Growth hormone (GH) secretion shows pulsatile nocturnal peaks. The primary hypothalamic peptides regulating GH secretion are GHRH (stimulatory) and somatostatin (inhibitory). Ghrelin, an endogenous GH secretagogue, acts primarily through which receptor and at which site?
- A IGF-1 receptor on pituitary somatotrophs, bypassing hypothalamic regulation
- B Somatostatin receptor subtypes 2 and 5 (SSTR2/5) on pituitary somatotrophs, causing paradoxical stimulation
- C Dopamine D2 receptor on hypothalamic neurons, reducing somatostatin release
- D GHS-R1a receptor on hypothalamic neurons (stimulating GHRH release) and directly on pituitary somatotrophs ✓
Explanation
Ghrelin, an octanoylated 28-amino-acid peptide secreted predominantly by gastric X/A-like cells, binds the growth hormone secretagogue receptor type 1a (GHS-R1a). It acts at both hypothalamic and pituitary levels: in the hypothalamus it stimulates GHRH release and suppresses somatostatin, while at the pituitary it directly stimulates somatotrophs via Gq-mediated Ca²⁺ signalling. Ghrelin also stimulates appetite through arcuate nucleus AgRP/NPY neurons — linking energy sensing to GH secretion. Peak ghrelin levels occur during fasting, reflecting GH's metabolic role in mobilising fatty acids.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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