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.