because the donor heart is believed to be surgically denervated , the peak heart rate response to exercise is dependent primarily on 1 ) an increase in the circulating levels of the catecholamines norepinephrine and epinephrine at peak exercise , and 2 ) the end-organ responsiveness of the sinoatrial ( sa ) node to beta-adrenergic stimulation .
C0028351
norepinephrine
C0014563
epinephrine