Altered beta-receptor control of in situ membrane potential in hypertensive rats

Sympathetic neural activation of vascular smooth muscle beta-receptors induces membrane hyperpolarization and arterial relaxation. This response, which likely is mediated by the Gs protein-adenylyl cyclase-cyclic AMP signaling cascade, is reduced in some hypertensive animal models and in human essential hypertension. Since reduced beta-receptor-mediated vasodilation is a potential mechanism for enhanced arterial resistance, this study was designed to identify which step (or steps) in the beta-receptor signaling cascade is altered in hypertension. Transmembrane potentials were recorded in situ in small first-order arterioles and venules of cremaster muscle from hypertensive, reduced renal mass rats and normotensive, sham-operated controls. Vascular muscle cells in arterioles and venules of hypertensive rats were 5-7 mV more depolarized than in respective vessels of control rats during superfusion with physiological salt solution. Hyperpolarization and depolarization responses were reduced in hypertensive rats during superfusion with a beta-receptor agonist and antagonist, respectively, suggesting attenuated beta-receptor responsiveness compared with normotensive rats. Furthermore, direct activation of Gs protein by 10 ng/mL cholera toxin did not affect arterial or venous transmembrane potential in hypertensive rats, but hyperpolarized arterial and venous vascular muscle in normotensive controls by 17 mV. However, when the Gs protein-adenylate cyclase coupling step of the beta-receptor cascade was bypassed by using 10(-5) M forskolin to directly activate adenylate cyclase, arterial and venous vascular muscle of hypertensive rats hyperpolarized by 25-27 mV.(ABSTRACT TRUNCATED AT 250 WORDS)