AJP - Regulatory, Integrative and Comparative Physiology, Vol 250, Issue 2 227-R234, Copyright © 1986 by American Physiological Society
Independent Na+ and Cl- active transport by urinary bladder epithelium of brook trout
W. S. Marshall
Brook trout (Salvelinus fontinalis) urinary bladder in vitro had a low
serosa-positive transepithelial potential (6.7 +/- 1.2 mV), low transmural
conductance (0.23 +/- 0.03 mS. cm-2), and net absorptive transport of Na+
and Cl-. The net flux of Na+ was equivalent to that of Cl- and much larger
than the membrane current, indicating neutral NaCl uptake. Na+ uptake was
not coupled to that of Cl-, since absorptive Na+ transport continued in
(Cl-)-free media. In the absence of Cl-, the net absorption of Na+ was
accompanied by net secretion of titratable acidity, suggestive of Na+-H+
exchange on the luminal surface. Active Cl- transport persisted in dilute
(2.0 mM) tetraethylammonium chloride with zero K+ and zero Na+, indicating
full independence of the Cl- transport from cation coupling and suggesting
the presence of Cl(-)-HCO-3 exchange. The kinetics of Cl- uptake (K1/2 =
35-37 mM, maximal transport rate = 3.0-3.4 mu eq . cm-2 . h-1) were not
significantly affected by removal of mucosal Na+ . Cl- uptake was inhibited
partially by 10(-4) M amiloride but not by 10(-4) M bumetanide. The results
strongly support a model for active NaCl transport involving paired ion
exchangers, likely located at the luminal membrane.
