Reflex adrenergic control of endocrine pancreas evoked by unloading of carotid baroreceptors in cats

The principle of 'brain energy on demand' and its predictive power for stress, sleep, stroke, obesity and diabetes

Does the brain actively draw energy from the body when needed? There are different schools of thought regarding energy metabolism. In this study, the various theoretical models are classified into one of two categories: (1) conceptualizations of the brain as being purely passively supplied, which we call 'P-models,' and (2) models understanding the brain as not only passively receiving energy but also actively procuring energy for itself on demand, which we call 'A-models.' One prominent example of such theories making use of an A-model is the selfish-brain theory. The ability to make predictions was compared between the A- and P-models. A-models were able to predict and coherently explain all data examined, which included stress, sleep, caloric restriction, stroke, type-1-diabetes mellitus, obesity, and type-2-diabetes, whereas the predictions of P-models failed in most cases. The strength of the evidence supporting A-models is based on the coherence of accurate predictions across a spectrum of metabolic states. The theory test conducted here speaks to a brain that pulls its energy from the body on-demand.

Proximal Disruption of Brain Energy Supply Raises Systemic Blood Glucose: A Systematic Review

This work joins a series that methodically tests the predictions of the Selfish-Brain theory. The theory postulates a vital ability of the mammalian brain, namely to give priority to its own energy metabolism. The brain behaves “selfishly” in this respect. For the cerebral artery occlusion studied here, the theory predicts an increase in blood glucose concentration, what becomes the hypothesis to be tested. We conducted a systematic review of cerebral-artery-occlusion papers to test whether or not the included studies could confirm this hypothesis. We identified 239 records, screened 231 works by title or abstract, and analyzed 89 by full text. According to strict selection criteria (set out in our PROSPERO preregistration, complying with PRISMA guidelines), 7 papers provided enough information to decide on the hypothesis. Our hypothesis could be fully confirmed for the 3 to 24 h after the onset of a transient 2 h or permanent occlusion. As for the mechanism, the theory predicts that the energy-deprived brain suppresses insulin secretion via the sympathoadrenal system, thereby preventing insulin-mediated glucose uptake into muscle and fat and, as a result, enhancing insulin-independent glucose uptake via the blood-brain barrier. Evidence from our included studies actually demonstrated cerebral insulin suppression. In all, the current work confirms the second major prediction of the Selfish-Brain theory that relates to a proximal bottleneck of the cerebral supply chain, cerebral artery occlusion. Its first major prediction relates to a distal supply bottleneck, caloric restriction, and is fulfilled as shown by our previous work, whereas the prediction of the long held gluco-lipostatic theory, which sees the brain as only passively supplied, is violated (Sprengell et al., 2021). The crucial point was that caloric restriction elicits smaller changes in mass (energy) in the brain than in the body. Taken together, the evidence from the current and previous work clearly shows that the most accurate predictions are possible with a theory that views the brain as an independently self-regulating energy compartment occupying a primary position in energy metabolism.

Hyperglycemic response to hemorrhage is modulated by baroreceptors unloading but not by peripheral chemoreceptors activation

The aim of this study was to assess the relative participation of carotid baro- and chemoreceptors on plasma glucose and lactate level in response to hemorrhagic hypotension. We also evaluated the effects of selective activation of carotid chemoreceptors. One week before the experiments, male Wistar rats (250-300 g) were submitted to bilateral total carotid denervation (BCD-group), or to bilateral ligature of the carotid body artery (ChD-group). During the same surgical procedure, a chronic jugular catheter for blood sampling and hemorrhage (1.2 mL/100 g/2 min) and polyethylene cannula was inserted into the left femoral artery for cardiovascular monitoring. One group submitted to fictitious surgery was used as a surgical control (Sham-group). Carotid chemoreceptors were selectively activated by sodium cyanide (NaCN, 40 microg/0.1 mL i.v.) in the Sham and ChD group. The results showed that hyperglycemic response to hemorrhage in the BCD-group was reduced whereas in the ChD-group there was no significant change in this parameter compared to the Sham group (8.6 +/- 0.5 mM, Sham-hemorrhaged, n = 8; 7.2 +/- 0.3 mM, BCD-hemorrhaged, n = 8 and 9.4 +/- 0.6 mM, ChD, n = 8, p < 0.05). Increased plasma lactate levels following hemorrhage were observed in all the three experimental groups throughout the experimental period and there were no differences between the groups. Chemoreceptor stimulation by NaCN also produced hyperglycemia, as well as an increase in blood pressure and bradycardia but did not affect plasma lactate concentration. Ligature of the carotid body artery annulled the cardiovascular responses induced by NaCN, but did not change the hyperglycemic response to hypoxia. In conclusion, our data indicate that carotid chemoreceptors do not play any major role in overall metabolic response to hypoxia or hemorrhagic hypotension. Furthermore, the results suggest that carotid baroreceptors unloading play a predominant role as main source of afferent impulses leading to the hyperglycemic response to hemorrhage. In addition our data shows that the metabolic response and cardiovascular adjustment to hypoxia can be dissociated by ligature of the carotid body artery.

