Role of protein phosphatase in exocytosis in adrenal chromaffin cells

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Tacrolimus (FK506) and cyclosporin A (CsA) are potent immunosuppressant drugs that have been used in the prophylaxis of allograft rejection in organ transplantation. These compounds inhibit calcineurin, Ca^<2+>/calmodulin-dependent protein phosphatase, thereby resulting in the inhibition of T-lymphocyte activation. Calcineurin is highly conserved and widely distributed in virtually all cell types. Calcineurin has been suggested to regulate the activity of ion channels, neurotransmitter and hormone release. In the present study, the effects of FK506 and CsA on catecholamine (CA) release were examined in cultured bovine adrenal chromaffin cells. The results obtained are as follows.1)In intact cells, FK506 (1-30μM) inhibited CA release stimulated by acetylcholine (ACh ; 100μM), 1,1-dimethyl-4-phenyl-piperadinium iodide (DMPP ; 10μM) or high K^+ (40mM). CsA (1-30μM) had a little inhibitory effect on the ACh- or DMPP-stimulated CA release, whereas it enhanced the high K^+ -stimulated CA release.2)In β-escin-permeabilized cells, FK506 inhibited CA release stimulated by Ca^<2+> (1 and 10μM) in the presence and absence of MgATP (2mM). CsA induced CA release under Ca^<2+> free condition and enhanced the Ca^<2+>-stimulated CA release in the presence and absence of MgATP.It is known that the Ca^<2+>-dependent exocytosis involves at least two distinct steps, ATP-requiring priming stage and ATP-independent fusion step in adrenal chromaffin cells. Therefore, it is suggested that FK506 inhibits the Ca^<2+>-dependent exocytosis probably at the fusion step whereas CsA induces CA release from bovine adrenal chromaffin cells. It is also suggested that their effects on CA release is not related to the inhibitory action of FK506 or CsA on calcineurin.