Vol. 53, Issue 3, 539-546, March 1998

Arylaminobenzoate Block of the Cardiac Cyclic AMP-Dependent Chloride Current

Kenneth B. Walsh and Chongmin Wang

Department of Pharmacology, University of South Carolina, School of Medicine, Columbia, South Carolina 29208

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl channel has been identified in the cardiac muscle of a number of mammalian species, including humans. The goal of this study was to begin quantifying the structural requirements necessary for arylaminobenzoate block of the CFTR channel. The cardiac cAMP-dependent Cl current (I_Cl) was measured using the whole-cell arrangement of the patch-clamp technique in guinea pig ventricular myocytes during stimulation of protein kinase A with forskolin. At drug concentrations below the IC₅₀ value for channel block, reduction of I_Cl by the arylaminobenzoates occurred in a strongly voltage-dependent manner with preferential inhibition of the inward currents. At higher drug concentrations, block of both the inward and outward I_Cl was observed. Increasing the length of the carbon chain between the benzoate and phenyl rings of the arylaminobenzoates resulted in a marked increase in drug block of the channel, with IC₅₀ values of 47, 17, and 4 µM for 2-benzylamino-5-nitro-benzoic acid, 5-nitro-2-(2-phenylethylamino)-benzoic acid, and 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB), respectively. Increasing the carbon chain length further with the compound 5-nitro-2-(4-phenylbutylamino)-benzoic acid, caused no additional increase in the potency of drug block (IC₅₀ = 4 µM). 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 intracellular dialysis of NPPB, and this action was not affected by raising the external pH.