Cell swelling activates basolateral membrane Cl and K conductances in rabbit proximal tubule

The ionic basis of volume regulation was assessed in the nonperfused rabbit proximal tubule (S2 segment) by use of simultaneous measurements of tubule volume via video-optical imaging techniques and basolateral membrane voltage (V_bl) and relative ionic conductance via conventional microelectrodes. Both cell volume (9.9 ± 0.70 nl/cm tubule length) and V_bl (-42.8 ± 3.6 mV) remained stable in the control isotonic Ringer solution (290 mosmol/kg). When the osmolality of the bathing medium was reduced to 150 mosmol/kg, tubules swelled 72% above base line within 1 min and subsequently regulated over the course of the next 4-6 min to a steady state 20 ± 3% above the initial base-line volume. Cell swelling was accompanied by a transient hyperpolarization of V_bl of -14.3 ± 2.0 mV (HCO₃-containing Ringer) and -10.0 ± 0.7 mV (HCO₃-free Ringer). Although the hyperpolarization was not inhibited by barium in the presence of bicarbonate buffer, addition of 2 mM Ba to a bicarbonate-free Ringer depolarized V_bl by +22 mV and abolished both the relative potassium conductance and the hyperpolarization accompanying cell swelling (ΔV_bl = -4.6 ± 0.6 mV). Furthermore, the relative conductance of K at the basolateral membrane increased from 0.16 in the isotonic control medium to 0.34 at the peak of cell swelling. Because the hyperpolarization of V_bl ensued after cells had swollen ∼10% above base line, a modest threshold volume and time delay may be involved in triggering the volume-dependent activation of the K conductance. In parallel studies, the change in V_bl on a rapid step-change in bath Cl (49 to 4.9 mM) averaged 5.3 ± 1.0 mV in the isotonic solution and increased to +11.3 ± 2.1 (P < 0.05) at the peak of cell swelling. This represented an increase in the relative Cl conductance of 0.08 to 0.20, which could only be attributed to an absolute increase in the basolateral membrane Cl conductance and not to a reduction in the other major basolateral membrane conductances. It is concluded that cell swelling results in an increase in both Cl and K conductance, which may underlie subsequent cell volume regulation.