Alpha-adrenergic blockade improves glucose-potentiated insulin secretion in non-insulin-dependent diabetes mellitus

Drug-induced hyperinsulinemic hypoglycemia: An update on pathophysiology and treatment

The initial step for the differential diagnosis of hypoglycemia is to determine whether it is hyperinsulinemic or non hyperinsulinemic. Existing literature discusses drug-related hypoglycemia, but it misses a focus on drug-induced hyperinsulinemic hypoglycemia (DHH). Here we reviewed the association existing between drugs and hyperinsulinemic hypoglycemia. We primarily selected on the main electronic databases (MEDLINE, EMBASE, Web of Science, and SCOPUS) the reviews on drug-induced hypoglycemia. Among the drugs listed in the reviews, we selected the ones linked to an increase in insulin secretion. For the drugs missing a clear association with insulin secretion, we investigated the putative mechanism underlying hypoglycemia referring to the original papers. Our review provides a list of the most common agents associated with hyperinsulinemic hypoglycemia (HH), in order to facilitate both the recognition and the prevention of DHH. We also collected data about the responsiveness of DHH to diazoxide or octreotide.

Endogenous α2A-Adrenoceptor-Operated Sympathoadrenergic Tones Attenuate Insulin Secretion via cAMP/TRPM2 Signaling

In pancreatic β-cells, pharmacological concentrations of catecholamines, including adrenaline, have been used to inhibit insulin release and explore the multiple mechanisms involved. However, the significance of these signaling pathways for physiological adrenergic functions in β-cells is largely unknown. In the process of glucose-induced insulin secretion, opening of background current through nonselective cation channels (NSCCs) might facilitate membrane depolarization by closure of the ATP-sensitive K⁺ channels. Here, we examined whether physiological insulinostatic adrenaline action is mediated via the transient receptor potential melastatin 2 (TRPM2) channel, a type of NSCC, in β-cells. Results showed that physiological concentrations of adrenaline strongly suppressed glucose-induced and incretin-potentiated cAMP production and insulin secretion and inhibited NSCCs current and membrane excitability via the α2A-adrenoceptor in wild-type mice; however, insulin secretion was not attenuated in TRPM2-knockout (KO) mice. Administration of yohimbine, an α2-adrenoceptor antagonist, failed to affect glucose tolerance in TRPM2-KO mice, in contrast to an improved glucose tolerance in wild-type mice receiving the antagonist. The current study demonstrated that a physiological concentration of adrenaline attenuates insulin release via coupling of α2A-adrenoceptor to cAMP/TRPM2 signaling, thereby providing a potential therapeutic tool to treat patients with type 2 diabetes.

Genotype-based treatment of type 2 diabetes with an α2A-adrenergic receptor antagonist

The feasibility of exploiting genomic information for individualized treatment of polygenic diseases remains uncertain. A genetic variant in ADRA2A, which encodes the α(2A)-adrenergic receptor (α(2A)AR), was recently associated with type 2 diabetes. This variant causes receptor overexpression and impaired insulin secretion; thus, we hypothesized that blocking α(2A)AR pharmacologically could improve insulin secretion in patients with the risk genotype. A total of 50 type 2 diabetes patients were recruited on the basis of ADRA2A genotype for a randomized placebo-controlled intervention study with the α(2A)AR antagonist yohimbine. The patients received 0, 10, or 20 mg of yohimbine at three separate visits. The primary endpoint was insulin secretion at 30 min (Ins30) during an oral glucose tolerance test (OGTT). Patients with the risk variant had 25% lower Ins30 than those without risk genotype. After administration of 20 mg of yohimbine, Ins30 was enhanced by 29% in the risk group, making secretion similar to patients carrying the low-risk allele. The corrected insulin response and disposition index in individuals with the high-risk (but not low-risk) allele were improved by 59 ± 18% and 43 ± 14%, respectively. The beneficial effect of yohimbine was not a consequence of improved insulin sensitivity. In summary, the data show that the insulin secretion defect in patients carrying the ADRA2A risk genotype can be corrected by α(2A)AR antagonism. The findings show that knowledge of genetic risk variants can be used to guide therapeutic interventions that directly target the underlying pathophysiology and demonstrate the potential of individualized genotype-specific treatment of type 2 diabetes.

Care of the Athlete With Type 1 Diabetes Mellitus: A Clinical Review

CONTEXT

Type 1 diabetes mellitus (T1DM) results from a highly specific immune-mediated destruction of pancreatic β cells, resulting in chronic hyperglycemia. For many years, one of the mainstays of therapy for patients with T1DM has been exercise balanced with appropriate medications and medical nutrition. Compared to healthy peers, athletes with T1DM experience nearly all the same health-related benefits from exercise. Despite these benefits, effective management of the T1DM athlete is a constant challenge due to various concerns such as the increased risk of hypoglycemia. This review seeks to summarize the available literature and aid clinicians in clinical decision-making for this patient population.

EVIDENCE ACQUISITION

PubMed searches were conducted for "type 1 diabetes mellitus AND athlete" along with "type 1 diabetes mellitus AND exercise" from database inception through November 2015. All articles identified by this search were reviewed if the article text was available in English and related to management of athletes with type 1 diabetes mellitus. Subsequent reference searches of retrieved articles yielded additional literature included in this review.

RESULTS

The majority of current literature available exists as recommendations, review articles, or proposed societal guidelines, with less prospective or higher-order treatment studies available. The available literature is presented objectively with an attempt to describe clinically relevant trends and findings in the management of athletes living with T1DM.

CONCLUSIONS

Managing T1DM in the context of exercise or athletic competition is a challenging but important skill for athletes living with this disease. A proper understanding of the hormonal milieu during exercise, special nutritional needs, glycemic control, necessary insulin dosing adjustments, and prevention/management strategies for exercise-related complications can lead to successful care plans for these patients. Individualized management strategies should be created with close cooperation between the T1DM athlete and their healthcare team (including a physician and dietitian).

Towards a genotype-based approach for a patient-centered pharmacologic therapy of type 2 diabetes

The recent data reported by Tang and colleagues in Science Translational Medicine suggest that alpha-2 adrenoceptors (α2AAR) genetic heterogeneity may explain diverging results regarding the effects of α2AAR antagonists on insulin secretion and glucose control in patients with type 2 diabetes. They first confirmed that the risk variant for rs553668 (the A allele for a single-nucleotide polymorphism in ADRA2A) is likely to cause defective insulin secretion in human pancreatic islets. Second they showed that blocking α2AAR with yohimbine dose-dependently improves the reduced insulin secretion during an oral glucose tolerance test in patients with the risk variant. The successful translation of genomic information into clinical intervention in patients with type 2 diabetes provides proof of concept for the feasibility of individualized treatment based on genotype.

