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.     

Conversion disorder presenting in a patient with an implantable morphine pump and an epidural abscess resulting in paraplegia

The purpose of this study was to determine long-term role of nitric oxide in modulating the chronic renal and arterial pressure responses to angiotensin II (AII). In normal dogs, intrarenal AII infusion (1.0 ng/KG/min) decreased renal plasma flow (RPF) by 31% and glomerular filtration rate (GFR) by 17% and increased mean arterial pressure (MAP) by 22%. In dogs with chronic intrarenal NO synthesis blockade with N(omega)-nitro-L-arginine methyl ester (3 micrograms/kg/min), AII decreased RPF by 25% and GFR by 19%, and increased MAP by 7%. These data indicate that chronic inhibition of NO synthesis within the kidney attenuated the long-term renal and arterial pressure responses by AII in dogs.     

Renal changes induced by nitric oxide and prostaglandin synthesis reduction: effects of trandolapril and verapamil

The benefits of the simultaneous administration of low doses of a calcium antagonist and a converting enzyme inhibitor in the treatment of hypertension and renal vasoconstriction are well established. The objective of this study was to evaluate whether the administration of low doses of a calcium antagonist and a converting-enzyme inhibitor have beneficial effects in treating the renal alterations induced by the acute administration of a cyclooxygenase inhibitor when nitric oxide synthesis is reduced. These effects were examined in anesthetized dogs before and during an acute sodium load. It was found that the intrarenal infusion of meclofenamate (5 microg x kg[-1] x min[-1]), simultaneously with a low dose of NG-nitro-L-arginine methyl ester (1 microg x kg[-1] x min[-1]), produced a 40% decrease of renal blood flow and glomerular filtration rate and a reduction in the renal excretory response to the sodium load. In a second group of dogs, intrarenal verapamil (0.5 microg x kg[-1] x min[-1]) was effective in blocking the effects of nitric oxide and prostaglandin synthesis inhibition on sodium excretion and glomerular filtration rate but did not modify the effects on renal blood flow. An intrarenal infusion of trandolapril (0.3 microg x kg[-1] x min[-1]) was effective in a third group of dogs in reducing the renal hemodynamic effects but not in preventing the antinatriuretic effect observed in the first group. Finally, in a fourth group, the simultaneous administration of verapamil and trandolapril was effective in treating all the renal changes induced by the cyclooxygenase inhibitor when nitric oxide synthesis was reduced. These results suggest that the combination of low doses of trandolapril and verapamil has additive effects in treating the renal vasoconstriction and antinatriuresis induced by the acute administration of a cyclooxygenase inhibitor, when nitric oxide synthesis is reduced.     

Autoradiographic localization of nitric oxide synthase binding sites in normal and diabetic rat corpus cavernosum

OBJECTIVES: To investigate density and distribution of nitric oxide synthase (NOS) binding sites in rat cavernosal tissue, and to assess any changes brought about by the onset of diabetes mellitus. METHODS: Hyperglycaemic non-ketonuric diabetes mellitus was induced in 5 rats using streptozotocin. The penises were excised from these rats 2 months after the administration of streptozotocin and stored at -70 degrees C. Longitudinal serial sections (6 microns) were cut in a cryostat and thaw mounted onto gelantinized microscope slides. Low- and high-resolution autoradiography was performed using a radioligand for NOS. Densitometric analysis was performed on the autoradiographs and the results compared with those obtained from 5 age-matched no-diabetic rats. RESULTS: NOS binding was primarily localized to the endothelium lining the cavernosal lacunar spaces. Significantly increased binding of NOS was seen in the diabetic cavernosal tissue 2 months after induction of diabetes mellitus. CONCLUSIONS: NOS binding is present on the endothelium of the rat corpus cavernosum and is increased in diabetic rats 2 months after streptozotocin administration. This increase in NOS binding may be part of the endothelial dysfunction which is reported in the corpus cavernosum of diabetic patients or rats.     

