Possible role of endogenous prostaglandins in glucagon secretion by isolated guinea-pig islets

Arachidonic acid metabolites and alpha 2-adrenoceptor-mediated glucagon secretion in rats

Effects of phospholipase A2 inhibitor, cyclooxygenase inhibitor and lipoxygenase inhibitor on glucagon secretion induced by the alpha 2-adrenergic agonist clonidine were studied in the isolated perfused rat pancreas. The phospholipase A2 inhibitor mepacrine at 25 and 50 mumol/l significantly inhibited glucagon secretion induced by 0.1 mumol/l clonidine (P less than 0.01, respectively), whereas 5 mumol/l mepacrine did not affect clonidine-induced glucagon secretion. Also, both 100 mumol/l acetylsalicylic acid (cyclooxygenase inhibitor) and 100 mumol/l caffeic acid (lipoxygenase inhibitor) significantly inhibited clonidine-induced glucagon secretion (P less than 0.01, respectively), whereas neither 10 mumol/l acetylsalicylic acid nor 10 mumol/l caffeic acid affected clonidine-induced glucagon secretion. None of the drugs at the tested concentrations affected insulin secretion at a glucose concentration of 5.5 mmol/l. These results suggest that not only cyclooxygenase metabolites but also lipoxygenase metabolites are involved in the stimulation of glucagon secretion mediated through the alpha 2-adrenergic receptors in perfused rat pancreas.

Effects of chronic sodium salicylate feeding on the impaired glucagon and epinephrine responses to insulin-induced hypoglycaemia in streptozotocin diabetic rats

The potential role of endogenous prostaglandins in glucagon and epinephrine responses to insulin-induced hypoglycaemia was studied in streptozotocin-diabetic and age-matched control adult male rats. Rats made diabetic with a single intravenous injection of streptozotocin (65 mg/kg) developed impaired glucagon and epinephrine responses to insulin-induced hypoglycaemia by 80-100 days. Plasma glucagon levels in response to insulin-induced hypoglycaemia in streptozotocin-diabetic rats (167 +/- 67 pg/ml) were significantly lower (p less than 0.01) than those in control rats (929 +/- 272 pg/ml). Similarly, plasma epinephrine levels in hypoglycaemic state in streptozotocin-diabetic rats (11 +/- 8 pmol/ml) were also significantly lower (p less than 0.01) compared to control rats (37 +/- 13 pmol/ml). Streptozotocin-diabetic rats provided with sodium salicylate (25 mg/100 ml) in their drinking water from day one of diabetes exhibited prevention of the blunted glucagon and epinephrine responses to insulin-induced hypoglycaemia. About 80-100 days after the chronic sodium salicylate treatment in streptozotocin-diabetic rats, both plasma glucagon levels (1080 +/- 169 pg/ml) and plasma epinephrine levels (39 +/- 8 pmol/ml) were essentially identical to plasma glucagon levels (1074 +/- 134 pg/ml) and plasma epinephrine levels (37 +/- 5 pmol/ml) in control rats in hypoglycaemic state. These animals also exhibited an improvement in the diabetic state in that they had less severe hyperglycaemia and lack of weight gain. These results suggest that the blunted glucagon and epinephrine responses to insulin-induced hypoglycaemia may be related to altered prostaglandin levels in streptozotocin-diabetes.

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.

Role of arachidonate lipoxygenase and cyclooxygenase products in insulin and glucagon secretion from rat pancreatic islets

