ARYLAMINOBENZOATE BLOCK OF THE CARDIAC CYCLIC-AMP-DEPENDENT CHLORIDE CURRENT

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- cha nnel has been identified in the cardiac muscle of a number of mammalia n species, including humans. The goal of this study was to begin quant ifying the structural requirements necessary for arylaminobenzoate blo ck of the CFTR channel. The cardiac cAMP-dependent Cl- current (I-Cl) was measured using the whole-cell arrangement of the patch-clamp techn ique in guinea pig ventricular myocytes during stimulation of protein kinase A with forskolin. At drug concentrations below the IC50 value f or channel block, reduction of I-Cl by the arylaminobenzoates occurred in a strongly voltage-dependent manner with preferential inhibition o f the inward currents. At higher drug concentrations, block of both th e inward and outward I-Cl was observed. Increasing the length of the c arbon chain between the benzoate and phenyl rings of the arylaminobenz oates resulted in a marked increase in drug block of the channel, with IC50 values of 47, 17, and 4 mu M for 2-benzylamino-5-nitrobenzoic ac id, 5-nitro-2-(2-phenylethylamino)-benzoic acid, and 5-nitro-2-(3-phen ylpropylamino)-benzoic acid (NPPB), respectively. Increasing the carbo n chain length further with the compound 5-nitro-2-(4-phenylbutylamino )-benzoic acid, caused no additional increase in the potency of drug b lock (IC50 = 4 mu M) Inhibition of I-Cl by the arylaminobenzoates was modulated by the pH of the external solution; increasing the pH from 7 .4 to 10.0 greatly weakened NPPB block, whereas decreasing the pH to 6 .4 enhanced block. In addition, block of I-Cl was observed during intr acellular dialysis of NPPB, and this action was not affected by raisin g the external pH.