Sphingosine is endogenous to cardiac and skeletal muscle.

Endogenous sphingosine levels have been determined for rabbit skeletal and cardiac muscle. The sphingosine content of whole muscle tissue was compared to cytosol fractions and to particulate fractions enriched in transverse tubules or sarcoplasmic reticulum membranes. Cardiac and skeletal muscle tissue exhibited high levels of sphingosine (17 pmol/g). Muscle cytosol fractions contained potentially significant levels of sphingosine when expressed as a concentration (0.6-1.3 microM). Isolated T-tubule membranes displayed high sphingosine contents (1400 pmol/mg), making the T-tubules a major source of sphingosine in the muscle cell. The presence of sphingosine in muscle cells suggests that sphingosine may be a physiologically important second messenger in cardiac and striated muscle.