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 and naloxone, the active ingredients in Suboxone, can be detected, but only if tested specifically. Neither component will appear as an opiate on standard testing for opiates. However, buprenorphine may be detected up to 4 days after ingestion.