Background and purpose: Ca(2+)-activated Cl(-) currents (I(Cl(Ca))) in arterial smooth muscle cells are inhibited by phosphorylation. The Ca(2+)-activated Cl(-) channel (Cl(Ca)) blocker niflumic acid (NFA) produces a paradoxical dual effect on I(Cl(Ca)), causing stimulation or inhibition at potentials below or above 0 mV respectively. We tested whether the effects of NFA on I(Cl(Ca)) were modulated by phosphorylation.
Experimental approach: I(Cl(Ca)) was elicited with 500 nM free internal Ca(2+) in rabbit pulmonary artery myocytes. The state of global phosphorylation was altered by cell dialysis with either 5 mM ATP or 0 mM ATP with or without an inhibitor of calmodulin-dependent protein kinase type II, KN-93 (10 microM).
Key results: Dephosphorylation enhanced the ability of 100 microM NFA to inhibit I(Cl(Ca)). This effect was attributed to a large negative shift in the voltage-dependence of block, which was converted to stimulation at potentials <-50 mV, approximately 70 mV more negative than cells dialysed with 5 mM ATP. NFA dose-dependently blocked I(Cl(Ca)) in the range of 0.1-250 microM in cells dialysed with 0 mM ATP and KN-93, which contrasted with the stimulation induced by 0.1 microM, which converted to block at concentrations >1 microM when cells were dialysed with 5 mM ATP.
Conclusions and implications: Our data indicate that the presumed state of phosphorylation of the pore-forming or regulatory subunit of Cl(Ca) channels influenced the interaction of NFA in a manner that obstructs interaction of the drug with an inhibitory binding site.