Excitation-Contraction Coupling in Crayfish

High-sensitivity recording techniques demonstrate a continuousrelation between the onset and magnitude ot tension and themembrane depolarization that is induced by increasing K in thebathing medium or by intracellularly applied outward currents.This finding is not consistent with the mechanism of signallinge-c coupling by electrotonic spread of a "critical" depolarizationinward along the membrane of the transverse tubular system.It is in accord, however, with the channelled current mechanismthat is based on the known anion-permselectivity of the membranein the terminals of the TTS. The channelled-current model alsopredicts a direct role of Cl and a possible interaction betweenCa and CI in e-c coupling. The initiation and maintenance oftension as well as its magnitude, are in fact dependent uponthe concentrations of Ca and Cl in the medium. Thus, both thesignalling to, and the activation of, the contractile systemappear to be performed by a flow of current in the loop: cellmembrane – cell interior – TTS membrane –TTS channels – exterior, as is envisaged in the channelled-currentmodel of e-c coupling.