Receptors. Epinephrine binds strongly to beta receptors which causes the heart rate to increase and heart to contract more forcefully. It also has Alpha receptor activity which causes blood vessels to contract which increases the blood pressure but norEpinephrine has a stronger effect on Alpha so it is used to increase blood pressure. Epinephrine would used in cases of heart failure to increase output.
1 extra carbon atom. Epinephrine has 1 more carbon & 2 more hydrogen atoms then norepinephrine. This small difference makes the molecules look different, so they interact differently with receptors, and have different effects. One example: norEpinephrine narrows blood vessels, increasing blood pressure; Epinephrine can narrow some arteries while dilating others, and can raise or lower blood pressure.