Antioxidants for hypertension

Pre- and post-menopausal women receiving oestrogen replacement therapy have a significantly reduced risk of cardiovascular disorders. It has been suggested that this protection might be partly a result of a direct relaxant effect of oestrogens on coronary arteries. This study examines and directly compares the effects of 17beta-oestradiol on rat isolated coronary and mesenteric vessels. The influence of nitric oxide on these responses was also investigated. 17Beta-oestradiol caused similar concentration-dependent relaxation of isolated coronary and mesenteric resistance arteries pre-contracted with either KCl (60 mM) or 9,11-dideoxy-11alpha,9alpha-epoxymethanoprostaglandin (U46619; 1 microM). The relaxation responses to 17beta-oestradiol were significantly reduced, but not totally inhibited, in the presence of N(omega)-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase; they were not altered by indomethacin, an inhibitor of prostaglandin synthesis. The responses to 17beta-oestradiol in the presence of L-NAME were not dependent on the vessel studied or the pre-contracting agent used. These results suggest that nitric oxide might contribute to the vasodilatory effects of 17beta-oestradiol in rat isolated coronary and mesenteric resistance arteries.     

Increased myogenic tone and diminished responsiveness to ATP-sensitive K+ channel openers in cerebral arteries from diabetic rats

Diabetes mellitus has profound adverse effects on vascular and, in particular, endothelial function. Although pressure-induced constriction ("myogenic tone") is a major contributor to the regulation of blood flow, little is known about the effects of diabetes on this response. Diabetes has been shown to diminish the dilation of cerebral arteries to synthetic ATP-sensitive K+ (KATP) channel openers. In this study, we explored the effects of diabetes induced in rats by streptozotocin on cerebral artery (250 to 300 microns) myogenic tone and on vasodilations to the synthetic KATP channel openers pinacidil and levcromakalim. Elevation of intravascular pressure caused a graded membrane potential depolarization and constriction, which was greater in arteries from diabetic rats compared with normal rats (at 60 mm Hg, 5 mV more depolarized and 22 microns more constricted). Pressurized arteries (at 60 mm Hg) from diabetic rats were 5- to 15-fold less sensitive to pinacidil and levcromakalim than were control arteries (EC50 values for pinacidil and levcromakalim were 1.4 and 0.6 mumol/L, respectively, in diabetic animals and 0.3 and 0.04, respectively, in control animals; P < .05). Removal of the endothelium or addition of a NO synthase inhibitor, NG-nitro-L-arginine (LNNA), in control arteries decreased the sensitivity to KATP channel openers and depolarized and constricted control arteries to levels similar to those observed in arteries from diabetic animals. Sodium nitroprusside caused a membrane potential hyperpolarization and enhanced the response to pinacidil in arteries from diabetic animals. Removal of the endothelium or LNNA had little effect on the apparent KATP channel opener sensitivity, the membrane potential, and pressure-induced constrictions of arteries from diabetic animals. The results are consistent with the hypothesis that this type of diabetes leads to a decrease in tonic NO release from the endothelium, which in turn causes membrane potential depolarization and vasoconstriction, resulting in a diminished response to KATP channel openers.     

Tissue ascorbic acid and polyol pathway metabolism in experimental diabetes

Previous studies demonstrating reduced plasma concentrations of ascorbic acid (AA) in diabetes and interactions between this vitamin and biochemical mechanisms such as synthesis of structural proteins, oxidative stress, polyol pathway and nonenzymatic glycation of proteins suggest that disturbed AA metabolism may be important in the pathogenesis of diabetic microangiopathy. However, limited information is available on the concentration of AA in tissues which develop diabetic complications. This study demonstrates reduced renal but not sciatic nerve or plasma AA concentration in two animal models of insulin-dependent diabetes mellitus, namely the STZ-diabetic rat and the spontaneously diabetic BB rat. Decreased lens AA concentration was also observed in STZ-diabetic rats. Improvement of glycaemic control by insulin treatment (albeit insufficient to achieve normoglycaemia) partially corrected lens and renal AA concentration in STZ-diabetic rats. AA treatment increased kidney and lens AA concentrations of STZ-diabetic and non-diabetic rats and corrected the abnormalities observed for untreated diabetic rats. Sciatic nerve AA concentration was not increased by AA treatment in any group. Tissue ratios of dehydroascorbic acid (DHAA)/AA, one index of oxidative stress, were not different between the diabetic and non-diabetic groups and were unaltered by AA supplementation. AA treatment of STZ-diabetic rats had no effect on elevated tissue concentrations of glucose, sorbitol and fructose or reduced myo-inositol concentration. The effect of reduced tissue AA levels in diabetes on either collagen synthesis or ability to combat increased free radical production is not known. However, correction of abnormal kidney and lens AA concentrations in experimental diabetes by AA supplementation suggests that if AA does have a role in the development or progression of the renal and ocular complications of diabetes, this treatment could be beneficial.     

