n 1  :  a muscle that on contracting is automatically checked and controlled by the opposing simultaneous contraction of another muscle - called also agonist muscle , prime mover  - compare ANTAGONISTa SYNERGIST2    2  :  a chemical substance (as a drug) capable of combining with a receptor on a cell and initiating a reaction or activity <binding of adrenergic s>  - compare ANTAGONISTb   .
 
 
 
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