ION-TRANSPORTING ACTIVITY IN THE MURINE COLONIC EPITHELIUM OF NORMAL ANIMALS AND ANIMALS WITH CYSTIC-FIBROSIS

Electrogenic ion transport in the isolated colonic epithelium from normal and transgenic mice with cystic fibrosis (CF mice) has been investigated under short-circuit current (I-sc) conditions. Normal tissues showed chloride secretion in response to carbachol or forskolin, which was sensitive to the Na-K-2CI cotransport inhibitor, frusemide. Responses to both agents were maintained for at least 12 h in vitro, but the responses to carbachol changed in format throughout this period. By contrast CF colons failed to show the normal secretory responses to carbachol and forskolin, most preparations showing a decrease in I-sc that was immediately reversed by frusemide. In CF colons addition of Ba2+ ions or tetraethylammonium (TEA(+)) to the apical bathing solution antagonised the reduction in I-sc caused by the secretagogues. It is concluded that the reduction in I-sc in CF colons is due to electrogenic K+ secretion and this was confirmed by flux studies using rubidium-86. In normal colons exposed to TEA(+) the responses to forskolin were greater, but not significantly so, presumably because the minor K+-secretory responses are dominated by major chloride-secretory responses. Again rubidium-86 fluxes showed an increase of K+ secretion in normal colons receiving forskolin. Since the amiloride-sensitive current was not different in CF and normal colons there was no evidence that the CF mice were stressed in a way that increased mineralocorticoid levels and hence K+ secretion. Knowledge of the phenotype of the colonic epithelium of the CF mouse sets the baseline from which attempts at gene therapy for the gut must be judged.