Studies on glucagon secretion using isolated islets of Langerhans of the rat

Comparative Impact of Various Fasting Periods on the Welfare of Sprague-Dawley Rats With or Without Supplementation

In nonclinical toxicology studies, lab animals are fasted typically overnight, to reduce variability in some clinical pathology parameters. However, fasting adds undue stress, and this is particularly concerning in rodents given their fast metabolic rates. Furthermore, as rodents are nocturnal animals, an overnight fasting may cause a protracted negative metabolic state even when the fasting has technically ended, given their minimal activity and food consumption during the day. Therefore, to evaluate the impacts of different fasting durations (±DietGel supplementation) on rats' welfare, we assessed the traditional and ancillary clinical pathology parameters in Sprague-Dawley rats, along with body weight, organ weight, and histopathology. Although most endpoints were comparable between the different fasting durations (±DietGel supplementation), the long fasting times (≥8 hr) without DietGel supplementation caused significant decreases in body weight, liver weight, liver glycogen content, serum glucose, triglyceride, and creatinine concentrations-all findings suggestive of a negative energy balance that could impact animal welfare and consequently, data quality; while the short fasting time (4 hr) and DietGel supplementation were associated with higher triglycerides variability. Hence, we propose that short fasting time should be adequate for most toxicology studies in rats, and long fasting times should only be accommodated with scientific justification.

Stimulatory short-term effects of free fatty acids on glucagon secretion at low to normal glucose concentrations

While free fatty acids (FFA) are well known as insulin secretagogues, their effects on pancreatic alpha cells have been mostly neglected. In the present study we therefore systematically analyzed the glucagon metabolism of rat pancreatic islets under the influence of FFA. Primary islets were incubated in the presence or absence of 200 micromol/L albumin-complexed palmitate or oleate at 2.8 mmol/L versus 16.7 mmol/L glucose and glucagon secretion was monitored over 8 hours. In addition to these time-course experiments, dose dependency of palmitate-induced effects was tested by a 2-hour incubation with 50 to 300 micromol/L albumin-complexed palmitate at 2.8 mmol/L and 5.6 mmol/L glucose. Apart from glucagon secretion, intracellular immunoreactive glucagon and cellular preproglucagon-mRNA (PPG-mRNA) content were determined from the remaining cell lysates. FFA, especially palmitate, induced a significant and dose-dependent increase of glucagon secretion (in average 2-fold above control) during the first 120 minutes of incubation at low to normal glucose (2.8 and 5.6 mmol/L). There was no significant glucagonotropic effect of FFA at concomitant 16.7 mmol/L glucose. Intracellular glucagon as well as cellular PPG-mRNA content were found to be dose-dependently diminished by palmitate when compared with untreated controls at 5.6 mmol/L glucose. The present analysis therefore points to a new role for FFA as a nutritient secretagogue and a modulator of alpha-cellular glucagon metabolism.

The neuropeptide PACAP contributes to the glucagon response to insulin-induced hypoglycaemia in mice

The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide in the autonomic nerves innervating the pancreatic islets and previous studies have shown that it stimulates insulin and glucagon secretion. It is known that autonomic nerve activation contributes to the glucagon response to hypoglycaemia. In the present study, we evaluated whether PACAP is involved in this glucagon response by examining the glucagon response to insulin-induced hypoglycaemia in mice genetically deleted of the specific PACAP receptor, the PAC1 receptor. We found that insulin (1 U kg-1 ip) reduced circulating glucose to a hypoglycaemic level of approximately 2.5 mmol L-1 in PAC1R-/- mice and their wild-type counterparts with no difference between the groups. However, the glucagon response to this hypoglycaemia was markedly impaired in the PAC1R-/- mice. Thus, after 120 min, plasma glucagon was 437 +/- 79 ng L-1 in wild-type mice vs. only 140 +/- 36 ng L-1 in PAC1R-/- mice (P=0.004). In contrast, the glucagon response to intravenously administered arginine (0.25 g kg-1) was the same in the two groups of mice. We conclude that PACAP through activation of PAC1 receptors contribute to the glucagon response to insulin-induced hypoglycaemia. Therefore, the glucagon response to hypoglycaemia is dependent not only on the classical neurotransmitters but also on the neuropeptide PACAP.

