Effect of forskolin on phosphorylation of a 25,000 Mr protein in perfused guinea pig hearts.

The effects of forskolin on phosphorylation of proteins of a 100,000 X g fraction was examined in isolated beating guinea pig hearts. Hearts were perfused with [32P] inorganic phosphate to label intracellular adenine nucleotides. Forskolin was injected into the coronary circulation and after freeze-clamping, phosphorylated proteins in a fraction were separated by sodium dodecyl sulfate-polyacrylamide gel electro-phoresis. Forskolin increased the incorporation into a 25,000 Mr protein approximately 15 fold over control. Incorporation of label was time and dose dependent and was temporally coincident with increases in developed tension. A sarcolemmal fraction prepared from perfused hearts contained a similar 25,000 Mr protein. The data provides evidence that forskolin induced inotropy is accompanied by cAMP-dependent protein kinase mediated phosphorylation. The phosphorylation may be of the same protein whose phosphorylation is associated with epinephrine-induced increase in contractility.