Adrenergic modulation of insulin and glucagon secretion from the isolated perfused rat pancreas

In order to observe the effect of the adrenergic system on pancreatic glucagon secretion in the isolated perfused rat pancreas, phenylephrine, an alpha-adrenergic agonist, and isoproterenol, a beta-adrenergic agonist, were added to the perfused solution. 1.2 microM phenylephrine suppressed glucagon secretion at 2.8 mM glucose, and it also decreased insulin secretion at 11.1 mM glucose. 240 nM isoproterenol enhanced glucagon secretion not only at 2.8 mM glucose, but also at 11.1 mM glucose, as well as insulin secretion at 11.1 mM. In order to study the role of intra-islet noradrenalin, phentolamine, an alpha-adrenergic antagonist, and propranolol, a beta-adrenergic antagonist, were infused with the perfused solution. 10 and 100 microM phentolamine caused an increase in insulin secretion, and 25 microM propranolol decreased insulin secretion, while they did not cause any change in glucagon secretion. From these results, it can be concluded that alpha-stimulation suppresses not only insulin but also glucagon secretion, while beta-stimulation stimulates glucagon secretion, as well as insulin secretion. Intra-islet catecholamine may have some effect on the B cell, whereas it seems to have no influence on the A cell.     

Stimulation of gastric glucagon secretion by epinephrine administration in dogs

To determine the response of gastric A-cells to adrenergic substances, immunoreactive glucagon was determined simultaneously in the jugular vein and in the left gastroepiploic vein of totally depancreatized dogs. Under basal conditions a significant gradient of glucagon concentrations between the jugular and gastric veins was observed, whereas plasma insulin values were almost undetectable. Intravenous administration of epinephrine elicits a prompt and significant increase in glucagon concentrations in the gastric vein which persist during the time of hormone infusion. To ensure adequate adrenergic blockade, blockers were infused before epinephrine administration. Accordingly, after phentolamine, the infusion of epinephrine failed to increase gastric glucagon concentrations, while after propranolol, epinephrine induced a significant release of gastric glucagon. These results indicate that epinephrine stimulates gastric glucagon secretion and that this effect is mediated through alpha-adrenergic receptors.     

Alpha-adrenergic influences on exocrine pancreatic secretion in the rabbit

Action of phenylephrine (35 micrograms/Kg/min) alone or previously blocked by phentolamine (100 micrograms/Kg/min) on exocrine pancreatic secretion of anaesthetized rabbits has been studied, in basal state or under stimulation by secretin (1 C.U./Kg/h) or by the octapeptide of cholecystokinin (OP-CCK) (0.15 Ivy dog units/Kg/h). Phenylephrine increased arterial pressure. This effect was blocked by phentolamine. However no variations were seen in pancreatic blood flow in any of the experimental conditions assayed. Phenylephrine produced a secretin-like effect on hydroelectrolytic secretion in basal conditions. This action was maintained after the infusion of secretin but not after OP-CCK. This effect was not blocked by phentolamine. Phenylephrine increased protein secretion in the basal state, an action that was blocked by phentolamine. After secretin or OP-CCK stimulation phenylephrine did not increase protein secretion. It is concluded that phentolamine blocks the effects of phenylephrine on acinar cells but not on ductular cells.     

Adrenergic effects on plasma levels of glucagon, insulin, glucose and free fatty acids in rabbits

Adrenergic effects on plasma levels of glucagon, insulin, glucose and free fatty acids were studied in fasted rabbits by infusing epinephrine, norepinephrine, isoproterenol, phentolamine (an adrenergic alpha-receptor blocking drug) and propranolol (an adrenergic beta-receptor blocking drug). The adrenergic effects on the plasma levels of insulin, glucose and free fatty acids were similar to those found in other species. The plasma levels of insulin were increased by beta-receptor stimulation (isoproterenol, phentolamine + epinephrine) and decreased by alpha-receptor stimulation (epinephrine, norepinephrine, propranolol + epinephrine). The plasma levels of glucose were increased by both alpha- and beta-receptor stimulation, and the epinephrine-induced hyperglycaemia was only blocked by combined infusions with phentolamine and propranolol. The plasma levels of free fatty acids were increased by saline and further increased by beta-receptor stimulation (isoproterenol), while epinephrine and norepinephrine gave variable results. Alpha-receptor stimulation (propranolol + epinephrine) slightly decreased the plasma levels of free fatty acids. The plasma levels of glucagon, however, were mainly increased by alpha-receptor stimulation (epinephrine, norepinephrine, propranolol + epinephrine) and increased only to a minor extent by beta-receptor stimulation (isoproterenol, phentolamine + epinephrine) in rabbits. This is in contrast to results reported for humans, where beta-receptor stimulation seems to be most important in stimulating glucagon release.     

