Role of an apical cation-selective channel in function of tight epithelia

Some features of oxytocin stimulation of a cation-selective channel of the apical membrane of amphibian tight epithelia were examined in an attempt to understand the channel's role in the regulation of epithelial transport. We first examined the ability of the channel to pass alkaline-earth cations. We found that oxytocin can stimulate the movement of alkaline-earth ions through the channel. This stimulation became greatly enhanced by treatment with Ag+. The stimulation of alkaline-earth movements is discussed together with recently reported experiments which suggest that the channel may be involved in K+ secretion. In addition we carried out comparative studies of the effects of oxytocin on the channel in a variety of epithelia obtained from different amphibians to examine whether the stimulation of ionic currents through the channel and the enhancement of hydrosmotic permeability caused by the hormone are linked. The results of our experiments showed that oxytocin activates the channel in the urinary bladders of Bufo marinus, and Rana catesbeiana as well as in the skin of B. marinus. It is well known that in all these tissues the hormone increases water permeability of the apical membrane. On the other hand, in skins of Rana catesbeiana, Rana pipiens, and Rana temporaria, where oxytocin does not have a hydrosmotic effect, the hormone did not increase the currents through the cation-selective channel.