Islet amyloid polypeptide inhibits glucagon release and exerts a dual action on insulin release from isolated islets

We have studied the influence of a wide concentration range of islet amyloid polypeptide (IAPP) on both glucagon and insulin release stimulated by various types of secretagogues. In an islet incubation medium devoid of glucose, the rate of glucagon release being high, we observed a marked suppressive action by low concentrations of IAPP, 10(-10) and 10(-8) M, on glucagon release. Similarly, glucagon release stimulated by L-arginine, the cholinergic agonist carbachol, or the phosphodiesterase inhibitor isobutylmethyl xanthine (IBMX), an activator of the cyclic AMP system, was inhibited by IAPP in the 10(-10) and 10(-8) M concentration range. Moreover, basal glucagon release at 7 and 10 mM glucose was suppressed by IAPP. In contrast, IAPP exerted a dual action on insulin release. Hence, low concentrations of IAPP brought about a modest increase of basal insulin secretion at 7 mM glucose and also of insulin release stimulated by carbachol. High concentrations of IAPP, however, inhibited insulin release stimulated by glucose (10 and 16.7 mM), IBMX, carbachol and L-arginine. In conclusion, our data suggest that IAPP has complex effects on islet hormone secretion serving as an inhibitor of glucagon release and having a dual action on insulin secretion exerting mainly a negative feedback on stimulated and a positive feedback on basal insulin release.     

Helodermin stimulates glucagon secretion in the mouse

Helodermin is structurally similar to VIP (vasoactive intestinal peptide) and PHI (peptide histidine isoleucine). Since VIP and PHI both stimulate insulin and glucagon secretion, we investigated the effects of helodermin on insulin and glucagon secretion in the mouse, both in the basal state and during administration of glucose and the cholinergic agonist carbachol. After intravenous injection at dose levels between 0.5 and 8.0 nmol/kg, helodermin markedly enhanced basal plasma glucagon levels, for example at 8 nmol/kg from 139 +/- 14 to 421 +/- 86 pg/ml (p less than 0.001) after 6 minutes, without affecting basal plasma insulin levels. Together with glucose (2.8 mmol/kg), helodermin (2 and 8 nmol/kg) augmented plasma glucagon levels but had no effect on plasma insulin levels. When injected together with the cholinergic agonist carbachol (0.16 mumol/kg), helodermin markedly potentiated the increase in plasma glucagon levels (more than three-fold; p less than 0.001), again without affecting the plasma insulin levels. Combined alpha- and beta-adrenoceptor blockade (yohimbine + L-propranolol) reduced the augmenting effect of helodermin on glucagon secretion by approximately 60%. It is concluded helodermin stimulates glucagon secretion in the mouse by an effect that is partially antagonized by combined alpha- and beta-adrenoceptor antagonism.     

Effect of leptin on insulin, glugacon and somatostatin secretion in the perfused rat pancreas.

We have investigated the effect of rat leptin as well as the 22-56 fragment of this molecule on pancreatic hormone secretion in the perfused rat pancreas. In pancreases from fed rats, leptin failed to alter the insulin secretion elicited by glucose, arginine or tolbutamide, but inhibited the insulin response to both CCK-8 and carbachol, secretagogues known to act on the B-cell by increasing phospholipid turnover. This insulinostatic effect was also observed with the 22-56 leptin fragment. In pancreases obtained from 24-hour fasted rats, no effect of leptin on carbachol-induced insulin output was found, perhaps as a consequence of depressed B-cell phospholipid metabolism. Leptin did not influence glucagon or somatostatin release. Our results do not support the concept of leptin as a major regulator of B-cell function. Leptin inhibition of carbachol-induced insulin output might reflect a restraining effect of this peptide on the cholinergic stimulation of insulin release.     

