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 (
G
max) 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 μmol/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.
Received: 27 July 1995/Revised: 7 December 1995
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