Effect of tetrodotoxin relaxation of cultured skeletal muscle on the sarcoplasmic reticulum Ca2+-transport ATPase

The spontaneous contractions of cultured chick skeletal muscle fibers were abolished by growth of cultures in the presence of tetrodotoxin (TTX). Inhibition of the contractile activity of cultured myofibers was associated with a marked reduction in the rate of azide-insensitive, ATP-dependent Ca2+ uptake by the total particulate fraction of cell homogenates and by purified sarcoplasmic reticulum. Myosin heavy chain (MHC) accumulation and azide-insensitive, ATP-dependent Ca2+ uptake into a total cell membrane fraction were measured simultaneously in the same culture dish. A decrease in the activity of the ATP-dependent Ca2+ uptake system preceded a significant reduction in MHC content of contraction-inhibited cultures. The reduced rate of Ca2+ uptake observed in the sarcoplasmic reticulum from TTX-treated cultures paralleled a decrease in the amount of enzymatically active Ca2+-transport ATPase. The cellular concentration of the ATPase was estimated from a measurement of the concentration of the Ca2+-dependent, hydroxylamine-sensitive, steady state level of phosphorylated intermediate formed in culture microsomes. In contrast to the changes observed in activity of the sarcoplasmic reticulum ATPase and MHC content of TTX-treated cultures, neither the specific activity of creatine kinase nor the accumulation of the MM isoenzyme were affected. It is therefore concluded that the contractile activity of muscle has a selective effect on the maintenance of the adult skeletal muscle phenotype.