Effects of different prostaglandins on glucose kinetics in the rat

The effects of arachidonic acid on the endocrine and osmoregulatory response of tilapia (Oreochromis mossambicus) acclimated to seawater and subjected to confinement stress

In previous studies in freshwater tilapia (Oreochromis mossambicus), dietary supplementation with arachidonic acid (ArA; 20:4n - 6) had considerable, opposing effects on the main ion-transporting enzyme Na(+)/K(+)-ATPase in gills and kidneys and changed the release of osmoregulatory hormones, such as cortisol. The present study was performed to assess the influence of dietary ArA on (1) the osmoregulatory capacity of tilapia acclimated to seawater (SW) (34‰) and (2) the osmoregulatory imbalance associated with acute stress. The increased ambient salinity was associated with significant alterations in the tissue fatty acid composition, particularly the n - 6 polyunsaturated fatty acids (PUFAs). Tissue levels of ArA were further increased as a result of dietary supplementation, whereas docosahexaenoic acid (DHA, 22:6n - 3) and eicosapentaenoic acid (EPA, 20:5n - 3) decreased in gills and kidneys. Basal plasma cortisol as well as lactate levels were elevated in the ArA-supplemented SW-acclimated tilapia compared with the control group. The 5 min of confinement (transient stress) increased plasma cortisol, glucose, and lactate levels with significantly higher levels in ArA-supplemented tilapia. Confinement was also associated with significantly elevated plasma osmolality, sodium, chloride, and potassium levels. ArA-supplemented tilapia showed markedly lower ionic disturbances after confinement, suggesting that dietary ArA can attenuate the hydromineral imbalance associated with acute stress. These results emphasize the involvement of ArA and/or its metabolites in the endocrine and osmoregulatory processes and the response to confinement stress.

Increased liver lipase activity in rats with essential fatty acid deficiency

Liver lipase activity was measured in EFA-deficient rats (long-term) and in control rats and rats fed an EFA-deficient diet for two weeks (short-term). Liver lipase activity was significantly enhanced by EFA deficiency, both in long-term and short-term experiments. The enhanced liver lipase activity could be normalized by feeding these rats normal laboratory chow for 14 days. Since during EFA deficiency prostaglandin synthesis is impaired, the possible involvement of prostaglandins in the observed changes in liver lipase activity during EFA deficiency was studied. Administration of the prostaglandin synthesis inhibitor indomethacin (5 mg/kg body weight, i.p.) to normally fed rats for two days led to an increase of liver lipase activity. Prostaglandin E2 was found to inhibit the secretion of liver lipase activity by freshly isolated parenchymal liver cells in vitro. These results indicate that the increase in liver lipase activity during EFA deficiency may be due to an impairment of the prostaglandin synthesis.

Effect of prostaglandins and their analogues on hormone-stimulated glycogenolysis in primary cultures of rat hepatocytes

Hepatocytes were isolated by collagenase perfusion method from adult male rats, cultured and then prelabeled with [14C]glucose. The [14C]glycogen-labeled cells were used in experiments for effect of prostaglandins on hormone-stimulated glycogenolysis. Prostaglandin E1, prostaglandin E2 and 16,16-dimethylprostaglandin E2, but not prostaglandin D2 or prostaglandin F2 alpha, inhibited glycogenolysis stimulated by glucagon, epinephrine, isoproterenol (beta-adrenergic agonist) or epinephrine in the presence of propranolol (beta-antagonist) in primary cultured hepatocytes. The inhibitory effects on day 2 of cultures were approx. twice those on day 1. Dimethylprostaglandin E2 (10(-6)M) caused 60-70% inhibitions of the stimulations by these substances. In the case of the stimulation by glucagon, the inhibition further increased by 80-100% on day 3 of culture. Prostaglandin E1 and prostaglandin E2 caused less inhibition than dimethylprostaglandin E2 of all these stimulations. Dinorprostaglandin E1 (9 alpha,13-dihydroxy-7-ketodinorprost-11-enoic acid), which is a hepatocyte-metabolite of prostaglandin E1 and prostaglandin E2, and arachidonic acid did not have any inhibitory effects. These data indicate that the E series of prostaglandins may function as the regulation of hepatic glycogenolysis stimulated by epinephrine and glucagon, and that their rapid degradation system may contribute to the modulation of the action in liver.

