Regulation of hepatic free fatty acid metabolism by glucagon and insulin

The Zinc Transporter Slc30a8/ZnT8 Is Required in a Subpopulation of Pancreatic α-Cells for Hypoglycemia-induced Glucagon Secretion

SLC30A8 encodes a zinc transporter ZnT8 largely restricted to pancreatic islet β- and α-cells, and responsible for zinc accumulation into secretory granules. Although common SLC30A8 variants, believed to reduce ZnT8 activity, increase type 2 diabetes risk in humans, rare inactivating mutations are protective. To investigate the role of Slc30a8 in the control of glucagon secretion, Slc30a8 was inactivated selectively in α-cells by crossing mice with alleles floxed at exon 1 to animals expressing Cre recombinase under the pre-proglucagon promoter. Further crossing to Rosa26:tdRFP mice, and sorting of RFP⁺: glucagon⁺ cells from KO mice, revealed recombination in ∼30% of α-cells, of which ∼50% were ZnT8-negative (14 ± 1.8% of all α-cells). Although glucose and insulin tolerance were normal, female αZnT8KO mice required lower glucose infusion rates during hypoglycemic clamps and displayed enhanced glucagon release (p < 0.001) versus WT mice. Correspondingly, islets isolated from αZnT8KO mice secreted more glucagon at 1 mm glucose, but not 17 mm glucose, than WT controls (n = 5; p = 0.008). Although the expression of other ZnT family members was unchanged, cytoplasmic (n = 4 mice per genotype; p < 0.0001) and granular (n = 3, p < 0.01) free Zn²⁺ levels were significantly lower in KO α-cells versus control cells. In response to low glucose, the amplitude and frequency of intracellular Ca²⁺ increases were unchanged in α-cells of αZnT8KO KO mice. ZnT8 is thus important in a subset of α-cells for normal responses to hypoglycemia and acts via Ca²⁺-independent mechanisms.

Glucose or insulin, but not zinc ions, inhibit glucagon secretion from mouse pancreatic alpha-cells

The mechanisms by which hypoglycemia stimulates glucagon release are still poorly understood. In particular, the relative importance of direct metabolic coupling versus paracrine regulation by beta-cell secretory products is unresolved. Here, we compare the responses to glucose of 1) alpha-cells within the intact mouse islet, 2) dissociated alpha-cells, and 3) clonal alphaTC1-9 cells. Free cytosolic concentrations of ATP ([ATP](c)) or Ca(2+) ([Ca(2+)](c)) were imaged using alpha-cell-targeted firefly luciferase or a green fluorescent protein-based Ca(2+) probe ("pericam"), respectively. Consistent with a direct effect of glucose on alpha-cell oxidative metabolism, an increase in glucose concentration (from 0 or 3 mmol/l to 20 mmol/l) increased [ATP](c) by 7-9% in alpha-cells within the intact islet and by approximately 4% in alphaTC1-9 cells. Moreover, glucose also dose-dependently decreased the frequency of [Ca(2+)](c) oscillations in both dissociated alpha-cells and alphaTC1-9 cells. Although the effects of glucose were mimicked by exogenous insulin, they were preserved when insulin signaling was blocked with wortmannin. Addition of ZnCl(2) slightly increased the frequency of [Ca(2+)](c) oscillations but failed to affect glucagon release from either islets or alphaTC1-9 cells under most conditions. We conclude that glucose and insulin, but not Zn(2+) ions, independently suppress glucagon secretion in the mouse.

Induction of ketogenesis and fatty acid oxidation by glucagon and cyclic AMP in cultured hepatocytes from rabbit fetuses. Evidence for a decreased sensitivity of carnitine palmitoyltransferase I to malonyl-CoA inhibition after glucagon or cyclic AMP treatment

The effects of pancreatic hormones and cyclic AMP on the induction of ketogenesis and long-chain fatty acid oxidation were studied in primary cultures of hepatocytes from fetal and newborn rabbits. Hepatocytes were cultivated during 4 days in the presence of glucagon (10(-6) M), forskolin (2 x 10(-5) M), dibutyryl cyclic AMP (10(-4) M), 8-bromo cyclic AMP (10(-4) M) or insulin (10(-7) M). Ketogenesis and fatty acid metabolism were measured using [1-14C]oleate (0.5 mM). In hepatocytes from fetuses at term, the rate of ketogenesis remained very low during the 4 days of culture. In hepatocytes from 24-h-old newborn, the rate of ketogenesis was high during the first 48 h of culture and then rapidly decreased to reach a low value similar to that measured in cultured hepatocytes from term fetuses. A 48 h exposure to glucagon, forskolin or cyclic AMP derivatives is necessary to induce ketone body production in cultured fetal hepatocytes at a rate similar to that found in cultured hepatocytes from newborn rabbits. In fetal liver cells, the induction of ketogenesis by glucagon or cyclic AMP results from changes in the partitioning of long-chain fatty acid from esterification towards oxidation. Indeed, glucagon, forskolin and cyclic AMP enhance oleate oxidation (basal, 12.7 +/- 1.6; glucagon, 50.0 +/- 5.5; forskolin, 70.6 +/- 5.4; cyclic AMP, 77.5 +/- 3.4% of oleate metabolized) at the expense of oleate esterification. In cultured fetal hepatocytes, the rate of fatty acid oxidation in the presence of cyclic AMP is similar to the rate of oleate oxidation present at the time of plating (85.1 +/- 2.6% of oleate metabolized) in newborn rabbit hepatocytes. In hepatocytes from term fetuses, the presence of insulin antagonizes in a dose-dependent fashion the glucagon-induced oleate oxidation. Neither glucagon nor cyclic AMP affect the activity of carnitine palmitoyltransferase I (CPT I). The malonyl-CoA concentration inducing 50% inhibition of CPT I (IC50) is 14-fold higher in mitochondria isolated from cultured newborn hepatocytes (0.95 microM) compared with fetal hepatocytes (0.07 microM), indicating that the sensitivity of CPT I decreases markedly in the first 24 h after birth. The addition of glucagon or cyclic AMP into cultured fetal hepatocytes decreased by 80% and 90% respectively the sensitivity of CPT I to malonyl-CoA inhibition. In the presence of cyclic AMP, the sensitivity of CPT I to malonyl-CoA inhibition in cultured fetal hepatocytes is very similar to that measured in cultured hepatocytes from 24-h-old newborns.

