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.
After all, in humans, buprenorphine is an opioid that is commonly used to wean people off addictions to other opioids, such as heroin. Fortunately, there are not a lot of cats who are smack addicts, and the dosages for cats are far smaller and prescribed with great care by certified veterinarians.