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 F2α was injected on day 11 ± 1 of the oestrous cycle and on the next day blood samples were collected at intervals of 15–20 min for 14 h, and i.m. injection of GnRH was given after 7 h. Treatments compared were control versus acute heat stress during blood sampling in winter, and cooled versus chronic heat stress in summer. Before GnRH injection, chronic heat stress in summer did not affect basal concentrations of plasma LH, but did lower LH pulse amplitude. However, in cows with low plasma oestradiol (1.9 ± 0.2 pg ml−1), the mean and basal concentrations and amplitude 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 high plasma oestradiol (6.3 ± 0.5 pg ml−1), these parameters were not affected. In chronically heat stressed cows in summer, GnRH-induced increases in plasma LH and FSH concentrations were the same as in the cooled controls. However, in cows with low plasma oestradiol, mean concentrations 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 50.7 versus 37.3 ng ml−1) and the shape of the GnRH-induced FSH and LH curves (treatment by time interaction) were significantly lower in non-cooled versus cooled controls. The GnRH-induced increase in LH secretion was unaffected by chronic heat stress in cows with high concentrations of oestradiol in plasma. In winter, acute heat stress depressed the mean concentration of FSH in plasma and decreased the GnRH-induced 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 gonadotrophins more in cows with low concentrations of oestradiol than in those with high concentrations of oestradiol in plasma.
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