Hyperglucagonemia and altered responsiveness of hepatic adenylate cyclase-adenosine 3′,3′-monophosphate system to hormonal stimulation during chronic ingestion of dl-ethionine

Basal activity and hormonal responsiveness of the adenylate cyclase-adenosine 3′,5′-monophosphate system were examined in premalignant liver from rat chronically fed the hepatic carcinogen DL-ethionine, and these data were correlated with endogenous levels of plasma glucagon. By 2 weeks basal hepatic cyclic AMP levels, determined in tissue quick-frozen in situ, were 2-fold higher in rats ingesting ethionine than in the pair-fed control. Enhanced tissue cyclic AMP content was associated with an increase in the adenylate cyclase activity of whole homogenates of fresh liver from rats fed ethionine (68 ± 5 pmol cyclic AMP/10 min per mg protein) compared to control (48 ± 4). Cyclic AMP-dependent protein kinase activity ratios were also significantly higher (control, 0.38 ± 0.04; ethionine 0.55 ± 0.05) and the percent glycogen synthetase activity in the glucose 6-phosphate-independent form was markedly reduced (control, 52 ± 7%; ethionine, 15 ± 1.5 %) in the livers of ethionine-fed rats compared to the controls, suggesting that the high total hepatic cyclic AMP which accompanied ethione ingestion was biologically effective. These changes persisted throughout the 38 weeks of drug ingestion. Immunoreactive glucagon levels, determined in portal venous plasma, were 8-fold higher than control after 2 weeks of the ethionine diet (contro, 185 ± 24 pg/ml; ethionine, 1532 ± 195). Analogous to the changes in hepatic parameters, plasma glucagon levels remained elevated during the entire period of drug ingestion until the development of hepatomas. The hepatic cyclic AMP response to a maximal stimulatory dose of injected glucagon was blunted in vivo in ethionine-fed rats (control, 14-fold increase over basal, to 8.63 ± 1.1 pmol/mg wet weight; ethionine, 4.6-fold rise over basal, to 5.42 ± 0.9). Reduced cyclic AMP responses to both maximal and submaximal glucagon stimulation were also evident in vitro in hepatic slices prepared from rats fed the drug, and the reduction was specific to glucagon. Absolute or relative hepatic cyclic AMP responses to maximally effective concentrations of prostaglandin E₁ or isoproterenol in hepatic slices from ethionine-fed rats were greater than or equal to those observed in control slices. Parallel alterations in hormonal responsiveness were observed in adenylate cyclase activity of whole homogenates of these livers, implying that the changes in cyclic AMP accumulation following hormone stimulation were related to an alteration in cyclic AMP generation in the premalignant tissue.In view of the recognized hepatic actions of glucagon and the desensitization of adenylate cyclase which can occur during sustained stimulation of the liver with this hormone, the endogenous hyperglucagonemia that accompanies ethionine ingestion could play a role in the pathogenesis of both the basal alterations in hepatic cyclic AMP metabolism and the reduced responsiveness to glucagon observed in liver from rats fed this carcinogen.     

Effect of fasting and insulin on the glucagon-induced orthophosphate incorporation to the isolated perfused rat liver.

Isolated perfused fed rat livers spontaneously liberated glucose and orthophosphate to the medium; 24-hr fasted rat livers did not exhibit these phenomena. In perfused fed rat livers, glucagon (2 mug) increased glucose output and promoted orthophosphate incorporation. In perfused fed rat livers, insulin (250 or 500 mU) inhibited the spontaneous liberation of glucose and orthophosphate. Comparable doses of insulin significantly reduced the glucagon (2 mug)-induced increase in glucose output from perfused fed rat liver, but did not affect orthophosphate uptake by the organ.     

Effect of ethynyl estradiol on the secretion of hepatic triglyceride.

The concentrations of triglyceride in the blood of female rats increased 2- and 4-fold during treatment with 5 and 15 mug/kg of ethynyl estradiol, respectively. The rate of secretion of triglyceride increased 66% over controls with livers obtained from the rats administered ethynyl estradiol. Ethynyl estradiol induced a hypocholesterolemia in the donor animals but the secretion of cholesterol into the perfusate from livers obtained from these animal was not affected. Adrenal corticosterone levels were depressed 48% in animals receivint of ethynyl estradiol on the liver or secondary to other hormonal changes.     