Gastrin releasing peptide (GRP): effects on basal and stimulated insulin and glucagon secretion in the mouse

GRP is a pancreatic neuropeptide and may be of importance for the neural control of insulin and glucagon secretion. In this study, we investigated the effects of GRP on basal and stimulated insulin and glucagon secretion in the mouse. Intravenous injections of GRP at dose levels exceeding 2.12 nmol/kg were found to rapidly increase basal plasma levels of both insulin and glucagon. Furthermore, at a low dose level without effect on basal plasma insulin levels, GRP was found to potentiate the insulin response to both glucose (by 40%; p less than 0.05) and to the cholinergic agonist carbachol (by 57%; p less than 0.01). Also, GRP was at this dose level found to potentiate the glucagon response to carbachol (p less than 0.01). Glucose abolished GRP-induced glucagon secretion. Moreover, methylatropine given at a dose level that totally abolishes carbachol-induced insulin secretion inhibited GRP-induced insulin secretion by 39% (p less than 0.05) and GRP-induced glucagon secretion by 25% (p less than 0.01). L-Propranolol at a dose level that totally abolishes beta-adrenergically-induced insulin secretion inhibited GRP-induced insulin secretion by 52% (p less than 0.01) and GRP-induced glucagon secretion by 15% (p less than 0.05). In summary, we have shown that GRP stimulates basal and potentiates stimulated insulin and glucagon secretion in mice, and that the stimulatory effects of GRP on insulin and glucagon secretion are partially inhibited by muscarinic blockade by methylatropine or by beta-adrenoceptor blockade by propranolol. We conclude that GRP activates potently both insulin and glucagon secretion in the mouse by mechanisms that are partially related to the muscarinic and the beta-adrenergic receptors.     

Alpha-adrenoceptor blockade by phentolamine inhibits beta-adrenergically and cholinergically induced glucagon secretion in the mouse

Glucagon secretion is known to be stimulated by activation of the alpha-adrenoceptors. In this study, we investigated whether alpha-adrenoceptor blockade by phentolamine affects basal and stimulated glucagon secretion in the mouse. Phentolamine was injected intraperitoneally to mice at dose levels varying from 2.6 to 260 mumol/kg. It was found that, while decreasing plasma glucose levels, phentolamine did not over this wide dose range affect basal glucagon concentrations indicating an inhibition of the hypoglycaemia-induced glucagon secretion. Further, phentolamine clearly inhibited the glucagon secretory response to beta-adrenergic or cholinergic stimulation. Thus, phentolamine (2.6 mumol/kg), impaired the glucagon secretory response to the beta 2-adrenoceptor agonist terbutaline by 51% (P less than 0.01), and to the cholinergic agonist carbachol by 44% (P less than 0.02). We conclude that alpha-adrenoceptor blockade by phentolamine inhibits the glucagon secretion following hypoglycaemia or stimulation by beta-adrenergic and cholinergic agonists. Thus, the alpha-adrenoceptors seem to be of great importance for glucagon secretion in the mouse.     

Effects of cholecystokinin octapeptide and carbachol on neonatal insulin secretory dynamics

The effects of glucose, sulfated cholecystokinin-octapeptide (CCK-8), or carbachol on insulin secretory dynamics were studied in pancreatic islets isolated from 1- and 3-day-old neonatal rats. When challenged with glucose, 1-day islets responded with a definite first phase and elevated secretion during the latter part of the stimulation period; 3-day islets had a first phase and a rising, sustained second phase. The presence of stimulatory concentrations of CCK-8 or carbachol in addition to glucose caused dramatic changes in the release pattern in both islet populations. In 1-day islets, carbachol stimulated mainly first phase secretion whereas CCK-8 enhanced first phase release and produced a definite second phase response. The two secretagogues increased significantly both phases of release in 3-day islets with no differences between the two agents in their effects. These results indicate that CCK-8 and carbachol differentially stimulate neonatal insulin secretion, possibly through different steps in the stimulus-secretion pathway. They also suggest that the cellular mechanism for second phase release is present in 1-day islets and can be activated by CCK-8.     

Galanin and pancreastatin inhibit stimulated insulin secretion in the mouse: comparison of effects