Immunohistochemical localization of advanced glycosylation end products in coronary atheroma and cardiac tissue in diabetes mellitus

Advanced glycosylation end products (AGEs) accumulate on long-lived extracellular matrix proteins and have been implicated in the micro- and macrovascular complications of diabetes mellitus. Within the arterial wall, AGE-modified proteins increase vascular permeability, inactivate nitric oxide activity, and induce the release of growth-promoting cytokines. Recently developed anti-AGE antibodies were used in an immunohistochemical analysis of coronary arteries obtained from type II diabetic and nondiabetic patients. High levels of AGE reactivity were observed within the atherosclerotic plaque present in vessels from selected patients with diabetes. Considered together with the pathological effects of AGEs on vascular wall homeostasis, these data support the role of advanced glycosylation in the rapidly progressive atherosclerosis associated with diabetes mellitus.     

Differential effects of serotonin (5-HT) lesions and synthesis blockade on neuropeptide-Y immunoreactivity and 5-HT1A, 5-HT1B/1D and 5-HT2A/2C receptor binding sites in the rat cerebral cortex

The present study was aimed at comparing the effects of serotonin (5-HT) synthesis blockade using chronic administration of p-chlorophenylalanine (PCPA) and 5,7-dihydroxytryptamine injections of variable volume (3 vs. 6 microl) on the density of NPY immunoreactive (Ir) neurons and binding of [3H]8-OH-DPAT, S-CM-G[125I]TNH2 and [125I]DOI to 5-HT1A, 5-HT1B/1D, and 5-HT2A/2C receptors in rat cortical regions. Three weeks after large but partial (89% depletion in 5-HT tissue concentration) lesions of 5-HT neurons no changes in neither NPY immunoreactivity nor 5-HT receptor binding were detected. The complete 5,7-DHT lesions produced increases in the number of NPY-Ir neurons in the upper regions of the cingular (134%), frontal (140%) and parietal cortex (48%) and corresponding decreases in 5-HT2A/2C binding (16-26%). No changes in 5-HT1A and 5-HT1B/1D binding were observed after lesions of this kind. After PCPA treatment, decreases in NPY-Ir neurons density (22-40%) and increases in 5-HT1A and 5-HT1B/1D receptor binding sites (20-50%) were distributed in both upper and deeper cortical regions. The lack of effect of the partial lesion suggests that spared 5-HT neurons may exert compensatory mechanisms up to a large extent. The changes in NPY immunoreactivity and 5-HT2A/2C binding detected in the upper regions of the cortex after complete 5-HT lesions probably result from local cellular rearrangements, whereas blocking 5-HT synthesis has more widespread influence on NPY neurons and on 5-HT1A and 5-HT1B/1D receptor subtypes. Moreover, decreases in DOPAC concentrations detected only after complete lesions suggest that the involvement of catecholaminergic transmission may also differentiate 5,7-DHT and PCPA treatments. Altogether, these data suggest that different receptor subtypes might be involved in 5-HT-NPY relationships.     

Dental chapters of Serefeddin Sabuncuoglu''s (1385-1468?) illustrated surgical book Cerrahiyyetu''l Haniyye

The goal of this study was to determine whether exogenous application of L-arginine could restore impaired agonist-induced increases in arteriolar diameter during diabetes mellitus. We used intravital microscopy to examine reactivity of cheek pouch arterioles (50 microns in diameter) in nondiabetic and diabetic (2 weeks after injection of streptozotocin) hamsters in response to histamine and substance P. In nondiabetic hamsters histamine (1.0 and 5.0 microM) dilated cheek pouch arterioles by 15 +/- 1 and 22 +/- 1%, respectively, and substance P (50 and 100 nM) dilated arterioles by 14 +/- 3 and 21 +/- 4%, respectively. In addition, dilatation of arterioles in response to histamine and substance P in nondiabetic hamsters was abolished by application of an enzymatic inhibitor of nitric oxide synthase (L-NMMA). In contrast, histamine- and substance P-induced increases in arteriolar diameter were markedly reduced in diabetic hamsters. Histamine (1.0 and 5.0 microM) dilated arterioles by only 5 +/- 1 and 4 +/- 2%, respectively, and substance P (50 and 100 nM) dilated arterioles by only 6 +/- 2 and 5 +/- 3%, respectively (p < 0.05 vs. nondiabetic hamsters). Nitroglycerin produced similar vasodilatation in nondiabetic and diabetic hamsters. Next, we examined whether exogenous application of L-arginine (100 microM) could restore impaired histamine- and substance P-induced increases in arteriolar diameter in diabetic hamsters. We found that L-arginine did not restore altered nitric oxide synthase-dependent vasodilatation in diabetic hamsters. These findings suggest that short-term diabetes mellitus alters agonist-induced increases in arteriolar diameter. In addition, the mechanism of altered arteriolar reactivity during diabetes mellitus does not appear to be related to an impaired availability of L-arginine.     

