Formation of partially phosphorylated phosphorylase in isoproterenol stimulated rat hearts

Phosphorylase ab hybrid was demonstrated in perfused rat hearts and during the in vitro conversion of purified rat heart phosphorylase b. Phosphorylase ab hybrid was determined in rat heart extracts by the activating effect of AMP in the presence of caffeine. These results were confirmed by the quantitative determination of incorporated 32P in vitro and through the characteristic inhibition of ab hybrid by glucose-6-phosphate. As shown by our results, in aerobically perfused control hearts only the ab hybrid represents the active form of phosphorylase, its activity reaching about 20% of the total. In response to isoproterenol (5-1000 ng), the amount of ab hybrid rose to about 30-40%, preceding the rise of the a form, which increased in a dose-dependent manner up to 45% of the total. The great sensitivity of the ab form to AMP activation and glucose-6-phosphate inhibition supports its physiological significance in heart under in vivo conditions as well. Our results strongly suggest that the activity ratio -AMP/ + AMP reflects rather the percentage ratio of phosphorylated subunits than that of the activated (partially or totally phosphorylated) phosphorylase molecules.