The compositions of sarcoplasmic reticulum (SR) membranes from rabbit caudofemoralis, tibialus, and soleus muscles (fast, mixed, and slow twitch, respectively) were analyzed. Compared to caudofemoralis (fast twitch) SR, soleus (slow twitch) SR contained a significantly greater percentage of cholesterol, phosphatidylinositol, and sphingomyelin and a lesser percentage of phosphatidylcholine. Correlations between properties reported for the SR isolated from different muscle types and our analyses of the compositions are discussed. We suggest that the greater cholesterol content and the greater sphingomyelin to phosphatidylcholine ratio present in soleus SR contribute to decreased bilayer fluidity and, hence, decreased Ca2+-ATPase activity.
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Borchman et al. (1982) studied this question.
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