Alpha 2-adrenergic modulation of glucagon and insulin secretions in sheep

The mechanism underlying the central glucagon-induced hyperglycemia and anorexia in chicks

We investigated the mechanism underlying central glucagon-induced hyperglycemia and anorexia in chicks. Male 8-day-old chicks (Gallus gallus) were used in all experiments. Intracerebroventricular administration of glucagon in chicks induced hyperglycemia and anorexia from 30 min after administration. However, the plasma insulin level did not increase until 90 min after glucagon administration, suggesting that glucose-stimulated insulin secretion from pancreatic beta cells may be suppressed by central glucagon. The plasma corticosterone concentration significantly increased from 30 min to 120 min after administration, suggesting that central glucagon activates the hypothalamic pituitary adrenal (HPA) axis in chicks. However, central administration of corticotropin-releasing factor (CRF), which activates the HPA axis in chicken hypothalamus, significantly reduced not only food intake but also plasma glucose concentration, suggesting that CRF and the activation of the HPA axis are related to the glucagon-induced anorexia but not hyperglycemia in chicks. Phentolamine, an α-adrenergic receptor antagonist, significantly attenuated the glucagon-induced hyperglycemia, suggesting that glucagon induced hyperglycemia at least partly via α-adrenergic neural pathway. Co-administration of phentolamine and α-helical CRF, a CRF receptor antagonist, significantly attenuated glucagon-induced hyperglycemia and anorexia. It is therefore likely that central administration of glucagon suppresses food intake at least partly via CRF-induced anorexigenic pathway in chicks.

Effects of central administration of glucagon on feed intake and endocrine responses in sheep

This study was conducted to investigate effects of glucagon intracerebroventricularly administered on feed intake and endocrine changes in sheep. Four male sheep (48-55 kg BW) were used. The animals were acclimatized to be fed alfalfa hay cubes at 12.00 hour. Human glucagon (40 and 80 microg/0.5 mL) was injected into the lateral ventricle at 12.00 hour. Blood samples were taken every 10 min from 30 min before to 180 min after the glucagon injection. Soon after the injection, the animals were given alfalfa hay cubes, and the amounts of the feed eaten within 2 h were measured. Feed intakes were significantly (P < 0.05) suppressed by 80 microg of glucagon. Plasma glucose levels in control animals were gradually decreased after the feeding, whilst those in glucagon-treated animals were temporarily elevated just after the feeding and then kept higher than control levels. Plasma insulin was abruptly elevated after the feeding and was maintained at higher levels than before the feeding in all treatments. Plasma NEFA concentrations were decreased after the feeding in all treatments. A tendency of increase in plasma cortisol levels occurred in glucagon-injected animals. The present study provides the first evidence that glucagon directly acts on the brain, then inhibiting feeding behavior and inducing endocrine responses in ruminants.

Partial neuroprotection with low-dose infusion of the alpha2-adrenergic receptor agonist clonidine after severe hypoxia in preterm fetal sheep

We have previously shown that brief alpha(2)-adrenergic receptor blockade increased neuronal injury after severe hypoxia in preterm fetal sheep. We now examine whether infusion of an alpha(2)-adrenergic receptor agonist, clonidine, is neuroprotective. Preterm fetal sheep (70% gestation) received either saline-vehicle or clonidine at either 10 microg/kg/h (low-dose) or 100 microg/kg/h (high-dose) from 15 min until 4 h after 25 min of umbilical cord occlusion. Both low- and high-dose clonidine infusions after sham-occlusion were associated with transient EEG suppression but no neuronal loss. Low-dose but not high-dose clonidine infusions after umbilical cord occlusion were associated with a significant overall increase in numbers of surviving neurons after three days' recovery. High-dose clonidine was associated with transient hyperglycemia and increased numbers of delayed electrographic seizures. These results provide further evidence that alpha(2)-adrenergic receptor activation shortly after perinatal hypoxia-ischemia can promote neural recovery, but highlight the complex dose-response of exogenous therapy.

Reduced blood glucose levels, increased insulin levels and improved glucose tolerance in alpha2A-adrenoceptor knockout mice

