Angiotensin II and angiotensin-(1-7) effects on free cytosolic sodium, intracellular pH, and the Na(+)-H+ antiporter in vascular smooth muscle.

Abstract

The aim of the present study was to define the effects of angiotensin II (Ang II) and Ang-(1-7) on free cytosolic Na+ (Na+i), intracellular pH (pHi), and the Na(+)-H+ antiporter in cultured vascular smooth muscle cells from rat aorta. Cells were loaded with either BCECF-AM or SBFI-AM for measurement of pHi and Na+i, respectively. Ang II (10(-6) mol/L) caused a rapid rise in Na+i followed by a progressive increase that peaked at about 10 minutes (from 11 +/- 1.5 to 16 +/- 1.5 mmol/L, P < .001), whereas Ang-(1-7) (10(-6) mol/L) did not affect Na+i significantly (from 11.5 +/- 1.1 to 11.8 +/- 0.07 mmol/L). The effect of Ang II on Na+i was concentration dependent (delta Na+i, 5.1 +/- 0.9, 3.8 +/- 0.6, 1.6 +/- 0.6, and 0.14 +/- 0.18 mmol/L with decreasing concentrations of 10(-6), 10(-7), 10(-8), and 10(-9) mol/L, respectively). Ang II caused a brief acidification followed by an increase in pHi (from 7.34 +/- 0.03 to 7.43 +/- 0.03 after 10 minutes, P < .005), and Ang-(1-7) had no significant effect on pHi (from 7.23 +/- 0.03 to 7.23 +/- 0.03). To investigate whether pHi and Na+i changes induced by Ang II were due to cell Na+ entry via stimulation of the Na(+)-H+ antiporter, we pretreated cells with EIPA (25 mumol/L) or ouabain (2.0 mmol/L). Ang II in the presence of ouabain caused a greater increase than that seen with ouabain alone (delta Na+i, 13 +/- 1.5 versus 6.3 +/- 1.2 mmol/L, P < .0025). EIPA by itself decreased Na+i and pHi. After EIPA, Ang II failed to increase both Na+i and pHi, demonstrating that the Na(+)-H+ antiporter is responsible for the rises in Na+i and pHi during stimulation with Ang II. To further characterize the mechanism of Ang II action, we exposed cells to an Ang II type I receptor antagonist (L-158,809, 10(-6) mol/L) or two different type 2 receptor antagonists (PD 123177 and CGP 421112A, 10(-6) mol/L). L-158,809 completely blocked the rise in pHi caused by Ang II, whereas PD 123177 and CGP 421112A did not. We conclude that Ang II increases both Na+i and pHi, and both effects are mediated by stimulation of the Na(+)-H+ antiporter. Ang-(1-7), by contrast, has no significant effect on Na+i, pHi, or the Na(+)-H+ antiporter. Stimulation of this antiporter by Ang II is exerted through the type 1 receptor.