agonist

noun3

1. •
   A molecule that can combine with a receptor on a cell to produce a physiological reaction.

   • Acetylcholine is an agonist at the cholinergic receptor.
2. •
   The muscle that contracts while the other relaxes.

   • When bending the elbow, the biceps is the agonist.
3. •
   Someone involved in a contest or battle (as in an agon), protagonist.