Do Australian Desert Frogs Co-Accumulate Counteracting Solutes with Urea During Aestivation?

Australian desert frogs of the genera Neobatrachus, Cyclorana and Heleioporus experience significant dehydration, and iono- and osmoconcentration, during aestivation in the laboratory and accumulate substantial amounts of urea (100–200 mmol l⁻¹). We expected a priori that aestivating frogs probably would not need to accumulate balancing osmolytes but would accumulate trimethylamine oxide (TMAO) or betaine as counteracting solutes to urea. These aestivating frogs did not co-accumulate a substantial quantity of any particular balancing osmolyte or counteracting solute, such as a methylamine [TMAO, trimethylamine amine (TMA), betaine, sarcosine, glycerophosphorylcholine (GPC)] or polyol (inositol, mannitol, sorbitol) in plasma or muscle relative to urea accumulation. However, for aestivating frogs, the total concentration of all measured methylamines and polyols (TMAO + TMA + betaine + sarcosine + GPC + inositol) in muscle was approximately 35–45 mmol kg⁻¹, and so it is possible that all of these solutes have a combined counteracting osmolyte role in aestivating frogs at a ratio to urea of approximately 1:2.5, as has been described for elasmobranch fishes. Alternatively, the absence of substantial co-accumulation with urea of any particular solute suggests that aestivating frogs might not require any major extracellular or intracellular counteracting solutes (TMAO, betaine, GPC). The enzyme systems of these aestivating frogs may be insensitive to the perturbing effects of urea, or the perturbing effects of accumulated urea may be a mechanism for metabolic depression, during aestivation.