The effect of citrate loading on exercise performance, acid-base balance and metabolism

Nine subjects (VO2max 65 +/- 2 ml.kg-1.min-1, mean +/- SEM) were studied on two occasions following ingestion of 500 ml solution containing either sodium citrate (C, 0.300 g.kg-1 body mass) or a sodium chloride placebo (P, 0.045 g.kg-1 body mass). Exercise began 60 min later and consisted of cycle ergometer exercise performed continuously for 20 min each at power outputs corresponding to 33% and 66% VO2max, followed by exercise to exhaustion at 95% VO2max. Pre-exercise arterialized-venous [H+] was lower in C (36.2 +/- 0.5 nmol.l-1; pH 7.44) than P (39.4 +/- 0.4 nmol.l-1; pH 7.40); the plasma [H+] remained lower and [HCO3-] remained higher in C than P throughout exercise and recovery. Exercise time to exhaustion at 95% VO2max was similar in C (310 +/- 69 s) and P (313 +/- 74 s). Cardiorespiratory variables (ventilation, VO2, VCO2, heart rate) measured during exercise were similar in the two conditions. The plasma [citrate] was higher in C at rest (C, 195 +/- 19 mumol.l-1; P, 81 +/- 7 mumol.l-1) and throughout exercise and recovery. The plasma [lactate] and [free fatty acid] were not affected by citrate loading but the plasma [glycerol] was lower during exercise in C than P. In conclusion, sodium citrate ingestion had an alkalinizing effect in the plasma but did not improve endurance time during exercise at 95% VO2max. Furthermore, citrate loading may have prevented the stimulation of lipolysis normally observed with exercise and prevented the stimulation of glycolysis in muscle normally observed in bicarbonate-induced alkalosis.     

Dehydration and serum biochemical changes in marathon runners

The effects of a competitive marathon race on serum biochemical and haematological parameters have been evaluated. Blood samples were obtained shortly before and immediately after the race; urine samples were also obtained before and after the race. Body weight was recorded pre- and post-race. During the race subjects consumed a total of 1.41 of either water or a dilute glucose-electrolyte solution. The average weight loss of the runners was 2.09 +/- 0.77 kg (mean +/- SD), corresponding to 2.9 +/- 0.8% of body weight. Small but significant increases in both haematocrit and haemoglobin concentration occurred; plasma volume was calculated to decrease by 4.7%. Serum potassium concentration showed no change, but the response was highly variable; serum sodium concentration increased in line with the decrease in plasma volume. In the group of subjects drinking water during the race, the pre-race plasma glucose concentration was 5.3 +/- 1.2 mmol . l-1, this was unchanged after the race (5.0 +/- 1.2 mmol . l-1). A significant increase (P less than 0.01) in the plasma glucose concentration, from 5.2 +/- 0.6 to 6.0 +/- 1.5 mmol . l-1 occurred in the group of subjects drinking the glucose-electrolyte solution. Apart from this, there were no significant differences between the two groups.     

Early fluid and protein shifts in men during water immersion

High precision blood and plasma densitometry was used to measure transvascular fluid shifts during water immersion to the neck. Six men (28-49 years) undertook 30 min of standing immersion in water at 35.0 +/- 0.2 degrees C; immersion was preceded by 30 min control standing in air at 28 +/- 1 degrees C. Blood was sampled from an antecubital catheter for determination of blood density (BD), plasma density (PD), haematocrit (Ht), total plasma protein concentration (PPC), and plasma albumin concentration (PAC). Compared to control, significant decreases (p less than 0.01) in all these measures were observed after 20 min immersion. At 30 min, plasma volume had increased by 11.0 +/- 2.8%; the average density of the fluid shifted from extravascular fluid into the vascular compartment was 1006.3 g.l-1; albumin moved with the fluid and its albumin concentration was about one-third of the plasma protein concentration during early immersion. These calculations are based on the assumption that the F-cell ratio remained unchanged. No changes in erythrocyte water content during immersion were found. Thus, immersion-induced haemodilution is probably accompanied by protein (mainly albumin) augmentation which accompanies the intravascular fluid shift.     

Haematology and metabolism of the blood of the little penguin, Eudyptula minor

1. We studied the haematology of the little penguin, Eudyptula minor, and measured red cell enzymes, and the rate of O2 and glucose consumption and CO2 and lactate production of whole penguin blood. 2. Little penguins have a very low RBC count (1.66 x 10(12) l-1) and very large red cells (MCV = 229 microns 3), resulting in a normal haemoglobin and haematocrit. 3. Despite the lower body temperature of penguins, the rate of O2 consumption by whole blood is close to that predicted from studies on other birds.     

