Analysis of angiotensin-stimulated sodium transport in cultured smooth muscle cells from rat aorta

Angiotensin peptides (AI, AII, AIII) increased the rate of Na+ accumulation by smooth muscle cells (SMC) cultured from rat aorta. The stimulatory effect of AII on Na+ uptake was observed when Na+ exodus via the Na+/K+ pump was blocked either by ouabain or by the removal of extracellular K+. AII was at least ten times more potent than AIII and about 100 times more potent than AI in stimulating Na+ uptake. Saralasin had little effect on Na+ uptake by itself but almost completely blocked the increase caused by AII. The stimulation of net Na+ entry by AI, but not AII, was prevented by protease inhibitors. The stimulation of Na+ uptake was almost completely blocked by amiloride. Tetrodotoxin, which prevented veratridine from increasing Na+ uptake, had no effect on the response to AII. Angiotensin increased the rate of ouabain-sensitive 86Rb+ uptake (Na+/K+ pump activity) but had no effect on ouabain-sensitive ATPase activity in frozen-thawed SMC or in microsomal membranes isolated from cultured SMC. The stimulation of ouabain-sensitive 86Rb+ uptake by AII was blocked by saralasin. Omitting Na+ from the external medium prevented AII from increasing 86Rb+ uptake. AII had no effect on cell volume or cyclic AMP levels in the cultured SMC. These results suggest that angiotensin peptides activate an amiloride-sensitive Na+ transporter which supplies the Na+/K+ pump with more Na+, its rate-limiting substrate.