PITUITARY AND THYROIDAL RESPONSES TO SYNTHETIC THYROTROPHIN RELEASING HORMONE IN TWO BREEDS OF CATTLE

Abstract

The effect of thyrotrophin releasing hormone (TRH) on thyroid and pituitary function has been investigated as a possible aid to the early selection of cattle suited to tropical conditions. Two groups of six calves were used, one a shorthorn breed (SH) adapted to temperate conditions and the other an Africander cross (AX) selected for tropical climates. The dose and time responses of serum thyroid-stimulating hormone (TSH), tri-iodothyronine (T₃), thyroxine (T₄) and prolactin to single, repeated or multiple doses of TRH were measured by radioimmunoassay. Levels of T₃ and T₄ before injection were lower in the SH than in the AX calves. After a single injection of TRH (0·4, 1, 2·5 or 5 μg/kg live weight) the percentage increase in T₃ in the AX group was significantly lower than that in the SH group. No relationship was observed between the various doses and the magnitude of the response. There was, however, a negative correlation between values of T₃ before injection and the maximum T₃ response. The T₄ : T₃ molar ratio was also significantly lowered between 2·5 and 4·5 h after injection of TRH. Changes in prolactin but not TSH concentrations were dependent upon the breed. After TRH injection, the initial increase in prolactin concentration was the same in both breeds but in the AX group the concentration then declined to values well below the pre-injection concentrations. In the SH group the prolactin concentrations returned to pre-injection levels. When TRH was injected on 2 successive days the T₃, T₄ and TSH responses were less on the second day in both breeds. The prolactin response differed between breeds in a similar manner to that following a single injection of TRH. Repeated hourly injections of increasing amounts of TRH for 4 h resulted in maximal increases of TSH after 2 h and of prolactin after 1 h. Despite continued injection the concentration of both hormones declined. In the AX breed T₃ and T₄ concentrations continued to increase for 8 h after the first TRH injection whereas in the SH group no further increase in the concentrations of these hormones occurred after the first 4 h. It is concluded that the information obtained by measuring resting serum T₃ and T₄ concentrations combined with changes in T₃ and prolactin concentrations after TRH injection may aid in the early selection of cattle adapted to tropical conditions.