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 withdrawal can start in the hours or days after you take your last dose. Generally, it is recommended that you gradually reduce your doses of buprenorphine over the period of 2-3 weeks before complete cessation so that the withdrawal symptoms are not as severe.