Embryonic and larval development of brown trout, Salmo trutta L.: exposure to trace metal mixtures in soft water

Freshly fertilized ova of brown trout, Salmo trutta L., were exposed to all possible mixtures of Al (6000 nmol 1¹), Cu (80 nmol 1⁻¹), Pb (50 nmol 1⁻¹) and Zn (300 nmol 1 ¹). In a separate experiment, newly hatched brown trout yolk-sac fry were exposed to Mn (1500 nmol 1⁻¹), Fe (2500 nmol 1 ¹), Ni (200 nmol 1⁻¹) or Cd (4 nmol 1 ¹), separately, and in mixtures with either Al or Cu. Both experiments were conducted in flowing, artificial softwater media nominally at pH 5.6 [Ca] 20 μmol 1 ¹ and 10° C.
Mortalities were high in fry subjected to treatments which contained both Al and Cu (31–72%), and to the Cu + Fe treatment (78%) compared with those from the other trace metal mixtures (0–22%). In all the treatments tested, fry exposed to trace metal mixtures containing Al and/or Cu had reduced whole body Ca, Na and K content, and seriously impaired skeletal calcification. Whole body Mg content was variable. In trace metal mixtures which contained Cu but not Al, the effects on fry survival and whole body mineral content were in general more deleterious than the corresponding mixtures but with Al present rather than Cu. The presence of Pb and/or Zn in mixtures with Al and/or Cu had a slight ameliorative effect in terms both of fry survival and whole body mineral content.     

Optimization of biomass, total carotenoids and astaxanthin production in Haematococcus pluvialis Flotow strain Steptoe (Nevada, USA) under laboratory conditions

The microalga Haematococcus pluvialis Flotow is one of the natural sources of astaxanthin, a pigment widely used in salmon feed. This study was made to discover optimal conditions for biomass and astaxanthin production in H. pluvialis from Steptoe, Nevada (USA), cultured in batch mode. Growth was carried out under autotrophic (with NaNO3, NH4Cl and urea) and mixotrophic conditions (with 4, 8, 12 mM sodium acetate) under two photon flux densities (PFD) (35 and 85 mumol m-2 s-1). The carotenogenesis was induced by 1) addition of NaCl (0.2 and 0.8%), 2) N-deprivation and 3) high PFD (150 mumol m-2 s-1). Total carotenoids were estimated by spectrophotometry and total astaxanthin by HPLC. Ammonium chloride was the best N-source for growth (k = 0.7 div day-1, 228-258 mg l-1 and 2.0 x 10(5)-2.5 x 10(5) cells ml-1 at both PFD, respectively). With increasing acetate concentration, a slight increment in growth occurred only at 85 mumol m-2 s-1. Light was the best inductive carotenogenic factor, and the highest carotenoid production (4.9 mg l-1, 25.0 pg cell-1) was obtained in cultures pre-grown in nitrate at low light. The NaCl caused an increase in carotenoid content per cell at increasing salt concentrations, but resulted in a high cell mortality and did not produce any increment in carotenoid content per volume compared to cultures grown at 150 mumol m-2 s-1. The highest carotenoid content per cell (22 pg) and astaxanthin content per dry weight (10.3 mg g-1) (1% w/w) were obtained at 85 mumol m-2 s-1 with 0.8% NaCl.     

Dietary sodium restriction and the development of two-kidney, one-clip hypertension in young versus adult rats.

To determine the effects of moderate versus severe dietary sodium restriction on the development of 2-kidney, 1-clip (2K,1C) hypertension, young male Wistar rats were placed on diets containing 9, 26, or 101 (control) mumol sodium/g food. Three days later, a solid silver clip (i.d. 0.20 mm) was placed on the left renal artery and diets were continued up to 6 weeks. Adult rats received a 0.25-mm clip. In young clipped rats receiving the 101 mumol/g diet, blood pressure (BP), plasma renin activity (PRA), and BP response to captopril were increased as early as 1 week after clipping and increased further over time. Moderate sodium restriction (26 mumol sodium/g) led to only a slight delay in the development of hypertension; the levels of BP and PRA, the BP response to captopril, and the extent of cardiac hypertrophy achieved by 6 weeks were not different between the 2K, 1C rats receiving 26 or 101 mumol sodium/g. Sodium restriction to 9 mumol/g decreased rate of growth and completely prevented the rise in BP and in left ventricular weight. At 3 and 6 weeks the severely sodium-restricted rats had significantly higher PRA levels than the 2K, 1C control group. However, the BP response to captopril was attenuated relative to the other hypertensive groups. In adult rats, this level of sodium restriction had a small, but significant effect on body weight, but still prevented the increase in BP and in left ventricular weight. In conclusion, dietary sodium restriction can prevent the development of 2K,1C hypertension in both young and adult rats, but only if the restriction is severe. This effect may relate to a marked reduction in the pressor effectiveness of the renin-angiotensin system by low sodium intake per se or by associated metabolic or other changes.     

