Relation between intracellular sodium and active sodium transport in rabbit colon: current-voltage relations of the apical sodium entry mechanism in the presence of varying luminal sodium concentrations.

The current-voltage relations of the amiloride-sensitive Na entry pathway across the apical membrane of rabbit descending colon, exposed to a high K serosal solution, were determined in the presence of varying mucosal Na activities, (Na)m, ranging from 6.2 to 99.4 mM. These relations could be closely fit to the "constant field" flux equation yielding estimates of the permeability of the apical membrane to Na, PmNa, and the intracellular Na activity, (Na)c. The following empirical relations emerged: (Na)c increased hyperbolically with increasing (Na)m; PmNa decreased hyperbolically with increasing (Na)m and linearly with increasing (Na)c; spontaneous variations in Na entry rate at constant (Na)m could be attributed entirely to parallel, spontaneous variations in PmNa; the rate of Na entry increased hyperbolically with increasing (Na)m obeying simple Michaelis-Menten kinetics; the relation between (Na)c and "pump rate," however, was sharply sigmoidal and could be fit by the Hill equation assuming strong cooperative interactions between Na and multiple sites on the pump; the Hill coefficient was 2-3 and the value of (Na)c at which the pump-rate is half-maximal was 24 mM. The results provide an internally consistent set of relations among Na entry across the apical membrane, the intracellular Na activity and basolateral pump rate that is also consistent with data previously reported for this and other Na-absorbing epithelia.