Cellular Signaling

From receptor to response

Most peptides discussed on this site act as messages, not as raw material. A peptide binds a receptor on the surface of a cell, and the receptor changes shape. That change gets relayed inward by a chain of enzymes and small signaling molecules until the cell does something different. It might build more muscle protein or release a hormone. Over hours to days, the signal can also change which genes the cell reads.

This page covers the middle of that story, from the receptor inward. The step before it, how peptides attach to their receptors, has its own page, and the mechanisms of action hub maps the rest of the category.

The relay usually runs in stages:

• Receptor activation. Binding shifts the receptor toward an active shape. Structural biologists who study G protein-coupled receptors describe the coupling as loose: an agonist makes the active shape more likely without locking the receptor into it (Weis and Kobilka, 2018). That is one reason two compounds acting on the same receptor can produce responses of different strength.
• Second messengers and kinases. The active receptor switches on enzymes inside the cell. Some make small "second messenger" molecules such as cyclic AMP. Others are kinases, enzymes that attach phosphate groups to other proteins and turn them on or off. Kinases often activate further kinases, so one occupied receptor can affect many molecules downstream.
• Gene and protein changes. The last links in the chain change what the cell makes. Fast effects act on proteins that are already there, such as the machinery that builds new muscle protein. Slower effects come from transcription factors that move into the nucleus and change gene expression.

Two receptor families that come up often

Two receptor types cover most of the peptides lifters ask about.

G protein-coupled receptors (GPCRs) sit in the cell membrane. When a ligand activates one, it hands the signal to a G protein, which splits into subunits that carry it onward. According to the same 2018 review, GPCRs handle most of a cell's responses to outside signals. The ghrelin receptor that growth hormone secretagogues act on is a GPCR, and so is the GLP-1 receptor. The growth hormone pathway page follows what happens after secretagogues act on the pituitary.

Receptor tyrosine kinases have the kinase built into the receptor itself. When a growth factor binds, the receptor's kinase domains switch on, tag each other with phosphate, and recruit signaling proteins (Lemmon and Schlessinger, 2010). The IGF-1 receptor belongs to this family. The same review notes that much is still unknown about the networks downstream of these receptors.

Different compounds, same pathway

Signaling pathways are shared machinery. A cell has a limited set of kinase cascades, and many different receptors feed into them. Compounds with nothing in common chemically can end up switching on the same pathway.

The mTOR pathway is the clearest example for lifters. In people, a bout of hard muscle contractions switches on mTORC1 signaling, and growth factor signaling through the kinase Akt feeds into the same complex. A 2009 study in human volunteers tested whether the pathway is needed. Giving rapamycin, a drug that blocks mTORC1, before exercise prevented the rise of about 40% in muscle protein synthesis normally seen 1-2 hours afterward (Drummond and colleagues, 2009).

The growth hormone axis works the same way, just with more steps. Secretagogues and GHRH analogs raise growth hormone, and growth hormone raises IGF-1. A compound that acts on the pituitary therefore reaches the IGF-1 receptor cascade indirectly. Direct IGF-1 analogs such as IGF-1 LR3 skip those steps. This site rates the evidence for IGF-1 LR3 as preliminary: it comes from cell and animal work, and as of October 2026 we found no published human pharmacokinetic study of the analog.

AMPK shows the same pattern from the energy side. AMPK is the cell's fuel gauge. It switches on when ATP runs low and slows growth processes, partly by blocking mTORC1 (Gwinn and colleagues, 2008, in cell studies). Anti-doping rules target AMPK as a pathway rather than as a list of molecules. As of October 2026, WADA's 2026 Prohibited List bans AMPK activators as a class under S4.4.1. Its examples range from the small molecules BAM15 and AICAR to the mitochondrial peptide MOTS-c.

MOTS-c is a good case study in how thin peptide evidence can be. In the 2015 paper that described it, MOTS-c acted mainly on skeletal muscle in mice and activated AMPK indirectly, by inhibiting the folate cycle and the purine synthesis tied to it. Treated mice were protected from diet-induced obesity and from age-related insulin resistance (Lee and colleagues, 2015). The evidence for MOTS-c remains almost entirely preclinical as of October 2026.

Shared pathways have practical consequences. Two compounds that reach the same pathway may add to each other, or one may make the other redundant. A pathway fed by many inputs is also hard to push in only one tissue. Community stacking guides often justify combinations by saying each compound "hits a different pathway," but compounds that look different on paper frequently meet at the same few kinases.

The pathway pages in this section

• The IGF-1 signaling cascade follows the route inward from the IGF-1 receptor. It also covers where growth hormone peptides and direct IGF-1 analogs enter that route.
• The mTOR pathway covers the protein synthesis switch that both mechanical loading and growth factor signals converge on.
• AMPK activation covers the cell's energy sensor and the peptides claimed to switch it on.

AMPK and mTOR tend to pull in opposite directions, so the last two pages are best read together.

Why cell and rodent signaling data don't settle human questions

Most signaling claims for research peptides come from cell culture or rodents. That evidence can show a mechanism is possible. It can't show that the same thing happens in a person at the amounts people actually use, or that it changes anything a lifter would notice.

Several gaps sit between a signaling result and a human outcome:

• A phosphorylated protein isn't a bigger muscle. Many studies measure a marker such as phosphorylated Akt or S6K1 in a dish or a biopsy. Those markers move within minutes to hours. Muscle growth takes weeks, and a pathway can light up without changing the end result.
• Concentration and access. Cells in culture can be exposed to concentrations that an injection may never produce inside muscle tissue. Without human pharmacokinetic data, as with IGF-1 LR3, nobody knows how the two compare.
• Rodents aren't small humans. In a 2001 study across several rodent models, blocking mTOR with rapamycin stopped load-induced muscle growth, and genetically switching on the Akt/mTOR pathway was enough to cause growth (Bodine and colleagues, 2001). That work established why the pathway matters. It didn't show that a compound nudging the pathway in a healthy adult would add muscle.

Human studies support the caution. In a 15-week study, 12 young men trained each arm on separate days. One arm trained under conditions that kept circulating anabolic hormones at baseline. The other trained right before a large leg workout that sharply raised those hormones. Muscle size grew about 12% in the low-hormone arm and 10% in the high-hormone arm. The increases in size and strength didn't differ between the two (West and colleagues, 2010).

Growth hormone itself shows the same gap. A 2008 systematic review of randomized trials in healthy, fit young adults found that growth hormone raised lean body mass by about 2.1 kg. Strength and exercise capacity didn't appear to improve, and soft tissue swelling and fatigue were more common (Liu and colleagues, 2008). Lean mass measurements include water, so some of that increase may be fluid.

What we still don't know

• For most research peptides, nobody has measured downstream signaling in human muscle after the amounts people report using. The mechanism claims rest on cell and rodent work.
• Whether shared pathways add up when people stack compounds is untested. Two compounds that each activate mTOR or AMPK in a dish may do nothing extra together in a trained person, or may add side effects.
• Whether MOTS-c triggers the AMPK signaling seen in mice when it is injected into people has not been shown in a published trial as of October 2026.