Ionic dependence of protein transport across the new-born pig intestine.

1. The everted small intestine of the new-born pig transports albumin to its serosal surface at rates which depend on the ionic composition of the bathing medium.2. Albumin transport takes place most rapidly at sodium and potassium concentrations of 120 and 6 mM respectively. Increasing the concentration of sodium or potassium, or decreasing the concentration of sodium, reduces the net transfer of albumin.3. Albumin can increase the serosal transfer of sodium, potassium and water. The calculated ionic composition of fluid transported in the presence of albumin closely resembles that presented to the mucosal surface over a wide range of sodium and potassium concentrations.4. Lowering the concentration of sodium reduces water transfer but only if albumin is present in the mucosal medium. Albumin-stimulated water transfer is only seen when the external concentration of sodium is high.5. Raising the concentration of potassium reduces water transport, whether or not albumin is present, but this reduction is too small to account for the inhibitory effect of potassium on albumin transport.6. The mutual interaction that exists between the transport of albumin and sodium is probably responsible for the increased potassium transport seen at low external concentrations of potassium. Higher concentrations of potassium appear directly to inhibit the interaction between the transport of sodium and albumin.