The role of the parasympathetic system in the control of insulin release in the conscious calf

Metabolic and Energy Imbalance in Dysglycemia-Based Chronic Disease

Metabolic flexibility is the ability to efficiently adapt metabolism based on nutrient availability and requirement that is essential to maintain homeostasis in times of either caloric excess or restriction and during the energy-demanding state. This regulation is orchestrated in multiple organ systems by the alliance of numerous metabolic pathways under the master control of the insulin-glucagon-sympathetic neuro-endocrine axis. This, in turn, regulates key metabolic enzymes and transcription factors, many of which interact closely with and culminate in the mitochondrial energy generation machinery. Metabolic flexibility is compromised due to the continuous mismatch between availability and intake of calorie-dense foods and reduced metabolic demand due to sedentary lifestyle and age-related metabolic slowdown. The resultant nutrient overload leads to mitochondrial trafficking of substrates manifesting as mitochondrial dysfunction characterized by ineffective substrate switching and incomplete substrate utilization. At the systemic level, the manifestation of metabolic inflexibility comprises reduced skeletal muscle glucose disposal rate, impaired suppression of hepatic gluconeogenesis and adipose tissue lipolysis manifesting as insulin resistance. This is compounded by impaired β-cell function and progressively reduced β-cell mass. A consequence of insulin resistance is the upregulation of the mitogen-activated protein kinase pathway leading to a pro-hypertensive, atherogenic, and thrombogenic environment. This is further aggravated by oxidative stress, advanced glycation end products, and inflammation, which potentiates the risk of micro- and macro-vascular complications. This review aims to elucidate underlying mechanisms mediating the onset of metabolic inflexibility operating at the main target organs and to understand the progression of metabolic diseases. This could potentially translate into a pharmacological tool that can manage multiple interlinked conditions of dysglycemia, hypertension, and dyslipidemia by restoring metabolic flexibility. We discuss the breadth and depth of metabolic flexibility and its impact on health and disease.

Cholinergic innervation of the pancreas in the sheep

Antibodies raised against vesicular acetylcholine transporter (VAChT) were applied to study the cholinergic innervation pattern of the pancreas of the sheep. To determine whether the cholinergic pancreatic neuronal elements contain tyrosine hydroxylase (TH), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP) or substance P (SP) double immunocytochemistry was used. A moderate number of VAChT-immunoreactive (IR) nerve terminals were distributed between the acini, whereas only single cholinergic nerve fibres innervated the interlobular connective tissue. VAChT-positive nerve fibres supplying the endocrine pancreas were found only occasionally. The pancreatic blood vessels and ducts system were devoid of VAChT-containing nerve endings. All intrapancreatic neurons studied showed immunoreactivity to VAChT, but intrapancreatic ganglia were not innervated with cholinergic nerve fibres. The colocalization of VAChT and TH or VAChT and SP was detected in distinct populations of nerve fibres localized amongst the acini, but not within the islet nor in the connective tissue. Single VAChT-IR nerve terminals co-expressing NPY were distributed around the acini, islets as well as in the connective tissue septa. A moderate number of VAChT-IR/VIP-IR nerve endings were located in the exocrine pancreas, whereas the islets and connective tissue were innervated with VAChT/VIP-containing nerve fibres only occasionally. In the vast majority of VAChT-positive intrapancreatic perikarya the presence of TH was additionally found. A moderate number of VAChT-IR intrapancreatic perikarya co-expressed NPY, SP or VIP. The results of the present study demonstrate species-dependent cholinergic innervation pattern of the pancreas of the sheep. The co-localization of VAChT with the neuropeptides suggests the existence of functional interactions influencing the ovine pancreas (mainly exocrine) activity.

