Potentiating and Depressing Actions of Caffeine on Twitch Tension in the Isolated Atrial Muscle of the Bullfrog
Caffeine shows the dual actions on the twitch tension of isolated atrial muscles of bullfrogs. When the preparation, which is repetitively stimulated at the rate of 6/min, is exposed to Ringer solutions containing caffeine of 1, 2, 5 and 10 mM, the twitch tension first increases in 0.52 min and then decreases exponentially with the time constants of 4.5―6.2 min. Namely, the actions of caffeine are biphasic; the initial potentiation and the following depression. The time constants are independent of concentrations of either caffeine or of external calcium. The initial potentiation is explained from the increased sensitivity of troponin to Ca and the following slow decay is explained from the depletion of stored Ca inside the sarcoplasmic reticulum. After washout of caffeine solution, the twitch tension further decreases transiently and then recovers slowly to the control tension. These tension changes during the recovery seem to be a mirror image of the tension changes observed during caffeine exposure. The 90% recovery time, however, is prolonged by an increase in caffeine concentration and is shortened by an increase in calcium concentration. The explanations are that the transient tension drop is produced by the removal of the potentiating action and that the slow tension recovery is produced by the reaccumulation of calcium into the sarcoplasmic reticulum.