Transport mechanism of some naturally occurring tetracyclines across everted rat gut.

Naturally occurring tetracycline antibiotics were tested for active transport in a standard everted rat gut preparation. The ratio of drug concentrations between serosal and mucosal solutions was measured as well as the rate of transport at different mucosal concentrations. Although a ratio of near unity was demonstrated by all the antibiotics, indicating passive transport, the rate of transport was linear only at low concentrations. A mechanism of more than simple diffusion was suggested. A carrier with limited capacity present in the intestinal tissue might be responsible for the transport of these antibiotics.