1979 Volume 26 Issue 5 Pages 533-539
We studied the effects of the administration of dexamethasone (DX) with or without thyrotropin releasing hormone (TRH), thyroxine (T4), growth hormone (GH), and prolactin (PRL) on the plasma 3, 5, 3'-triiodothyronine (T3), 3, 3', 5'-triiodothyronine (rT3), T4, thyrotropin (TSH), GH, PRL, insulin, and milk production in36 normal lactating cows. A single subcutaneous injection of 10mg DX resulted in 21% reduction in milk production on the following day. While plasma rT3level increased 1.4 fold, plasma T3 concentration reduced by 52% 24hr after DX injection (p<0.001). Reduction in plasma TSH and T4 levels was small and no significant depression of plasma GH and PRL was detected. A prompt and prolonged increase in plasma insulin concentration was observed. Simultaneous injections of 10mg DX with 2mg TRH followed by an additional injection of the same dose of TRH5hr later, resulted in no depression of milk production. Plasma T3, insulin, GH, and PRL levels 5hr after the first hormone injection was significantly elevated. Plasma T4and rT3 concentrations 24hr after the injection increased 1.78 and 3.2 fold, respectively (p<0.001), but the levels of T3, GH, and PRL returned to their pre-treatment levels. The mean plasma GH level significantly increased. In the control cows, neither plasma hormone levels nor milk production significantly changed. The injection of 10mg T4 or 20mg GH twice a day with an administration of 10mg DX reduced the milk production by less than 9% on the following day. On the other hand, no preventive effect of concomitant administration of PRL (54mg×2) with 10mg DX on depression in milk production was detected. Plasma T3 level 24hr after the simultaneous administration of DX with T4 was 1.65times as high as the pre-injection level and the level continued for several days. However, plasma T3 reduced by 40% 24hr after the administration of either DX+GH or DX+PRL. The preventive effect of TRH on the depression of milk production caused by DX might be partly derived from a synergistic action of the enhanced circulating levels of thyroid hormones and GH. A possibility of additional favourable TRH actions on lactation other than through adenohypophysis also exists.