Glucagon-stimulated calcium efflux in the isolated perfused rat liver is dependent on cellular redox potential

In the absence of any exogenous substrates, glucagon (1 X 10(-9) M) stimulated 45Ca2+ efflux from perfused livers derived from fed rats but not in livers of 24-h-fasted animals. In livers of 24-h-fasted animals perfused under conditions which would decrease cellular NAD(P)H/NAD(P)+ ratio (pyruvate (2.0 mM) or acetoacetate (10.0 mM], glucagon (1 X 10(-9) M) did not stimulate 45Ca2+ efflux. Similarly, in livers of 24-h-fasted animals perfused with substrates which increase cellular NAD(P)H content (lactate (2.0 mM) or beta-hydroxybutyrate (10.0 mM], glucagon (1 X 10(-9) M) did not increase 45Ca2+ efflux. Glucagon (1 X 10(-9) M) elicited an increase in 45Ca2+ efflux from livers of 24-h-fasted animals, only when the livers were perfused with [lactate]/[pyruvate] and [beta-hydroxybutyrate]/[acetoacetate] ratios similar to those reported for livers of fed rats. Stimulation of 45Ca2+ efflux elicited by either 8-CPT-cAMP, a cAMP analog, or high glucagon concentrations (1 X 10(-8) M) was not affected whether livers were perfused with pyruvate (2.0 mM) or lactate (2.0 mM). Administration of isobutylmethylxanthine (50 microM) alone, or glucagon (1 X 10(-9) M) in the presence of isobutylmethylxanthine (50 microM) stimulated 45Ca2+ efflux from livers of 24-h-fasted animals perfused with pyruvate (2.0 mM) but not from livers perfused with lactate (2.0 mM). The ability of glucagon (1 X 10(-9) M) to elevate tissue cAMP levels was also regulated by the oxidation-reduction state of the livers. The data indicate that glucagon-stimulated 45Ca2+ efflux from perfused livers is mediated via cAMP and is dependent on the oxidation-reduction state of the livers.     

Effects of beta-adrenergic stimulation on 1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine-mediated vasoconstriction and glycogenolysis in the perfused rat liver

The beta-adrenergic agonist isoproterenol inhibited the glycogenolytic response of platelet-activating factor (AGEPC, 1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine) in perfused livers derived from fed rats. AGEPC-stimulated hepatic vasoconstriction, measured by increases in portal vein pressure, also was inhibited by prior isoproterenol infusion. Isoproterenol-mediated inhibition of these hepatic responses to AGEPC was not apparent when isoproterenol (10 microM) was coinfused with the beta-receptor antagonist propranolol (75 microM) or when isoproterenol was replaced with the alpha-adrenergic agonist phenylephrine (10 microM). alpha-Agonist-induced glycogenolysis and vasoconstriction in the perfused liver was unaffected by isoproterenol infusion. Glucagon (2.3 nM) had no effect on the glycogenolytic or vasoconstrictive responses of the liver to AGEPC despite the fact that glucagon increased hepatic cAMP levels to a far greater extent than isoproterenol. Additionally, inhibition of the hepatic responses to AGEPC by isoproterenol occurred in perfused livers from mature rats (i.e. greater than 300 g) in which liver parenchymal cells lack functional beta-adrenergic receptors. The data presented in this study illustrate a specific inhibition of AGEPC-induced hepatic glycogenolysis and vasoconstriction by beta-adrenergic stimulation of the perfused liver. This inhibition appears to be mediated by interaction of isoproterenol with nonparenchymal cells within the liver. These findings are consistent with the concept that AGEPC stimulates hepatic glycogenolysis by an indirect mechanism involving hepatic vasoconstriction.     

Comparison of metabolism of free fatty acid by isolated perfused livers from male and female rats.

