Glucose-lowering effect of BTS 67 582

A model of controlled acute hyperglycemia in rats: Effects of insulin and glucagon-like peptide-1 analog

A rodent model of controlled acute hyperglycemia that is sensitive to glucose-lowering agents insulin and glucagon-like peptide-1 (GLP-1) analog has been developed. The studies show that anesthesia could be induced in fasted rats with ketamine (100 mg/kg) plus a low dose of xylazine (5 mg/kg) without inducing the acute hyperglycemia typically associated with these agents. Under these conditions, continuous infusion of glucose (10 and 20%) via the jugular vein for 30 to 150 min induced hyperglycemia in a time-dependent fashion. Administration of "loading" boluses of glucose (0.2-0.6 ml of a 20% solution) prior to continuous infusion of 10% glucose produced more immediate and sustained hyperglycemia. Plasma levels of a variety of glucoregulatory and stress hormones such as insulin, growth hormone, glucagon, and corticosterone were determined. Only glucagon levels changed significantly during induction and maintenance of hyperglycemia. The infusion of insulin (0.1 U/kg/h) or GLP-1 analog (10 microg/kg/h) effectively lowered blood glucose from its elevated levels. Insulin produced a significant increase in glucagon levels, and GLP-1 analog produced a significant increase in insulin levels without any change in other glucoregulatory and stress hormone levels. In conclusion, the present studies identified a novel approach for the induction of anesthesia and surgical manipulations without inducing hyperglycemia and further defined an approach for producing acute hyperglycemia in a controlled fashion in rodents. This model will be beneficial to study the influence of hyperglycemia in acute models of critical illness where hyperglycemia develops following the precipitating event. This model was responsive to insulin and GLP-1 analog, both of which were effective in ameliorating hyperglycemia.

Essential role of the imidazoline moiety in the insulinotropic effect but not the KATP channel-blocking effect of imidazolines; a comparison of the effects of efaroxan and its imidazole analogue, KU14R

AIMS/HYPOTHESIS

Imidazolines are a class of investigational antidiabetic drugs. It is still unclear whether the imidazoline ring is decisive for insulinotropic characteristics.

MATERIALS AND METHODS

We studied the imidazoline efaroxan and its imidazole analogue, KU14R, which is currently classified as an imidazoline antagonist. The effects of both on stimulus secretion-coupling in normal mouse islets and beta cells were compared by measuring KATP channel activity, plasma membrane potential, cytosolic calcium concentration ([Ca2+]c) and dynamic insulin secretion.

RESULTS

In the presence of 10 mmol/l but not of 5 mmol/l glucose, efaroxan (100 micromol/l) strongly enhanced insulin secretion by freshly isolated perifused islets, whereas KU14R (30, 100 or 300 micromol/l) was ineffective at both glucose concentrations. Surprisingly, the insulinotropic effect of efaroxan was not antagonised by KU14R. KATP channels were blocked by efaroxan (IC50 8.8 micromol/l, Hill slope -1.1) and by KU14R (IC50 31.9 micromol/l, Hill slope -1.5). Neither the KATP channel-blocking effect nor the depolarising effect of efaroxan was antagonised by KU14R. Rather, both compounds strongly depolarised the beta cell membrane potential and induced action potential spiking. However, KU14R was clearly less efficient than efaroxan in raising [Ca2+]c in single beta cells and whole islets at 5 mmol/l glucose. The increase in [Ca2+]c induced by 10 mmol/l glucose was affected neither by efaroxan nor by KU14R. Again, KU14R did not antagonise the effects of efaroxan.

CONCLUSIONS/INTERPRETATION

The presence of an imidazole instead of an imidazoline ring leads to virtually complete loss of the insulinotropic effect in spite of a preserved ability to block KATP channels. The imidazole compound is less efficient in raising [Ca2+]c; in particular, it lacks the ability of the imidazoline to potentiate the enhancing effect of energy metabolism on Ca2+-induced insulin secretion.

