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[Effect of the ionophore A23187 on the contractility and slow action potential of the guinea pig papillary muscle].

Calcium ionophore A23187 (2.10(-6) M) increases contractility of the isolated guinea-pig papillary muscle 3.13 +/- +/- 0.47-fold at 25 degrees C and 1.88 +/- 0.28-fold at 37 degrees C. Prolonged exposure (30 min) to 2 mM of caffeine failed to eliminate positive inotropic effect of ionophore. A23187 (2.10(-6) M) also increased slow response action potential overshoot (18 mM K+ in Tyrode solution, resting potential 46 +/- 1 mV); at 37 degrees C the peak overshoot value was reached by the 2nd min, being equal to 5.2 +/- 0.4 mV; at 25 degrees C the peak value was attained by the 15th min, being equal to 15.5 +/- 0.9 mV with an abrupt increase in the repolarization rate. Overshoot enhancement within the first minutes of A23187 action is concluded to be due to the increased slow inward Ca-current rather than to the decreased outward currents. It is assumed that A23187 facilitates Ca2+ transport to the lipid phase thereby increasing the amount of sarcolemma bound Ca2+ that participates in the development of Ca-current.