EFFECT OF INTERMITTENT WEIGHT-BEARING ON SOLEUS FIBER FORCE-VELOCITY-POWER AND FORCE-PCA RELATIONSHIPS

Rat permeabilized type I soleus fibers displayed a 33% reduction in pe ak power output and a 36% increase in the free Ca2+ concentration requ ired for one-half maximal activation after 14 days of hindlimb non-wei ght bearing (NWB). We examined the effectiveness of intermittent weigh t bearing (IWB; consisting of four 10-min periods of weight bearing/da y) as a countermeasure to these functional changes. At peak power outp ut, type I fibers from NWB animals produced 54% less force and shorten ed at a 56% greater velocity than did type I fibers from control weigh t-bearing animals while type I fibers from the IWB rats produced 26% m ore 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 d ifference was observed in the average peak power of fibers from the IW B and NWB animals. Hill plot analysis of force-pCa relationships indic ated that fibers from the IWB group required similar levels of free Ca 2+ to reach half-maximal activation in comparison to fibers from the w eight-bearing group. However, at forces <50% of peak force, the force- pCa curve for fibers from the IWB animals clearly fell between the rel ationships observed for the other two groups. In summary, IWB treatmen ts 1) attenuated the NWB-induced reduction in fiber Ca2+ sensitivity b ut 2) failed to prevent the decline in peak power that occurs during N WB because of opposing effects on fiber force (an increase vs. NWB) an d shortening velocity (a decrease vs. NWB).