Studies on the calcium antagonistic action of tetrandrine: III. Effect of tetrandrine on positive inotropic action of isoproterenol and Ca⁺⁺ and on excitation-contraction coupling in isolated cat papillary muscles

Abstract

The effects of tetrandrine, verapamil, propranolol and practolol on isoproterenol dose-response relationships and influence of tetrandrine, verapamil and nifedipine on calcium dose-response relationships and on excitation-contraction coupling were investigated in isolated cat papillary muscles.

Propranolol and practolol competitively antagonized isoproterenol. Tetrandrine and verapamil also antagonized isoproterenol but in a non-competitive manner. Tetrandrine, verapamil and nifedipine shifted calcium dose-response curves to the right, and acted as non-competitive antagonists of calcium, as a plot of log (x-1) vs.log [B] resulted in straight lines with slopes of -0.43, -0.55 and -0.42, respectively.

Tetrandrine, verapamil and nifedipine resulted in excitation-contraction uncoupling. It is suggested that tetrandrine inhibited the inward displacement of calcium through the voltage-dependent channel.

The results are consistent with our previous supposition that tetrandrine is a calcium antagonist of plant origin.

Keywords

tetrandrine / verapamil / nifedipine / calcium-antagonist / isoproterenol / surface electrogram of cat papillary muscle / excitation-contraction coupling

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