Alpha-adrenoceptors and insulin release from pancreatic islets of normal and diabetic rats

Pheochromocytoma - update on disease management

Pheochromocytomas are rare endocrine tumors that can present insidiously and remain undiagnosed until death or onset of clear manifestations of catecholamine excess. They are often referred to as one of the 'great mimics' in medicine. These tumors can no longer be regarded as a uniform disease entity, but rather as a highly heterogeneous group of chromaffin cell neoplasms with different ages of onset, secretory profiles, locations, and potential for malignancy according to underlying genetic mutations. These aspects all have to be considered when the tumor is encountered, thereby enabling optimal management for the patient. Referral to a center of specialized expertise for the disease should be considered wherever possible. This is not only important for surgical management of patients, but also for post-surgical follow up and screening of disease in patients with a hereditary predisposition to the tumor. While preoperative management has changed little over the last 20 years, surgical procedures have evolved so that laparoscopic resection is the standard of care and partial adrenalectomy should be considered in all patients with a hereditary condition. Follow-up testing is essential and should be recommended and ensured on a yearly basis. Managing such patients must now also take into account possible underlying mutations and the appropriate selection of genes for testing according to disease presentation. Patients and family members with identified mutations then require an individualized approach to management. This includes consideration of distinct patterns of biochemical test results during screening and the appropriate choice of imaging studies for tumor localization according to the mutation and associated differences in predisposition to adrenal, extra-adrenal and metastatic disease.

Pharmacological modulation of dopamine receptor D2-mediated transmission alters the metabolic phenotype of diet induced obese and diet resistant C57Bl6 mice

High fat feeding induces a variety of obese and lean phenotypes in inbred rodents. Compared to Diet Resistant (DR) rodents, Diet Induced Obese (DIO) rodents are insulin resistant and have a reduced dopamine receptor D2 (DRD2) mediated tone. We hypothesized that this differing dopaminergic tone contributes to the distinct metabolic profiles of these animals. C57Bl6 mice were classified as DIO or DR based on their weight gain during 10 weeks of high fat feeding. Subsequently DIO mice were treated with the DRD2 agonist bromocriptine and DR mice with the DRD2 antagonist haloperidol for 2 weeks. Compared to DR mice, the bodyweight of DIO mice was higher and their insulin sensitivity decreased. Haloperidol treatment reduced the voluntary activity and energy expenditure of DR mice and induced insulin resistance in these mice. Conversely, bromocriptine treatment tended to reduce bodyweight and voluntary activity, and reinforce insulin action in DIO mice. These results show that DRD2 activation partly redirects high fat diet induced metabolic anomalies in obesity-prone mice. Conversely, blocking DRD2 induces an adverse metabolic profile in mice that are inherently resistant to the deleterious effects of high fat food. This suggests that dopaminergic neurotransmission is involved in the control of metabolic phenotype.

Bromocriptine-induced hyperglycemia in nonobese diabetic mice: kinetics and mechanisms of action

The effects of bromocriptine (10 mg/kg), known to inhibit prolactin secretion and lower autoimmune processes, were studied on glucose homeostasis in non-fasted non-obese diabetic mice, a spontaneous model of type 1 diabetes. Hyperglycemia was observed 120 and 240 min after i.p. but not s.c. injection. Bromocriptine administration i.p. led to rapid and marked hyperglycemia characterized by sexual dimorphism with males having higher glycemia than females. Bromocriptine induced a rapid but transient decrease in insulinemia in males only and biphasic increases in glucagon levels and a sustained stimulatory effect on circulating corticosterone in both sexes. Bromocriptine-induced hyperglycemia involved D2-dopaminergic receptors, as demonstrated by the inhibitory effect of the D2-dopamine antagonist, metoclopramide (10 mg/kg). Simultaneous injection of bromocriptine and metoclopramide also blocked the rise in blood corticosterone. In conclusion, by inducing hyperglycemia, i.p. bromocriptine administration to prediabetic autoimmune mice may counteract its beneficial anti-immunostimulatory effects.

The effects of sympathectomy and dexamethasone in rats ingesting sucrose

Both high-sucrose diet and dexamethasone (D) treatment increase plasma insulin and glucose levels and induce insulin resistance. We showed in a previous work (Franco-Colin, et al. Metabolism 2000; 49:1289-1294) that combining both protocols for 7 weeks induced less body weight gain in treated rats without affecting mean daily food intake. Since such an effect may be explained by an increase in caloric expenditure, possibly due to activation of the sympathetic nervous system by sucrose ingestion, in this work, and using 10% sucrose in the drinking water, male Wistar rats were divided into 4 groups. Two groups were sympathectomized using guanethidine (Gu) treatment for 3 weeks. One of these groups of rats received D in the drinking water. Of the 2 groups not receiving Gu, one was the control (C) and the other received D. After 8 weeks a glucose tolerance test was done. The rats were sacrificed and liver triglyceride (TG), perifemoral muscle lipid, and norepinephrine (NE) levels in the liver spleen, pancreas, and heart were determined. Gu-treated rats (Gu and Gu+D groups) showed less than 10% NE concentration compared to C and D rats, less daily caloric intake and body-weight gain, more sucrose intake, and better glucose tolerance. The area under the curve after glucose administration correlated significantly with the mean body weight gain of the rats, except for D group. Groups D (D and Gu+D) also showed less caloric intake and body-weight gain but higher liver weight and TG concentration and lower peripheral muscle mass. The combination of Gu+D treatments showed some peculiar results: negative body weight gain, a fatty liver, and low muscle mass. Though the glucose tolerance test had the worst results for the D group, it showed the best results in the Gu+D group. There were significant interactions for Guan X Dex by two-way ANOVA test for the area under the curve in the glucose tolerance test, muscle mass, and muscle lipids. The results suggest that dexamethasone catabolic effect is not caused by sympathetic activation.

