Hypoxia and Posthypoxic Recovery in Insects: Physiological and Metabolic Aspects
G. Wegener
Abstract
Insects, the most successful class of invertebrates, have evolved striking physiological capacities that are based on intensive aerobic energy metabolism. Lack of oxygen, therefore, brings about dramatic reactions in insects. If made to breathe pure nitrogen, adult insects, like mammals, suffer a rapid loss of CNS function and body coordination. However, unlike mammals which are irreversibly damaged if anoxia is maintained for more than a few minutes, insects can fully recover from many hours of strict anoxia. This shows that high standard metabolic rate and rapid loss of neuronal function are not necessarily connected with a low tolerance of anoxia. Insects are, therefore, interesting experimental animals with respect to the question of which physiological properties are crucial for preventing tissue damage from hypoxia. Unlike in anoxia-tolerant lower vertebrates, the functions of all organs are suspended in anoxic insects. Heartbeat and circulation of hemolymph stop rapidly and nerve cell activity ceases. The metabolic rate, i.e., ATP turnover, is dramatically decreased during anoxia, metabolic flux is retarded and, compared to vertebrates, only modest amounts of anaerobic products are formed during prolonged anoxia. The cellular energy status is not stabilized during anaerobiosis, the content of ATP in insect tissues falls to very low levels, and the animals reach a state resembling that of rigor mortis. Unlike in mammals, however, products of ATP breakdown are not easily lost into the circulation but are predominantly retained in the tissues of anoxic insects; hence, ATP can readily be resynthesized when oxygen is readmitted.