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 sarcoplasmi c reticulum, transverse tubules, and triads isolated from frog skeleta l muscle, as well as in triads isolated from rabbit skeletal muscle. W e found that acid extracts of both transverse tubules and triads conta ined 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 ve sicles. The identity of inositol 1,4,5-trisphosphate in the extracts w as confirmed by comigration with [H-3]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-ki nase present in muscle extracts. These findings may have important phy siological implications. First, the results indicate that the muscle p lasma membrane regions differentiated as transverse tubules contain hi gh amounts of inositol 1,4,5-trisphosphate, suggesting that they might possess a high density of binding sites for this compound. Second, si nce inositol 1,4,5-trisphosphate has been proposed as a chemical trans mitter in excitation-contraction coupling in skeletal muscle, our find ing that this second messenger is present in high density at the site of coupling may contribute to the understanding of its role in this pr ocess.