On the existence of epithelial chloride channels in intact frogs and crayfish

Chloride channels in isolated amphibian skins have been shown to be open (high PCl) when the transepithelial potential (TEP) across the skin is > 30 mV, inside positive, and [Cl-] in the external medium is high. In the absence of either of these signals PCl is much lower. Using Cl- efflux as a criterion, it is shown here that when the same cues are presented to intact frogs, channels open as in the in vitro preparation. The augmented efflux is partially inhibited by high I- in both preparations. However, salt-depleted frogs do not show channel opening, differing in this regard from isolated skins. Elevating external [Cl-] resulted in an even larger efflux stimulation in crayfish than in frogs. The TEP in these animals is usually small and sometimes blood negative; it does not appear to play a role in augmenting the efflux. Gluconate and NO3- also caused Cl efflux to increase markedly. Salt-depletion reduced the stimulation by high [Cl-] but did not abolish it as in frogs. Iodide had no effect on chloride efflux in crayfish. The results may indicate that gated channels exist in these animals, but interpretation is complicated by the possibility that the augmented efflux is due to exchange diffusion.