Propanolol (and other beta blockers) … blocks a type of` receptor found on cells call beta adrenergic receptors. In the heart, beta blockers work to help with rate control.
Cocaine activates beta and alpha adrenergic receptors. Activating alpha adrenergic receptors can cause vasoconstriction.

Combining them, you get unopposed alpha activity, leading to coronary artery constriction, and a significant increase in risk of heart attack.

edited to change “ion channel” to “receptor”