DEVELOPMENTAL-CHANGES IN OXYGEN-CONSUMPTION REGULATION IN LARVAE OF THE SOUTH-AFRICAN CLAWED FROG XENOPUS-LAEVIS

Well-developed larval Xenopus laevis (NF stages 58-66) are oxygen regu lators, at least during mild hypoxia. When and how they change from ox ygen conformers (the presumed condition of the fertilized egg) to oxyg en regulators is unknown, Also unknown is how anaerobic metabolic capa bilities change during development, especially in response to acute hy poxia, and to what extent, if any, anaerobiosis is used to supplement aerobic metabolism, Consequently, we have investigated resting rates o f oxygen consumption (MO(2)) and concentrations of whole-body lactate (lactic acid) during development In normoxia and in response to acute hypoxia in Xenopus laevis, nio, increased in an episodic, non-linear f ashion during development. Resting, normoxic MO(2) increased about ten fold (to approximately 0.20 mu mol g(-1) h(-1)) between NF stages 1-39 and 40-44, and then another tenfold between NF stages 45-48 and 49-51 (to approximately 2.0 mu mol g(-1) h(-1)), remaining at about 2 mu mo l g(-1) h(-1) for the remainder of larval development, MO(2) reached i ts highest level in newly metamorphosed frogs (nearly 4 mu mol g(-1) h (-1)), before decreasing to about 1.0 mu mol g(-1) h(-1) in large adul ts, X. laevis embryos and larvae up to NF stage 54-57 were oxygen conf ormers when exposed to variable levels of acute hypoxia, The only exce ption was NF stage 45-48 (external gills present yet body mass still v ery small), which showed some capability of oxygen regulation, All lar vae older than stage 54-57 and adults were oxygen regulators and had t he lowest values of Pc,it (the oxygen partial pressure at which nio, b egins to decline). Whole-body lactate concentration in normoxia was ab out 1 mu mol g(-1) for all larval groups, rising to about 12 mu mol g( -1) in adults, Concentrations of lactic acid in NF stages 1-51 were un affected by even severe ambient hypoxia. However, whole-body lactate l evels in NF stages 52-66 increased in response to severe hypoxia, indi cating that some anaerobic metabolism was being used to supplement dim inishing aerobic metabolism, The largest increases in concentration of lactate occurred in late larvae and adults.