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.
Starting 36 hours after the last does of methadone, the oral drug buprenorphine can be used to reduce symptoms of withdrawal. Initial dose is 2-4 mg of buprenorphine. If that is tolerated, a second 2-4 mg does is given an hour later, and then 4 mg 6-8 hours later, for a total day 1 dose of 8-12 mg.