Effect of KATP channel blocker U37883A on renal function in experimental diabetes mellitus in rats

An increase in glomerular filtration rate (GFR) in early diabetes mellitus is considered a risk factor for the development of diabetic nephropathy. Insulin deficiency may increase the activity of ATP-sensitive potassium channels (KATP), which could promote afferent arteriolar vasodilation und thus contribute to glomerular hyperfiltration in early diabetes mellitus. To further elucidate this hypothesis we performed renal clearance experiments in anesthetized rats at 2 and 6 weeks after onset of streptozotocin-induced insulin-treated diabetes mellitus and studied the acute effect of the putative KATP channel blocker 4-morpholinecarboximidine-N-1-adamantyl-N'-cyclohexylhydr ochloride (U37883A) on renal function. In control rats, application of U37883A (1.5 mg/kg i.v. bolus plus 1.5 mg/kg/hr) induced a significant reduction in heart rate, but did not affect or even slightly increased mean arterial blood pressure. Furthermore, U37883A did not significantly affect renal vascular resistance, renal blood flow or GFR, but caused an eukaliuretic diuresis and natriuresis and lowered plasma renin activity. Diabetic rats at both 2 or 6 weeks after streptozotocin exhibited essentially an identical response to U37883A; in particular, RVR and glomerular hyperfiltration remained unchanged. These results show that in both control and diabetic rats, the renal excretory function, renin secretion and pace setting in the heart were sensitiv to U37883A, implying a functional contribution of KATP channel activity. However, in both control and diabetic rats, renal vascular resistance, renal blood flow, or GFR were not altered by U37883A. These results argue against a substantial role for KATP channels in the basal control of renal hemodynamics in both nondiabetic and diabetic rats.     

Emergency medicine in the New Delhi area, India

OBJECTIVES: To examine the role of nitric oxide in the cardiovascular system in spontaneous hypertension. In particular, we wanted to know whether the production of nitric oxide in the cardiovascular system of the spontaneously hypertensive rat is different from that of the normotensive Wistar-Kyoto rat and whether nitric oxide is biologically effective in this system. DESIGN: We studied various aspects of the L-arginine-nitric oxide pathway in the cardiovascular system of spontaneously hypertensive rats and Wistar-Kyoto rats. METHODS: To address the first objective we analysed the expression of endothelial nitric oxide synthase in the heart by Western blotting and the activity of constitutive nitric oxide synthase in resistance microvessels obtained from the mesenterium, both from spontaneously hypertensive rats and Wistar-Kyoto rats aged 14-18 weeks. We also analysed the concentration of the oxidative product of nitric oxide, nitrate, in plasma from these rats. To address the second objective, that is, to assess the bioactivity of nitric oxide, we studied the accumulation in tissue of cyclic guanosine 3',5'-monophosphate (GMP), as well as the acute and chronic effects of withdrawing the nitric oxide vasodilatory tone with the inhibitor of nitric oxide synthesis NG-nitro-L-arginine methyl ester on Wistar-Kyoto rats and spontaneously hypertensive rats. RESULTS: We found that the expression of endothelial nitric oxide synthase in the heart, the activity of constitutive nitric oxide synthase in resistance microvessels and the concentration of nitrate in plasma were all significantly higher in the spontaneously hypertensive rats. In contrast, neither cyclic GMP levels nor the effects of NG-nitro-L-arginine methyl ester were greater in the spontaneously hypertensive rat than they were in the Wistar-Kyoto rat. CONCLUSIONS: The nitric oxide pathway is upregulated in the resistance circulation and the heart of the spontaneously hypertensive rat by a mechanism involving induction of the constitutive nitric oxide synthase and overproduction of nitric oxide. However, nitric oxide is not sufficiently bioactive to stimulate the formation of cyclic GMP and to maintain an adequate nitric oxide-dependent vasodilatory tone.     