Changes in arterial blood pressure, heart rate and haematocrit during acute hyperkalaemia in conscious sheep.

The systolic, diastolic and mean arterial blood pressures, heart rate and haematocrit were measured at 15 minute intervals before, during and after 2 hour infusions of 0-4 mol.l-1 NaCl at 2-2 ml min-1 into conscious intact sheep and 0-4 mol. l-1 KCl at 2-2 ml. min-1 into conscious sheep which were either intact or adrenalectomized. The haemotocrit was also measured in splenectomized sheep receiving 0-4 mol. l-1 KCl. The NaCl infusion had no significant effect on blood pressure(BP), heart rate and haematocrit. Both intact and adrenalectomized sheep were able to withstand an increase in plasma potassium concentration in excess of 50% of the preinfusion concentration before any substantial fall in BP occurred. In intact and adrenalectomized sheep, heart rate and haematocrit increased rapidly and progressively throughout the potassium infusions and at maximum plasma potassium concentration the mean increments in these parameters for both groups of sheep were 21-6+/-2-69 beats/min and 7-5+/-0-47% respectively. Heart rate and haematocrit were more closely correlated with the plasma potassium concentration than with any other variable measured in these experiments. Adrenalectomy did not reduce the ability of the sheep to maintain their BP or to increase their heart rate and haematocrit. As the mean increase in haematocrit during potassium infusion into splenectomized sheep was 1-3+/-0-45% most of the increase in haematocrit observed in the potassium-infused intact and adrenalectomized sheep was caused by ejection of red cells from the spleen into the circulation.     

Evaluating in vitro and in vivo the interference of ascorbate and acetaminophen on glucose detection by a needle-type glucose sensor.

The aim of this work was to assess, in vitro and in vivo, the interference of ascorbate and acetaminophen on glucose measurements by a needle-type glucose sensor detecting hydrogen peroxide generated during the enzymatic oxidation of glucose, and to ascertain whether the protection against interference by the membranes used in the construction of the electrode is feasible. The oxidation of ascorbate and acetaminophen on a platinum electrode set at a 650 mV potential yielded a current representing 75 +/- 5% and 25 +/- 6% of that generated by the oxidation of an equimolar concentration of hydrogen peroxide, respectively. The bias introduced by the presence of 100 mumol l-1 ascorbate on the reading of 5 mmol l-1 glucose by the complete sensor (electrode + membranes) would be minimal (approximately 0.4 mmol l-1). By contrast, the bias introduced by 200 mumol l-1 of acetaminophen (a plasma concentration easily reached in clinical practice) was about 7 mmol l-1. The sensor was implanted subcutaneously in anaesthetized rats (n = 3). Using the current generated in the presence of a plasma acetaminophen concentration of about 200 mumol l-1 for glucose monitoring would lead to a major underestimation (approx. 6 mmol l-1) of subcutaneous glucose concentrations.     

Effects of pH and/or calcium-enriched freshwater on plasma levels of vitellogenin and Ca2+ and on bone calcium content during exogenous vitellogenesis in rainbow trout (Salmo gairdneri)

1. Three year old rainbow trout were exposed to low pH (5.1) and/or calcium-enriched (1.52 mM) freshwater for 10 weeks. 2. Plasma was collected periodically from individually-marked fish for analysis of total calcium and alkaline-labile phosphate (vitellogenin). 3. After the last sample gonadosomatic and hepatosomatic indices were measured and the caudal vertebrae centra were analysed for total calcium content. 4. Female trout exposed to calcium-enriched freshwater had increased plasma vitellogenin levels compared to females in soft water, whereas there was a tendency for low pH to decrease plasma vitellogenin in these fish. 5. The gonadosomatic and hepatosomatic indices were reduced in female trout exposed to acidified water. 6. There was no evidence of bone demineralization in trout exposed to low pH.     

