BASOLATERAL K+,CL-, AND HCO3(-) CONDUCTANCES AND CELL-VOLUME REGULATION IN RABBIT PCT

The relationship between changes in cellular volume, intracellular pH (pH(i)), basolateral membrane potential (V(BL)), and membrane partial basolateral conductances to K+ (t(K)) and Cl- (t(Cl)) and mediated by the Na-HCO3 cotransporter (t(NaHCO3)) was determined in the collapsed proximal convoluted tubule (PCT) submitted to a 125-mosmol/kg hypotonic shock. The shock that produces a rapid swelling followed by partial volume regulation was accompanied by a rapid and transient V(BL) hyperpolarization of 10.0 +/- 1.5 mV and a second gradual hyperpolarization of 5.0 +/- 0.7 mV with respect to a control value of -44.0 +/- 4.6 mV. t(K) was 0.12 +/- 0.03 in control, increased transiently to 0.15 +/- 0.03, and then gradually increased to reach 0.32 +/- 0.06 at the end of hypotonic shock. In contrast, t(Cl) was 0.03 +/- 0.01 in control, increased rapidly to a maximum of 0.16 +/- 0.01, and then decreased slowly to 0.08 +/-0.02. During the same period, t(NaHCO3) decreased rapidly from 0.41 +/- 0.04 to a minimum of 0.11 +/- 0.02 and slowly reincreased to reach 0.16 +/- 0.01. pH(i) increased transiently from 7.09 +/- 0.03 in control to 7.24 +/- 0.05 to come back gradually to 7.15 +/- 0.05 at the end of the hypotonic period. The membrane absolute conductance mediated by the Na-HCO3 cotransporter was found to increase only slightly in hypotonic conditions, whereas that to K+ and Cl-, G(K) and G(Cl), increased by at least factors of 8 and 17, respectively, with the increase of G(Cl) being much faster than that of G(K). In addition, the temporal variations in G(Cl) followed closely those of the cellular water efflux. We conclude that the hypotonic swelling leads to important increases in the conductive pathways for K+ and Cl- and that the Cl- conductance pathway appears to be the rate limiting step in triggering and supporting regulatory volume decrease.