Developmental changes in the actions of phosphatase inhibitors on calcium current of rabbit heart cells

We used whole-cell voltage clamp to compare the modulation of calcium current density (I_Ca, picoampere per picofarad) of freshly isolated, adult and newborn rabbit heart in response to intracellular application of microcystin and okadaic acid, both of which block phosphatase activity of phosphatase types 1 and 2A. Newborn cells showed a much larger response to the intracellular application of either microcystin or okadaic acid than did adult cells. In newborn cells, the application of microcystin produced an increase in I_Ca which appeared to maximize I_Ca, as shown by the rise in I_Ca to levels which could be reached by application of 10 M forskolin or by the intracellular application of 200 M 3,5-cyclic adenosine monophosphate (cAMP). In adult cells, the maximal response to microcystin was considerably less than that obtainable with forskolin or cAMP. After achieving a maximal response with microcystin, the addition of forskolin increased I_Ca further in adult cells but elicited no additional response in newborn cells. The treatment of cells with 0.1 M isoproterenol, a concentration approximately equal to that required for a half-maximal response, strongly potentiated the effect of microcystin in newborn cells, but not in adult cells. We propose that newborn rabbit heart cells compared with adult rabbit heart cells have a greater level of protein phosphatase activity (perhaps combined with a somewhat greater kinase activity), a greater proportion of the protein phosphatase activity in the form of protein phosphatase type 1 (which is inhibited by isoproterenol) and a greater dependence on the inhibition of protein phosphatase as a mechanism of action of isoproterenol, compared with the increase in kinase activity on calcium channels.