Interactions of acetylcholine and epinephrine on the dynamics of insulin release in vitro

The adenosine, adrenergic and opioid pathways in the regulation of insulin secretion, beta cell proliferation and regeneration

Insulin, a key hormone produced by pancreatic beta cells precisely regulates glucose metabolism in vertebrates. In type 1 diabetes, the beta cell mass is destroyed, a process triggered by a combination of environmental and genetic factors. This ultimately results in absolute insulin deficiency and dysregulated glucose metabolism resulting in a number of detrimental pathophysiological effects. The traditional focus of treating type 1 diabetes has been to control blood sugar levels through the administration of exogenous insulin. Newer approaches aim to replace the beta cell mass through pancreatic or islet transplantation. Type 2 diabetes results from a relative insulin deficiency for the prevailing insulin resistance. Treatments are generally aimed at reducing insulin resistance and/or augmenting insulin secretion and the use of insulin itself is often required. It is increasingly being recognized that the beta cell mass is dynamic and increases insulin secretion in response to beta cell mitogens and stress signals to maintain glycemia within a very narrow physiological range. This review critically discusses the role of adrenergic, adenosine and opioid pathways and their interrelationship in insulin secretion, beta cell proliferation and regeneration.

Decreased [3H] YM-09151-2 binding to dopamine D2 receptors in the hypothalamus, brainstem and pancreatic islets of streptozotocin-induced diabetic rats

In the present study dopamine was measured in the hypothalamus, brainstem, pancreatic islets and plasma, using HPLC. Dopamine D2 receptor changes in the hypothalamus, brainstem and pancreatic islets were studied using [3H] YM-09151-2 in streptozotocin-induced diabetic and insulin-treated diabetic rats. There was a significant decrease in dopamine content in the hypothalamus (P<0.001), brainstem (P<0.001), pancreatic islets (P<0.001) and plasma (P<0.001) in diabetic rats when compared to control. Scatchard analysis of [3H] YM-09151-2 in the hypothalamus of diabetic rats showed a significant decrease in Bmax (P<0.001) and Kd, showing an increased affinity of D2 receptors when compared to control. Insulin treatment did not completely reverse the changes that occurred during diabetes. There was a significant decrease in Bmax (P<0.01) with decreased affinity in the brainstem of diabetic rats. The islet membrane preparation of diabetic rats showed a significant decrease (P<0.001) in the binding of [3H] YM-09151-2 with decreased Kd (P<0.001) compared to control. The increase in affinity of D2 receptors in hypothalamus and pancreatic islets and the decreased affinity in brainstem were confirmed by competition analysis. Thus our results suggest that the decreased dopamine D2 receptor function in the hypothalamus, brainstem and pancreas affects insulin secretion in diabetic rats, which has immense clinical relevance to the management of diabetes.

Decreased alpha1-adrenergic receptor binding in the cerebral cortex and brain stem during pancreatic regeneration in rats

The purpose of this study was to investigate the role of brain alpha1-adrenergic receptor binding in the rat model of pancreatic regeneration using 60-70% pancreatectomy. The alpha1-adrenergic receptors kinetics was studied in the cerebral cortex and brain stem of sham operated, 72 h pancreatectomised and 7 days pancreatectomised rats. Scatchard analysis with [3H]prazosin in cerebral cortex and brain stem showed a significant decrease (P < 0.01), (P < 0.05) in maximal binding (Bmax) with a significant decrease (P < 0.001), (P < 0.01) in the Kd in 72 h pancreatectomised rats compared with sham respectively. Competition analysis in cerebral cortex and brain stem showed a shift in affinity during pancreatic regeneration. The sympathetic activity was decreased as indicated by the significantly decreased norepinephrine level in the plasma (P < 0.001), cerebral cortex (P < 0.01) and brain stem (P < 0.001) of 72 h pancreatectomised rats compared to sham. Thus, from our results it is suggested that the central alpha1-adrenergic receptors have a functional role in the pancreatic regeneration mediated through the sympathetic pathway.

