• Notes
• Kenhub

• The location and proportion of each major hormone-secreting cell type in the islets of Langerhans are:

1. Alpha cells:

   • Make up about 20% of the islet cells.
   • Secrete glucagon and are typically found near the periphery of the islet.

2. Beta cells:

   • Make up 60-75% of the islet cells and are typically located near the center of the islet.
   • These cells synthesize preproinsulin, which is cleaved to form proinsulin. This, in turn, splits into insulin and C peptide. Both are secreted in equal amounts.

3. Delta cells:

   • Make up about 5% of the islet cells and are dispersed between the alpha and beta cells.
   • They secrete somatostatin.

   

   Insulin.

   Synthesis.

   The insulin gene directs the synthesis of preproinsulin in the ribosomes. Preproinsulin consists of four peptides in sequence: the signal peptide, the B chain of insulin, a connecting peptide (C Peptide), and the A chain of insulin. As preproinsulin enters the endoplasmic reticulum, the signal peptide is split and the remaining molecule forms proinsulin.

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   • During its packaging by the Golgi apparatus, proinsulin is converted into insulin and C-peptide inside the granules. These are secreted in equal quantities. While C-peptide is not known to have a specific function, it serves as an indicator of endogenous insulin secretion. This is particularly useful when externally administered insulin interferes with the measurement of endogenous insulin. Both insulin and C-peptide levels increase with insulinoma and sulfonylurea use, whereas exogenous insulin lacks C-peptide.

   

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Functions.

Binds insulin receptors (tyrosine kinase activity), inducing glucose uptake (carrier-mediated transport) into insulin-dependent tissue and gene transcription:

o ↑ glucose transport in skeletal muscle and adipose tissue. o ↑ glycogen synthesis and storage. o ↑ triglyceride synthesis. o ↑ Na retention (kidneys). o ↑ protein synthesis (muscles). o ↑ cellular uptake of K and amino acids.

o ↓ glucagon release. o ↓ lipolysis in adipose tissue.

Notes

▪ Insulin pumps potassium into cells. Despite the process not being well understood, insulin increases the activity of Na/K-ATPase in most body tissues. ▪ This potassium-lowering action of insulin is used to treat acute, life-threatening hyperkalemia. For instance, hyperkalemia of renal failure can sometimes be lowered by the simultaneous administration of insulin and glucose. The glucose is given to prevent the development of severe insulin-induced hypoglycemia.

Glucose transporters.

• Glucose serves as the primary energy source for all cellular functions. It is typically transported from areas of high concentration outside the cell to areas of lower concentration inside the cell. However, glucose can't significantly diffuse across the cell membrane without assistance. This is where carrier proteins come into play.