REGULATION OF CA2-AND SLOW-TWITCH SKELETAL-MUSCLE FIBERS( HANDLING BYPHOSPHORYLATION STATUS IN MOUSE FAST)

The effects of phosphorylation status on Ca2+ release and Ca2+ removal were studied in fast-twitch flexor digitorum brevis and slow-twitch s oleus skeletal muscle fibers enzymatically isolated from wild-type and phospholamban knockout (PLBko) mice. In all fibers the adenosine 3',5 '-cyclic monophosphate-dependent protein kinase (PKA) inhibitor H-89 d ecreased the peak amplitude of the intracellular Ca2+ concentration ([ Ca2+]) transient for a single action potential, and the PKA activator dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP) reversed this effect, indicating modulation of Ca2+ release by phosphorylation statu s in all fibers. H-89 decreased the decay rate constant of the [Ca2+] transient and DBcAMP reversed this effect only in phospholamban-expres sing fibers (wild-type soleus), indicating modulation of Ca2+ removal only in the presence of phospholamban. A high basal level of PKA phosp horylation in soleus fibers maintained under our control conditions wa s indicated by the lack of effect of direct application of DBcAMP on C a2+ release or Ca2+ removal in wild-type or PLBko soleus fibers and wa s confirmed by analysis of phospholamban from wild-type soleus fibers.