This occurs when a substance passes from the fluid in the tubular lumen through the apical membrane, across the cytosol , and then into the interstitial fluid.
Yes, it's potent stuff. Cat receive it either as an injection or orally. It takes effect in roughly half an hour, lasting about 8 hours before another administration is necessary. Cats usually react negatively to opiates, but buprenorphine is an exception. That's one reason vets use it if they can receive the necessary special certification.
Buprenorphine is typically provided to a cat in very small doses and dependent upon the animal's weight, overall health, age, and the specific source of pain that it is designed to treat. It can either be given as an oral medicine, which is generally preferred, or as a direct injection to the site of the pain.