Sodium-coupled sugar transport: effects on intracellular sodium activities and sodium-pump activity.

Intracellular sodium activities, (Na)c, were determined in Necturus small intestine before and after addition of galactose to the mucosal bathing solution. In the absence of galactose, (Na)c averaged 12 millimoles per liter. Within 2 minutes after the addition of galactose to the mucosal solution, (Na)c increased to a mean value of 20 millimoles per liter and then declined, in parallel with an increase in transcellular sodium transport, to a value that did not differ significantly from that observed in the absence of the sugar. The final steady state in the presence of galactose was characterized by a three- to fourfold increase in the rate of transcellular Na+ transport in the absence of a significant increase in (Na)c. Thus, the increase in steady-state basolateral pump activity cannot be attributed to an increase in the intracellular sodium transport pool.