Effect of adrenergic agonists on human chorionic gonadotropin release by human trophoblast cells obtained from first trimester placenta

The cultured syncytiotrophoblast cells from human first trimester placenta were used to determine the effect of adrenergic agonists on human chorionic gonadotropin (hCG) production in vitro. Beta-adrenergic agonists isoproterenol, ritodrine and isoxsuprine increased the hCG release during the 2 h incubation period, however, alpha-agonists norepinephrine and phenylephrine and a beta 1-agonist dobutamine had no effect. The effect of isoproterenol was blocked by propranolol and butoxamine, but less efficiently by phentolamine and atenolol. These results indicate that placental hCG production can be modulated by stimulation of beta-, possibly beta 2-adrenoceptors but not by alpha-adrenoceptors.     

The effect of alpha and beta adrenergic receptor stimulation on the adenylate cyclase activity of human adipocytes.

The effects of the mixed agonist epinephrine and the beta agonist isoproterenol, each alone and in combination with the alpha adrenergic blocker phentolamine and the beta blocker propranolol on the adenylate cyclase activity of human adipocyte membrane fragments were determined in a calcium free buffer. Neither phentolamine (10 muM) nor propranolol (32 muM) affected basal adenylate cyclase activity. Epinephrine (10 muM) stimulated adenylate cyclase activity and this effect was slightly enhanced by phentolamine. The combination of epinephrine plus propranolol depressed adenylate cyclase below the basal level. Isoproterenol (10 muM) markedly stimulated adenylate cyclase; the addition of phentolamine caused an equivocal further increase while the addition of propranolol depressed adenylate cyclase activity to, but not below, the basal level. These findings are consistent with the hypothesis that human adipocytes have both alpha and beta adrenergic receptors and that these receptors are associated with the cell membrane adenylate cyclase system.     

Altered hormone control of cyclic AMP formation in isolated parenchymal liver cells from rats treated with 2-acetylaminofluorene

The hormone control of cyclic AMP-formation in isolated parenchymal liver cells from rats fed the carcinogen 2-acetylaminofluorene (0.025% for 4–8 weeks) was studied. The cells from the carcinogen-treated animals responded much more strongly to adrenergic agents than cells from control animals, while no significant difference was found for the glucagon effect. Of the adrenergic substances studied, the order of potency was isoprenalin ⩾ adrenalin > phenylephrine; stimulation was blocked by propranolol, but not by phentolamine. The effects of supramaximal concentrations of isoprenalin and glucagon were not additive.     

Synchronization of secretion of the evoked transmitter quanta as mechanism of the facilitating action of sympathomimetics

Noradrenaline, isoproterenol, dobutamine were found to modulate kinetics of quanta secretion so as to synchronize the transmitter release. This effect could be prevented with blocking agents of beta-adrenoreceptor (atenolol, propranolol). Activators of beta-adrenoreceptors klonidine and phenylephrine did not change the kinetics of quanta secretion, whereas phentolamine did not affect the synchronizing effect of noradrenaline. The change in the time course of the secretion induced by noradrenaline increased the end-plate current amplitude. There seems to exist a specific presynaptic mechanism involving beta-adrenoreceptors for facilitation of effects of sympathomimetics.     

Effects of cholinergic and adrenergic agonists on the secretion of fluid and protein by submandibular glands of the guinea-pig and the mouse

Significant differences were observed between the guinea-pig and the mouse in terms of the secretion of fluid, protein and secretory granules from submandibular glands in response to pilocarpine, phenylephrine and isoproterenol. In both the guinea-pig and the mouse, the secretory responses induced by pilocarpine, phenylephrine and isoproterenol were inhibited by pretreatment with 4-DAMP, phentolamine and propranolol, respectively. The results suggest that the submandibular glands of the guinea-pig and the mouse have M₃-cholinoreceptors, as well as α- and β-adrenoceptors, and that these receptors play different roles in the secretion of fluid, protein and secretory granules from guinea-pig and mouse submandibular glands.     

