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, K(ATP), 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 K(ATP) channel open probability (P(o)) is low, 0.15 +/- 0.06 (n = 15) at a -V(p) = -100 mV (V(p) is the pipette potential), and increases slightly with depolarization. The gating kinetics are characterized by one open time constant (tau(o) = 5.0 +/- 1.9 ms, n = 6) and two closed time constants (tau(C1) = 5. 2 +/- 1.5 ms, tau(C2) = 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 (tau(o) = 96 +/- 19 ms, tau(C) = 10.5 +/- 3.6 ms; n = 4). Extracellular perfusion of 40 mM taurine fails to affect sK channels, whereas K(ATP) 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 K(ATP) 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.