Ultrastructural features of the columellar muscle and contractile protein analyses in different muscle groups of Megalobulimus abbreviatus (Gastropoda, Pulmonata)

In Megalobulimus abbreviatus, the ultrastructural features and the contractile proteins of columellar, pharyngeal and foot retractor muscles were studied. These muscles are formed from muscular fascicles distributed in different planes that are separated by connective tissue rich in collagen fibrils. These cells contain thick and thin filaments, the latter being attached to dense bodies, lysosomes, sarcoplasmic reticulum, caveolae, mitochondria and glycogen granules. Three types of muscle cells were distinguished: T1 cells displayed the largest amount of glycogen and an intermediate number of mitochondria, suggesting the highest anaerobic metabolism; T2 cells had the largest number of mitochondria and less glycogen, which suggests an aerobic metabolism; T3 cells showed intermediate glycogen volumes, suggesting an intermediate anaerobic metabolism. The myofilaments in the pedal muscle contained paramyosin measuring between 40 and 80 nm in diameter. Western Blot muscle analysis showed a 46-kDa band that corresponds to actin and a 220-kDa band that corresponds to myosin filaments. The thick filament used in the electrophoresis showed a protein band of 100 kDa in the muscles, which may correspond to paramyosin.