Na+‐dependent uridine transport into liver plasma membrane vesicles from partially hepatectomized and sham‐operated rats was studied. Preparations purified 6 h after 70% hepatectomy exhibited an increased V max of uridine uptake (3.7 vs. 1.4 pmol/mg prot/3 s) without any change in Km (6μM). Incubation of the vesicles in the presence of monensin decreased uridine uptake although the differences between both experimental groups remained identical. It is concluded that uridine transport is induced early after partial hepatectomy by a mechanism which does not involve changes in the transmembrane Na+ gradient. This is the first evidence in favor of modulation of nucleoside transport into liver cells.
Na+,K+-ATPase expression has been studied in the early phase of liver growth after partial hepatectomy to ascertain whether its increased activity is due to stable effects, involving de novo synthesis and insertion of pumps into the plasma membrane. Na+,K+-ATPase activity progressively increases after partial hepatectomy, reaching a three-fold induction above basal values 12 h after surgery, mRNA amounts of both a~ and/31 subunits are rapidly increased up to two-fold for a~ and nearly three-fold for/31, at 9 and 12 h post-hepatectomy, respectively. This correlates with increased abundance of both subunit proteins. The results prove that the increase of Na+,K+-ATPase activity correlates with higher expression of both subunit proteins and mRNAs, although the characteristics of the induction suggest that some translational and post-translational events may be equally involved in the increased activity of the pump.Key words: Na+,K+-ATPase; Liver regeneration; Gene expression is under complex control, involving transcriptional, translational and post-translational steps. At this stage it is not clear whether the increased Na ÷, K+-ATPase activity occurring in early phases of liver cell proliferation correlates with enhanced expression of their subunits. The aim of this study was to address this issue by monitoring the changes in the amount of mRNA and protein for both subunits, alpha and beta, during the pre-replicative phase of liver growth. Materials and methods Animals and surgeryOvernight fasted male Wistar rats (200~240 g) were used. After pentobarbital anaesthesia (60 mg/kg b.wt. i.p.) rats were laparotomized and partial hepatectomy (approximately 70%) carried out as previously described [8]. A second set of animals that did not undergo hepatectomy but only liver extrusion was used as a group of sham-operated controls. At the indicated times, animals were killed by decapitation and the liver immediately excised and used for the biochemical analysis detailed below. The piece of liver excised during the hepatectomy was also used for these analysis and considered to be a control at zero time.
Alanine disposal by liver parenchymal and haematopoietic cells from 21-day fetuses, newborns and adult rats was studied. Preparations selectively enriched in either haematopoietic cells or hepatocytes were obtained by direct perfusion of fetal- and neonatal-rat livers. L-Alanine transport into liver parenchymal cells was best fitted to two Na(+)-dependent saturable systems. The high-affinity system showed a much higher activity (Vmax.) in hepatocytes from fetuses and newborns than in those from adult rats (2.4, 4.3 and 0.3 nmol/8 min per 10(6) cells for fetuses, newborns and adults respectively). Vmax. for the low-affinity component was slightly lower during the perinatal period than in the adult (about 30 nmol/8 min per 10(6) cells for hepatocytes from fetuses and newborns, versus 48 nmol/8 min per 10(6) cells for adult rat parenchymal cells). Haematopoietic cells from fetal-rat livers showed significant Na(+)-dependent L-alanine uptake which was completely abolished after birth. These results show that the transport systems involved in L-alanine uptake by liver parenchymal cells are fully developed before birth. This probably contributes to fulfilling the high requirement for neutral amino acids for protein synthesis during development. Haematopoietic cells may play an important role in liver amino acid metabolism during fetal life.
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