Livers from normal, fed male and female rats were perfused with different amounts of [1-14C]oleate under steady state conditions, and the rates of uptake and utilization of free fatty acid (FFA) were measured. The uptake of FFA by livers from either male or female rats was proportional to the concentration of FFA in the medium. The rate of uptake of FFA, per g of liver, by livers from female rats exceeded that of the males for the same amount of FFA infused. The incorporation by the liver of exogenous oleic acid into triglyceride, phospholipid, and oxidation products was proportional to the uptake of FFA. Livers from female rats incorporated more oleate into triglyceride (TG) and less into phospholipid (PL) and oxidation products than did livers from male animals. Livers from female rats secreted more TG than did livers from male animals when infused with equal quantities of oleate. The incorporation of endogenous fatty acid into TG of the perfusate was inhibite) by exogenous oleate. At low concentrations of perfusate FFA, however, endogenous fatty acids contributed substantially to the increased output of TG by livers from female animals. Production of 14CO2 and radioactive ketone bodies increased with increasing uptake of FFA. The partition of oleate between oxidative pathways (CO2 production and ketogenesis) was modified by the availability of the fatty acid substrate with livers from either sex. The percent incorporation of radioactivity into CO2 reached a maximum, whereas incorporation into ketone bodies continued to increase. The output of ketone bodies was dependent on the uptake of FFA, and output by livers from female animals was less than by livers from male rats. The increase in rate of ketogenesis was dependent on the influx of exogenous FFA, while ketogenesis from endogenous sources remained relatively stable. The output of glucose by the liver increased with the uptake of FFA, but no difference due to sex was observed. The output of urea by livers from male rats was unaffected by oleate, while the output of urea by livers from females decreased as the uptake of FFA increased. A major conclusion to be derived from this work is that oleate is not metabolized identically by livers from the two sexes, but rather, per gram of liver, livers from female rats take up and esterify more fatty acid to TG and oxidize less than do livers from male animals; livers from female animals synthesize and secrete more triglyceride than do livers from male animals when provided with equal quantities of free fatty acid.     

Stimulation by vasopressin of glycogen breakdown and gluconeogenesis in the perfused rat liver

1. Vasopressin (anti-diuretic hormone, [8-arginine]vasopressin) stimulated the breakdown of glycogen in perfused livers of fed rats, at concentrations (50-600muunits/ml) that have been reported in the blood of intact rats, especially during acute haemorrhagic shock. 2. In perfused livers from starved rats, vasopressin (30-150muunits/ml) stimulated gluconeogenesis from a mixture of lactate, pyruvate and glycerol. 3. Vasopressin prevented accumulation of liver glycogen in the perfused liver of starved rats, or in starved intact rats. 4. The action of vasopressin on hepatic carbohydrate metabolism thus resembles that of glucagon; the minimum effective circulating concentrations of these hormones are of the same order (100pg/ml). 5. The stimulation of hepatic glucose output by vasopressin is discussed in connexion with the release of glucose and water from the liver.     

Carbohydrate metabolism of the perfused rat liver

1. The rates of gluconeogenesis from most substrates tested in the perfused livers of well-fed rats were about half of those obtained in the livers of starved rats. There was no difference for glycerol. 2. A diet low in carbohydrate increased the rates of gluconeogenesis from some substrates but not from all. In general the effects of a low-carbohydrate diet on rat liver are less marked than those on rat kidney cortex. 3. Glycogen was deposited in the livers of starved rats when the perfusion medium contained about 10mm-glucose. The shedding of glucose from the glycogen stores by the well-fed liver was greatly diminished by 10mm-glucose and stopped by 13.3mm-glucose. Livers of well-fed rats that were depleted of their glycogen stores by treatment with phlorrhizin and glucagon synthesized glycogen from glucose. 4. When two gluconeogenic substrates were added to the perfusion medium additive effects occurred only when glycerol was one of the substrates. Lactate and glycerol gave more than additive effects owing to an increased rate of glucose formation from glycerol. 5. Pyruvate also accelerated the conversion of glycerol into glucose, and the accelerating effect of lactate can be attributed to a rapid formation of pyruvate from lactate. 6. Butyrate and oleate at 2mm, which alone are not gluconeogenic, increased the rate of gluconeogenesis from lactate. 7. The acceleration of gluconeogenesis from lactate by glucagon was also found when gluconeogenesis from lactate was stimulated by butyrate and oleate. This finding is not compatible with the view that the primary action of glucagon in promoting gluconeogenesis is an acceleration of lipolysis. 8. The rate of gluconeogenesis from pyruvate at 10mm was only 70% of that at 5mm. This ;inhibition' was abolished by oleate or glucagon.     

Studies on the mechanism of action of acetazolamide in the prophylaxis of hyperkalemic periodic paralysis.

