Hypothalamic Peptide Regulators
| Peptide | Effect on GH | Mechanism of Action |
|---|---|---|
| Growth Hormone Releasing Hormone (GHRH) | Stimulates | - Primary stimulator of GH release. - Binds G-protein-coupled receptor on somatotropes. - Stimulates adenylate cyclase, increases intracellular cAMP. - Upregulates somatotrope proliferation. |
| Somatostatin (SRIF) | Inhibits | - Primary inhibitor of GH release. - Inhibits adenylate cyclase activity. - Reduces intracellular calcium concentrations. - Regulates timing and amplitude of pulsatile secretion. |
| Ghrelin | Stimulates | - Endogenous ligand for GH secretagogue receptor (GHS-R). - Secreted predominantly by stomach enteroendocrine cells. - Functions synergistically with GHRH. - Requires octanoylation for normal function. |
Endocrine and Feedback Regulators
Feedback Inhibition
- Insulin-Like Growth Factor-1 (IGF-1):
- Exerts long-loop1 negative feedback.
- Inhibits spontaneous GH secretion.
- Growth Hormone (GH):
- Exerts short-loop2 negative feedback.
- Acts directly on hypothalamic GH receptors.
- Activates periventricular somatostatin neurons.
Hormonal Modulators
| Hormone | Effect on GH | Clinical Context / Mechanism |
|---|---|---|
| Sex Steroids (Estrogens/Androgens) | Stimulate | - Cause rise in GH secretion characteristic of puberty. - Increase GH pulse amplitude. - Estrogen mediates increased GH production. |
| Thyroid Hormone | Stimulates (Permissive) | - Prerequisite for normal GH synthesis. - Hypothyroidism blunts spontaneous and provocative GH secretion. |
| Glucocorticoids | Inhibit | - Excess blunts spontaneous and provocative GH secretion. - Inhibit GH release. |
Neurotransmitter and Neuropeptide Regulation
- Multiple neurotransmitters modulate release of GHRH and somatostatin.
Inputs to Hypothalamic Neurons
| Neurotransmitter/Input | Target Neuron | Resultant Effect on GH Secretion |
|---|---|---|
| Galanin | GHRH Neuron (Stimulates) | Stimulates GH |
| Gamma-Aminobutyric Acid (GABA) | GHRH Neuron (Stimulates) | Stimulates GH |
| Alpha2-Adrenergic stimuli | GHRH Neuron (Stimulates) | Stimulates GH |
| Dopaminergic inputs | GHRH Neuron (Stimulates) | Stimulates GH |
| Beta2-Adrenergic stimuli | Somatostatin Neuron (Stimulates) | Inhibits GH |
| Corticotropin-Releasing Hormone (CRH) | Somatostatin Neuron (Stimulates) | Inhibits GH |
| Muscarinic Acetylcholine (ACh) | Somatostatin Neuron (Inhibits) | Stimulates GH |
| 5-HT-1D receptor ligands | Somatostatin Neuron (Inhibits) | Stimulates GH |
- Additional modulators altering GH secretion include serotonin, histamine, vasoactive intestinal peptide (VIP), gastrin, neurotensin, substance P, calcitonin, neuropeptide Y (NPY), and vasopressin.
Metabolic and Physiological Factors
Physiological States
| State / Factor | Effect on GH | Underlying Mechanism |
|---|---|---|
| Sleep | Stimulates | - Maximal secretion during first slow-wave sleep (stages III/IV). |
| Rapid-Eye-Movement (REM) Sleep | Inhibits | - Associated with low GH secretion. |
| Exercise / Physical Stress | Stimulates | - Physiologic stress response triggers GH release. |
| Trauma / Acute Illness | Stimulates | - Acute stress elevates GH as counterregulatory defense. |
Metabolic and Nutritional Factors
| Factor | Effect on GH | Mechanism / Clinical Context |
|---|---|---|
| Hypoglycemia | Stimulates | - GH acts as delayed counterregulatory hormone. - Mobilizes stored fuels, reduces glucose utilization. |
| Fasting | Stimulates | - Increases number and amplitude of GH secretory bursts. - Mediated by decreased somatostatin secretion. |
| Malnutrition / Undernutrition | Stimulates | - Causes functional GH insensitivity. - Increased GH concentrations secondary to decreased IGF -1 negative feedback. |
| Hyperglycemia | Inhibits | - Suppresses GH release. |
| Free Fatty Acids (FFAs) | Inhibit | - Exert negative feedback control at pituitary level. |
| Obesity | Inhibits | - Decreases GH secretion. - Reduces number of secretory bursts and pulse amplitude. - Blunts peak responses during stimulation testing. |
Pharmacological Stimulators (Provocative Testing)
- Evaluation of GH deficiency requires pharmacologic provocation due to pulsatile nature of GH.
- Agents identified as GH secretagogues rapidly increase GH levels.
Agents Used for Provocative Testing
- Insulin: Induces acute hypoglycemia; triggers counterregulatory GH surge.
- Arginine HCl: Intravenous infusion stimulates GH.
- Ornithine HCl: Intravenous infusion stimulates GH.
- Clonidine: Alpha2-adrenergic agonist; stimulates GHRH release.
- Levodopa (L-Dopa): Dopaminergic agonist; stimulates GHRH release.
- Glucagon: Intramuscular or subcutaneous administration stimulates GH.
- Macimorelin: Oral ghrelin agonist; stimulates GH secretion at hypothalamic and pituitary levels.
- GHRH: Direct intravenous bolus stimulates pituitary somatotropes.
Footnotes
-
This is the most common form of regulation. It occurs when the final hormone produced by a target gland travels through the bloodstream back to the “top” of the chain to inhibit further production. ↩
-
This loop is “shorter” because it involves the middle step of the hierarchy signaling the first step. ↩