Glycogen depletion of the intrafusal fibers in a mouse muscle
spindle during prolonged swimming.
Yoshimura, Atsushi, Yoshiharu Shimomura, Taro Murakami, Masumi
Ichikawa, Naoya Nakai, Chie Fujitsuka, Miki Kanematsu, and Noriaki
Fujitsuka.
Department of Bioscience, Nagoya Institute of Technology, Gokiso
-cho, Showa-ku, Nagoya 466, Japan
APStracts 3:0079R, 1996.
This study investigated the recruitment of different types of
intrafusal fibers during prolonged swimming at 60 - 75% of Vo2max. We
used 56 male adult mice and examined depletion of glycogen in soleus
(SOL) and extensor digitorum longus (EDL) muscle spindles by visual
inspection and a newly developed optical scanning method. More than
80% of all spindles from six mice consisted of four fibers: one type
I nuclear bag (bag1) fiber, one type II nuclear bag (bag2) fiber, and
two nuclear chain fibers. Glycogen content was estimated in muscle
fibers from groups of 6 mice that had rested or swum for either 0.5,
1, 2, 4, or 8 h. The optical scanning intensity of periodic acid
Schiff (PAS) stained sections was correlated with their biochemically
determined glycogen content (r = 0.93). Both methods showed
fundamentally the same result: Each type of intrafusal fiber has its
own typical recruitment pattern during exercise. In the initial phase
(0 - 0.5 h), glycogen depletion was largest in nuclear bag1 fibers
and insignificant in the bag2 and chain fibers. With the bag1 fibers
having become fatigued, mainly nuclear bag2 fibers took over during
the middle phase (2 - 4 h). During the last phase (4 - 8 h), only the
glycogen content of chain fibers decreased significantly during the
last phase (4 - 8 h). There were significant correlations between the
recruitment pattern of bag1 and extrafusal type I fibers in both SOL
and EDL, between nuclear bag2 and type IIA fibers in SOL, and between
nuclear chain and type IIB fibers in EDL. This suggests that during
moderately intense exercise, glycogen depletion occurs first in the
slow, then the intermediate and finally the fast intrafusal fibers.
Received 16 August 1995; accepted in final form 26 February 1995.
APS Manuscript Number R512-5.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 20 March 96