any agent that causes a narrowing of an opening of a blood vessel: cold or stress or nicotine or epinephrine or norepinephrine or angiotensin or vasopressin or certain drugs; maintains or increases blood pressure | the semantic role of the animate entity that instigates or causes the happening denoted by the verb in the clause |
| an active and efficient cause; capable of producing a certain effect |
| a businessman who buys or sells for another in exchange for a commission |
| a representative who acts on behalf of other persons or organizations |
| any agent or representative of a federal agency or bureau |
| a substance that exerts some force or effect |