[Effect of catecholamines on the electrical and mechanical activity of frog myocardium after increasing the duration of action potentials with nickel ions].

The "chemical current--clump" method revealed that, after nickel ions (0.5 mM) had produced prolongation of AP, catecholamines (CA) increased the force and shortened the relaxation phase of isometric tension in the frog ventricular myocardium. Additional blockade of Ca++--channels with 2 mM NiCl2 or niphedipin inversed the positive inotropic effect of CA. Prevention of the ability of CA to increase the Ca--current and AP--duration unmasks the action of CA on intracellular stores.