Modulatory role of endothelial and nonendothelial nitric oxide in 5-hydroxytryptamine-induced contraction in cerebral arteries after subarachnoid hemorrhage

OBJECTIVE: Endothelial dysfunction is claimed to play a role in the pathogenesis of delayed cerebral vasospasm after subarachnoid hemorrhage (SAH). We have examined the effect of experimental SAH on the modulatory action of endothelial and nonendothelial nitric oxide (NO) in the contractile response of goat middle cerebral artery to 5-hydroxytryptamine (5-HT). METHODS: We compared the 5-HT-induced contractile responses of cerebral arteries from control goats and from goats with SAH that had been experimentally induced 3 days earlier by delivery of autologous arterial blood into the subarachnoid space. Contractile responses were examined by recording the isometric tension in isolated cerebral arteries. To assess the influence of endothelium, this cell layer was mechanically removed in some of the arteria, segments (rubbed arteries) from both control goats and goats with SAH. RESULTS: In arteries from control goats, contractile responses to 5-HT were significantly higher in rubbed arteries than in arteries with intact endothelium; 5-HT-induced contractions were significantly enhanced by a competitive inhibitor of NO synthesis, NG-nitro-l-arginine, in arteries both with and without endothelium. In arteries from goats with SAH, 5-HT contracted cerebral arteries without showing significant differences between segments with endothelium and those that had been rubbed; in both cases, 5-HT-induced contractions were significantly higher than those obtained in arteries from control goats. NG-Nitro-l-arginine significantly enhanced the contractile response to 5-HT of cerebral arteries from goats with SAH. CONCLUSION: These results suggest that cerebral arteries after SAH exhibit hyperreactivity to 5-HT via a mechanism that involves the absence of the modulatory role of endothelial NO, that SAH does not modify the modulatory role of nonendothelial NO, and that impairment of the modulatory action of endothelial NO on vascular responses to 5-HT could contribute to the pathogenesis of cerebral vasospasm after SAH.     

Long-term treatment in vivo with NOX-101, a scavenger of nitric oxide, prevents diabetes-induced endothelial dysfunction

Substantial evidence exists that diabetes results in impaired endothelial dysfunction suggesting diminished nitric oxide production from diabetic endothelium. It is not known what factors contribute to the development of this defect. In this study, we tested whether chronic treatment in vivo with NOX-101, a water-soluble nitric oxide scavenger, prevents endothelial dysfunction in diabetes. Sprague-Dawley rats were made diabetic by an intravenous injection of streptozotocin. A subgroup of control or diabetic animals received twice daily subcutaneous injections of 80 mg/kg NOX-101 beginning at 48 h after streptozotocin was injected and throughout 8 weeks of diabetes. Body weights and glucose concentrations were monitored weekly. At the end of 8 weeks, blood glucose and glycosylated haemoglobin was raised in diabetic rats but serum insulin concentrations were reduced. Treatment with NOX-101 did not alter glucose or insulin concentrations in control or diabetic rats; however, total glycosylated haemoglobin was partially reduced compared with untreated rats. In a subgroup of 2-week diabetic and age-matched rats fasted for 24 h, NOX-101 abolished total urinary nitrate plus nitrite (an index of nitric oxide production in vivo). In isolated tissue baths, relaxation to the endothelium-dependent vasodilator, acetylcholine, was impaired in diabetic aortic rings and relaxation to nitroglycerin was unaltered. Treatment of control rats with NOX-101 did not alter maximum relaxation to acetylcholine but shifted the response curve slightly to the right. In contrast in diabetic rats, NOX-101 prevented the impairment in endothelium-dependent relaxation but had no effect on relaxation induced by nitroglycerin. These data suggest the possibility that diabetes-induced endothelial dysfunction in diabetes results, in part, from a paradoxical increase in nitric oxide production during the course of the disease. This suggests a novel pathway of vascular complications.     

