Coumestrol inhibits carotid sinus baroreceptor activity by cAMP/PKA dependent nitric oxide release in anesthetized male rats.

Phytoestrogens could offer multiple beneficial effects on the cardiovascular system. Here, we have examined the effects of coumestrol (CMT) on carotid baroreceptors activity (CBA) and the possible mechanisms in male rats. The functional parameters of carotid baroreceptors were measured by recording sinus nerve afferent discharge in anesthetized male rats with perfused isolated carotid sinus. The levels of protein expression were determined by using ELISA and Western blotting. CMT (1 to 100μmolL(-1)) inhibited CBA, which shifted the functional curve of the carotid baroreceptor to the right and downward, with a marked decrease in the peak slope and the peak integral value of carotid sinus nerve discharge in a concentration dependent manner. These effects were not blocked by a specific estrogen receptor antagonist ICI 182,780, but were completely abolished by nitric oxide (NO) synthase inhibitor l-NAME (N(G)-nitro-l-arginine methyl ester). Furthermore, a NO donor, SIN-1(3-morpholion-sydnon-imine), could potentiate these inhibitory effects of CMT. CMT stimulated the phosphorylation of Ser(1176)-eNOS (endothelial nitric oxide synthase) in a dose-dependent manner in carotid bifurcation tissue over a perfusion period of 15min. The rapid activation of eNOS by CMT was blocked by a highly selective PKA (protein kinase A) inhibitor H89. In addition, inhibition of PI3K (phosphatidylinositol-3-kinase) and ERK (extracellular signal-regulated kinase) pathways had no effect on eNOS activation by CMT. CMT inhibited CBA via eNOS activation and NO synthesis. These effects were mediated by the cAMP/PKA pathway and were unrelated to the estrogenic effect.