Short Communication the Spleen in Hypoxic and Exercised Rainbow Trout

The teleost spleen is a discrete organ containing, in addition to erythrocytes sequestered from the circulation, erythropoietic tissue involved in the synthesis of new erythrocytes. In this division into supply and synthesis, there appear to be some species differences. The eel spleen is an erythrocyte reservoir but is not thought to be a major erythropoietic organ (Johansson-Sjobeck, 1979). The splenic mass in goldfish does not change with induced anaemia (Houston etal. 1988). The trout spleen, in contrast, plays an erythropoietic role during anaemia (Lane, 1979), in addition to serving as a store of erythrocytes (Randall and Daxboeck, 1982). Exercise in trout is accompanied by haemoconcentration (Black etal. 1966; Stevens, 1968) and the spleen appears to contribute to elevated circulating haemoglobin levels (Stevens, 1968). Capture stress induced a 25% reduction in spleen haemoglobin concentration, [Hb], in the marine teleost Girella tricuspidata, and splenic histology revealed melano-macrophagic centres and erythropoietic tissue indicative of a major role in erythrocyte destruction and synthesis (Ling, 1984). Neither role can be played by the spleen of the icefish, which lacks haemoglobin (Wells et al. 1990). Although storage and synthesis of erythrocytes in the spleen have been examined in few fish, several intraspecific factors affect the size of the spleen. These include reproductive status and body weight (Yamamoto and Itazawa, 1989), exercise (Yamamoto et al. 1980; Yamamoto, 1988) and hypoxia (Yamamoto et al. 1985; Wells et al. 1989). Active fish appear to have highly contractile spleens that assist in the expulsion of erythrocytes (Nilsson and Grove, 1974). Rainbow trout, more than any other fish, have received the close attention of researchers, and yet we can find little information concerning the possible role of the trout spleen in erythrocyte supply during exercise and hypoxia. Furthermore, no attempt has been made to discern whether erythrocytes stored in the spleen are qualitatively different from those in general circulation. In this study we have addressed these problems. Female rainbow trout (Salmo gairdneri Richardson) were obtained from a