ipamorelin mastery course
Unit 2 of 11

the GH axis and the ghrelin receptor

somatotrophs, pulses, and the slow-wave-sleep peak

three inputs, one cell, discrete pulses

Before any drug enters the story, the GH axis already has its own architecture. Somatotrophs in the anterior pituitary receive three inputs: GHRH from the hypothalamus says go, somatostatin says stop, and ghrelin says go louder. The result is not a steady GH level -- it is a discrete pulse every 3-5 hours, with the biggest one timed to the first cycle of slow-wave sleep. Every ipamorelin protocol is an attempt to hijack this rhythm.

the 24-hour view

a top-down dial of where natural GH pulses fall across the clock face.

sleep band 0 6 12 18 circadian GH sleep band 10p-6a shaded nocturnal SWS-1 surge (1:00 AM) pre-dawn second pulse (4:30 AM) fasted-morning pulse (8:00 AM) midday trough pulse (1:00 PM) late-afternoon pulse (5:30 PM) pre-sleep pulse (10:00 PM)

6 GH pulses across 24 hours. amplitude = marker size. hover any marker to name the pulse.

24-hour GH pulse timeline

explore the daily pulse rhythm and where ipamorelin doses can fit relative to natural peaks.

24-hour GH pulse timeline

key terms

tap to expand.

Ssomatotrophcell
the GH-producing cell of the anterior pituitary. about 40% of anterior-pituitary cells. expresses GHRH-R, somatostatin receptors SSTR2/5, and GHS-R1a on its surface -- so it integrates three competing inputs into a single GH-pulse decision.
GGHRHhormone
growth-hormone-releasing hormone -- 44 amino acids, made in the hypothalamic arcuate nucleus, released into the pituitary portal blood. binds GHRH-R on somatotrophs (Gs-coupled, cAMP/PKA pathway) to trigger GH transcription and exocytosis.
Ssomatostatinhormone
a 14- or 28-amino-acid inhibitory peptide also from the hypothalamus. binds SSTR2 and SSTR5 on somatotrophs (Gi-coupled, cAMP reduction) and shuts the GH pulse off. it is the rising somatostatin tone after a meal that blunts postprandial GH -- and the reason ipamorelin is dosed away from large meals.
Gghrelinhormone
a 28-residue stomach-derived peptide with an octanoyl modification on serine-3. binds GHS-R1a to drive a GH pulse and, via the arcuate nucleus, to drive appetite. ipamorelin mimics the GH-releasing arm of ghrelin without engaging the appetite arm.
SSTAT5bsignaling
a transcription factor activated by GH signaling. STAT5b reads the pulse pattern, not the average GH level -- which is why pulsatile and continuous GH produce different gene-expression programs in the liver. mutations in STAT5b cause GH-insensitivity short stature.
IIGF-1growth factor
insulin-like growth factor 1 -- produced mainly by hepatocytes in response to GH binding. IGF-1 mediates most of GH's anabolic effects on tissue and also closes the feedback loop by inhibiting GHRH and stimulating somatostatin. chronic GH-axis stimulation raises tonic IGF-1.
SSWSsleep stage
slow-wave sleep -- the deep N3 phase of non-REM sleep. it produces the largest, most reliable daily GH pulse via vagal afferent and hypothalamic mechanisms. sleep deprivation flattens this pulse and is one of the most replicated lifestyle determinants of GH-axis tone.

the somatotroph pulse story -- the simple version

three inputs, one cell, a yes/no decision every few hours.

The somatotroph is a small cell with a big job: decide, every few hours, whether to fire a GH pulse. It listens to three voices. GHRH from the hypothalamus says go -- it raises cAMP inside the cell. Somatostatin says stop -- it lowers cAMP. And ghrelin (or any GHS-R1a agonist like ipamorelin) says go through a separate channel that raises intracellular calcium directly.

The vote is dynamic. When GHRH is high and somatostatin is low, the cell fires a pulse. When somatostatin is high (after a meal, after stress), the cell stays quiet even if GHRH is present. Layering a GHS-R1a agonist on top of a low-somatostatin moment produces a much bigger pulse than GHRH alone.

