Determination of sensitivity to metocurine in exercised horses

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 O2 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 constant-rate 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) microgram/ml in exercised horses, and 0.58 +/- 0.22 microgram/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.