Fuelling the athlete with type 1 diabetes

People with type 1 diabetes (T1DM) want to enjoy the benefits of sport and exercise, but management of diabetes in this context is complex. An understanding of the physiology of exercise in health, and particularly the control of fuel mobilization and metabolism, gives an idea of problems which may arise in managing diabetes for sport and exercise. Athletes with diabetes need to be advised on appropriate diet to maximize performance and reduce fatigue. Exercise in diabetes is complicated both by hypoglycaemia and hyperglycaemia in particular circumstances and explanations are advanced which can provide a theoretical underpinning for possible management strategies. Management strategies are proposed to improve glycaemic control and performance.

Islet G protein-coupled receptors as potential targets for treatment of type 2 diabetes

Islet dysfunction - characterized by a combination of defective insulin secretion, inappropriately high glucagon secretion and reduced beta-cell mass - has a central role in the pathophysiology of type 2 diabetes. Several G protein-coupled receptors (GPCRs) expressed in islet beta-cells are known to be involved in the regulation of islet function, and therefore are potential therapeutic targets. This is evident from the recent success of glucagon-like peptide 1 (GLP1) mimetics and dipeptidyl peptidase 4 (DPP4) inhibitors, which promote activation of the GLP1 receptor to stimulate insulin secretion and inhibit glucagon secretion, and also have the potential to increase beta-cell mass. Other islet beta-cell GPCRs that are involved in the regulation of islet function include the glucose-dependent insulinotropic peptide (GIP) receptor, lipid GPCRs, pleiotropic peptide GPCRs and islet biogenic amine GPCRs. This Review summarizes islet GPCR expression, signalling and function, and highlights their potential as targets for the treatment of type 2 diabetes.

Diabetes management for intense exercise

PURPOSE OF REVIEW

People with type 1 diabetes want to enjoy the benefits of sport and exercise, but management of diabetes in this context is complex. An understanding of the physiology of exercise in health, and particularly the control of fuel mobilization and metabolism, gives an idea of problems that may arise in managing diabetes for sport and exercise.

RECENT FINDINGS

Exercise is complicated both by hypoglycaemia and hyperglycaemia in particular circumstances. Recent data demonstrate both early and late hypoglycaemia associated with endurance exercise and also give new insights into fuel use during exercise in diabetes. These data also provide potential explanations for the reduction in maximal exercise capacity sometimes observed in people with diabetes, although it should be noted that this observation is by no means universal.

SUMMARY

Advances in the understanding of exercise physiology allow the development of management strategies that aim to help athletes with diabetes achieve appropriate metabolic control during exercise. These metabolic strategies, coupled with observations from each athlete's own experience, give a basis for individualized advice that will help athletes with diabetes to fulfil their full potential.

G-protein-coupled receptors and islet function-implications for treatment of type 2 diabetes

Islet function is regulated by a number of different signals. A main signal is generated by glucose, which stimulates insulin secretion and inhibits glucagon secretion. The glucose effects are modulated by many factors, including hormones, neurotransmitters and nutrients. Several of these factors signal through guanine nucleotide-binding protein (G protein)-coupled receptors (GPCR). Examples of islet GPCR are GPR40 and GPR119, which are GPCR with fatty acids as ligands, the receptors for the incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), the receptors for the islet hormones glucagon and somatostatin, the receptors for the classical neurotransmittors acetylcholine (ACh; M(3) muscarinic receptors) and noradrenaline (beta(2)- and alpha(2)-adrenoceptors) and for the neuropeptides pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP; PAC(1) and VPAC(2) receptors), cholecystokinin (CCK(A) receptors) and neuropeptide Y (NPY Y1 receptors). Other islet GPCR are the cannabinoid receptor (CB(1) receptors), the vasopressin receptors (V1(B) receptors) and the purinergic receptors (P(2Y) receptors). The islet GPCR couple mainly to adenylate cyclase and to phospholipase C (PLC). Since important pharmacological strategies for treatment of type 2 diabetes are stimulation of insulin secretion and inhibition of glucagon secretion, islet GPCR are potential drug targets. This review summarizes knowledge on islet GPCR.

Imidazoline receptors: new targets for antihyperglycaemic drugs

In both animal models of Type 2 diabetes and in man, it has been evident for many years that certain imidazoline drugs can stimulate insulin secretion and improve glycaemia. This suggests that they may be useful new reagents for use in the management of Type 2 diabetes. However, despite their promise, no imidazoline compound has yet come into clinical use as an effective therapeutic agent in diabetes. This should not be taken as evidence of a flaw in the basic hypothesis, but derives, in part, from continuing ignorance about the molecular characteristics of imidazoline binding proteins, and the precise structure-activity relationships of their ligands. In this review, the mode of action of antihyperglycaemic imidazoline compounds is considered, and the possibility discussed that these agents may interact with a unique subtype of imidazoline binding site associated with ATP-sensitive potassium channels. The functional consequences of this interaction are summarised together with evidence that the binding site may actually lie within the channel complex. Additional data implicating the participation of alpha2-adrenoceptors in some actions of imidazolines are evaluated, and examples of relevant drugs having encouraging therapeutic profiles are highlighted. The possibility that some anti-diabetic imidazoline reagents may exert extra-pancreatic effects is also considered. Overall, the article aims to highlight important developments within the field but also draws attention to those areas where controversy remains.

Survival and hospitalization in heart failure patients with or without diabetes treated with beta-blockers

BACKGROUND

Physicians are still concerned about prescribing beta-blockers in diabetic patients with heart failure.

METHODS

In the outcome research study (the Beta-Blockers in Patients With Congestive Heart Failure: Guided Use in Clinical Practice [BRING-UP] study), the responsible clinicians could decide whether to start beta-blocker treatment and which agent to use. A total of 3091 patients were enrolled by 202 cardiologic centers: 25% of the recruited patients were already on beta-blockers, 28% started treatment at the enrollment visit, and 47% were not started on beta-blockers.

RESULTS

The 1-year mortality, hospitalization rate, and the combined end point of mortality or hospitalization were higher in diabetic patients (15.8% versus 10.9%; relative risk [RR] = 1.44; 95% confidence intervals [CI] 1.16-1.78, P =.001) (31.0% versus 24.0%; RR = 1.28; 95% CI 1.11-1.49; P =.0009) (40.5% versus 30.1%; RR = 1.35; 95% CI 1.19-1.51; P =.0001). The event-free analysis of the 4 groups (diabetic patients not treated with beta-blockers, diabetic patients treated with beta-blockers, nondiabetic patients not treated with beta-blockers, nondiabetic patients treated with beta-blockers) showed that patients treated with beta-blockers had a higher event-free probability than patients not treated with beta-blockers regardless the presence of diabetes (P <.0001).