The effects of the two intrapancreatic peptides galanin and pancreastatin on basal and stimulated insulin and glucagon secretion in the mouse were compared. It was found that at 2 min after intravenous injection of galanin or pancreastatin (4.0 nmol/kg), basal plasma glucagon and glucose levels were slightly elevated. Galanin was more potent than pancreastatin to elevate basal plasma glucagon levels: they increased from 60 +/- 15 to 145 +/- 19 pg/ml (p less than 0.01) after galanin compared to from 35 +/- 5 to 55 +/- 8 pg/ml (p less than 0.05) after pancreastatin. Plasma insulin levels were lowered by galanin (p less than 0.05), but not by pancreastatin. CCK-8 (6.3 nmol/kg) or terbutaline (3.6 mumol/kg) markedly increased the plasma insulin levels. Galanin (4.0 nmol/kg) completely abolished the insulin response to CCK-8 (p less than 0.001), but pancreastatin (4.0 nmol/kg) was without effect. Galanin inhibited the insulin response to terbutaline by approximately 60% (p less than 0.01), but pancreastatin inhibited the insulin response to terbutaline by approximately 35% only (p less than 0.05). CCK-8 and terbutaline did both elevate plasma glucagon levels by moderate potencies: neither pancreastatin nor galanin could affect these responses. Thus, in the mouse, galanin and pancreastatin both inhibit basal and stimulated insulin secretion, and stimulate basal glucagon secretion. Galanin is thereby more potent than pancreastatin. The study also showed that galanin potently inhibits insulin secretion stimulated by the octapeptide of cholecystokin and by the beta 2-adrenoceptor agonist terbutaline, and that pancreastatin inhibits terbutaline-induced insulin secretion.     

Muscarinic cholinergic receptor blockade impairs free fatty acid mobilization during fasting in pigeons (<Emphasis Type="Italic">Columba livia</Emphasis>)

The effects of intracerebroventricular pretreatment with muscarinic (scopolamine or methylscopolamine; 2.7 nmol or 5.4 nmol) or nicotinic (mecamylamine, 2.7 nmol or 5.4 nmol) cholinergic receptor antagonists on plasma free fatty acid increases induced by intracerebroventricular injections of carbachol in conscious resting pigeons (Columba livia) were examined. Plasma glucose levels were also measured throughout the experiments. Pretreatment with methylscopolamine suppressed the lipolytic effect of carbachol injections, while mecamylamine left this response unchanged. Neither carbachol treatment alone, nor the pretreatments with cholinergic agents affected glucose levels. Subsequently, the effects of intracerebroventricular injections of methylscopolamine were investigated in 24-h food-deprived pigeons. The increase in free fatty acid levels after fasting was of a magnitude similar to that observed after carbachol treatment; intracerebroventricular injections of methylscopolamine (5.4 nmol) transiently but powerfully decreased plasma free fatty acids in 24-h food-deprived pigeons to levels comparable to those of free-feeding animals. The fasting-induced decrease in glucose levels was not affected by this treatment. These data indicate that the lipolytic response induced by carbachol may be mediated by central muscarinic cholinergic receptors and that this central cholinergic mechanism partially contributes to plasma free fatty acid increases observed during fasting. Furthermore, the absence of effects on glucose levels suggests that these cholinergic mechanisms participate selectively in the lipolytic component of the metabolic response to fasting.     

Functional CCK-A and Y2 receptors in guinea pig esophagus

Effects of cholecystokinin octapeptide (CCK-8), peptide YY (PPY), neuropeptide Y (NPY) and their analogs on muscle contractions of esophageal strips were investigated. CCK-8 induced a tetrodotoxin and atropine-sensitive contraction. The relative potencies for CCK related peptides to induce contractions were CCK-8 > desulfated CCK-8 > gastrin-17-I. The CCK-A receptor antagonist L-364,718 was 300-fold more potent than the CCK-B receptor antagonist L-365,260 at inhibiting CCK-8-induced contraction. These indicate that neural CCK-A receptors mediate this contraction. PYY or NPY did not cause muscle contraction or inhibit muscle contraction induced by carbachol, endothelin-1 or KCl. However, both PYY and NPY concentration-dependently inhibited contraction induced by CCK-8. This inhibition was not affected by nitric oxide (NO) synthase inhibitors L-NMMA or L-NAME. The relative potencies of PYY related peptides to inhibit CCK-8 induced contraction were PYY > NPY > NPY13-36 > [Leu(31), Pro(34)]NPY > pancreatic polypeptide (PP). We conclude that CCK interacts with neural CCK-A receptors to cause esophageal muscle contraction. PYY and NPY interact with Y2 receptors to inhibit this CCK-induced muscle contraction by an effect not related to NO.     

