Mechanisms for nongenomic and genomic effects of aldosterone on Na+/H+ exchange in vascular smooth muscle cells

OBJECTIVES: We have reported that exposure of vascular smooth muscle cells (VSMCs) to aldosterone for 3 and 24 h activated Na+/H+ exchange (NHE) via nongenomic and genomic mechanisms, respectively. The present study determined whether aldosterone-induced nongenomic and genomic NHE activation depends on the number of transporters, the turnover rate of a single transporter, and/or the change in intracellular pH (pHi) sensitivity of the transporter, and whether aldosterone-induced NHE activation is inhibited by the selective mineralocorticoid receptor (MR) antagonist (eplerenone). METHODS: Using a fluorescent dye, we assessed NHE activity by Na-dependent acid extrusion rates (JH) after an acid load in the absence of CO2/HCO3- in VSMCs treated with aldosterone. RESULTS: Treatment with aldosterone for 3 and 24 h increased JH at the wide pHi range, and shifted the JH versus pHi in the alkaline direction. Without affecting the apparent Km for external Na+, the Vmax increased in VSMCs treated with aldosterone for 3 and 24 h. Both eplerenone and spironolactone inhibited only aldosterone-induced genomic NHE activation, but the IC50 of eplerenone was smaller than that of spironolactone. CONCLUSION: We demonstrated that: (1) both nongenomic and genomic stimulatory effects of aldosterone on NHE activity in VSMCs occur by an increase in the number of NHEs and the alkaline shift in pHi sensitivity of the NHE; (2) only the aldosterone-induced genomic NHE activation occurs via MR; and (3) both eplerenone and spironolactone inhibit the aldosterone-induced genomic NHE activation, but eplerenone is more effective than spironolactone, based on the IC50 value in VSMCs.