Alpha(2)-Adrenoceptors regulate insulin secretion and sympathetic output. In the present study, alpha(2A)-adrenoceptor knockout (alpha(2A)-KO) mice and their C57BL/6J wild-type (WT) controls were used to assess the glucoregulatory role of the alpha(2A)-adrenoceptor subtype in vivo. Fasting and glucose-stimulated blood glucose and plasma insulin levels were determined with or without (+/-)-propranolol (5 mg/kg) or atropine (10 mg/kg) pre-treatment. Intraperitoneal glucose (1 g/kg) and insulin (0.5 and 1.0 IU/kg) tolerance tests were performed. Fasting plasma glucagon and corticosterone levels were measured. Blood glucose levels (mean+/-S.E.M.) were lower in alpha(2A)-KO males (7.2+/-0.6 mM) and females (7.2+/-0.2 mM) than in WT males (9.8+/-0.3 mM) and females (9.1+/-0.3 mM). Plasma insulin levels were higher in alpha(2A)-KO males (2.2+/-0.5 microg/l) and females (1.7+/-0.3 microg/l) than in WT males (0.7+/-0.1 microg/l) and females (0.8+/-0.2 microg/l). These differences remained after pharmacological beta-adrenoceptor and muscarinic acetylcholine receptor inhibition. In spite of a tendency for slightly decreased insulin sensitivity in alpha(2A)-KO mice, glucose tolerance in alpha(2A)-KO mice was significantly better than in WT mice. However, glucose-stimulated insulin secretion was not increased in alpha(2A)-KO mice compared to WT controls. Plasma glucagon levels, but not corticosterone levels, were elevated in alpha(2A)-KO mice. These results suggest that lack of inhibitory pancreatic beta-cell alpha(2A)-adrenoceptor function results in hyperinsulinaemia, reduced blood glucose levels and improved glucose tolerance in alpha(2A)-KO mice, and demonstrate a key role for the alpha(2A)-adrenoceptor in adrenergic regulation of blood glucose and insulin homeostasis.

Postoperative alpha 2-adrenergic stimulation attenuates protein catabolism

The metabolic effects of continuous intravenous (IV) application of the alpha 2 agonist clonidine were evaluated by assessment of nitrogen economy and postaggression endocrine patterns. Twenty-four patients undergoing abdominothoracic esophageal cancer resection were studied. Thirteen of these patients with alcohol abuse were treated postoperatively with IV clonidine for prevention of alcohol withdrawal syndrome. Eleven patients who were not treated with clonidine served as controls. All patients were treated in a standardized manner in regard to surgical technique, balanced anesthesia, and postoperative intensive care treatment, including thoracic epidural analgesia with bupivacaine and fentanyl. Isonitrogenous and isocaloric nutrition was comparable in all patients. A significantly improved cumulated 6-day nitrogen balance was found in clonidine-treated patients (-1.5 +/- 4.9 g nitrogen) compared to the control group (-17.6 +/- 4.2 g nitrogen) (P < 0.05). The main reason for improved nitrogen economy may be clonidine-induced growth hormone (GH) release. The pattern of insulin-like growth factor I (IGF-I) and insulin-like growth factor binding protein 3 (IGFBP-3) concentrations could support this hypothesis.

Effects of alpha 2- and beta-adrenergic agonism on glucagon secretion from perfused pancreata of normal and streptozocin-induced diabetic rats

Insulin secretion is known to be inhibited by alpha 2-adrenergic agonism and stimulated by beta-adrenergic agonism in both experimental animals and humans. In contrast, adrenergic regulation of glucagon secretion remains controversial. This study was designed to determine the effects of alpha 2- and beta-adrenergic agonism on islet alpha cells, using isolated perfused pancreata of normal and streptozocin-induced diabetic (STZ-D) rats. The alpha 2-adrenoceptor agonist clonidine at a concentration of 10(-7) mol/L significantly stimulated glucagon secretion as compared with basal levels in both normal (1,286 +/- 90 v 417 +/- 53 ng/L, P < .01) and STZ-D rats (551 +/- 86 v 130 +/- 19 ng/L, P < .01). Also, the beta-adrenoceptor agonist isoproterenol at a concentration of 10(-7) mol/L significantly stimulated glucagon secretion as compared with basal levels in both normal (751 +/- 130 v 347 +/- 41 ng/L, P < .05) and STZ-D rats (182 +/- 22 v 92 +/- 20 ng/L, P < .01). Furthermore, these alpha 2- and beta-agonistic effects were almost completely inhibited in the presence of the alpha 2-adrenoceptor antagonist yohimbine and the beta-adrenoceptor antagonist propranolol at a concentration of 10(-6) mol/L, respectively. Insulin secretion was markedly reduced in STZ-D rats. These results suggest that even in a severely diabetic state, not only beta- but also also alpha 2-adrenergic agonism stimulates glucagon secretion from rat pancreatic alpha cells.

Mechanism of insulin secretion by midaglizole

Midaglizole was introduced as a hypoglycemic agent, but its insulin releasing mechanism remains unknown. In the present study, the effect of midaglizole upon the B cell function of the pancreas was investigated, using an in situ local circulation of the canine pancreas. The graded doses of midaglizole (0.2, 1.0 and 2.0 mg/kg) revealed a dose-related response of plasma insulin. The administration of yohimbine, a classical alpha 2-antagonist (1.11 mg/kg) revealed a similar increase in plasma insulin to that with midaglizole of equimolar amount. During the clonidine infusion midaglizole did not elicit any significant rise in plasma insulin, whereas yohimbine increased plasma insulin significantly. During glucagon infusion plasma insulin increased following midaglizole infusion but not by yohimbine. The simultaneous administration of diazoxide (K-channel opener) suppressed the midaglizole-induced insulin secretion. These results obtained in the present experiments revealed a different mechanism of insulin releasing action of midaglizole from that of yohimbine. Furthermore, the finding with diazoxide administration suggests that midaglizole stimulates insulin release through an interaction of K(+)-channel of the pancreatic B-cell.