Rat thyrotrophin levels during heat stress and stimulation by thyrotrophin releasing factor

The negative feedback action of the hypothalamic\p=m-\hypophysial\p=m-\thyroidsystem is abolished in rats kept at 37 \s=deg\C,presumably due to the stress of the high temperature(Weller, Dikstein & Sulman 1969; Tal & Sulman, 1971). The present investigation compares pituitary thyroid-stimulating hormone (TSH) levels in rats kept at room temperature (23 \s=deg\C)or at higher temperatures (34 \s=deg\Cor 37 \s=deg\C). Six groups of 12 male albino rats of the Hebrew University Sabra strain, 42 days old and weighing 120 \m=+-\5 g each, were housed six to a cage in three rooms at 23 \m=+-\1 \s=deg\C, 34 \m=+-\1 \s=deg\Cand 37 \m=+-\1 \s=deg\Crespectively. The rats received standard laboratory pellets and water ad libitum. Synthetic thyrotrophin releasing factor (TRF) (300 ng/kg body wt) (pyroglutamyl-histidyl-prolinamide, Farbwerke Hoechst A.G., Frankfurt) was injected i.v. (Kendall, Rees & Kramer, 1971) on the 4th day in the experimental groups. The rats were killed 1 h later and blood and pituitaries were removed. Serum and homogenates of the pituitaries were assayed for TSH (McKenzie, 1958) using bovine TSH (NIH-B2) as standard. Serum thyroxine (T4) was assayed by the Tetralute-Ames-Yissum method (Braverman, Vagenakis, Foster & Ingbar, 1971). Pituitary TSH content was also measured in another six groups of 12 rats kept unde