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.