A chemical link in excitation-contraction coupling in skeletal muscle.

The process of EC coupling at the T-SR junction in skeletal muscle shows the characteristic features observed in other chemical junctions; namely, there is a considerable temperature-dependent latency between prejunctional and postjunctional events. This evidence gives further support to the possible involvement of inositol phosphates, InsP3 in particular, in the aforementioned process. Although the kinetics of Ca release in response to InsP3 microinjections have not been appropriately studied yet, our preliminary evidence does not rule out the proposed role of InsP3 as the chemical transmitter. The biochemical analysis of inositol phosphates and phosphoinositides in control and stimulated muscles demonstrates significant changes in the concentration of the key moieties proposed to be relevant in several other preparations. Apparently, the enzymatic machinery for the relevant biochemical reactions of the inositol phosphate/phosphoinositide cycle is present at high levels in muscle. It remains to be investigated whether some aspects of this cycle may have been selectively adapted for the fast process of Ca release in muscle.