Decreased alpha2-adrenergic receptor in the brain stem and pancreatic islets during pancreatic regeneration in weanling rats

Sympathetic stimulation inhibits insulin secretion. alpha(2)-Adrenergic receptor is known to have a regulatory role in the sympathetic function. We investigated the changes in the alpha(2)-adrenergic receptors in the brain stem and pancreatic islets using [(3)H]Yohimbine during pancreatic regeneration in weanling rats. Brain stem and pancreatic islets of experimental rats showed a significant decrease (p<0.001) in norepinephrine (NE) content at 72 h after partial pancreatectomy. The epinephrine (EPI) content showed a significant decrease (p<0.001) in pancreatic islets while it was not detected in brain stem at 72 h after partial pancreatectomy. Scatchard analysis of [(3)H]Yohimbine showed a significant decrease (p<0.05) in B(max) and K(d) at 72 h after partial pancreatectomy in the brain stem. In the pancreatic islets, Scatchard analysis of [(3)H]Yohimbine showed a significant decrease (p<0.001) in B(max) and K(d) (p<0.05) at 72 h after partial pancreatectomy. The binding parameters reversed to near sham by 7 days after pancreatectomy both in brain stem and pancreatic islets. This shows that pancreatic insulin secretion is influenced by central nervous system inputs from the brain stem. In vitro studies with yohimbine showed that the alpha(2)-adrenergic receptors are inhibitory to islet DNA synthesis and insulin secretion. Thus our results suggest that decreased alpha(2)-adrenergic receptors during pancreatic regeneration functionally regulate insulin secretion and pancreatic beta-cell proliferation in weanling rats.

Enhanced β-adrenergic receptors in the brain and pancreas during pancreatic regeneration in weanling rats

Adrenergic stimulation has an important role in the pancreatic beta-cell proliferation and insulin secretion. In the present study, we have investigated how sympathetic system regulates the pancreatic regeneration by analyzing Epinephrine (EPI), Norepinephrine (NE) and beta-adrenergic receptor changes in the brain as well as in the pancreas. EPI and NE showed a significant decrease in the brain regions, pancreas and plasma at 72 hrs after partial pancreatectomy. We observed an increase in the circulating insulin levels at 72 hrs. Scatchard analysis using [(3)H] propranolol showed a significant increase in the number of both the low affinity and high affinity beta-adrenergic receptors in cerebral cortex and hypothalamus of partially pancreatectomised rats during peak DNA synthesis. The affinity of the receptors decreased significantly in the low and high affinity receptors of cerebral cortex and the high affinity hypothalamic receptors. In the brain stem, low affinity receptors were increased significantly during regeneration whereas there was no change in the high affinity receptors. The pancreatic beta-adrenergic receptors were also up regulated at 72 hrs after partial pancreatectomy. In vitro studies showed that beta-adrenergic receptors are positive regulators of islet cell proliferation and insulin secretion. Thus our results suggest that the beta-adrenergic receptors are functionally enhanced during pancreatic regeneration, which in turn increases pancreatic beta-cell proliferation and insulin secretion in weanling rats.

Decreased 5-HT1A receptor gene expression and 5-HT1A receptor protein in the cerebral cortex and brain stem during pancreatic regeneration in rats

The present study was to investigate the role of central 5-HT and 5-HT(1A) receptor binding and gene expression in a rat model of pancreatic regeneration using 60% pancreatectomy. The pancreatic regeneration was evaluated by 5-HT content and 5-HT(1A) receptor gene expression in the cerebral cortex (CC) and brain stem (BS) of sham operated, 72 h and 7 days pancreatectomised rats. 5-HT content significantly increased in the CC (P < 0.01) and BS (P < 0.05) of 72 h pancreatectomised rats. Sympathetic activity was decreased as indicated by the significantly decreased norepinephrine (NE) and epinephrine (EPI) level (P < 0.001 and P < 0.05) in the plasma of 72 h pancreatectomised rats. 5-HT(1A) receptor density and affinity was decreased in the CC (P < 0.01) and BS (P < 0.01). These changes correlated with a diminished 5-HT(1A) receptor mRNA expression in the brain regions studied. Our results suggest that the brain 5-HT through 5-HT(1A) receptor has a functional role in the pancreatic regeneration through the sympathetic regulation.

Decreased 5-HT2C receptor binding in the cerebral cortex and brain stem during pancreatic regeneration in rats

The purpose of this study was to investigate the role of central 5-HT2C receptor binding in rat model of pancreatic regeneration using 60-70% pancreatectomy. The 5-HT and 5-HT2C receptor kinetics were studied in cerebral cortex and brain stem of sham operated, 72 h pancreatectomised and 7 days pancreatectomised rats. Scatchard analysis with [3H] mesulergine in cerebral cortex showed a significant decrease (p < 0.05) in maximal binding (Bmax) without any change in Kd in 72 h pancreatectomised rats compared with sham. The decreased Bmax reversed to sham level by 7 days after pancreatectomy. In brain stem, Scatchard analysis showed a significant decrease (p < 0.01) in Bmax with a significant increase (p < 0.01) in Kd. Competition analysis in brain stem showed a shift in affinity towards a low affinity. These parameters were reversed to sham level by 7 days after pancreatectomy. Thus the results suggest that 5-HT through the 5-HT2C receptor in the brain has a functional regulatory role in the pancreatic regeneration.

