Mechanisms of Na transport at the basolateral pole of rat enterocyte

Basolateral membranes isolated from rat jejunal enterocytes and enriched 14 times were found present as unsealed and sealed right side out (RSO) or inside out (IO) vesicles in the ratio 2:2:1. Entrance of 1 mM Na (JNa) into basolateral membrane vesicles was measured in the presence and in the absence of 5 mM ATP by a rapid filtration technique, under different experimental conditions. JNa across the basolateral membrane is cis-inhibited by K, transstimulated by K and further stimulated by ATP (activation of the Na pump). Both K- and ATP-positive effects are also obtained by other cations: the relative stimulation sequence is K greater than Rb = NH4 greater than Cs. The ATP effect can be suppressed by vanadate and strophanthidin; moreover the K effect on JNa is not influenced by blocking the mechanism of the Na pump with ouabain, strophanthidin and adenosine 5'-[beta,gamma-imido]triphosphate. The transmembrane potential does not influence the K-stimulated JNa. In addition to the Na pump this study demonstrates the existence of an exchange mechanism between Na and K, which seems to be different from the Na pump. There appears to be no Na-K cotransport.