Effects of caffeine and isoprenaline on mammalian ventricular muscle.

1. Caffeine, 0.6-20.0 mM, altered the duration of the action potential recorded from kitten papillary muscles; low concentrations shortened and high concentrations prolonged the action potential.2. Caffeine, 20 mM, prolonged the action potential by delaying the final phase of repolarization.3. Caffeine, 2.0-20.0 mM increased the tension developed and the duration of the isometric contraction.4. When large stimulating electrodes were used, all concentrations of caffeine increased the duration of the action potential; this effect was probably due to the interactions of caffeine and released endogenous catecholamines.5. Concentrations of caffeine and isoprenaline, which separately caused little change in the duration of the action potential, greatly prolonged the action potential when used together.6. The effects of caffeine may be due to an increase in membrane calcium current in addition to an action on intracellular calcium stores.