CONCLUSIONS

On the basis of post hoc analysis, diabetic patients with chronic heart failure benefit from beta-blockers even if at a lower degree. Thus, there are no justifications to avoid beta-blockers in heart failure patients in the presence of diabetes.

Pharmacological agents that directly modulate insulin secretion

Blood glucose levels are sensed and controlled by the release of hormones from the islets of Langerhans in the pancreas. The beta-cell, the insulin-secreting cell in the islet, can detect subtle increases in circulating glucose levels and a cascade of molecular events spanning the initial depolarization of the beta-cell membrane culminates in exocytosis and optimal insulin secretion. Here we review these processes in the context of pharmacological agents that have been shown to directly interact with any stage of insulin secretion. Drugs that modulate insulin secretion do so by opening the K(ATP) channels, by interacting with cell-surface receptors, by altering second-messenger responses, by disrupting the beta-cell cytoskeletal framework, by influencing the molecular reactions at the stages of transcription and translation of insulin, and/or by perturbing exocytosis of the insulin secretory vesicles. Drugs acting primarily at the K(ATP) channels are the sulfonylureas, the benzoic acid derivatives, the imidazolines, and the quinolines, which are channel openers, and finally diazoxide, which closes these channels. Methylxanthines also work at the cell membrane level by antagonizing the purinergic receptors and thus increase insulin secretion. Other drugs have effects at multiple levels, such as the calcineurin inhibitors and somatostatin. Some drugs used extensively in research, e.g., colchicine, which is used to study vesicular transport, have no effect at the pharmacological doses used in clinical practice. We also briefly discuss those drugs that have been shown to disrupt beta-cell function in a clinical setting but for which there is scant information on their mechanism of action.

Evidence that the novel imidazoline compound FT005 is an alpha(2)-adrenoceptor agonist

1: The aim of this study was to determine whether the hyperglycaemic action of the novel imidazoline compound FT005 could be mediated by activation of alpha(2)-adrenoceptors, using a variety of in vivo and in vitro methods including radioligand binding. 2: FT005 produced a dose-dependent increase in blood glucose levels of CBA/Ca mice (0.125-25 mg kg(-1), i.p.). The time course of this hyperglycaemic effect matched that of adrenaline (1 mg kg(-1)) more closely than glucagon (1 mg kg(-1)) or the K(ATP) channel opener diazoxide (25 mg kg(-1)). The hyperglycaemic effect of FT005 (1 mg kg(-1)) was significantly reduced by the alpha(2)-adrenoceptor antagonist rauwolscine (0.5 mg kg(-1)). 3: FT005 produced a significant reduction in plasma insulin levels of mice 30 min after administration. The hyperglycaemic effect of FT005 (25 mg kg(-1)), although still present, was significantly less in fasted mice in which insulin levels are lower, suggesting that a reduction of insulin secretion contributes to the action of FT005. 4: When studied in the mouse isolated vas deferens preparation, FT005 produced a complete inhibition of neurogenic contractions, which was blocked by rauwolscine. This is consistent with activation of pre-synaptic alpha(2)-adrenoceptors. 5: In radioligand binding studies FT005 completely displaced the alpha(2)-adrenoceptor antagonist [(3)H]-RX821002 from mouse whole brain homogenates. The displacement was best described by a two-site model of interaction comprising high and low affinity components. 6: The results indicate that FT005 is an agonist at alpha(2)-adrenoceptors. A reduction in insulin secretion contributes to the hyperglycaemic action of FT005, although an additional mechanism can not be excluded.

The potential antidiabetic activity of some alpha-2 adrenoceptor antagonists

The effect of alpha-2 adrenoceptor antagonists, yohimbine and efaroxan, on the plasma glucose and insulin levels was studied in non-diabetic control, type-I (insulin-dependent) and type-II (non-insulin-dependent) diabetic rats. Pretreatment with either yohimbine or efaroxan potentiated glucose-induced insulin release in non-diabetic control rats and produced an improvement of the oral glucose tolerance and potentiated glucose-induced insulin release in type-II but not in type-I diabetic rats. Treatment with either yohimbine or efaroxan reduced the plasma glucose level and increased the plasma insulin level of non-diabetic control and type-II diabetic rats but not of type-I diabetic rats. Effects of efaroxan were more marked. Pretreatment of non-diabetic control and type-II diabetic rats with either yohimbine or efaroxan inhibited clonidine-induced hyperglycaemia and suppressed or reversed clonidine-induced hypoinsulinaemia. Also, pretreatment of these animals with either yohimbine or efaroxan enhanced the hypoglycaemic and insulinotropic effects of glibenclamide. The combination of glibenclamide and efaroxan led to a synergistic increase in insulin secretion, while that of glibenclamide and yohimbine led to an additive increase. The hyperglycaemic effect of diazoxide in non-diabetic control and type-II diabetic rats was inhibited by pretreatment with either yohimbine or efaroxan. The hypoinsulinaemic effect of diazoxide in these animals was antagonized and reversed by pretreatment with yohimbine and efaroxan, respectively. In type-I diabetic rats, there was no change in the plasma glucose and insulin levels induced by the treatment of animals with each of clonidine or diazoxide alone or in combination with either yohimbine or efaroxan. Glibenclamide produced a slight decrease in the plasma glucose level of type-I diabetic rats, at the end of the 120 min period of investigation but there was no change in the plasma insulin level. Pretreatment of these animals with either yohimbine or efaroxan produced no change in glibenclamide effects. Additionally, bath application of efaroxan or glibenclamide inhibited the relaxant effects of different concentrations of diazoxide on the isolated norepinephrine-contracted aortic strips, while the application of yohimbine produced insignificant changes. The combination of glibenclamide and efaroxan led to complete inhibition of the relaxant effects of different concentrations of diazoxide, while that of glibenclamide and yohimbine did not produce such an effect. It is concluded that yohimbine, via blockade of postsynaptic alpha-2 adrenoceptors, and efaroxan, via blockade of postsynaptic alpha-2 adrenoceptors and adenosine triphosphate-sensitive potassium channels in the pancreatic beta-cell membrane, produce insulinotropic and subsequent hypoglycaemic effects.