Glycogenolytic and haemodynamic responses to heat-aggregated immunoglobulin G and prostaglandin E2 in the perfused rat liver

Infusion of heat-aggregated immunoglobulin G (HAG) into perfused livers from fed rats caused transient increases in hepatic glycogenolysis and portal-vein pressure, accompanied by a transient increase in hepatic glycogen phosphorylase alpha content. The hepatic responses to HAG were inhibited by indomethacin (2 microM). In contrast, HAG was without effect on phosphorylase alpha content and glucose output in isolated hepatocytes. HAG infusion caused a transient decrease in hepatic cyclic AMP. Lowering the extracellular Ca2+ concentration to 6 or 50 microM attenuated markedly the glycogenolytic and haemodynamic responses to HAG; efflux of Ca2+ from the liver was not observed in response to HAG. Co-infusion of the specific platelet-activating-factor antagonist U-66985 (1-O-octadecyl-2-O-acetyl-sn-glycero-3-phosphoric acid 6'-trimethylammoniumhexyl ester) did not attenuate the glycogenolytic response to HAG. Infusion of prostaglandin E2 caused increases in glucose output, portal-vein pressure and the reduction state of the cytosolic NAD(H) redox couple similar to those seen with HAG. The present study suggests that the glycogenolytic activation after HAG infusion may be an indirect consequence of the haemodynamic response of the hepatic vasculature to stimulation of the reticuloendothelial cells of the liver.

Kinetics of prostaglandin E metabolism in isolated hepatocytes

The kinetics of prostaglandin E metabolism were investigated in isolated hepatocytes from male Sprague-Dawley rats. [3H]Prostaglandin E1 or [3H]prostaglandin E2 was incubated with cells in suspension, aliquots were collected over time and samples extracted and chromatographed using reverse-phase high-performance liquid chromatography. Both prostaglandins E1 and E2 were rapidly metabolized by the hepatocytes and at low prostaglandin E concentrations (10(-12) M) both have similar apparent half-lives (prostaglandin E1, 1.24; prostaglandin E2, 1.68 min). Disappearance of counts from prostaglandin E1 and E2 fractions was accompanied by the appearance of counts in earlier fractions of the chromatograms, consistent with metabolism to compounds which are more polar than the native prostaglandins. The half-life for prostaglandin E1 was unchanged over a prostaglandin E1 concentration range of 10(-12) to 10(-6) M. In contrast, increasing prostaglandin E2 concentration resulted in a prolonged half-life over the same concentration range. Decreasing cell concentrations from 5 X 10(6) to 5 X 10(5) cells/ml resulted in decreases in the rates of metabolism of both prostaglandins E1 and E2.

Glucoregulatory and metabolic responses to heat exposure in rats

To determine the possible role of altered secretion and effects of insulin in fuel homeostasis during heat exposure, the hormonal and metabolic milieu of three groups of rats were studied. The first was placed at 35 degrees C for 12 days (HE), the second was pair-fed (PF) to the first but maintained at 23 degrees C, and the third was allowed to eat ad libitum at 23 degrees C (C). Plasma insulin, glucagon, glucose, and free fatty acids (FFA), and blood lactate, pyruvate, 3-hydroxybutyrate, and individual amino acids were determined. To further characterize glucoregulation, an intraperitoneal glucose tolerance test (1 mg/g body wt) and isotopic glucose turnover (primed infusion of [3-3H]glucose) were performed. In HE rats, weight was constant for the last third of the period, and metabolic state 4 h after food removal was characterized by euglycemia but hypoinsulinemia, elevated blood pyruvate and FFA, and normal 3-hydroxybutyrate compared with C. Lowered levels of branched-chain amino acids and arginine were found. Fourteen hours after food removal glucose turnover was decreased. However, glucose intolerance accompanied by hyperinsulinemia was also found. Many of these changes were also seen in PF, including constant weight, fasting euglycemia, hypoinsulinemia, elevated FFA, and lowered valine and isoleucine. In contrast, pyruvate concentrations were normal, that of 3-hydroxybutyrate was elevated, and the decrement in glucose turnover was smaller than in HE rats. The glucose tolerance was similar to that of HE but accompanied by hypoinsulinemia. The results in HE suggest decreased energy metabolism, insulin secretion altered in a complex manner, and altered insulin action.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of glucagon-stimulated hepatic glycogenolysis by E-series prostaglandins

The effect of E-series prostaglandins (PGE) on hepatic glucose metabolism is controversial. This study uses isolated rat hepatocytes and exogenously added PGE analogs or frequent native PGE additions (to compensate for hepatic PGE degradation) to define PGE's effect on hepatic glycogenolysis. 16,16-Dimethyl PGE2, 15(S),15-methyl PGE2, PGE1 and PGE2 all inhibit glucagon-stimulated glycogenolysis. It is concluded that E-series prostaglandins can act directly on the liver to inhibit glycogenolysis.