Pancreatic sympathetic nerves contribute to increased glucagon secretion during severe hypoglycemia in dogs

To determine if pancreatic sympathetic nerves can contribute to increased glucagon secretion during hypoglycemia, plasma glucagon and pancreatic glucagon secretion in situ were measured before and during insulin-induced hypoglycemia in three groups of halothane-anesthetized dogs. All dogs were bilaterally vagotomized to eliminate the input from pancreatic parasympathetic nerves. One group of dogs received only vagotomy (VAGX). A second group was vagotomized and adrenalectomized (VAGX + ADX). A third group received vagotomy, adrenalectomy, plus surgical denervation of the pancreas (VAGX + ADX + NERVX) to prevent activation of pancreatic sympathetic nerves. In dogs with VAGX only, hypoglycemia increased plasma epinephrine (Epi), pancreatic norepinephrine (NE) output (+320 +/- 140 pg/min, P < 0.05), arterial plasma glucagon (+28 +/- 12 pg/ml, P < 0.01), and pancreatic glucagon output (+1,470 +/- 370 pg/min, P < 0.01). The addition of ADX eliminated the increase of Epi but did not increase pancreatic NE output (+370 +/- 190 pg/min, P < 0.025), arterial plasma glucagon (+20 +/- 5 pg/ml, P < 0.01), or pancreatic glucagon output (+810 +/- 200 pg/min, P < 0.01). In contrast, the addition of pancreatic denervation eliminated the increase of pancreatic NE output (-20 +/- 40 pg/min, P < 0.05 vs. VAGX), the arterial glucagon (+1 +/- 2 pg/ml, P < 0.01 vs. VAGX), and pancreatic glucagon output responses (+210 +/- 280 pg/min, P < 0.025 vs. VAGX) to hypoglycemia. Thus activation of pancreatic sympathetic nerves can contribute to the increased glucagon secretion during severe insulin-induced hypoglycemia in dogs.

Adrenoceptor antagonists, but not guanethidine, reduce glucopenia-induced glucagon secretion from perfused rat pancreas

This study was designed to investigate (1) whether norepinephrine is released in response to glucopenia in vitro, thereby stimulating glucagon secretion and, (2) the modulating effects of norepinephrine on insulin and glucagon secretion, using isolated perfused rat pancreas preparations. Simultaneous addition of the adrenergic receptor antagonists yohimbine, prazosin and propranolol, each at a concentration of 10-(5) mol/l, significantly potentiated glucose-stimulated insulin secretion (6.23 +/- 0.76 vs. 2.11 +/- 0.72 (control) nmol/min, P < 0.01), and suppressed glucopenia-induced glucagon secretion (0.59 +/- 0.10 vs. 1.34 + 0.18 (control) ng/min, P < 0.05). Also, 10-(5) mol/l yohimbine alone significantly potentiated glucose-stimulated insulin secretion (4.86 +/- 0.50 nmol/min, P < 0.05). The norepinephrine release inhibitor, guanethidine, significantly inhibited tyramine-induced secretion of both norepinephrine (7.86 +/- 0.77 vs. 49.7 +/- 2.3 nmol/min, P < 0.01) and glucagon (0.31 +/- 0.08 vs. 1.21 +/- 0.15 ng/min, P < 0.01), but exerted no effects on glucopenia-induced secretion of either norepinephrine or glucagon. We conclude that these results further support the concept that the neurotransmitter norepinephrine is released in response to glucopenia in vitro, and modulates insulin and glucagon secretion. Our data do not, however, provide evidence indicating that glucopenia-induced glucagon secretion is mainly mediated by activation of sympathetic nerve terminals around the alpha-cells in the isolated perfused rat pancreas.

Redundant parasympathetic and sympathoadrenal mediation of increased glucagon secretion during insulin-induced hypoglycemia in conscious rats

Both the parasympathetic and sympathoadrenal inputs to the pancreas can stimulate glucagon release and are activated during hypoglycemia. However, blockade of only one branch of the autonomic nervous system may not reduce hypoglycemia-induced glucagon secretion, because the unblocked neural input is sufficient to mediate the glucagon response, ie, the neural inputs are redundant. Therefore, to determine if parasympathetic and sympathoadrenal activation redundantly mediate increased glucagon secretion during hypoglycemia, insulin was administered to conscious rats pretreated with a muscarinic antagonist (methylatropine, n = 7), combined alpha- and beta-adrenergic receptor blockade (tolazoline + propranolol, n = 5) or adrenergic blockade + methylatropine (n = 7). Insulin administration produced similar hypoglycemia in control and antagonist-treated rats (25 to 32 mg/dL). In control rats (n = 9), plasma immunoreactive glucagon (IRG) increased from a baseline level of 125 +/- 11 to 1,102 +/- 102 pg/mL during hypoglycemia (delta IRG = +977 +/- 98 pg/mL, P < .0005). The plasma IRG response was not significantly altered either by methylatropine (delta IRG = +677 +/- 141 pg/mL) or by adrenergic blockade (delta IRG = +1,374 +/- 314 pg/mL). However, the IRG response to hypoglycemia was reduced to 25% of the control value by the combination of adrenergic blockade + methylatropine (delta IRG = +250 +/- 83 pg/mL, P < .01 v control rats). These results suggest that the plasma glucagon response to hypoglycemia in conscious rats is predominantly the result of autonomic neural activation, and is redundantly mediated by the parasympathetic and sympathoadrenal divisions of the autonomic nervous system.

