Short Communication: Urate does not Accumulate in the Haemolymph of Exercised Blue Crabs, Calunectes Sapidus

L-Lactate, the only known anaerobic end-product in decapod crustaceans (Gade, 1983), increases haemocyanin oxygen-affinity (Tnichot, 1980; Mangum, 1983; Bridges and Morris, 1986). In exercised blue crabs, Callinectes sapidus, the decrease in haemocyanin oxygen-affinity induced by metabolic acidosis via the Bohr shift was substantially balanced by the opposing effect of L-lactate (Booth etal, 1982). Since this study, Morris et al. (1985) have discovered that urate also enhances haemocyanin oxygen-affinity in the crayfish Austropotamobius pallipes. The importance of urate in regulating oxygen transport during environmental hypoxia has recently been stressed in the crab Carcinus maenas and the prawn Penaeus japonicus (Lallier and Truchot, 1989a,b). Since Callinectes sapidus haemocyanin exhibits a urate effect (De Fur et al. 1990), urate may also play a role during functional anaerobiosis. In the land crab Birgus latro, which is mostly uricotelic (Greenaway and Morris, 1989), there is no increase in haemolymph urate concentration following exercise (Greenaway et al. 1988). Because aquatic crustaceans are ammonotelic animals, little attention has been devoted to variations in urate levels in this group. However, it has recently been shown that urate accumulates in the haemolymph of decapod crustaceans exposed to environmental hypoxia (Lallier et al. 1987; Czietrich et al. 1987; De Fur et al. 1990) or elevated temperature (Lallier et al. 1987). Its probable origin, even though not directly demonstrated in vivo, may be the degradation of purine nucleotide compounds, such as AMP, GMP or IMP, via the purinolytic pathway. Xanthine dehydrogenase (XDH; EC 1.2.1.37), the enzyme required to produce urate via xanthine from these substrates, is present in the hepatopancreas of both Carcinus maenas (Dykens and Shick, 1988) and Callinectes sapidus [maximum activity 17.5±7.2nmol of urate produced per minute and per gram hepatopancreas (mean±s.E., N=7), F. Lallier and P. J. Walsh, unpublished results]. Alternatively, xanthine oxidase (XOD, EC 1.2.3.2), which uses oxygen rather than NAD as a cofactor, may play the role of XDH.

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