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 delayed onset, even when given IV, that needs to be taken into account for best effect. Given IV, peak effect occurs in about 30 minutes. Given IM, peak effect occurs in 45 to 60 minutes[xii].