Near-normoglycaemic remission in African-Americans with Type 2 diabetes mellitus is associated with recovery of beta cell function

AIMS

To prospectively determine the frequency of remission and possible mechanism of beta cell recovery in non-Whites with Type 2 diabetes mellitus in the setting of intensive glycaemic regulation using pharmacological agents.

METHODS

Twenty-six consecutive, newly diagnosed African-American, Type 2 diabetic patients presenting primarily for severe hyperglycaemia (31.0+/-12.8 mmol/l) were followed for at least 1 year. Initial hospitalization included treatment with insulin, fluids and electrolytes. Outpatient intensive glycaemic regulation included insulin or glibenclamide, diabetes education and diet that altered nutrient content. Plasma glucose and C-peptide responses to an oral glucose tolerance test and HbA1c were measured at < 14, 15-56 and 57-112 days after presentation. Remission was defined as a HbA1c < or = 6.3% and fasting plasma glucose < 6.9 mmol/l, 3 months after discontinuing all pharmacological agents.

RESULTS

Eleven of 26 patients (42.3%) developed remission after a mean of 83 days of pharmacological treatment and remained in remission during follow-up for 248-479 days; one relapsed after 294 days. Fifteen patients who did not develop a remission and were followed for 168-468 days, required continuing pharmacological therapy to be well-controlled. (mean HbA1c = 7.1%). There was no significant difference in age, sex, plasma glucose at presentation, initial glycaemic regulation, final body mass index, magnitude of weight change or pharmacological agents used for treatment between the two groups. Plasma C-peptide response to oral glucose was initially (< 14 days) suppressed in all subjects and subsequently increased. The increase was significantly greater in those who underwent a remission than those who did not. Neither significant weight loss nor severe hypoglycaemia was observed in either group during intensive treatment.

CONCLUSIONS

Forty-two per cent of newly diagnosed, unselected African-Americans with Type 2 diabetes, treated intensively using pharmacological agents, education and diet developed near-normoglycaemic remission. Remission was associated with a greater recovery of glucose-stimulated insulin secretion suggesting that therapies directed at promoting beta cell recovery and preservation are potentially useful approaches to the treatment of Type 2 diabetes mellitus.

Imidazolines and pancreatic hormone secretion

A range of imidazoline derivatives are known to be effective stimulators of insulin secretion, and this response correlates with closure of ATP-sensitive potassium channels in the pancreatic beta-cell. However, mounting evidence indicates that potassium channel blockade may form only part of the mechanism by which imidazolines exert their effects on insulin secretion. Additionally, it remains unclear whether members of this class of drugs can bind directly to potassium channel components and whether occupation of a single binding site accounts for their functional activity. This review considers recent developments in the field and highlights evidence that does not fit readily with the concept that a single mechanism of action is sufficient to mediate the effects of imidazolines on pancreatic hormone secretion.

Drug treatment of hypertension complicating diabetes mellitus

Hypertension and diabetes mellitus are both common conditions associated with a high morbidity and mortality. When the two conditions occur together, as they do in 50% of diabetic individuals, the result is a 7.2-fold increase in mortality. If hypertension occurs in association with diabetes mellitus and diabetic nephropathy, mortality rises to 37-fold above that of a healthy population. Despite the increase in incidence of nephropathy, cardiovascular disease remains the major cause of death in diabetic individuals. Therapy should therefore take into consideration the results of large, placebo-controlled trials which have shown reduction in cardiovascular morbidity and mortality as a result of active treatment. Although studies with the newer antihypertensive agents such as calcium antagonists and angiotensin converting enzyme (ACE) inhibitors are ongoing, only diuretics and beta-adrenoceptor antagonists have been clearly shown to reduce cardiovascular risk. Despite concerns regarding adverse metabolic effects and loss of hypoglycaemic awareness, beta-blockers and diuretics do have a role in the management of diabetic patients. While it is clear that ACE inhibitors reduce the progression of diabetic nephropathy, evidence suggests that diuretics may be just as effective. However, unlike diuretics or beta-blockers, ACE inhibitors have no proven benefit in the prevention of stroke of myocardial infarction. Despite the claims of metabolic neutrality made for many antihypertensive agents there appears to be no advantage in their use in the majority of hypertensive diabetic patients, except where there exist specific contraindications to established therapies.

Altered activity of the autonomous nervous system as a determinant of the impaired beta-cell secretory response after protein-energy restriction in the rat

Glucose-induced insulin secretion in vivo is known to be severely blunted in the rat as a consequence of protein-energy restriction starting early in life. We have recently reported in such malnourished rats (M rats) that the release of the counterregulatory hormones that defend against hypoglycemia was severely disturbed, and their plasma levels of epinephrine and norepinephrine were prominently increased. Knowing that the autonomic nervous system has the potential to play a major role in the control of insulin secretion in response to glucose in vivo, we therefore determined whether protein-energy restriction starting after weaning could alter sympathetic and/or parasympathetic nerve activities, and whether these changes could be responsible for the lack of response to glucose of their beta-cells in vivo. When tested in the basal postabsorptive state, the malnourished rats exhibited profound alterations of both parasympathetic and sympathetic nerve activities; the firing rates of the vagus nerve and the superior cervical ganglion were dramatically decreased and increased, respectively. Under the same conditions, insulin secretion in vivo in response to a glucose load (deltaI/deltaG) was severely decreased in M rats compared with that in control (C) rats. When evaluated after administration of acetylcholine, deltaI was amplified to the same extent in M rats as in C rats. After administration of the alpha2A-adrenergic agonist oxymetazoline, glucose-induced insulin release in M rats was not significantly affected, whereas it was sharply decreased in C rats. Finally, administration of yohimbine, an alpha2-adrenergic antagonist, partially restored the lack of reactivity of the beta-cells to glucose in the M rats, as deltaI/deltaG was amplified by 6-fold in the M group and by 3.3-fold in the C group. We conclude that protein-energy restriction starting early in life in rats brings about changes in the overall activity of the autonomic nervous system that, in turn, are responsible at least in part for the acquisition/maintenance of decreased beta-cell reactivity to glucose in vivo.

