Effect of chloride channel blockers on the cardiac CFTR chloride and L-type calcium currents

Objectives: The aim of this study was to determine the effects of Cl- channel blockers on the cardiac cystic fibrosis transmembrane conductance regulator (CFTR) Cl- current (I-Cl) and the protein kinase A-regulated L-type calcium current (PKA-I-Ca). Methods: Whole-cell I-Cl and I-Ca were recorded from isolated guinea pig ventricular myocytes using the patch clamp technique during stimulation of PKA by forskolin (1 or 2 mu M). Results: The inhibitory effects of clofibric acid, p-chlorophenoxy propionic acid, gemfibrozil, diphenylamine-2-carboxylate (DPC), anthracene-9-carboxylate, 4,4'dinitrostilbene-2,2'-disulfonic acid and indanyloxyacetic acid 94 were examined on the two currents. Clofibric acid (1 mM), p-chlorophenoxy propionic acid (1 mM) and gemfibrozil (250 mu M) produced an approximate 50% decrease in I-Cl, but had no effect on the PKA-I-Ca. Surprisingly, application of DPC (500 mu M and 1 mM) and anthracene-9-carboxylate (500 mu M) strongly reduced both currents. However, inhibition of the Ca2+ and Cl- channels by DPC could be differentiated in two important ways. First, increasing the pH of the external solution from 7.4 to 10.0 prevented the block of I-Cl by DPC, but did not attenuate the reduction in the PKA-I-Ca. Second, DPC inhibited the PKA-I-Ca in mouse atrial myocytes which lacked I-Cl. Neither 4,4'dinitrostilbene-2,2'-disulfonic acid (100 mu M) nor indanyloxyacetic acid 94 (50 mu M) caused any change in either of the guinea pig ventricular currents. Conclusions: Drugs such as DPC and anthracene-9-carboxylate which block the cardiac CFTR Cl- channel also inhibit the regulation of the L-type I-Ca. During beta-adrenergic stimulation, changes produced by these drugs on the cardiac action potential duration will be attributable to inhibition of both the Cl- and Ca2+ currents. Analogues of clofibric acid may serve as selective blockers of the CFTR Cl- channel that can be used to determine the physiological function of I-Cl in cardiac excitation.