Inhibition of Na+/Ca2+ exchange in membrane vesicle and papillary muscle preparations from guinea pig heart by analogs of amiloride.

Na+/Ca2+ exchange is inhibited in both guinea pig cardiac membrane vesicles and papillary muscles in a concentration-dependent fashion by several analogs of the pyrazine diuretic amiloride. Structure/activity studies based on transport measurements in vesicles prepared from guinea pig left ventricle indicate that hydrophobic substitutions at the terminal nitrogen atom of the guanidinium moiety of amiloride improved the inhibitory potency almost 100-fold over that of the parent compound. 3',4'- Dichlorobenzamil ( DCB ) is one of the most active inhibitors (IC50 = 17 microM). In electrically stimulated papillary muscles isolated from guinea pig heart, 10-40 microM DCB decreases contractile force. At 100 microM inhibitor, diastolic tension is significantly increased. The positive inotropic responses to veratridine and ouabain are inhibited by 20 and 40 microM DCB . Since the responses to these interventions were a consequence of increased intracellular Na+ concentration, these data indicate that DCB is an inhibitor of Na+-dependent Ca2+ influx in the intact tissue. Interpretation of mechanical responses elicited by paired pulses suggests that 40 microM but not 100 microM DCB decreases release of Ca2+ from the sarcoplasmic reticulum. The mechanical data obtained with concentrations of DCB that inhibited Na+/Ca2+ exchange in vesicles suggest that a significant amount of Ca2+ can enter the cardiac cell via Na+/Ca2+ exchange under normal conditions and that this transport system may be an important source of Ca2+ supplying the sarcoplasmic reticulum in guinea pig heart. Moreover, these amiloride analogs function as potent inhibitors of the positive inotropic effect caused by increased intracellular Na+ concentration.