Effect of dopamine on amylase secretion from guinea pig pancreatic acinar cells in vitro

Dopamine has been shown to effect pancreatic flow, protein output and amylase secretion in a variety of species. However, there is conflicting evidence regarding the role of dopamine on amylase release in vitro. Specific studies were conducted to evaluate the effect of dopamine and to compare its effects with other substances on basal- and secretagogue-stimulated amylase secretion in a guinea pig dispersed pancreatic acinar cells preparation. Dopamine (10(-6) M) induced a small, but significant (P less than 0.05) increase of amylase secretion. Established secretagogues (10(-6) M) including bombesin, cholecystokinin-octapeptide (CCK-8) and carbachol as anticipated induced significantly larger responses. Other substances tested (10(-6) M) including thyrotropin-releasing hormone (TRH) and muscimol were without effect. Complete dose-response studies (10(-11)-10(-3) M) in the presence of bombesin, CCK-8 and carbachol revealed that dopamine does not affect amylase release in response to these secretagogues. These findings suggest that dopamine is a weak stimulant of amylase secretion in vitro, and that it may therefore play a minor role in regulation of pancreatic enzyme secretion. Several factors including vascular, hormonal and neural have been implicated in regulation of pancreatic exocrine secretion. In particular, autonomic nervous system activity, notably cholinergic, has been shown to affect the secretory status of the pancreatic acinar cell. In addition, several biologically active peptides including bombesin, cholecystokinin (CCK), secretin, vasoactive intestinal peptide (VIP), substance P, gastrin and stimulation of cholinergic (muscarinic) receptors with carbachol have been shown to stimulate pancreatic enzyme secretion both in vivo and in vitro. Certain controversy regarding the role of the sympathetic nervous system in regulation of pancreatic exocrine secretion does exist. For example, several studies with agonists and antagonists of noradrenergic and dopaminergic receptor subtypes suggest a stimulatory effect on pancreatic fluid, electrolyte and enzyme secretion. However, these responses are species-specific and variations inherent to the model have been described. Dopamine administration has been shown to stimulate pancreatic bicarbonate and enzyme secretion in a variety of species including mice, dogs, and man. Radioligand binding studies with 3H-dopamine have revealed the presence of high- and low-affinity dopamine binding sites in dog pancreatic acinar cells. Stimulation of these receptors has been correlated with dose-dependent increases in intracellular cAMP levels.(ABSTRACT TRUNCATED AT 400 WORDS)     

The cyclic AMP-protein kinase A pathway restrains islet phospholipase A(2) activation

Although phospholipase A(2) (PLA(2)) is of importance for insulin secretion, it is not established how it relates to other signalling mechanisms. This study examined the crosstalk between PLA(2) and the cyclic AMP (cAMP)-protein kinase A (PKA) pathway in isolated rat islets. Forskolin, IBMX, and dbcAMP reduced [(3)H]arachidonic acid ([(3)H]AA) efflux from prelabelled islets during PLA(2) activation by mellitin or cholecystokinin (CCK-8), while efflux induced by carbachol was unaffected. The PKA inhibitor myrPKI(14-22) prevented this reduction of CCK-8-induced efflux. Glucagon-like peptide-1 (GLP-1), gastric inhibitory polypeptide (GIP), and vasoactive intestinal polypeptide (VIP) diminished CCK-8-induced efflux. Also in the absence of Ca(2+), forskolin/IBMX and dbcAMP reduced CCK-8-induced efflux. In parallel with effects on [(3)H]AA, the expected additive insulin secretion induced by mellitin or CCK-8 in combination with forskolin or GLP-1, respectively, was reduced. In conclusion, the cAMP-PKA pathway restrains both Ca(2+)-dependent and Ca(2+)-independent PLA(2) activation, indicating a regulating crosstalk between these two pathways.     

Stimulatory effect of xenin-8 on insulin and glucagon secretion in the perfused rat pancreas

Xenin is a 25-amino acid peptide of the neurotensin/xenopsin family identified in gastric mucosa as well as in a number of tissues, including the pancreas of various mammals. In healthy subjects, plasma xenin immunoreactivity increases after meals. Infusion of the synthetic peptide in dogs evokes a rise in plasma insulin and glucagon levels and stimulates exocrine pancreatic secretion. The latter effect has also been demonstrated for xenin-8, the C-terminal octapeptide of xenin. We have investigated the effect of xenin-8 on insulin, glucagon and somatostatin secretion in the perfused rat pancreas. Xenin-8 stimulated basal insulin secretion and potentiated the insulin response to glucose in a dose-dependent manner (EC(50)=0.16 nM; R(2)=0.9955). Arginine-induced insulin release was also augmented by xenin-8 (by 40%; p<0.05). Xenin-8 potentiated the glucagon responses to both arginine (by 60%; p<0.05) and carbachol (by 50%; p<0.05) and counteracted the inhibition of glucagon release induced by increasing the glucose concentration. No effect of xenin-8 on somatostatin output was observed. Our observations indicate that the reported increases in plasma insulin and glucagon levels induced by xenin represent a direct influence of this peptide on the pancreatic B and A cells.     