The hypothalamic paraventricular nucleus and carotid receptors modulate hyperglycemia induced by hemorrhage

The aim of this study was to assess the role of cholinergic transmission in the paraventricular nucleus of the hypothalamus (PVN) and carotid body receptors in mediating a rise in plasma glucose levels in response to hemorrhagic hypotension in rats. Methylatropine (1x10(-9) mol) or 0.15 M NaCl (0.2 microl) was injected into the PVN of Wistar rats weighing 250-300 g bearing a chronic jugular catheter for blood sampling and hemorrhage (1.2 ml/100 g/2 min). Polyethylene cannulae (PE-10) were inserted into the left femoral artery for cardiovascular monitoring. In the other experimental protocol, hemorrhage was performed on rats submitted to bilateral carotid receptor denervation (H-CD). The results show that the hyperglycemic response to hemorrhage was decreased by either methylatropine (H-MA) treatment or bilateral carotid receptor denervation (10.3+/-0.4 mM, control, n=15 vs. 7.7+/-0.2 mM, H-MA, n=12, and 7.6+/-0.3 mM, H-CD, n=5, p<0.01). Furthermore, methylatropine did not affect the recovery of blood pressure after hemorrhage-induced hypotension, suggesting that the metabolic and pressor adjustments have different efferent pathways. Our data demonstrate that cholinergic input from the PVN and carotid receptors (chemo- and/or baroreceptors) might participate in the same neural pathway activated by hemorrhage-induced hypotension that produces hyperglycemia.

Theophylline enhances glucose recovery after hypoglycemia in healthy man and in type I diabetic patients

The principal mediators of glucose counterregulation (glucagon and epinephrine) use intracellular cyclic adenosine monophosphate (cAMP) to mediate glucose release. Since theophylline increases cAMP (by inhibiting its decomposition), we investigated the effect of theophylline on glucose recovery after insulin-induced hypoglycemia. Eleven healthy subjects and nine type I (insulin-dependent) diabetic patients each participated in two experiments in randomized order, receiving on both days an insulin bolus of 0.15 IU/kg soluble insulin. On one day, theophylline (intravenous [IV] bolus of 220 mg followed by IV infusion of 1 mg/kg/h) was administered from 1 hour before induction of hypoglycemia until the end of the study period. On the other day, NaCl was administered. Plasma glucose before induction of hypoglycemia was equal on the 2 study days. The plasma glucose area under the curve (AUC) was larger with theophylline than with NaCl (P = .04 for diabetic patients and P = .003 for healthy subjects). During the most active phase of glucose counterregulation, the rate of increase of plasma glucose was larger with theophylline (P = .003 for diabetic patients and P = .03 for healthy subjects). The incremental AUC for cAMP was larger with theophylline for diabetic patients (P = .01). For healthy subjects, cAMP was greater with theophylline 30 minutes after insulin (P = .03). In conclusion, glucose recovery after hypoglycemia is significantly increased when theophylline is administered in an asthma dosage before hypoglycemia is induced. This may be due to a significant enhancement of the cAMP response.

Pituitary adenylate cyclase-activating polypeptide (PACAP): occurrence in rodent pancreas and effects on insulin and glucagon secretion in the mouse

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide that occurs in several tissues, e.g., in the gut. We have studied PACAP-like immunoreactivity in the pancreas of rat and mouse, and the effects of PACAP-38 on basal and stimulated insulin and glucagon secretion in the mouse. Immunofluorescence staining demonstrated the presence of PACAP-like immunoreactivity in nerve fibers in both the rat and mouse pancreas. The nerve fibers were seen in the exocrine pancreas and surrounding the islets. Occasionally, the nerve fibers occurred within the islets. Most PACAP-positive nerve fibers innervated the intrapancreatic ganglia, although no nerve cell bodies contained PACAP-like immunoreactivity. In-vivo experiments in mice revealed that basal plasma glucagon levels were increased by PACAP-38 injected intravenously at dose levels exceeding 1.8 nmol/kg. Furthermore, PACAP-38 (7 nmol/kg) potentiated the plasma glucagon response to the cholinergic agonist carbachol (0.16 mumol/kg). This potentiation was reduced to simple addition by pretreatment with a combined alpha- and beta-adrenergic blockade by phentolamine (35 mumol/kg) and propranolol (8.5 mumol/kg). Moreover, PACAP-38 inhibited a carbachol-induced increase in the level of plasma insulin in the absence but not in the presence of adrenergic blockade. PACAP-38 increased basal plasma insulin levels and increased basal plasma glucose levels 6 min and 10 min, respectively, after injection of the peptide. We conclude that PACAP-like immunoreactivity exists in nerve fibers innervating the mouse and rat pancreas, particularly the intrapancreatic ganglia, and that PACAP-38 augments both basal and carbachol-stimulated glucagon secretion in the mouse.

