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

Abstract

In a previous communication, we showed that beta-adrenergic stimulation of cardiac muscles 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 (fmin) is taken as a measure of the rate of crossbridge cycling. In this paper, we test the hypothesis that the beta-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 fmin in rat papillary muscles during Ba(2+)-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 beta-adrenergic stimulation: it potentiated the isometric twitch force, reduced the time to peak tension and time to half relaxation, and shifted fmin by a factor of 1.8 +/- 0.1 (n = 5). Isobutylmethylxanthine at up to 1.1 mM also acted in the same manner, increasing fmin 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 fmin changed (n = 4). Our findings support the hypothesis that a beta-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.

Mechanism of action of endothelin in rat cardiac muscle: cross‐bridge kinetics and myosin light chain phosphorylation

“…This insensitivity of fmin to endothelin is consistent with the lack of effect of endothelin on the twitch-time parameters, TTPT and T1Ï2R, in the light of unchanged Ca¥ transients. This contrasts with the mechanism of action of inotropic agents that work by increasing the level of intracellular cAMP, such as IBMX, and â-adrenergic receptor agonists, which result in increasing cross-bridge kinetics as indicated by an increase in fmin (Hoh et al 1991) and abbreviation of the time course of the isometric twitch. The lack of change in these stiffness spectra also makes it unlikely that endothelin alters the cross-bridge on-time, and further reinforces the interpretation of the tension-to-stiffness data that endothelin does not change the force per unit cross-bridge.…”

“…These results point to a species difference in the effect of endothelin on cardiac tissue. Potentiating the magnitude of the isometric twitch-force profile without altering its time course is also characteristic of another inotropic agent, ouabain (Hoh et al 1991). In contrast with ouabain, whose inotropic action can be attributed to an increase in the magnitude of the calcium transient (Allen & Blinks, 1978;Wier & Hess, 1984), no appreciable change in the calcium transient is associated with the action of endothelin on rat ventricular myocytes (Kelly et al 1990).…”

“…been established in previous studies Hoh et al 1988Hoh et al , 1991) that ventricular muscle from juvenile rats has an fmin value of 2 Hz at 25°C. This was again confirmed in this study where control fmin values were 2·0 ± 0·1 Hz (n = 4).…”

“…Endothelin caused no significant variation in the absolute values of fmin (2·0 ± 0·1 Hz, n = 13) from the control values, even when the total exposure time of the muscles to endothelin was extended to 60 min with the alternate protocol. IBMX has been previously shown (Hoh et al 1991) to increase cross-bridge kinetics in cardiac muscle, as indicated by an increase in fmin. We verified, by using IBMX, that each papillary muscle reported here was able to have its cross-bridge kinetics changed by this inotropic intervention (data not shown).…”

Mechanism of the effects of acetylcholine on the contractile properties and Ca2+ transients in ferret ventricular muscles.

“…Since cGMP derivatives did not mimic the effect of ACh on the CCR and IAP treatment blocked the effect of ACh on the CCR, the same mechanism as that in Ca2" sensitivity is involved in the regulation of the CCR in muscarinic receptor stimulation. Hoh, Rossmanith & Hamilton (1991) suggest that ,3-adrenoceptor stimulation increases the CCR and this effect is due to an increase in the intracellular cAMP level which is produced by the activation of adenylate cyclase. Although the details of the mechanism of ,-adrenoceptor stimulation which effects the CCR are not fully clarified (Hoh et al 1991), it is conceivable that muscarinic receptor stimulation inhibits the activation of adenylate cyclase via GTP-binding proteins (Gi and/or Go) and the decrease in the cAMP concentration is the mechanism which antagonizes the effect of ,3-adrenoceptor stimulation on the CCR.…”