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

The possible interdependence of the effects of angiotensin II (Ang II) on intracellular pH (pH(i)), free cytosolic calcium (Ca-i(2+)) and free cytosolic sodium (Na-i(+)) 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 pH(i), Na-i(+) or Ca-i(2+), respectively. Superfusing with a HEPES solution containing Ang II (1 mu M) caused a progressive increase in Na-i(+). Concurrent pH(i) 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 Ca-i(2+) changes measured in parallel experiments showed that Ang II caused a prompt rise in Ca-i(2+) with a peak response at similar to 30 seconds followed by a rapid recovery. Obliteration of the Ang II-induced rise in Ca-i(2+) using a Ca2+ chelator (BAPTA), prevented the initial cell acidification but did not prevent the delayed rise in pH(i), which reflects stimulation of the Na+/H+ antiporter. Our data thus show that in cultured VSMC neither the rise in Ca-i(2+) nor the fall in pH(i) caused by Ang II is a total prerequisite for activation of the Na+/H+ antiporter by this agonist.