The biguanide compound metformin prevents desensitization of human pancreatic islets induced by high glucose

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Abstract

Pancreatic islet desensitization by high glucose concentrations is a temporary and reversible state of beta-cell refractoriness to glucose (and possibly other secretagogues), due to repeated or prolonged pre-exposure to increased glucose concentrations. We evaluated whether the oral antidiabetic agent metformin affects this phenomenon in isolated, human pancreatic islets, and whether the possible effects of the biguanide are influenced by the presence of a sulphonylurea, glyburide. Islets prepared from five human pancreases were incubated for 24 h in M199 culture medium containing either 5.5 or 22.2 mmol/l glucose, with or without a therapeutic concentration (2.4 μg/ml) of metformin. Then, the islets were challenged with either 3.3 mmol/l glucose, 16.7 mmol/l glucose, or 3.3 mmol/l glucose+10 mmol/l arginine, and insulin release was measured. After incubation in the absence of metformin, the human islets exposed to 22.2 mmol/l glucose showed no significant increase in insulin release when challenged with 16.7 mmol/l glucose (confirming that hyperglycemia desensitizes pancreatic beta-cells). In the presence of metformin, the islets fully maintained the ability to significantly increase their insulin release in response to glucose, even when previously exposed to 22.2 mmol/l glucose. No major effect on arginine-induced insulin release was observed, whatever the culture conditions. The protective action of metformin was observed also when glyburide was present in the incubation medium, whereas the sulphonylurea alone did not affect insulin release from the islets previously exposed to high glucose concentrations. These in vitro results suggest that metformin can prevent the desensitization of human pancreatic islets induced by prolonged exposure to increased glucose concentrations.

Introduction

The harmful consequences of high glucose concentrations on pancreatic beta-cells have been recently differentiated into glucose toxicity and glucose desensitization (Robertson et al., 1994). Glucose toxicity has been defined as a condition of irreversible damage of the beta-cells, that occurs after chronic exposure to high glucose concentrations, and is possibly due to the deleterious effects of the hexose on insulin gene transcription and/or expression (Robertson et al., 1994). The concept of glucose desensitization implies a temporary, readily induced and reversible condition of refractoriness of the beta-cell to glucose that follows repeated or prolonged exposure to hyperglycemia. Glucose desensitization has been observed in several experimental models (Weir et al., 1986; Purrello et al., 1989; Rossetti et al., 1990; Davalli et al., 1991; Eizirik et al., 1992), and seems to be due to defects at the level of insulin exocytosis mechanisms or insulin granule stores (Robertson et al., 1994). Much less information is available about the effects of high glucose concentrations on islet sensitivity to secretagogues other than glucose. In a previous paper it was suggested that insulin release in response to arginine is not changed after preculture with increased glucose levels (Davalli et al., 1991).

Glucose toxicity and glucose desensitization may adversely affect residual insulin secretion in diabetic patients as well, triggering a vicious cycle in which hyperglycemia is a consequence and also a cause of the altered pancreatic beta-cell function (Unger and Grundy, 1985). Indeed, a reduction of blood glucose levels by any means improves beta-cell sensitivity to glucose (Shah et al., 1989). Unfortunately, even when adequate therapy is provided, most diabetic patients experience periods of mild to severe hyperglycemia, and this could further damage the residual islet insulin secretion.

Metformin (dimethylbiguanide) is an oral antidiabetic drug that can be used, alone or in combination, to lower blood glucose in patients with non-insulin dependent diabetes (Marchetti and Navalesi, 1989; Bailey, 1996; Scheen and Lefebvre, 1998). The therapeutic action of the drug is attributed mainly to its effects at the hepatocyte level, with decreased glucose production, and the muscle cell level, with enhanced glucose uptake. Metformin is also able to affect the function of many other cell types, including pancreatic beta-cells (Gregorio et al., 1989; Marchetti et al., 1996a). Indeed, although this issue is still a matter of debate (Bailey, 1992; Hermann et al., 1994), under certain conditions the drug can influence insulin release from the perfused rat pancreas (Gregorio et al., 1989) and isolated human islets (Marchetti et al., 1996a). In view of these latter findings, we considered it of interest to evaluate whether metformin can affect the phenomenon of islet desensitization by high glucose concentrations. To address this issue, we prepared purified human islets and incubated them for 24 h with various glucose concentrations, with or without a therapeutic concentration of metformin. At the end of the incubation period, islet insulin release in response to glucose and arginine was measured. In addition, since in the treatment of non-insulin dependent diabetes metformin can be used in combination with a sulphonylurea, we also evaluated whether the possible effect of the biguanide could be affected by the presence of glyburide in the incubation medium.

Section snippets

Preparation of the islets

The procedures for the preparation of the islets were initially developed for the pancreas of large mammals (Marchetti et al., 1995, Marchetti et al., 1996b), and then applied, with minor modifications, to the human pancreatic gland. In the present study we used the islets from the pancreases of five human cadaver donors (three males, two female, aged 18 to 48 years), obtained through the local organ procurement organization, AIRT, with permission of our local Ethics Committee.

The enzyme

Results

Insulin secretion in response to 3.3 and 16.7 mmol/l glucose from the islets cultured for 24 h with the various glucose concentrations, with or without metformin, is detailed in Table 1 (pmol/islet/45 min) and Table 2 (percent of insulin content). The release of insulin was significantly higher with 16.7 mmol/l glucose than with 3.3 mmol/l glucose islets precultured with 5.5 mmol/l glucose, either with or without metformin, and from the islets precultured with 22.2 mmol/l glucose and metformin,

Discussion

This study confirms that human islets exposed to high glucose loose the ability to increase their insulin release at increased glucose concentrations, i.e., they become glucose desensitized (Robertson et al., 1994). In keeping with a previous report (Davalli et al., 1991), we also found that the human islets cultured with 22.2 mmol/l glucose did not change their sensitivity to arginine stimulation significantly, even though a trend to a decrease in insulin secretion in response to 3.3 mmol/l+10

Acknowledgements

This work was supported by grants from the Italian National Research Council, Ministero Università e Ricerca Scientifica e Tecnologica, and Regione Toscana.

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