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

Stimulatory effect of interleukin-1 upon hepatic metabolism

The liver plays an important role in the acute-phase response to sepsis and injury, and host survival often depends upon an adequate hepatic response. Many of the metabolic sequelae to sepsis and injury are mediated by interleukin-1. This study was undertaken to investigate the impact of interleukin-1 upon hepatic metabolism and whether this mediator acted directly upon the liver. Interleukin-1 (5 rabbit pyrogen dose units) was administered to male Fisher F344 rats (175 to 200 g), and hepatocytes were isolated at three time periods; 2 to 4, 6 to 10, and 12 to 14 hours following an intraperitoneal injection. Alanine transport, gluconeogenesis, nonsecretory protein synthesis, and oxygen consumption were measured simultaneously in freshly isolated hepatocytes. Interleukin-1 stimulated initial rates of alanine uptake over a four-minute period. Peak stimulation of gluconeogenesis occurred at six to ten hours (0.52 +/- .14 v 0.08 +/- .01 nmol alanine converted/10(6) cells/min, P less than 0.05); nonsecretory protein synthesis was significantly stimulated at 12 to 14 hours (2.1 +/- .7 v 0.7 +/- 0.1 nmol valine converted/10(6) cells/min, P less than 0.05). These enhanced metabolic processes were associated with an increased oxygen consumption, with peak oxygen utilization occurring at six to ten hours (69 +/- 2 v 25 +/- 7 nmol of oxygen consumed 10(6) cells/min, P less than 0.05). In order to examine if interleukin-1 exerted its effect directly upon the liver, hepatocytes from normal rats were incubated in vitro with this mediator for two hours. Under these experimental conditions, interleukin-1 did not reproduce the stimulatory effect obtained following in vivo administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of hypophysectomy and GH administration on bovine and human GH binding to rat liver membranes

Experiments were conducted to investigate the specific binding of highly purified bovine and human growth hormones (bGH and hGH) to purified liver plasma membranes of male rats at various times after hypophysectomy and after the acute intravenous administration of bGH. Liver membranes prepared from hypophysectomized male rats showed a two- to threefold increase in the specific binding of either [125I]iodo-bGH or [125I]iodo-hGH, when compared with membranes prepared from the livers of age-matched normal male rats. The increase in GH binding was apparent within 3 days after hypophysectomy and persisted for a number of weeks after the operation. The increase in GH binding produced by hypophysectomy appeared to be due to an increase in the number of binding sites present on the membranes. The intravenous injection of 200 micrograms of bGH into hypophysectomized male rats 5-60 min before they were killed markedly reduced the ability of liver membranes prepared from these animals to bind [125I]iodo-bGH specifically. This decrease in GH binding seen after the injection of bGH may have been due to the development of a slowly dissociating hormone-binding site complex, which thereby reduced the number of available binding sites. This conclusion is supported by the finding that bGH, which is bound in vitro to isolated liver membranes, dissociates slowly and incompletely in the presence of an excess of unlabeled hormone. Moreover, the degree to which the bound hormone can dissociate appears to depend on the length of time that association is allowed to occur.(ABSTRACT TRUNCATED AT 250 WORDS)

Structures of the somatotropin receptor and prolactin receptor on rat hepatocytes characterized by affinity labelling

Human somatotropin competed for 125I-human somatotropin binding to hepatocytes from female or male rats. Bovine somatotropin and prolactin each inhibited part, but not all, of the uptake of 125I-human somatotropin. The binding of 125I-prolactin was inhibited by human somatotropin and prolactin, but not by bovine somatotropin. Bovine somatotropin and human somatotropin, but not prolactin, competed for 125I-bovine somatotropin binding sites. 125I-labelled hormones were covalently coupled to membrane receptors with higher efficiency on hepatocytes from female than from male rats, allowing structural descriptions of lactogenic and somatogenic binding sites that had not been possible previously. Disuccinimidyl suberate covalently coupled 125I-human somatotropin into saturable complexes of Mr 300 000, 220 000, 130 000, 65 000 and 50 000. Bovine somatotropin inhibited the incorporation of 125I-human somatotropin into complexes of Mr 300 000, 220 000 and 130 000, whereas low concentrations of prolactin competed for incorporation into the 65 000- and 50 000-Mr species. 125I-bovine somatotropin was incorporated into complexes of Mr 300 000, 220 000 and 130 000. Human somatotropin and bovine somatotropin, but not prolactin, inhibited the production of these complexes. 125I-prolactin binding produced complexes of Mr 65 000 and 50 000. Native prolactin and human somatotropin, but not bovine somatotropin, inhibited uptake of 125I-prolactin into these species. Thus direct affinity labelling, as well as competition for covalent coupling, suggests that the 300 000-, 220 000- and 130 000-Mr species are components of the somatotropin receptor and that the 65 000- and 50 000-Mr complexes result from hormone binding to the prolactin receptor. By subtracting the Mr of prolactin, it was calculated that the hormone was bound to species of Mr 43 000 and 28 000. These Mr values were not affected by reduction of solubilized membranes, suggesting that the structure of the prolactin receptor is not stabilized by interchain disulphide bonds between subunits. Subtracting the Mr of somatotropin from somatogenic complexes indicated that the hormone had bound to species of Mr 280 000, 200 000 and 100 000. The 300 000- and 220 000-Mr complexes were not isolated from reduced membranes, whereas the amount of the 130 000-Mr species was augmented. These observations could suggest that a major component of the somatotropin receptor is a trimeric aggregate in which some subunits are retained in a larger complex by interchain disulphide bonds.

