Endocrinology | Receptor Pathways

The video introduces two main types of hormones: peptide hormones and steroid hormones. Peptide hormones are water-soluble and require membrane receptors, while steroid hormones are lipid-soluble and can pass directly through the cell membrane to bind to intracellular receptors. This distinction is crucial for understanding how each hormone type exerts its effects on a cell.

The G stimulatory protein (Gs) pathway begins when a hormone (like epinephrine) binds to a G protein-coupled receptor, initiating a change that replaces GDP with GTP on the Gs protein. This activated Gs protein then binds to and activates adenylate cyclase, which converts ATP into cyclic AMP (cAMP). cAMP, as a second messenger, activates protein kinase A, leading to the phosphorylation of various proteins and enzymes. This phosphorylation can alter membrane permeability, regulate metabolic pathways, and control gene transcription and cell growth.

The GQ protein pathway is activated when a hormone (like oxytocin) binds to its receptor, causing GDP to be replaced by GTP on the Gq protein. The activated Gq protein then activates phospholipase C, an enzyme that cleaves PIP2 into DAG (diacylglycerol) and IP3 (inositol triphosphate). DAG activates protein kinase C, leading to phosphorylation of various proteins with effects similar to protein kinase A. IP3, on the other hand, binds to receptors on the smooth endoplasmic reticulum, causing the release of calcium ions into the cytoplasm. This increased calcium can bind to calmodulin, activating other kinases that regulate processes like muscle contraction (e.g., in the case of oxytocin).

Steroid hormones (e.g., testosterone), being lipid-soluble, can diffuse directly through the cell membrane into the cytoplasm or nucleus. Inside the cell, they bind to intracellular receptors, displacing heat shock proteins. The hormone-receptor complex then binds to specific DNA sequences called hormone response elements (HREs), leading to changes in gene expression. This can cause DNA replication, protein synthesis, and ultimately influence cell growth, metabolism, and other cellular functions.

To prevent these pathways from continuously stimulating the cell, inhibitory mechanisms are in place. Enzymes like phosphodiesterase (PDE) break down cyclic AMP, thereby turning off the G stimulatory pathway. Similar phosphodiesterases can also act on phospholipase C-related pathways to regulate their activity, ensuring that cellular responses are properly controlled and terminated.