Na uptake across the mucosal border of the colon of Bufo marinus

The unidirectional influx of Na from the mucosal medium into the mucosal cell layer of the toad colon was measured using the technique of Biber and Curran. Na influx was measured under short circuit conditions at mucosal Na concentrations of 11.5, 25.8, 50.2, 81.5 and 109.8mM. The relationship between Na influx and Na concentration can be described by a curve consisting of the sum of a saturating and a linear component, i.e. J(i)(Na)=[J(m) x (Na)/K(Na) + (Na)] + α(Na). The values obtained for the parameters were: Jm=2.8±.2 μEq/cm² hr, K=31.0±5.3mM, and α=0.020±.001 cm/hr. At each Na concentration the relationship between J(i)(Na) short circuit current can be described by a straight line with a positive slope and positive intercept. The value of the intercept increases linearly with increasing Na concentration and the relationship between the intercept and Na concentration can be described by a straight line with an intercept near zero and a slope of 0.020 cm/hr. The effect of lithium ions on sodium uptake was studied at Li concentrations of 25, 50, and 87mM. Li in the mucosal medium inhibited the uptake of Na and an analysis of the degree of inhibition as a function of Li concentration suggests that Li competitively inhibits Na influx.