Calcium-dependent changes of apical membrane sodium channel density and open probability in frog skin

Blocker-induced noise analysis of Na$\sp{+}$ channels at apical membranes of intact and isolated epithelia of frog skin was carried out to investigate whether treatment of tissues with ionomycin, quinine, or acidification of basolateral solution shared similar mechanisms of inhibition of Na$\sp{+}$ absorption. Experiments were done using staircase protocols (SC) that allowed evaluation of the dependence on blocker concentration of the macroscopic rates of Na$\sp{+}$ transport I$\sbsp{\rm Na}{\rm B}$, single channel Na$\sp{+}$ current (i$\sbsp{\rm Na}{\rm B}$), on and off blocker rate coefficients as well as determination of channel densities in open and blocked states and the Na$\sp{+}$ channel open probability in the absence of blocker ($\beta\sp\prime$). In addition to SC experiments, time course (TC) or paired time course (PTC) experiments allowed an assessment of the rates of change of the blocker rate coefficients and the changes of I$\sbsp{\rm Na}{\rm B}$, i$\sbsp{\rm Na}{\rm B}$, and the channel densities. Ionomycin, quinine, and acidification of the basolateral solution, despite their differing methods of increase of cytosolic Ca$\sp{++}$ activity, caused in common inhibition of total channel density (N$\sb{\rm T}$) and stimulation of open channel density (N$\sb{\rm o}$) by way of increase of open channel probability ($\beta\sp\prime$). Time course experiments revealed that inhibition of N$\sb{\rm T}$ and stimulation of $\beta\sp\prime$ were delayed by several minutes, with relatively large changes of N$\sb{\rm T}$ and $\beta\sp\prime$ occurring over 30 to 60 minutes. These observations support the view that Ca$\sp{++}$-related mechanisms of regulation of N$\sb{\rm T}$ and $\beta\sp\prime$ are slow with respect to reported rates of increase of intracellular Ca$\sp{++}$ and are thus incompatible with a direct effect of Ca$\sp{++}$ on the epithelial Na$\sp{+}$ channel.U of I OnlyETDs are only available to UIUC Users without author permissio