Neural control of islet function by norepinephrine and sympathetic neuropeptides

It is clear that the sympathoadrenal system has a role in the regulation of endocrine pancreatic function and that the sympathetic nerves of the pancreas can change pancreatic hormone secretion to increase the availability of metabolic fuels. It seems likely that the classical sympathetic neurotransmitter, NE, acts in concert with peptide co-transmitters, such as galanin and NPY. Each is released during the stimulation of pancreatic sympathetic nerves and each is capable of influencing either islet function or pancreatic blood flow. There is considerable indirect evidence that the sympathetic innervation of the pancreas is activated during acute stress and influences the endocrine pancreas. However, proving such a physiologic role is difficult because of redundant mechanisms that influence the secretion of the metabolically-crucial hormones, insulin and glucagon. Such definitive proof therefore awaits the development of new techniques to dissect and dissociate these mechanisms.

The role of alpha- and beta-adrenoceptor subtypes in mediating the effects of catecholamines on fasting glucose and insulin concentrations in the rat

1. The role of alpha- and beta-adrenoceptor subtypes in the regulation of plasma glucose and immunoreactive insulin (IRI) levels has been investigated in normal conscious fasted rats by employing selective agonists and antagonists. 2. Adrenaline (0.2 mg kg-1)-induced hyperglycaemia was abolished by the selective alpha 2-adrenoceptor antagonist idazoxan (1.0 mg kg-1), unaltered by non-selective beta-adrenoceptor blockade (propranolol, 1.0 mg kg-1) and potentiated by the selective alpha 1-adrenoceptor antagonist prazosin (0.3 mg kg-1). Adrenaline increased plasma IRI levels in the presence of idazoxan but not in the presence of either prazosin or propranolol. 3. The selective alpha 2-adrenoceptor agonists UK 14304 (0.1 and 0.3 mg kg-1) and BHT-920 (0.2 and 0.5 mg kg-1) elicited dose-dependent hyperglycaemic responses, but did not alter plasma IRI levels. UK 14304 (0.1 mg kg-1)-evoked hyperglycaemia was blocked by idazoxan but not by prazosin. 4. The selective alpha 1-adrenoceptor agonists methoxamine (0.3 mg kg-1) and phenylephrine (0.3 mg kg-1) failed to modify either plasma glucose or IRI levels. 5. Isoprenaline (0.2 mg kg-1) elicited hyperglycaemic and insulinotropic responses which were attenuated by propranolol (1.0 mg kg-1) and the selective beta 2-adrenoceptor antagonist ICI 118551 (1.0 mg kg-1), but not by the beta 1-selective antagonists atenolol (1.0 mg kg-1) and betaxolol (1.0 mg kg-1). 6. None of the antagonists per se affected basal plasma glucose or IRI concentrations, except prazosin (1.0 mg kg-1). 7. The results indicate that adrenoceptors do not appear to be involved in regulating basal plasma glucose and IRI concentrations in the fasted rat. However, the effects of catecholamines on these parameters are mediated by alpha 2- and beta 2-adrenoceptors, whereas alpha,- or beta l-adrenoceptors do not appear to be involved.

Pancreatic noradrenergic nerves are activated by neuroglucopenia but not by hypotension or hypoxia in the dog. Evidence for stress-specific and regionally selective activation of the sympathetic nervous system

To determine if acute stress activates pancreatic noradrenergic nerves, pancreatic norepinephrine (NE) output (spillover) was measured in halothane-anesthetized dogs. Central neuroglucopenia, induced by intravenous 2-deoxy-D-glucose [( 2-DG] 600 mg/kg + 13.5 mg/kg-1 per min-1) increased pancreatic NE output from a baseline of 380 +/- 100 to 1,490 +/- 340 pg/min (delta = +1,110 +/- 290 pg/min, P less than 0.01). Surgical denervation of the pancreas reduced this response by 90% (delta = +120 +/- 50 pg/min, P less than 0.01 vs. intact innervation), suggesting that 2-DG activated pancreatic nerves by increasing the central sympathetic outflow to the pancreas rather than by acting directly on nerves within the pancreas itself. These experiments provide the first direct evidence of stress-induced activation of pancreatic noradrenergic nerves in vivo. In contrast, neither hemorrhagic hypotension (50 mmHg) nor hypoxia (6-8% O2) increased pancreatic NE output (delta = +80 +/- 110 and -20 +/- 60 pg/min, respectively, P less than 0.01 vs. neuroglucopenia) despite both producing increases of arterial plasma NE and epinephrine similar to glucopenia. The activation of pancreatic noradrenergic nerves is thus stress specific. Furthermore, because both glucopenia and hypotension increased arterial NE, yet only glucopenia activated pancreatic nerves, it is suggested that a regionally selective pattern of sympathetic activation can be elicited by acute stress, a condition in which sympathetic activation has traditionally been thought to be generalized and nondiscrete.

