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.
The FDA's approval was for patients who have already stabilized their dependency by taking 8 mg or less per day of buprenorphine, but doctors can, and do, often prescribe medications for off-label uses, which in this case would include new users of buprenorphine.