In the Navy, medical corps is 2105 for USNR and 2100 for USN. HPSP I think is 2105 USNR and I think everyone gets augmented to 2100 USN upon graduation (used to be just USUHS grads were automatically augmented to USN).
Don't know if it's different for AF or Army. I thought either AF or Army med students were medical service corps or something until graduating into the medical corps, but I should probably stop speculating about that as I don't know what I'm talking about.