Activation of chloride currents in murine portal vein smooth muscle cells by membrane depolarization involves intracellular calcium release

The present study describes the first characterization of Ca²⁺-activated Cl^– currents (I_ClCa) in single smooth muscle cells from a murine vascular preparation (portal veins). I_ClCa was recorded using the perforated patch version of the whole cell voltage-clamp technique and was evoked using membrane depolarization. Generation of I_ClCa relied on Ca²⁺ entry through dihydropyridine-sensitive Ca²⁺ channels because I_ClCa was abolished by 1 µM nicardipine and enhanced by raising external Ca²⁺ concentration or by application of BAY K 8644. I_ClCa was characterized by the sensitivity to Cl^– channel blockers and the effect of altering the external anion on reversal potential. Activation of I_ClCa after membrane depolarization was dependent on Ca²⁺ release from intracellular stores. Thus the amplitude of I_ClCa was diminished by the SR-ATPase inhibitor cyclopiazonic acid, the inositol 1,4,5-trisphosphate receptor antagonist 2-aminoethoxydiphenyl borate (2-APB), and the ryanodine receptor blocker tetracaine. The degree of inhibition produced by the application of 2-APB and tetracaine together was significantly greater than the effect of each agent applied alone. In current-clamp mode, injection of depolarizing current elicited a biphasic action potential, with the later depolarization being sensitive to niflumic acid (NFA; 10 µM). In isometric tension recordings, NFA inhibited spontaneous contractions. These data support a role for this conductance in portal vein excitability.