Triads and transverse tubules isolated from skeletal muscle contain high levels of inositol 1,4,5-trisphosphate.

We measured the content of inositol 1,4,5-trisphosphate in sarcoplasmic reticulum, transverse tubules, and triads isolated from frog skeletal muscle, as well as in triads isolated from rabbit skeletal muscle. We found that acid extracts of both transverse tubules and triads contained significant amounts of inositol 1,4,5-trisphosphate, in the range of 300-400 pmol/mg of protein as determined by a radioreceptor assay, whereas no detectable amounts were found in sarcoplasmic reticulum vesicles. The identity of inositol 1,4,5-trisphosphate in the extracts was confirmed by comigration with [3H]inositol 1,4,5-trisphosphate on polyethyleneimine-cellulose plates, and by phosphorylation to inositol 1,3,4,5-tetrakisphosphate using the inositol 1,4,5-trisphosphate 3-kinase present in muscle extracts. These findings may have important physiological implications. First, the results indicate that the muscle plasma membrane regions differentiated as transverse tubules contain high amounts of inositol 1,4,5-trisphosphate, suggesting that they might possess a high density of binding sites for this compound. Second, since inositol 1,4,5-trisphosphate has been proposed as a chemical transmitter in excitation-contraction coupling in skeletal muscle, our finding that this second messenger is present in high density at the site of coupling may contribute to the understanding of its role in this process.