AMILORIDE ANALOGS INDUCE RESPONSES IN ISOLATED RAT CARDIOVASCULAR TISSUES BY INHIBITION OF NA+/CA2+ EXCHANGE

The role of inhibition of Na+/Ca2+ exchange in the positive inotropic, negative chronotropic and vasorelaxant responses to amiloride and some of its analogues was investigated in isolated cardiovascular tissues from female Wistar rats. The compounds tested were amiloride, 5-(N-ethyl-N-isopropyl)amiloride (EIPA, a potent inhibitor of Na+/H+ exchange), phenamil and 2',4'-dimethylbenzamil (DMB), both potent Na+ channel inhibitors with activity against Na+/Ca2+ exchange, and 5-(N-4-chlorobenzyl)-2',4'-dimethylbenzamil (CBDMB), a potent inhibitor of Na+/Ca2+ exchange with reduced activity against Na+ channels compared with its parent compound DMB. Phenamil, DMB and CBDMB increased the force of contraction of right ventricular papillary muscles with similar potencies (-log EC50 values: 4.77 +/- 0.06, 5.09 +/- 0.09, 4.97 +/- 0.17 respectively), while amiloride and EIPA gave small negative inotropic responses. All compounds gave negative chronotropic responses at similar concentrations to those which exerted inotropic effects. Inhibition of KCl contraction of endothelium-free aortic rings was observed with all compounds tested. Phenamil, DMB and CBDMB but not amiloride or EIPA showed a shift to the left of the concentration-response curves in the presence of intact endothelium. These results provide further evidence for positive inotropic and endothelium-dependent vasorelaxant effects of amiloride analogues mediated by inhibition of Na+/Ca2+ exchange.