Biliary secretion of methotrexate in rats and its inhibition by probenecid.

The biliary secretion of methotrexate was investigated in rats under steady-state conditions. The transport system involved was saturable and displayed Michaelis-Menten-type kinetics. Values for the maximal rate of transport and a transport constant analogous to the Michaelis constant were 12 mg/hr and 32 mg/liter (7 x 10-5 M), respectively. The inhibition of this transport mechanism by probenecid also was investigated, and the relationship between the plasma concentration of probenecid and the biliary clearance of methotrexate was elucidated. The value of Ki, the dissociation constant for the transport carrier-inhibitor complex, was 23 mug/ml (8 x10-5 M).