Theophylline affects the inotropic status of guinea-pig left atria by actions on the sarcolemma and sarcoplasmic reticulum.

Theophylline (0.1, 0.3 mM) produced a positive inotropism which was maximum by 15 min but was partially or completely absent by 45 min. The mechanism for this response was investigated using post-rest potentiation and positive staircase, phenomenon associated with the sarcoplasmic reticulum and sarcolemma, respectively. Staircase was enhanced at 15 min, but not after 45 min. Post-rest potentiation was attenuated by theophylline. The data suggest that theophylline's effect on cardiac contractility results from opposing actions on the sarcolemma and sarcoplasmic reticulum.