Potent antihyperglycaemic property of a new imidazoline derivative S-22068 (PMS 847) in a rat model of NIDDM

Recent data suggest that some imidazoline derivatives can lower plasma glucose in experimental animal models of diabetes. We studied the activity of an imidazoline S-22068, in rat model of non-insulin-dependent diabetes mellitus (NIDDM) produced with a low dose of streptozotocin (35 mg kg(-1), i.v.) in the adult. The respective increase over basal value in glucose (deltaG) and insulin (deltaI), and the rate of glucose disappearance (K), were measured during a 30 min intravenous glucose tolerance test. After an intraperitoneal injection of S-22068 (24 mg kg(-1)), deltaG (mM min(-1)) was decreased (91.67+/-5.83 vs 120.5+/-3.65; P<0.001), whereas K was increased (1.74+/-0.09 vs 1.18+/-0.05; P<0.001). Although insulinaemia was increased at time-point 0 of the test, deltaI was unchanged. During oral glucose tolerance tests (OGTT), S-22068 (24 mg kg(-1), p.o.) improved glucose tolerance, and its efficiency was potentiated after chronic treatment (15 days). Basal glycaemia was unaffected by the treatment. Under the same conditions, a higher dose of S-22068 (40 mg kg(-1)) further improved glucose tolerance without causing hypoglycaemia. Binding experiments revealed that S-22068 displays no affinity for either adrenoceptors or the two imidazoline receptors I1 or I2. These results demonstrate that S-22068 improves glucose tolerance without causing hypoglycaemia. Thus S-22068 represents a new potential option in the treatment of NIDDM.

Effects of acute alpha 2-blockade on insulin action and secretion in humans

We tested whether acute alpha 2-blockade affects insulin secretion, glucose and fat metabolism, thermogenesis, and hemodynamics in humans. During a 5-h epinephrine infusion (50 ng.min-1.kg-1) in five volunteers, deriglidole, a selective alpha 2-receptor inhibitor, led to a more sustained rise in plasma insulin and C-peptide levels (+59 +/- 14 vs. +28 +/- 6, and +273 +/- 18 vs. +53 +/- 14 pM, P < 0.01 vs. placebo) despite a smaller rise in plasma glucose (+0.90 +/- 0.4 vs. +1.5 +/- 0.3 mM, P < 0.01). Another 10 subjects were studied in the postabsorptive state and during a 4-h hyperglycemic (+4 mM) clamp, coupled with the ingestion of 75 g of glucose at 2 h. In the postabsorptive state, hepatic glucose production, resting energy expenditure, and plasma insulin, free fatty acid (FFA), and potassium concentrations were not affected by acute alpha 2-blockade. Hyperglycemia elicited a biphasic rise in plasma insulin (to a peak of 140 +/- 24 pM), C-peptide levels (1,520 +/- 344 pM), and insulin secretion (to 410 +/- 22 pmol/min); superimposed glucose ingestion elicited a further twofold rise in insulin and C-peptide levels, and insulin secretion. However, alpha 2-blockade failed to change these secretory responses. Fasting blood beta-hydroxybutyrate and glycerol and plasma FFA and potassium concentrations all declined with hyperglycemia; time course and extent of these changes were not affected by alpha 2-blockade. Resting energy expenditure (+25 vs. +16%, P < 0.01) and external cardiac work (+28% vs. +19%, P < 0.01) showed larger increments after alpha 2-blockade. We conclude that acute alpha 2-blockade in humans 1) prevents epinephrine-induced inhibition of insulin secretion, 2) does not potentiate basal or intravenous- or oral glucose-induced insulin release, 3) enhances thermogenesis, and 4) increases cardiac work.

Drug treatment of non-insulin-dependent diabetes mellitus in the 1990s. Achievements and future developments

Non-insulin-dependent diabetes mellitus (NIDDM, type 2 diabetes) is a heterogeneous disease resulting from a dynamic interaction between defects in insulin secretion and insulin action. There are various pharmacological approaches to improving glucose homeostasis, but those currently used in clinical practice either do not succeed in restoring normoglycaemia in most patients or fail after a variable period of time. For glycaemic regulation, 4 classes of drugs are currently available: sulphonylureas, biguanides (metformin), alpha-glucosidase inhibitors (acarbose) and insulin, each of which has a different mode and site of action. These standard pharmacological treatments may be used individually for certain types of patients, or may be combined in a stepwise fashion to provide more ideal glycaemic control for most patients. Adjunct treatments comprise a few pharmacological approaches which may help to improve glycaemic control by correcting some abnormalities frequently associated with NIDDM, such as obesity (serotoninergic anorectic agents) and hyperlipidaemia (benfluorex). There is intensive pharmaceutical research to find new drugs able to stimulate insulin secretion (new sulphonylurea or nonsulphonylurea derivatives, glucagon-like peptide-1), improve insulin action (thiazolidinediones, lipid interfering agents, glucagon antagonists, vanadium compounds) or reduce carbohydrate absorption (miglitol, amylin analogues, glucagon-like peptide-1). Further studies should demonstrate the superiority of these new compounds over the standard antidiabetic agents as well as their optimal mode of administration, alone or in combination with currently available drugs.

Insulin secretagogues with an imidazoline structure inhibit arginine-induced secretion from isolated glucagon secretion from isolated rat islets of Langerhans

It is well documented that imidazoline compounds such as efaroxan and phentolamine act as potent insulin secretagogues both in vivo and in vitro, an effect which is mediated principally by blockade of ATP-sensitive potassium channels in the pancreatic B-cell. However, little is known about the effects of these drugs on the secretion of other pancreatic hormones and, in the present work, we have investigated the effects of selective imidazoline compounds on glucagon release from isolated rat islets of Langerhans. None of several imidazoline compounds tested (efaroxan, phentolamine, idazoxan, antazoline) affected glucagon secretion from islets incubated with 4 mM glucose. However, when the rate of glucagon release was stimulated by L-arginine (20 mM) efaroxan caused a rapid, sustained and dose-dependent inhibition of the secretory response (EC50 approximately 30 microM). This effect was seen under both static incubation and islet perifusion conditions. Antazoline and phentolamine also inhibited arginine-induced glucagon secretion, whereas idazoxan (an imidazoline which does not affect insulin secretion) failed to alter glucagon release. The inhibitory effects of imidazolines on glucagon release were not secondary to changes in insulin secretion. Taken together, the results indicate that pancreatic A-cells express functional imidazoline receptors which can regulate the secretory activity of the cells.

