Elsevier

Regulatory Peptides

Volume 118, Issue 3, 15 May 2004, Pages 143-150
Regulatory Peptides

Effects of ghrelin on insulin and glucagon secretion: a study of isolated pancreatic islets and intact mice

https://doi.org/10.1016/j.regpep.2003.12.001Get rights and content

Abstract

We combined in vitro and in vivo methods to investigate the effects of ghrelin, a novel gastric hormone, on insulin and glucagon release. Studies of isolated mouse islets showed that ghrelin concentrations in the physiological range (0.5–3 nmol l−1) had no effect on glucose-stimulated insulin release, while low ghrelin concentrations (1–100 pmol l−1) inhibited and high (0.1 and 1 μmol l−1) stimulated. The insulin response to glucose was enhanced in the presence of a high ghrelin concentration (100 nmol l−1). Glucagon release was stimulated by ghrelin (0.1 pmol l−1 to 1 μmol l−1); this effect was maintained in the presence of glucose (0–20 mmol l−1). In intact mice, basal plasma insulin was suppressed by 1 and 10 nmol kg−1 of ghrelin, 2 and 6 min after i.v. injection. Ghrelin (0.2–10 nmol kg−1 i.v.) suppressed also the glucose-stimulated insulin response and impaired the glucose tolerance (at a ghrelin dose of 3.3 nmol kg−1). Ghrelin (1 or 10 nmol kg−1 i.v.) inhibited the insulin response to the phospholipase C stimulating agent carbachol and enhanced the insulin response to the phosphodiesterase inhibitor isobutyl-methylxanthine (IBMX) but did not affect the response to the membrane-depolarizing amino acid l-arginine. These observations suggest that the inhibitory effect of ghrelin on glucose-induced insulin release is in part exerted on phospholipase C pathways (and not on Ca2+entry), while the stimulatory effect of high doses of ghrelin depends on cyclic AMP. In contrast to the spectacular glucagon-releasing effect of ghrelin in vitro, ghrelin did not raise plasma glucagon. Carbachol, IBMX and l-arginine stimulated glucagon release. These responses were impaired by ghrelin, suggesting that it suppresses the various intracellular pathways (phospholipase C, cyclic AMP and Ca2+), that are activated by the glucagon secretagogues. Together these observations highlight (but do not explain) the different effects of ghrelin on glucagon release in vitro and in vivo. The results show that ghrelin has powerful effects on islet cells, suggesting that endogenous ghrelin may contribute to the physiological control of insulin and glucagon release. However, the narrow “window” of circulating ghrelin concentrations makes this doubtful.

Introduction

There is a suspicion that the stomach harbours peptide hormones that contribute to the control of pancreatic hormone release. This suspicion is based on reports describing impaired insulin release following resection of the stomach both in man and experimental animals [1], [2], [3]. Recently, a growth hormone-releasing peptide named ghrelin was identified in extracts of the rat stomach [4] and found to be produced and stored in a population of peptide hormone-producing cells referred to as A-like cells [5], [6], [7]. Circulating concentrations of ghrelin are elevated by food deprivation and lowered by food intake [6], [8], [9]. The signals that regulate the secretion of ghrelin from the A-like cells have not yet been identified. Whether the A-like cells and ghrelin participate in the control of islet hormone secretion remains an unresolved issue although some reports have appeared, suggesting that ghrelin either stimulates [10], [11] or inhibits [12], [13], [14] insulin secretion. There have been claims that insulin (and/or glucose) lowers circulating ghrelin [15], [16], [17], [18], [19] (for a different view, see Ref. [20]). There are surprisingly few data on the effect of ghrelin on glucagon release; however, it was recently reported that ghrelin is without effect on glucagon secretion in the perfused rat pancreas [13].

The purpose of the present study was to examine the effects of ghrelin on insulin and glucagon release from freshly isolated pancreatic islets from mice and to further investigate effects of ghrelin on insulin and glucagon secretion in intact, freely moving mice using different blood sampling times and different doses with or without administration of glucose and various insulin and glucagon secretagogues.

Section snippets

Drugs and chemicals

Rat ghrelin-28 was a kind gift from Professors N. Yanaihara and C. Yanaihara at the Yanaihara Institute, Shizuoka, Japan. Collagenase (CLS-4) from Worthington Biochemicals (Freehold, NJ, USA) was used to prepare the pancreatic islets. Bovine serum albumin (BSA) was from ICN Biomedical (High Wycombe, UK). All other chemicals were from British Drug Houses (Poole, UK) or Merck (Darmstadt, Germany). Radioimmunoassay kits for determination of insulin were obtained from Diagnostika (Falkenberg,

Insulin and glucagon secretion in response to varying concentrations of ghrelin at a fixed glucose concentration

Fig. 1 illustrates the concentration–response curves for the effect of ghrelin on insulin and glucagon secretion from isolated islets in the presence of a glucose concentration (12 mmol l−1) that in itself has a modest stimulating effect on insulin secretion. Insulin release was suppressed in the presence of low concentrations of ghrelin (1–100 pmol l−1), while concentrations of 0.1 and 1 μmol l−1 stimulated insulin secretion. However, within the range of circulating ghrelin concentrations (0.5

Discussion

In an earlier report [3], we suggested that the stomach harbours a peptide hormone that serves to enhance the glucose-stimulated insulin response and potentiate the glucose-induced suppression of glucagon secretion. This suggestion is in line with the fact that the oxyntic mucosa of the stomach is rich in peptide hormone-producing endocrine cells, such as ECL cells and A-like cells [29], [30]. The putative peptide hormone of the ECL cells is unknown, whereas ghrelin has been identified as the

Acknowledgements

The study was supported by grants from the Swedish Research Council (grants 4286 and 04x-1007), the Albert Påhlsson foundation, the Crafoord foundation, the Golje foundation, the Magnus Bergvall foundation, the Novo Nordisk foundation and the Medical Faculty of Lund.

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