Effect of sodium-potassium pump inhibitors and membrane-depolarizing agents on sodium nitroprusside-induced relaxation and cyclic guanosine monophosphate accumulation in rat aorta

Abstract

The purpose of this study was to investigate the relationship between sodium nitroprusside-induced relaxation, inhibition of the sodium-potassium pump, and cyclic guanosine monophosphate. Exposure of rat thoracic aorta to ouabain, or potassium- or magnesium-free Krebs-Ringer bicarbonate solution, procedures which presumably inhibit the sodium-potassium pump, or to potassium chloride or tetraethylammonium, membrane-depolarizing agents, inhibited relaxation to nitroprusside. These conditions had little or no effect on the elevated cyclic guanosine monophosphate levels at a concentration of nitroprusside (0.1 μM) that relaxed norepinephrine contracted tisues by 80%. However, at a maximum relaxant concentration of nitroprusside (1.0 μM), these conditions decreased the elevation of cyclic guanosine monophosphate. The inhibition of elevated cyclic guanosine monophosphate levels was independent of the endothelium, extracellular calcium, and the cyclic guanosine monophosphate phosphodiesterase inhibitor, M&B 22,948. The inhibitory effects of ouabain and of potassium- and magnesium-free solution on the increased levels of cyclic guanosine monophosphate caused by 1.0 μM nitroprusside were abolished when tissues were incubated without norepinephrine, or with norepinephrine in the presence of the α-adrenergic blocker, phentolamine. In contrast, a β-adrenergic blocker, propranolol, had no effect on the ouabain-induced inhibition of elevated cyclic guanosine monophosphate levels, with norepinephrine present. These results are consistent with the hypothesis that membrane events regulate cyclic guanosine monophosphate synthesis. At nitroprusside concentrations greater than 0.1 μM, the formation of cyclic guanosine monophosphate appears to be coupled with the status of the smooth muscle cell membrane and integrity of the sodium-potassium pump. Furthermore, the elevated cyclic guanosine monophosphate levels induced by nitroprusside may be compartmentalized and, depending upon the intracellular concentration, may be associated with several functions. These functions may include activation of the sodium-potassium pump and/or membrane hyperpolarization. However, an effect of cyclic guanosine monophosphate on sodium potassium pump activity and/or membrane potential remains to be demonstrated.