The structure of Ca(2+) release units in arthropod body muscle indicates an indirect mechanism for excitation-contraction coupling.
The relative disposition of ryanodine receptors (RyRs) and L-type Ca(2+) 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 Ca(2+) channels involved in excitation-contraction (e-c) coupling, based on their similarity in size and location with the L-type Ca(2+) 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.
Takekura
Hiroaki
H
Department of Physiological Sciences, National Institute of Fitness and Sports, Kanoya, Kagoshima 891-23, Japan.
Franzini-Armstrong
Clara
C
D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000737:chemistry; D002462:Cell Membrane / Q000378:metabolism; D004175:Diptera; D005614:Freeze Fracturing; D007313:Insecta; D018482:Muscle, Skeletal / Q000378:metabolism; D009132:Muscles / Q000378:metabolism*; D009206:Myocardium / Q000378:metabolism; D012605:Scorpions
D020746:Calcium Channels, L-Type; D002118:Calcium