抄録
The effects of administration of synthetic thyrotropin-releasing hormone (TRH) on circulating growth hormone (GH), prolactin (PRL) and triiodothyronine (T3) levels of lactating dairy cows, non-lactating dairy heifers, and beef cows were studied. Intravenous administration of 0.1, 1, and 5μg of TRH per kg of body weight (bw) elevated plasma GH and PRL levels of lactating cows within 5min. The plasma GH and PRL levels increased in proportion to the dose of TRH and reached a peak 10 to 30min after TRH injection. Intravenous administration of 1μg of TRH per kg of bw to 7 non-lactating heifers, 14 lactating dairy cows, and 5 non-lactating beef cows elevated plasma GH level to peak values after 15min, the increase rates being 6.9, 5.6, and 3.8times as high as those in the pretreatment levels. The mean maximum value was also in that order. Plasma T3 levels of non-lactating dairy heifers at pre-and post-injection of TRH were significantly higher than those of lactating cows. The peak values of plasma PRL were obtained between 5 to 30min after TRH administration. The increase rates of lactating dairy cows, heifers, and beef cows were 19.2, 13.9, and 20.9times as high as those in the pretreatment. In contrast to GH and T3, plasma PRL levels of both pre- and post-injection with TRH in lactating cows and heifers were significantly higher in May than in October, though the increase rates were similar. Plasma PRL levels of lactating dairy cows at pre- and post-injection with TRH were significantly higher than those of non-lactating heifers. Subcutaneous administration of TRH was also effective to increase plasma GH, PRL, and T3 levels in lactating cows. No significant change of GH or PRL response to TRH was observed after a short-term pretreatment of thyroid hormones.
