Long-term exposure of mouse pancreatic islets to oleate or palmitate results in reduced glucose-induced somatostatin and oversecretion of glucagon

Abstract

Aims/hypothesis: Long-term exposure to NEFAs leads to inhibition of glucose-induced insulin secretion. We tested whether the release of somatostatin and glucagon, the two other major islet hormones, is also affected.

Methods: Mouse pancreatic islets were cultured for 72 h at 4.5 or 15 mmol/l glucose with or without 0.5 mmol/l oleate or palmitate. The release of glucagon and somatostatin during subsequent 1 h incubations at 1 or 20 mmol/l glucose as well as the islet content of the two hormones were determined. Lipid-induced changes in islet cell ultrastructure were assessed by electron microscopy.

Results: Culture at 15 mmol/l glucose increased islet glucagon content by approximately 50% relative to that observed following culture at 4.5 mmol/l glucose. Inclusion of oleate or palmitate reduced islet glucagon content by 25% (at 4.5 mmol/l glucose) to 50% (at 15 mmol/l glucose). Long-term exposure to the NEFA increased glucagon secretion at 1 mmol/l glucose by 50% (when islets had been cultured at 15 mmol/l glucose) to 100% (with 4.5 mmol/l glucose in the culture medium) and abolished the inhibitory effect of 20 mmol/l glucose on glucagon secretion. Somatostatin content was unaffected by glucose and lipids, but glucose-induced somatostatin secretion was reduced by approximately 50% following long-term exposure to either of the NEFA, regardless of whether the culture medium contained 4.5 or 15 mmol/l glucose. Ultrastructural evidence of lipid deposition was seen in <10% of non-beta cells but in >80% of the beta cells.

Conclusions/interpretation: Long-term exposure to high glucose and/or NEFA affects the release of somatostatin and glucagon. The effects on glucagon secretion are very pronounced and in type 2 diabetes in vivo may aggravate the hyperglycaemic effects due to lack of insulin.

Fig. 1

Glucagon (a) and somatostatin (c) content in islets treated with low (4.5 mmol/l) or high (15 mmol/l) glucose, in the presence or absence of palmitate (P) or oleate (O). Data are presented as mean ± SEM of four experiments. *p < 0.05 for control islets at 15 mmol/l glucose vs all other groups. Glucagon (b) and somatostatin (d) hormone secretion in islets treated as indicated (a, c). Secretion was measured at 1 (black) and 20 mmol/l (grey) glucose during 1 h incubation following the 72 h culture at the indicated conditions. Data are presented as mean ± SEM of four experiments. *p < 0.05 (at least) for comparisons with secretion measured at 1 mmol/l glucose in the respective groups; ^† p < 0.05 (at least) for comparison with secretion measured at 1 mmol/l glucose in islets cultured at 4.5 mmol/l glucose; ^‡ p < 0.05 (at least) for comparison of secretion measured in the presence of 20 mmol/l glucose in islets cultured for 72 h at 4.5 mmol/l glucose; ^§ p < 0.05 (at least) for comparison of secretion measured in the presence of 20 mmol/l glucose in islets cultured for 72 h at 15 mmol/l glucose

Fig. 2

Electron micrograph through mouse islets incubated for 72 h in 15 mmol/l glucose alone (a) and in 15 mmol/l glucose with 0.5 mmol/l palmitate (b, c) or 0.5 mmol/l oleate (d). Alpha, beta and delta cells are indicated. The presence of crescent-like structures in palmitate-treated cells (b, c) and oleate-induced lipid droplets are indicated by white arrows. The cell borders are indicated schematically by the dashed white lines. Micrographs and data presented in the text are representative of four to seven islets obtained from two different mice. Scale bar, 1 μm