C. Hidalgo et al., TRIADS AND TRANSVERSE TUBULES ISOLATED FROM SKELETAL-MUSCLE CONTAIN HIGH-LEVELS OF INOSITOL 1,4,5-TRISPHOSPHATE, The Journal of biological chemistry, 268(20), 1993, pp. 5111-5117
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.