Determination of sensitivity to metocurine in exercised horses

David A. White From the Department of Anesthesiology, School of Medicine, (White, Fung, Gronert), and the Departments of Anesthesiology (Hildebrand) and Physiological Sciences (Jones), School of Veterinary Medicine, University of California, Davis, CA 95616.

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Sue V. Hildebrand From the Department of Anesthesiology, School of Medicine, (White, Fung, Gronert), and the Departments of Anesthesiology (Hildebrand) and Physiological Sciences (Jones), School of Veterinary Medicine, University of California, Davis, CA 95616.

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James H. Jones From the Department of Anesthesiology, School of Medicine, (White, Fung, Gronert), and the Departments of Anesthesiology (Hildebrand) and Physiological Sciences (Jones), School of Veterinary Medicine, University of California, Davis, CA 95616.

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Dennis L. Fung From the Department of Anesthesiology, School of Medicine, (White, Fung, Gronert), and the Departments of Anesthesiology (Hildebrand) and Physiological Sciences (Jones), School of Veterinary Medicine, University of California, Davis, CA 95616.

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Gerald A. Gronert From the Department of Anesthesiology, School of Medicine, (White, Fung, Gronert), and the Departments of Anesthesiology (Hildebrand) and Physiological Sciences (Jones), School of Veterinary Medicine, University of California, Davis, CA 95616.

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Summary

On the basis of results in dogs, conditioning exercise may increase sensitivity to nondepolarizing muscle relaxants. Five Thoroughbreds were exercised/conditioned 3 times weekly on a treadmill for 8 months. Increasing maximal rate of 02 consumption verified that the horses were responding to exercise conditioning. Six nonexercised Thoroughbreds served as the control group. Studies were done with horses under general anesthesia by use of halothane during partial paralysis by a brief constantrate infusion with the muscle relaxant, metocurine iodide. Quantification of degree of paralysis of the hoof twitch (eg, digital extensor) occurred with simultaneous quantification of blood values of metocurine. Pharmacokinetic and pharmacodynamic analyses of the data were done by a nonlinear regression program, using the Hill equation. There were no differences in findings between exercised and nonexercised horses. The mean blood concentration for the 50% paralyzing dose of metocurine was 0.44 ± 0.11 (sd) μg/ml in exercised horses, and 0.58 ± 0.22 μg/ml in nonexercised horses. Despite evidence for a response to conditioning, a significant change in the sensitivity of the neuromuscular junction to metocurine was not found.

Summary

On the basis of results in dogs, conditioning exercise may increase sensitivity to nondepolarizing muscle relaxants. Five Thoroughbreds were exercised/conditioned 3 times weekly on a treadmill for 8 months. Increasing maximal rate of 02 consumption verified that the horses were responding to exercise conditioning. Six nonexercised Thoroughbreds served as the control group. Studies were done with horses under general anesthesia by use of halothane during partial paralysis by a brief constantrate infusion with the muscle relaxant, metocurine iodide. Quantification of degree of paralysis of the hoof twitch (eg, digital extensor) occurred with simultaneous quantification of blood values of metocurine. Pharmacokinetic and pharmacodynamic analyses of the data were done by a nonlinear regression program, using the Hill equation. There were no differences in findings between exercised and nonexercised horses. The mean blood concentration for the 50% paralyzing dose of metocurine was 0.44 ± 0.11 (sd) μg/ml in exercised horses, and 0.58 ± 0.22 μg/ml in nonexercised horses. Despite evidence for a response to conditioning, a significant change in the sensitivity of the neuromuscular junction to metocurine was not found.

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