Ca2+-activated force production and calcium handling by the sarcoplasmic reticulum of crustacean muscles during molt-induced atrophy

SYNOPSIS. Growth in crustaceans is an intermittent process centered around the principal event of ecdysis. A'major problem facing decapod crustaceans at the time of ecdysis is the withdrawal of the large muscle mass of the chelae through the narrow basi-ischial joints. To overcome this problem the muscle undergoes an atrophy triggered by the molt, which reduces the muscle mass. Once the animal is freed from the old exoskeleton, the muscle fibers, must elongate to accommodate the new larger exoskeleton. Despite this major myofibrillar remodification, the muscles are thought to remain functional over the molt cycle. Studies using skinned muscle fibers have shown that long-sarcomere fibers maintain their function over the molt cycle while the contractile properties of the short-sarcomere fibers are modified, as fibers could not withstand maximal activation with Ca2+ during the prcrnolt stage. In this study the maximum Ca2+-activated force production and the ability of the sarcoplasmic reticulum (SR) to release accumulated Ca2+ has been investigated in the two major fiber types in the claw muscle of Cherax destructor, in the stages just prior to ecdysis and during intermolt. In both long- and shortsarcomere fibers, the amount of Ca2+ released by the SR was not different in premolt and intermolt stages. However, the maximum releasing capacity of the SR was reached in a shorter time during the premolt suggesting that Ca2+ is being accumulated at a faster rate. The force production was greatly reduced and was graded during the premolt in both fiber types. This modulation of force appears to be the most likely candidate regulating the magnitude of the force development in the periods when fibers are undergoing myofibrillar remodification and thus may serve to prevent fiber damage.