The initial rate of Na uptake by the turtle colon from the mucosal bathing solution consists of two operationally distinct components. One component is a linear function of mucosal Na concentration, is unaffected by amiloride, and appears to represent Na uptake into the paracellular shunt path. The major component of Na uptake is abolished by amiloride and is virtually equal to the short-circuit current over a wide range of. mucosal Na concentrations, suggesting that this portion of Na uptake represents Na movement into Na-transporting cells of the colon. The amiloride-sensitive component of Na uptake, at low mucosal Na concentrations, was unaffected if net Na transport was abolished by ouabain. Similarly, at low mucosal Na concentrations the amiloride-sensitive conductance of the colon was identical in the presence and in the absence of net Na transport.
These results show that the isolated turtle colon behaves, as two distinct barriers to transmural Na transport, an apical barrier blocked by amiloride and a more basal-lying barrier where active, transmural Na transport is blocked by ouabain. In addition, these experiments appear to provide the first unambiguous demonstration that the initial-rate isotope uptake technique can provide adirect measure of the properties of the amiloridesensitive barrier to transmural Na movement, presumably the apical membranes of the Na-transporting cells. The results are consistent with the notion that the rate of transmural active Na transport and the conductance of the active Na-transport path are determined by the properties of the apical membrane.