Na+ transport across abdominal skins of the frog speciesRana esculenta andRana pipiens was analyzed by recording short-circuit current (Isc), transepithelial conductance (Gt), and the current noise generated by the random blockage of apical Na+ channels by the diuretic, amiloride.
Specific Na+ current (INa) and conductance (GNa), as reflected by the amiloride-sensitive part ofIsc andGt, respectively, were markedly depressed after addition of some osmotically active substances, like sugars or alcohols to the mucosal Na+-Ringer solution. These hypertonicity-induced reactions were fast and fully reversible, even at mucusal osmolarities of 1 Osmol.
With muscosal solutions of moderate hyperosmolarity a recovery ofINa andGNa was observed in presence of the osmotic gradient. This “regulatory” current showed to be carried by Na+ through the Na+-specific apical channels. Contrary to the fast current drop during the intial phase of hyperosmotic shocks, the “osmoregulation” was considerably slower. The recovery ofINa was only complete at smaller osmotic gradients but became more and more suppressed at higher osmolarities.
Steady-state analysis of the kinetics of the Na+-specific current revealed that the current depression by osmotic shocks obeys Michaelis-Menten kinetics. This current depression at high osmolaritics, as well as during the initial phase before “osmoregulation” with small osmotic gradients, can be described in terms of a non-competitive inhibition. This was also suggested by Na+-concentration jump experiments indicating a reduction of the maximal, apical Na+ permeability as mechanism of the hypertonicity-induced drop inINa. TheINa kinetics after complete “osmoregulation” were, however, indistinguishable from the isotonic control condition.
In the presence of the Na+-channel blocker, amiloride, the relaxation, or Lorentzian, noise component inINa was analyzed to yield microscopic parameters like single Na+-channel current (i), channel density (M), as well as the association (k01) and dissociation (k10) rate constants for the blocker reaction between amiloride and its receptor at the apical Na+ channel. With mucosal hypertonicity amiloride blocks less effectively, becausek01 is markedly reduced whereas changes ink10 could not be demonstrated. Furthermore, hypertonicity dramatically reducesM andi. While, after “osmoregulation” with small osmotic gradients, the reduction ink01 is still clearly observed,M andi gain their original values as if they were in isotonic condition.