Bilateral carotid occlusion and the sympathetic regulation of insulin secretion

After a 2-min bilateral carotid arterial occlusion (BCO) in puppies, a centrally originating, sympathetic discharge takes place which increases heart rate and blood pressure. We examined the specificity of this sympathetic neural outflow by determining whether it also caused a direct neurally mediated inhibition of insulin release from the pancreas. The effects of this BCO on portal venous insulin concentrations, as well as on heart rate and blood pressure, were examined during i.v. glucose infusions of 0 (saline), 5 and 15 mg/kg X min-1. To determine changes in splanchnic blood flow and to more closely estimate pancreatic insulin secretion rates, a major vein draining the pancreas, the gastroduodenal, was catheterized. Blood flows and the amount of insulin traversing this vein per min (insulin flow rate) were followed before, during and after BCO. BCO decreased portal vein insulin concentrations during i.v. glucose infusions of 5 and 15 mg/kg X min-1, but not when saline was infused. Since bilateral splanchnicotomy altered this result little and since BCO increased blood flow and the insulin flow rate in the gastroduodenal vein, it appears that the lower portal venous insulin concentrations during BCO are secondary, not to sympathetically induced decrease in insulin secretion rates but, to dilution of pancreatic effluent blood. We conclude that while BCO causes appropriate changes in heart rate and blood pressure, this central stimulus to the sympathetic system does not provide a direct neuroendocrine reflex change in insulin secretion. BCO alters portal venous insulin concentration indirectly, and the alteration depends on the plasma glucose concentration and an enhancement in the splanchnic blood flow.

Suckling in lactating women stimulates the secretion of insulin and prolactin without concomitant effects on gastrin, growth hormone, calcitonin, vasopressin or catecholamines

The levels of growth hormone, vasopressin, prolactin, calcitonin, gastrin, insulin, epinephrine, norepinephrine and dopamine were measured in six lactating women during breast feeding. Prolactin levels increased in response to suckling as expected. In addition, insulin levels rose two-fold. No consistent changes were observed in the levels of the other hormones. It is suggested that the suckling related insulin release is either secondary to a reflexly induced activation of the vagal nerves or to the increased circulating levels of prolactin. Furthermore, it is suggested that the insulin release in response to suckling participates in the stimulation of milk production.

The relative contribution of the nervous system, hormones, and metabolites to the total insulin response during a meal in the rat

An attempt was made to quantitatively determine the relative contribution of the nervous system, hormonal factors, and metabolites to the total peripheral plasma insulin response (integrated incremental area) during a ten-minute liquid meal in conscious freely moving rats. The neurally mediated insulin response, as measured in the gastric-fistula bearing sham-feeding rat, amounted to at least 26%. The possible contribution of neural mechanisms triggered by the gastric, intestinal, and postabsorptive phases of the meal were, however, not determined. Hormonal factors were found to contribute at least 30% to the total insulin response on the basis of the insulin response to real feeding in atropinized rats, in the absence of any increases of plasma glucose and with only small elevations of plasma alpha-amino nitrogen. A possible atropine-suppressible hormonal factor was not isolated in the present study. Finally, the relative contribution of rising plasma glucose as determined by intravenous glucose infusions was found to amount to no more than 20%; however, the contribution of rising plasma amino acids was not determined. Thus, 23% of the total insulin response could not be segregated, but it is thought that a good part of it can be attributed to synergistic mechanisms. Because of such interactions, the sum of the effects of the isolated factors is less than the effect of the combined factors.

Release of gastrin and insulin in response to suckling in lactating dogs

The concentration of gastrin and insulin was determined in peripheral blood of 4 lactating dogs during suckling. Suckling induced an immediate rise of gastrin and insulin levels, twofold and threefold, respectively. A peak was reached at approximately 5 min after suckling was started, and basal levels were reached again within 15-20 min. The increase in gastrin and insulin levels coincided with the let-down reflex, i.e. it occurred when the puppies started to obtain milk from their mothers. Sham feeding and feeding induce a vagally mediated increase of gastrin and insulin release in dogs. We suggest that the changes in the gastrin and insulin levels observed by us in the lactating dogs, reflect a similar vagal activation induced by suckling. Possible effects of the suckling induced release of gastrin and insulin on milk ejection and secretion are discussed.

