Abstract
The secretion of glucagon by pancreatic alpha cells is regulated by a number of external and intrinsic factors. While the electrophysiological processes linking a lowering of glucose concentrations to an increased glucagon release are well characterized, the evidence for the identity and function of the glucose sensor is still incomplete. In the present study we aimed to address two unsolved problems: (1) do individual alpha cells have the intrinsic capability to regulate glucagon secretion by glucose, and (2) is glucokinase the alpha cell glucose sensor in this scenario. Single cell RT-PCR was used to confirm that glucokinase is the main glucose-phosphorylating enzyme expressed in rat pancreatic alpha cells. Modulation of glucokinase activity by pharmacological activators and inhibitors led to a lowering or an increase of the glucose threshold of glucagon release from single alpha cells, measured by TIRF microscopy, respectively. Knockdown of glucokinase expression resulted in a loss of glucose control of glucagon secretion. Taken together this study provides evidence for a crucial role of glucokinase in intrinsic glucose regulation of glucagon release in rat alpha cells.
Highlights
The secretion of glucagon by pancreatic alpha cells is regulated by a number of external and intrinsic factors
We have addressed this question by manipulation of glucokinase activity and expression in purified single alpha cells combined with measurements of glucagon release by total internal reflection fluorescence (TIRF) microscopy
To test which hexokinase isoforms are expressed in rat pancreatic alpha cells, we analysed the expression of all four hexokinases in single islet cells
Summary
The secretion of glucagon by pancreatic alpha cells is regulated by a number of external and intrinsic factors. It has been demonstrated that hypoglycaemia stimulates glucagon secretion by a mechanism involving a decrease of the cytoplasmic ATP/ADP ratio, a slight activation of ATP-sensitive potassium channels (KATP-channels), an increased activity of P/Q-type C a2+-channels, resulting in a C a2+-influx[9,10]. These findings demonstrate the importance of the electrophysiological mechanisms that link glucose metabolism to electrical activity and hormone secretion. We have addressed this question by manipulation of glucokinase activity and expression in purified single alpha cells combined with measurements of glucagon release by total internal reflection fluorescence (TIRF) microscopy
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