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We studied the relationships between the positive inotropic effects of isoproterenol, increased frequency of contraction or paired electrical stimulation, and cyclic AMP concentration and phosphorylase activity in isolated guinea pig papillary muscles. The minimum concentration of isoproterenol (10 nM) that augmented isometric force development increased cyclic AMP concentration. However 100 nM isoproterenol was required to increase the phosphorylase activity ratio (-AMP/+AMP) from 0.15 +/- 0.03 to 0.25 +/- 0.03. After addition of 1 muM isoproterenol to the bath, cyclic AMP increased within 0.5 minute from 0.58 +/- 0.03 to 1.04 +/- 0.13 mol/kg (wet weight), peak contractile force was elevated 2-fold at 1 minute, and the phosphorylase activity ratio rose to 0.40 +/- 0.02 in 4 minutes. Although an increase in contraction frequency (6/min to 36/ min) and paired stimulation produced more than a 3-fold increase in peak contractile force, there were no changes in cyclic AMP and phosphorylase activity. The cyclic AMP concentration during diastole was 0.60 +/- 0.04 and in midsystole, 0.55 +/- 0.03 mumol/kg. Anoxia increased the phosphorylase activity ratio from 0.19 +/-0.02 to 0.41 +/- 0.04 without elevation of cyclic AMP concentration. Removal of Ca2+ from the bathing medium prevented active force development and the anoxic increase in phosphorylase activity, but did not prevent the isoproterenol-induced increase in cyclic AMP and phosphorylase. These results suggest that cyclic AMP is a factor in the catecholamine-induced enhancement of inotropic state. However, it does not appear to play a role in the maintained augmentation of inotropic state produced by increased contraction frequency and paired stimulation, nor does the concentration of the cycle nucleotide appear to vary during the contraction cycle or during anoxia. Extracellular Ca2+ is required for contraction, the positive inotropic aciton of catecholamines and phosphorylase b to a conversion by anoxia.
Dobson et al. (1976) studied this question.