Effects of adrenergic blockade on the release of insulin, glucagon and somatostatin from the pancreas in response to splanchnic nerve stimulation in cats

The effects of alpha-, beta- or alpha + beta-adrenergic blockade on arterial plasma concentrations of insulin, glucagon and somatostatin in response to splanchnic nerve stimulation were studied in anesthetized cats. In control experiments splanchnic nerve stimulation caused a marked rise in plasma glucose and glucagon concentrations and a marked fall in insulin but somatostatin was unaffected. Pretreatment with phentolamine significantly increased basal plasma insulin concentration but the response pattern to splanchnic nerve stimulation was not altered. Propranolol attenuated both the glucose and insulin responses. Combined alpha-and beta-blockade abolished the hyperglycemia and hypoinsulinemia induced by splanchnic nerve stimulation, whereas the rise in plasma glucagon was not affected. It is concluded that insulin release from the pancreas and glucose release from the liver is controlled by adrenergic mechanisms whereas pancreatic glucagon and somatostatin secretion is relatively insensitive to splanchnic nerve stimulation in cats.

Sham feeding-induced cephalic phase insulin release in the rat

The effect of the cephalic phase of food ingestion on plasma insulin and glucagon concentration was assessed in the sham-feeding rat, bearing chronically implanted gastric drainage fistulas. It was found that continuous sham feeding produced a significant and phasic peripheral insulin response in the absence of any significant changes of glycemia. The response was almost completely blocked by prior intravenous administration of 2 mg/kg of atropine methyl nitrate and potentiated by prior intravenous administration of 1.0 or 2.5 mg/kg of phentolamine. In spite of the larger insulin response after phentolamine, there was no hypoglycemia detected. Furthermore, continuous sham feeding did not produce a significant glucagon response, whereas real feeling did. The results demonstrate that cholinergic insulin release is triggered phasically by continuous ingestion of familiar food and that this insulin response is inhibited by an alpha-adrenergic sympathetic tone. It is further concluded that the increased glucose disposal produced by the neurally released insulin is not counteracted by a concomitant glucagon response or by direct adrenergic stimulation of hepatic glucose production.

Reflex activation of the sympatho-adrenal system inhibits the gastrin release caused by electrical vagal stimulation in cats

The present study on anesthetized cats was performed to investigate how vagally induced gastrin release is influenced by simultaneous activation of the sympatho-adrenal system. The gastrin concentration in portal blood and in antral perfusates was measured with radioimmunoassay. The vagi were activated electrically and the sympatho-adrenal system by bilateral clamping of the carotid arteries. It was found that the vagally induced release of gastrin both into the circulation and into the antral lumen, was markedly inhibited (30% of control values) by simultaneous sympathetic activation, both at neutral and at acid intraantral pH. Infusions of adrenaline or noradrenaline did not mimick the effect on gastrin release induced by clamping of the carotid arteries. It is possible that the inhibitory effect of sympathetic activation on stimulated gastrin secretion is mediated via release of an inhibitory peptide, e.g. somatostatin, from the splanchnic nerves.

Metabolic and hormonal adjustments during hemorrhage in cats after interference with the sympatho-adrenal system

The relative contribution of the splanchnic sympathetic innervation and the adrenal medulla for metabolism and hormone secretion during two different levels of hemorrhagic hypotension was investigated in 3 groups of anesthetized cats, viz, intact, adrenalectomized and splanchnicotomized (adrenalectomy + cutting of splanchnic nerves). In intact cats, hemorrhage caused very marked elevations of arterial plasma glucose, adrenaline, noradrenaline, dopamine, lactate, cAMP, glycerol and glucagon concentrations whereas plasma insulin fell to only 20% of control values. Adrenalectomy attenuated the glucose, adrenaline, noradrenaline and cAMP responses whereas the normal insulin inhibition was abolished. Splanchnicotomy further reduced the hemorrhagic glucose and glycerol responses and, possibly, also that of glucagon. It is concluded that the adrenergic system as a whole is important for the adjustments of the release of glucose, cAMP, glycerol, insulin and glucagon that occur during hemorrhage in cats. The adrenal medulla seems to be of particular importance for the regulation of cAMP release.

Nervous control of pancreatic endocrine secretion in pigs. V. Influence of the sympathetic nervous system on the pancreatic secretion of insulin and glucagon, and on the insulin and glucagon response to vagal stimulation

Electrical stimulation of the splanchnic nerves in anesthetized pigs was found to stimulate markedly the pancreatic secretion of glucagon. The response pattern was glucose dependent, the glucagon responses at blood glucose concentrations below 4.5 mmol x l-1 being significantly greater than those noted during stimulation at higher concentrations. Insulin secretion was stimulated weakly and variably and only at higher glucose levels. The magnitude of the glucagon response was comparable to that obtained by electrical stimulation of the thoracic vagus nerves with the same frequency. The glucagon response to combined vagal and splanchnic stimulation was nearly identical to the sum of the responses to the two types of stimulation, whereas splanchnic stimulation abolished or reduced the increase in insulin secretion elicited by vagal stimulation. Combined alpha- and beta-adrenergic blockade markedly reduced the glucagon responses to splanchnic stimulation.

