Ion channels in the basolateral membrane of intralobular duct cells of mouse mandibular glands

We have used single-channel patch-clamp techniques to study the ion channels in the basolateral membranes of intralobular duct cells from the mouse mandibular gland. In 39% of cell-attached patches, we observed a K⁺ channel that had an inwardly rectifying current/voltage (I/V) relation with a maximum slope conductance of 123±9 pS (n=12) and a zero current potential of +49.4±3.4 mV (n=5) relative to the resting cell potential. The selectivity sequence of this channel, as estimated by zero current potential measurements, was: K⁺ (1) > Rb⁺ (0.38) > NH ₄ ⁺ (<0.34), Cs⁺ (<0.16) > Na⁺ (<0.028). The activity of the channel was not affected by changes in membrane potential, nor was it affected by changes in the free Ca²⁺ concentration on the cytosolic side of inside-out excised patches in the range 1 nmol/l to 1 mol/l. In 38% of cell-attached patches we observed a second K⁺ channel type with a maximum slope conductance of 62±3 pS (n=12) and an inwardly rectifyingI/V relation. The selectivity sequence of this channel was K⁺ (1) > Rb⁺ (<0.5) > NH ₄ ⁺ (<0.2) > Na⁺ (<0.09). The activity of this channel type was not affected by changes in membrane potential. In 18% of excised patches, we also observed a non-selective cation channel that was not demonstrable in cell-attached patches. It had a slope conductance of 22±2 pS (n=6) and was blocked by the non-selective cation channel blocker, flufenamate (10 mol/l). A fourth channel type, observed only in 5% of patches was a Cl^– channel with a slope conductance of 40 pS and a linearI/V relation. The K⁺ channels observed in this study seem likely to underlie the K⁺ conductance described in the basolateral membrane of extralobular ducts by in vitro perfusion studies. Our finding that they are inwardly rectifying suggests that they may not be the sole route of K⁺ transport across the basolateral membrane.