Hormonal changes and their influence on metabolism and nutrition in the critically ill

This is a brief review of the observed hormonal alterations following trauma and sepsis. The major changes noted in the metabolic status of the stressed patient have been characterized by deranged carbohydrate metabolism, altered metabolic rate as measured by oxygen consumption and increased ureagenesis. Each of these phenomena are regulated to a large extent by the specific hormonal profile of the patient. Failure of insulin and growth hormone production have been associated with glucose intolerance, excessive urinary nitrogen loss and a fatal outcome. Glucagon, cortisol and catecholamines exhibit sustained elevation and have been associated with increased metabolic rate and excessive ureagenesis. These changes are usually self limited following trauma but will persist if the patient enters a septic phase. The use of specific nutritional support, namely hypertonic glucose versus a balanced fat emulsion system in the face of sepsis is considered.

Effects of adrenaline on ketogenesis from long- and medium-chain fatty acids in starved rats

1. Injection of adrenaline into 24 h-starved rats caused a 69% decrease in blood [ketone-body] (3-hydroxybutyrate plus acetoacetate), accompanied by a decreased [3-hydroxybutyrate]/[acetoacetate] ratio. Blood [glucose] and [lactate] increased, but [alanine] was unchanged. 2. Adrenaline also decreased [ketone-body] after intragastric feeding of both long- and medium-chain triacylglycerol. The latter decrease was observed after suppression of lipolysis with 5-methylpyrazole-3-carboxylic acid, indicating that the antiketogenic action of adrenaline was not dependent on the chain length of the precursor fatty acid. 3. The actions of adrenaline to decrease blood [ketone-body] and to increase blood [glucose] were not observed after administration of 3-mercaptopicolinate, an inhibitor of gluconeogenesis. This suggests that these effects of the hormone are related. 4. The possible clinical significance of the results is discussed with reference to the restricted ketosis often observed after surgical or orthopaedic injury.

The effect of glucagon treatment and starvation of virgin and lactating rats on the rates of oxidation of octanoyl-L-carnitine and octanoate by isolated liver mitochondria

1. Oxygen-consumption rates owing to oxidation of octanoate or octanoylcarnitine by isolated mitochondria from livers of fed, starved and glucagon-treated virgin or 12-day-lactating animals were measured under State-3 and State-4 conditions, in the presence or absence of l-malate and inhibitors of tricarboxylic acid-cycle activity (malonate and fluorocitrate). 2. Mitochondria from fed lactating animals had a slightly lower rate of octanoylcarnitine oxidation than did those of fed virgin animals, whereas the rates of octanoate oxidation were unaffected. 3. Starvation of virgin animals for 24h or 48h resulted in a large (70-100%) increase in mitochondrial octanoylcarnitine oxidation; rates of octanoate oxidation were either unaffected (24 and 48h starvation in the absence of malonate and fluorocitrate) or diminished by 30% (48h starvation in the presence of inhibitors). In lactating animals, 24h starvation resulted in a smaller increase in the rate of octanoylcarnitine oxidation than that obtained for mitochondria from virgin rats. 4. Glucagon treatment (by intra-abdominal injection) of fed virgin and lactating rats increased the rate of mitochondrial oxidation of both octanoylcarnitine and octanoate. Injection of glucagon into 48h-starved virgin rats did not increase further the already elevated rate of octanoylcarnitine oxidation, but reversed the inhibition of octanoate beta-oxidation observed for these mitochondria in the presence of malonate and fluorocitrate. 5. It is suggested that glucagon activates octanoylcarnitine oxidation by increasing the activity of the carnitine/acylcarnitine transport system [Parvin & Pande (1979) J. Biol. Chem.254, 5423-5429] and that the increase in octanoate oxidation by mitochondria from glucagon-treated animals is caused by the increased rate of ATP synthesis in these mitochondria. 6. The results are discussed in relation to the increased capacity of the liver to oxidize long-chain fatty acids and carnitine esters of medium-chain fatty acids under conditions characterized by increased ketogenesis.