Lack of suppression of insulin secretion by exercise in patients with insulinoma

Serum glucose, insulin and glucagon concentrations were measured in three patients with insulin-producing tumours of the pancreas while performing an exercise test. In contrast to the normal adrenergic inhibition of insulin release in response to exercise, plasma insulin concentration remained at a constant and high level during exercise in patients with insulinomas. Their plasma glucose concentrations fell during exercise and in the post-exercise period. No significant changes occurred in plasma glucagon concentration. An exercise test may be a useful new diagnostic tool in organic hyperinsulinism.

Nucleus ambiguus stimulation increases plasma insulin levels in the rat

The ventral lateral brainstem has been explored for sites that facilitate insulin release unilaterally, using electrical stimulation techniques in male rats anesthetized with alpha-chloralose/urethane. Stimulation in the region of nucleus ambiguus (amb) produced a rapid rise (by 1 min) in plasma insulin levels, whereas stimulation of brainstem regions further than 500 micrometers from amb had no consistent effect on insulin levels. The amb-induced rise in insulin was markedly exaggerated by the alpha-adrenergic antagonist, phentolamine, and was greatly diminished by bilateral cervical vagotomy or atropine pretreatment. These results strongly suggest that the amb is one source of vagal motoneurons that facilitate insulin secretion. However, amb electrical stimulation alone also activated an apparent sympathetic efferent output whose inhibitory effect on insulin release could be blocked by phentolamine.

Effects of autonomic blockade by methylatropine and optical isomers and propranolol on plasma insulin levels in the basal state and after stimulation

The effects of autonomic blockade on plasma concentrations of immunoreactive insulin in the basal state and after stimulation with 4 different secretagogues were investigated in vivo in conscious mice. The muscarinic blocker methylatropine slightly depressed basal insulin, and almost totally abolished the insulin response to the cholinergic agonist carbachol, whereas the insulin response to glucose and glucagon was unaffected. Contrary to these findings, the insulin response to the beta 2-adrenoceptor agonist terbutaline was potentiated, by about 75%. The beta-adrenoceptor blocker L-propranolol depressed basal insulin by about 60%, and totally abolished the insulin responses to glucagon and terbutaline. The insulin response to glucose was slightly reduced and that to carbachol was unaffected by L-propranolol. The stereoisomer D-propranolol which is devoid of beta-adrenoceptor blocking activity but exerts local anaesthetic effect of the same potency as the Lpisomer, was without effect on basal insulin levels and the insulin responses to the different secretagogues. It is concluded that basal concentrations of immunoreactive insulin is substantially dependent on intact beta-adrenoceptors and more moderately dependent on intact muscarinic receptors. The insulin responses to terbutaline and glucagon are closely connected to beta-adrenoceptors, and the response to carbachol to the muscarinic receptors. Glucose-induced insulin response seems largely unaffected by autonomic receptor blockade although a slight reduction after beta-adrenoceptor blockade is demonstrable.

Adrenalectomy and chemical sympathectomy by 6-hydroxydopamine. Effects on basal and stimulated insulin secretion

Influences of the sympatho-adrenal system on basal and stimulated insulin secretion were studied in vivo in the conscious mouse and rat. In the mouse, adrenalectomy or chemical sympathectomy, induced by 6-hydroxydopamine, lowered basal insulin concentrations moderately. A marked depression of basal insulin concentration (about 50%) was seen after the combined treatment of chemical sympathectomy and adrenalectomy. In short-term experiments in mice, insulin secretion stimulated by glucose or the cholinergic agonist carbachol was enhanced after chemical sympathectomy and/or adrenalectomy, whereas insulin release induced by the synthetic octapeptide of cholecystokinin (CCK-8) was inhibited. The promoting influences on the insulin secretory response to carbachol displayed a rapid development whereas those to glucose developed more slowly. In contrast, the inhibiting effect on CCK-8 stimulated insulin release vanished with time. The insulin secretory response to the beta 2-adrenoceptor stimulator, terbutaline, was increased after chemical sympathectomy, unaffected by adrenalectomy, and decreased after chemical sympathectomy plus adrenalectomy. The glucose elimination rate after 6 weeks of chemical sympathectomy was increased in mice and decreased in rats. The insulin secretory response to glucose was enhanced in mice, whereas it tended to diminish in rats after long-term sympathectomy. In conclusion, the sympatho-adrenal system is involved in regulation of basal insulin concentrations in the mouse, and apparently is of great importance for stimulated insulin secretion; the influence being dependent on the nature of the secretagogue.