Differences in energy metabolism between normal weight 'large-eating' and 'small-eating' women

Nine 'large-eating' (approximately 12 MJ/d) and nine 'small-eating' (approximately 5.3 MJ/d) women were selected from the population on the basis of diet and activity diaries. At rest and in the post-absorptive state the rate of oxygen consumption (VO2)/kg fat-free mass (FFM) and rate of carbon dioxide production (VCO2)/kg FFM were 9-17% higher (P < 0.05) in the 'large-eaters' than in the 'small-eaters'. As energy expenditure was increased by walking at 2.4, 3.9 and 5.4 km/h the differences between the two experimental groups for both VO2/kg FFM and VCO2/kg FFM were decreased to negligible values, but energy expended on a body-weight basis (MJ/kg per min) remained significantly higher (5-10%) in 'large-eaters'. Oral temperature was also consistently higher (up to 0.5 degrees) in this group both at rest and during sitting, standing and walking activities. Although the average thermic effect of a standardized liquid meal tended to be higher (27%; not significant) in the 'small-eaters', the other results demonstrate that the 'large-eating' females had a markedly higher rate of energy expenditure at rest and during light physical activities.

Adrenergic modulation of pancreatic glucagon and insulin secretion in goats

Five goats were used to investigate adrenergic influences on the secretion of both glucagon and insulin. The secretion of glucagon was augmented via alpha-adrenergic stimulation. The secretion of insulin was enhanced by stimulation of beta-adrenergic receptors and inhibited by alpha-adrenergic stimulation.

Studies on the mechanisms of somatostatin release after insulin induced hypoglycaemia in man

Insulin-induced hypoglycaemia, which stimulates gastric acid secretion, is associated with an increase in circulating somatostatin levels in man. In order to assess the mechanisms involved in this rise, six normal volunteers connected to a Biostator for continuous glucose monitoring were studied, on three separate occasions. On each occasion after basal blood sampling, 0.15 i.u./kg body weight of insulin was administered i.v. and further samples were obtained intermittently over 150 min. On one occasion, dextrose was infused by the Biostator to prevent hypoglycaemia, while on the other two, a constant infusion of either normal saline or the specific H2 antagonist cimetidine was administered. Insulin plus dextrose caused no significant changes in circulating somatostatin levels, whereas insulin plus saline was associated with a marked, sustained and significant rise in all subjects; insulin plus cimetidine also produced a rise but it was delayed; the area under the curve was significantly (P less than 0.05) greater with insulin plus saline than with insulin plus cimetidine. These results show that in man insulin itself does not stimulate somatostatin secretion directly, but indirectly via hypoglycaemia. Further, the inhibition of gastric acid secretion with cimetidine reduces somatostatin release during insulin-induced hypoglycaemia. This suggests that gastric acid may mediate somatostatin secretion associated with insulin-induced hypoglycaemia.

Effect of insulin on glucagon secretion mediated via glucose metabolism of pancreatic A cells in ducks

A possible action of insulin via glucose metabolism on the pancreatic A cell response to glucose, was studied in ducks. 2-Deoxyglucose, a nonmetabolizable analogue of glucose was used. In normal ducks, the hyperglycaemia induced by 2-deoxyglucose (IV: 0.5 g/kg) resulted in hyperglucagonaemia, while the same degree of hyperglycaemia, induced by glucose infusion (IV injection 25 mg/kg, and infusion 5 mg/kg/min) immediately suppressed glucagon secretion. In diabetic ducks, two days after subtotal pancreatectomy, glucose responsiveness of the A cell was abolished, but could be restored by insulin treatment before (IM 0.2 U/kg insulin + 8 micrograms/kg glucagon every 6 h) and during (IV 3.6 mU/kg + infusion 0.9 mU/kg/min) the glucose test (IV: 0.5 g/kg). The normal response of the A cell to glucose was not observed in diabetic insulin-treated ducks after the administration of 2-deoxyglucose (IV: 0.5 g/kg). These data suggest an inhibitory effect of the metabolism of glucose on the release of glucagon. In addition, the action of insulin on the A cell may be mediated by its effect on glucose metabolism within the A cell.