SUMMARY1. Frog proximal tubular cells were fused into giant cells. We measured membrane potential (Vm), its changes (AVm), and current-induced voltage changes (Av/) in single cells, during control and experimental states. Each cell served as its own control.2. In the presence of a physiological Ringer solution, the transference number for potassium (tK) was 0-50. Barium (3 mM) reduced membrane conductance (Gm) by 50 %; low-Cl-solutions and low-Na+ solutions also diminished Gm, by 52 and 30 %, respectively. The association of barium and low-NaCl solutions decreased Gm to 38 % of control, indicating that the impermeant substitute of a physiological ion may interact with other pathways; alternatively, blockade of steady-state conductances may activate physiologically silent processes.3. In an attempt to enhance the contribution of the partial K+ conductance (GK) to Gm, fused cells were exposed to low-Cl-solutions, containing in addition 0.1 mMmethazolamide, to inhibit the rheogenic Na'-HCO3 symport, and 1 /aM-amiloride, to block Na' conductance (GNa). tK went up to 0 83. 4. The high tl preparation was challenged with barium (3 mm) or quinine (Quin, 1 mM). These blockers produced large depolarizations (~60 mV), however, although Gm decreased along early-and mid-depolarization, Gm plateaued and eventually it increased with larger and larger depolarization.5. Depolarization-associated increase in Gm reflects activation of other conductances. These are Na+, cationic, and K+ conductance(s) poorly sensitive to quinine or barium. In the presence of Ba2+-or Quin-induced depolarization, injection of depolarizing current produces delayed increase in conductance.6. Depolarization-induced activation of cationic conductance (Gcat) and GNa results in enlargement of the K+ electrochemical potential difference, to about 70 mV; this difference allows recycling of K+ ions outwards, since a GK is still detected and may contribute up to 38 % of the total conductance.
Triflocin, applied at millimolar concentration hyperpolarizes the basolateral membrane of Necturus proximal convoluted tubular cells, in vivo. Barium, 2.5 × 10−3 m, ouabain, 10−3 m, or amiloride 10−4 m, fail to prevent this hyperpolarization. Triflocin has no effect on the intracellular chloride activity. In physiological acid base conditions, Triflocin increases intracellular pH. Upon an acute isohydric hypercapnia, Triflocin depolarizes the basolateral membrane potential. It is concluded that, Triflocin inhibits the basolateral electrogenic Na‐(HCO3)n > 1 cotransport in proximal tubules.
1 Amiloride, applied at millimolar concentrations, results in the blockade of K+ conductance in amphibian proximal convoluted cells (PCT), fused into giant cells. 2 Amiloride results directly in a blockade of K+ conductance that is not related to inhibition of the Na+-H+ antiport, which would lower intracellular pH, adversely affecting K+ conductance. On the contrary, high amiloride concentrations promote entry of this lipophilic base in the cell, leading to higher cell pH. 3 Under voltage clamp conditions, control vs. amiloride, current-voltage curves from PCT fused giant cells intersect at -86.2 ± 3.4 mV, a value close to the equilibrium potential for potassium. 4 Hexamethylene amiloride, 10-5 M, irreversibly depolarizes the membrane potential. 5 Barium decreased by 50% the initial slope of realkalinization, following removal of a solution containing NH4Cl, as did amiloride. In addition, these blockers reduced membrane conductance by 40%, suggesting that a fraction of the amiloride-suppressible NH4' efflux may be conductive. 6 Amiloride does not directly inhibit the Na+-K+, ATPase in our preparation, contrary to the prevalent -belief. 7 In vivo studies show that amiloride interferes with an apical K+ conductance but it does not alter basolateral K+ conductance.
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