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, I_Na, by rabbit descending colon was examined when I_Na was varied over a wide range by chronic dietary Na⁺ deprivation. (Na)_c averaged 13 mM and was independent of I_Na over a sixfold range. Further, the ratios of the slope resistance of the apical membrane (r^m) to that of the basolateral membrane (r^s) (i.e. r^m/r^s) 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.