A fluctuation analysis study of the development of amiloride-sensitive Na+ transport in the skin of larval bullfrogs (Rana catesbeiana).

In the presence of the Na+ -channel blocker amiloride, the short-circuit current across the skins of bullfrog tadpoles in metamorphic stages XIX-XXIV was subjected to fluctuation analysis. The resulting power spectra contained a Lorentzian component of which the plateau value (S0) decreased while the corner frequency (fc) increased as the mucosal amiloride concentration was increased from 0.5 to 24 microM. From the linear relationship between the fc values and the amiloride concentrations it was possible to determine the binding (k'01) and unbinding (k10) constants for amiloride to its receptor on the Na+ channel. With these parameters as well as short-circuit current and S0 values, the current through the individual Na+ channels (i) was calculated (average 0.58 pA). It did not increase significantly during late metamorphosis. The density of Na+ channels (M) in the apical membrane, on the other hand, increased significantly. It would appear that the increase in short-circuit current which occurs at this time is due primarily to an increase in amiloride-blockable Na+ channels. Unexpectedly, a Lorentzian component could be fitted to power spectra in amiloride-treated skins (stages XIX-XXI) which showed no amiloride-sensitive short-circuit current. Moreover, the typical increase in fc with the amiloride concentration did not occur in these animals.