pH i -Dependent membrane conductance of proximal tubule cells in culture (OK): Differential effects on K+- and Na+-conductive channels

Abstract

Confluent monolayers of the established opossum kidney cell line were exposed to NH4Cl pulses (20 mmol/liter) during continuous intracellular measurements of pH, membrane potential (PDm) and membrane resistance (R'm) in bicarbonate-free Ringer. The removal of extracellular NH4Cl leads to an intracellular acidification from a control value of 7.33 +/- 0.08 to 6.47 +/- 0.03 (n = 7). This inhibits the absolute K conductance (gK+), reflected by a decrease of K+ transference number from 71 +/- 3% (n = 28) to 26 +/- 6% (n = 5), a 2.6 +/- 0.2-fold rise of R'm, and a depolarization by 24.2 +/- 1.5 mV (n = 52). In contrast, intracellular acidification during a block of gK+ by 3 mmol/liter BaCl2 enhances the total membrane conductance, being shown by R'm decrease to 68 +/- 7% of control and cell membrane depolarization by 9.8 +/- 2.8 mV (n = 17). Conversely, intracellular alkalinization under barium elevates R'm and hyperpolarizes PDm. The replacement of extracellular sodium by choline in the presence of BaCl2 significantly hyperpolarizes PDm and increases R'm, indicating the presence of a sodium conductance. This conductance is not inhibited by 10(-4) mol/liter amiloride (n = 7). Patch-clamp studies at the apical membrane (excised inside-out configuration) revealed two Na(+)-conductive channels with 18.8 +/- 1.4 pS (n = 10) and 146 pS single-channel conductance. Both channels are inwardly rectifying and highly selective towards Cl-. The low-conductive channel is 4.8 times more permeable for Na+ than for K+. Its open probability rises at depolarizing potentials and is dependent on the pH of the membrane inside (higher at pH 6.5 than at pH 7.8).

Sodium Conductance in Cultured Proximal Tubule Cells (OK) Is Inhibited by Extracellular Cationic Amino Acids

“…The lower conductive channel increases its open probability at depolarizing membrane poten tials [15]. This matches the increase in so dium conductance at positive intracellular potentials described in the present paper and suggests that sodium conductance is probably mainly due to that carrier population.…”

“…Cell culture, microelectrodc design, and signal processing have been de scribed previously [ 15], In the present study, the patch clamp technique was applied to highly confluent mon olayers of OK cells (7 days in conflucncy) in the whole cell configuration [ 16]. The cells were constantly superfused at room temperature with control or experimen tal solutions at a constant flow' rate (2 ml/min).…”

“…as well as the local ization and activation of the sodium-proton antiporter [13. 14], Our own studies with excised apical membrane patches [15] give evidence of 2 populations of sodium-conduc tive channels in the apical membrane of con fluent OK cells that are different from the amiloride-blockable distal tubular sodium channel. These channels are sensitive to changes of the intracellular pH.…”

K⁺-, Cl^–- and Nonselective Channels in Opossum Kidney Cells

“…NPPB had no effect on this channel. This finding is in con trast to other findings of NPPB in NS chan nels [15,41], An NS channel with a mean conductance of 120 pS [42] was not seen in our experiments. This type of channel with a selectivity for Na+ over K+ has thus far been found only by Schwegler et al [42].…”

Properties and Regulation of pH-Dependent Cation Channels in the Apical Membrane of Cultured Proximal Tubule Cells

“…OK cells serve as a model for proximal tubule transport function since they reveal many similarities to proximal tubule cells [4,7,8], OK cells have a pH-dependent Na conduct ance [9], We now show the existence of a nonselective cation channel in the apical membrane of OK cells which is activated by decreased pH,.…”