The pulses themselves are stored, not made on demand. GH sits in secretory granules waiting for the calcium spike that triggers exocytosis. This is why a GH pulse is fast -- the protein is already packaged -- and why the receptor desensitizes for a few hours afterward while the granule pool is replenished.

advanced: Gq / PLC / IP3 cascade and pulse exocytosis

signaling chain

GHS-R1a is a Gq-coupled GPCR. ipamorelin binding activates phospholipase C-beta, which hydrolyzes PIP2 into DAG and IP3. IP3 then releases Ca2+ from the endoplasmic reticulum into the cytosol.

why the pulse is fast

the calcium spike triggers fusion of pre-loaded GH secretory granules with the plasma membrane. the GH protein is already packaged, so the pulse is exocytotic rather than synthetic.

numbers

Raun 1998 measured in-vitro EC50 of 1.3 +/- 0.4 nM at rat pituitary cells, with in-vivo ED50 of 80 +/- 42 nmol/kg in anesthetized rats and 2.3 +/- 0.03 nmol/kg in conscious swine.

advanced: pulse-pattern programming of liver gene expression

what the liver reads

Norstedt and Palmiter 1984 showed that the shape of the GH pulse pattern, not the time-integrated dose, programs sex-dimorphic liver gene expression. continuous GH produces feminized hepatic transcription; pulsatile GH produces masculinized transcription.

STAT5b kinetics

Waxman's lab spent the 1990s-2010s mapping the STAT5b activation kinetics that read the pulse. STAT5b integrates pulse shape and frequency, not average GH level.

why this matters for protocols

compressed-window ipamorelin dosing preserves the pulsatile signal. the CJC-1295 DAC variant, which clamps GHRH tone high for days, plausibly degrades that signal -- though no clinical-endpoint trial has confirmed it.

advanced: receptor desensitization and inter-pulse spacing

desensitization timescale

GHS-R1a undergoes agonist-induced internalization and beta-arrestin-mediated desensitization on a timescale of minutes to hours. full responsiveness recovers within roughly 3-4 hours.

dosing implication

this kinetics is the pharmacological basis for dosing ipamorelin 2-3 times daily at 4-6 hour intervals. closer spacing or continuous infusion degrades the per-pulse GH response by holding the receptor desensitized.

contrast with DAC

continuous GHRH-R stimulation (CJC-1295 DAC) does not desensitize the same way GHS-R1a does -- another reason the pulsatile vs continuous philosophy diverges between the two ligands.

the somatotroph is the converged point of every drug in this course. CJC-1295 and sermorelin hit GHRH-R on it. ipamorelin hits GHS-R1a on it. tesamorelin and MK-677 also act here. every "stack" debate is really an argument about how to layer inputs on the same cell.

where this has been studied

classical endocrinology, decades old, well-replicated.

pulsatility in humans
Hartman et al., Am J Physiol 1991, mapped the temporal structure of GH secretion across 24 hours in healthy adults using frequent sampling and deconvolution analysis. the result -- discrete pulses every 3-5 hours with a dominant SWS-1 peak -- is the canonical reference for what natural GH release looks like.
GHRH + GHS synergy
Bowers et al., JCEM 1990, demonstrated in healthy men that GHRH + GHRP-6 produced a GH pulse markedly larger than either alone -- the foundational synergy result that grounds every modern GHRH-analog + GHS-R1a-agonist stack including ipamorelin + CJC-1295.
GHS-R1a deorphanization
Howard et al., Science 1996, identified GHS-R1a as the receptor through which GHRP-class drugs release GH -- three years before Kojima and Kangawa identified its endogenous ligand (ghrelin) in Nature 1999. the receptor was characterized before the natural ligand was known.
sleep-GH coupling
multiple decades of polysomnography-coupled GH sampling consistently show the largest daily GH pulse aligning with the first SWS cycle. the coupling is robust enough that sleep deprivation is one of the most reliable ways to flatten daily GH output in healthy adults.