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

Abstract

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.

Basolateral membrane potential and conductance in frog skin exposed to high serosal potassium

“…Consideration of transcellular current-voltage relationships has led to the view that apical Na entry obeys the GoldmanHodgkin-Katz "constant field" equation in the frog skin (Fuchs et al, 1977), the toad urinary bladder (Palmer et al, 1980), and the rabbit descending colon (Thompson et al, 1982), permitting the evaluation of the apical Na permeability PNa and the cell Na activity. This has led to inferences concerning the apical entry process and mechanism of action of hormones (Fuchs et al, 1977;Li et al, 1982;Palmer et al, 1982;Thompson et al, 1982;Turnheim et al, 1983). The presumed minimization of apical electrical forces has facilitated fluctuation analysis of the area density and turnover rate of apical Na channels (Lindemann & Van Driessche, 1977) and the characterization of the mechanism of channel ionic fluxes (Palmer, 1982a,b).…”

Mechanism of Cl secretion in canine trachea: Changes in intracellular chloride activity with secretion

“…4 The assumption that the ion movements across the apical membrane can be described by the GHK equation has not yet been independently verified. There is evidence that the characteristics of Na channels in certain Na-transporting epithelia do fit the GHK equation at least over the ranges in potentials with which we are concerned (Fuchs, Larsen & Lindemann, 1977;Palmer, Edelman & Lindemann, 1980;Thompson, Suzuki & Schultz, 1982;Turnheim, Thompson & Schultz, 1983). To date, 3 Chloride fluxes have been measured under epinephrinestimulated conditions (A1- Bazzaz & Cheng, 1979).…”

Relationships among sodium current, permeability, and Na activities in control and glucocorticoid-stimulated rabbit descending colon

“…Distal colon and other tight epithelia exhibit an adaptive response to changes in (Na)m such that a lowering of (Na)m results in a reciprocal increase in P~a (Fuchs et al, 1977;Palmer et al, 1982;Thomas et al, 1983;Turnheim et al, 1983). Comparisons of the dependence of P~a on (Na)m between control and MP-treated tissues (Fig.…”

“…4) reveals that this adaptation response is clearly also present in MPtreated tissues and, moreover, that it appears to be augmented. For both controls and MP tissues the curvilinear relation between P~a and (Na)m may be described empirically by the expression (Fuchs et al, 1977;Turnheim et al, 1983): For both controls and MP tissues the curvilinear relation between P~a and (Na)m may be described empirically by the expression (Fuchs et al, 1977;Turnheim et al, 1983):…”