[Biphasic action of caffeine on contractile activity of frog myocardium]

Action of caffeine on the isometric contractions and intracellular potentials of the frog heart muscle involved the positive inotropic effect only at high concentrations (0.1-20 mM). The "dose-action" curve for the twitch responses reached plateau at caffeine concentration higher than 4 mM. Two phases of the caffeine effect on the frog myocardium contractility were identified: transient increase of the contraction amplitude (the 1st phase) was followed by the inhibition of contractions and shortening of the APs (the 3nd phase). The magnitude of the contractile changes and the duration of each phase depended on the caffeine concentration.