Pathophysiologic mechanisms of early changes in renal hemodynamics in diabetes mellitus

The early stages of diabetes mellitus are in some patients associated with renal haemodynamic changes resulting in increased glomerular filtration. This "diabetic hyperfiltration" is considered to be one of pathophysiological mechanisms and risk factors for the development of diabetic nephropathy. The aim of this paper is to review some contemporary views on pathophysiological mechanisms leading to this disorder with emphasis on the role impaired activity of humoral factors influencing renal haemodynamics. In addition to poor metabolic control due to insulinopenia there is a convincing experimental evidence suggesting the role of atrial natriuretic factor and endothelium-derived nitric oxide in mediating renal haemodynamic changes in diabetes. Enhanced renal activity of angiotensin I converting enzyme resulting in local overproduction of angiotensin II and accelerated degradation of kinins may be another factor contributing to the genesis of diabetic hyperfiltration. Hyperglycaemia induces changes in cellular signalling of these vasoactive systems. Furthermore, diabetes is a state of decreased capability of renal vascular bed to autoregulate blood flow likely due to altered activity of tubuloglomerular feedback and ion channels.     

Contrast media-induced nephrotoxicity--questions and answers

The intravascular administration of contrast media (CM) can produce acute haemodynamic changes in the kidney characterized by an increase in renal vascular resistance and a decrease in the glomerular filtration rate (GFR). These changes may lead to clinically significant reduction in renal function in patients with pre-existing risk factors such as diabetic nephropathy, congestive heart failure and dehydration. The pathophysiology of the renal haemodynamic effects of CM involves activation of the tubuloglomerular feedback (TGF) mechanism and the modulation of the intrarenal production of vasoactive mediators such as prostaglandins, nitric oxide, endothelin and adenosine. The TGF response is osmolality-dependent and accounts for about 50% of the acute functional effects of high osmolar CM on the kidney. Reduction in the synthesis of the endogenous vasodilators nitric oxide and prostaglandins increases the nephrotoxicity of CM. Endothelin and adenosine play a crucial role in mediating the acute functional effects of CM. Antagonists of these mediators attenuate the reduction in renal function induced by contrast agents. Vacuolization of the cells of the proximal tubules and necrosis of those of the medullary ascending limbs of loops of Henle are the main structural effects of CM in the kidney. The reduction in renal function induced by CM could be minimized by the use of low osmolar CM and adequate hydration. The prophylactic administration of calcium channel blockers and adenosine antagonists such as theophylline may also offer some protective effect.     

Direct vasodilatory effect of insulin on isolated retinal arterioles

PURPOSE: To test the hypothesis that insulin has a direct vasodilatory effect on retinal arteries and their branches and to investigate the mechanisms involved. METHODS: Segments of porcine retinal arteries were dissected, cannulated, and perfused. Vessel diameter was measured continuously on-line. Vessels were precontracted to 66% +/- 0.9% (SEM, n = 148) of their original diameter by perfusing with 124 mM K(+)-Krebs solution. Dose-response curves to insulin (2 to 2000 microU/ml) were compared for extraluminal (EL), intraluminal (IL), and combined IL-EL application. The effect of cyclooxygenase and nitric oxide synthase inhibition on the insulin response was determined, as was Ca2+ channel involvement. RESULTS: EL insulin alone had no significant effect on vessel diameter. IL insulin produced a dose-dependent dilatation of 5.6% +/- 2.9% (n = 22) of the K+ contracted diameter at 200 microU/ml and up to 12.4% +/- 3.6% (n = 22) by 2000 microU/ml, whereas combined IL-EL insulin application caused dilatation at all concentrations, rising to 15.1% +/- 2.9% (n = 44) at 200 microU/ml and 19.7% +/- 3% (n = 44) at 2000 microU/ml. IL indomethacin (5 x 10(-5) M) had no significant effect on the insulin-induced dilatation, whereas IL L-NAME (10(-4) M) inhibited insulin dilatation completely. The addition of EL verapamil (10(-6) M) during insulin-induced dilatation resulted in further dilatation to 37.8% +/- 4.2% (n = 18). However, the addition of insulin to verapamil-dilated vessels caused no further dilatation. Exposure to EL insulin while the IL K+ contraction dose-response curve was measured had no effect. Results in main arteries and branches did not differ. CONCLUSIONS: The IL application of insulin dilates potassium-contracted pig retinal arteries. This effect was enhanced by the EL presence of insulin, which did not result in dilatation when it was administered alone. The dilatation response was mediated by nitric oxide but not by prostaglandins. There was some evidence for the involvement of Ca2+ channels in insulin-induced dilatation. These results imply that insulin is a vascular regulator in normal conditions and may have relevance to the vascular changes occurring in diabetes and hypertension in the retina.     