对牛胰多肽抑制胰酶分泌作用的离体研究

为探讨胰多肽抑制胰酶分泌的机制,我们利用大鼠离体胰腺泡制备观察了牛胰多肽(BPP)在细胞受体水平对氨甲酰胆碱等促分泌物作用的影响。实验结果显示,BPP 对氨甲酰胆碱诱导的胰腺泡淀粉酶分泌具有抑制作用,并存在剂量反应关系。BPP0.1μmol/L 和0.2μmol/L,可分别使氨甲酰胆碱诱导淀粉酶分泌的效价降低3倍和10倍;BPP 还可抑制氨甲酰胆碱刺激胰腺泡释放~(45)Ca。以上结果提示,BPP 对胰腺泡的胆碱能 M 受体具有拮抗作用。此外,BPP 对促胰液素及其同类激动剂和氨甲酰胆碱协同作用诱导的胰腺泡淀粉酶分泌具有抑制作用,提示胰多肽在整体对促胰液素诱导的胰酶分泌的抑制,可能是通过拮抗胰腺泡细胞上的 M 受体而抑制了促胰液素和胆碱能刺激协同作用引起的胰酶分泌。     

Effects of glicentine on insulin secretion

The glucagon-like immunoreactivity of the gastrointestinal tract is heterogeneous, probably including several different peptides. One of these peptides, glicentine, has recently been extracted and highly purified. Furthermore, by immunocytochemistry a glicentine-like peptide has been reported to occur in the glucagon cell of the pancreatic islets. In the present study we investigated the effects of pure glicentine on insulin release in vivo in mice. The effects were compared with effects of two other peptides, glucagon and GIP. It was found that glicentine had no influence on basal insulin secretion. This was in contrast to equimolar doses of glucagon and GIP, which both stimulated the secretion of insulin. Glucose-induced insulin release was partially inhibited by glicentine. D-glucose, in a dose selected to give a response of 25% of its maximal, raised the plasma insulin concentrations by 44.0 +/- 5.9 microU/ml. The corresponding rise for glicentine plus D-glucose was 22.3 +/- 3.7 microU/ml, i.e. glicentine inhibited glucose-induced insulin released by about 50% (p < 0.01). GIP, on the other hand, enhanced glucose-induced insulin release. This enhancement was diminished by glicentine, a reflection of the inhibition by glicentine of the glucose-induced insulin release. Neither glicentine nor GIP in the doses tested had any effect on insulin secretion induced by cholinergic stimulation. In conclusion, glicentine seems to have no effect on basal insulin release in the mouse, but it partially inhibits glucose-induced insulin secretion. Thus, if the recently demonstrated glicentine-like peptide in the glucagon cell is authentic glicentine, the glucagon cell of the pancreatic islets may contain peptides with stimulatory (glucagon) as well as inhibitory (glicentine) effects on insulin secretion induced by glucose.     

Stimulation of the glucose transport system in isolated mouse pancreatic acini by cholecystokinin and analogues.

Cholecystokinin and analogues increased the uptake of 2-deoxy-D-glucose and 3-O-methylglucose into isolated mouse pancreatic acini. This uptake was mediated by a facilitated glucose transport system that was saturable, stereospecific, and was inhibited by both phloretin and cytochalasin B. In agreement with previous studies of acinar function, caerulein was more potent and pentagastrin less potent than cholecystokinin in increasing sugar transport. The cholinergic analogue carbachol mimicked the effect of caerulein; atropine completely abolished the effects of carbachol but was without influence on the effects of the polypeptide hormones. In contrast, secretion, as well as dibutyryl cyclic AMP and dibutyryl cyclic GMP, had no effect on 2-deoxy-D-glucose uptake. Two lines of evidence suggested that hormonal stimulation of this sugar transport system was related to mobilization of cellular Ca2+. First, depletion of cellular Ca2+ by incubation of acini with ethylene glycol bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) reduced the effect of caerulein. Second, the Ca2+ ionophore A23187 mimicked the effects of caerulein on 2-deoxy-D-glucose uptake when Ca2+ was present in the medium.     

