"Please, if I send you an email labeled as urgent, I expect you to treat it urgently and respond/reply (to it) as soon as you could."
Instead of "could", you could also use "can" or "as soon as possible" or "as soon as you're able to". I think "could" sounds a bit awkward.
"You may stop working on whatever you're working on and reply to/respond to/do it right away. Then you can resume your previous assignment after replying/responding (to it)."
You could make them two separate sentences or join them "...right away, and then..."
"If you could rephrase it to sound more professional, I'll be indebted to you."
Are you a boss, employer, manager, or in some other position of authority?