Abnormal glucagon secretion is often associated with diabetes mellitus. However, the mechanisms by which nutrients modulate glucagon secretion remain poorly understood. Paracrine modulation by β- or δ-cells is among the postulated mechanisms. Herein we present further evidence of the paracrine mechanism. First, to activate cellular metabolism and thus hormone secretion in response to specific secretagogues, we engineered insulinoma INS-1E cells using an adenovirus-mediated expression system. Expression of the Na+-dependent dicarboxylate transporter (NaDC)-1 resulted in 2.5- to 4.6-fold ( P < 0.01) increases in insulin secretion in response to various tricarboxylic acid cycle intermediates. Similarly, expression of glycerol kinase (GlyK) increased insulin secretion 3.8- or 4.2-fold ( P < 0.01) in response to glycerol or dihydroxyacetone, respectively. This cell engineering method was then modified, using the Cre- loxP switching system, to activate β-cells and non-β-cells separately in rat islets. NaDC-1 expression only in non-β-cells, among which α-cells are predominant, caused an increase (by 1.8-fold, P < 0.05) in glucagon secretion in response to malate or succinate. However, the increase in glucagon release was prevented when NaDC-1 was expressed in whole islets, i.e., both β-cells and non-β-cells. Similarly, an increase in glucagon release with glycerol was observed when GlyK was expressed only in non-β-cells but not when it was expressed in whole islets. Furthermore, dicarboxylates suppressed basal glucagon secretion by 30% ( P < 0.05) when NaDC-1 was expressed only in β-cells. These data demonstrate that glucagon secretion from rat α-cells depends on β-cell activation and provide insights into the coordinated mechanisms underlying hormone secretion from pancreatic islets.
Functional MR Microimaging of Pancreatic β-Cell Activation