The role of the Ca2+-dependent phosphatase calcineurin (CaN) in the modulation of Ca2+-dependent Cl- channels (ClCa) was studied in freshly isolated rabbit coronary arterial myocytes. Immunocytochemical experiments showed that calmodulin-dependent protein kinase II (CaMKII) and CaN were distributed evenly throughout the cytoplasm of coronary myocytes at rest and translocated to the plasmalemma when intracellular Ca2+ was increased. ClCa currents (ICl(Ca)) elicited by cell dialysis with fixed intracellular Ca2+ levels up to 500 nM were inhibited by 10 microM cyclosporin A (CsA), a specific inhibitor of CaN, in a voltage-dependent manner, whereas currents evoked by 1 microM Ca2+ were not affected. Inhibition of CaN with CsA also led to a significant reduction in Ca2+ sensitivity of the channel at +50 mV; half-maximal activation increased from 363 +/- 16 nM Ca2+ in control to 515 +/- 40 nM Ca2+ in the presence of CsA. Similar effects were observed on ICl(Ca) when a specific peptide fragment inhibitor of CaN (CaN-AF, 5 microM) was dialysed into the cell via the pipette (500 nM Ca2+). Application of KN-93 (10 microM), a specific inhibitor of CaMKII, enhanced ICl(Ca) in myocytes dialysed with 1 microM Ca2+ but produced no significant effect on this current when the cells were dialysed with 350 or 500 nM Ca2+. These results are consistent with the notion that in coronary arterial cells, the activity of ClCa is enhanced by dephosphorylation of the channel or a closely associated regulatory protein. Moreover the balance of CaN and CaMKII regulating ICl(Ca) is dependent on the level of Ca2+ used to activate ICl(Ca).