Free radicals in diabetic endothelial cell dysfunction

Several studies have shown impairment of endothelium-dependent relaxations as well as increased release of vasoconstrictor prostanoids in arteries from diabetic animals and humans. This impairment is restored towards normal by prostaglandin (PG) H2/thromboxane A2 receptor blockade or superoxide dismutase, indicating that the PGH2 and/or superoxide anion (O2-.) generated contributes to the abnormality. Of particular note is that PGH2 impairs endothelium-dependent relaxations and causes contractions by a mechanism that involves generation of O2-. in the endothelium. The effects of elevated glucose are exacerbated by increased aldose reductase activity leading to depletion of NADPH and generation of reactive oxidants. Because NADPH is required for generation of nitric oxide from L-arginine, the depletion of NADPH leads to reduced nitric oxide formation. In a manner similar to that observed with elevated glucose, oxygen-derived free radicals or activation of protein kinase C also cause impairment of endothelium-dependent relaxations, smooth muscle contractions, and release constrictor prostanoids, indicating that a common mechanism for the impairment of endothelial cell function may be operative in diabetes. In this review the cumulative effects of oxidative stress on diabetic endothelial cell dysfunction, together with the complex interrelationship of cyclooxygenase catalysis, protein kinase C activity, and flux through the polyol pathway, are considered.     

Ischemia causes rapidly progressive nephropathy in the diabetic rat

We examined the influence of renal ischemia in rats with diabetes mellitus (DM). Male Wistar rats were rendered diabetic by streptozotocin treatment. Two weeks later, 30 minutes of complete ischemia was induced in the left kidney of DM and non-DM animals. Both groups were evaluated functionally and morphologically four or eight weeks post-ischemia. In non-DM animals renal function and morphology showed almost complete recovery. In the DM animals, however, this comparatively short period of ischemia caused a substantial loss of renal function. Four weeks post-ischemia inulin clearance in the DM kidneys rendered ischemic was only 20% of that in the corresponding non-DM kidneys, and after eight weeks the DM kidneys were completely anuric. Extensive inflammation and tubulointerstitial fibrosis were evident in DM kidneys four weeks after ischemia and seemed to increase over time. After eight weeks, tubular atrophy was found in the ischemic DM kidneys, resulting in a substantial loss of kidney mass. We conclude that in diabetic rats renal ischemia causes rapidly progressive kidney damage that in several respects resembles diabetic nephropathy in humans. Since advanced renal lesions similar to those seen in human diabetic nephropathy never develop in the rat solely as a result of DM, the present study may provide an experimental model for further studies on renal failure in diabetes mellitus.     

The use and abuse of routine stool microbiology: a College of American Pathologists Q-probes study of 601 institutions

BACKGROUND/AIMS: In the present study we have evaluated the role of nitric oxide and prostaglandins in the renal vascular response to a vasoconstrictor (methoxamine) and to endothelium-dependent (acetylcholine) and independent (sodium nitroprusside) vasodilators. METHODS: The experiments were performed in isolated and perfused kidneys of portal vein ligated and sham rats under various treatments. RESULTS: Baseline renal perfusion pressure was lower in the portal vein ligated than in the sham group (37.2 +/- 2.6 vs 48.4 +/- 2.5 mmHg). Indomethacin (10(-5)M) did not modify baseline renal perfusion pressure in any group, but the nitric oxide inhibitor N(W)-Nitro-L-Arginine (10(-4) M) increased it in both sham and portal vein ligated kidneys, but without abolishing the differences between them. The vasoconstrictor renal response to methoxamine was blunted in portal vein ligated rats compared to controls. Indomethacin did not modify this renal hyporesponsiveness, but N(W)-Nitro-L-Arginine completely abolished it. In another set of experiments, both acetylcholine and nitroprusside caused dose-dependent vasodilation in kidneys, preconstricted with methoxamine, from sham and portal vein ligated rats, and there were no significant differences between them. N(W)-Nitro-L-Arginine reduced acetylcholine-induced vasodilation and did not modify the vasodilation evoked by nitroprusside. CONCLUSIONS: These results indicate that the renal vasculature of portal vein ligated rats shows a basal reduction in perfusion resistance that is not related to nitric oxide or prostaglandins. However, increased nitric oxide production interferes with the effects of the alfa-agonist methoxamine. This suggests that nitric oxide plays an important role in the modulation of the renal vascular responses to vasoconstrictors in portal hypertension.