Intracellular potassium activity measurements in single proximal tubules ofNecturus kidney

Summary

The intracellular potential (E _M) and potassium activity (a ⁱ_K ) ofNecturus proximal tubule were measured with double barreled liquid ion-exchange microelectrodes. Axial heterogeneity along the length of the accessible proximal tubule was observed, withE _M anda ⁱ_K averaging −63.6±6.0 mV and 59.6±8.5 meq/liter in early segments, and −75.4±6.9 mV and 71.5±7.8 meq/liter in late segments. In both segmentsa ⁱ_K was above electrochemical equlibrium and thus actively accumulated within the cells. Increasing extracellular [K⁺] in increments from 2.5 to 50.0 meq/liter causedE _M to depolarize progressively from −63.9±5.8 to −15.8±3.8 mV, whilea ⁱ_K increased only slightly from 63.1±7.0 to 69.0±8.7 meq/liter. The response ofE _M to increasing extracellular [K⁺] was reduced when extracellular [Na⁺] was decreased from 101 to 13 meq/liter. Treatment of tubules with ouabain for 1–2 hr caused a dose-dependent depolarization of the cell potential and a fall in intracellularK ⁺. With 10⁻⁴ m ouabainE _M decreased from −61.1±7.6 to −28.1±5.6 mV, anda ⁱ_K decreased from 62.7±5.7 to 10.2±4.0 meq/liter. However, when sodium entry into tubule cells was curtailed by perfusion with low-sodium solutions, or by replacement of chloride with a poorly permeant anion, cellular potassium activity remained unchanged. Taken together, the results of these studies clearly demonstrate that K⁺ is actively accumulated within the cells of theNecturus proximal tubule and that this accumulation is dependent upon Na⁺−K⁺-ATPase. In addition, the basolateral cell membrane has a relatively large K⁺ conductive pathway, which is subject to modulation by extracellular sodium. Finally, significant axial heterogeneity of the peritubular potential and ofa ⁱ_K along the proximal tubule were noted.