EFFECT OF BASOLATERAL OR APICAL HYPOSMOLARITY ON APICAL MAXI-K CHANNELS OF EVERTED RAT COLLECTING TUBULE

We are able to evert and perfuse rat cortical collecting tubules (CCT) at 37 degrees C. Patch-clamp techniques were used to study high-condu ctance potassium channels (maxi K) on the apical membrane. Under contr ol conditions (150 mM Na+ and 5 mM K+ in pipette and bathing solutions ), the slope conductance averaged 109.8 +/- 6.6 pS (12 channels), and reversal potential (expressed as pipette voltage) was +26.3 +/- 2.4 mV . The percent of time the channel spends in the open state and unitary current when voltage was clamped to 0 mV were 1.4 +/- 0.7% and 3.12 /- 0.42 pA, respectively. In six patches voltage clamped to 0 mV, the isosmotic solution perfused through the everted tubule (basolateral su rface) was exchanged for one made 70 mosmol/ kgH(2)O hyposmotic to the control saline. Open probability increased from 0.019 to 0.258, an in crease of 0.239 +/- 0.065 (P < 0.005). In four patches where a maxi a channel was evident, no increase in open probability was observed when a hyposmotic saline was placed on the apical surface. However, when v asopressin was present on the basolateral surface, apical application of hyposmotic saline resulted in a series of bursts of channel activit y. The average increase in open probability during bursts was (0.055 /- 0.017, P < 0.005). We conclude that one function of the maxi K chan nel located in the apical membrane of the rat CCT may be to release in tracellular solute (potassium) during a volume regulatory decrease ind uced by placing a dilute solution on the basolateral surface or when t he apical osmolarity is reduced in the presence of vasopressin. These data are consistent with the hypothesis that the physiological role of the channel is to regulate cell volume during water reabsorption.