Details

Autor(en) / Beteiligte

• Cogburn, Larry A.
• Freeman, Ray M.

Titel

Response surface of daily thyroid hormone rhythms in young chickens exposed to constant ambient temperature

Ist Teil von

• General and comparative endocrinology, 1987-10, Vol.68 (1), p.113-123

Ort / Verlag

San Diego, CA: Elsevier Inc

Erscheinungsjahr

1987

Quelle

Elsevier Journal Backfiles on ScienceDirect (DFG Nationallizenzen)

Beschreibungen/Notizen

• Response surface models were developed by multiple regression techniques to describe daily rhythms of plasma thyroid hormones in young chickens during acclimation to a wide range of ambient temperatures ( T a). Seventy-two Leghorn cockerels were raised in six environmental chambers under a 12L:12D light/dark cycle. Beginning at 21 days of age, two replicate chambers provided T a treatments of 10, 24, or 38°. During Day 6 of acclimation, blood samples were obtained at 0800, 1200, 1600, 2000, 2400, and 0400 hr for thyroid hormone analyses. Plasma triiodothyronine (T 3) levels showed a convex response to the experimental region (10 to 38°, and 0800 to 0400 hr under a 12L:12D light/dark cycle), whereas the plasma thyroxine (T 4) response surface was concave. In particular, heat exposure (38°) depressed ( P < .01) plasma T 3 levels (2.8 ng/ml) below those of birds held at 24° (4.5 ng/ml), whereas elevated ( p < 0.05) T 3 concentrations were found in the 10° treatment (5.0 ng/ml). Plasma T 3 levels rose gradually during photophase and slowly declined during scotophase. Plasma T 4 concentrations followed an opposite pattern with the daily zenith in scotophase and the nadir during photophase. Plasma T 4 levels were sharply elevated in birds maintained at 38°, particularly during scotophase. Although T a did not change the daily amplitude or circadian frequency of the plasma T 3 rhythm, an increased frequency or ultradian rhythm of plasma T 4 was observed in cold-acclimated birds. The counter-responses of T 3 and T 4 to heat exposure and phase of the light/dark cycle were best seen in the molar ratio of plasma T 3 T 4 levels. The average daily T 3 T 4 molar ratio for heat-exposed birds was 0.16 while the average T 3 T 4 ratio was higher ( P < 0.01) for the 10° (0.46) and 24° (0.41) treatments. The opposing responses of plasma T 3 and T 4 levels to a wide range of T a and phase of the light/dark cycle support a major role for circulating T 3 in metabolic adjustments that chickens make to changes in the photothermal environment.