Interactions between imidazoline compounds and sulphonylureas in the regulation of insulin secretion

1. Imidazoline alpha 2-antagonist drugs such as efaroxan have been shown to increase the insulin secretory response to sulphonylureas from rat pancreatic B-cells. We have investigated whether this reflects binding to an islet imidazoline receptor or whether alpha 2-adrenoceptor antagonism is involved. 2. Administration of (+/-)-efaroxan or glibenclamide to Wistar rats was associated with a transient increase in plasma insulin. When both drugs were administered together, the resultant increase in insulin levels was much greater than that obtained with either drug alone. 3. Use of the resolved enantiomers of efaroxan revealed that the ability of the compound to enhance the insulin secretory response to glibenclamide resided only in the alpha 2-selective-(+)-enantiomer; the imidazoline receptor-selective-(-)-enantiomer was ineffective. 4. In vitro, (+)-efaroxan increased the insulin secretory response to glibenclamide in rat freshly isolated and cultured islets of Langerhans, whereas (-)-efaroxan was inactive. By contrast, (+)-efaroxan did not potentiate glucose-induced insulin secretion but (-)-efaroxan induced a marked increase in insulin secretion from islets incubated in the presence of 6 mM glucose. 5. Incubation of rat islets under conditions designed to minimize the extent of alpha 2-adrenoceptor signalling (by receptor blockade with phenoxybenzamine; receptor down-regulation or treatment with pertussis toxin) abolished the capacity of (+)- and (+/-)-efaroxan to enhance the insulin secretory response to glibenclamide. However, these manoeuvres did not alter the ability of (+/-)-efaroxan to potentiate glucose-induced insulin secretion. 6. The results indicate that the enantiomers of efaroxan exert differential effects on insulin secretion which may result from binding to effector sites having opposite stereoselectivity. Binding of (-)-efaroxan (presumably to imidazoline receptors) results in potentiation of glucose-induced insulin secretion, whereas interaction of (+)-efaroxan with a second site leads to selective enhancement of sulphonylurea-induced insulin release.

Antazoline increases insulin secretion and improves glucose tolerance in rats and dogs

In vivo effects of an imidazoline devoid of alpha2-adrenoceptor antagonistic properties, antazoline, on insulin secretion and glycemia were investigated both in fasted rats and dogs. In both species, antazoline (1.5 mg/kg i.v.) transiently increased insulinemia without affecting basal plasma glucose levels. In contrast, during an i.v. glucose tolerance test, antazoline markedly potentiated insulin release and thus increased the glucose disappearance rate. In rats, during an oral glucose tolerance test, the intragastric administration of antazoline (1.5 mg/kg) clearly enhanced insulin secretion and reduced hyperglycemia. In dogs provided with a venous pancreatico-duodenal bypass, antazoline (0.5 mg/kg i.v.) induced an immediate and transient increase in insulin and somatostatin but not in glucagon pancreatico-duodenal outputs. In conclusion, intravenously and orally administered, the imidazoline antazoline is able to stimulate insulin secretion in vivo and improve glucose tolerance. The imidazoline compounds could therefore have a potential therapeutic relevance as new antihyperglycemic insulinotropic agents.

Therapeutic challenges in the obese diabetic patient with hypertension

Obesity, diabetes mellitus, and hypertension are common and interrelated medical problems in Westernized, industrialized societies. These medical conditions are associated with an increased risk of cardiovascular disease and are more prevalent among minorities, such as African-American and Hispanic populations. The associated cardiovascular risks of these problems are more thoroughly addressed in another review in this supplement. Obesity markedly enhances the development of type II diabetes. Moreover, it enhances the cardiovascular risk associated with other risk factors, such as hypertension and dyslipidemia. Weight reduction in association with an aerobic exercise program improves metabolic abnormalities and reduces blood pressure in individuals with diabetes and hypertension. Frequently, however, pharmacologic treatment is required to lower blood pressure. Individual therapy with an angiotensin-converting enzyme (ACE) inhibitor is preferred initially in these individuals, with the addition of either a low dose diuretic or a nondihydropyridine calcium antagonist if additional blood pressure reduction is required. These additive agents are recommended, since each has been shown individually to reduce cardiovascular morbidity and to preserve renal function among diabetic patients. Other issues, such as aggressive therapy of lipids and adequate glycemic control, are also important strategies for reducing cardiovascular and renal morbidity and mortality in this very high-risk population.

MB fraction of cumulative creatine kinase correlates with insulin secretion in patients with acute myocardial infarction: insulin as a possible determinant of myocardial MB creatine kinase

To test whether insulin is a regulatory factor of myocardial MB creatine kinase content, we investigated the correlation between the ability of insulin secretion and the MB fraction of cumulative CK released in patients with acute myocardial infarction. We analyzed 18 patients who underwent successful direct angioplasty within 10 hours of the onset of their first myocardial infarction. Exclusion criteria were age more than 75 years, heart failure, severe obesity, multivessel disease, and history of diabetes mellitus. Cumulative activity of serum MB CK divided by that of total CK was defined as MB%, which was considered to represent myocardial MB CK content. Two weeks or more after the onset of myocardial infarction, 75 gm oral glucose tolerance test with serial determination of plasma glucose and serum insulin (0, 0.5, 1, 2, 3 hours) was done. Urinary and plasma catecholamines and echocardiographic left ventricular (LV) mass were measured. MB% significantly correlated with insulinogenic index (r = 0.564, p = 0.019), insulin area (r = 0.594, p = 0.012), insulin area/glucose area (r = 0.630, p = 0.007), and urinary adrenaline (r = -0.542, p = 0.025) and tended to correlate with plasma adrenaline (r = -0.431, p = 0.084). Age, body mass index, infarct size, glucose metabolism, and LV mass were not significant univariate predictors of MB%. Multivariate analysis showed that the ability of insulin secretion contributed to MB% more than catecholamines did and that insulin area/glucose area was the strongest independent predictor of MB% (t = 3.01, p = 0.015). Thus MB fraction of cumulative CK released, indicative of Myocardial MB CK distribution, strongly related to the ability of insulin secretion in subjects without overt insulin resistance. Regulation by insulin of myocardial MB CK is suggested.

Diabetes and hypertension: prognostic and therapeutic considerations

Hypertension and diabetes are common disorders which frequently co-exist. Both are risk factors for atherosclerotic vascular disease and their combination is associated with an increased incidence of nephropathy, ischaemic heart disease, peripheral vascular disease, and stroke. Several trials such as the HDFP and SHEP studies that included diabetic patients have demonstrated the beneficial effects of antihypertensive therapy in reducing mortality. In diabetes, studies have focussed predominantly on the efficacy of antihypertensive therapy in reducing the progression of diabetic kidney disease. Such therapy has been shown to decrease albuminuria in the setting of "normal" and elevated blood pressure in both type I and type II diabetic patients. This reduction in albuminuria has been observed in microalbuminuric diabetic patients and also in those with overt renal disease. Recent studies in type I diabetic patients with overt nephropathy indicate that these effects on urinary albumin excretion are associated with reduction in the rate of decline in renal function and development of end-stage renal failure. Indeed, several groups have shown that the initiation of antihypertensive therapy improves the prognosis of type I diabetic patients with nephropathy. While certain classes of drugs may reduce the rate of progression of complications such as nephropathy, others have side effect profiles that are disadvantageous in patients with diabetes.

