title: Tuning of shortening speed in coleoid cephalopod muscle: no evidence for tissue-specific muscle myosin heavy chain isoforms.
abstract: The contractile protein myosin II is ubiquitous in muscle. It is widely accepted that animals express tissue-specific myosin isoforms that differ in amino acid sequence and ATPase activity in order to tune muscle contractile velocities. Recent studies, however, suggested that the squidDoryteuthis pealeiimight be an exception; members of this species do not express muscle-specific myosin isoforms, but instead alter sarcomeric ultrastructure to adjust contractile velocities. We investigated whether this alternative mechanism of tuning muscle contractile velocity is found in other coleoid cephalopods. We analyzed myosin heavy chain transcript sequences and expression profiles from muscular tissues of a cuttlefish,Sepia officinalis, and an octopus,Octopus bimaculoides, in order to determine if these cephalopods express tissue-specific myosin heavy chain isoforms. We identified transcripts of four and six different myosin heavy chain isoforms inS. officinalisandO. bimaculoidesmuscular tissues, respectively. Transcripts of all isoforms were expressed in all muscular tissues studied, and thusS. officinalisandO. bimaculoidesdo not appear to express tissue-specific muscle myosin isoforms. We also examined the sarcomeric ultrastructure in the transverse muscle fibers of the arms ofO. bimaculoidesand the arms and tentacles ofS. officinalisusing transmission electron microscopy and found that the fast contracting fibers of the prey capture tentacles ofS. officinalishave shorter thick filaments than those found in the slower transverse muscle fibers of the arms of both species. It thus appears that coleoid cephalopods, including the cuttlefish and octopus, may use ultrastructural modifications rather than tissue-specific myosin isoforms to adjust contractile velocities.
authors.affiliation: Department of Biology, University of North Carolina, Chapel Hill, NC 27599.
authors.forename: Justin F
authors.initials: JF
authors.lastname: Shaffer
authors.affiliation: Department of Biology, University of North Carolina, Chapel Hill, NC 27599.
authors.forename: William M
authors.initials: WM
authors.lastname: Kier
keywords: contraction; cuttlefish; myofilament; octopus