Effects of S20787 on pHi-regulating mechanisms in isolated rat ventricular myocytes

Intracellular pH (pHi) regulation in the heart relies on the activity of three membrane mechanisms: the Na+/H+ exchange and an Na+, HCO3(-)-dependent transport, both activated after an acid load, and the Cl-/HCO(3-) exchange, activated by an intracellular alkalinization. Whereas several specific inhibitors of Na+/H+ exchange exist, distinguishing between the two HCO3(-)-dependent mechanisms remains difficult, especially near the steady state, because of the lack of specific inhibitors. To detect one such inhibitor, we tested the effects of S20787 on pHi regulation in the rat isolated ventricular myocytes. Intracellular pH was recorded with the fluorescent probe carboxy-SNARF-1. The NH4Cl (10 mM) prepulse method was used to induce an acid load to activate the dual acid extrusion system; Cl-/HCO3- exchange was activated with the acetate (40 mM) prepulse method. Our results showed that (a) a high dose (5.10(-6) M) of S20787 did not change intracellular intrinsic buffering power, beta i; (b) the dual acid extrusion system was unaffected by S20787 in the concentration range of 10(-11)-10(-6) M; and (c) S20787 partially inhibited (approximately 50%) the activity of Cl-/HCO3- exchange in a dose-dependent manner, with an IC50 of 8.8 x 10(-10) M. This inhibitory action of S20787 did not change the steady-state pHi after 5-10 min application. Our results demonstrate that S20787 is a specific and potent partial inhibitor of Cl-/HCO3- exchange in cardiac cells.