Correlation between ultrastructural and functional changes in sarcoplasmic reticulum during chronic stimulation of fast muscle.

Chronic indirect stimulation of fast twitch rabbit muscle (tibialis anterior and extensor digitorum longus) with a frequency of 10 Hz induced a progressive transformation of the sarcoplasmic reticulum (SR). Ultrastructural changes as studied by electron microscopy of freeze-fractured vesicles consisted in a decrease of intramembranous particles of the concave (A) face and an increase of particles in the convex (B) face. The asymmetry of the membrane proved to be lowered. Changes in the particle density of the A face were mainly confined to the 7-9 nm particles. Electrophoretic analyses revealed a decrease in the 115,000-Mr Ca2+ transport ATPase. The reduced density of the 7-9 nm particles correlated well with decreased activities in Ca2+-dependent ATPase as well as with decreases in initial and maximum Ca2+ uptake.