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.

Desensitization of insulin secretion

Desensitization of insulin secretion describes a reversible state of decreased secretory responsiveness of the pancreatic beta-cell, induced by a prolonged exposure to a multitude of stimuli. These include the main physiological stimulator, glucose, but also other nutrients like free fatty acids and practically all pharmacological stimulators acting by depolarization and Ca2+ influx into the beta-cell. Desensitization of insulin secretion appears to be an important step in the manifestation of type 2 diabetes and in the secondary failure of oral antidiabetic treatment. In this commentary, the basic concepts and the controversial issues in the field will be outlined. With regard to glucose-induced desensitization, two fundamentally opposing concepts have emerged. The first is that desensitization is the consequence of functional changes in the beta-cell that impair glucose-recognition. The second is that long-term increased secretory activity leads to a depletion of releasable insulin, often in spite of increased insulin synthesis. The latter concept is more appropriately termed beta-cell exhaustion. The same dichotomy applies to the desensitization evoked by pharmacological stimuli: again the relative contributions of a decreased insulin content versus alterations in signal transduction are in dispute. The action of tolbutamide on beta-cells may be an example of desensitization caused by a lack of releasable insulin since the signaling mechanisms are nearly unchanged, whereas the action of phentolamine, an imidazoline, induces a strong desensitization without reducing insulin content or secretory granules, apparently by abolishing Ca2+ influx. With pharmacological agents it seems that both, alterations in signal transduction and decreased availability of releasable insulin, can contribute to the desensitized state of the beta-cell, the relative contribution being variable depending upon the exact nature of the secretory stimulus.

Pathophysiology and pharmacological treatment of insulin resistance

Diabetes mellitus type 2 is a world-wide growing health problem affecting more than 150 million people at the beginning of the new millennium. It is believed that this number will double in the next 25 yr. The pathophysiological hallmarks of type 2 diabetes mellitus consist of insulin resistance, pancreatic beta-cell dysfunction, and increased endogenous glucose production. To reduce the marked increase of cardiovascular mortality of type 2 diabetic subjects, optimal treatment aims at normalization of body weight, glycemia, blood pressure, and lipidemia. This review focuses on the pathophysiology and molecular pathogenesis of insulin resistance and on the capability of antihyperglycemic pharmacological agents to treat insulin resistance, i.e., a-glucosidase inhibitors, biguanides, thiazolidinediones, sulfonylureas, and insulin. Finally, a rational treatment approach is proposed based on the dynamic pathophysiological abnormalities of this highly heterogeneous and progressive disease.

Novel sulfonylurea and non-sulfonylurea drugs to promote the secretion of insulin

The onset of type 2 diabetes is characterized by two determining factors: the insufficient ability to secrete insulin and/or the resistance to its biological action. Although in a very small proportion of individuals, one of those two metabolic abnormalities is the leading cause of diabetes, in most subjects, the coexistence of both appears to be necessary for the clinical manifestation of diabetes. Current biomedical research continues to clarify the relative contributions of these defects to the pathogenesis of type 2 diabetes, and novel pharmacological agents are specifically designed to correct either the impaired insulin secretory activity or the resistance to the action of insulin. The aim of this article is to provide a critical review of new sulfonylurea and non-sulfonylurea drugs that have been recently introduced for the treatment of diabetes, as well as drugs that are still under investigation and are likely to be made available in the near future.