Levels of glycogen and cyclic 3′, 5′-adenosine monophosphate (cAMP) were determined in livers of rats treated with 10, 25, 50 or 100 mg/kg of acetazolamide (Diamox). When compared with livers of untreated rats, there were significant decreases in liver glycogen content and significant increases in cAMP levels at all doses of the drug. When liver slices were incubated in the presence of 10^?5 to 10^?3 molar acetazolamide, no difference was found between treated and untreated slices.Plasma insulin and blood glucose levels were also determined and it was found that although plasma insulin levels were significantly increased at all four doses of acetazolamide, blood glucose remained unchanged.These data suggest that acetazolamide induces glycogenolysis through an indirect mechanism dependent upon the release of some endogenous factor, e.g., glucagon or epinephrine, which, in turn, increases levels of cAMP. However, because insulin levels are increased, the increased glycogenolysis does not elevate blood glucose. Thus, it is suggested that acetazolamide stabilizes blood glucose levels while stimulating insulin secretion to potentiate the movement of potassium across muscle membranes and thereby correct the defect which causes attacks of hyperkalemic periodic paralysis.     

Effects of serum proteins on estrogen action in the perfused rat liver

To determine the effects of serum proteins on the biologic activity of estrogens, we perfused isolated livers from ovariectomized female rats with oxygenated Krebs-Henseleit-bicarbonate buffer (KHBB), with and without 4% human serum albumin (4% HSA), with and without added estrogens, or with charcoal-stripped human serum (CSHS) with and without added estradiol. At the end of the perfusions, the cytosolic and nuclear estrogen receptors were measured by an exchange assay. When added to KHBB, estradiol 10(-9) or 10(-8) M or estrone 10(-8) M did not cause any significant increase in the percent of receptors measured in the nucleus. When the livers were perfused with KHBB containing 4% HSA and estradiol 10(-9) to 10(-7) M or estrone 10(-8) M, there was an increase in nuclear receptors. Perfusion with estradiol 10(-8) M in CSHS resulted in significantly less receptor in the nucleus than after estradiol in KHBB plus 4% HSA. We conclude that the presence of 4% HSA in the perfusion medium increases the biologic activity of estradiol and estrone on the isolated rat liver, and this increase is inhibited in the presence of sex hormone-binding globulin. The exact mechanism by which HSA increases the biologic activity is uncertain, but may be due in part to better diffusion of estrogen through the liver.     

The rate of gluconeogenesis from various precursors in the perfused rat liver

1. The rates of gluconeogenesis from many precursors have been measured in the perfused rat liver and, for comparison, in rat liver slices. All livers were from rats starved for 48hr. Under optimum conditions the rates in perfused liver were three to five times those found under optimum conditions in slices. 2. Rapid gluconeogenesis (rates of above 0·5μmole/g./min.) were found with lactate, pyruvate, alanine, serine, proline, fructose, dihydroxyacetone, sorbitol, xylitol. Unexpectedly other amino acids, notably glutamate and aspartate, and the intermediates of the tricarboxylic acid cycle (with the exception of oxaloacetate), reacted very slowly and were not readily removed from the perfusion medium, presumably because of permeability barriers which prevent the passage of highly charged negative ions. Glutamine and asparagine formed glucose more readily than the corresponding amino acids. 3. Glucagon increased the rate of gluconeogenesis from lactate and pyruvate but not from any other precursor tested. This occurred when the liver was virtually completely depleted of glycogen. Two sites of action of glucagon must therefore be postulated: one concerned with mobilization of liver glycogen, the other with the promotion of gluconeogenesis. Sliced liver did not respond to glucagon. 4. Pyruvate and oxaloacetate formed substantial quantities of lactate on perfusion, which indicates that the reducing power provided in the cytoplasm was in excess of the needs of gluconeogenesis. 5. Values for the content of intermediary metabolites of gluconeogenesis in the perfused liver are reported. The values for most intermediates rose on addition of lactate. 6. The rates of gluconeogenesis from lactate and pyruvate were not affected by wide variations of the lactate/pyruvate ratio in the perfusion medium.     

Differential effect of glucagon on gluconeogenesis in periportal and pericentral regions of the liver lobule.

