Diabetes enhances activity of alanine transport in liver plasma membrane vesicles

The SNAT4 isoform of the system A amino acid transporter is functional in human placental microvillous plasma membrane

Placental system A activity is important for the supply of neutral amino acids needed for fetal growth. There are three system A isoforms: SNAT1, SNAT2 and SNAT4, but the contribution of each to system A-mediated transport is unknown. Here, we have used immunohistochemistry to demonstrate that all three isoforms are present in the syncytiotrophoblast suggesting each plays a role in amino acid transport across the placenta. We next tested the hypothesis that the SNAT4 isoform is functional in microvillous plasma membrane vesicles (MVM) from normal human placenta using a method which exploits the unique property of SNAT4 to transport both cationic amino acids as well as the system A-specific substrate MeAIB. The data show that SNAT4 contribution to system A-specific amino acid transport across MVM is higher in first trimester placenta compared to term (approx. 70% and 33%, respectively, P < 0.01). Further experiments performed under more physiological conditions using intact placental villous fragments suggest a contribution of SNAT4 to system A activity in first trimester placenta but minimal contribution at term. In agreement, Western blotting revealed that SNAT4 protein expression is higher in first trimester MVM compared to term (P < 0.05). This study provides the first evidence of SNAT4 activity in human placenta and demonstrates the contribution of SNAT4 to system A-mediated transport decreases between first trimester and term: our data lead us to speculate that at later stages of gestation SNAT1 and/or SNAT2 are more important for the supply of amino acids required for normal fetal growth.

Regulatory and molecular aspects of mammalian amino acid transport

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Growth hormone regulates amino acid transport in human and rat liver

Human growth hormone (GH) has been shown to improve nitrogen balance in surgical patients and to decrease urea production. This has been thought to be due primarily to an increase in protein synthesis in skeletal muscle. Little attention has focused on the liver as a possible site where GH may modulate amino acid uptake and thereby divert nitrogen away from urea-genesis. The authors hypothesized that GH regulates amino acid transport in hepatocytes at the plasma membrane level. They studied hepatic amino acid transport in 20 healthy surgical patients that received saline, low-dose GH (0.1 mg/kg/day), or high-dose (0.2 mg/kg/day) GH for 3 days before operation. At operation, a 5- to 10-g wedge biopsy of the liver was obtained, and hepatocyte plasma membrane vesicles were prepared by Percoll density gradient centrifugation. Vesicle transport of [3H]-MeAIB, a highly selective system A substrate, and [3H]-glutamine, a selective system N substrate, was measured, employing a rapid mixing/filtration technique. Hepatocyte plasma membrane vesicles were also prepared from 14 rats treated with saline or one of three different GH treatment regimens: (A) 12 hours after chronic GH treatment (6 mg/kg every 12 hours x 4 doses); (B) 4 hours after acute (1 dose) GH treatment; and (C) 4 hours after chronic GH treatment. In human liver vesicles, low-dose GH resulted in a 13% decrease in system A activity (p = not significant), whereas high-dose GH caused a marked 79% decrease (6.7 +/- 1.7 pmol/mg protein/10 seconds in control patients versus 1.4 +/- 0.7 in GH, p less than 0.05). System N was unaffected. Kinetic analysis of MeAIB transport by vesicles from high-dose GH patients showed the reduction in transport to be due to a 63% decrease in the Vmax (maximal transport velocity) with no alteration in the transport Km (carrier affinity). Vesicles from rats treated chronically with GH using a protocol similar to that used for human subjects exhibited decreased system A transport activity (10.4 +/- 0.4 pmol/mg pro/10 seconds in controls versus 7.5 +/- 0.2 in GH, p less than 0.05) secondary to a 59% reduction in the transport Vmax. Chronic growth hormone treatment decreases the activity of system A in both human and rat hepatocytes. This may be one mechanism by which GH diminishes hepatic urea-genesis and spares amino acids for peripheral protein synthesis.

Transport of L-glutamine and L-glutamate across sinusoidal membranes of rat liver. Effects of starvation, diabetes and corticosteroid treatment

There is increasing evidence that membrane transporters for glutamine and glutamate are involved in control of liver metabolism in health and disease. We therefore investigated the effects of three catabolic states [starvation (60 h), diabetes (4 days after streptozotocin treatment) and corticosteroid (8-day dexamethasone) treatment] associated with altered hepatic amino acid metabolism on the activity of glutamine and glutamate transporters in sinusoidal membrane vesicles from livers of treated rats. In control preparations, L-[14C]glutamine uptake was largely Na(+)-dependent, but L-[14C]glutamate uptake was largely Na(+)-independent. Vmax. values for Na(+)-dependent uptake of glutamine and/or glutamate exceeded control values (by about 2- and 12-fold respectively) in liver membrane vesicles from starved (glutamine), diabetic (glutamate) or steroid-treated (glutamine and glutamate) rats. The Km values for Na(+)-dependent transport of glutamine or glutamate and the rates of their Na(+)-independent uptake were not significantly altered by any treatment. Na(+)-independent glutamate uptake appeared to include a dicarboxylate-exchange component. The patterns of inhibition of glutamine and glutamate uptake by other amino acids indicated that the apparent induction of Na(+)-dependent amino acid transport in catabolic states included increased functional expression of systems A, N (both for glutamine) and X-ag (for glutamate). The results demonstrate that conditions resulting in increased secretion of catabolic hormones (e.g. corticosteroid, glucagon) are associated with increased capacity for Na(+)-dependent transport of amino acids into liver cells from the blood. The modulation of hepatic permeability to glutamine and glutamate in these situations may control the availability of amino acids for intrahepatic metabolic processes such as ureagenesis, ammonia detoxification and gluconeogenesis.

Expression of rat liver Na+/L-alanine co-transport in Xenopus laevis oocytes. Effect of glucagon in vivo

Poly(A)+ RNA (mRNA) isolated from rat liver was injected into Xenopus laevis oocytes, and expression of Na+/L-alanine transport was assayed by measuring Na(+)-dependent uptake of L-[3H]alanine. Expression of Na+/L-alanine transport was detected 3-7 days after mRNA injection, and was due to an increment of the Na(+)-dependent component. After injection of 40 ng of total mRNA, Na(+)-dependent uptake of L-alanine was 2.5-fold higher than in water-injected oocytes. In contrast with Na+/L-alanine transport by water-injected oocytes, expressed Na+/L-alanine transport was inhibited by N-methylaminoisobutyric acid, was inhibited by an extracellular pH of 6.5 and was saturated at approx. 1 mM-L-alanine. After sucrose-density-gradient fractionation, highest expression of Na+/L-alanine uptake was observed with mRNA of 1.9-2.5 kb in length. Compared with mRNA isolated from control rats, mRNA isolated from glucagon-treated rats showed a approx. 2-fold higher expression of Na+/L-alanine transport. The results demonstrate that both liver Na+/L-alanine transport systems (A and ASC) can be expressed in X. laevis oocytes. Furthermore, the data obtained with mRNA isolated from glucagon-treated rats suggest that glucagon regulates liver Na+/L-alanine transport (at least in part) via the availability of the corresponding mRNA.