Effect of hindlimb unloading on rat soleus fiber force, stiffness
and calcium sensitivity.
McDonald, Kerry S., and Robert H. Fitts.
Biology Department, Marquette University, Milwaukee, Wisconsin
53233
APStracts 2:0315A, 1995.
The purpose of this study was to examine the time course of change in
soleus muscle fiber peak force (N), tension, Po (kN.m-2), elastic
modulus (Eo), and force-pCa and stiffness-pCa relationships.
Following 1, 2 or 3 wk of hindlimb unloading (HU), muscle bundles
were isolated and placed between a motor arm and a transducer and
fiber diameter, peak absolute force (N), Po (kN.m-2), Eo (an
indicator of the number of myofibrillar cross-bridges per fiber
cross-sectional area), and force-pCa and stiffness-pCa relationships
were characterized. One wk HU resulted in a significant reduction in
fiber diameter (68+/-2 vs 57+/-1 [mu]m), force (3.59+/-0.15 vs
2.19+/-0.12 x 10+/--4 N), Po (102+/-4 vs 85+/-2 kN.m-2), Eo (1.96+/
-0.12 vs 1.37+/-0.13 x 107 N.m-2), and 2 wk HU caused a further
decline in fiber diameter (45+/-1 [mu]m), force (1.31+/-0.06 x 10-4
N), and Eo (0.96+/-0.09 x 107 N.m-2). Although the mean fiber
diameter and absolute force continued to decline through 3 wk HU, Po
recovered to values not significantly different from the control. The
Po/Eo ratio was significantly increased following 1 (5.5+/-0.3 to
7.1+/-0.6), 2, and 3 wk HU, and the 2 (9.5+/-0.4) and 3 (9.4+/-0.8)
wk values were significantly greater than 1 wk values. The force-pCa
and stiffness-pCa curves were rightward shifted following 1, 2, and 3
wk HU. At one week HU, the Ca2+ sensitivity of isometric force,
assessed by [Ca2+]50 (the Ca2+ required for half maximal force) was
increased from the control value of 1.83+/-0.12 to 2.30+/-0.10 [mu]M.
Two and three wk of HU had no further effect on [Ca2+]50. Following 3
wk HU, the slope of the force-pCa relationship below [Ca2+]50 (n2 of
the Hill plot) decreased. In conclusion, following HU the decrease in
soleus fiber Po can be explained by a reduction in the number of
myofibrillar cross-bridges per CSA. Our working hypothesis is that
the loss of contractile protein reduces the number of cross-bridges
per CSA, and increases the filament lattice spacing. The increased
spacing reduces both cross-bridge force and stiffness, but Po/Eo
increases due to a quantitatively greater effect on stiffness.
Received 6 September 1994; accepted in final form 12 June 1995.
APS Manuscript Number A936-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 18 July 1995.