Plasma Creatine Kinase and Aspartate Aminotransferase Activities Before, During, and After 5 Days of Intramuscular Penicillin Administration in Sedentary Horses

Muscle damage can result in leakage of intracellular enzymes such as creatine kinase (CK) and aspartate transaminase (AST) into plasma. There are no controlled documentations of the effects of intramuscular antibiotic drug administration on plasma CK and AST activities in horses. The objective of this experiment was to test the hypothesis that 5 days of intramuscular procaine penicillin G injection in normal horses would result in increased plasma activities of CK and AST. Nine healthy adult horses were sampled for 7 days preceding, 5 days during, and 32 days following procaine penicillin G (22,000 IU/kg) administration intramuscularly twice daily. Heparinized jugular venous blood samples were obtained daily before treatment and were analyzed the same day for plasma activities of CK and AST. Repeated measures ANOVA and post hoc Tukey's Test were used to identify days where CK or AST were elevated compared to control means at a significance level of P < .05. Beginning the day after first injection, plasma CK increased above the reference range, peaking at 2,046 ± 627 U/L after 3 days, and returned to 227 ± 57.3 U/L (within the reference range) 9 days after treatment began. Beginning the day after first injection, plasma AST increased, peaking at 703 ± 135 U/L on the day after the last injection. Plasma AST did not return to the reference range in all individual horses until 29 days after the last injection (mean 247 ± 33 U/L). Compared to the control period, plasma CK and AST elevations lasted for 8 and 28 days, respectively, after the onset of treatment (P < .001 to P = .03) and lasted for 4 and 24 days, respectively, after the last day of treatment (P < .001 to P = .03).