Cholinergic and VIPergic effects on thyroid hormone secretion in the mouse

The thyroid gland is known to harbor cholinergic and VIPergic nerves. In the present study, the influences of cholinergic stimulation by carbachol, cholinergic blockade by methylatropine and stimulation with various VIP sequences on basal, TSH-induced and VIP-induced thyroid hormone section were investigated in vivo in mice. The mice were pretreated with 125I and thyroxine; the subsequent release of 125I is an estimation of thyroid hormone secretion. It was found that basal radioiodine secretion was inhibited by both carbachol and methylatropine. Furthermore, TSH-induced radioiodine secretion was inhibited already by a low dose of carbachol. Moreover, a high dose of carbachol could inhibit VIP-induced radioiodine secretion. Methylatropine did not influence TSH- or VIP-stimulated radioiodine secretion, but counteracted the inhibitory action of carbachol on TSH- and VIP-induced radioiodine release. In addition, contrary to VIP, six various synthesized VIP fragments had no effect on basal or stimulated radioiodine release. It is concluded that basal thyroid hormone secretion is inhibited by both cholinergic activation and blockade. Furthermore, TSH-induced thyroid hormone secretion is more sensitive to inhibition with cholinergic stimulation than is VIP-induced thyroid hormone secretion. In addition, the VIP stimulation of thyroid hormone secretion seems to require the full VIP sequence.     

The effects of γ-hexachlorocyclohexane on amylase secretion and inositol phospholipid metabolism in mouse pancreatic acini

Dispersed mouse and guinea-pig pancreatic acini were used to examine the effects of the inositol analogue, γ-hexachlorocyclohexane (lindane) on agonist-stimulated amylase secretion. Secretion from mouse acini in response to carbachol and cholecystokinin octapeptide (CCK-8) was reduced by lindane. Similarly, amylase release from guinea-pig acini stimulated by carbachol was abolished by lindane. These acini, however, still remained responsive to dibutyryl-cAMP with only a slightly diminished secretion to this agent. Inositol phospholipid synthesis and hydrolysis was stimulated in mouse acini by both carbachol and CCK-8. Although hydrolysis of these lipids in response to CCK-8 was reduced by only 18%, stimulation of inositol phospholipid synthesis by either agonist was abolished by lindane. Dose-response curves for inositol phospholipid synthesis stimulated by carbachol and CCK-8 in mouse acini were biphasic and superimposable with those of amylase secretion. In contrast, the dose-response curve for phosphoinositide hydrolysis was sigmoid and clearly separable from that of synthesis. Reducing the external Ca²⁺ concentration caused the dose-response curves for carbachol- and CCK-8-induced inositol phospholipid synthesis to be displaced to the right, as has been observed for amylase secretion. A23187 was also found to induce amylase secretion and inositol phospholipid synthesis, and both of these responses were inhibited by lindane. Amylase secretion and inositol phospholipid synthesis may, therefore, be closely related events in the exocrine pancreas. Lindane may provide a valuable tool with which to determine the role of inositol phospholipid metabolism in stimulus-response coupling.     

Increased Insulin Secretion by Muscarinic M1 and M3 Receptor Function from Rat Pancreatic Islets in vitro

Parasympathetic system plays an important role in insulin secretion from the pancreas. Cholinergic effect on pancreatic beta cells exerts primarily through muscarinic receptors. In the present study we investigated the specific role of muscarinic M1 and M3 receptors in glucose induced insulin secretion from rat pancreatic islets in vitro. The involvement of muscarinic receptors was studied using the antagonist atropine. The role of muscarinic M1 and M3 receptor subtypes was studied using subtype specific antagonists. Acetylcholine agonist, carbachol, stimulated glucose induced insulin secretion at low concentrations (10⁻⁸–10⁻⁵ M) with a maximum stimulation at 10⁻⁷ M concentration. Carbachol-stimulated insulin secretion was inhibited by atropine confirming the role of muscarinic receptors in cholinergic induced insulin secretion. Both M1 and M3 receptor antagonists blocked insulin secretion induced by carbachol. The results show that M3 receptors are functionally more prominent at 20 mM glucose concentration when compared to M1 receptors. Our studies suggest that muscarinic M1 and M3 receptors function differentially regulate glucose induced insulin secretion, which has clinical significance in glucose homeostasis.     

