Endocrine

Which hormone classes use intracellular (nuclear/cytosolic) receptors rather than cell-surface receptors?

Lipophilic hormones — steroid hormones (cortisol, aldosterone, sex steroids), thyroid hormone, and vitamin D. They diffuse across the membrane and bind intracellular receptors that act as transcription factors, giving a slow, sustained genomic response.

Which second messenger is increased by Gs-coupled hormone receptors, and decreased by Gi-coupled receptors?

cAMP. Gs activates adenylyl cyclase (raising cAMP → activates protein kinase A); Gi inhibits adenylyl cyclase (lowering cAMP). Examples of Gs: glucagon, ACTH, FSH, LH, TSH, PTH, calcitonin, GHRH, ADH (V2).

Which signaling pathway is used by Gq-coupled hormone receptors?

Gq activates phospholipase C → cleaves PIP2 into IP3 and DAG. IP3 raises intracellular Ca2+; DAG activates protein kinase C. Examples: GnRH, TRH, histamine (H1), angiotensin II, ADH (V1), oxytocin, α1-adrenergic, M1/M3. (GHRH is Gs-coupled, not Gq.)

Which hormones signal through receptor tyrosine kinases or JAK/STAT rather than G proteins?

Insulin, IGF-1, and most growth factors use intrinsic receptor tyrosine kinases (MAP kinase / PI3K pathways). Prolactin, GH, leptin, erythropoietin, and most cytokines use JAK/STAT (non-receptor tyrosine kinase) pathways. Atrial natriuretic peptide uses guanylyl cyclase (cGMP).

In a negative feedback axis, how does a peripheral hormone usually regulate its own secretion?

The peripheral hormone (e.g., cortisol, thyroid hormone) inhibits both the hypothalamic releasing hormone and the anterior pituitary trophic hormone, reducing further stimulation. This keeps hormone levels within a set range and underlies the use of paired hormone levels to localize disease.