The phorbol ester TPA increases the affinity of exocytosis for calcium in 'leaky' adrenal medullary cells.

Exposure of 'leaky' bovine adrenal medullary cells to the phorbol ester TPA causes a shift in the calcium-activation curve to lower calcium concentrations without altering the levels of secretion at the extremes of the activation curve. These results are consistent with a role for protein kinase C in exocytosis.