Uptake of fractionated [3H]heparin by rat parenchymal hepatocytes in primary culture: effects of alpha-globulin, temperature, and pH.

The mechanism of uptake of fractionated [3H]heparin in conjunction with the effect of alpha-globulin (the major binding protein of heparin) was investigated in primary cultures of rat parenchymal hepatocytes. The uptake clearances were estimated from the initial linear phase, up to 1 min, of the uptake-versus-time profile. The uptake clearance for fractionated [3H]heparin was 11.0 mL/min/g of liver at 7.5 nM fractionated [3H]heparin in the absence of alpha-globulin. This value decreased with increasing concentration of alpha-globulin in the incubation solution, a fact suggesting that the rate of uptake of alpha-globulin-bound, fractionated [3H]heparin is lower than that of unbound [3H]heparin, as generally assumed for hepatic drug elimination. However, the uptake clearance in the presence of alpha-globulin at 8 mg/mL, where free, fractionated [3H]heparin was supposed to be negligible according to the results of our in vitro study, was approximately 12% of that in the absence of alpha-globulin. These results suggest that alpha-globulin-bound, fractionated [3H]heparin also contributes to the uptake of fractionated [3H]heparin. This effect on uptake could be explained by the protein-mediated transport concept rather than the traditional assumption that protein-bound drugs are not transported. The uptake clearance was reduced significantly by reducing the incubation temperature from 37 to 4 degrees C. However, neither the pH of the incubation solution (6.4-8.4) nor several inhibitors of active transport had any clear effects on the uptake clearance.