The effect of glucagon on gluconeogenesis was measured in periportal and pericentral regions of the liver lobule by monitoring changes in rates of O2 uptake on the surface of the perfused liver with miniature O2 electrodes after infusion of lactate. When lactate (2 mM) was infused into livers from starved rats perfused in the anterograde direction, O2 uptake was increased 2.5-fold more in periportal than in pericentral regions, reflecting increased energy demands for glucose synthesis. Under these conditions, glucagon infusion in the presence of lactate increased O2 uptake exclusively in periportal regions of the liver lobule. Thus, when perfusion is in the physiological anterograde direction, the metabolic actions of glucagon predominate in periportal regions of the liver lobule under gluconeogenic conditions in the starved state. When livers were perfused in the retrograde direction, however, glucagon stimulated O2 uptake exclusively in pericentral regions. Thus glucagon only stimulates gluconeogenesis in 'upstream' regions of the liver lobule irrespective of the direction of flow.     

The action of anoxia and cyanide on glycogen breakdown in the liver of the gsd/gsd rat

Perfusion of normal rat livers under anoxic conditions or the addition of KCN to aerobic perfusions activated phosphorylase and stimulated glycogen breakdown and glucose output. Livers from rats with a deficiency of liver phosphorylase kinase (gsd/gsd) showed a much smaller activation of phosphorylase with anoxia or KCN and produced glucose at about half the rate of normal livers. The increase in phosphorylase a in gsd/gsd livers was insufficient to account for the increase in glucose output. The addition of KCN to normal hepatocytes, activated phosphorylase and stimulated glucose output almost as effectively as glucagon. Hepatocytes from gsd/gsd rats showed only a very small increase in phosphorylase a on the addition of KCN, and glucose output did not increase. We conclude that in the perfused liver, anoxia and KCN stimulate glycogen breakdown and glucose output, at least in part, by a mechanism that does not involve conversion of phosphorylase b to phosphorylase a. In isolated hepatocytes KCN stimulates glucose output only by increasing the content of phosphorylase a.     

Cholesterol is required for secretion of very-low-density lipoprotein by rat liver.

To study potential effects of hepatic cholesterol concentration on secretion of very-low-density lipoprotein (VLDL) by the liver, male rats were fed on unsupplemented chow, chow with lovastatin (0.1%), or chow with lovastatin (0.1%) and cholesterol (0.1%) for 1 week. Livers were isolated from these animals and perfused in vitro, with a medium containing [2-14C]acetate, bovine serum albumin and glucose in Krebs-Henseleit buffer, and with an oleate-albumin complex. With lovastatin feeding, the hepatic concentrations of cholesteryl esters and triacylglycerols before perfusion were decreased, although free cholesterol was unchanged. However, hepatic secretion of all the VLDL lipids was decreased dramatically by treatment with lovastatin. Although total secretion of VLDL triacylglycerol, phospholipid, cholesterol and cholesteryl esters was decreased, the decrease in triacylglycerol was greater than that in free cholesterol or cholesteryl esters, resulting in secretion of a VLDL particle enriched in sterols relative to triacylglycerol. In separate studies, the uptake of VLDL by livers from control animals or animals treated with lovastatin was measured. Uptake of VLDL was estimated by disappearance of VLDL labelled with [1-14C]oleate in the triacylglycerol moiety, and was observed to be similar in both groups. During perfusion, triacylglycerol accumulated to a greater extent in livers from lovastatin-fed rats than in control animals. The depressed output of VLDL triacylglycerols and the increase in triacylglycerol in the livers from lovastatin-treated animals was indicative of a limitation in the rate of VLDL secretion. Addition of cholesterol (either free cholesterol or human low-density lipoprotein) to the medium perfusing livers from lovastatin-fed rats, or addition of cholesterol to the diet of lovastatin-fed rats, increased the hepatic concentration of cholesteryl esters and the output of VLDL lipids. The concentration of cholesteryl esters in the liver was correlated with the secretion of VLDL by the liver. These data suggest that cholesterol is an obligate component of the VLDL required for its secretion. It is additionally suggested that cholesteryl esters are in rapid equilibrium with a small pool of free cholesterol which comprises a putative metabolic pool available and necessary for the formation and secretion of the VLDL. Furthermore, the specific radioactivity (d.p.m./mumol) of the secreted VLDL free cholesterol was much greater than that of hepatic free cholesterol, suggesting that the putative hepatic metabolic pool is only a minor fraction of total hepatic free cholesterol.     

