Mechanisms of desensitization of vasodilatation induced by platelet-activating factor in hypertensive rats

Differential expression of α2D-adrenoceptor and eNOS in aortas from early and later stages of diabetes in Goto-Kakizaki rats

The aim of the present study was to compare vascular dysfunction between the early (12 wk old) and later (36 wk old) stages of spontaneous diabetes in Goto-Kakizaki (GK) rats. We also evaluated the aortic expression of the α2D-adrenoceptor and endothelial nitric oxide synthase (eNOS). Vascular reactivity was assessed in thoracic aortas from age-matched control rats and 12- and 36-wk GK rats. Using RT-PCR and immunoblots, we also examined the changes in expression of the α2D-adrenoceptor and eNOS. In aortas from GK rats (vs. those from age-matched control rats): 1) the relaxation response to ACh was enhanced at 12 wk but decreased at 36 wk; 2) the relaxation response to sodium nitroprusside was decreased at both 12 and 36 wk, 3) norepinephrine (NE)-induced contractility was decreased at 12 wk but not at 36 wk, 4) the expressions of α1B- and α1D-adrenoceptors were unaffected, whereas those of α2D-adrenoceptor and eNOS mRNAs were increased at both 12 and 36 wk; and 5) NE- and ACh-stimulated NOx (nitrite and nitrate) levels were increased at 12 wk, although at 36 wk ACh-stimulated NOx was lower, whereas NE-stimulated NOx showed no change. These results clearly demonstrate that enhanced ACh-induced relaxation and impaired NE-induced contraction, due to NO overproduction via eNOS and increased α2D-adrenoceptor expression, occur in early-stage GK rats and that the impaired ACh-induced relaxation in later-stage GK rats is due to reductions in both NO production and NO responsiveness (but not in eNOS expression).

Mechanisms underlying attenuated contractile response of aortic rings to noradrenaline in fructose-fed mice

We hypothesized that an impairment of endothelial dysfunction and an increased response to alpha-adrenoceptor agonists may occur in fructose-fed, insulin-resistant mice. The aim of the present study was to assess the relationship between endothelial dysfunction and agonist-induced contractile responses in such mice. The acetylcholine-induced relaxation was significantly attenuated in streptozotocin-diabetic and fructose-fed mice. The contractile response to noradrenaline was significantly weaker than the control in fructose-fed but not in streptozotocin-diabetic mice; treatment with N(G)-nitro-L-arginine effectively restored this response. Incubating aortic rings with noradrenaline increased the NO(x) [nitrite (NO(2)(-)) and nitrate (NO(3)(-))] level and this level was significantly higher in fructose-fed mice than in control mice. Clonidine induced a dose-dependent relaxation in aortic rings pre-contracted with prostaglandin F(2alpha) that was completely abolished by N(G)-nitro-L-arginine; this relaxation was markedly enhanced in fructose-fed mice. In both control and fructose-fed mice, the clonidine-induced relaxation was significantly attenuated and the noradrenaline-induced contraction augmented by pertussis toxin. These results suggest that endothelial function is attenuated in both fructose-fed and streptozotocin-diabetic mice. It is suggested that the decreased noradrenaline contractile response in fructose-fed mice (compared to both controls and streptozotocin-diabetic mice) may be due to an increase in nitric oxide formation mediated by endothelial GTP-binding-coupled alpha(2)-adrenoceptors.

Possible involvement of endothelium-derived hyperpolarizing factor (EDHF) in the depressor responses to platelet activating factor (PAF) in rats

1. In anaesthetized rats, platelet activating factor (PAF; 1 microg kg(-1)) decreased mean arterial blood pressure by around 60 mmHg (n=18). This depressor response was completely blocked by the PAF antagonist, CV-6209 (1 mg kg(-1)), indicating the role of PAF-specific receptor in the response. 2. N(G)-nitro-L-arginine methyl ester (L-NAME; 50 mg kg(-1)), an NO synthase inhibitor, profoundly elevated systemic blood pressure (n=19), indicating an important role of NO in the basal blood pressure regulation. The depressor response to PAF (1 microg kg(-1)) normalized against that to sodium nitroprusside (SNP) (10 microg kg(-1)) was not substantially different between rats treated without and with L-NAME (n=4). In contrast, the depressor effect of acetylcholine (0.03 - 1.0 microg kg(-1)) normalized against that of SNP (10 microg kg(-1)) was significantly attenuated by L-NAME (n=5). 3. Charybdotoxin (0.4 mg kg(-1)) plus apamin (0.2 mg kg(-1)) significantly attenuated the depressor response to PAF (1 microg kg(-1)) (n=5) without affecting the blood pressure change due to SNP (1 mg kg(-1)) (n=3). Charybdotoxin (0.4 mg kg(-1)) (n=4) or apamin (0.2 mg kg(-1)) (n=4) alone did not affect the PAF-induced depressor response. 4. These findings suggest that EDHF may make a significant contribution to the depressor response to PAF in rats. Although NO plays the determinant role in the basal blood pressure regulation, its contribution to PAF-produced depressor response seems to be less as compared with that to the depressor response to acetylcholine.