Molecular and functional distributions of chloride conductances in rabbit ventricle

The regulation of cardiac electrical activity is critically dependent on th e distribution of ion channels in the heart. For most ion channels, however , the patterns of distribution and what regulates these patterns are not we ll characterized. The most likely candidates for the genes that encode the cAMP- and swelling-activated chloride conductances in the heart are an alte rnatively spliced variant of CFTR and ClC-3, respectively. In this study we have 1) measured the density of CFTR and ClC-3 mRNA levels across the left ventricular free wall (LVFW) of the rabbit heart using in situ hybridizati on and 2) measured the corresponding current density of cAIVIP- and swellin g-activated chloride channels in myocytes isolated from subepicardial, midm yocardial, and subendocardial regions of the LVFW. There was a highly signi ficant gradient in the whole cell slope conductance of cAMP-activated chlor ide currents; normalized slope conductance at 0 mV was 15.7 +/- 1.8 pS/pF ( n = 9) in subepicardial myocytes, 7.8 +/- 1.5 pS/pF (n = 11) in midmyocardi al myocytes, and 4.9 +/- 1.1 pS/pF (n = 9) in subendocardial myocytes. The level of CFTR mRNA was closely correlated with the density of cAMP-activate d chloride conductances in different regions of the heart, with the level o f CFTR mRNA being three times higher in the subepicardium than in the suben docardium. The whole cell slope conductance of swelling-activated chloride channel activity, measured 3-5 min after the commencement of cell swelling, was higher in myocytes isolated from the subepicardium than in myocytes is olated from the midmyocardium or subendocardium. In contrast, there was a u niform expression of ClC-3 mRNA across the LVFW of the rabbit heart. These results suggest that the control of gene expression is an important contrib utor in regulating the distribution of cAMP-activated chloride channels in the rabbit heart but that it may be less important for the swelling-activat ed chloride channels.