Abstract

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 μM) and transiently augmented in a concentration-independent manner (10 μM to 1 mM) ∼2-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 μM) affinity sites could reproduce the properties of the NFA-modulated I_Cl(Ca) fairly well.