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.
How long buprenorphine withdrawal lasts varies depending on how frequently you used the drug and how large the doses were. If you were a heavy user, burprenorphine withdrawal may be more intense and last longer than someone using buprenorphine for a few months.