Osmorégulation et métabolisme de l'urée : comparaison entre Bufo viridis et Rana temporaria

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Abstract
A comparison is made between the euryhaline species Bufo viridis, which can survive in more than 50% sea water, and the stenohaline species Rana temporaria, which cannot. The elevated blood urea, which B. viridis uses for osmoregulation, comes from a temporary reduction in the amount of urea excreted and possibly from an increased urea production. Below 40% seawater B. viridis shows no change in body weight but in 30% seawater there is a marked reduction in urea excretion and an increase in liver arginase activity. High salinity does not increase arginase activity further. The toads do show a weight loss with each increment of salinity above 40% seawater but the daily urea excretion returns towards the level of freshwater controls. In 60% seawater (546 milliosmolal) the bladder urine concentration is 575 ± 51 mosmolal with urea accounting for 107 ± 8 mosmolal and that of the contents of the large intestine is 582 ± 41 mosmolal but with only traces of urea. Nematodes (Aplectana sp.) in the large intestine are unaffected by 60% seawater.On the other hand, R. temporaria shows an increase in body weight when placed in 10% and in 20% seawater. In 30% seawater there is weight loss and increased urea excretion but no increase in liver arginase activity.