Sodium absorption and potassium secretion in rabbit colon during sodium deficiency

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.