Pharmacological characterisation of the antihyperglycaemic properties of Tinospora crispa extract

The efficacy of Tinospora crispa (Menispermaceae) extract for the treatment of diabetes has previously been verified in animal models. In order to substantiate the antidiabetic effect, we characterised the antihyperglycaemic properties by studying its effect on intestinal glucose absorption and glucose uptake into adipocytes. We also performed experiments to characterise in more detail the mechanism of T. crispa-evoked insulin release by challenging it with insulin secretory antagonists viz. adrenaline, somatostatin, verapamil and nifedipine. In addition, we also performed experiments to determine the effect of the extract on cAMP content. The results clearly showed that the antihyperglycaemic effect is not due to interference with intestinal glucose uptake or uptake of the sugar into the peripheral cells. Rather, the antihyperglycaemic effect of T. crispa is probably due to stimulation of insulin release via modulation of beta-cell Ca2+ concentration. That the insulinotropic effect of T. crispa is physiological suggests that the extract contains compounds which could be purified for use in the treatment of type II diabetes.

Effects of cholinergic agonists on diacylglycerol and intracellular calcium levels in pancreatic beta-cells

We have studied the effects of cholinergic agonists on the rates of insulin release and the concentrations of diacylglycerol (DAG) and intracellular free Ca2+ ([Ca2+]i) in the beta-cell line MIN6. Insulin secretion was stimulated by glucose, by glibenclamide and by bombesin. In the presence of glucose, both acetylcholine (ACh) and carbachol (CCh) produced a sustained increase in the rate of insulin release which was blocked by EGTA or verapamil. The DAG content of MIN6 beta-cells was not affected by glucose. Both CCh and ACh evoked an increase in DAG which was maximal after 5 min and returned to basal after 30 min; EGTA abolished the cholinergic-induced increase in DAG. ACh caused a transient rise in [Ca2+]i which was abolished by omission of Ca2+ or by addition of devapamil. Thus, cholinergic stimulation of beta-cell insulin release is associated with changes in both [Ca2+]i and DAG. The latter change persists longer than the former and activation of protein kinase C and sensitization of the secretory process to Ca2+ may underlie the prolonged effects of cholinergic agonists on insulin release. However, a secretory response to CCh was still evident after both [Ca2+]i and DAG had returned to control values suggesting that additional mechanisms may be involved.

Cross-talk between muscarinic- and adenosine-receptor signalling in the regulation of cytosolic free Ca2+ and insulin secretion

The effects of A1-adenosine-receptor occupation on Ca2+ handling in the insulin-secreting RINm5F cell line were investigated. The selective A1-agonist N6-cyclopentyladenosine (CPA) had no effect itself on the cytosolic free Ca2+ concentration in cells loaded with Fura 2. However, CPA (1) attenuated the rise due to activation of voltage-gated Ca2+ channels with Bay K 8644, and (2) caused a secondary increase (EC50 approx. 300 nM) if added after the primary Ca(2+)-mobilizing agonists vasopressin or carbamoylcholine (carbachol). Prior addition of CPA (10 microM) also potentiated (by approx. 20%) the subsequent Ca2+ peak due to maximal (100 microM) carbachol, but did not alter the EC50 of the carbachol response. Detailed analysis of the secondary rise in Ca2+ revealed further features. First, it was due to mobilization from intracellular stores, since it persisted in the absence of extracellular Ca2+. Second, it was associated with a rapid (5-15 s) increase in phospholipase C (PLC) activity, as measured by h.p.l.c. analysis of Ins(1,4,5)P3. This increase was only apparent after prior stimulation with carbachol. Third, and unlike the response to carbachol, it was mediated by a pertussis-toxin-sensitive G-protein. Fourth, it was not secondary to a decrease in cyclic AMP. Fifth, it was absolutely dependent on continued occupation of the primary receptor, since it was abolished if carbachol was displaced with the antagonist atropine. This implies a dynamic cross-talk between the two receptor coupling systems, rather than covalent modification as a result of the prior activation of PLC. Sixth, it was not associated with any desensitization of the ability of CPA to inhibit forskolin-stimulated adenylate cyclase activity. Glyceraldehyde (10 mM)-induced insulin secretion was also potently inhibited by CPA > 10 nM, but the secretory response to 100 microM carbachol was unaffected up to 10 microM. The results suggest that, in vivo, adenosine would inhibit secretion due to carbohydrate nutrients much more effectively than that due to stimuli which activate PLC.

