Isoproternol only affects B1 and B2. Through B1, it increases heart rate and through B2 it vasodilates. The vasodilation cause drop in blood pressure.
Compare with norepinephrine which affects alpha 1,2 and beta 1 so it vasoconstricts via alpha1 and there's a reflex decrease in heart rate because of that.
I know you took this from FA and that little paragraph underneath sums it up.