Pharmacokinetics of Heparin V: In Vivo and In Vitro Factors Affecting the Relationship Between Concentration and Anticoagulant Effect of Heparin in Rat Plasma

Abstract

□ There are appreciable interindividual variations in rats of baseline activated partial thromboplastin time (APTT) and of the anticoagulant effect of heparin added to plasma (as reflected by the slope of the regression line describing the essentially linear relationship between In APTT and heparin concentration). Determination of baseline APTT and slope value on two occasions, 7 days apart, in the same rats revealed that (unlike in humans) these characteristics were subject also to considerable intraindividual variation. To explore the possible reasons for the observed variability, the effect of citrate concentration (acid citrate solution is used as a blood anticoagulant in the collection of plasma), calcium concentration (in the recalcifying solution used to initiate coagulation), and plasma incubation time (for activating the coagulation system) was determined. All three variables had pronounced effects on the anticoagulant response to heparin. Since rat erythrocytes are almost totally impermeable to citrate, hematocrit is a determinant of plasma citrate concentration when acid citrate solution is added in constant proportion to rat blood. Accordingly, inter- and intraindividual differences in baseline APTT and slope values were measured in another experiment in which the citrate solution to plasma (rather than blood) volume ratio was held constant and blood samples were obtained 30 days apart to permit the return of hematocrit values to normal. Intraindividual variation of the coagulation characteristics was appreciably decreased under these conditions. There are important differences between rats and humans with respect to the effect of citrate concentration and plasma incubation time on baseline APTT and on the anticoagulant action of heparin, as well as with respect to the relationship between these two characteristics.