Alanine uptake by liver at midpregnancy in rats

Abstract

The participation of the liver to the increase in alanine utilization seen at midpregnancy was studied in 9- and 12-day pregnant rats. Liver fractional extraction of alanine was assessed in vivo from the changes in concentration in afferent and efferent vessels. Hepatic active transport of alanine was determined in vitro using isolated plasma-membrane vesicles. Compared with nonpregnant controls, alanine fractional extraction was significantly increased on day 12 but not on day 9 of pregnancy. Vesicles isolated from 9- and 12-day pregnant animals had a greater capacity for Na+-dependent transport than those from controls. Eadie-Hofstee plotting showed that this increase was due to an increase in Vmax with no change in Km. Both A and ASC systems contributed to the Vmax increase. These results indicate that, although by day 9 the liver has developed an increased capacity for alanine uptake, the actual extraction is seen only by day 12 of pregnancy. At this stage the liver participates actively in the turnover of alanine and the development of hypoalaninemia.

Sensitivity of system A and ASC transport activities to thiol-group-modifying reagents in rat liver plasma-membrane vesicles. Evidence for a direct binding of N-ethylmaleimide and iodoacetamide on A and ASC carriers

“…The activities of 5'-nucleotidase (Aronson & Touster, 1974), glucose-6-phosphatase (Baginsky et al, 1974), N-acetyl-/I-Dglucosaminidase (Carroll, 1978) and cytochrome c oxidase (Yanetani, 1962) were measured in liver homogenate and in isolated membranes. The enrichment of plasma membrane in the vesicle preparation, as assessed by 5'-nucleotidase activity, was 7-fold (Table 1), comparable with previous reports (Van Amelsvoort et al, 1978;Pastor-Anglada et al, 1987). Under those conditions, the activity of glucose-6-phosphatase, a microsomal marker, was not significantly enriched (Table 1).…”

“…Plasma-membrane vesicles were prepared from the livers of 200-300 g male Wistar rats fed ad libitum by the method of Van Amelsvoort et al (1978) as modified by Pastor-Anglada et al (1987). In brief, rats were killed by decapitation without anaesthesia and livers were immediately excised and minced in 5 vol.…”

Nucleoside uptake in rat liver parenchymal cells

“…The K m for uridine in vesicles is in the range 6-9 µM [28,35]. Other workers, using adenosine as substrate, have reported K m values of 8 µM [27] and 14 µM [29].…”

“…A few results on plasma membrane vesicles are presented for comparative purposes only. Because the method and conditions used have been explained in detail previously [29,35,36], we refer to earlier studies for more information about the experimental procedure.…”

Early induction of Na+‐dependent uridine uptake in the regenerating rat liver

“…Plasma membrane vesicles were isolated by the method described in [24]. This procedure involves a Percoll density centrifugation; it has been widely used in our laboratory and the characteristics of the plasma membrane vesicles have been published elsewhere [24--271.…”