Alpha-adrenergic system in the modulation of pancreatic A and B cell function in normal rats

The role of the alpha-adrenergic system in the control of pancreatic A and B cell function was investigated in an isolated perfused rat pancreas model. Two experimental procedures were performed. In the first one we evaluated the effects of two distinct concentrations (10(-8) M and 10(-7) M) of five adrenergic substances, with varying degrees of potency on the alpha-adrenergic presynaptic receptor, on insulin (IRI) and glucagon (IRG) release induced by arginine (20 mM) plus glucose (6.6 mM). In the second procedure we studied the effects of the two alpha-blocking agents yohimbine (alpha 2-blocker) and prazosin (alpha 1-blocker) at 10(-7) M on epinephrine-modulated IRI and IRG response to the same combined metabolic stimulus. The inhibitory activity on basal and metabolically induced IRI secretion of the agonists was superimposable on their potency on the presynaptic alpha 2-adrenergic receptors. Similarly, the alpha 1-blocking agent prazosin was less effective than the alpha 2-blocker yohimbine in counteracting the inhibitory effects of epinephrine on basal and arginine plus glucose-induced insulin release. The alpha-cell activity was clearly stimulated by epinephrine, whereas selective alpha-adrenergic drugs showed no significant action on IRG secretion. Both alpha-blockers were ineffective on basal IRG release, while they had some potentiating effect on the epinephrine-induced glucagon release in basal state and during the metabolic stimulus, without a significant difference between the two drugs. We conclude that, at least in the isolated perfused rat pancreas, alpha 2-adrenergic receptors are involved in the inhibition of IRI release induced by catecholamines. On the contrary, the alpha-adrenergic system does not seem to play an essential role in the regulation of IRG secretion; the potentiation of the epinephrine-induced stimulation of A cell function by the alpha-adrenergic blockade could be accounted for by a greater availability of the catecholamine at the beta-receptor binding sites.

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.

Insulin secretory dynamics during development of rat pancreas

The dynamics of insulin secretion during development of the fetal rat pancreas were investigated. The time of onset of glucose-induced insulin secretion was of special interest. Pancreases from 15- to 22-day-old fetal rats were perifused in vitro with low (0.5 or 0.9 mg/ml) or high (5 mg/ml) concentrations of glucose in the presence or absence of arginine and leucine. Levels of insulin in the perifusate were determined by radioimmunoassay. At day 17, a significant increase in perfusate insulin level was observed in response to arginine and leucine (each at 5 mM), This response was independent of a high concentration of glucose. In addition, perifusate insulin levels were augmented when the concentration of amino acids were kept constant and the glucose concentration was changed from a high level to a low level. On day 20, a monophasic, rapid-onset short-duration rise in insulin release with a high glucose concentration was observed. This response was enhanced by acetylcholine (2.7 x 10(-9) M). At days 21 and 22, insulin levels rose rapidly in the presence of high glucose and remained elevated. The results show that there is considerable precision in the timing of the onset and maturation of the glucose-induced insulin secretory response prenatally and reaffirm that insulin secretion by the fetal beta-cell varies with the stimulus applied.

Somatostatin- and epinephrine-induced modifications of 45Ca++ fluxes and insulin release in rat pancreatic islets maintained in tissue culture

The effects of somatostatin and epinephrine have been studied with regard to glucose-induced insulin release and (45)Ca(++) uptake by rat pancreatic islets after 2 days in tissue culture and with regard to (45)Ca(++) efflux from islets loaded with the radio-isotope during the 2 days of culture. (45)Ca(++) uptake, measured simultaneously with insulin release, was linear with time for 5 min. (45)Ca(++) efflux and insulin release were also measured simultaneously from perifused islets. Glucose (16.7 mM) markedly stimulated insulin release and (45)Ca(++) uptake. Somatostatin inhibited the stimulation of insulin release by glucose in a concentration-related manner (1-1,000 ng/ml) but was without effect on the glucose-induced stimulation of (45)Ca(++) uptake. Similarly, under perifusion conditions, both phases of insulin release were inhibited by somatostatin while no effect was observed on the pattern of (45)Ca(++) efflux after glucose.Epinephrine, in contrast to somatostatin, caused a concentration-dependent inhibition of the stimulation of both insulin release and (45)Ca(++) uptake by glucose. Both phases of insulin release were inhibited by epinephrine and marked inhibition could be observed with no change in the characteristic glucose-evoked pattern of (45)Ca(++) efflux (e.g., with 10 nM epinephrine). The inhibitory effect of epinephrine on (45)Ca(++) uptake and insulin release appeared to be mediated via an alpha-adrenergic mechanism, since is was abolished in the presence of phentolamine. Somatostatin inhibits insulin release without any detectable effect upon the handling of calcium by the islets. In contrast, inhibition of insulin release by epinephrine is accompanied by a partial inhibition of glucose-induced Ca(++) uptake.