Simultaneous measurements were made of crayfish muscle Ca2+ currents (ICa) and the intracellular Ca2+ transients they elicit due to Ca(2+)-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR). Ca2+ concentration ([Ca2+]) elevations produced by Ca2+ entry via ICa were much more effective in triggering CICR than were ongoing release or homogeneous elevations of Ca2+ produced by photolysis of caged Ca2+. This suggests that [Ca2+] gradients exist when Ca2+ is elevated by ICa and that, during Ca2+ entry, [Ca2+] at the activation site of the release channels must be much greater than spatially averaged [Ca2+] reported by the indicator. Analysis of voltage dependencies of ICa inactivation and SR Ca2+ release suggest that both Ca(2+)-dependent processes are controlled by ICa via the nearest T tubule Ca2+ channel rather than by total ICa entry. The contribution of SR Ca2+ release to ICa inactivation studied with a two-pulse protocol was less than predicted if Ca2+ derived from SR Ca2+ release and from T tubule Ca2+ channels have equal access to the Ca2+ binding site controlling ICa inactivation. These results can be explained in terms of a scheme where sites for release activation and ICa inactivation are located in the same junctional gap subdomain, closer to the cytoplasmic mouth of the T tubule Ca2+ channel than to the cytoplasmic mouth of the SR Ca2+ release channels. Such a scheme could provide an explanation for the graded nature and selective control of CICR in this preparation as well as in vertebrate cardiac muscle.
The Triple Innervation of the Crayfish Muscle
