Effects of N-chlorobenzyl analogues of amiloride on myocardial contractility, Na-Ca-exchange carrier and other cardiac enzymatic activities

Summary

1. In electrically driven guinea pig left atria, micromolar concentrations (2 μmol/l to 80 μmol/l) of N-chlorobenzyl derivatives of amiloride (o-chlorobenzamil and 3′,4′-dichlorobenzamil) produced quantitatively similar positive inotropic effects. Contracture developed with 3′,4′-dichlorobenzamil. Endogenously released catecholamines contributed 30% to the positive inotropic effect of ochlorobenzamil but did not contribute at all to the effect of 3′,4′-dichlorobenzamil. When tested in the presence of the inhibitor of phosphodiesterase isobutylmethylxanthine, ochlorobenzamil antagonized its positive inotropic effect, whereas 3′,4′-dichlorobenzamil potentiated it. o-Chlorobenzamil also antagonized the positive inotropic effect of ouabain in that it shifted its concentration-effect curve to the right. Moreover, o-chlorobenzamil prevented the appearance of ouabain toxicity in terms of a rise in the resting force. 2. Also, in electrically driven guinea pig papillary muscle, micromolar concentrations (5 μmol/l to 30 μmol/l) of both N-chlorobenzyl derivatives of amiloride produced a positive inotropic effect. This effect was more marked with 3′,4′-dichlorobenzamil than with o-chlorobenzamil and was associated for both compounds with lengthening of relaxation time. 3. o-Chlorobenzamil and 3′,4′-dichlorobenzamil influenced, though not to the same extent, several systems involved in the onset and in the control of cardiac contractility. 3′,4′-Dichlorobenzamil inhibited with the same potency Na-K-ATPase, sarcotubular Ca-ATPase, Na-Ca-exchange carrier, cAMP-dependent phosphodiesterase isolated from bovine heart and oxidative phosphorylation of mitochondria isolated from rat liver. Low micromolar concentrations of o-chlorobenzamil mainly inhibited Na-Ca-exchange carrier and cAMP-dependent phosphodiesterase. 4. The results suggest that 3′,4′-dichlorobenzamil is a quite unspecific compound and its cardiac effects are the result of an interference with several enzymatic and transport systems. In contrast, both the inhibition of the Na-Ca-exchange carrier and cAMP-dependent phosphodiesterase can contribute to the increase in the force of contraction induced by o-chlorobenzamil. Finally, the antagonism of o-chlorobenzamil against the cardiac effects of ouabain can be explained by the inhibition of the Na-Ca-exchange carrier.