Purification and characterization of (2S)-flavanone 3-hydroxylase from Petunia hybrida

(2S)-Flavanone 3-hydroxylase from flowers of Petunia hybrida catalyses the conversion of (2S)-naringenin to (2R,3R)-dihydrokaempferol. The enzyme could be partially stabilized under anaerobic conditions in the presence of ascorbate. For purification, 2-oxoglutarate and Fe2+ had to be added to the buffers. The hydroxylase was purified about 200-fold by a six-step procedure with low recovery. The Mr of the enzyme was estimated by gel filtration to be about 74,000. The hydroxylase reaction has a pH optimum at pH 8.5 and requires as cofactors oxygen, 2-oxoglutarate, Fe2+ and ascorbate. With 2-oxo[1-14C]glutarate in the enzyme assay dihydrokaempferol and 14CO2 are formed in a molar ratio of 1:1. Catalase stimulates the reaction. The product was unequivocally identified as (+)-(2R,3R)-dihydrokaempferol. (2S)-Naringenin, but not the (2R)-enantiomer is a substrate of the hydroxylase. (2S)-Eriodictyol is converted to (2R,3R)-dihydroquercetin. In contrast, 5,7,3',4',5'-pentahydroxy-flavanone is not a substrate. Apparent Michaelis constants for (2S)-naringenin and 2-oxoglutarate were determined to be respectively 5.6 mumol X l-1 and 20 mumol X l-1 at pH 8.5. The Km for (2S)-eriodictyol is 12 mumol X l-1 at pH 8.0. Pyridine 2,4-dicarboxylate and 2,5-dicarboxylate are strong competitive inhibitors with respect to 2-oxoglutarate with Ki values of 1.2 mumol X l-1 and 40 mumol X l-1, respectively.     

Blood pressure and endocrine responses of healthy subjects in cold pressor test after acutely increased dietary sodium intake

The objective of the study was to compare blood pressure and endocrine responses in a cold pressure test in young healthy subjects who had shown increased blood pressure during an acutely increased sodium intake. Subjects (n = 53) added 121 mmol sodium into their normal diet for one week. If the mean arterial pressure had increased by a minimum of 5 mmHg compared to the control measure, they were selected for the experiments. The selected subjects (n = 8) were given 121 mmol supplemental sodium d-1 for 14 days after which they immersed the right hand into a cold (+10 degrees C) water bath for 5 min. The blood pressure increased (P < 0.05) during the test and was independent of the sodium intake. The plasma noradrenaline increased from 2.41 +/- 0.38 nmol l-1 to 2.82 +/- 0.42 nmol l-1 (P < 0.05) with normal diet and from 1.85 +/- 0.29 nmol l-1 to 2.40 +/- 0.37 nmol l-1 (P < 0.05) with high sodium diet. The starting concentrations and the endpoint concentrations were statistically similar. The plasma levels of natriuretic peptides (NT-proANP, ANP and BNP) did not change during the test, and the concentrations were independent of the sodium diet. To conclude, acutely increased sodium intake does not change blood pressure or hormonal responses in a cold pressor test in young healthy subjects.     

Characteristics of sinusoidal uptake and biliary excretion of cysteinyl leukotrienes in perfused rat liver

In single-pass perfused rat liver, the sinusoidal uptake of infused 3H-labelled leukotriene (LT) C4 (10 nmol.l-1) was inhibited by sulfobromophthalein. Inhibition was half-maximal at sulfobromophthalein concentrations of approximately 1.2 mumol.l-1 in the influent perfusate and leukotriene uptake was inhibited by maximally 34%. Sulfobromophthalein (20 mumol.l-1) also decreased the uptake of infused [3H]LTE4 (10 nmol.l-1) by 31%. Indocyanine green (10 mumol.l-1) inhibited the sinusoidal [3H]LTC4 uptake by 19%. Replacement of sodium in the perfusion medium by choline decreased the uptake of infused [3H]LTC4 (10 nmol.l-1) by 56%, but was without effect on the uptake of sulfobromophthalein. The canalicular excretion of LTC4, LTD4 and N-acetyl-LTE4 was inhibited by sulfobromophthalein. In contrast, the proportion of polar omega-oxidation metabolites recovered in bile following the infusion of [3H]LTC4 was increased. Taurocholate, which had no effect on the sinusoidal leukotriene uptake, increased bile flow and also the biliary elimination of the radioactivity taken up. With increasing taurocholate additions, the amount of LTD4 recovered in bile increased at the expense of LTC4. Following the infusion of [3H]LTD4 (10 nmol.l-1), a major biliary metabolite was LTC4 indicating a reconversion of LTD4 to LTC4. In the presence of taurocholate (40 mumol.l-1), however, this reconversion was completely inhibited. The findings suggest the involvement of different transport systems in the sinusoidal uptake of cysteinyl leukotrienes. LTC4 uptake is not affected by bile acids and has a sodium-dependent and a sodium-independent component, the latter probably being shared with organic dyes. Sulfobromophthalein also interferes with the canalicular transport of LTC4, LTD4 and N-acetyl-LTE4, but not with the excretion of omega-oxidized cysteinyl leukotrienes. The data may be relevant for the understanding of hepatic leukotriene processing in conditions like hyperbilirubinemia or cholestasis.     

