The energy metabolism of the foot muscle of the jumping cockle,Cardium tuberculatum: Sustained anoxia versus muscular activity

Summary

• 1. When touched by a starfish, the cockleCardium tuberculatum shows a typical escape response which results in a series of jumps. These are achieved by contractions of the foot muscle.C. tuberculatum can also survive periods of prolonged anoxia.

• 2. The concentrations of adenosine nucleotides, arginine phosphate, arginine, and various other metabolites related to anaerobic metabolism were determined in the foot muscle at rest, after jumping (7–11 and 19–31 contractions) and after 6 and 10 h of anoxia.

• 3. Jumping resulted in only small changes in the level of the adenylates with a slight drop of the energy charge, but the arginine phosphate store was depleted by more than 50%. Concurrently the concentration of arginine increased, while octopine accumulated as the main anaerobic end product; no accumulation of succinate ord- andl-lactate was observed.

• 4. During anoxia no significant change in the energy charge was observed. However, the level of arginine phosphate fell to 74% (6 h) and 53% (10 h) of the value in resting muscle with a concomitant increase in arginine concentration, but with virtually no formation of octopine. Instead,d-lactate, but notl-lactate, accumulated as the main glycolytic end product, despite the low activity of lactate dehydrogenase (7 U/g wet wt.), compared to octopine dehydrogenase (120 U/g wet wt.) measured in vitro. Succinate levels also increased.

• 5. During 10 h of anoxia 55% of the energy required is contributed by breakdown of arginine phosphate, whereas during muscular contraction 82% of the energy supply is derived from this reaction.

• 6. It is concluded that in the foot muscle ofC. tuberculatum anaerobic metabolism is different during anoxia and during muscular activity. In the first situation ATP is produced by formation ofd-lactate in glycolysis, and by the breakdown of arginine phosphate. During exercise, however, octopine represents the end product of glycogen breakdown, but the phosphagen is also utilized.

Pyruvate forms a branchpoint for the formation ofd-lactate or production of octopine; the mechanisms for the regulation of the lactate dehydrogenase versus octopine dehydrogenase reactions remain unknown.