Fused cells of frog proximal tubule: I. Basic membrane properties

Proximal tubular cells of the frog (Rana esculenta) kidney were fused within an isolated tubule portion to giant cells according to the polyethylene-glycol fusion method. Cell membrane potentials (V_m) were measured while cells were superfused with varioús experimental solutions. Rapid concentration stepchanges of different ions allowed to calculate the respective transference numbers (t_ion). In some experiments the specific cell membrane resistances (R_m) were evaluated by measuring V_m induced by short current pulses injected into the cell with a second electrode. The experiments reveal: i) Fused cells of the proximal tubule exhibit a V_m of -49.5±1.6 mV (n=65). ii) Addition of glucose to the perfusate yields a transient depolarization, consistent with a rheogenic Na/glucose cotransport system. iii) In absence of organic substrates the whole cell membrane conductance is made up of K⁺ and HCO₃, iv) There is a positive relationship between V_m and tK⁺ and a negative relationship between V_m and tHCO₃^-. v) HCO₃^--induced V_m changes are attenuated or abolished when Na⁺ is replaced with choline⁺, consistent with a rheogenic Na⁺/HCO₃^- cotransport system. vi) Replacement of Na⁺ by choline⁺ depolarizes V_m and increases R_m by about 50%; addition of 3 mmol/liter Ba²⁺ to the Na⁺-free perfusate increases R_m by about 58% compared to the initial control value. vii) There is no measurable cell membrane Cl^- conductance. We conclude that fused cells of proximal tubule exert both luminal and peritubular membrane properties. In absence of organic substrates the cell membrane potential is determined by the HCO₃^- and K⁺ transport systems.