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 μM) 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 ~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.