Control of the amiloride-sensitive Na+ current in salivary duct cells by extracellular sodium

We have previously reported that intralobular salivary duct cells contain an amiloride-sensitive Na+ conductance (probably located in the apical membranes). Since the amiloride-sensitive Na+ conductances in other tight epithelia have been reported to be controlled by extracellular (luminal) Na+, we decided to use whole-cell patch clamp techniques to investigate whether the Na+ conductance in salivary duct cells is also regulated by extracellular Na+. Using Na(+)-free pipette solutions, we observed that the whole-cell Na+ conductance increased when the extracellular Na+ was increased, whereas the whole-cell Na+ permeability, as defined in the Goldman equation, decreased. The dependency of the whole-cell Na+ conductance on extracellular Na+ could be described by the Michaelis-Menten equation with a K(m) of 47.3 mmol/1 and a maximum conductance (Gmax) of 2.18 nS. To investigate whether this saturation of the Na+ conductance with increasing extracellular Na+ was due to a reduction in channel activity or to saturation of the single-channel current, we used fluctuation analysis of the noise generated during the onset of blockade of the Na+ current with 200 mumol/l 6-chloro-3,5-diaminopyrazine-2-carboxamide. Using this technique, we estimated the single channel conductance to be 4 pS when the channel was bathed symmetrically in 150 mmol/l Na+ solutions. We found that Na+ channel activity, defined as the open probability multiplied by the number of available channels, did not alter with increasing extracellular Na+. On the other hand, the single-channel current saturated with increasing extracellular Na+ and, consequently, whole-cell Na+ permeability declined. In other words, the decline in Na+ permeability in salivary duct cells with increasing extracellular Na+ concentration is due simply to saturation of the single-channel Na+ conductance rather than to inactivation of channel activity.