Chronic heat stress and prenatal development in sheep: I. Conceptus growth and maternal plasma hormones and metabolites.
Pregnant ewes were chronically exposed to thermoneutral (TN; 20 degrees C, 30% relative humidity) or hot (H; 40 degrees C 9 h/d, 30 degrees C 15 h/d, 40% relative humidity) environments between d 64 and 136 to 141 of pregnancy. They were sampled for blood at 14-d intervals during this period for measurement of plasma metabolites and hormones, then slaughtered and dissected to measure conceptus weights, dimensions and fetal organ weights. Rectal temperatures of H ewes were elevated .3 to 1.0 C degrees above those of TN ewes throughout the experiment. Voluntary feed intakes were not altered by heat exposure except after 120 d of pregnancy, when feed intake was about 25% lower (P less than .10) by H than by TN ewes. Blood 3-hydroxybutyrate concentrations were not affected by heat, but plasma glucose concentrations were greater in H than in TN animals after 120 d (P less than .05). Placental weight, reduced by 54% (P less than .001) by heat exposure of ewes, was correlated positively with fetal weight and correlated negatively with fetal/placental weight ratio, fetal brain/liver weight ratio and fetal relative heart weight. Late in pregnancy, plasma concentrations of progesterone, cortisol and placental lactogen were reduced (P less than .01) in H ewes, whereas triiodothyronine levels were markedly lower (P less than .03) at all stages of pregnancy. Plasma concentrations of prolactin were elevated dramatically (P less than .01) and a modest increase (P less than .03) in somatotropin levels was recorded in H ewes. These results are consistent with our hypothesis that heat-induced fetal growth retardation is secondary to a primary reduction in placental growth; this could be mediated partly by reduced peripheral activity of thyroid hormones. Heat-induced reductions in secretion of progesterone and ovine placental lactogen more likely were a consequence than a cause of placental stunting.
Bell
A W
AW
Dept. of Anim. Sci., Cornell University, Ithaca, NY 14853.
McBride
B W
BW
Slepetis
R
R
Early
R J
RJ
Currie
W B
WB
D000818:Animals; D001831:Body Temperature; D004435:Eating; D005314:Embryonic and Fetal Development; D004958:Estradiol / Q000097:blood; D005260:Female; D006728:Hormones / Q000097:blood*; D006358:Hot Temperature; D006854:Hydrocortisone / Q000097:blood; D008098:Litter Size; D010929:Placentation; D011247:Pregnancy; D011248:Pregnancy Complications / Q000097:blood / Q000378:metabolism / Q000662:veterinary*; D011374:Progesterone / Q000097:blood; D012756:Sheep; D012757:Sheep Diseases / Q000097:blood / Q000378:metabolism*; D013312:Stress, Physiological / Q000097:blood / Q000378:metabolism / Q000662:veterinary*; D013974:Thyroxine / Q000097:blood; D015430:Weight Gain
D006728:Hormones; D011374:Progesterone; D004958:Estradiol; D013974:Thyroxine; D006854:Hydrocortisone