Disuse and d???Tubocurarine Sensitivity in Isolated Muscles

The effect of immobilization on subsequent sensitivity of limb muscles to d-tubocurarine was examined in a guinea pig model. A foreleg was immobilized in a cast for 1-4 weeks. At the end of this period, isolated nerve-lumbrical muscle preparations were set up and sensitivity to d-tubocurarine assayed. Immobilization increased the ED50 of the drug not only in muscles from the limb in the cast but also those from contralateral forelimbs and from hindlimbs. The increase in ED50 was not accompanied by atrophy. In a parallel series of experiments measuring the effect of d-tubocurarine on depolarization by carbachol, the apparent d-tubocurarine-receptor dissociation constant was determined. Immobilization produced no change in this parameter. Because immobilization of one extremity produced sensitivity changes in others, the anesthesiologist must be careful not to assume that monitoring from any limb will yield normal values in a patient with partial immobility.