Insulin participates in the control of the energy feeling and, in particular, the feeling of glucose. It is the only hypoglycemic hormone. Its structure has been remarkably conserved during evolution. Its production and its secretion by the β cells of the endocrine pancreas are very controlled. The transcription of the insulin gene is under the control of many factors which provided in trans on the promoter region. Glucose is the most important regulator, but hormones (glucagon-like peptide 1, growth hormone, leptin, prolactin, etc.) are also capable of modulating the expression of the insulin gene. The final hormone is first synthesized in the form of a high molecular weight protein, pro-insulin, which is stored in microvesicles where its conversion into insulin begins. The release of insulin requires the initiation of the process of exocytosis of the secretory vesicles. The control of hormone secretion calls upon an elementary regulation loop which links the concentration of nutrients, primarily glucose, to the secretion of insulin. Modulating, hormonal or nervous agents are grafted onto this loop to reduce or amplify this secretion. Glucose is the most potent stimulant of insulin secretion and conditions the action of all other stimuli. Its increase in the β cell has generated cofactors including in particular triphosphoric adenosine, at the origin of membrane electrical phenomena and ionic movements resulting in the massive entry of calcium into the cell and the stimulation of exocytosis. The factors that modulate the secretory response to glucose essentially concern G or Gi proteins (modulation of the concentration of cyclic monophosphoric adenosine) or Gq proteins (activation of phospholipase C). The adaptation of the coordinated functioning of β cells to variations in glycemic balance is a fine example of the specialization of a micro-organ for the optimal short- and long-term control of energy homeostasis.
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