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 ch annels in the basolaeral 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) rel ation with a maximum slope conductance of 123 +/- 9 pS (n = 12) and ze ro current potential of + 49.4 +/- 3.4 mV (n = 5) relative to the rest ing cell potential. The selectivity sequence of this channel, as estim ated by zero current potential measurements, was: K+ (1) > Rb+ (0.38 > NH4+ (< 0.34), Cs+ (< 0.16) > Na+ (< 0.028). The activity of the chan nel was not affected by changes in membrane potential, nor was it affe cted by changes in the free Ca2+ concentration on the cytosolic side o f inside-out excised patches in the range 1 nmol/l to 1 mu mol/l. In 3 8% 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 re ctifying I/V relation. The selectivity sequence of this channel was K (1) > Rb+ (< 0.5) > NH4+ (< 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 th at was not demonstratable in cell-attached patches. It had a slope con ductance of 22 +/- 2 pS (n = 6) and was blocked by the nonselective ca tion channel blocker, flufenamate (10 mu mol/l). A fourth channel type , observed only in 5% of patches was a Cl- channel with a slope conduc tance of 40 pS and a linear I/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 stud ies. Our finding that they are inwardly rectifying suggests that they may not be the sole route of K+ transport across the basolateral membr ane.