Rat pancreatic islets incubated in nutrient medium were used to study the role of endogenous arachidonic acid metabolism in pancreatic hormone secretion. Both glucose and fetal calf serum stimulated radioimmunoassayable PGE2 production and insulin secretion from islets. These effects were abolished by the phospholipase inhibitor p-bromophenacyl bromide or by concurrent inhibition of cyclooxygenase and lipoxygenase by flurbiprofen plus nordihydroguaiaretic acid (NDGA), respectively. Bromophenacyl bromide also inhibited glucagon secretion. When used alone, flurbiprofen caused a significant enhancement of glucose-induced insulin secretion that was attributed to reactive stimulation of lipoxygenase-product formation rather than to selective cyclooxygenase inhibition. NDGA given alone in the presence of stimulatory concentrations of glucose suppressed the normal eight-fold rise in insulin secretion, but caused a marked enhancement in glucagon secretion that could be overcome by simultaneous inclusion of flurbiprofen. We concluded that: (1) Increased metabolism of arachidonic acid in pancreatic islets amplifies the secretion of insulin and glucagon. (2) The lipoxygenase as well as the cyclooxygenase pathways of arachidonate metabolism participate in the amplification of insulin secretion. (3) The observations made in this study are inconclusive with respect to the involvement of the lipoxygenase and cyclooxygenase pathways in glucagon secretion; an inhibitory role for lipoxygenase pathway products is suggested.

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.

Maturity-onset diabetes mellitus--toward a physiological appropriate management

"Eumetabolic" therapies of diabetes are defined as those which promote and potentiate a normal physiological pattern of insulin activity--in contrast to the less subtle effects of exogenous insulin or sulfonylureas. Effective eumetabolic therapy would provide better metabolic control and reduce the risk of secondary complications, would be more convenient to administer, and would be especially appropriate for preventive use in the early stages of diabetes. Possible components of a eumetabolic therapy include: aspirin, as a potentiator of glucose-stimulated insulin secretion; GTF, to directly enhance the efficacy of insulin; weight loss, exercise, and fasting, to help reduce tissue resistance to insulin; mitochondrial "metavitamins", to optimize the oxidative disposal of excess substrate; a high-fiber, low-fat diet, which appears superior to traditional diabetic diets as a promoter of glucose tolerance. Following a prolonged fast, obese diabetics show substantial improvement in most parameters of insulin function--an effect which is to some degree independent of weight loss; long-term remission of diabetes may be possible if the benefits of therapeutic fasting are conserved by appropriate eumetabolic measures.

Effect of indomethacin on the metabolic and hormonal response to a standardized breakfast in normal subjects

We have investigated the influence of a single oral administration of indomethacin on blood glucose, plasma free fatty acids (FFA), alpha-amino-nitrogen, insulin and glucagon concentrations in young healthy subjects. Two groups of 6 subjects were studied, the first received a standardized 500 kcal mixed meal without any previous drug administration (controls) whereas the second group received 50 mg indomethacin 2 h before ingesting an identical meal. Plasma indomethacin concentration reached its maximum (2.36 +/- 0.36 micro g/ml) 15 min after administration and declined to 0.45 +/- 0.04 micro g/ml after 2 h. Indomethacin ingestion was followed by a significant increase in blood glucose and plasma FFA reaching their maximum value at 45 min and returning to basal levels at 120 min. No simultaneous changes in plasma alpha-amino-nitrogen, insulin or glucagon levels were detected during this period. The meal was followed by a rise in blood glucose and plasma insulin as well as by a decrease in plasma FFA concentration. No significant differences were detected between the controls and the subjects receiving indomethacin. In controls, the meal was followed by a rise in plasma alpha-amino-nitrogen and a modest although significant increase in glucagon levels. In indomethacin-treated subjects, the increment of alpha-amino-nitrogen was less marked and the increase in plasma glucagon was not observed. Thus, indomethacin by itself can exert several metabolic effects; however, it does not deteriorate the blood glucose or insulin profile after a regular meal. The present work is the first to demonstrate that an inhibitor of prostaglandin synthesis inhibits the plasma glucagon rise occurring after a physiological stimulus such as a normal meal. On the basis of previous in vitro experiments, we suggest that this effect results from an inhibition of glucagon secretion by the PG synthesis inhibitor.

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.

Stimulation of gastric-glucagon release by prostaglandin E1

The canine gastric fundus has recently been demonstrated to contain and release a material identical by all criteria to pancreatic glucagon (1, 2). Since exogenous prostaglandins have been reported to stimulate pancreatic glucagon release (3) and endogenous prostaglandins are suspected to play a role in the control of glucagon secretion (4), we were interested in seeing if prostaglandin E1 (PGE1) would affect gastric-glucagon release.