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1. Effects of caffeine on the electrical and mechanical properties of the atrial muscle of guinea pig were investigated.2. In normal Krebs solution, 3mM caffeine caused a more than 5-fold potentiation of contraction. The potentiation, however, became smaller the higher the concentration, and 50mM caffeine depressed the contraction somewhat in its later phase.3. In 3mM caffeine, the duration of action potential was prolonged up to 140% at a height of 80% of the spike, and in 50mM caffeine, up to 230%, whereas the membrane slope resistance was decreased. The rate of rise of the action potential appeared enhanced at the later phase.4. In sodium-free Tris-maleate Krebs solution, even 3mM caffeine evoked a contracture, and the contracture tension tended to increase with concentration of caffeine.5. By soaking the muscle in low calcium Tris-maleate Krebs solution the amplitude of caffeine contracture decreased, first rapidly and then slowly with time, suggesting the contributions of surface membrane and internal calcium store towards contractile activation.6. These results are interpreted as indicating that:a) Caffeine acts on both the surface membrane and the sarcoplasmic reticulum in the myocardium.b) Enhancing the inward calcium movement, caffeine augments the contraction both directly and indirectly through the sarcoplasmic reticulum.c) In normal Krebs solution, the amount of the accumulated calcium is small but is increased by sodium depletion.

EFFECTS OF CAFFEINE ON THE MEMBRANE POTENTIALS AND CONTRACTILITY OF THE GUINEA PIG ATRIUM