Diabetes attenuates the alterations in both venous endothelial and smooth muscle cell function induced by prolonged hypercholesterolemia in an experimental model

Enhanced histamine-mediated contraction of rabbit penile dorsal artery in diet-induced hypercholesterolemia

The present study was designed to establish whether penile dorsal arteries isolated from rabbits fed a high cholesterol diet show an enhanced contractile and/or impaired vasodilator response to histamine, and to characterize the histamine receptor subtype involved through in vitro isometric techniques. New Zealand White rabbits were fed a normal diet or a 1% cholesterol diet for 16 weeks. Arteries from cholesterol-fed rabbits retained the ability to relax in response to acetylcholine, whereas histamine and noradrenaline induced a greater contraction response compared to that observed in controls. In both groups, histamine-induced contraction was unaffected by the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME), its precursor L-arginine or the cyclooxygenase inhibitor indomethacin. Treatment of arterial rings in the control and hypercholesterolemia groups with the H1 receptor antagonist, mepyramine, unmasked a vasodilation response to histamine. This was followed by contraction at higher concentrations showing a leftward displacement of the histamine curve compared to controls. The histamine receptor that induced contraction in preparations from the hypercholesterolemic animals was of the H1 subtype, whereas the receptor involved in histamine-induced relaxation was H2. The affinity of histamine receptor agonists was comparable to their effects in control animals, and receptor antagonists showed the same potency in both groups. Our findings indicate a preserved endothelial function and enhanced contraction in response to histamine in penile dorsal arteries, probably due to a change in the sensitivity of the contractile machinery of smooth muscle but not a mechanism mediated by a receptor.

Mechanisms of decreased bradykinin- induced vasodilation in experimental hyperlipemia-hyperglycemia: contribution of nitric oxide and Ca2+-activated K+ channels

Common complications of diabetes are accelerated atherosclerosis and vascular disturbances. We investigated whether the simultaneous insult of hyperlipemia-hyperglycemia affects the reactivity of the resistance arteries to bradykinin (BK), and if so, what are the mechanisms responsible for this disturbance. Experiments were conducted on male Golden Syrian hamsters rendered hyperlipemic (H) by a fat-rich diet, diabetic (D) by streptozotocin injection, or simultaneously hyperlipemic-diabetic (HD). Normal age-matched animals were used as controls (C). At 24 weeks after the induction of disease(s) the vascular reactivity of the mesenteric resistance arteries to BK (10(-8)-10(-4) M) was assayed by the myograph technique. To explore the role of nitric oxide (NO) in modulating the endothelium-dependent BK-induced relaxation, two experimental approaches were employed: (i) in vivo administration of L-arginine (622.14 mg/kg bw) to H, D, and HD hamsters (for 12 weeks); (ii) in vitro blockage of nitric oxide synthase by N(omega)-nitro- L-arginine methyl ester (10(-4) M). To evaluate the contribution of Ca2+-activated K+ channel(s) to BK-induced relaxation, the resistance arteries were exposed to 10(-3) M tetraethylammonium. Comparatively, the endothelium-independent relaxation was assayed using sodium nitroprusside (10(-8)-10(-4) M). The results showed that compared to the H and D groups, the HD hamsters exhibited the most reduced vasodilation of the resistance arteries to BK (34.09 +/- 1.5%). The diminished vasodilation was found to be due to a dual mechanism: an L-arginine:NO pathway and a NO-independent process, mediated via Ca2+-activated K+ channels. In vivo administration of L-arginine had favourable effects especially in the HD group, which manifested (i) an; 30% improvement of attenuated BK relaxation, (ii) an increase in sensitivity of the response to BK, (iii) a 3-fold diminishment of plasma hyperglycemia. Collectively, these data explain in part, the mechanisms and possible ways to correct the arterial endothelial dysfunction when diabetes is complicated with hyperlipemia.