Receptor- and non-receptor-mediated uptake and degradation of insulin by hepatocytes

Native insulin inhibits the binding and degradation of (125)I-labelled insulin in parallel. Half-maximal inhibition of degradation occurs with 10nm-insulin, a hormone concentration sufficient to saturate the insulin receptor. The proportion of bound hormone that is degraded increases as the insulin concentration is increased, suggesting that low-affinity uptake is functionally related to degradation. Since only a small fraction (approx. 10%) of the overall degradation occurs at the plasma membrane, or in the extracellular medium, translocation of bound hormone into the cell is the predominant mechanism mediating the degradation of insulin. In the presence of 0.6nm-insulin, a concentration at which most cell-associated hormone is receptor-bound, chloroquine increases the amount of (125)I-labelled insulin retained by hepatocytes. However, chloroquine increases the retention of degradation products of insulin in incubations containing sufficient hormone (6nm) to saturate the receptor and permit occupancy of low-affinity sites. Glucagon does not compete for the interaction of (125)I-labelled insulin (1nm) with the insulin receptor. In contrast, 20mum-glucagon inhibits 75% of the uptake of insulin (0.1mum) by low-affinity sites. A fraction of the cell-bound radioactivity is not intact insulin throughout a 90min association reaction at 37 degrees C. During dissociation, fragments of (125)I-labelled insulin are released to the medium more rapidly than is intact hormone. The production and transient retention of degradation products of the hormone complicates the characterization of the insulin receptor by equilibrium or kinetic methods of assay. It is proposed that insulin degradation occurs by receptor- and non-receptor-mediated pathways. The latter may be related to the action of glutathione-insulin transhydrogenase, with which both insulin and glucagon interact.

Placental amino acid uptake. VI. Regulation by intracellular substrate

Amino acid uptake by human placental tissue is regulated by intracellular amino acids. alpha-Aminoisobutyric acid (AIB) uptake was reduced at intracellular AIB concentrations of 0.8 mM. The magnitude of reduction increased sharply between 1 and 3 mM and reached a maximum of 45% at 5 mM. Suppression was specific to the "A" system. It occurred only when both the amino acid used for preloading and that used as an uptake substrate were active with that system. In the "L" system, facilitation apparently occurs, and in the "ASC" system there is no apparent effect. The system specificity as well as other evidence indicated that suppression is caused by substrate present intracellularly rather than by dilution of extracellular substrate. Suppression was independent of inhibitors of protein synthesis and was not seen in membrane vesicles prepared from preloaded tissue, indicating that intracellular substrate interacts directly with the carrier (transinhibition) rather than altering its synthesis or degradation. The A system transinhibition has the potential to regulate syncytial uptake in vivo and limit variation due to changes in maternal plasma amino acid concentration.

Consequences of 125I-labeled insulin degradation by hepatocytes on the interpretation of receptor binding studies

The association of 125I-labeled insulin with hepatocytes was assayed by filtration or microcentrifugation. Assay by centrifugation resulted in a greater amount of retained radioactive label throughout the course of association of 125I-labeled insulin with hepatocytes. Similarly, saturation experiments assayed by microcentrifugation suggested greater binding than filtration. During dissociation, cells isolated by centrifugation release a greater amount of rapid-dissociating radioactive label. Control experiments of [3H]-inulin exclusion with cell pellets, which were isolated during microcentrifugation, demonstrated that the difference between the methods was not due to extracellular trapping of radioactivity. Therefore, the data suggested that there was more low-affinity retention when binding was assayed by centrifugation than filtration. The integrity of the 125I-labeled insulin extracted from hepatocytes was determined by column chromatography. A substantially greater proportion of the extracted radioactivity was fragments of 125I-labeled insulin in cells isolated by centrifugation. It is suggested that the extensive washing of the cells during filtration removes more fragments than does centrifugation. During dissociation, the low-affinity component of radioactivity, which was observed in the centrifugal assay, resulted from the transient retention of insulin fragments. The extensive degradation of insulin, which was assayed by either method, and the differences observed between these methods, should be considered in the interpretation of binding experiments with cells.