Alpha 2-adrenoceptors and the regulation of glucose, insulin and amylin levels in diabetic rats

The effects of the selective alpha 2-adrenoceptor agonist UK 14.304 on glucose, insulin and amylin levels were examined in neonatal streptozotocin-induced diabetic rats. UK 14.304 (0.03 to 0.3 mg/kg i.p.) induced a dose-dependent reduction of both insulin and amylin levels in normal rats while UK 14.304 at 0.03 mg/kg had already a maximal effect in diabetic rats. Amylin/insulin molar ratios rose after UK 14.304 administration in diabetic rats but remainded constant in normal rats. The alpha 2-adrenoceptor antagonist deriglidole (3 mg/kg i.p.) slightly increased insulin and amylin levels in the two groups of rats but glucose levels were more markedly decreased in diabetic rats. Deriglidole completely antagonized UK 14.304 (0.1 mg/kg i.p.)-induced changes thereby normalizing amylin/insulin molar ratios in diabetic rats. These results suggest that insulin and amylin are both under inhibitory control via alpha 2-adrenoceptor though the responses may be differentially regulated. It is further suggested that diabetes in the neonatal streptozotocin-induced rat model is associated with a hypersensitivity of the pancreas to alpha 2-adrenoceptor stimulation.

Drugs on the horizon for treatment of type 2 diabetes

The only new pharmaceutical therapy for Type 2 (non-insulin-dependent) diabetes that has become available for clinical use in the last 40 years is the alpha-glucosidase inhibitor, acarbose, which reduces postprandial glucose levels by retarding digestion of complex carbohydrates in the gut. It has proved difficult to find other new metabolically active drugs that lack toxicity. Agents that reduce insulin resistance include the thiazolidinediones, which are very effective in animals. Of these, the only one that has been maintained in clinical evaluation appears from preliminary data to have an effect that although still useful, is not greater than that reported for current oral agents. Agents that reduce non-esterified fatty acid levels by inhibiting lipolysis, thereby allowing increased peripheral uptake of glucose, have so far given minimal reduction in glycaemia. The development of fatty acid oxidation inhibitors to reduce gluconeogenesis in the liver has been hampered by toxicity, but additional new agents are being studied. The most promising new approach for enhancing insulin secretion has been suggested by the demonstration that pharmacological doses of GLP-1 (7-36 amide), a natural enteric incretin hormone, improves pancreatic beta-cell and alpha-cell sensitivity to glucose and can induce normal basal glucose levels in diabetic man. The future development of GLP-1 agonists will be of great interest. This is timely as other insulin secretogogues, such as alpha 2 adrenergic blockers have proved relatively ineffective. Anti-obesity agents would in theory be beneficial, but have either had limited efficacy or have been avoided because of concern about long-term safety. Until new pharmaceutical agents become available, if near-normal glycaemia is to be achieved, many more Type 2 diabetic patients will need insulin therapy. When full insulin replacement therapy is not feasible, reducing the fasting blood glucose level towards normal with a single daily basal insulin supplement, either alone or in combination with oral agents, could become a more widely used therapy.

The effects of a peripherally selective alpha 2-adrenoceptor antagonist, MK-467, on the metabolic and cardiovascular response to exercise in healthy man

1. A double-blind, placebo controlled study has been conducted to investigate the consequences of i.v. treatment with MK-467, a peripherally selective alpha 2-adrenoceptor antagonist in exercising healthy male subjects. In particular, the effects on blood pressure, heart rate, circulating catecholamines (noradrenaline (NA) and adrenaline (A)), insulin, glucose and free fatty acids (FFA) were determined. 2. Exercise produced increases in catecholamines, blood pressure and heart rate. FFA increased at the start of the exercise but then declined as exercise progressed. 3. MK-467 significantly increased NA, in a dose-dependent manner, before and during exercise without altering A. Blood pressure and heart rate were not affected by drug treatment. 4. The insulin and FFA response to exercise was significantly enhanced by MK-467 although glucose was unaltered by drug. 5. It is concluded that both pre- and post-junctional peripheral alpha 2-receptors play an important role in the metabolic response to exercise in man.

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

Stimulation of insulin secretion by imidazoline compounds is not due to interaction with non-adrenoceptor idazoxan binding sites

1. The potency of interaction of several imidazoline compounds with non-adrenoceptor idazoxan binding sites (NAIBS) in rat liver membranes was compared with their ability to alter insulin secretion from rat pancreatic islets. 2. NAIBS could be labelled specifically with [3H]-idazoxan in both rat liver membranes and in rat islet homogenates. Liver binding sites exhibited a KD for [3H]-idazoxan of 24 nM and a Bmax of 264 fmol mg-1 protein. 3. Binding of [3H]-idazoxan to NAIBS in rat liver membranes was displaced effectively by unlabelled idazoxan (IC50 0.1 microM) and by UK14304 (IC50 0.5 microM). However, two other imidazoline compounds efaroxan and RX821002, which are related in structure to idazoxan, were much less effective as displacers. 4. In insulin secretion experiments, the ATP-sensitive potassium channel agonist diazoxide (250 microM) was able to suppress the rise in insulin secretion induced by 20 mM glucose. Both efaroxan and RX821002 (100 microM) antagonized the inhibitory effect of diazoxide on glucose-induced insulin secretion. By contrast, neither idazoxan (100 microM) nor UK14304 (50 microM), was able to overcome significantly the inhibitory effect of diazoxide. 5. The ability of 100 microM efaroxan to antagonize the suppression of insulin secretion mediated by diazoxide, was not prevented by idazoxan (up to 100 microM) or by UK14304 (up to 50 microM). 6. The results indicate that the stimulatory effects of imidazoline compounds on insulin secretion are not due to interaction with NAIBS similar to those present in rat liver.