Blood volume in inbred strain BALB/c, CBA/J and C57BL/10 mice determined by means of 59Fe-labelled red cells and 59Fe bound to transferrin.

Circulating red blood cell (RBC) and plasma volume was determined in male inbred strain BALB/c, CBA/J and C57BL/10 mice by parallel use of the 59Fe-labelled RBC dilution and the dilution of 59Fe bound to transferrin. The whole blood volumes values derived from the venous haematocrit and plasma volume were about double the values calculated from the venous haematocrit and circulating RBC volume. Comparison of the two methods thus explains the marked differences in different studies of blood volume in mice and shows that correct values can be obtained only by parallel measurements of RBC and plasma volume by separate methods, or by correcting the venous haematocrit to whole body haematocrit. Combination of the labelled RBC method and the 59Fe-transferrin method showed the blood volume values in the above strains of mice to be 10.35 +/- 0.16, 7.32 +/- 0.10 and 7.94 +/- 0.15 ml/g b.w. respectively. The ratio of whole body to venous haematocrit in these strains was was 57.3 +/- 1.6%, 68.0 +/- 1.8% and 69.5 +/- 2.2%. Significant interstrain differences were demonstrated in RBC, plasma and blood volume and in the venous and whole body haematocrit and their ratio.     

Effects of sampling conditions on selected blood variables of rainbow trout, Salmo gairdneri Richardson

The effects of two methods of specimen immobilization (MS 222 anaesthesia and stunning), two types of anticoagulant (EDTA and heparin), two storage temperature ranges (0–2°C and 22–25°C) and four sample storage periods (0, 1, 3, and 24 h) on the haemoglobin, haematocrit, plasma and packed cell sodium, potassium and chloride ion concentrations and packed cell ATP levels of rainbow trout were examined. Stored samples exhibited increases in cell volume, net transfer of sodium and chloride from plasma into cells, net loss of potassium to plasma and rapid depletion of ATP. Room temperature conditions and prolonged storage exacerbated these changes. Use of EDTA, particularly in combination with MS 222, frequently led to haemolysis. Least change in most variables was observed in samples drawn from stunned specimens, treated with heparin and refrigerated before use or preparation for deep cold storage.     

Content and deposition of major mineral elements in different tissues and empty body of growing bulls of the german simmental breed

54 bulls of the German Simmental breed were fed either on a high energy level (maize silage ad libitum and 1.8 kg concentrate) or on a low energy level (maize silage restrictively and 1.0 kg concentrate). In dependence on feeding intensity a mean daily weight gain of 870 or 1210 g was obtained. Animals were slaughtered with a live mass of 200 kg, 350 kg, 500 kg, 575 kg and 650 kg. Empty body was divided into 13 cuts and afterwards separated into lean, bone and adipose tissues and tendons. Major mineral element content was determined in these tissues as well as in the noncarcass parts.

In the lean tissue the mean content (200 kg) of 0.3 g calcium, 10 g phosphorus, 1 g magnesium, 2.3 g sodium and 14.8 g potassium/kg dry matter decreased slightly with rising live mass (200–650 kg). The contents of major mineral elements were much higher in bone tissue. For the fattening period from 200 to 650 kg of live mass mean contents of 151.5 g calcium, 71.3 g phosphorus, 3.2 g magnesium, 5.1 g sodium and 1.1 g potassium per kg DM were analysed. Mineral element content of bone tissue increased with rising live mass as well as animals on low feeding intensity showed a higher mineral content than on high energy level. In all, major mineral element content in fat tissue was very low. In noncarcass parts head and legs calcium and phosphorus had analogous to bone tissue the highest concentration. Hide showed a high content of sodium, whereas organs and digestive tract had a high content of potassium and phosphorus. Total mass of major mineral elements in the different tissues increased above all in the fattening period of 200 to 350 kg. In carcass as well as in empty body, mass of calcium and phosphorus was much higher than magnesium, sodium and potassium. Also animals on low feeding intensity showed a higher mass of major mineral elements in carcass and empty body than animals on high energy feeding intensity. The intensively fed bulls had a mean deposition of 12.7 g calcium, 6.9 g phosphorus, 0.37 g magnesium, 1.2 g sodium and 2.1 potassium per 1000 g of empty body weight gain, whereas restrictively fed bulls deposited in average 15.0 g calcium, 7.8 g phosphorus, 0.4 g magnesium, 1.2 g sodium and 2.4 g potassium per 1000 g of empty body weight gain.     

