Glucagon stimulates the A system for neutral amino acid transport in isolated hepatocytes of adult rat

Hormonal regulation of amino acid transport and cAMP production in monolayer cultures of rat hepatocytes

The effects of insulin, glucagon of Dexamethasone (DEX) and of glucagon with insulin or DEX were examined on the uptake of 2-amino [1-14C]isobutyric acid (AIB) and N-Methyl-2-amino [1-14C]isobutyric acid (NMe AIB) in monolayer cultures of rat hepatocytes. Insulin and glucagon stimulated the uptake of both the amino acids and DEX inhibited it, showing that all three of these hormones regulate the A system (the sodium-dependent system that permits the transport of NMe AIB) for amino acid transport in these cultures. Experiments investigating the transport of aminocyclopentane-1-carboxylic acid, 1- [carboxyl-14C] in the presence of excess AIB or in the absence of sodium showed that insulin had no effect on the activity of the L system (the sodium-independent system that prefers leucine). Experiments on the uptake of AIB in the presence of excess NMe AIB showed insulin had no effect on the transport activity of the ASC system (the sodium-dependent system that does not transport NMe AIB). Insulin concentrations ranging from 0.1 nM to 100 nM did not antagonize the stimulatory effect of optimum or suboptimum concentrations of glucagon on the uptake of either AIB or NMe AIB. Similarly, glucagon did not antagonize the stimulatory effect of optimum or suboptimum concentrations of insulin on the uptake of both the amino acids. The combined effect of insulin and glucagon was additive on the rate as well as the cumulative uptake of both AIB and NMe AIB. DEX alone inhibited the transport of both AIB and NMe AIB by about 25%, while glucagon caused a 2--3-fold increase; however, the addition of glucagon to cultures containing DEX caused a 7--8-fold increase in the uptake of both AIB and NMe AIB when compared to cultures containing DEX alone. The effect of insulin on the levels of cAMP was also investigated. Insulin had no effect on the cAMP levels in cultures treated or untreated with optimum or suboptimum concentrations of glucagon.

Nitrogen handling in adult hypothalamic obese rats

Several aspects of nitrogen metabolism have been studied in adult nonhyperphagic, hyperinsulinemic, ventromedial hypothalamus (VMH)-lesioned rats. Ten days postoperatively, VMH-lesioned rats had high plasma levels of urea and low plasma tyrosine, while plasma alanine, glutamine, total amino acid, and protein levels were unaltered. Urea production and excretion were increased in VMH-lesioned rats. Increased urea synthesis could not be attributed to enhanced peripheral release of amino acids. In vitro, measurements of hepatic amino acid uptake and liver protein synthesis did not reveal any disturbances of these processes in VMH-lesioned rats. However, hepatic transamination and lipogenesis from amino acids were increased following VMH-lesions. In addition to the hyperinsulinemia previously reported to occur in nonhyperphagic VMH-lesioned rats, this study showed an increased glucagon secretion from perfused pancreases of these animals. These data together suggest that the excess of the two hormones at the portal vein of VMH-lesioned rats would favor uptake and deamination of amino acids, their diversion to lipid synthesis and possibly other pathways, with consequent increase in urea production and reduced supply of amino acids to peripheral tissues available for nitrogen retention.

Amino acid transport in isolated hepatocytes after partial hepatectomy in the rat

The effect of partial (70%) hepatectomy on amino acid transport was investigated by measuring the uptake of the nonmetabolizable amino acid analogue alpha-amino-[1(-14C]isobutyric acid in hepatocytes freshly isolated from the liver remnant. In hepatocytes from partially hepatectomized rats, both the influx and the efflux of alpha-aminoisobutyric acid were increased. Quantitative analysis of the relationship between the initial rate of uptake and the substrate concentration indicated that a high-affinity component was responsible for the increased rate of amino acid entry in hepatocytes from partially hepatectomized rats. This component could be detected 90 min after operation, but its Vmax increased with time. The high-affinity component was strictly dependent on sodium and had the properties of a pure A system; its mode of energization partly involved cationic transmembrane gradients. The L system of amino acid transport was not affected after partial hepatectomy. The rapid emergence of a high-affinity amino acid transport system in the liver remnant following partial hepatectomy may play an important role in the regulation of liver regeneration.

