Structure and Function of the Transverse Tubular System in Crustacean Muscle Fibers

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Abstract
The structure and function of the transverse tubular system (TTS) in two types of crustacean muscle fibers are examined. Giant fibers from the barnacle,Balanus nubilus, which are gradedly contracting, are compared with allor-none twitch fibers from the crab, Carcinus maenas. Both fiber types were found to have deep sarcolemmal invaginations which serve both to increase the fiber surface area and to kfeep the length of the tubules short enough for electrotonic propagation.The ultrastructure of the tubular system in both types of fiber is compared.The system is better developed in Carcinus than in Balanus, but the slow Balanus fibers do have a relatively well developed TTS and sarcoplasmic reticulum in contrast to slow vertebrate fibers. The apparent high, membrane-capacitance values of crustacean fibers are the result of investigators not taking into consideration the large increase in surface area due to the sarcolemmal infoldings.The tubular membranes in Carcinus fibers were found to be permselective to chloride ions, and could be made to swell (as confirmed by electron microscopy) by establishing an outward gradient for chloride across them. The capacitance of the tubular membrane relative to the plasma membrane was found to increase when the tubuleswere swollen. The implication of a fiber having two spatially separated, differentially permeable membranes on excitation-contraction coupling is discussed.
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