Cell signaling and metabolism – Which protein signaling molecule primarily increases cellular glucose uptake, and on what types of cells are its receptors found?

Introduction:
Insulin is a peptide hormone central to glucose homeostasis. It promotes glucose uptake and storage and coordinates carbohydrate, lipid, and protein metabolism. This question examines recognition of insulin's role and the breadth of its receptor distribution across tissues that utilize glucose.

Given Data / Assumptions:

• Insulin is secreted by pancreatic beta cells in response to rising blood glucose.
• The insulin receptor is a receptor tyrosine kinase.
• Many tissues respond to insulin via increased glucose uptake or metabolic reprogramming.

Concept / Approach:
Insulin binds its receptor on numerous target cells, triggering tyrosine kinase signaling and downstream pathways (e.g., PI3K–Akt). In skeletal muscle and adipose tissue, insulin promotes translocation of GLUT4 to the plasma membrane, increasing glucose uptake. The liver, while not using GLUT4, still responds to insulin by enhancing glycogenesis and glycolysis and suppressing gluconeogenesis. Therefore, insulin receptors are broadly expressed on many glucose-utilizing cells, not restricted to beta cells.

Step-by-Step Solution:

Verification / Alternative check:
Physiological responses to insulin—postprandial glucose disposal in muscle/adipose and hepatic glycogen synthesis—demonstrate widespread receptor-mediated effects across tissues.

Why Other Options Are Wrong:

• Only beta cells: beta cells secrete insulin; they are not the principal uptake target.
• PDGF in blood: PDGF is a growth factor, not a primary regulator of glucose uptake.
• NGF in reflex neurons: neurotrophin, unrelated to systemic glucose uptake.
• Glucagon exclusively hepatocytes/adipocytes: glucagon raises blood glucose; it is not the main uptake hormone.

Common Pitfalls:
Assuming insulin only acts on classic GLUT4 tissues; while uptake effects are tissue-specific, insulin signaling is broadly influential.

Final Answer:
Insulin; many different cell types that use glucose for fuel