Dissociation of E prostaglandin effects on liver glycogenolysis and cyclic AMP levels

Some metabolic effects of prostaglandins have been related to their alteration of adenosine-3',5'-monophosphate (cyclic AMP) metabolism in different tissues. Prostaglandins E1 and E2 stimulate liver adenylate cyclase in vitro, but conflicting reports have been made about metabolic changes caused by E prostaglandins in hepatic tissue. We have attempted to resolve these issues by comparing the effects of PGE1 with those of glucagon using broken-cell homogenates, intact hepatocytes, liver slices and perfused liver. Prostaglandin E1 (PGE1) increased cyclic AMP in liver slices and in perfused liver without increasing glycogenolysis, but PGE1 had no discernible effect on carbohydrate or cyclic AMP metabolism in isolated hepatocytes. Glucagon caused predictable increases in cyclic AMP and glycogenolysis using hepatocytes, liver slices or perfused liver. These data can be explained by the absence of PGE effects on cyclic AMP metabolism in hepatocytes. The concentration of E prostaglandins (PGEs) increased 1.75-fold during incubations (37 degrees C) of hepatocyte suspensions, but cyclic AMP remained constant. Addition of exogenous arachidonate and indomethacin to cell suspensions increased and decreased PGEs, respectively, but cyclic AMP and glycogen metabolism were unchanged. Arachidonate and indomethacin likewise did not alter glucagon-stimulated glycogenolysis or cyclic AMP biosynthesis. The production of E prostaglandins and cyclic AMP appears to be unrelated in hepatocytes.

Interrelationships between PGE1 and PGI2 binding and stimulation of adenylate cyclase

The effects of prostaglandin E1 (PGE1) and prostacyclin (PGI2) on hepatic adenylate cyclase were studied in plasma membranes isolated from Sprague-Dawley rat livers. Both PGE1 and PGI2 stimulated this enzyme complex to the same maximal levels and with approximately the same EC50 (10(-7) M). Maximally stimulating concentrations of PGE1 and PGI2 were examined alone and together; their effects were not additive, indicating that the same enzyme complex was shared. Although a receptor for PGE1 could be demonstrated with a dissociation constant of 1 X 10(-8) M, PGI2 was only 1/100 as effective in competing for PGE1 binding sites (KD, 1 X 10(-6) M), indicating that these two prostaglandins may act via separate membrane receptors. PGI2 is known to be unstable at neutral pH; however, we have determined its half-life during these assays by a sensitive bioassay and concluded that the degradation of PGI2 is not sufficient to account for its inability to dissociate [3H]PGE1 binding. Further evidence that PGI2 might act through a distinct receptor was found in animals whose PGE1 receptors were 40% downregulated with a corresponding 28% decrease in PGE1-sensitive adenylate cyclase activity. These membranes had no such decrease in PGI2-sensitive adenylate cyclase activity. We conclude that 1) hepatic adenylate cyclase is equally sensitive to PGE1 and PGI2; 2) the same adenylate cyclase complex responds to both prostaglandins; and 3) PGE1 and PGI2 interact with separate membrane receptors in rat liver.

Factors affecting tryptophan-induced hypoglycaemia in rats

The mechanisms whereby tryptophan administration leads to hypoglycaemia in some groups of rats but not others have been investigated. Animals insensitive to tryptophan are rendered responsive by adrenalectomy. This effect is reversed by steroid replacement. Turnover studies with [2-3H]glucose show that hypoglycaemia in sensitive animals is associated with a decrease in glucose synthesis. Tryptophan administration causes a marked and sustained increase in plasma glucagon concentrations in all animals. The locus of the inhibition of gluconeogenesis in tryptophan-sensitive animals is the reaction catalysed by phosphoenolpyruvate carboxykinase. The sensitivities to tryptophan of gluconeogenesis in isolated hepatocytes from normal and adrenalectomized animals were similar. Cells from chronically streptozotocin-diabetic animals required higher concentrations of the amino acid for the same effect. These results are discussed in relation to previous discrepancies in the literature, and a unifying hypothesis for tryptophan-induced hypoglycaemia is proposed.

