Elevation of basolateral K+ induces K+ secretion by apical maxi K+ channels in Ambystoma collecting tubule

We previously reported that exposure of aquatic-phase Ambystoma tigrinum to a solution containing 50 mM K+ (K+ adaptation) caused a nearly 10-fold increase in the number of detectable maxi K+ channels on the apical membrane of their initial collecting tubules. In apparent contradiction to the notion that maxi K+ channels contribute to K+ secretion, these channels were not routinely active at the resting membrane potential (0 mV voltage clamp). To test the possibility that hyperkalemia yields maxi K+ channels that are secreting K+ (i.e., active at 0 mV), we patch-clamped the apical membranes of initial collecting tubules under conditions of elevated basolateral K+ (15 mM). Seven patches containing maxi K+ channels were studied. Six of the seven patches showed maxi K+ channel activity when voltage was clamped at 0 mV. Open probability and unitary current averaged 0.059 +/- 0.016 and 1.65 +/- 0.50 pA, respectively. This activity, together with the high density of channels observed (1.06 channels/micrometer2), indicates that after K+ adaptation, maxi K+ channels contribute to the ability of the late distal nephron of amphibians to secrete K+.