EFFECT OF HEAT-STRESS ON TONIC AND GNRH-INDUCED GONADOTROPIN-SECRETION IN RELATION TO CONCENTRATION OF ESTRADIOL IN PLASMA OF CYCLIC COWS

Effects of acute and seasonal heat stress on tonic and GnRH-induced LH and FSH secretion were examined during the early follicular phase of the oestrous cycle of cows (n = 40). Prostaglandin F-2 alpha was injec ted on day 11 +/- 1 of the oestrous cycle and on the next day blood sa mples were collected at intervals of 15-20 min for 14 h, and i.m. inje ction of GnRH was given after 7 h. Treatments compared were control ve rsus acute heat stress during blood sampling in winter, and cooled ver sus chronic heat stress in summer. Before GnRH injection, chronic heat stress in summer did not affect basal concentrations of plasma LH, bu t did lower LH pulse amplitude. However, in cows with low plasma oestr adiol (1.9 +/- 0.2 pg ml(-1)), the mean and basal concentrations and a mplitude of tonic LH pulses were reduced by heat stress (3.1, 2.1 and 4.8 versus 1.9, 1.4 and 2.5 ng ml(-1), respectively). In cows with hig h plasma oestradiol (6.3 +/- 0.5 pg ml(-1)), these parameters were not affected. In chronically heat stressed cows in summer, GnRH-induced i ncreases in plasma LH and FSH concentrations were the same as in the c ooled controls. However, in cows with low plasma oestradiol, mean conc entrations of FSH in plasma (31.8 versus 25.5 ng ml(-1)), the peak of the GnRH-induced FSH and LH surge (FSH 47.4 versus 35.6 ng ml(-1) LH 5 0.7 versus 37.3 ng ml(-1)) and the shape of the GnRH-induced FSH and L H curves (treatment by time interaction) were significantly lower in n on-cooled versus cooled controls. The GnRH-induced increase in LH secr etion was unaffected by chronic heat stress in cows with high concentr ations of oestradiol in plasma. In winter, acute heat stress depressed the mean concentration of FSH in plasma and decreased the GnRH-induce d release of FSH in cases with low but not with high concentrations of oestradiol in plasma. The peak of the GnRH-induced surge of LH in all acutely heat stressed cows was significantly lower in winter than in control cows, irrespective of concentrations of oestradiol in plasma. These results show that heat stress affects the secretion of gonadotro phins more in cows with low concentrations of oestradiol than in those with high concentrations of oestradiol in plasma.