Schistosoma mansoni: in vitro uptake and incorporation of glycine by adult worms

1. The uptake of [3H]glycine into an ethanol-extractable intracellular pool and its simultaneous incorporation into protein (TCA insoluble fraction) have been measured in adult male and female Schistosoma mansoni incubated, in vitro, in complex media. 2. The patterns of uptake and incorporation were similar in male and female schistosomes incubated in media containing serum, and also in male worms incubated in the presence or absence of serum. 3. The amounts of [3H]glycine uptake and incorporation were directly proportional to the extracellular glycine concentration and were identical in male worms (in the presence or absence of serum) and female worms. 4. Emetine at a concentration producing a 95% inhibition of protein synthesis was without effect on the uptake of [3H]glycine into the ethanol-extractable pool in male worms.

Evidence for control of protein synthesis in HeLa cells via the elongation rate

The effects of fresh medium and serum on protein synthesis in suspension-cultured HeLa cells after growth to high cell density (> 5 x 10(5) cells/ml) were studied. Cells which were resuspended in fresh medium plus serum and grown for 24 hours (control) were compared with cells grown for 2 hours after resuspension (stimulated). The spectrum of proteins being synthesized by control and stimulated cells does not appear to be grossly different; that is, the weight and number average molecular weights of newly synthesized whole-cell protein are about the same in both cultures. Also, no significant differences were observed in the number of ribosomes per polysome or in the fraction of total ribosomes in polysomes. However, the transit times (combined elongation and termination times) were found to differ significantly; the average transit time for control cells was 2.24 minutes, while the average transit time for stimulated cells was 1.26 minutes. (An appendex evaluating the methodology involved in measuring the transit time is included.) In aggreement with the difference in transit time, the absolute rate of protein synthesis in stimulated cells was approximately 1.8 times the rate measured in control cells. These data are taken as evidence that under certain conditions, the rate of elongation and/or termination of polypeptide chains limits the overall rate of translation, and that cells can respond to growth conditions by changing the elongation and/or termination rate of protein synthesis.

Effect of glucose, independent of changes in insulin and glucagon secretion, on alanine metabolism in the conscious dog

To study the effects of hyperglycemia on the metabolism of alanine and lactate independent of changes in plasma insulin and glucagon, glucose was infused into five 36-h-fasted dogs along with somatostatin and constant replacement amounts of both insulin and glucagon. Hepatic uptakes of alanine and lactate were calculated using the arteriovenous difference technique. [14C]Alanine was infused to measure the conversion of alanine and lactate into glucose. Hyperglycemia (delta 115 mg/dl) of 2 h duration caused the plasma alanine level to increase by over 50%. This change was caused by an increase in the inflow of alanine into plasma since the net hepatic uptake of the amino acid did not change. Taken together, the above findings indicate that glucose per se can significantly impair the fractional extraction of alanine by the liver. Hepatic extraction of lactate was also affected by hyperglycemia and had fallen to zero within 90 min of starting the glucose infusion. This fall was associated with a doubling of arterial lactate level. Conversion of [14C]-alanine and [14C]lactate into [14C]glucose was suppressed by 60 +/- 11% after 2 h of hyperglycemia, and because this fall could not be entirely accounted for by decreased lactate extraction an inhibitory effect of glucose on gluconeogenesis within the liver is suggested. These studies indicate that the plasma glucose level per se can be an important determinant of the level of alanine and lactate in plasma as well as the rate at which they are converted to glucose.

Sodium-dependent alanine transport in plasma-membrane vesicles from rat liver

L-Alanine transport was studied in plasma-membrane vesicles from rat liver. A gradient of NaSCN, but not of KSCN, stimulated alanine uptake. Monensin plus carbonyl cyanide p-trifluoromethoxyphenylhydrazone abolished the observed overshoot in uptake. After equilibration of alanine, NaSCN induced uphill transport.