Short-term regulation of multidrug resistance-associated protein 3 in rat and human hepatocytes

The short-term regulation of multidrug resistance-associated protein 3 (Mrp3/MRP3) by cAMP and PKC was investigated in sandwich-cultured rat and human hepatocytes and isolated perfused rat livers. The modulator glucagon (500 nM) and the phorbol ester PMA (0.1 muM) were utilized to increase intracellular cAMP and PKC levels, respectively. In glucagon-treated rat hepatocytes, efflux of the Mrp3 substrate 5-(6)-carboxy-2',7'-dichlorofluorescein (CDF) increased approximately 1.5-fold, even in hepatocytes treated with the organic anion transporter (Oatp) inhibitor sulfobromophthalein (BSP). Confocal microscopy revealed more concentrated Mrp3 fluorescence in the basolateral membrane (less diffuse staining pattern) with glucagon treatment. PMA had no effect on Mrp3 activity or localization in sandwich-cultured rat hepatocytes. Glucagon and PMA treatment in isolated perfused rat livers resulted in a threefold increase (14 +/- 4.6 mul.min(-1).g liver(-1)) and a fourfold decrease (1.3 +/- 0.3 mul.min(-1).g liver(-1)) in CDF basolateral clearance compared with control livers (4.7 +/- 2.3 mul.min(-1).g liver(-1)), whereas CDF biliary clearance was not statistically different. In sandwich-cultured human hepatocytes, glucagon treatment resulted in a 1.3-fold increase in CDF efflux and a concomitant increase in MRP3 fluorescence in the basolateral membrane. In summary, cAMP and PKC appear to be involved in the short-term regulation of Mrp3/MRP3, as demonstrated by alterations in activity and localization in rat and human hepatocytes.     

Hormonal regulation of glutathione efflux

The efflux of GSH has been shown previously to be a saturable process in both isolated rat hepatocytes and perfused liver, suggesting a carrier-mediated transport mechanism. The possibility in hormonal regulation of this process has been raised by recent reports. Our present work examined the role of hormones known to affect intracellular signal transduction mechanisms on GSH efflux in cultured rat hepatocytes and perfused rat livers. We found that cAMP-dependent factors, such as cholera toxin (CT), dibutyryl cAMP, forskolin, and glucagon all stimulated GSH efflux in cultured rat hepatocytes. The efflux kinetics were compared in cultured cells incubated with or without CT; the stimulation of GSH efflux was related to a near doubling of the Vmax while exhibiting no significant alteration of the Km. The increase in intracellular cAMP level associated with the threshold for this stimulatory effect was 25% above control. The stimulatory effect of CT could not be blocked by cyclohexamide pretreatment or reversed by colchicine treatment. The stimulatory effect of glucagon was abolished in the presence of ouabain but not in the presence of barium. On the other hand, hormones which act through Ca2+ and protein kinase C, such as phenylephrine and vasopressin, had no effect on GSH efflux in the cultured cells. In the perfused liver model, glucagon (10 nM) and dibutyryl cAMP (8 microM) stimulated sinusoidal GSH efflux to 130 and 144% of control values, respectively, and increased bile flow while not affecting biliary GSH efflux. Finally, the physiological significance of glucagon-mediated stimulation of sinusoidal GSH efflux was assessed by both plasma GSH and glucose levels in response to in vivo glucagon infusion. The threshold dose of glucagon for significant increase in plasma GSH (5.21 pmol/min) was lower than for glucose (15.61 pmol/min). At the highest glucagon infusion rate (261 pmol/min), plasma GSH level doubled while glucose level increased 80%. In conclusion, increased cAMP stimulates GSH efflux in cultured rat hepatocytes and perfused livers. The stimulatory effect of cAMP is exerted at the sinusoidal pole and appears to be mediated by hyperpolarization of hepatocytes by stimulation of Na(+)-K(+)-ATPase. In vivo studies confirmed the importance of cAMP-mediated stimulation of sinusoidal GSH efflux as it resulted in significant elevation of the plasma GSH level.     

