A transplanted heart does not respond to atropine
This is because atropine affects the vagus nerve (parasympathetic innervation of the heart), not the heart itself.
Since a transplanted heart is not connected to the recipient’s nerves, atropine will have limited or no effect on the donor heart. Neither will vagal stimulation like Valsalva maneuvre or carotid massage
Eventually however, there may be some regeneration of nervous innervation to the new heart; mostly sympathetic, but also parasympathetic nerves. In that case, atropine might work.
But the treatment of choice in hemodynamically significant bradycardia in heart recipients, is pacing (according to ACLS guidelines).