Abstract
To evaluate circadian changes in thyroid hormones of piau, Leporinus obtusidens Valenciennes, 1847, (Osteichthyes, Anostomidae) after feeding were determined the plasma levels of thyroid hormones (TH) of 128 fishes, same age, immature, both sexes, distributed into four classes of weight. They were kept in 16 aquaria (100 L), with artificial aeration and 2 L min. -1 water flow, from March to August, 1996, in Aquaculture Station of the Universidade Federal de Lavras (UFLA). Daily, feed was supplied at 5% body weight. For blood samples, the fishes were anesthetized with benzocaine 10%, and plasmatic levels of TH were determined at 1, 4, 7, 10, 13, 16 and 22h after food supply during 4 consecutive days in June and in August, period of lowest temperature. The classes of weight 63, 75, 82 and 91 g were considered homogeneous, showing that plasmatic concentrations of T 3 and T 4 were body weight independent for this fish species. The daily cycle of plasma TH were correlated with feed intake, reaching the highest levels 7 h after feeding supply (T 3 = 1.75 ± 0.07 ng mL -1 and T 4 = 14.9 ± 1.59 ng mL -1 ). It was also possible to verify that the daily intake is directly correlated with water temperature which is affected by day-light cycle
Highlights
The plasmatic levels of thyroid hormones in teleosts has been studied in last decades by Spieler and Noeske (1979; 1981; 1984) with goldfish; Stacey et al (1984) with white sucker; Rydevik et al (1984; 1989), Youngson and McLay (1989) with salmon; White and Henderson (1977), Leatherland et al (1978), Osborn et al (1978), Eales et al (1981), Flood and Eales (1983), Mc Cormick and Naiman (1984), Acta Scientiarum
The photoperiod presented a constant reduction through this period, reaching the lowest value (10.7 hours) in 14 to 26 June (Figure 3), from that on, started a gradual increase
In June and August there was an increase in T3 plasmatic level seven hours after food supply (1.75 ± 0.07 ng mL-1 and 1.58 ± 0.12 ng mL-1, respectively), decreasing at 10 hours, and kept invariable in subsequent times. These results clearly show that there is a cycle in T3 plasmatic level related to feeding
Summary
The plasmatic levels of thyroid hormones in teleosts has been studied in last decades by Spieler and Noeske (1979; 1981; 1984) with goldfish; Stacey et al (1984) with white sucker; Rydevik et al (1984; 1989), Youngson and McLay (1989) with salmon; White and Henderson (1977), Leatherland et al (1978), Osborn et al (1978), Eales et al (1981), Flood and Eales (1983), Mc Cormick and Naiman (1984), Acta Scientiarum. Many works have shown the influence of food as a factor that produces alterations in biological rhythms of fishes; rhythms that are controlled by hormones such as thyroid hormones. The action of food ingestion in teleosts hypothalamic-hypophysis-thyroid axis, in peripheral metabolism of thyroid hormones, is not well described, such as the nutritional conditions that affect the thyroid system (CUENCA; GALLEGO, 1987)
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