HYPOTONIC STRESS-INDUCED RELEASE OF KHCO3 IN FUSED RENAL EPITHELOID (MDCK) CELLS

Mechanisms of cell volume regulation induced by the reduction of the osmolality of the Ringer solution by one-third were studied in fused Madin-Darby canine kidney (MDCK) cells. Intracellular HCO3-, K+ and Cl- concentrations [ion]i in parallel with cell membrane potential (PD), cell membrane conductance (G(m)) and conductances of individual ions (G(m)ion) were evaluated with microelectrode techniques. Fused cells regulate their cell volume by about 50%. G(m) increased from 0.43 +/- 0.03 mS/cm2 in isotonic Ringer solution to 4.3 +/- 0.3 mS/cm2 in the steady state phase of cell swelling. G(m)Cl was 0.31 +/- 0.03 mS/cm2 in isotonic Ringer solution and thus was the dominant individual ion conductance. In the initial phase of cell swelling G(m)K increased transiently 64-fold to 0.32 +/- 0.03 mS/cm2, and consequently PD hyperpolarized. At peak hyperpolarization G(m)Cl transiently decreased by 15%. Cell swelling increased G(m)Cl 11-fold and G(m)HCO3 28-fold to 0.95 +/- 0.1 mS/cm2 in the steady state phase of cell swelling. In this phase G(m)Cl and G(m)HCO3 were dominating, whereas G(m)K was only slightly increased compared to isotonic conditions. The hyperpolarization of PD was paralleled by cytoplasmic acidification. At peak acidification [HCO3-]i decreased by 6.4 mmol/kg H2O. Cl- extrusion was not detectable in the initial phase of cell swelling. In isotonic Ringer solution [K+]i was 125 +/- 5 mmol/kg H2O. During the initial phase of cell swelling 23 +/- 5 mmol/kg H2O K+ was extruded, indicating that yet unknown anions participated in cell volume regulation in this phase of cell swelling. In the steady state phase of cell swelling [pH]i was normalized by replenishing [HCO3-]i, whereas Cl- was extruded. We conclude that fused renal epitheloid cells acutely release KHCO3 in response to hypotonicity, but then regain pH homeostasis in the steady state phase of cell swelling.