Two types of K+ channels at the basolateral membrane of proximal tubule: inhibitory effect of taurine.

The cell-attached configuration of the patch-clamp technique was used to investigate the effects of taurine on the basolateral potassium channels of rabbit proximal convoluted tubule. In the absence of taurine, the previously reported ATP-blockable channel, KATP, was observed in 51% of patches. It is characterized by an inwardly rectifying current-voltage curve with an inward slope conductance of 49 ± 5 pS ( n = 15) and an outward slope conductance of 13 ± 6 pS ( n = 15). The KATP channel open probability ( P o) is low, 0.15 ± 0.06 ( n = 15) at a - V p = -100 mV ( V pis the pipette potential), and increases slightly with depolarization. The gating kinetics are characterized by one open time constant (τo = 5.0 ± 1.9 ms, n = 6) and two closed time constants (τC1 = 5.2 ± 1.5 ms, τC2 = 140 ± 40 ms; n = 6). In 34% of patches, a second type of potassium channel, sK, with distinct properties was recorded. Its current-voltage curve is characterized by a sigmoidal shape, with an inward slope conductance of 12 ± 2 pS ( n = 4). Its P o is voltage independent and averages 0.67 ± 0.03 ( n = 4) at - V p = -80 mV. Both its open time and closed time distributions are described by a single time constant (τo = 96 ± 19 ms, τC = 10.5 ± 3.6 ms; n = 4). Extracellular perfusion of 40 mM taurine fails to affect sK channels, whereas KATP channel P o decreases by 75% (from 0.17 ± 0.06 to 0.04 ± 0.02, n = 7, P < 0.05). In conclusion, the absolute basolateral potassium conductance of rabbit proximal tubules is the resulting combination of, at least, two types of potassium channels of roughly equal importance: a high-conductance low-open probability KATP channel and a low-conductance high-open probability sK channel. The previously described decrease in the basolateral absolute potassium conductance by taurine is, however, mediated by a single type of K channel: the ATP-blockable K channel.