Inhibition of insulin secretion

Reports of the effects of amylin and amylin agonists on insulin secretion have varied widely. Some confusion can be attributed to the use of human amylin, which has been shown to readily fall out of solution resulting in low estimates of bioactivity. Some confusion can be resolved by assessing the probability that this had happened. The view taken here, supported by authors using reliable and well-characterized ligands (representing the preponderance of recent studies), is that exogenously administered amylin agonists inhibit insulin secretion, at least partly via activation of an amylin-like receptor linked to Gi-mediated inhibition of cAMP in islets. There may additionally be autonomic extrapancreatic effects of amylin on insulin secretion that derive from its action at the area postrema. Studies with amylin receptor antagonists, including human studies, indicate that endogenously secreted amylin may physiologically inhibit beta-cell secretion (insulin and amylin) via feedback inhibition that is characteristic of many other hormones. Part of this inhibition may be local (paracrine), as indicated by the amylin sensitivity of isolated preparations and the fact that the concentration of secreted products in the islet interstitium can be over 100-fold higher than in the circulation (Bendayan, 1993).

Involvement of nitric oxide in neuroglycopenia-induced insulin and glucagon secretion in the mouse

Neuroglycopenia induced by administration of 2-deoxy-D-glucose is known to stimulate the secretion of both insulin and glucagon in mice by a mechanism that is dependent on neural activity. In the present study, we examined whether the neurotransmitter nitric oxide (NO) is involved in this process. Therefore, 2-deoxy-D-glucose (500 mg/kg) was injected intravenously alone or together with the inhibitor of NO synthase, NG-nitro-L-arginine methyl ester (50 mg/kg) to conscious mice. It was found that NG-nitro-L-arginine methyl ester inhibited the increased plasma levels of both insulin (by 26%; P = 0.039) and glucagon (by 45%; P < 0.001) at 10 min after injection of 2-deoxy-D-glucose. Similarly, the NO synthase inhibitor, NG-nitro-L-arginine, which is devoid of the anticholinergic property of NG-nitro-L-arginine methyl ester, inhibited the responses of both insulin (by 53%; P = 0.026) and glucagon (by 57%; P = 0.003) to 2-deoxy-D-glucose. In contrast, the stereoisomer of NG-nitro-L-arginine methyl ester, NG-nitro-D-arginine methyl ester, which is devoid of NO synthase inhibitory activity, was without effect on 2-deoxy-D-glucose-induced insulin and glucagon secretion. Plasma levels of adrenaline and noradrenaline after administration of 2-deoxy-D-glucose were also reduced by NG-nitro-L-arginine methyl ester. In contrast, the insulin and glucagon secretory responses to intravenous injection of arginine (250 mg/kg), glucose (500 mg/kg) or the cholinergic agonist, carbachol (30 micrograms/kg), were not influenced by NG-nitro-L-arginine methyl ester, NG-nitro-D-arginine methyl ester or NG-nitro-L-arginine.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide-related pancreatic endocrine responses to hyperglycaemia in the conscious calf

Mean plasma insulin concentration was reduced and mean plasma glucose concentration increased following the administration of N-nitro-L-arginine methyl ester (L-NAME; 100 mumol kg-1 i.a.) in conscious calves given continuous infusions of exogenous glucose (30-60 mumol min-1 kg-1 i.v.). It is concluded that the rise in plasma insulin concentration which occurs in these animals in response to glucose is mediated, at least in part, by a nitric oxide-related factor (NOx).

The role of the autonomic nervous system in mediating pancreatic endocrine responses to arginine in the calf

The release of insulin which occurred in response to arginine, in the conscious calf, differed from that which occurs in response to glucose in that it was not significantly affected by either adrenergic or muscarinic blocking agents. Release of pancreatic glucagon was reduced by pretreatment with phentolamine.

Pancreatic neuroendocrine responses to butyrate in conscious sheep

The role of the autonomic innervation in the control of pancreatic endocrine responses to I.V. infusions of butyrate (0.02 mmol kg-1 min-1 for 10 min) has been investigated in conscious 4-6 month old weaned lambs. Intravenous butyrate produced a small rise in mean arterial plasma pancreatic glucagon concentration which was unlikely to have had any physiological effect and produced no consistent or statistically significant changes in mean plasma pancreatic polypeptide concentration in any of the groups studied. In contrast, butyrate produced an abrupt and substantial rise in mean plasma insulin concentration, which rose to a peak incremental value of about 300 pmol l-1 in normal control lambs. This response was unaffected by pre-treatment with either phentolamine or propranolol alone, but was significantly reduced by simultaneous administration of these drugs, as it was either by pre-treatment with atropine or prior section of the splanchnic nerves. The rise in mean plasma insulin concentration was inhibited most effectively by combined treatment with propranolol, phentolamine and atropine, which was significantly more effective than administration of atropine to lambs with cut splanchnic nerves (P less than 0.01). It is concluded that both divisions of the autonomic nervous system are likely to be involved in mediating the release of insulin from the pancreas, in response to butyrate, in this species under these experimental conditions and also in the resting state.

