Excitation-contraction coupling in skeletal muscle fibers injected with the InsP3 blocker, heparin.

Heparin, an inhibitor of inositol trisphosphate (InsP3)-induced Ca2+ release in smooth muscle and non-muscle cells, was injected into intact frog skeletal muscle fibres. Ca2+ release from the sarcoplasmic reticulum was elicited by the normal action potential mechanism and monitored by both fura-2 fluorescence and an intrinsic birefringence signal. Both optical signals, and hence Ca2+ release, were unaffected by high concentrations of heparin. This result argues against a major physiological role of InsP3 as a chemical messenger of excitation-contraction coupling in skeletal muscle.