CNS modulation of pancreatic endocrine function : Multiple modes of expression

The involvement of the CNS in pancreatic hormone release has been studied. 1.) It has been shown that one source of vagal efferent fibers capable of facilitating insulin secretion originated in the rostral half of the nucleus ambiguus. 2.) Acute lesions of the ventromedial hypothalamus resulted in hyperinsulinaemia that could be abolished by acute vagotomy. 3.) Chronic lesions of the ventromedial hypothalamus increased secretion of insulin and glucagon, and decreased secretion of somatostatin when the pancreas was subsequently isolated and perfused. These changes were attributed to altered cholinergic activity related to previous ventromedial hypothalamic lesions as they could be reversed toward normal by atropine infusion or mimicked by the cholinergic agonist, methacholine. 4.) Electrical stimulation of the lateral hypothalamus in anaesthetized rats produced both an inhibitory component of insulin secretion, probably related to adrenergic stimulation, and a stimulatory component, probably due to the release into the blood of factor(s) that promote insulin secretion. 5.) The anatomical organization of brain of the genetically obese (ob/ob) mice is abnormal. These abnormalities could be involved in the endocrinological disturbances of these animals.

Influence of the sympatho-adrenal system and somatostatin on the secretion of insulin in the rat

1. The effects of somatostatin on insulin secretion in anaesthetized rats subjected to different manipulations of the sympatho-adrenal system have been investigated.2. Somatostatin (0.1 mug/min) inhibited the secretion of insulin in intact rats both in the basal state and after inducing an enhanced insulin release by infusion of the alpha-adrenoceptor-blocker phentolamine.3. Combined surgical splanchnicotomy and adrenalectomy caused an increase in the basal plasma insulin concentration. Somatostatin (0.1 mug/min) inhibited basal insulin release also in these rats. After infusion of phentolamine, however, the dose of somatostatin had to be raised five fold (0.5 mug/min) to achieve a comparable inhibition of insulin release. On the other hand, a similar rate of insulin secretion induced by glucose in intact rats could be inhibited by the lower dose of somatostatin.4. Administration of the beta-adrenoceptor-blocking agent propranolol to splanchnicotomized-adrenalectomized rats lowered basal insulin secretion to the same level as seen in intact rats. In these beta-adrenoceptor-blocked rats somatostatin (0.1 mug/min) inhibited insulin release both in the presence and absence of alpha-adrenoceptor blockade.5. Rats subjected to chemical sympathectomy through pre-treatment with 6-hydroxydopamine together with adrenalectomy displayed plasma insulin concentrations slightly above the normal range, but the values were much lower than in splanchnicotomized-adrenalectomized rats. Infusion of phentolamine to the chemically sympathectomized rats did not further increase insulin secretion, and somatostatin (0.1 mug/min) depressed insulin release both in the absence and presence of alpha-adrenoceptor blockade.6. It is suggested that an inhibitory tone exerted by the splanchnic nerves modulates the basal insulin secretion in the rat. Somatostatin and the sympathoadrenal system show a complex interaction on the insulin cells in that the sensitivity to somatostatin in splanchnicotomized-adrenalectomized rats with intact beta-adrenoceptors is decreased in the presence of the alpha-adrenoceptor-blocker phentolamine. The exact mechanism behind this decreased sensitivity remains unclear.

Nervous control of pancreatic endocrine secretion in pigs. III. The effect of acetylcholine on the pancreatic secretion of insulin and glucagon

We studied the effect of intraarterial administration of acetylcholine on insulin and glucagon secretion in anesthetized splanchnicotomized pigs and on insulin, glucagon, and pancreatic polypeptide secretion from the isolated perfused porcine pancreas and the isolated perfused duodeno-pancreatic block of pigs and dogs. In the pigs acetylcholine stimulated insulin and glucagon secretion in a glucose dependent manner similar to vagal stimulation; however, the response was completely resistant to hexamethonium and abolished by atropine. Acetylcholine stimulated insulin and pancreatic polypeptide secretion of the isolated perfused porcine pancreas, and inhibited glucagon secretion, whether the duodenum was present or not, whereas the glucagon secretion of the isolated perfused canine pancreas was stimulated by acetylcholine.

Nervous control of pancreatic endocrine secretion in pigs. II. The effect of pharmacological blocking agents on the response to vagal stimulation

The glucose dependent glucagon and insulin responses to electrical vagal stimulation in anesthetized splanchnicotomized young pigs were studied after administration of various pharmacological blocking agents. Hexamethonium completely abolished the responses, regardless of the glucose level. Atropine was without effect on the glucagon as well as the insulin response, regardless of stimulation frequency, glucose level, or dose of atropine. Neither propranolol nor a combination of propranolol and phenoxybenzamine inhibited the response. Our findings indicate that neither adrenergic fibres nor fibres impinging on muscarinic cholinergic receptors are involved in the pancreatic endocrine response to vagal stimulation.

