HYPOGRAVITY INCREASES CYCLOPIAZONIC ACID SENSITIVITY OF RAT SOLEUS MUSCLE

The functional capacity of skeletal muscle sarcoplasmic reticulum was explored in slow rat soleus muscle after 21 days of hindlimb suspensio n. The sarcoplasmic reticulum function was assessed in intact and sapo nin-skinned fibers by using cyclopiazonic acid, a specific Ca2+-adenos inetriphosphatase inhibitor. After hindlimb unweighting, the sensitivi ty to cyclopiazonic acid of intact and skinned soleus fibers becomes s imilar to that found in fast-twitch muscles. This change could be rela ted to the expression of fast Ca2+-adenosinetriphosphatase-pump protei n in unloaded soleus muscles and agrees with a transformation of soleu s muscle from slow- to fast-twitch type. These results also indicate t hat specific pharmacological tools, hire cyclopiazonic acid, could be used to analyze subcellular functional changes due to hindlimb unweigh ting.