Taurocholate transport and Na⁺-K⁺-ATPase activity in fetal and neonatal rat liver plasma membrane vesicles

The ontogenesis of Na⁺-K⁺-ATPase activity and Na⁺-taurocholate cotransport was studied in basolateral plasma membrane vesicles from fetal and neonatal rat liver. Membrane vesicles from each age group were 30-fold enriched in the basolateral marker enzyme Na⁺-K⁺-ATPase, 4- to 7-fold enriched in the bile canalicular membrane marker enzymes alkaline phosphatase and Mg²⁺ ATPase, and not significantly enriched in activities of marker enzymes for intracellular organelles. Na⁺-K⁺-ATPase activity was significantly lower in basolateral membranes from late fetal (day 21-22) and neonatal (day 1) rat liver. Kinetic analysis of Na⁺-K⁺-ATPase activity at various concentrations of ATP revealed that the maximum velocity of enzyme reaction (V(max)) for Na⁺-K⁺-ATPase was 70 and 90% of adult activity in the fetus and the neonate, respectively. The ATP K(m) was significantly lower in the neonate than the adult, suggesting a higher affinity of the neonatal enzyme for ATP. In contrast to the early maturation of Na⁺-K⁺-ATPase, transport of taurocholate was markedly lower in both fetal and neonatal vesicles compared with the adult. Taurocholate uptake on day 19 of gestation did not differ in the presence of a Na⁺ or K⁺ gradient, and uphill transport, as indicated by an overshoot, did not occur. On day 20 taurocholate uptake was stimulated by a Na⁺ compared with a K⁺ gradient, and accumulation of isotope above equilibrium was demonstrated. Na⁺-dependent transport of taurocholate by late fetal (day 22) and neonatal vesicles was saturable but the V(max) at each age was significantly lower and the apparent K(m) higher in developing compared with adult membrane vesicles. We conclude from these studies that the specific carrier that mediates Na⁺-bile acid cotransport is expressed on the basolateral plasma membrane of fetal rat liver during late gestation. Bile acid uptake by the perinatal liver is limited by an immaturity of bile acid carriers and to a lesser extent by the underlying driving force provided by the Na⁺ pump.