Enhancement of Net Sodium Transport in Isolated Bullfrog Intestine by Sugars and Amino Acids

Summary

When isolated bullfrog small intestine is mounted between identical Ringer solutions in which chloride is replaced by sulfate, short circuit current and net mucosal-serosal Na⁺ flux are identical. This identity is preserved in the presence of actively transported sugars and amino acids. In the presence of a constant concentration of Na⁺, both sugars and amino acids increase the transmural pd and short circuit current when added to the mucosal bathing fluid. Addition of these substances to the serosal medium only has no effect. The increases in transmural pd and short circuit current show identical saturation characteristics with respect to the concentration of added solute and can be described in terms of simple Michaelis-Menten kinetics. These effects are not dependent on metabolism of the added solute. Addition of a second actively transported sugar to the mucosal solution during maximal stimulation by another sugar had no effect on transmural pd or short circuit current. Addition of an amino acid under these conditions caused an additional saturable increase in these parameters. Similarly, addition of an actively transported sugar during maximal stimulation by an amino acid caused a saturable increase in transmural pd and short circuit current. The kinetics of the response of the short circuit current to L-valine were unaffected by D-glucose. L-Valine decreased both the K _m and the maximal short circuit current obtained with glucose.