Further studies on the mechanism of phosphorylase activation in rabbit liver in response to splanchnic nerve stimulation

1. The mechanism of activation of liver phosphorylase after splanchnic nerve stimulation has been investigated in rabbits and compared with the effects of intraportal injections of noradrenaline.2. The increase in the activity of liver phosphorylase-a, the active form of this key glycogenolytic enzyme, in response to injections of noradrenaline was blocked by beta-adrenergic antagonists, but not by alpha-adrenergic antagonists, suggesting that the effect of noradrenaline is mediated mainly through beta-adrenoceptors in vivo. In contrast, the increase in phosphorylase activity in response to stimulation of the peripheral end of the splanchnic nerve was resistant to both alpha- and beta-adrenoceptor blockade.3. When diltiazem and verapamil, selective Ca(2+) antagonists that restrict calcium influx across the cell membrane, were infused intraportally, the phosphorylase response to splanchnic nerve stimulation was virtually abolished, while the response to noradrenaline was unaltered.4. Infusion of the prostaglandin-synthesis inhibitor indomethacin at a dose of 3.4 mug/min was found to block the activation of liver phosphorylase in response to stimulation of the splanchnic innervation.5. These results suggest that the mechanism whereby stimulation of the sympathetic innervation to the liver leads to activation of phosphorylase is not mediated by either alpha- or beta-adrenoceptors, but appears to depend upon prostaglandin formation and influx of Ca(2+) ions into the hepatocytes.

Prostaglandins and the alpha-cell

Experimental evidence has recently accumulated indicating that administration of some prostaglandins (PGs), particularly those of the E series, can evoke release of glucagon by the pancreatic alpha-cells. Virtually, all the in vitro studies (isolated perfused rat pancreas, isolated guinea-pig islets) agree that PGs can increase both basal and stimulated glucagon release. On the other hand, inhibition of PG synthesis with indomethacin blocks glucagon release. In rats and in normal humans, PGE1, but not PGA2 or PGF2a, causes a progressive rise of plasma glucagon levels. While in the rat this response seems independent of the adrenergic nervous system, in man the hyperglucagonemia induced by PGE1 is easily suppressed by propranolol, suggesting an interaction between the prostaglandin and the beta-receptors of the alpha-cell. Studies with inhibitors of PG synthesis in vivo have yielded conflicting results, depending on the particular experimental protocol used and on the type of inhibitor tested. In normal humans, it seems that acetylsalicylic acid (ASA) has no effect on glucagon response to arginine, tolbutamide and insulin-induced hypoglycemia. Conversely, a stimulator of PG synthesis, such as furosemide, increases glucagon response to an arginine pulse in man. In insulin-dependent diabetics, who present an exaggerated glucagon response to stimulants, ASA fails to alter glucagon response to arginine, but completely blunts the glucagon response to salbutamol, a weak beta-2 receptor agonist. In conclusion, these observations provide evidence in support to a role for PGs, at least PGE, in the contro of glucagon release.

Prostaglandin synthesis inhibitors reverse alpha-adrenergic inhibition of acute insulin response to glucose

