The recovery from anaerobic metabolism in invertebrates

Abstract

Abstract The duration and completeness of recovery from anaerobic metabolism is of important functional consequence in invertebrates. Two basic processes occur during recovery: (1) recharging of the phosphagen and ATP pools and (2) disposal of end products by excretion, oxidation, or conversion back to anaerobic substrates. The recovery process in invertebrates is largely aerobic, as is evidenced by the near universality of the oxygen debt phenomenon. However, there is often a lack of correlation between end product accumulation and the magnitude of the oxygen debt. Typically, ATP and phosphagen levels are recharged very rapidly during recovery–ATP (0.25 to 1.0 hr) and phosphagens (0.5 to 4 hr). Excretion of propionate, acetate, and, occasionally succinate, is relatively common in annelids and has also been observed in a number of bivalve molluscs. Nitrogen‐containing end products do not appear to be excreted. Cephalopods and large arthropods show elevated hemolymph end product levels following anoxia/hypoxia. In arthropods, these end products display a protracted return to control levels during recovery. In cephalopods, some octopine may be rapidly taken up and metabolized by the heart, brain, and gill. The sites of lactate metabolism in arthropods are unclear. In contrast to the situation in cephalopods and arthropods, most end products in the adductor muscles of bivalve molluscs do not appear to be exported, but are metabolized within the tissue itself. End ducts such as octopine, succinate, and alanine thus appear to be metabolized at the site of their production. Octopine and lactate appear to be gluconeogenic substrates in cephalopods and arthropods, respectively. Although many of the enzymes involved in gluconeogenesis are present in bivalve molluscs, this process has not been demonstrated in this group. Recovery from anaerobic metabolism remains a fruitful area for future research. Of special interest is the identification of sites of gluconeogenesis in the larger, more active species.