AJP - Renal Physiology, Vol 260, Issue 3 311-F316, Copyright © 1991 by American Physiological Society
Electrophysiological characterization of upper portion of descending limb of long-looped nephron
K. Yoshitomi and M. Imai
Department of Pharmacology, Jichi Medical School, Tochigi, Japan.
The upper portion of the descending limb of long-looped nephron (LDLu) of
the hamster is characterized by high water and ion permeabilities. Although
the paracellular route is considered to be the major pathway representing
cation permselectivity of this segment, ion transport mechanisms through
the transcellular pathway are unknown. To study this issue; we applied
cable analysis and conventional microelectrode technique to the hamster
LDLu perfused in vitro. The transmural voltage (VT) was not different from
zero, and transmural resistance (RT) was very low, 18.3 +/- 2.0 omega.cm2
(n = 12). The basolateral membrane voltage was -80 +/- 2 mV (n = 55), and
fractional apical membrane resistance was 0.92 +/- 0.23 (n = 5). Ouabain
(0.1 mM) in the bath decreased basolateral membrane voltage (VB) by 23 +/-
3 mV (n = 6, P less than 0.001). Increase in K+ concentration in bath and
in lumen from 5 to 50 mM decreased VB by 39 +/- 2 (n = 7, P less than 0.01)
and apical membrane voltage (VA) by 10 +/- 1 mV (n = 7, P less than 0.001),
respectively. Addition of 2 mM Ba2+ to bath and to lumen decreased VB by
-47 +/- 2 (n = 11, P less than 0.001) and decreased VA by 8 +/- 1 mV,
respectively. Reduction of HCO3- in bath from 25 to 2.5 mM decreased VB by
4 +/- 1 mV (n = 7, P less than 0.005). Reduction of bath Cl- did not cause
any rapid deflection of VB. No appreciable Na+ conductance was detected in
the apical membrane.(ABSTRACT TRUNCATED AT 250 WORDS)
