Abstract
The effects of mating type, age, and environmental temperature on the biological activity of injected L-thyroxine (T4) and L-triiodothyronine (T3) were studied with White Plymouth Rock (WR) and the cross-bred of White Cornish male × White Plymouth Rock female (WC×WR).
The birds were reared in the electric brooders at 30°C for the first 2 weeks and at 25°C thereafter. Ten days before starting the first experiment, eighty 5-week-old WR chicks were devided into four groups. The first two groups were maintained in a temperature controlled room (25±1°C). Another two groups were reared in a natural room temperatre. Forty WC×WR chicks were devided into 2 groups and were maintained in the controlled room. Each group was composed of 10 male and 10 female chicks. Thyroxine secretion rate (TSR) was measured at 6 and 12 weeks of age. For the determination of TSR the same groups of birds were used both at 6 and 12 weeks of age. During the period of the first TSR determination the natural environmental temperature fluctuated between maximum 28 and 22°C, whereas in the second TSR determination, it fiuctuated between maximum 21 and minimum 14°C.
Thyroxine secretion rate was estimated by the method described by TANASE and KOMIYAMA (1962)10). Thyroxine was injected subcutaneously in the increasing doses, 0, 0.5, 1.0 1.5, 2.0, 3.0, 4.0μg/100g body weight per day at the first experiment at 6 weeks of age. At 12 weeks of age, thyroxine was injected in the increasing doses, 0, 0.25, 0.75, 1.25, 1.75, 2.0, 3.0μg/100g body weight per day. Triiodothyronine was injected in the same manner.
In the 6-week-old WR chicks reared in the controlled temperature, the difference between the responses to T4 and T3 was statistically significant. It was revealed that, by subcutaneous injection, T4 was 1.30 times as active biologically as T3. In the 6-week-old WC×WR chicks, T4 was observed to be 1.30 times as active as T3, but this difference was not statically significant. In the other sets of comparisons the differences between the responses to T4 and T3 were not statistically significant (Table 1).
Comparison of biological activities of T4 and T3 on a basis of the number of molecules is presented in Table 2. Except the 12-week-old WC×WR chicks reared in the controlled room, all the groups showed statistically significant differences between the responses to T4 and T3. Ratio of T3. to T4 on a basis of the number of molecules was higher than that on the μg basis.
It thus appears that the biological activities of T4 and T3 are nearly the same, or the former is slightly higher than the latter on the μg basis, but, on a basis of the numher of molecules, the biological activity of T4 is higher than that in T3. These results agree with those in earlier studies3, 4, 5). However, the results in the present study do not accord with those in the recent studies by SRIVASTAVA and TURNH6, 7)
