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.
Do not self-inject this medicine if you do not fully understand how to give the injection and properly dispose of used needles, syringes, IV tubing, and other items used to inject the medicine. Buprenorphine is usually given at evenly spaced intervals, up to 6 hours apart.