Increments in glucose, glucagon and insulin after morphine in rats, and naloxone blocking of this effect.

In awake rats adapted to experimental conditions and allowed food ad libitum, hyperglycemia was induced by the administration of morphine 10 mg/kg through indwelling catheters in the external jugular vein. High glucose values were measured at 5, 15 and 25 min. Glucagon values were high at 5 and 15 min, and again at basal level at 25 min. Insulin was increased after morphine both at 5, 15 and 25 min, whereas somatostatin levels did not change after morphine. When morphine was administered together with naloxone after an initial 10 min period of naloxone administration, there was no increment in glucose, insulin or somatostatin values; neither at 5, 15 or 25 min. There was a remarkable glucagon decrease after naloxone and morphine remaining from 5 to 25 min. Then, one of the possible mechanisms for the hyperglycemic response after morphine may be an opioid effect on pancreas, stimulating glucagon and thereby causing hepatic glucose output.     

Islet-cell metabolism during insulin release. Effects of glucose, citrate, octanoate, tolbutamide, glucagon and theophylline

1. Concentrations of glucose 6-phosphate and 6-phosphogluconate were studied in islets of Langerhans isolated from rat pancreas and incubated in the presence of various agents that induce insulin release. 2. In response to rising concentrations of extracellular glucose (2-10mm) there is a linear increase in the intracellular concentration of glucose 6-phosphate, though this is not the case for 6-phosphogluconate, the intracellular concentration of which only increases when the external glucose concentration exceeds 5mm. 3. Tolbutamide, octanoate and citrate, all of which promote insulin secretion from isolated islets, increase the intracellular concentrations of glucose 6-phosphate and 6-phosphogluconate. The results obtained in the presence of octanoate and citrate are compatible with an inhibitory effect of citrate on islet-cell phosphofructokinase. 4. Theophylline and glucagon when incubated with islets in vitro promote insulin release and cause a rise in 6-phosphogluconate concentration and not in that of glucose 6-phosphate. 5. It is suggested that the further metabolism of glucose 6-phosphate through a pathway other than glycolysis is essential for insulin release. One such pathway involves its oxidation to 6-phosphogluconate, which seems to be a necessary accompaniment of insulin secretion due to glucose. The possibility that agents other than glucose promote insulin release by enhancing the oxidation of glucose 6-phosphate through this pathway is discussed.     

Proteolytic degradation of insulin and glucagon.

The degradation of insulin and glucagon by a highly purified enzyme isolated from rat skeletal muscle was investigated. A sensitive assay for proteolytic degradation of insulin and glucagon using fluorescamine to detect an increase in primary amine groups was established. As measured by an increase in fluorescamine reactive materials, insulin was rapidly degraded by this highly purified enzyme without requiring initial disulfide cleavage. Associated with the increase in fluorescamine reactive materials was a decrease in immunoassayable insulinmglucagon wal also proteolytically degraded by this enzyme but a number of other peptides and proteins including proinsulin, and A and B chains of insulin were not degraded. Thus, we have demonstrated that insulin (and glucagon) can be proteolytically degraded by an enzyme isolated from an insulin sensitive tissue, skeletal muscle. Proteolytic degradation by this enzyme requires the intact insulin molecule rather than separate A and B chains.     

Intestinal motility responses to insulin and glucagon in streptozotocin diabetic rats

Myoelectrical and mechanical activities were chronically recorded by use of nichrome electrodes and miniaturized strain-gage transducers sutured on the serosa of the antrum, the duodenum, and the jejunum. In a first experiment (n = 6 rats) the early (0-6 h) and late (greater than 4 days) effects of streptozotocin (65 mg/kg i.v.) was recorded. In addition, the effect of insulin (1-5 IU/kg) and glucagon (6-200 micrograms/kg) administered intravenously were studied separately each in groups of seven normal and streptozotocin-induced diabetic-fed and fasted rats. The results indicated that within the 30 min following streptozotocin administration there was a significant stimulation of the duodenal and jejunal motility lasting 46 +/- 8 min. When diabetes was established as shown by the basal blood glucose level obtained in those rats (2.30 +/- 0.84 g/L), a progressive decrease of the frequency of the migrating myoelectric complex was observed along with a disorganization of the regular spiking activity phases without disturbing the basal electrical rhythm. Comparing with the basal level, a significant increase in the gastrointestinal motility indexes (MI) appeared both in fasted (p less than 0.01) and fed (p less than 0.05) normal animals, 13.1 +/- 1.6 min after an i.v. injection of 1 IU/kg insulin. Motor effects of glucagon were related to the dose. When used at 25 microgram/kg a disorganization of the spiking activity was observed with a stimulation of the contractile activity in the jejunum. At higher dosages, i.e., 100 micrograms/kg, it induced an immediate and significant decrease of motility at any level tested and lasting up to 20 +/- 7 min. The motility responses to both hormones were lower in diabetic than in normal rats.