Effects of Na +, K +-pump inhibitors and membrane depolarizing agents on acetylcholine-induced endothelium-dependent relaxation and cyclic GMP accumulation in rat aorta

Abstract

The purpose of this study was to investigate the effects of inhibitors of the Na +,K +-pump and membrane depolarizing agents on endothelium-dependent relaxation and elevated cyclic GMP levels induced by acetylcholine in rat thoracic aorta. Ouabain or exposure to K +-free or Mg 2+-free Krebs-Ringer bicarbonate solution, agents and procedures known to inhibit the Na +,K +-pump, inhibited acetylcholine-induced relaxation and the associated increased levels of cyclic GMP. However, the inhibitory effect of ouabain on cyclic GMP levels was abolished in the absence of norepinephrine or in the presence of norepinephrine and the α-adrenergic receptor antagonist phentolamine. The membrane depolarizing agents KCl and tetraethylammonium also inhibited the acetylcholine-induced relaxation and the elevated cyclic GMP levels. Exposure to norepinephrine reduced the increased levels of cyclic GMP due to acetylcholine as compared to rested controls. This effect was inhibited by prior exposure to phentolamine, but not by the β-adrenergic receptor antagonist, propranolol. These results suggest that (1) increased activity of the Na +,K +-pump may mediate, in part, endothelium-dependent relaxation; (2) inhibition of relaxation may be due to membrane depolarization; (3) the endothelium-dependent increased levels of cyclic GMP may increase Na +,K +-pump activity; (4) a complex interaction exists between membrane polarization, the Na +,K +-pump and α-adrenergic stimulation in regulation of cyclic GMP accumulation and relaxation.