Low and High Power Output Modes of Anaerobic Metabolism: Invertebrate and Vertebrate Strategies

Abstract

During muscular contraction ATP is hydrolysed. The cell does not store large amounts of ATP and this compound has, therefore, to be almost simultaneously resynthesised at about the same rate. Depending on the type and function of the muscle, the driving force is mainly delivered by either aerobic or anaerobic processes. In vertebrates three primary types of muscle are found: cardiac muscle, smooth muscle and skeletal muscle. Smooth muscle which lack a regular pattern of banding contract slowly. Vertebrate skeletal and heart muscle show cross-striation due to the orderly arrangement of the myofibrils within the fibers. The same muscle types are also common in invertebrates. Based on differences in the pattern of innervation muscle fibres are classified as tonic (slow) fibres or twitch fibres. Tonic fibres receive multiple innervation and their contractile response is confined to the immediate region of the nerve-muscle junction. Twitch fibres receive only a single motor nerve terminal on their surface and its impulse causes contraction throughout the whole length. On the basis of the contraction time two types of muscles can be distinguished: the fast twitch and the slow twitch muscles. The latter are red, while the fast twitch muscles are often pale in colour. Red muscles have a high myoglobin and mitochondria content and are particularly suited for sustained aerobic activity. Red muscle can be either of the slow and of the fast twitch type. The slow type is almost exclusively aerobic, while the fast red muscle has also a high anaerobic capacity and is, therefore, called intermediate type. Fast twitch white fibres are highly anaerobic and have together with the intermediate fibre type a high glycolytic and ATPase activity and considerable amounts of phosphagen and glycogen. The fast twitch fibres are primarily suited for burst activity which can be maintained only for a short time due to endogenous substrate depletion and the accumulation of lactate (and/or other acids). A specific muscle may contain different fibre types depending on the work they perform. Uniform fibres within one muscle are often located within distinct layers or bundles. Examples of invertebrates are squid mantle muscle (Bone et al. 1981), lobster claw closer muscles (Costello and Govind 1983) and adductor muscle of bivalves.