The effects of hypophysectomy on osmoregulation in the euryhaline flounder Platichthys flesus (L.)

Euryhaline teleostean fish. Inhabit a range of aquatic environments in which the external osmolality varies from less than Im0sm/kg in freshwater to circa l000m0sm/kg in sea water. Teleosts in hypoosmotic media are subject to an osmotic influx of water and a passive efflux of electrolytes, whereas in hyperosmotic conditions these fluxes operate in the reverse direction. Against these adverse effects euryhaline teleosts are able to maintain a regulated internal osmolality within the region of 300m0sm/kg. A homeostatic control of water content and electrolyte composition is achieved by the activities of the three osmo(iono-)regulatory organs - the gill, gut and kidney. A scheme for teleostean osmoregulation was proposed originally by Smith (1930, 1932) and Krogh (l939). Comprehensive reviews of more recent advances in this field are available (Black, 1957; Potts and Parry, 1964; Parry, 1966; Maetz, 1968,1970a; Potts, 1968; Conte, 1969; Hickman and Trump, 1969; Holmes and Donaldson, 1969). In sea water teleosts an osmotic efflux of water is compensated by the drinking and subsequent intestinal absorption of the external medium. Monovalent ions absorbed from the gut are excreted at the gill and divalent ions are lost primarily via the kidney, which also conserves water. In freshwater teleosts drinking is reduced and the osmotic influx of water is excreted by the kidney, which conserves ions to 2 produce a copious, dilute urine. The passive efflux and renal depletion of ions is compensated by their active absorption by the gill from the external medium