The Effects of Theophylline on Aequorin Light Transients and Force in the Isolated Dog Right Ventricular Myocardium

Experiments were carried our to investigate the changes in intracellular Ca2+ transients associated with biphasic contractions that were elicited during interaction of theophylline with isoproterenol in the dog ventricular myocardium. For this purpose, effects of theophylline and isoproterenol on aequorin light transients and isometric contractions were assessed in the isolated canine ventricular trabeculae, superficial cells of which had been microinjected with the Ca2+ sensitive bioluminescent protein aequorin. The positive inotropic effect of theophylline (0.1-0.3 mM) was consistently associated with an increase in the amplitude of aequorin light transients. Theophylline at concentrations of 0.6 mM and higher decreased the amplitude of aequorin light transients, but the force of contraction increased further in association with a prominent prolongation of time to peak force. Theophylline (0.3 mM) enhanced the forskolin-induced increase in aequorin light transients and force. Theophylline (2 mM) inhibited the isoproterenol-induced increase in aequorin light transients associated with early phase of contraction in a reversible manner. A late phase of aequorin light transients was induced in association with late phase of contraction in the presence of both isoproterenol and theophylline. Thus, both the early and late phase of contraction were accompanied by corresponding phases of aequirin light transients. The relation between the amplitude of force and Ca2+ transients was markedly different and the late phase of contraction was associated with much lower aequorin light transients. The late phase of aequorin light transients induced by theophylline at a high concentration (10 mM) was enhance by isoproterenol. These results indicate that theophylline (0.1-0.3 mM) increases the amplitude of Ca2+ transients through an accumulation of cyclic AMP by inhibition of the cyclic AMP phosphodiesterase activity. In concentrations of 0.6 mM and higher theophylline decreases the amplitude of the early phase aequorin light transients probably by inhibition of release of Ca2+ from the sarcoplasmic reticulum and induces simultaneously the late phase of contraction that may be associated with an increase in responsiveness to Ca2+ of myofibrils.