AJP - Renal Physiology, Vol 250, Issue 2 235-F245, Copyright © 1986 by American Physiological Society
Sodium absorption and potassium secretion in rabbit colon during sodium deficiency
K. Turnheim, H. Plass, M. Grasl, P. Krivanek and H. Wiener
Reducing the daily Na intake of rabbits from approximately 4.4 to 0.1
meq/kg body wt increases plasma aldosterone levels and the rate of
amiloride-sensitive Na transport in the descending colon two- to threefold.
The stimulation of Na transport is a result of an increase in the maximum
transport capacity of the epithelium, whereas the affinity of Na to its
transport system is not altered. Simultaneous with enhanced Na absorption,
there is statistically significant K secretion of 0.25 mu eq . cm-2 . h-1
under short-circuit conditions. Transepithelial current-voltage relations
in the absence and presence of amiloride were used to determine the Na
permeability of the apical membrane and the intracellular Na activity of
the Na-transporting cells. The Na content of the amiloride-sensitive cells
was estimated from the kinetics of absorptive Na tracer fluxes. The
stimulation of active Na transport under conditions of dietary Na
restriction is associated with parallel increases in apical membrane Na
permeability and the Na content of the amiloride-sensitive cells, but the
intracellular Na activity and the activity of the epithelial Na-K-ATPase
are not significantly altered. Taken together, these results suggest that
endogenous aldosterone increases the number of conducting Na entry sites in
the apical membrane of colonic epithelium and that there is activation of
additional Na pump units in the basolateral membrane, brought about by cell
swelling and possibly by an increase in the fraction of epithelial cells
that participate in active Na transport.
