Effects of Chronic Stimulation at Low Frequency on the Lipid Phase of Sarcoplasmic Reticulum in Rabbit Fast‐Twitch Muscle

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Abstract
The transformation of sarcoplasmic reticulum in fast‐twitch rabbit muscle by low‐frequency indirect stimulation was studied. Peptide pattern analyses and ATPase activities revealed transformation of the sarcoplasmic reticulum into a slow‐twitch muscle type to be completed after 30 days. Concomitant changes in the lipid phase consisted in a decrease of the phospholipid/protein ratio, a decrease in the phosphatidylcholine content and in changes of the saturated fatty acid moiety of the phospholipid fraction. Significant increases were seen in content of lysophosphatidylcholine (lysolecithin) and in nonesterified fatty acid. Parallel increases in lysolecithin acyl‐ transferase activity indicated an enhanced phospholipid turnover. The changes in the phospholipid matrix did not lead to a complete transformation. Moreover, the decrease in Ca2+ uptake and Ca2+‐dependent ATPase activity preceded most of the changes in the phospholipids. It appears thus that these alterations occurred independently.