ANGIOTENSIN-II ACTIVATES THE NA+/HCO3- SYMPORT THROUGH A PHOSPHOINOSITIDE-INDEPENDENT MECHANISM IN CARDIAC-CELLS

Angiotensin II (AngII) is a hormone that alters contractility as well as myocyte growth in heart. Since many hormones that regulate cardiac contractility have also been found to modulate intracellular pH (pH(i)) the goal of this study was to determine if AngII altered pH(i) in cultured neonatal rat ventricular myocytes. Changes in pH(i) were monitored in single cells using the fluorescent pH indicator carboxy-seminaphthorhodafluor-1. Application of 100 nM AngII resulted in a rapid, receptor-mediated alkalinization of 0.08 +/- 0.02 pH unit. The Na+/H+ exchanger was not involved since the response was HCO3--dependent and amiloride-insensitive. Ammonia rebound experiments showed AngII increased the initial rate of recovery from an imposed acid load by 3.15-fold and showed that the hormone led to the selective activation of the Na+/HCO3- symport. In contrast, phorbol ester activation of protein kinase C led to the selective activation of Na+/H+ antiport in these cells. Pharmacological studies showed that the alkalinization was independent of the AngII receptor subtype 1 (AT(1)) phosphoinositide signaling path. In contrast, AngII activation of the symport was blocked by nanomolar AT(2) receptor antagonist PD 123319. Superfusion of the myocytes with exogenous arachidonic acid (5 mu M) mimicked the AngII-mediated alkalinization, further suggesting that the AT(2) signaling pathway underlies the response. In summary, while most of the known actions of AngII in heart are mediated through AT(1) receptors, activation of the Na+/HCO3- symport occurs through a distinct alternative path that is likely related to fatty acid production.