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.
[PDF] BupSR-LAB_InfoSheet_2014.pdf. ZooPharm can provide sustained release Buprenorphine SR-LAB by prescription in a patented, sustained release system. Buprenorphine SR-LAB releases over 72 hours and provides blood levels greater than 1.0 nanogram/ml in rodents for post operative analgesia.