Inactivation of hepatic acetyl-CoA carboxylase by catecholamine and its agonists through the alpha-adrenergic receptors

The effects of adrenergic agonists on acetyl-CoA carboxylase and fatty acid synthesis were studied in isolated rat hepatocytes from mature rats (300 to 350 g). Norepinephrine and phenylephrine inactivate acetyl-CoA carboxylase activity and inhibit fatty acid synthesis. The effects of both norepinephrine and phenylephrine were blocked by the alpha-adrenergic receptor blockers, phentolamine and phenoxybenzamine, and unaffected by the beta-receptor blocker propranolol. This inactivation was not mimicked by the beta-agonist isoproterenol. The measurable increase in cyclic AMP levels caused by norepinephrine and phenylephrine was abolished by the alpha-antagonist phentolamine and diminished by the beta-antagonist propranolol. Calcium depletion potentiated the increase in cyclic AMP levels by phenylephrine but abolished the phenylephrine inactivation of the carboxylase. The inactivation of acetyl-CoA carboxylase by phenylephrine was correlated with an increase in the incorporation of [32P]phosphate into the enzyme. Thus, catecholamines and their agonists promote phosphorylation and inactivation of acetyl-CoA carboxylase through the alpha-adrenergic receptor, and the inactivation requires calcium.     

Roles of alpha and beta adrenergic receptors in control of glucose oxidation in hamster epididymal adipocytes.

Oxidation of [14C] glucose in isolated epididymal adipocytes from Golden hamsters was stimulated by isoproterenol, epinephrine and norepinephrine, which all interact with beta-adrenergic receptors and by adrenocorticotrophic hormone. In contrast alpha-receptor agonists, such as phenylephrine, methoxamine or clonidine did not increase basal glucose oxidation. The beta-adrenergic blocking drug propranolol inhibited both lipolysis and glucose oxidation when these had been stimulated by isoproterenol, epinephrine or norepinephrine. Conversely, the alpha-adrenergic blocking drugs phentolamine and phenoxybenzamine did not influence lipolysis or glucose oxidation when isoproterenol provided the stimulus and increased both lipolysis and glucose metabolism in the present of either epinephrine or norepinephrine. All alpha-adrenergic agonists tested (phenylephrine, methoxamine and clonidine) lowered lipolysis and glucose oxidation isolated adipocytes exposed to isoproterenol. However, when adrenocorticotropin provided the stimulus for glucose oxidation and lipolysis, only clonidine produced a significant reduction in lipolysis and glucose oxidation. None of the alpha-agonists influenced glucose metabolism which had been increased by insulin. These data confirm the presence of both alpha and beta adrenergic receptors on hamster epididymal adipocytes and suggest that they exert antagonistic influences on lipolysis and glucose oxidation. These data are also consistent with the view that adrenergic stimulation of glucose oxidation and lipolysis in adipocytes are both mediated through beta receptors.     

The failure of aminophylline to modulate glucagon release in man

There are conflicting results regarding the impact of cyclic AMP on pancreatic glucagon release. The effect of aminophylline, a phosphodiesterase inhibitor, on glucagon secretion was studied in four non-obese, non-diabetic, healthy young male volunteers. The subjects received separate infusions of: 1) aminophylline; 2) aminophylline and propranolol; 3) arginine; 4) aminophylline and arginine; 5) insulin; 6) aminophylline and insulin; and 7) aminophylline and isoproterenol. Aminophylline not only failed to alter glucagon levels but also did not affect the glucagon responses observed after arginine and insulin-induced hypoglycemia. The concurrent infusion of isoproterenol and aminophylline also failed to cause a glucagon response. Although glucagon release has been evoked by cyclic AMP in some $\text{in vitro}$ systems, administration of aminophylline to human subjects does not enhance secretion. These results indirectly suggest that cyclic AMP is of little importance in the control of glucagon secretion in man, though the effects of aminophylline at the cellular level may be complex.     

Temperature and adrenoceptors in the frog heart

1. Cardiac adrenergic receptors in a frog, Rana tigrina, were examined in winter and summer months using isolated atria preparation maintained at 24 degrees, 14 degrees and 6 degrees C. Treatments included an examination of the atrial responses to selective alpha and beta adrenergic agonists (phenylephrine and isoproterenol respectively) and antagonists (phentolamine and propranolol). 2. Basal atrial beating rates differed between summer and winter months and increased with temperature. 3. Phenylephrine produced dose-dependent increases in the atrial beating rate and tension in the winter frogs only at 6 degrees C. These increases were blunted by phentolamine. 4. Isoproterenol produced positive chronotropic effects of 14 degrees and 24 degrees C but not at 6 degrees C in both summer and winter frogs; these effects were abolished by propranolol. Further, at 6 degrees C, the contractile response of the atrial tissue to isoproterenol was very sensitive. 5. Data suggests that the alpha adrenoceptor might be physiologically important to the frog in the low temperature environment of the cold season, during which period the cardiac beta adrenergic activity would be minimal or even absent.     

