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.
Urine tests are the most commonly used, especially by employers. Buprenorphine will become detectable in the urine just 40 minutes after consumption, and for long-term, heavy users, the drug will show up in these tests for up to 2 weeks later.