Does sexual maturity alter the clearance of creatine kinase from female chicken plasma?

Previous studies in this laboratory have demonstrated an apparent myoprotective effect of estrogen in female chickens both at normal sexual maturity and following administration of exogenous steroid. This action is characterized by a marked decrease in the plasma activity of the muscle enzyme creatine kinase (CK) although the precise mechanism remains to be elucidated. It has been suggested that alterations in the elimination of the enzyme from the circulation may contribute to this phenomenon. The present study examined this hypothesis. Two groups of female chickens of an Institute-bred brown leghorn line (J-line) were used. One group was designated immature (16-18 weeks of age) and the other mature (26-30 weeks of age) and were in lay. Birds were anaesthetized with urethane and a cannula was inserted in to the brachial vein and baseline blood samples were obtained. Birds from each age group received a 0.5ml bolus of a semi-purified preparation of skeletal muscle CK of known enzyme activity. Control birds received an equal volume of the Tris-EDTA muscle homogenization buffer. Serial blood samples were taken over a total period of 6 hours and plasma activity of CK was determined by spectrophotometry. Sexual maturity and associated elevated circulating estrogen was confirmed by measurement of plasma concentrations of triglyceride (TG), zinc (Zn) and calcium (Ca). Use of an appropriate model allowed calculation of total volume of distribution (Vd) of CK, the half-life of elimination (t½) and the plasma clearance (CCK). Sexually mature females had reduced plasma CK activity and elevated plasma TG, Zn and Ca compared to immature birds. The Vd for CK was 75ml in immature and 88ml in mature birds. The corresponding t½ values were 233 and 252 minutes with CK values of 11ml/kg/hr and 10ml/kg/hr. There were no significant differences in these parameters between groups. It is concluded that estrogen induced decreases in plasma CK activity are primarily a consequence of reduced release from muscle tissue and not from alterations in elimination or clearance.