AJP - Renal Physiology, Vol 245, Issue 2 167-F174, Copyright © 1983 by American Physiological Society
Potassium secretion in the rabbit proximal straight tubule
A. G. Wasserstein and Z. S. Agus
The renal handling of potassium is generally thought to involve proximal
reabsorption and distal secretion. To evaluate transport in the pars recta,
we perfused S2 and S3 segments from superficial and juxtamedullary proximal
straight tubules isolated from the rabbit kidney. The data indicate net
potassium secretion in the isolated perfused perfused proximal straight
tubule (PST). K+ secretion (JK, pmol X mm-1 X min-1) was -2.51 +/- 0.53 in
superficial PST S2 segments, -2.80 +/- 1.05 in superficial PST S3 segments,
and -1.36 +/- 0.84 in juxtamedullary PST. Secretion was inhibited by 10(-5)
M ouabain in the bath in superficial S2 and S3 segments. When a solution
resembling late proximal tubular fluid was perfused in superficial PST, JK
fell from -3.86 +/- 1.77 to -0.45 +/- 0.63 pmol X mm-1 X min-1. When
luminal flow rate was varied in the physiologic range in individual
superficial S2 and S3 segments, JK varied directly; K+ secretion increased
by -0.5 pmol X mm-1 X min-1 per 1 nl X min-1 increment in luminal flow,
while collected K+ concentration did not vary significantly. When a
favorable bath-to-lumen K+ gradient (10 vs. 5 mM) was imposed, K+ secretion
was markedly enhanced; when an equal but oppositely directed gradient was
imposed, net K+ reabsorption was observed. These data are consistent with a
gradient-limited process. In midcortical tubule segments (S2 and S3),
10(-3) M amiloride in perfusate inhibited net K+ secretion from -2.77 +/-
0.52 to -0.18 +/- 1.08 pmol X mm-1 X min-1 and fluid absorption from 0.42
+/- 0.10 to 0.18 +/- 0.05 nl X mm-1 X min-1. Net K+ secretion in S2 and S3
segments of PST may contribute to previously reported K+ secretion prior to
the bend of Henle's loop. The magnitude of this process in vivo is
uncertain in the absence of measurements of interstitial K+ concentration
in the milieu of the PST.