Characterization of the hormone-sensitive Ca2+ uptake activity of the hepatic endoplasmic reticulum

The characteristics and kinetics of calcium uptake activity were studied in isolated hepatic microsomes. The sustained accumulation of calcium was ATP- and oxalate-dependent. Glucagon increased microsomal Ca2+ uptake upon either in vivo injection, or in vitro perfusion of the hormone in the liver. In contrast, the effect of insulin depended on the route of administration. Calcium accumulation by subsequently isolated hepatic microsomes increased when insulin was injected intraperitoneally whereas it decreased when the hormone was perfused directly into the liver. These effects of glucagon and insulin were dose dependent. When insulin was added to the perfusate prior to the addition of glucagon, insulin blocked the glucagon-stimulated increase in microsomal Ca2+ uptake. Cyclic AMP mimicked the effect of glucagon on microsomal Ca2+ accumulation when the cyclic nucleotide was perfused into the liver. The effects of glucagon and insulin on the kinetics of hepatic microsomal Ca2+ uptake were investigated. In microsomes isolated from perfused rat livers treated with glucagon the V of the uptake was significantly increased over the control values (12.2 vs. 8.6 nmol Ca2+ per min per mg protein, P less than 0.02). In contrast, the addition of insulin to the perfusate significantly decreased the V of Ca2+ uptake by subsequently isolated microsomes (6.8 vs. 8.3 nmol Ca2+ per min per mg protein, P less than 0.05). However, neither hormone had an effect on the apparent Km for Ca2+ (4.1 +/- 0.5 microM) of the reaction. The effect of these hormones on the activity of Ca2+-stimulated ATPase was also studied. No significant changes in either V or Km for Ca2+ of the enzymatic reaction were detected.     

Rapid stimulation of calcium uptake into rat liver by L-tri-iodothyronine.

The short-term effect of L-tri-iodothyronine (T3) on hepatic Ca2+ uptake from perfusate was compared with changes induced by T3 on cellular respiration and glucose output in isolated perfused livers from fasted and fed rats. The same parameters were also studied after the addition of glucagon or vasopressin. T3 (1 microM) induced Ca2+ uptake from the perfusate into the liver within minutes, and the time course was similar to that for stimulation of respiration and gluconeogenesis in livers from fasted rats, and for the stimulation of respiration and glucose output in livers from fed rats. The effects were dose-dependent in the range 1 microM-0.1 nM. Similar changes in the same parameters could be observed with glucagon and vasopressin, but with a completely different time course. Also, the influence of the T3 analogues L-thyroxine (L-T4), 3,5-di-iodo-L-thyronine (L-T2) and 3,3',5-tri-iodo-D-thyronine (D-T3) on hepatic energy metabolism was examined. Whereas D-T3 had practically no effect, L-T4 and L-T2 caused changes in Ca2+ uptake, O2 consumption and gluconeogenesis in livers from fasted rats similar to those with T3. It is concluded that changes in mitochondrial and cytosolic Ca2+ concentrations are involved in the stimulation of respiration and glucose metabolism observed with T3, glucagon and vasopressin.     

The estradiol-stimulated lipoprotein receptor of rat liver. A binding site that membrane mediates the uptake of rat lipoproteins containing apoproteins B and E

Hepatic catabolism of lipoproteins containing apolipoproteins B or E is enhanced in rats treated with pharmacologic doses of 17 alpha-ethinyl estradiol. Liver membranes prepared from these rats exhibit an increased number of receptor sites that bind 125I-labeled human low density lipoproteins (LDL) in vitro. In the present studies, this estradiol-stimulated hepatic receptor was shown to recognize the following rat lipoproteins: LDL, very low density lipoproteins obtained from liver perfusates (hepatic VLDL), and VLDL-remnants prepared by intravenous injection of hepatic VLDL into functionally eviscerated rats. The receptor also recognized synthetic lamellar complexes of lecithin and rat apoprotein E as well as canine high density lipoproteins containing apoprotein E (apo E-HDLc). It did not recognize human HDL or rat HDL deficient in apoprotein E. Much smaller amounts of this high affinity binding site were also found on liver membranes from untreated rats, the number of such sites increasing more than 10-fold after the animals were treated with estradiol. Each of the rat lipoproteins recognized by this receptor was taken up more rapidly by perfused livers from estrogen-treated rats. In addition, enrichment of hepatic VLDL with C-apoproteins lowered the ability of these lipoproteins to bind to the estradiol-stimulated receptor and diminished their rate of uptake by the perfused liver of estrogen-treated rats, just as it did in normal rats. The current data indicate that under the influence of pharmacologic doses of estradiol the liver of the rat contains increased amounts of a functional lipoprotein receptor that binds lipoproteins containing apoproteins B and E. This hepatic lipoprotein receptor appears to mediate the uptake and degradation of lipoproteins by the normal liver as well as the liver of estradiol-treated rats. The hepatic receptor bears a close functional resemblance to the LDL receptor previously characterized on extrahepatic cells.