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Department of Biology, Marquette University, Milwaukee, Wisconsin 53201-1881
Received 30 September 1996; accepted in final form 7 February 1997.
Bangart, J. J., J. J. Widrick, and R. H. Fitts. Effect
of intermittent weight bearing on soleus fiber force-velocity-power and
force-pCa relationships. J. Appl.
Physiol. 82(6): 1905-1910, 1997.
Rat
permeabilized type I soleus fibers displayed a 33% reduction in peak
power output and a 36% increase in the free
Ca2+ concentration required for
one-half maximal activation after 14 days of hindlimb non-weight
bearing (NWB). We examined the effectiveness of intermittent weight
bearing (IWB; consisting of four 10-min periods of weight bearing/day)
as a countermeasure to these functional changes. At peak power output,
type I fibers from NWB animals produced 54% less force and shortened
at a 56% greater velocity than did type I fibers from control
weight-bearing animals while type I fibers from the IWB rats produced
26% more absolute force than did fibers from the NWB group and
shortened at a velocity that was only 80% of the NWB group mean. As a
result, no difference was observed in the average peak power of fibers from the IWB and NWB animals. Hill plot analysis of force-pCa relationships indicated that fibers from the IWB group required similar
levels of free Ca2+ to reach
half-maximal activation in comparison to fibers from the weight-bearing
group. However, at forces <50% of peak force, the force-pCa curve
for fibers from the IWB animals clearly fell between the relationships
observed for the other two groups. In summary, IWB treatments
1) attenuated the NWB-induced
reduction in fiber Ca2+
sensitivity but 2) failed to prevent
the decline in peak power that occurs during NWB because of opposing
effects on fiber force (an increase vs. NWB) and shortening velocity (a
decrease vs. NWB).
isotonic contractile properties; hindlimb suspension; muscle atrophy; countermeasures
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