Dynamics of Ca2+-dependent Cl- channel modulation by niflumic acid in rabbit coronary arterial myocytes

Calcium-activated chloride channels (Cl-Ca) are crucial regulators of vascular tone by promoting a depolarizing influence on the resting membrane potential of vascular smooth muscle cells. Niflumic acid (NFA), a potent blocker of Cl-Ca in vascular myocytes, was shown recently to cause inhibition and paradoxical stimulation of sustained calcium-activated chloride currents [I-Cl(Ca)] in rabbit pulmonary artery myocytes. The aims of the present study were to investigate whether NFA produced a similar dual effect in coronary artery smooth muscle cells and to determine the concentration-dependence and dynamics of such a phenomenon. Sustained I-Cl(Ca) evoked by intracellular Ca2+ clamped at 500 nM were dose-dependently inhibited by NFA (IC50 = 159 muM) and transiently augmented in a concentration-independent manner (10 muM to 1 mM) similar to2-fold after NFA removal. However, the time to peak and duration of NFA-enhanced I-Cl(Ca) increased in a concentration-dependent fashion. Moreover, the rate of recovery was reduced by membrane depolarization, suggesting the involvement of a voltage-dependent step in the interaction of NFA, leading to stimulation of I-Cl(Ca). Computer simulations derived from a kinetic model involving low (K-i = 1.25 mM) and high (K-i < 30 mu M) affinity sites could reproduce the properties of the NFA-modulated I-Cl(Ca) fairly well.