THE EFFECT OF CALCIUM-CHANNEL BLOCKERS AND CALCIUM ON METHOTREXATE ACCUMULATION IN RAT HEPATOCYTES

The effects of diltiazem (DIL), verapamil (VRP) and Ca2+ on the accumu lation of methotrexate (MTX) were investigated in isolated rat hepatoc ytes. At the physiological 2 mM Ca2+, the calcium-channel blockers DIL (100 mu M) and VRP (50 mu M) significantly reduced the hepatocellular accumulation of MTX: By increasing the Ca concentration to 7 mM contr ol MTX levels(at 2 mM Ca2+) were restored with VRP, and resulted in MT X levels above the controls for DIL. Ca2+ at 7 mM significantly enchan ced MTX accumulation in the hepatocyte suspensions after 60 min. The c oncentration time curves for MTX indicated that for the first 10 min i nflux was the dominating process. Dixon plot analysis of this uptake p hase revealed Ki values of 140 mu M for DIL and 75 mu M for VRP. The d ata suggested that DIL was a non-competitive, and VRP a competitive in hibitor of MTX influx Hence, the inhibitory effect on MTX accumulation mediated by DIL and VRP could be due to different mechanisms.