Influence of adenosine or adrenergic agonists on growth hormone stimulated lipolysis by chicken adipose tissue in vitro

In vitro lipolysis by chicken adipose explants was stimulated by growth hormone (GH) or glucagon. Adenosine or the adenosine agonist, N⁶-phenylisopropyladenosine (PIA), inhibited GH stimulated lipolysis, the effect of adenosine not being observed in the presence or adenosine deaminase. Glucagon induced lipolysis was also reduced by PIA. It is suggested that adenosine may act by G_i linked to either adenylate cyclase (for glucagon) or the signal transduction mechanism for GH. Lipolysis was not stimulated by GH in the presence of phenylephrine (α₁ adrenergic agonist), isoproterenol (β adrenergic agonist), adrenaline or glucagon. Although the presence of p-amino clonidine (α₂ adrenergic agonist) depressed basal lipolysis, a response to GH was still present. Either glucagon or β-adrenergic agonists (isoproterenol, adrenaline) stimulated lipolysis. In both cases, GH attenuated the lipolytic response to these hormones, which act via a cyclic adenosine monophosphate signal transduction mechanism.     

Adrenergic control of lipogenesis and lipolysis in the chicken in vitro

The adrenergic inhibition of lipogenesis and stimulation of lipolysis in the avian has been examined using chicken hepatocytes and adipose tissue explants in vitro. Lipogenesis was inhibited by adrenergic agonists: epinephrine (alpha + beta) greater than isoproterenol (beta 1/beta 2) greater than norepinephrine (alpha 1/alpha 2, beta 1) greater than metaproterenol (beta 2), phenylephrine (alpha 1). Dobutamine (beta 1 agonist) and dopamine (dopaminergic agonist) did not significantly affect [14C]acetate incorporation into lipid, while clonidine and para-aminoclonidine (alpha 2 agonists) were slightly stimulatory. Lipolysis in young and adult chicken adipose tissue was stimulated by epinephrine, isoproterenol, phenylephrine, dobutamine and metaproterenol, but was inhibited by clonidine and para-aminoclonidine. Both the antilipogenic and lipolytic effects of epinephrine were partially blocked by phentolamine (alpha 1 = alpha 2 antagonist) or propranolol (beta 1 = beta 2 antagonist), but completely inhibited by phentolamine and propranolol administered together.     

Roles of alpha and beta adrenergic receptors in control of glucose oxidation in hamster epididymal adipocytes

Oxidation of [¹⁴C]glucose in isolated epididymal adipocytes from Golden hamsters was stimulated by isoproterenol and norepinephrine, which all interact with β-adrenergic receptors and by adrenorticotrophic hormone. In contrast α-receptor agonists, such as phenylephrine, methoxamine or clonidine did not increase basal glucose oxidation. The β-adrenergic blocking drug propranolol inhibited both lipolysis and glucose oxidation when these had been stimulated by isoproterenol, ephinephrine and phenoxybenzamine did not the α-adrenergic blocking drugs phentolamine and phenoxybenzamine did not influence lipolysis or glucose oxidation when isoproterenol provided the stimulus and increased both liposlysis and glucose metabolism in the presence of either epinephrine or norepinephrine. All α-adrenergic agonists tested (phenylephrine, methoxamine and clonidine) lowered liposlysis and glucose oxidation in isolated adipocytes exposed to isoproterenol. However, when adrenorcortropin provided the stimulus for glucose oxidation and lipolysis, only clonidine produced a significant reduction in lipolysis and glucose oxidation. None of the α-agonists influenced glucose metabolism which had been increased by insulin. These data confirm the presence of both α and β adrenergic receptors on hamster epididymal adipocytes and suggests that they exert antagonistic influences on lipolysis and glucose oxidation. These data are also consistent with the view that adrenergic stimulation of glucose oxidation and lipolysis in adipocytes are both mediated through β receptors.     