Prostaglandin E (PGE) has several effects on glucose homoeostasis and insulin secretion. The same events can be induced by alpha-adrenergic stimulation, which is known to stimulate PGE synthesis. To evaluate the hypothesis that PGE may be one intracellular mediator for certain alpha-adrenergic events, we examined the effects of a known PG synthesis inhibitor Sodium salicylate (SS) (40 mg/min iv) on the alpha-adrenergic effects of epinephrine (Epi) at two doses (3 and 6 micrograms/min) in normal male subjects. The lower dose of epinephrine diminished the acute insulin response (AIR) after a 20-g intravenous glucose pulse (control, 463 +/- 149; epinephrine, 97 +/- 38% of basal insulin, mean +/- SE, n = 6, P < 0.02); SS markedly augmented the AIR during epinephrine towards control values (339 +/- 137%; P < 0.02). In 12 subjects, the higher dose of Epi abolished the AIR. When similar studies were performed during a SS infusion, the AIR was partially restored (96 /+- 27% of basal insulin, n = 12, P < 0.01). Similarly, partial reversal of this alpha-adrenergic effect of Epi was observed with indomethacin, another inhibitor of PG synthesis. At both doses of Epi, SS augmented the glucose disappearance rate (KG) after the glucose pulse (P < 0.001). Sodium salicylate also increased basal glucagon levels (P < 0.05). In contrast, SS did not affect the glycemic response, the suppression of basal insulin levels, or the hemodynamic responses induced by adrenergic stimulation. We conclude that two prostaglandin synthesis inhibitors partially reverse the alpha-adrenergic inhibition of the AIR to glucose caused by Epi, without affecting other adrenergic events. The data are compatible with a role for prostaglandins in alpha-adrenergic events selectively in the pancreatic islet.

Down-regulation in vivo of PGE receptors and adenylate cyclase stimulation

Down-regulation in vivo of liver plasma membrane receptors for prostaglandin E (PGE) was investigated in Sprague-Dawley rats using the 16,16-dimethyl analogue of PGE2, This analogue was used for subcutaneous injections because it escapes the rapid pulmonic degradation characteristic of PGE and was recognized well by liver plasma membrane receptors. Following treatment with the analogue, the concentration of PGE receptors was significantly decreased (-37%, P less than 0.001), but the binding affinity was not altered. There was no evidence for carry-through of the analogue into the isolated plasma membrane preparation. It was also demonstrated that GTP decreased the binding affinity between PGE and its receptor. Down-regulation of receptor concentration was associated with a significant decrease (P less than 0.001) in PGE1-stimulated plasma membrane adenylate cyclase activity. These data provide the novel demonstration that rat liver plasma membrane receptor for PGE can be down-regulated in vivo and that this causes a corresponding decrease in PGE-induced plasma membrane adenylate cyclase activity.

Some peculiarities of the glucoregulatory response to glucose infusion in the rat

The glucoregulatory response to the i.v. infusion of different doses of glucose and glucose plus insulin was studied in anesthetized rats by using the primed constant infusion of glucose-2-3H. Infusion of glucose at the rate of 10 mg/kg/min induced a rise of about 100% in blood glucose, while the hepatic release of glucose showed only a small and transient decrease. A proportional increase of glycemia and glucose utilization (Rd) was observed without any appreciable change in the metabolic clearance rate (MCR) of glucose; a two-fold increase in plasma insulin was recorded at all times. In the group of rats receiving 20 mg/kg/min of glucose, changes in the above parameters were slightly greater; MCR showed a moderate increment in spite of the six-fold rise of plasma insulin. Finally, the influsion of large doses of insulin together with 20 mg/kg/min of glucose resulted in complete cessation of glucose release by the liver and in a remarkable increase of Rd and MCR. These results suggest a poor adaptability of the glucoregulatory system of the rat in response to glucose infusion as compared to other mammalian species.

In vivo and in vitro experiments on relationships between PGA1 and glucose utilization

The in vivo and in vitro effects of PGA1 on glucose utilization were investigated in normal rats and in rats with alloxan-diabetes (50 mg/kg i.v. administered 48 hrs before experiment). The animals were divided into two groups. The first group -- which included both normal and diabetic animals -- was submitted to an IVGIT after a 12-h fast and during a sodium chloride infusion. In the second group -- which equally included normal and diabetic rats -- the same GTT was performed during a sodium chloride infusion in which PGA1 had been diluted, so that a dose of 0.5 g/kg/min was administered. This dose is devoid of any effect on cardiovascular activity. For in vitro experiments, glucose utilization was studied in the rat diaphragm incubated with insulin (200 muU/ml) and PGA1 (10 and 100 ng): results demonstrated that PGA1 enhances the insulin effect on glucose utilization and the enhancement is dose-dependent. The same results were observed also in the in vivo experiments: in normal rats PGA1 really improves glucose utilization without any interference with insulin secretion from B-cells. On the other hand, PGA1 has no effect on this utilization in diabetic rats. From our experiments it can therefore be concluded that PGA1 improves glucose utilization, showing a synergic action with the increased quantity of insulin secreted in response to a glucose load. No effect is noted when insulin secretion from B-cells is reduced or absent.