EFFECT OF CYCLOPIAZONIC ACID ON CONTRACTILE RESPONSES IN SLOW AND FAST BUNDLES OF CREMASTER SKELETAL-MUSCLE FROM THE FERRET

The effects of cyclopiazonic acid (CPA) on twitch force, calcium (Ca2) uptake and release by the sarcoplasmic reticulum (SR), and Ca2+ sens itivity of contractile apparatus were studied using intact and chemica lly skinned cremaster fibers and compared with those on the extensor d igitorum longus and soleus. In cremaster muscles treated with CPA (0.5 -5 mu M) a potentiation of the twitch was observed, associated with an increase in time to peak and in time of relaxation. In Triton-skinned fibers, CPA, at concentrations less than 10 mu M, exerted no signific ant effect on the contractile apparatus of either slow- or fast-twitch fibers. In slow-twitch fibers, a dose-dependent increase in Ca2+ sens itivity was associated with a decrease in maximal tension, at CPA conc entrations > 10 mu M. In saponin-skinned fibers, during the uptake pha se, CPA at > 10 mu M induced a dose-dependent decrease in caffeine con tracture. The possibility of an action on the SR Ca2+ release channel was excluded by testing the effect of CPA during the releasing phase. The enhancing effect of CPA (0.5-5 mu M) on mechanical activity could be explained by an inhibition of the SR Ca2+ ATPase in skeletal muscle cells without an effect on the contractile proteins. Our results stro ngly suggest that CPA (< 10 mu M) has a highly specific effect on the SR Ca2+ pump in the fast- and slow-twitch fibers and therefore could b e a good tool to study the mechanisms of Ca2+ regulation in skeletal m uscles. Furthermore, the study of the SR properties, using CPA, has sh own no significant differences in the SR function of ferret cremaster fibers in comparison with extensor digitorum longus and soleus muscles .