On the sympathetic innervation to the cat's liver and its role for hepatic glucose release

Morphology and function of the adrenergic innervation of the liver were studied in cats. Fluorescence microscopy revealed a dense network of adrenergic nerve fibres in association with interlobular vessels and a sparse, but unequivocal innervation of the hepatocytes. These parenchymal adrenergic nerve fibres were more frequent in kittens (2 months old) than in adult cats. Electrical stimulation of the hepatic sympathetic nerves in the adult adrenalectomized cat evoked a small but insignificant increment (1-2 mM) of arterial plasma glucose concentration. When both hepatic and pancreatic sympathetic nerves were stimulated simultaneously, arterial plasma glucose concentration increased significantly by about 6 mM. We conclude that the pronounced hyperglycemic effect of activation of the sympathetic nervous system in the cat is mediated mainly via an adrenergic influence on the release of insulin and glucagon from the pancreas. The sympathetic innervation of the cat liver parenchyma seems to contribute to the hyperglycemia to a minor extent only.

Effects of autonomic stimulation on the release of vasoactive intestinal peptide from the gastrointestinal tract in the calf

1. The effects of autonomic stimulation on the release of vasoactive intestinal peptide (VIP) from the gastrointestinal tract have been investigated in adrenalectomized claves 2-5 weeks after birth.2. Stimulation of the peripheral ends of the splanchnic nerves (10 Hz for 10 min) caused a small fall in the concentration of VIP in portal and arterial plasma, together with a rise in the concentration in intestinal lymph. None of these changes achieved statistical significance.3. The effects of stimulation of the peripheral ends of the thoracic vagi, below the heart (10 Hz for 10 min), were found to depend in part upon the integrity of the splanchnic sympathetic innervation. A substantial rise in the concentration of VIP in intestinal lymph occurred whether or not the splanchnic nerves had been cut whereas an associated rise in arterial plasma VIP was only observed in calves in which the splanchnic nerves had been sectioned.4. The rise in the concentration of VIP in intestinal lymph, in response to vagal stimulation, was unaffected by concomitant stimulation of the splanchnic nerves, although the associated rise in arterial plasma VIP concentrations was suppressed. The response was also found to be resistant to atropine.5. The minimum estimated concentration of VIP in the extracellular fluid of the gastrointestinal tract was estimated to be about 60 p-mole/l. at rest and to rise by 70-120 p-mole/l. in response to vagal stimulation.6. Intravenous infusions of VIP at a dose of 50 ng kg(-1) min(-1) (16 p-mole kg(-1) min(-1)), which raised the minimum estimated concentration of VIP in the gastro-intestinal tract to the highest range encountered during stimulation, produced no significant changes in the concentrations of glucose, insulin, pancreatic glucagon or pancreatic polypeptide in the arterial plasma.7. It is concluded that a small amount of VIP is released from the gastrointestinal tract in response to vagal stimulation. In contrast, release of VIP is unaffected by stimulation of the splanchnic nerves except in so far as the rate at which the peptide passes into the circulation is reduced by adrenergic vasoconstriction.

The role of the adrenergic innervation to the pancreatic islets in the control of insulin release during exercise in man

The normal depression of plasma insulin concentration during exercise has been ascribed to adrenergic inhibition of insulin release and the role of humoral catecholamines in this hormonal adjustment has repeatedly been stressed. In the present study this contention has been investigated in 6 bilaterally adrenalectomized patients and in 6 sex- and age-matched controls who undertook exercise on an ergometer until they were exhausted. No differences were observed in any cardiovascular or metabolic adjustments between the two groups during strenous exercise. Mean plasma insulin concentration fell by about 50% in both groups. Phentolamine effectively abolished the fall in plasma insulin concentration during exercise in 2 adrenalectomized patients. The results suggest that the adrenergic nerves that supply the B-cells have a functional role in man during exercise.

Reflex adrenergic gastrin release evoked by unloading of carotid baroreceptors in cats

Gastric acid and pepsin output in response to 0.10 and 6.0 microgram . kg-1h-1 of pentagastrin response to complete elimination of the carotid baroreceptor discharge by cutting the bilateral sinus nerves. Such unloading of the baroreceptors simulates a blood pressure drop to 50--60 mmHg, resulting in a pronounced reflex increase in the activity of the sympatho-adrenal system. Cutting the sinus nerves caused an increased level of immuno-reactive gastrin in the portal plasma, and this elevated gastrin concentration was virtually extinguished by bilateral adrenalectomy. We conclude that the carotid baroreceptors can influence the gastrin release by a reflex adjustment of the release of adrenal catecholamines.