Regional adaptations of rabbit diaphragm muscle fibers to
unilateral denervation.
Zhan, Wen-Zhi, Gaspar A. Farkas, Mark A. Schroeder, Luc E. Gosselin
and Gary C. Sieck.
Departments of Anesthesiology, Physiology and Biophysics, &
Thoracic Physiology Research Unit, Mayo Clinic and Foundation,
Rochester, Minnesota 55905
APStracts 2:0187A, 1995.
Morphological adaptations of diaphragm (DIA) muscle fibers after
unilateral denervation (DNV) have been attributed to either a direct
effect of DNV itself or the effect of passive length changes imposed
by continued inspiratory contractions of the intact contralateral
side. We hypothesized that if the mid-costal and sternal regions of
the rabbit DIA displayed different passive length changes after
unilateral DNV, the morphological and contractile adaptations of
these two regions would differ if passive length changes were the
primary determinant underlying the adaptations. Conversely, regional
adaptations would be similar if due to DNV itself. Morphological and
contractile adaptations of the mid-costal and sternal DIA regions
were examined one and four weeks after unilateral DNV. DIA length
changes were measured using sonomicrometry, and length of the two
regions were compared during spontaneous breathing before and after
DNV. Passive stress imposed on the paralyzed DIA was estimated based
on passive force-length relationships measured in vitro. Following
DNV, the mid-costal region passively lengthened by 3-5% of resting
length (Lr), with length changes never exceeding optimal fiber length
(Lo), while the sternal region shortened by 4-5% of Lr. Despite
differences in passive length changes between mid-costal and sternal
regions, minimal stress was associated with these passive length
changes in both regions. The proportions of type I and IIb fibers in
both regions did not change after DNV, while the proportion of type
IIa fibers was slightly decreased. By one week after DNV, type I
fibers in both regions hypertrophied while type IIb fibers atrophied.
The cross sectional area (CSA) of type IIa fibers was unaffected.
After four weeks of DNV, type I fibers further hypertrophied, while
the CSA of type IIa and IIb fibers remained unchanged. Consequently,
type I fibers contributed relatively more to total DIA muscle mass
after DNV, whereas the relative contributions of type IIa and IIb
fibers were reduced. Isometric contractile changes after DNV were
similar in both DIA regions. Twitch contraction and 1/2 relaxation
times increased; force-frequency relationships shifted leftward; and
maximum tetanic force decreased. We conclude that passive length
changes and mechanical stress are not the main determinants of the
morphological and contractile adaptations of the DIA after unilateral
DNV, but that these adaptations result from DNV itself.
Received 4 November 1994; accepted in final form 28 April 1995.
APS Manuscript Number A1140-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 9 May 1995.