Biochemical and electrophysiological mechanisms of the positive inotropic effect of calyculin A, a protein phosphatase inhibitor.

Calyculin A (CyA; 1 microM) increased the force of contraction in isolated guinea pig papillary muscles to 144% of control without affecting contraction parameters. The effect of CyA on L-type calcium channels was assessed in cell-attached patches of guinea pig ventricular cardiomyocytes. Unitary Ba++ current recordings revealed that CyA at micromolar concentrations enhanced channel availability almost 2-fold, whereas the duration of individual openings and closures remained unchanged. In whole-cell recordings with Ca++ as the charge carrier, intracellular dialysis of 1 microM CyA enhanced peak current to a similar extent. In homogenates from guinea pig ventricles, 1 microM CyA completely inhibited phosphorylase phosphatase activity. In isolated [32P]-labeled guinea pig ventricular cardiomyocytes, 1 microM CyA increased the phosphorylation state of phospholamban (to 267% of control), that of the inhibitory subunit of troponin (to 182% of control) and those of various additional proteins. We conclude that the effects of CyA are likely to be mediated by increasing the phosphorylation state of several regulatory proteins.