Effect of insulin on amino acid transport in isolated rat hepatocytes

The effects of insulin on amino acid transport were studied in freshly prepared suspensions of isolated hepatocytes from adult rats. Insulin stimulated the active transport of alpha-aminoisobutyric acid by increasing the influx. The onset of the insulin effect was delayed by thirty to sixty min. Insulin increased by Vmax of transport by about 60% without affecting the Km. Cycloheximide and actinomycin D inhibited hormonal action by 60 to 80%. Only the "A" system of transport was affected by insulin. Half-maximal stimulation of transport was observed with insulin at 2 to 3nmol/l, a concentration which also occupies about 50% of insulin-specific binding sites at steady state. Insulin did not antagonize the stimulatory effect of glucagon on amino acid transport.

Intracellular translocation of iodine-125-labeled insulin: direct demonstration in isolated hepatocytes

Insulin labeled with iodine-125 binds to receptors on isolated rat hepatocytes. At low temperatures initial binding is restricted to the plasma membrane as detected by direct quantitative autoradiographic analysis with the electron microscope. With increasing time and temperature of incubation there is a systematic and progressive translocation of autoradiographic grains to a highly limited area of the cell periphery representing no more than 15% of the radius of the cell.

The effects of bioregulators upon amino acid transport and protein synthesis in isolated rat hepatocytes

Isolated rat hepatocytes prepared by an enzyme perfusion technique possess a functional amino acid transport system and retain the capacity to synthesize protein. Amino acid transport was studied using the non-metabolizable amino acid analog alpha-aminoisobutyric acid. The transport process was time, temperature and concentration dependent. Similarly, leucine incorporation into protein was time and temperature dependent being optimal at 3m degrees C. Amino acid, fetal calf serum, growth hormone and glucose all produced small, reproducible increases in protein synthesis rates. Bovine serum albumin diminished the uptake of alpha-aminoisobutyric acid and leucine incorporation into protein. The amino acid content on either side of the cell membrane was found to affect transport into or out of the cellular compartment (transconcentration effects). High cell concentrations decreased transport and protein synthesis as a result of isotopic dilution of labelled amino acids with those released by the hepatocytes. This was consistent with the capacity of naturally occurring amino aicds to compete with alpha-aminoisobutyric acid for uptake into the hepatocyte. In order to define more precisely the effects of bioregulators on transport and protein synthesis it will be necessary to define and subfractionate cellular compartments and proteins which are the specific targets of cellular regulation.

Hormonal regulation of hepatic amino acid transport

The transport of 2-aminoisobutyric acid (AIB) into liver tissue was increased by both insulin and glucagon. We have now shown that these hormones do not stimulate the same transport system. Glucagon, possibly via cAMP, increased the hepatic uptake of AIB by a mechanism which resembled system A. This glucagon-sensitive system could be monitored by the use of the model amino acid MeAIB. In contrast, the insulin-stimulated system exhibited little or no affinity for MeAIB and will be referred to as system B. On the basis of other reports that the hepatic transport of AIB is almost entirely Na+ dependent and the present finding that the uptake of 2-aminobicyclo [2,2]heptane-2-carboxylic acid (BCH) was not stimulated by either hormone, we conclude that system B is Na+ dependent. Furthermore, insulin added to the perfusate of livers from glucagon-pretreated donors suppressed the increase in AIB or MeAIB uptake. Depending upon the specificities of systems A and B, both of which are unknown for liver tissue, the insulin/glucagon ratio may alter the composition of the intracellular pool of amino acids.