The role of the sympathetic system in the control of insulin release in response to hyperglycaemia in conscious calves

The effects of the adrenoceptor blocking agents, phentolamine and propranolol, on the release of pancreatic glucagon and insulin in response to exogenous glucose have been investigated in conscious 2--6-week-old calves. There were no significant differences between the changes in mean plasma glucagon or insulin concentration, which occurred in response to hyperglycaemia, in calves pre-treated with both propranolol and phentolamine, phentolamine alone and normal calves. However, the rise in mean plasma insulin concentration was effectively abolished by pre-treatment with propranolol alone and there was a small but significant fall in mean plasma glucagon concentration which was not observed in any of the other groups. The results are discussed in relation to previous studies of the role of the autonomic innervation to the endocrine pancreas in the control of insulin release during hyperglycaemia in conscious calves and weaned lambs.

Effects of certain metabolites on pancreatic endocrine responses to stimulation of the vagus nerves in conscious calves

The effects of exogenous glucose (0.05 mmol/kg . min) and vamin (0.02 mmol/kg . min) on the pancreatic endocrine responses to stimulation of the peripheral ends of the vagus nerves have been investigated in conscious 3-6-week-old calves with cut splanchnic nerves. Exogenous glucose potentiated both the basal release of insulin and that which occurred in response to vagal stimulation, while inhibiting both the basal release of glucagon and that during vagal stimulation. Vamin significantly inhibited basal release of insulin but not that which occurred during vagal stimulation although it significantly inhibited vagal release of glucagon. The inhibitory effect of exogenous glucose on the basal and vagally stimulated release of pancreatic glucagon were both significantly reduced in the presence of vamin. Neither glucose nor mixed amino acids were found to affect the release of pancreatic polypeptide either at rest or during nerve stimulation. It is concluded that the effects of vagal activity on the alpha- and beta-cells of the islets of Langerhans are normally modified by the existing concentration of both glucose and amino acids in these animals.

Characteristics of the neuroendocrine responses to stimulation of the splanchnic nerves in bursts in the conscious calf

Neuroendocrine responses to splanchnic nerve stimulation in bursts (at 40 Hz for 1 s at 10 s intervals for 10 min) have been investigated in conscious adrenalectomized calves, 3-6 weeks after birth, in the presence of various pharmacological blocking agents. Preganglionic blockade with hexamethonium abolished all the neuroendocrine responses that were monitored. Pre-treatment with phentolamine significantly reduced, but failed to eliminate, the release of both pancreatic glucagon and pancreatic polypeptide. In the presence of phentolamine splanchnic nerve stimulation produced a massive rise in arterial plasma insulin concentration. None of these pancreatic neuroendocrine responses was significantly affected by additional pre-treatment with propranolol. The rise in mean plasma insulin concentration which occurred in calves pre-treated with both phentolamine and propranolol was significantly reduced by atropine. Release of bombesin-like immunoreactivity (BLI) was unaffected by total post-ganglionic adrenergic and cholinergic blockade. The results indicate that pancreatic endocrine responses to splanchnic nerve stimulation may be attributable, at least in part, to release of BLI in this species.

