Sodium transport by the colon of Bufo marinus: Na uptake across the mucosal border.

Na transport by the isolated toad colon has been studied by measuring transmural Na fluxes and by direct measurement of the Na influx across the mucosal border. Net Na transport accounts for 88% of the short circuit current in the presence and in the absence of exogenous aldosterone. Na influx across the mucosal border appears to consist of two components. One component is highly correlated with short circuit current, is a saturable function of mucosal Na concentration, and is inhibited by lithium ions in the mucosal medium. The second component is a linear function of mucosal Na concentration, is unaffected by lithium, and is apparently not related to net Na transport by the tissue.