Cell Na+ activities and transcellular Na+ absorption by descending colon from normal and Na+-deprived rabbits.

The relation between intracellular Na+ activities, (Na)c, determined employing Na+-selective microelectrodes, and the rates of active Na+ absorption, INa, by rabbit descending colon was examined when INa was varied over a wide range by chronic dietary Na+ deprivation. (Na)c averaged 13 mM and was independent of INa over a sixfold range. Further, the ratios of the slope resistance of the apical membrane (rm) to that of the basolateral membrane (rs) (i.e. rm/rs) in low-transporters (control diet) and high-transporters (Na+-deprived) did not differ significantly inspite of the fact that the Na+ conductance of the apical membranes of high-transporters was, on the average, three times greater than that of the low-transporters. These findings, together with the results reported by other laboratories, strongly suggest that the aldosterone-induced increase in the conductance of the apical membrane to Na+ and, in turn, the rate of entry of Na+ into the absorptive cells are followed by parallel increases in the ability of cells to extrude Na+ across the basolateral membrane in the absence of a sustained increase in (Na)c as well as the conductance of that barrier.