Okadaic acid, a protein phosphatase inhibitor, increases the calcium transients in isolated ferret ventricular muscle.

Okadaic acid is a protein phosphatase inhibitor which has been found to produce a marked positive inotropic effect in isolated cardiac muscle. Using aequorin-injected ferret papillary muscles, we demonstrate that the increase in tension seen with okadaic acid is accompanied by a simultaneous increase in the amplitude of the calcium transients. By comparison with the effects of changing the extracellular calcium concentration, it is shown that the increase in calcium transient amplitude can account for the inotropic effect of okadaic acid.