quadriceps moments (extension), and concentric hamstring to eccentric quadriceps moments (flexion). We compared functional and conventional isokinetic hamstring: quadriceps strength ratios and examined their relation to knee joint angle and joint angular velocity. Peak and angle-specific (50 degrees, 40 degrees, and 30 degrees of knee flexion) moments were determined during maximal concentric and eccentric muscle contractions (10 degrees to 90 degrees of motion; 30 and 240 deg/sec). Across movement speeds and contraction modes the functional ratios for different moments varied between 0.3 and 1.0 (peak and 50 degrees), 0.4 and 1.1 (40 degrees), and 0.4 and 1.4 (30 degrees). In contrast, conventional hamstring:quadriceps ratios were 0.5 to 0.6 based on peak and 50 degrees moments, 0.6 to 0.7 based on 40 degrees moment, and 0.6 to 0.8 based on 30 degrees moment. The functional hamstring:quadriceps ratio for fast knee extension yielded a 1:1 relationship, which increased with extended knee joint position, indicating a significant capacity of the hamstring muscles to provide dynamic knee joint stability in these conditions. The evaluation of knee joint function by use of isokinetic dynamometry should comprise data on functional and conventional hamstring:quadriceps ratios as well as data on absolute muscle strength.     

Cardiomyocyte apoptosis and cardiac angiotensin-converting enzyme in spontaneously hypertensive rats

A comparative study of the effects of heating by microwave radiation and water-bath heating up to temperature 35-75 degrees C on the structural state of bovine serum albumin is presented. Microwave radiation perturbs the surface of an albumin molecule. This essentially affects aggregation properties of bovine serum albumin and enhances structural transformations which accompany the process of thermal denaturation.