Endocrine responses to exogenous bombesin and gastrin releasing peptide in conscious calves

The effects of I.V. infusions of synthetic amphibian bombesin and porcine gastrin releasing peptide (GRP), at a dose of 5 pmol/kg. min for 30 min, have been investigated in conscious calves 3-6 weeks after birth. The protocols produced a closely similar rise in the bombesin-like immuno-reactivity of the arterial plasma of 208 +/- 14 pmol/l (bombesin) and 210 +/- 32 pmol/l (GRP) which fell exponentially with a half-life of about 3 min when the infusions were terminated. Neither peptide produced a discernible change in mean heart rate or aortic blood pressure, or in the mean arterial plasma concentrations of enteroglucagon, gastric inhibitory peptide (GIP), gastrin or cholecystokinin (CCK). GRP, but not bombesin, produced a small but significant rise in the mean plasma somatostatin concentration. Both peptides produced a significant rise in mean plasma pancreatic glucagon and pancreatic polypeptide concentration and proved to be exceptionally potent insulinotropic agents. These responses were associated with a rise in plasma glucose concentration which could not be attributed to a direct action of GRP on the liver. The distribution of bombesin-like immunoreactivity in the gastrointestinal tract was consistent with the findings of other workers who have concluded that it is restricted to nerve terminals. However, our other findings show that GRP is capable of acting as a true hormone.

Effects of the autonomic system on insulin release in response to exogenous glucose in weaned lambs

1. The role of the autonomic innervation to the pancreas in mediating the release of insulin in response to exogenous glucose has been investigated in conscious weaned lambs 4-5 months after birth. 2. The incremental rise in mean plasma insulin concentration was consistently lower in lambs with cut splanchnic nerves, which had been pre-treated with atropine (0.2 mg kg-1), than in normal control lambs, although the differences did not achieve statistical significance. 3. Pre-treatment with propranolol (0.25 mg kg-1) effectively suppressed release of insulin in response to exogenous glucose in atropinized lambs with intact splanchnic nerves, but not in atropinized lambs with cut splanchnic nerves. 4. The rise in mean plasma insulin concentration was significantly greater in lambs pre-treated with atropine (0.2 mg kg-1) and phentolamine (0.1 mg kg-1 initially, followed by 0.02 mg kg-1 min-1) than in atropinized lambs with cut splanchnic nerves. 5. It is concluded that release of insulin in response to exogenous glucose is normally modified by both sympathetic alpha-adrenergic inhibition and beta-adrenergic excitation in this species.

Pancreatic endocrine function in cortisol-treated thyroidectomized calves

1. Pancreatic endocrine function has been investigated in thyroidectomized calves given exogenous cortisol (2.0 mg.kg-1.day-1) in order to produce overt signs of diabetes. 2. Whenever this diabetic syndrome was induced it was associated with falling plasma insulin concentrations. A few days later, there was a significant rise in the post-absorptive concentration of both pancreatic glucagon and pancreatic polypeptide (PP) in the arterial plasma. Elevated levels of both hormones invariably persisted until the animals were given thyroxine. 3. Each of the pancreatic endocrine responses to cortisol was reversed by daily administration of thyroxine (25 microgram.kg-1. day-1) and the plasma glucose concentration was restored to normal within a few days. 4. Starvation was found to be an extremely effective way of reducing both the plasma glucose and glucagon concentration of diabetic calves without apparently affecting the concentration of either insulin of PP. 5. Neurally mediated release of insulin in response to 2-deoxyglucose, but not of either pancreatic glucagon or PP, was found to be defective in diabetic calves and recovered in response to thyroxine. 6. These results suggest that the primary defect that leads to the development of this diabetic syndrome in cortisol-treated thyroidectomized calves is failure of insulin release but that this is associated with consequential changes in the rates at which both glucagon and PP are released from the pancreas.

Pancreatic endocrine responses to stimulation of the peripheral ends of the vagus nerves in conscious calves

1. The effects of stimulation of the peripheral ends of both vagus nerves below the heart (10 Hz for 10 min) were investigated in conscious calves 2-5 weeks after birth. 2. Stimulation was invariably below behavioural threshold and caused a prompt increase in the concentrations of pancreatic glucagon, insulin and pancreatic polypeptide (PP) in the arterial plasma. Each of these responses was blocked by pretreatment with atropine (0.2 mg/kg). 3. Administration of exogenous glucose by continuous I.V. infusion (ca. 0.08 mmol kg-1 min-1) greatly enhanced the release of insulin in response to vagal stimulation without significantly changing that of pancreatic glucagon or PP. 4. Vagal stimulation also caused a significant rise in the concentration of vasoactive intestinal peptide (VIP) in the intestinal lymph and this response was found to persist in the presence of atropine.