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.
Sometimes the long half life of buprenorphine makes it possible to delay doses or to switch to alternate day dosing. And with an average half-life of approximately 35 hours, 48 hour and 72 hour dosing is theoretically possible.