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 has a slow onset, mild effect, and is very long acting with a half-life of 24 to 60 hours. Once a patient has stabilized on the drug, there are two treatment options; continual use/maintenance, or medically supervised withdrawal from the physical dependence to the drug.