Blocker-induced noise analysis of apical membrane Na channels of epithelia of frog skin was carried out with the electroneutral blocker (CDPC, 6-chloro-3,5-diamino-pyrazine-2-carboxamide) that permitted determination of the changes of single-channel Na currents and channel densities with minimal inhibition of the macroscopic rates of Na transport (Baxendale, L. M., and S. I. Helman. 1986. Biophys. J. 49:160a). Experiments were designed to resolve changes of channel densities due to mass law action (and hence the kinetic scheme of blocker interaction with the Na channel) and to autoregulation of Na channel densities that occur as a consequence of inhibition of Na transport. Mass law action changes of channel densities conformed to a kinetic scheme of closed, open, and blocked states where blocker interacts predominantly if not solely with open channels. Such behavior was best observed in "pulse" protocol experiments that minimized the time of exposure to blocker and thus minimized the contribution of much longer time constant autoregulatory influences on channel densities. Analysis of data derived from pulse, staircase, and other experimental protocols using both CDPC and amiloride as noise-inducing blockers and interpreted within the context of a three-state model revealed that Na channel open probability in the absence of blocker averaged near 0.5 with a wide range among tissues between 0.1 and 0.9.

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