Basic haematological values in the Cameroon goat (Capra hircus)

1. Basic haematological values in 165 Cameroon goats (Capra hircus) are reported. 2. The erythrocyte count, mean 14.36 x 10(12) l-1, ranged from 8.24 to 24.7 x 10(12) l-1; the haematocrit values, mean 0.304, varied from 0.20 to 0.38; the haemoglobin content, mean 113.4 g/l, was in the range from 83.0 to 143.0 g/l and the leukocyte count, mean 13.67 x 10(9) l-1, had lowest and highest values between 5.4 and 24.5 x 10(9) l-1. 3. Comparing these blood constituents in 47 male and 118 female Cameroon goats we demonstrated statistically significant lower values of the haematocrit and haemoglobin content and a statistically significant higher proportion of lymphocytes in the female animals. 4. In 16 pregnant and 30 non-pregnant female Cameroon goats, all animals older than 3 years, no statistically significant differences of the red blood picture were noted. 5. Also in three groups, assorted according to age, no significant changes in basic haematological parameters were seen. 6. During one year follow-up of some haematological parameters, statistically significant seasonal changes were found. 7. All presented data are compared with values abstracted from the literature and discussed.     

Progressive changes in haematology and tissue water of sexually mature trout, Salmo gairdneri Richardson during the autumn and winter

Rainbow trout of the Kamploops variety were sampled at intervals from October to the end of March. During this period a decline was noted in red cell count, haematocrit, haemoglobin concentration and plasma osmotic concentration. Increases were seen, however, in mean cellular volume, mean corpuscular haemoglobin, mean corpuscular haemoglobin concentration and the water content of liver and dorsal muscle. Sexual differences were found in all values with the exception of mean corpuscular haemoglobin concentration and the water content of both tissues. Males always had higher values in those parameters in which sexual differences were noted. All trends, with the exception of male haemoglobin and mean cellular volume and female mean cellular volume and osmotic concentration, were significantly linear.
None of the findings in this study could be correlated with temperature or photoperiod. Neither could the declining plasma osmotic concentration be correlated with the rising mean cellular volume or tissue water content. However, correlations were noted between haemoglobin and haematocrit, red cell count and haematocrit and between osmotic concentration and haematocrit. A negative correlation was seen between mean corpuscular haemoglobin concentration and mean cellular volume.     

Triiodothyronine is needed for the acclimatization of brown trout (Salmo trutta) or rainbow trout (Oncorhynchus mykiss) to seawater]

Brown and rainbow trout, held in freshwater at 13 +/- 1 degrees, were injected, every 3 days, with iopanoic acid (IOP: 5 mg/100 g body wt), an inhibitor of deiodination of thyroxine (T4) to triiodothyronine (T3). One group of IOP-treated rainbow trout was immersed in T3 (20 micrograms/l water). In IOP trout, plasma T3 fell to very low levels by day 7, while changes in T4 levels were less marked. In IOP + T3 trout plasma T3 increased fivefold, plasma T4 being unchanged. No mortality occurred and plasma osmolarity (OP) was not altered by any treatment. After direct transfer to seawater (30/1000), IOP trout were unable to acclimate to salinity: all died within 2 or 3 days, while the survival at day 3 was 100% in control brown trout and 45 and 74% in control and IOP + T3 rainbow trout respectively. OP increased more in IOP and less in IOP + T3 than in controls. There was a significant inverse correlation between T3, but not T4, plasma level, at the time of transfer and the OP 1 day later. In conclusion, although T3 does not play a significant role in osmoregulation in freshwater, T3 and therefore the deiodination of T4 into T3, were required for the development of hypo-osmoregulatory capacity involved in acclimation of trout to seawater.