The role of AT(1) receptor signaling to maintain cerebrovascular relaxation mechanisms.

摘要

Elevated dietary salt intake is recognized as a risk factor for many cardiovascular diseases, including hypertension and stroke. In the cerebral microcirculation, high-salt intake has been demonstrated to decrease vascular relaxation to several vasodilator stimuli, possibly contributing to the pathogenesis of cerebrovascular ischemia and stroke. However, since high-salt intake is known to selectively affect certain vasodilator responses, this loss appears to result from changes in an active signaling process rather than a general loss of vascular function. The hypothesis of this dissertation is that a high-salt diet impairs cerebral vascular relaxation mechanisms by chronically altering intracellular signal transduction in middle cerebral arteries.;High-salt intake appears to mediate its adverse effects on the micro circulation through suppression of angiotensin II (ANG II), since cerebral vasodilator responses can be restored in animals fed a high-salt diet with a chronic, sub-pressor infusion of the hormone. These vasodilator responses can also be restored by returning animals fed a high-salt diet to a low-salt diet for two weeks. However, while giving high-salt fed animals a chronic infusion of ANG II or returning these animals to a low-salt diet are known to restore vascular relaxation, the mechanisms mediating these restored responses are unknown. The first specific goal of this dissertation was to characterize the vascular relaxation mechanisms that mediate these restored vasodilator responses.;The action of ANG II to restore vascular relaxation is mediated through the AT1 receptor, since the effect is abolished by an AT1 receptor antagonist such as losartan. It also appears likely that AT 1 receptor activation plays a housekeeping role to maintain vascular reactivity under normal physiological conditions. However, little is known about the mechanism by which the AT1 receptor carries out this restoration and maintenance of vascular relaxation mechanisms. The second specific goal of this dissertation was to identify critical signaling components downstream of the AT1 receptor that are necessary to restore and maintain normal vascular relaxation responses.;The first set of experiments examined the mechanisms of middle cerebral artery (MCA) responses in four groups of rats: (1) animals fed a low-salt (0.4% NaCl) diet; (2) animals fed a high-salt (4% NaCl) diet for 3 days; (3) animals fed a high-salt diet for 3 days before being returned to a low-salt diet for 2 weeks (HS→LS); and (4) animals fed a high-salt diet for 3 days before receiving a sub-pressor infusion of ANG II (HS + ANG II; 5 ng/kg/min) for 3 additional days while remaining on a high-salt diet. On the day of the experiment, isolated MCA were treated with either (1) no enzyme inhibitor, (2) the nitric oxide synthase inhibitor No-nitro-L-arginine methyl ester (L-NAME; 10-4 M), or (3) the cyclooxygenase (COX) inhibitor indomethacin (INDO; 10-6 M). Responses of isolated MCA from each treatment group were then assessed to acetylcholine (ACh; 10 -9-10-5 M), reduced PO2, the NO donor sodium nitroprusside (SNP; 10-9-10-5 M), the stable analogue of prostacyclin---iloprost (10-16-10-11 g/mL), the stable analogue of prostglandin E2---butaprost (10-14-10-7 M), the stimulatory G-protein (G s) activator cholera toxin (10-13-10-7 g/mL), and the adenylyl cyclase activator forskolin (10-12-10 -6 M).;For endothelium-dependent responses to ACh, it was determined that the normal dilation in animals fed a low-salt diet was mediated by nitric oxide (NO). The ACh-induced dilation in the HS→LS and HS + ANG II groups was also NO-mediated. MCA responses to ACh were abolished in animals fed a high-salt diet, and treatment with enzyme inhibitors did not affect this response. Relaxation in response to reduced PO2 was mediated by endothelial release of a COX-generated mediator in animals fed a low-salt diet, and this COX-dependent mechanism returned to mediate hypoxic dilation in the HS→LS and HS + ANG II groups. On the other hand, MCA from animals fed a high-salt diet constricted in response to hypoxia, and this constriction was also COX-dependent. These results indicate that returning animals fed a high-salt diet to a low-salt diet or infusing high-salt fed animals with ANG II restores the normal mechanisms of endothelium-dependent vasodilation. (Abstract shortened by UMI.).