Meeting Abstract
P2.137 Jan. 5 The contractile properties of mitochondria-rich and mitochondria-poor circular muscle fibers in the mantle of the long-finned squid Loligo pealei THOMPSON, J.T.**; SZCZEPANSKI, J.A.; Franklin & Marshall College; St. Joseph\’s University joseph.thompson@fandm.edu
The circular muscles of the mantle provide power for the low-amplitude mantle contractions important in ventilating the mantle cavity, as well as for the large-amplitude contractions important for jet locomotion. In many species of squids (including the loliginid squids whose deep-fried form is, perhaps, more familiar than the living animal), there are two types of circular muscle fibers: centrally-located, mitochondria-poor (CMP) fibers and superficially-located, mitochondria-rich (SMR) fibers. Metabolic enzyme activity studies suggest that CMP and SMR fibers are analogous to vertebrate white muscle and red muscle fibers, respectively. EMG analysis indicates that SMR fibers are active during slow jetting and ventilation of the mantle cavity while CMP fibers are active during rapid jetting. We were able to test hypotheses about CMP and SMR function by measuring the contractile properties of bundles of live CMP and SMR muscle fibers isolated from the mantles of sexually mature long-finned squid Loligo pealei. Relative to CMP fibers, SMR muscle fibers produced higher average peak isometric stress (SMR=265mN/mm2; CMP=64mN/mm2), had a lower average twitch:tetanus ratio (SMR=0.073; CMP=0.18) and had a lower average unloaded maximum shortening velocity (SMR=1.6L/s; CMP=5.2L/s). These differences in muscle performance correlate with differences in the average thick filament lengths of the CMP and SMR fibers. Initial results indicate that the thick filaments of SMR muscle fibers range between 2 and 4 μm in length, while CMP thick filaments are between 1 and 2 μm. Our results highlight what may be a fundamental difference in the way squid modulate the contractile properties of their muscles relative to vertebrates and arthropods � via the dimensions of the components of the sarcomere as opposed to biochemically.
