Skeletal Muscle

Activin Receptor Type IIB

The receptor myostatin uses to put the brakes on muscle growth. Every myostatin inhibitor works by keeping the ligand off this receptor rather than by acting on it directly.

Androgen Receptor

The receptor every anabolic-androgenic steroid is designed to hit. Binding moves it into the nucleus where it switches on genes for muscle protein synthesis, and in other tissues for hair loss, prostate growth and oil production.

Glucocorticoid Receptor

The cortisol receptor. Some androgens bind it too, and whether they act as agonist or blocker at a given tissue is a large part of why compounds with similar AR affinity feel so different.

Growth Hormone Receptor

Where growth hormone itself acts. Binding pairs the receptor and fires JAK2/STAT5, which in the liver means IGF-1 output and in fat means lipolysis. Injected GH works here; secretagogues work one step upstream.

IGF-1 Receptor

The receptor that carries almost all of growth hormone's anabolic effect, one step downstream of it. It is also the reason the IGF-1 analogs are the peptides with the most serious theoretical cancer question: the same signal that grows muscle grows anything already growing.

Glucose Uptake

Moving glucose out of the blood and into muscle. It can happen through insulin or, independently of it, through the AMPK pathway that exercise uses.

Muscle Protein Synthesis

The rate at which muscle is built. Androgen receptor activation raises it and holds it raised, which is the entire point of the exercise.