Question

(B) Insulin is a peptide hormone synthesised and secreted from pancreatic islets. Describe the control of nutrient-mediated insulin release by gut hormones, neurotransmitters and paracrine factors.

Answer 1

• Insulin is a peptide hormone synthesised by the beta cells of the islets of langerhans in the pancreas.The insulin plays an important role in regulation of the glucose level in the blood within favorable limits.The Insulin secretion is governed by the interaction of nutrients, hormones, and the autonomic nervous system.
• Glucose and other sugars stimulates the release of insulin. The glucose concentration plays a major role in the regulation of the insulin secretion.
• The insulin secretion can be controlled only by the sugars that are metabolized by the islets of Langerhans
• The glucose transport into the beta cell through the GLUT2 glucose transporter and phosphorylation of glucose by means of glucokinase play an important role in the regulation of insulin secretion.The defects in these steps causes various metabolic disorders like Diabetes.
• Cyclic-AMP is involved in stimulating insulin release.
• Translocation of calcium ions into the cytoplasm plays a key role in the contractile forces that propel insulin to the cell surface. Other ions like potassium and magnesium, are also involved in the insulin secretion.
• Amino acids,Free fatty acids and ketone bodies also stimulate insulin release.
• Gut factors (incretins) also helps in regulating the insulin secretion.
  • gastrointestinal polypeptide (GIP), pancreatic glucagon and the glucagon-like peptides (GLP) play a major role in stimulating insulin release.
  • Somatostatin, produced in the delta cells of islets, inhibits insulin and glucagon release and reduces splanchnic blood flow.
  • Growth hormone is involved in insulin synthesis and storage.
  • Glucocorticoids and estrogens increases insulin secretion and also induces the peripheral insulin resistance
• Insulin secretion is constantly modulated by the autonomic nervous system.
  • The parasympathetic arm (through the vagus nerve) directly stimulates insulin release.
  • Sympathetic arm : Activation of β2 receptors  stimulates insulin secretion by cyclic AMP generation.Activation of α-adrenergic receptors decreases insulin secretion.
  • Epinephrine and norepinephrine stimulate predominantly α-adrenergic receptors in islets, resulting in impaired insulin secretion