Ionic effects of angiotensin II and their role in the activation of the Na+/H+ antiporter

Abstract

The possible interdependence of the effects of angiotensin II (Ang II) on intracellular pH (pHi), free cytosolic calcium (Cai2+) and free cytosolic sodium (Nai+) was studied in cultured vascular smooth muscle cells (VSMC) from rat aorta. Cells were loaded with either BCECF/AM, SBFI/AM or Fura-2 for measurement of pHi, Nai+ or Cai2+, respectively. Superfusing with a HEPES solution containing Ang II (1 microM) caused a progressive increase in Nai+. Concurrent pHi changes measured in parallel experiments showed that Ang II caused a brief cell acidification followed by a delayed alkalinization, which is due to activation of the Na+/H+ antiporter. Concurrent Cai2+ changes measured in parallel experiments showed that Ang II caused a prompt rise in Cai2+ with a peak response at approximately 30 seconds followed by a rapid recovery. Obliteration of the Ang II-induced rise in Cai2+ using a Ca2+ chelator (BAPTA), prevented the initial cell acidification but did not prevent the delayed rise in pHi, which reflects stimulation of the Na+/H+ antiporter. Our data thus show that in cultured VSMC neither the rise in Cai2+ nor the fall in pHi caused by Ang II is a total prerequisite for activation of the Na+/H+ antiporter by this agonist.