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.
Trying to estimate exactly how long buprenorphine is detectable in the body depends on many variables, including which kind drug test is being used. Buprenorphine - also known as Suboxone, Subutex, Buprenex, Temgesic, Bupe - can be detected for a shorter time with some tests, but can be visible for up to three months in other tests.