Imidazoline antagonists of alpha 2-adrenoceptors increase insulin release in vitro by inhibiting ATP-sensitive K+ channels in pancreatic beta-cells

1. Islets from normal mice were used to study the mechanisms by which imidazoline antagonists of alpha 2-adrenoceptors increase insulin release in vitro. 2. Alinidine, antazoline, phentolamine and tolazoline inhibited 86Rb efflux from islets perifused with a medium containing 3 mM glucose, i.e. under conditions where many adenosine 5'-triphosphate (ATP)-sensitive K+ channels are open in the beta-cell membrane. They also reduced the acceleration of 86Rb efflux caused by diazoxide, an opener of ATP-sensitive K+ channels. 3. ATP-sensitive and voltage-sensitive K+ currents were measured in single beta-cells by the whole-cell mode of the patch-clamp technique. Antazoline more markedly inhibited the ATP-sensitive than the voltage-sensitive current, an effect previously observed with phentolamine. Alinidine and tolazoline partially decreased the ATP-sensitive K+ current. 4. The four imidazolines reversed the inhibition of insulin release caused by diazoxide (through opening of ATP-sensitive K+ channels) or by clonidine (through activation of alpha 2-adrenoceptors) in a concentration-dependent manner. Only the former effect correlated with the ability of each drug to increase control insulin release stimulated by 15 mM glucose alone. 5. It is concluded that the ability of imidazoline antagonists of alpha 2-adrenoceptors to increase insulin release in vitro can be ascribed to their blockade of ATP-sensitive K+ channels in beta-cells rather than to their interaction with the adrenoceptor.

The effects of sympathetic nervous system activation and psychological stress on glucose metabolism and blood pressure in subjects with type 2 (non-insulin-dependent) diabetes mellitus

The sympathetic nervous system may contribute to excessive hepatic glucose output in Type 2 (non-insulin dependent) diabetes mellitus and could be implicated in the interrelated problem of hypertension. The aim of these studies was to determine whether subjects with Type 2 diabetes had normal sensitivity (compared with age- and weight-matched non-diabetic subjects) to noradrenaline infusion (60 ng.kg-1.min-1 for 60 min) and to compare the responses with oral tyramine administration (800 mg), and psychological stress (using competitive computer games). Noradrenaline infusion caused significantly greater plasma glucose (mean increment 2.1 +/- 0.4 vs 0.6 +/- 0.1 mmol/l, p less than 0.005) and pressor responses (mean systolic increment 21 +/- 3 vs 11 +/- 1 mmHg, p less than 0.02) in the diabetic subjects. The excessive glycaemia was due to increased hepatic glucose output rather than reduced glucose disposal. Tyramine administration caused significantly increased hepatic glucose output and plasma glucose levels, but with similar responses in the diabetic and non-diabetic subjects; the pulse and pressor responses were also similar between the groups. The psychological stressor induced significant increases in pulse, blood pressure and non-esterified fatty acid levels in the combined group of subjects (p less than 0.01) but did not influence plasma glucose levels in either diabetic or non-diabetic subjects. We conclude that pharmacologically-induced sympathetic nervous stimulation can induce hyperglycaemia. Subjects with uncomplicated Type 2 diabetes have increased sensitivity to exogenous noradrenaline but may not hyperrespond to endogenous sympathetic activation.

Mechanisms involved in stimulation of insulin secretion by the hypoglycaemic alpha-adrenergic antagonist, DG-5128

The selective alpha 2-antagonist DG-5128 provoked a dose-dependent stimulation of insulin release from isolated rat islets. DG-5128 was only weakly effective as an antagonist of noradrenaline-induced inhibition of insulin secretion but, surprisingly, was able to reverse the suppression of secretion and increase in 86Rb efflux from preloaded islets, mediated by diazoxide. These effects were not reproduced with more effective alpha-antagonists, suggesting that stimulation of insulin secretion by DG-5128 is independent of alpha-receptor blockade.

An insulin-releasing property of imidazoline derivatives is not limited to compounds that block alpha-adrenoceptors

As we have demonstrated previously phentolamine stimulates the release of additional insulin from isolated mouse islets and raises plasma insulin levels in the whole rat. This effect was independent of the well known property of phentolamine to block alpha-adrenoceptors. In experiments on isolated pancreatic islets from mice we now demonstrate that tolazoline and antazoline which are chemically closely related to phentolamine, share its ability to potentiate insulin release. The following results were taken as evidence that this effect does not result from an alpha-adrenoceptor blocking action of imidazoline compounds. More than 10 times higher concentrations of phentolamine were required to liberate additional insulin from isolated islets than were effective in counteracting the inhibitory effect of clonidine on insulin release. The newly introduced alpha 2-adrenoceptor antagonist BDF 8933, which is an imidazoline derivative, stimulates insulin release as well, while the irreversible alpha-adrenoceptor blocking agent benextramine of different structure failed to do so, even when being present in concentrations blocking the alpha 2-adrenoceptor-mediated effects of clonidine. Antazoline shared the ability of phentolamine to stimulate insulin release despite having no or only very little alpha-adrenoceptor blocking activity. When used under our conditions, it almost entirely failed to alleviate the inhibition of insulin release induced by clonidine. We conclude that the response of the islet cells to imidazoline derivatives is not limited to those capable of blocking alpha-adrenoceptors. On the other hand, alpha-adrenoceptor blocking agents of different chemical structure fail to induce the release of additional insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of phenoxybenzamine on insulin secretion from isolated rat islets of Langerhans

In isolated rat islets the alpha 2-adrenergic antagonist phenoxybenzamine was found to be only partially effective at relieving the inhibition of glucose-induced insulin secretion mediated by noradrenaline. Further experiments revealed a direct inhibitory effect of phenoxybenzamine itself on the secretory response to glucose. At concentrations above 1 microM the antagonist inhibited insulin secretion in a dose-dependent manner, with greater than 50% inhibition at 50 microM. The inhibition of secretion developed rapidly in perifused islets, and was not altered when islets were also incubated with idazoxan or benextramine, suggesting that it did not reflect binding of phenoxybenzamine to the alpha 2-receptor. Paradoxically phenoxybenzamine significantly increased the basal secretion rate in the presence of 4 mM glucose. The results demonstrate that phenoxybenzamine can exert direct effects on insulin secretion which are unrelated to its alpha-antagonist properties.

Phentolamine, a deceptive tool to investigate sympathetic nervous control of insulin release

We investigated the effects of phentolamine and another more selective alpha 2-adrenoceptor antagonist, rauwolscine, on insulin release in vivo (in female Wistar-rats) and in vitro (in perfused rat pancreas and in isolated perifused mouse islets). Phentolamine was found to significantly increase glucose-induced insulin release. On the other hand, rauwolscine failed to do so, when applied in a concentration that effectively antagonized the inhibitory effect of clonidine. These results demonstrate that phentolamine is capable of directly stimulating insulin release. This effect is thus not mediated by alpha-adrenoceptors. For this reason phentolamine is not an appropriate tool to study possible inhibitory effects of the sympathetic nervous system on insulin release. An enhanced insulin response as may be observed in animals and in man in the presence of phentolamine does not furnish evidence for a tonic inhibitory control of the islet cells by the sympathetic nervous system.