Sodium uptake by rainbow trout gills has been investigated with a small-volume system enabling rapid, successive flux measurements in different solutions. Sodium influx obeys a Michaelis-Menten type relation, with a Km of 0.46 mM, and uptake proceeds unimpaired in the absence of penetrating counterions. This suggests a coupled cation exchange. Ammonia output is about the same as the Na+ influx when external [Na+] is 1 mM, but at higher or lower Na+ influxes, the correlation does not hold. A progressive downward shift in the pH of the irrigating medium as Na+ influx increases indicates that the exchanging cation is hydrogen. In support of this, acetazolamide, which inhibits Na+ uptake, also prevents the downward pH shift. The potential across the gill is about 10 my, body fluids positive, in NaC1 solutions up to 10 mM, and is little affected by changes in Na+ concentration below that. Finally, evidence for locating the rate-limiting step at the outer membrane of the epithelium is presented.

Total lipid and lipid class composition of selected tissues and serum were determined for juvenile steelhead trout Sulmo guirdneri, during the smolting period between 25 January and 19 April 1982. The total lipid content ofthe serum, liver, light muscle and dark muscle was significantly depleted during smolt transformation. Mesenteric fat total lipid concentration fluctuated little over the sampling period. Phospholipids of muscle and mesenteric fat exhibited little seasonality. Cholesterol was significantly depleted from dark muscle and liver. Large stores of triacylglycerol (TGL) in the parrs' mesenteric fat, dark muscle and liver implicate these tissues as lipid depot organs. In those tissues depleted of lipid, TGL concentrations were reduced more than any other lipid class. This supports the hypothesis that increased catabolism is the cause of loss of body fat during salmonid smolt transformation.

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Rainbow trout, immunized by the immersion method against Vihrio unguillurum. were used as donors of plasma and celis for transfer to unimmunized trout. Recipients were then challenged with a 20 min exposure to a virulent Vibrio unguillurum suspension, and the mortality was compared to that of trout receiving plasma and cells from unimmunized donors. Immunity was successfully conferred by plasma taken from immune donors 16 days post-immunization (PI), and by pronephros and spleen cells from Day 20 and 21 PI donors. Plasma was most effective in transferring vibriosis protection, followed by pronephros cells. Splenocytes gave variable results, while trials with thymus cells were negative in all cases.

No abstract