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

Abstract

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 (Vmax) for Na+-K+-ATPase was 70 and 90% of adult activity in the fetus and the neonate, respectively. The ATP Km 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 Vmax at each age was significantly lower and the apparent Km higher in developing compared with adult membrane vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)

The Role of the Sodium-Taurocholate Cotransporting Polypeptide (NTCP) and of the Bile Salt Export Pump (BSEP) in Physiology and Pathophysiology of Bile Formation

“…Complete glycosylation of the protein is only observed about 4 weeks after birth [109]. Functionally, sodium-dependent bile salt transport is observed at day 20 of gestation [304] coinciding with the first basolateral protein expression of Ntcp in fetal liver. This relatively late appearance of Ntcp during ontogenesis is reproduced in primary cultures of small hepatocytes, which expand and differentiate into mature hepatocytes [243,280].…”

Development of the Bile Canalicular System in Rat Liver: with Special Reference to Bile Canalicular Actin Filaments and Mg2+, Ca2+-ATPase Activity.

“…The uptake of unconjugated bile acids from the sinusoidal membrane is carried out by Na+, K+-ATPase (localized in the sinusoidal membrane) which mediates carrier protein transport system (16,30).…”

Age‐dependent glycosylation of the sodium taurocholate cotransporter polypeptide: From fetal to adult human livers

“…On the N‐terminal domain, two N‐linked glycosylation sites (Asn 5, Asn 11) facing the extracellular lumen have been characterized15, 16 and are important for NTCP localization on the plasma membrane 17. Current knowledge of NTCP ontogenesis is mostly based on experiments conducted on rats and shows that i) compared to adult levels, Ntcp messenger RNA (mRNA) levels at birth are 35%‐76%; ii) in newborns, Ntcp protein expression reaches adult levels during the first postnatal week; iii) Ntcp expression levels and plasma membrane localization do not correlate with protein functional status; iv) NTCP activity is delayed approximately 4 weeks after birth 6, 18, 19, 20, 21. However, extrapolations in humans are limited due to the known differences among species in NTCP expression regulation.…”