Phosphoinositides in giant barnacle muscle fibers: a quantitative analysis at rest and following electrical stimulation.

Quantitative data are presented on the composition of the major phospholipids in isolated giant barnacle muscle fibers. It is shown, using internal perfusion techniques, that the high specific activity of labeling of polyphosphoinositides in vivo is attained by the activities of specific kinases. Electrical stimulation causes a reduction in the specific activity of labeling of PtdInsP2. This phospholipid, which is the immediate precursor for the release of InsP3, is found at a significant concentration (40 nmol/g wet weight) in single barnacle muscle fibers, sufficient to support a role as precursors of second messengers. The rapid catabolization of PtdInsP2 in the absence of external Ca2+ suggests that E-C coupling in barnacle muscle may be associated with a voltage-dependent, Ca(2+)-independent, activation of the breakdown of polyphosphoinositides.