MUIR, T.J.*; COSTANZO, J.P.; LEE , R.E.; Miami University; Miami University; Miami University: Hypometabolism induced by urea in a terrestrially-hibernating frog

Accumulation of urea in defense of body water is an adaptation present in a diverse array of ureotelic animals. Some authors have proposed that urea may contribute to a metabolic depression in hyperuremic animals which do not co-accumulate counteracting solutes, as is likely the case in estivating and terrestrially-hibernating amphibians. However, to our knowledge the direct metabolic effects of hyperuremia in dormant amphibians have never been tested. We used the wood frog, Rana sylvatica, a terrestrial hibernator which accumulates urea in winter, to examine the hypometabolic effect of physiological levels of urea at the whole animal and tissue levels. Compared to hydrated controls, mildly-dehydrated, winter-acclimated frogs had higher (p<0.01) levels of plasma urea (28.0 vs. 7.7 mM) and 30% lower (p<0.01) rates of oxygen consumption. Preliminary in vitro studies measuring the metabolism of isolated tissues using the non-toxic, oxidation-reduction indicator dye, alamarBlue demonstrated a marked hypometabolism of liver and skeletal muscle when incubated in the presence of urea. Additional tests with a microrespirometry system corroborated these findings by showing ~35% and ~20% reductions in urea-treated liver (p<0.05) and skeletal muscle (p<0.01) metabolic rates, respectively. Increased osmolality in the absence of urea did not result in hypometabolism, suggesting the decrease in energy use was caused specifically by urea and not simply an increase in solute concentration. To our knowledge this is the first empirical evidence of hypometabolism directly induced by physiological levels of urea in any organism. We suggest this novel role of urea as a metabolic inhibitor has adaptive significance for energy conservation in the hibernating wood frog as well as other seasonally dormant animals. NSF IOB 0416750.