Role of temperature in determining effects of thyroxine and propylthiouracil on the motor unit

Abstract

Neuromuscular function and body temperature were measured in five groups of rats: 1) hyperthyroid, 2) thyroidectomized hypothyroid, 3) saline control (euthyroid), 4) propylthiouracil hypothyroid and 5) propylthiouracil control (thyroxine plus propylthiouracil). Significant differences occurred among the five groups in twitch and 30/sec. tensions and percentage tension remaining after 5 seconds of stimulation (but not in tetanic tension); the groups ranking in the order one to five (hyperthyroid lowest). Body temperature differences occurred in the inverse order. Twitch and 30/sec. tension, and tension remaining at 5 seconds showed significant inverse temperature coefficients. When the contribution of temperature differences among groups is removed, only the tension elicited during 30/sec. stimulation is a function of treatment. Propylthiouracil exerts nonthyroidally mediated neuromuscular effects, blocked by curare. It is hypothesized that hyperthyroidism reduces muscle tension by reducing the duration of active state of muscle by a) a direct effect on the mechanism controlling the duration and/or b) an indirect effect on the mechanism by an increase in muscle temperature.