| Abstract: | The calcium ionophore A23187, in a concentration of 2.5  M, caused a two- to threefold increase in the strength of the contraction of the isolated papillary muscle of the guinea pig heart, stimulated at a frequency of 0.2 Hz. The ionophore A23187 reduced the resting voltage of the preparation in the intertrial interval and reduced the total duration of contraction. The increase in the strength of contraction was not accompanied by any change in the amplitude or duration of the transmembrane action potential. In the presence of the ionophore the ascending phase of a single contraction cycle showed a discontinuity separating the development of the twitch into two phases; differentiation of the twitch gave two positive maxima. The substance D-600, which blocks the calcium current, reduced the duration of the action potential and inhibited the second phase of twitch development, but caused no change in the magnitude or rate of the first phase of contraction. It is suggested that under the influence of the ionophore the component of the twitch which is not blocked by D-600 is caused by liberation of calcium from the sarcoplasmic reticulum.Laboratory of Electrophysiology of the Heart and Laboratory of Myocardial Metabolism, All-Union Cardiologic Scientific Center, Academy of Medical Sciences of the USSR, Moscow. Department of Pharmacology, Vanderbilt University, Nashville, USA. (Presented by Academician of the Academy of Medical Sciences of the USSR V. V. Zakusov.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 85, No. 6, pp. 690–693, June, 1978. |
