Uptake of Fractionated [3H]Heparin by Rat Parenchymal Hepatocytes in Primary Culture: Effects of α‐Globulin, Temperature, and pH

Abstract

AbstractThe mechanism of uptake of fractionated [3H]heparin in conjunction with the effect of α‐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 7.5nM fractionated [3H]heparin in the absence of α‐globulin. This value decreased with increasing concentration of α‐globulin in the incubation solution, a fact suggesting that the rate of uptake of α‐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 α‐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 ∼12% of that in the absence of α‐globulin. These results suggest that α‐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°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.