Effect of potassium concentration on bicarbonate reabsorption in the rabbit proximal convoluted tubule

The direct effects of acute changes in K+ concentration on HCO-3 (JnettCO2) and volume reabsorption (Jv) were examined in isolated perfused rabbit proximal convoluted tubules (PCT). Increasing ambient K+ concentration from 5 to 8 mM did not change JnettCO2 (94.5 +/- 16.1 vs. 98.8 +/- 17.7 pmol X mm-1 X min-1) or Jv (1.27 +/- 0.15 vs. 1.24 +/- 0.16 nl X mm-1 X min-1). In contrast, reducing ambient K+ concentration from 5 to 2 mM inhibited JnettCO2 by 22% and Jv by 29%. Reducing luminal K+ concentration from 5 to 0 mM with constant bath K+ concentration at 5 mM did not affect JnettCO2 or Jv. Further reductions in bath K+ concentration to 0.5 and 0 mM showed a similar dependence of both fluxes on K+ concentration. Half-maximum inhibition of JnettCO2 was obtained at 1.1 mM ambient K+ concentration and of Jv at 0.85 mM. At zero bath K+ concentration JnettCO2 was 6.6 +/- 2.5 pmol X mm-1 X min-1 and Jv was 0.03 +/- 0.04 nl X mm-1 X min-1. To determine whether this rate of acidification was significantly different from zero, we examined the ability of the PCT to generate tCO2 concentration gradients with zero bath K+ concentration at slow perfusion rates. The tCO2 concentration gradient generated (0.94 mM) was not different from that found when the perfusate was inserted directly into the collection pipette in the absence of a tubule (0.71 mM). These data are consistent with the view that HCO-3 reabsorption is totally dependent on the Na+-K+-ATPase pump system.