Muscle function in a patient with Brody's disease

Adductor pollicis muscle function of a 21-year-old man with genetically con firmed Brody's disease (sarcoplasmic reticulum [SR] -Ca(2+)ATPase deficienc y) was investigated to study the possible effects of reduced SR-Ca(2+)ATPas e activity on muscle relaxation and force production. Following maximal ele ctrical activation of the ulnar nerve, tetanic muscle half-relaxation time was greater in the patient (246 +/- 10 ms) than control subjects (97 +/- 4 ms, n = 8), During repetitive activation, there was a similar decline in ma ximal shortening velocity in the patient and controls, indicating a compara ble reduction in cross-bridge cycling rate. The finding that the slowing of relaxation was greater in the patient (329 ms versus 138 +/- 20 ms) sugges ts that there was a further reduction of SR-Ca(2+)ATPase activity in the pa tient's muscle during fatigue. Following a voluntary contraction, involunta ry activity of the antagonist muscles facilitated force decline and masked the impaired relaxation in the patient. This antagonist-induced relaxation indicates that it might be difficult to establish impaired muscle relaxatio n with voluntary contractions. (C) 1999 John Wiley & Sons, Inc.