cAMP-induced potassium channel activity in apical membrane of cultured A6 kidney cells

In the present study, we report an adenosine 3',5'-cyclic monophosphate (cAMP)-induced potassium channel in the apical membrane of cultured A6 kidney cells grown on impermeable supports. The channel is present in approximately 10% of untreated cell-attached patches. After treatment with 1 mM dibutyryl-cAMP, the channel is present in greater than 70% of the same patches. The characteristics of this channel are 1) the channel is highly selective for potassium; 2) the channel has a unit conductance of 13 +/- 2 pS; 3) the probability of a channel opening increases in the presence of membrane permeable analogues of cAMP and with increasing depolarization of the cell interior; 4) channels are blocked by Ba2+; 5) the channel loses activity rapidly in excised patches; and 6) the channel has at least one open and two closed states. The mean open time is 3.5 +/- 1.0 ms, whereas the mean durations of the closed states are 3.2 +/- 1.4 and 29.4 +/- 3.4 ms. The channels could mediate potassium secretion in A6 cells, if the channels are normally present under transporting conditions; however, surface expression of the channels appears to depend on growth substrate and the state of cellular differentiation, since the channels are not observed in cells grown on permeable supports.