Distribution of cAMP-activated chloride current and CFTR mRNA in the guinea pig heart

Guinea pig ventricular myocytes exhibit a Cl--selective current regulated by the cAMP-dependent pathway. We have investigated the distribution of cAMP-activated Cl- channel current density and cystic fibrosis transmembrane-conductance regulator (CFTR) mRNA in three regions of the guinea pig heart: the atrium, and the epicardium and endocardium of the free wall of the left ventricle. The regional differences in the Cl- current density were investigated in enzymatically isolated myocytes using the whole-cell patch-clamp technique. Forskolin (1 mu mol/L) activated Cl--selective currents in all ventricular myocytes and 21% of atrial myocytes examined. The conductance density, estimated as the outward chord conductance normalized to cell capacitance, was greatest in epicardial myocytes (79.8 +/- 8.3 pS/pF, n = 21) and significantly lower in endocardial (59.8 +/- 9.5 pS/pF, n = 22) and atrial(10.9 +/- 5.0 pS/pF, n = 38) myocytes. The regional differences in CFTR mRNA expression levels were investigated by competitive reverse-transcribed polymerase chain reaction. The regional distribution of the mRNA levels was similar to that of the Cl- conductance density, ie, highest in the epicardium (23 230 +/- 1840 molecules/mu gl total RNA, n = 3), significantly lower in endocardium (10 610 +/- 780 molecules/mu g total RNA, n = 3), and lowest in atrium (1450 +/- 290 molecules/mu g total RNA, n = 3). The data indicate that regional differences in CFTR mRNA expression in the guinea pig heart are responsible, at least in part, for the regional differences in cAMP-activated Cl- current density.