The rate of elimination and distribution volume of rabbit muscle creatine phosphokinase.

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Abstract
An in vivo study on the elimination of rabbit muscle creatinine phosphokinase (CPK) following i.v. administration was carried out with 9 rabbits selected for low endogenous CPK fluctuation character. A 2-compartment open model is proposed for the disposition of CPK. The equation Cp = Ae-.alpha.t + Be-.beta.t was used to fit the profile of CPK activity in the plasma. The pharmacokinetic parameters of CPK in rabbits were determined at low, medium and high dose levels, and the factors which affect the estimation of pharmacokinetic parameters are discussed. The elimination of CPK appeared to be independent of the injected dose, but the intersubject variability was significant. The pharmacokinetic parameters .alpha. and .beta. at high injected dose (860 U/kg body wt, n = 9) were estimated to be 0.432 .+-. 0.274 and 0.099 .+-. 0.031 h-1, respectively. This value for .beta. in vivo is significantly larger than the inactivation rate constant of CPK in vitro. There must be some other mechanism involved in vivo in addition to simple inactivation in the circulation by the body temperature. The distribution volume of injected CPK in the steady state was estimated to be .apprx. 4.3 .+-. 1.1% of body wt. Injected CPK apparently is distributed into a major compartment (plasma) and a minor one (assumed to be interstitium).