Abstract

The effect of epinephrine on the biochemical sequence of events leading to the activation of glycogen phosphorylase was studied in the isolated, perfused rat heart. In arrested preparations, perfused with a medium either devoid of Ca⁺⁺ or containing a depolarizing concentration of K⁺, the epinephrine-induced activation of phosphorylase was markedly diminished. In the absence of Ca⁺⁺, however, the epinephrine-induced rise in cyclic adenosine 3',5'-monophosphate (cyclic AMP) concentration and the activation of phosphorylase kinase were not greatly altered. This suggested that the effect of omission of Ca⁺⁺ occurred at the phosphorylase b to a conversion step.

The effect of epinephrine on phosphorylase activation was blocked simultaneously with the cessation of contraction following perfusion with a Ca⁺⁺-deficient medium. The store of Ca⁺⁺ required for the maintenance of contractile activity appeared to be similar to that necessary for the conversion of phosphorylase b to a.

Elevation of Ca⁺⁺ in the perfusion medium resulted in activation of phosphorylase but not of phosphorylase kinase; nor was there an increase in cyclic AMP concentration. This observation also suggested that Ca⁺⁺ is required for the catalytic activity of phosphorylase kinase.

In the heart depolarized with K⁺, only a small and transient increase in cyclic AMP concentration occurred in response to epinephrine. Activation of phosphorylase kinase was considerably less than that observed in the beating heart. This suggested that an excess of K⁺ interfered with the action of epinephrine at a site at or before adenyl cyclase.

Thus the activation of phosphorylase in cardiac muscle can be the consequence of one of the following events: (a) the activation of phosphorylase kinase as a result of a rise in cyclic AMP concentration; (b) an increase in the catalytic activity of phosphorylase kinase as a result of an increase in the concentration of Ca⁺⁺ available to the enzyme; or (c) a combination of these two mechanisms.