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.
1 A dose of 12-16 mg is reached by day 2 or 3 and should suppress most symptoms of withdrawal from methadone, with additional symptoms treated by clonidine.  Once the patientâs withdrawal symptoms are stabilized, buprenorphine can be continued as a long-term maintenance drug, or can be tapered off over 2 to 3 weeks.