The Structure of Ca2+ Release Units in Arthropod Body Muscle Indicates an Indirect Mechanism for Excitation-Contraction Coupling

Abstract

AbstractThe relative disposition of ryanodine receptors (RyRs) and L-type Ca2+ channels was examined in body muscles from three arthropods. In all muscles the disposition of ryanodine receptors in the junctional gap between apposed SR and T tubule elements is highly ordered. By contrast, the junctional membrane of the T tubule is occupied by distinctive large particles that are clustered within the small junctional domain, but show no order in their arrangement. We propose that the large particles of the junctional T tubules represent L-type Ca2+ channels involved in excitation-contraction (e-c) coupling, based on their similarity in size and location with the L-type Ca2+ channels or dihydropyridine receptors (DHPRs) of skeletal and cardiac muscle. The random arrangement of DHPRs in arthropod body muscles indicates that there is no close link between them and RyRs. This matches the architecture of vertebrate cardiac muscle and is in keeping with the similarity in e-c coupling mechanisms in cardiac and invertebrate striated muscles