Aged transgenic mice with increased glucocorticoid sensitivity in pancreatic beta-cells develop diabetes

Glucocorticoids are diabetogenic hormones because they decrease glucose uptake, increase hepatic glucose production, and inhibit insulin release. To study the long-term effects of increased glucocorticoid sensitivity in beta-cells, we studied transgenic mice overexpressing the rat glucocorticoid receptor targeted to the beta-cells using the rat insulin I promoter. Here we report that these mice developed hyperglycemia both in the fed and the overnight-fasted states at 12-15 months of age. Progression from impaired glucose tolerance, previously observed in the same colony at the age of 3 months, to manifest diabetes was not associated with morphological changes or increased apoptosis in the beta-cells. Instead, our current results suggest that the development of diabetes is due to augmented inhibition of insulin secretion through alpha(2)-adrenergic receptors (alpha(2)-ARs). Thus, we found a significantly higher density of alpha(2)-ARs in the islets of transgenic mice compared with controls, based on binding studies with the alpha(2)-AR agonist UK 14304. Furthermore, incubation of islets with benextramine, a selective antagonist of the alpha(2)-AR, restored insulin secretion in response to glucose in isolated islets from transgenic mice, whereas it had no effect on control islets. These results indicate that the chronic enhancement of glucocorticoid signaling in pancreatic beta-cells results in hyperglycemia and impaired glucose tolerance. This effect may involve signaling pathways that participate in the regulation of insulin secretion via the alpha(2)-AR.

Two generations of insulinotropic imidazoline compounds

The imidazoline RX871024 increased basal- and glucose-stimulated insulin release in vitro and in vivo. The compound inhibited activity of ATP-sensitive K(+) channels as well as voltage-gated K(+) channels, which led to membrane depolarization, an increase in the cytosolic Ca(2+) concentration ([Ca(2+)](i)), and insulin release. Importantly, RX871024 also enhanced the insulinotropic effect of glucose in cells with clamped [Ca(2+)](i) but in the presence of high ATP and Ca(2+)concentration inside the cell. We believe that the latter effect on insulin exocytosis was at least in part mediated by a rise in diacylglycerol, which then activated protein kinase C (PKC) and increased the generation of arachidonic acid (AA) metabolites. Activation of both the PKC and AA pathways resulted in potentiation of glucose effects on insulin secretion. Unlike RX871024, the novel imidazoline BL11282 did not block ATP-dependent K(+) channels, but similarly to RX871024, it stimulated insulin secretion in depolarized or permeabilized islets. Accordingly, BL11282 did not influence glucose and insulin levels under basal conditions either in vitro or in vivo, but it markedly enhanced the insulinotropic effects of glucose. BL11282 restored the impaired insulin response to glucose in islets from spontaneously diabetic GK rats. We conclude that BL11282 belongs to a new class of insulinotropic compounds that demonstrate a strong glucose-dependent effect on insulin exocytosis.

Regulation of in vitro maturation of stimulus-secretion coupling in fetal rat islet beta-cells

We have studied the maturation of a glucose-responsive insulin release from fetal rat islets, and specifically investigated the impact of nutrients, alpha-adrenoceptors, imidazoline receptors, and cyclic adenosine monophosphate (cAMP). Islets were isolated from 21 -d-old fetal rats and maintained for 7 d in tissue culture at 3.3 or 11.1 mM glucose and various supplements. Culture in the presence of the nonglucidic nutrient alpha-ketoisocaproic acid (KIC), markedly enhanced both basal and stimulated insulin release from islets cultured at either low or high glucose. Additionally, KIC significantly elevated the insulin content of islets maintained in low glucose, whereas it slightly lowered it in islets cultured at high glucose. Culture with phentolamine, an antagonist of alpha-adrenergic and imidazoline receptors, markedly amplified both basal and glucose-stimulated insulin secretion when added with islets cultured in either low or high glucose. By contrast, the pure alpha2-adrenoceptor antagonist benextramine had no such effects. Addition to culture media of a membrane-permeant agonist (Sp-cAMP[S]) or antagonist (Rp-cAMP[S]) of cAMP-dependent protein kinases types I and II failed to influence basal or glucose-responsive insulin secretory rates at either glucose concentration during culture as well as islet insulin content. In conclusion, islet beta-cell differentiation and functional maturation of the stimulus-secretion coupling can be accelerated in vitro in fetal rat pancreatic tissue by nutrient stimulation, and by interference with imidazoline receptors, whereas cAMP seems virtually ineffective in this respect. These effectors may be of regulatory significance in the in vivo development of glucose-sensitive beta-cells.