Induced desensitization of the insulinotropic effects of antidiabetic drugs, BTS 67 582 and tolbutamide

Acute and chronic mechanisms of action of novel insulinotropic antidiabetic drug, BTS 67 582 (1, 1-dimethyl-2-(2-morpholinophenyl)guanidine fumarate), were examined in the stable cultured BRIN-BD11 cell line. BTS 67 582 (100 - 400 microM) stimulated a concentration-dependent increase (P<0.01) in insulin release at both non-stimulatory (1.1 mM) and stimulatory (8. 4 mM) glucose. Long-term exposure (3 - 18 h) to 100 microM BTS 67 582 in culture time-dependently decreased subsequent responsiveness to acute challenge with 200 microM BTS 67 582 or 200 microM tolbutamide at 12 - 18 h (P<0.001). Similarly 3 - 18 h culture with the sulphonylurea, tolbutamide (100 microM), also effectively suppressed subsequent insulinotropic responses to both BTS 67 582 and tolbutamide. Culture with 100 microM BTS 67 582 or 100 microM tolbutamide did not affect basal insulin secretion, cellular insulin content, or cell viability and exerted no influence on the secretory responsiveness to 200 microM of the imidazoline, efaroxan. While 18 h BTS 67 582 culture did not affect the insulin-releasing actions (P<0.001) of 16.7 mM glucose, 10 mM arginine, 30 mM KCl, 25 microM forskolin or 10 nM phorbol-12-myristate 13-acetate (PMA), significant inhibition (P<0.001) of the insulinotropic effects of 10 mM 2-ketoisocaproic acid (KIC) and 10 mM alanine were observed. These data suggest that BTS 67 582 shares a common signalling pathway to sulphonylurea but not imidazoline drugs. Desensitization of drug action may provide an important approach to dissect sites of action of novel and established insulinotropic antidiabetic agents.

New agents for Type 2 diabetes

Current agents for the treatment of Type 2 diabetes mellitus improve the metabolic profile but do not reinstate normality. They also reduce chronic diabetic complications, but they do not eliminate them. Thus, new agents with novel actions are required to complement and extend the capabilities of existing treatments. Insulin resistance and beta-cell failure, which are crucial components in the pathogenesis of Type 2 diabetes, remain the underlying targets for new drugs. Recently introduced agents include a short-acting non-sulphonylurea insulin-releaser, repaglinide, which synchronizes insulin secretion with meal digestion in order to reduce post-prandial hyperglycaemia. The thiazolidinedione drugs, troglitazone, rosiglitazone and pioglitazone represent a new class of agonists for the nuclear receptor peroxisome proliferator-activated receptor-gamma (PPARgamma). PPARgamma increases the transcription of certain insulin-sensitive genes, thereby improving insulin sensitivity. The intestinal lipase inhibitor orlistat and the satiety-inducer sibutramine are new weight-reducing agents that may benefit glycaemic control in obese Type 2 diabetes patients. Several further new insulin-releasing agents, and agents to retard carbohydrate digestion and modify lipid metabolism stand poised to enter the market. The extent to which they will benefit glycaemic control remains to be seen. However, the prospect of permanently arresting or reversing the progressive deterioration of Type 2 diabetes continues to evade therapeutic capture.

Biphasic insulin-releasing effect of BTS 67 582 in rats

BTS 67 582 (1,1-dimethyl-2(2-morpholinophenyl)guanidine fumarate) is being developed as a short-acting anti-diabetic insulin secretagogue. The effect of BTS 67 582 on the phasic pattern of insulin release was assessed in anaesthetized normal rats by measuring arterial plasma insulin concentrations while arterial glucose concentrations were fixed at 6, 8.5 and 12.5 mM. Intravenous BTS 67 582 (10 mg kg(-1)) induced an immediate but transient increase in insulin concentrations which declined by 10 min (first phase). This was followed by a smaller but sustained increase in insulin concentrations (second phase). The increment from basal to peak insulin release (0-2 min) was independent of glucose, but the first phase was maintained for longer and the second phase was greater at the highest concentration of glucose (12.5 mM). BTS 67 582 also extended the first-phase insulin response to a standard intravenous glucose challenge and enhanced the rate of glucose disappearance by approximately 12%. Thus BTS 67 582 causes biphasic stimulation of insulin release and augments the insulin-releasing effect of glucose.