Effect of dexamethasone on sodium channel block and densities in A6 cells.

The association (ON) and dissociation (OFF) rates of either positively charged amiloride or its uncharged analogue, CDPC (6-chloro-3, 5-diaminopyrazine-2-carboxamide), with the apical Na+ channel protein of renal A6 cells were analysed during exposure to the synthetic glucocorticoid, dexamethasone, using noise analysis. These rates were further used to reach specific conclusions about single-channel current, channel density and open probability of the channel in the absence of the blocker. Short-term exposure (3 h) to 10(-7) mol/l dexamethasone at the basolateral side increased the short-circuit current, Isc by 85%, without a change in the ON and OFF rates of the interaction between amiloride and the Na+ channel. A longer incubation (24 h) with dexamethasone tripled the current with a notable increase in the ON rate of the interaction between amiloride and the and channel. The OFF rate remained constant. The effects of dexamethasone on the rate constants of the reaction of amiloride with the channel did not match with the expected changes in membrane potential. On the other hand, ON and OFF rates of the interaction between neutral CDPC and the channel were not influenced by a 24-h incubation with dexamethasone. Further calculations disclosed that the gain in macroscopic current after a 24-h incubation with dexamethasone might be explained by an increase in Na+ channel density, and, to a lesser extent, by a rise in single-channel current. This all occurred without a change in the fraction of time spent by the channel in the conducting state in the absence of the blocker.