Fusion of cultured dog kidney (MDCK) cells: II. Relationship between cell pH and K+ conductance in response to aldosterone.

We have chosen the MDCK cell line to investigate aldosterone action on H+ transport and its role in regulating cell membrane K+ conductance (GKm). Cells grown in a monolayer respond to aldosterone indicated by the dose-dependent formation of domes and by the alkalinization of the dome fluid. The pH sensitivity of the plasma membrane K+ channels was tested in "giant cells" fused from individual MDCK cells. Cytoplasmic pH (pHi) and GKm were measured simultaneously while the cell interior was acidified gradually by an extracellular acid load. We found a steep sigmoidal relationship between pHi and GKm (Hill coefficient 4.4 +/- 0.4), indicating multiple H+ binding sites at a single K+ channel. Application of aldosterone increased pHi within 120 min from 7.22 +/- 0.04 to 7.45 +/- 0.02 and from 7.15 +/- 0.03 to 7.28 +/- 0.02 in the absence and presence of the CO2/HCO-3 buffer system, respectively. We conclude that the hormone-induced cytoplasmic alkalinization in the presence of CO2/HCO-3 is limited by the increased activity of a pHi-regulating HCO-3 extrusion system. Since GKm is stimulated half-maximally at the pHi of 7.18 +/- 0.04, internal H+ ions could serve as an effective intracellular signal for the regulation of transepithelial K+ flux.