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The effects ql ionic and/or osmotic change on skeletal muscle mitochondrial performance were investigated. Two substrates, pyruvate and glutamate, and variation in osmotic pressure from 205 to 360 mosm in KCl or mannitol/sucrose media had no effect on maximal respiratory rate (state 3) or coupling (respiratory control ratio) in either species. Over an equivalent range of osmolalities associated with dehydration, organismic maximal 02-consumption rates are severely diminished with increasing osmolalities. The data do not support a mitochondria/limit to organismic 02 consumption under dehydration. There were interspecies differences in state 3 respiration and coupling that were similar to differences noted in mitochondria isolated from fish red and white muscle, with toad mitochondria behaving more like red muscle and frog mitochondria behaving more like white muscle.

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