Taurine supplementation restores glucose and carbachol-induced insulin secretion in islets from low-protein diet rats: involvement of Ach-M3R, Synt 1 and SNAP-25 proteins

Isolated islets from low-protein (LP) diet rats showed decreased insulin secretion in response to glucose and carbachol (Cch). Taurine (TAU) increases insulin secretion in rodent islets with a positive effect upon the cholinergic pathway. Here, we investigated the effect of TAU administration upon glucose tolerance and insulin release in rats fed on a normal protein diet (17%) without (NP) or with 2.5% of TAU in their drinking water (NPT), and LP diet fed rats (6%) without (LP) or with TAU (LPT). Glucose tolerance was found to be higher in LP, compared to NP rats. However, plasma glucose levels, during ipGTT, in LPT rats were similar to those of controls. Isolated islets from LP rats secreted less insulin in response to increasing glucose concentrations (2.8-22.2 mmol/L) and to 100 μmol/L Cch. This lower secretion was accompanied by a reduction in Cch-induced internal Ca(2+) mobilization. TAU supplementation prevents these alterations, as judged by the higher secretion induced by glucose or Cch in LPT islets. In addition, Ach-M3R, syntaxin 1 and synaptosomal associated protein of 25 kDa protein expressions in LP were lower than in NP islets. The expressions of these proteins in LPT were normalized. Finally, the sarcoendoplasmatic reticulum Ca(2+)-ATPase 3 protein expression was higher in LPT and NPT, compared with controls. In conclusion, TAU supplementation to LP rats prevented alterations in glucose tolerance as well as in insulin secretion from isolated islets. The latter effect involves the normalization of the cholinergic pathway, associated with the preservation of exocytotic proteins.     

Distribution of neurotransmitters and their effects on glucagon secretion from the in vitro normal and diabetic pancreatic tissues

The distribution of adrenergic, cholinergic and amino acid neurotransmitters and/or their enzymes were examined in both the normal and diabetic pancreatic tissues in rat using immunohistochemistry to determine whether changes in the pattern of distribution of nerves containing these neurotransmitters will occur as a result of diabetes mellitus. In addition to this, the effect of noradrenaline (NA), adrenaline (ADR), acetylcholine (ACh) and gamma-amino butyric acid (GABA) on glucagon secretion from the isolated normal and diabetic pancreatic tissues was also investigated. Pancreatic fragments from the tail end of normal and diabetic rats were removed and incubated with different concentrations (10(-8)-10(-4) M) of these neurotransmitters. Glucagon secretion into the supernatant was later determined by radioimmunoassay. NA at 10(-6) M evoked a three-fold increase in glucagon secretion from normal pancreatic tissue fragments. In diabetic pancreatic tissue, NA at 10(-6) M was able to increase glucagon secretion 1.5 times the value obtained from diabetic basal. ADR (10(-8) M) increased glucagon secretion slightly but not significantly in normal pancreatic tissue. ADR inhibited glucagon secretion from diabetic pancreas at all concentrations. ACh (10(-8) M) induced a five-fold increase in glucagon secretion from normal pancreatic tissue. In a similar way, ACh evoked a two-fold increase in glucagon secretion from diabetic pancreas at 10(-4) M. In normal pancreatic tissue, GABA produced a slight but not significant increase in glucagon secretion at 10(-4) M. In contrast to this it inhibited glucagon secretion from diabetic pancreatic tissue fragments at all concentrations. In summary, tyrosine hydroxylase- and choline acetyltransferase-positive nerves are equally well distributed in both normal and diabetic rat pancreas. There was an increase in the number of glucagon positive cells and a decrease in the number of GABA-positive cells in diabetic pancreas. NA and ACh have a potent stimulatory effect on glucagon secretion from normal pancreatic tissue fragments, whereas ADR and GABA produced a small but not significant increase in glucagon secretion from normal pancreas. NA and GABA stimulated glucagon secretion from diabetic pancreas. In contrast, ADR and ACh inhibited glucagon secretion from diabetic pancreas. Neurotransmitters vary in their ability to provoke glucagon secretion from either normal or diabetic pancreas.