Myoneural interactions affect diaphragm muscle adaptations to
inactivity.
Miyata, Hirofumi, Wen-Zhi Zhan, Y. S. Prakash, and Gary C. Sieck.
Departments of Anaesthesiology and Physiology & Biophysics, Mayo
Clinic and Mayo Medical School, Rochester Minnesota 55905
APStracts 2:0276A, 1995.
We hypothesized that inactivity effects on diaphragm (DIA) muscle
contractile and morphometric properties are attenuated if phrenic
motoneurons are also inactive. Three models of rat DIA inactivity
were compared: 1) Spinal isolation (SI); 2) Tetrodotoxin (TTX) nerve
blockade; and 3) Denervation (DNV). Motoneuron and muscle fiber
inactivities were matched only in SI animals. After 2 weeks, maximum
tetanic force decreased in all three groups compared to controls
(CTL), but to a greater extent in TTX and DNV animals. Fatigue
resistance improved and maximum unloaded shortening velocity slowed
only in TTX and DNV groups. Type IIa fiber proportions decreased in
all three groups and type IIx fiber proportions increased in TTX and
DNVanimals. Type I fiber CSA increased in all three groups, but to a
greater extent in TTX and DNV animals. Type IIa fibers hypertrophied
while type IIx and IIb fibers atrophied only in TTX and DNV groups.
These results support the hypothesis that muscle adaptations to
prolonged inactivity are attenuated when muscle fiber and motoneuron
inactivities are matched.
Received 10 March 1995; accepted in final form 14 June 1995.
APS Manuscript Number A268-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 11 July 1995.