agonist

n. muscle whose activity is balanced by another related muscle (Anatomy); competitor for a prize (as in ancient Greece)

For other meanings of 'agonist' or 'agonism', see agonism (disambiguation). In pharmacology an agonist is a substance that binds to a specific receptor and triggers a response in the cell. It mimics the action of an endogenous ligand (such as hormone or neurotransmitter) that binds to the same receptor.
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Agonist /ag·o·nist/ 1. one involved in a struggle or competition. 2. agonistic muscle. 3. in pharmacology, a drug that has an affinity for and [more]Agonist - Community and Resources

Noun
1. the principal character in a work of fiction
(synonym) protagonist
(hypernym) fictional character, fictitious character, character
(hyponym) antihero
2. someone involved in a contest or battle (as in an agon)
(antonym) adversary, antagonist, opponent, opposer, resister
(hypernym) contestant
3. a muscle that contracts while another relaxes; "when bending the elbow the biceps are the agonist"
(hypernym) antagonistic muscle
4. (biochemistry) a drug that can combine with a receptor on a cell to produce a physiological reaction
(hypernym) drug
(classification) biochemistry

(n.)
One who contends for the prize in public games.