http://www.wgrz.com/news/local/first-responders-find-alternative-for-costly-epipens/305259463
The results shown in the figure are what happens at a low dose of epi (0.001 mg/min). At lower circulating concentrations of epi, the major effect on vessels is dilation of arterioles (mainly in skeletal muscle) mediated by activation of beta 2 receptors. This is also the major response of circulating epi in the fight or flight response.
An epi pen produces higher circulating concentrations of epi (dose is 0.3 mg) which are sufficient to activate alpha-1 receptors, and increase TPR. If norepi were injected, alpha-1 activation would occur but norepi would not activate beta-2 receptors in the lungs to dilate the airways as an epi pen will do.
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