Increase of cyclic AMP in subcellular fractions of rat heart muscle after beta-adrenergic stimulation: prenalterol and isoprenaline caused different distribution of bound cyclic AMP.

Although prenalterol is a partial beta-agonist, it has been reported to produce only a marginal, non-significant increase of cAMP in rat hearts. We studied the effect of prenalterol and isoprenaline on free and bound cAMP content in subcellular fractions of perfused rat hearts to see whether prenalterol increased cAMP and whether there were differences in the subcellular distribution of the cAMP contents after stimulation with the drugs. The in situ binding characteristics of cAMP to the protein kinases at different degree of beta-adrenergic stimulation were elucidated. Prenalterol increased significantly both total cAMP content in homogenate, bound cAMP in a 100,000 g particulate fraction and bound, free and total cAMP contents in the resulting supernatant. While prenalterol increased bound cAMP in the particulate fraction and in the supernatant proportionally, isoprenaline caused a relatively greater increase of bound cAMP in the supernatant. At equieffective concentrations regarding inotropic and lusitropic effects, prenalterol and isoprenaline increased bound cAMP in the particulate fraction to the same degree, while the increase in the other fractions was greater after isoprenaline. Thus, the cAMP bound in the particulate fraction seemed to determine the inotropic state of the cell after beta-adrenergic stimulation, supporting a compartmentation of cAMP in myocardium. In controls 31 to 34% of the total binding capacity for cAMP in the particulate fraction was occupied, increasing to 77% after maximal beta-stimulation. The binding capacity in the supernatant was only 15 to 18% saturated in the basal situation and about half saturated after maximal beta-stimulation. cAMP bound in the particulate fraction is of special interest when cAMP and functional effects are studied.