Tracheal mucosal blood flow responses to autonomic agonists

In lightly anesthetized adult sheep, we determined tracheal mucosal blood flow (Qtr) by measuring the steady-state uptake of dimethyl ether from a tracheal chamber created by an endotracheal tube provided with two cuffs. Qtr normalized for carotid arterial pressure [Qtr(n)] was determined before and after the exposure of the tracheal mucosa to aerosolized phenylephrine (0.25-2.0 mg), isoproterenol (0.05-0.8 mg), and methacholine (2.5-20 mg). The same doses of methacholine were also administered during the intravenous infusion of vasopressin. The measurements were repeated after intravenous pretreatment with the respective antagonists phentolamine, propranolol, and atropine. Mean +/- SE base-line Qtr(n) was 1.2 +/- 0.1 ml.min-1.mmHg-1.10(2). The autonomic antagonists had no effect on mean Qtr(n). Phenylephrine produced a dose-dependent decrease in mean Qtr(n) (-70% at the highest dose), which was blunted by phentolamine, and isoproterenol produced a dose-dependent increase in mean Qtr(n) (40% at the highest dose), which was blocked by propranolol. Methacholine failed to alter mean Qtr(n) even when Qtr was first decreased by vasopressin. We conclude that in lightly anesthetized adult sheep 1) base-line Qtr(n) is not under adrenergic or cholinergic control, 2) a locally administered alpha-adrenergic agonist decreases and beta-adrenergic agonist increases Qtr(n) via specific receptor activation, and 3) a locally administered cholinergic muscarinic agonist has no effect on Qtr(n).     

Central catecholaminergic control of ACTH secretion

Plasma adrenocorticotropic hormone (ACTH) has been measured after an intra-third ventricular administration of noradrenaline, an adrenergic agonist or an adrenergic antagonist. Centrally administered noradrenaline caused a significant increase in ACTH secretion. The alpha-agonist phenylephrine also increased the ACTH level. However, neither the alpha-antagonist phentolamine nor beta-agonist isoproterenol affected the ACTH level. The beta-antagonist propranolol evoked a significant elevation in ACTH. Passive immunoneutralization was examined with anti-rat corticotropin-releasing factor (CRF) rabbit serum, anti-arginine vasopressin (AVP) rabbit serum and normal rabbit serum (NRS) on the intra-third ventricular noradrenaline-induced ACTH secretion to study the involvement of endogenous CRF. An intra-third ventricular administration of noradrenaline caused a significant increase of ACTH levels in NRS-injected rats and anti-AVP-injected rats, whereas an i.v. anti-rat CRF injection significantly reduced the intra-third ventricular noradrenaline-induced ACTH secretion. These results suggest that central catecholamine stimulated ACTH secretion via the alpha-adrenergic mechanism and that endogenous CRF is at least partly involved in the noradrenaline-induced ACTH secretion.     

Effect of glucagon antiserum on somatostatin,glucagon and insulin release from the isolated perfused rat pancreas

In order to elucidate the effect of glucagon antiserum on the endocrine pancreas, the release of somatostatin, glucagon, and insulin from the isolated perfused rat pancreas was studied following the infusion of arginine both with and without pretreatment by glucagon antiserum. Various concentrations of arginine in the presence of 5.5 mM glucose stimulated both somatostatin and glucagon secretion. However, the responses of somatostatin and glucagon were different at different doses of arginine. The infusion of glucagon antiserum strongly stimulated basal secretion in the perfusate total glucagon (free + antibody bound glucagon) and also enhanced its response to arginine, but free glucagon was undetectable in the perfusate during the infusion. On the other hand, the glucagon antiserum had no significant effect on either insulin or somatostatin secretion. Moreover, electron microscopic study revealed degrannulation and vacuolization in the cytoplasm of the A cells after exposure to glucagon antiserum, suggesting a hypersecretion of glucagon, but no significant change was found in the B cells or the D cells. We conclude that in a single pass perfusion system glucagon antiserum does not affect somatostatin or insulin secretion, although it enhances glucagon secretion.     

Evidence for alpha-1 adrenergic receptor regulation of atriopeptin release from the isolated rat heart

Atrial myocardium is the source of a recently described peptide hormone termed atriopeptin. Atriopeptin is thought to have a role in the regulation of systemic arterial pressure, fluid balance and plasma electrolyte homeostasis. Isolated rat hearts release atriopeptin into the coronary effluent, and we have found that this release is stimulated by the administration of norepinephrine, a compound with alpha and beta adrenergic properties. Infusion of the pure beta-receptor agonist, isoproterenol, failed to stimulate the release; however, the alpha-1 receptor agonist phenylephrine induced the release in a dose-dependent manner. The stimulation of atriopeptin release by norepinephrine and phenylephrine was inhibited by alpha-blockade with phentolamine. Administration of BHT-920, a selective alpha-2 agonist, had no effect on atriopeptin release. We conclude that atriopeptin secretion by the atrial myocyte is stimulated by activation of the alpha-1 adrenergic receptor. This finding suggests an involvement of the sympathetic nervous system in the physiologic regulation of the secretion of this hormone.