On administration of an agent,heart rate increases because of a direct effect on the heart.However,total peripheral resistance decreases dramatically.As a consequence,systolic blood pressure increases only slightly,and mean arterial pressure typically falls.This agent is

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This response is characteristic of a pure b-adrenergic agonist,of which isoproterenol is an example.The primary actions are stimulation of the heart and relaxation of the vessels supplying the skeletal muscles.This leads to tachycardia and decreased peripheral resistance.Epinephrine is a mixed a- and b-adrenergic agonist.This balance leads to little change or an increase in mean arterial pressure.Norepinephrine and phenylephrine are á-adrenergic agonists,which constrict blood vessels and lead to a clear rise in mean arterial pressure.

On Administration of an Agent,heart Rate Increases Because of a Direct