Effects of dibutyryl cyclic AMP, ouabain and xanthine derivatives on crossbridge kinetics in rat cardiac muscle

In a previous communication, we showed that β-adrenergic stimulation of cardiac muscle was associated with an increase in the rate of cycling of crossbridges as measured by perturbation analysis in the frequency domain. In this analysis, the frequency at which dynamic stiffness is a minimum (f(min)) is taken as a measure of the rate of crossbridge cycling. In this paper, we test the hypothesis that the β-adrenergic receptor-induced increase in crossbridge cycling rate is mediated by elevation of the intracellular level of cyclic AMP. The approach taken is to compare the effects on I(min) in rat papillary muscles during Ba²⁺-activated contractures of 1) an agonist of cyclic AMP that can easily penetrate the cell, namely, dibutyryl cyclic AMP, 2) agents that block cyclic AMP phosphodiesterase, namely, the xanthine derivatives isobutylmethylxanthine and caffeine, and 3) an inotropic agent that does not affect the intracellular level of cyclic AMP, namely, ouabain. Our results showed that dibutyryl cyclic AMP at a dose of 5 mM has the same actions as β-adrenergic stimulation: it potentiated the isometric twitch force, reduced the time to peak tension and time to half relaxation, and shifted f(min) by a factor of 1.8 ± 0.1 (n = 5). Isobutylmethylxanthine at up to 1.1 mM also acted in the same manner, increasing f(min) by a factor of 1.8 ± 0.2 (n = 6), but ouabain, at a dose (0.03 mM) sufficient to potentiate twitch force by 40 ± 2% (n = 4), was without effect on the time course of the twitch nor was f(min) changed (n = 4). Our findings support the hypothesis that a β-adrenergic receptor-mediated increase in crossbridge cycling rate is due to an increase in intracellular cyclic AMP level and illustrate the usefulness of the frequency domain analysis approach in the analysis of the mechanism of action of inotropic agents.