The role of myofibrillar atpase and thyroid hormones in the development of thermoregulation in the domestic fowl, (Gallus Domesticus)

Abstract

The development of homeothermy was studied in the domestic fowl by measuring changes in oxygen consumption (\1°2) and body temperature (Tre; chicks only) with decreasing ambient temperatures (Ta) during the last week of incubation for embryos and first week of life for chicks. During the same period, myofibrillar ATPase (m-ATPase) activity and thyroid hormones (Thyroxine, T4; and triiodothyronine, T3) were also measured. \102 for 15, 16, and 17 day old embryos diminished with decreasing Ta. Embryos at 18, 19, 20 and 21 days, initially maintained their \102 for decrements of 3, 3, 4, and SOC in Ta respectively, but their \102 fell with decreasing Ta thereafter. For chicks, \102 increased with decreasing Ta upto a peak and then fell with further decrements in Ta. The difference between the initial and final Tre taken before and after the experiments decreased as the chicks became older. Myofibrillar ATPase activity increased with incubation to reach a critical value in 18 day old embryos and thereafter undulated about the same level. Both T3 and T4 levels increased exponentially during the last week of incubation and peaked at hatching. T4 fell below prehatching levels to a nadir 2 days posthatching and thereafter rose slowly, while T3 never fell below prehatching levels. T3/T 4 ratio increased exponentially from day 19 of incubation and peaked 2 days post-hatching. It is concluded that up to day 17 of incubation, chicken embryos are poikilothermic and incipient endothermic homeothermy starts on day 18. At hatching, the qualitative adult homeothermic response is present and it approximates that of adults 7 days post-hatching. The strong correlation between \102 and m-ATPase during the embryonic stages and the critical value of m-ATPase activity attained on day 18 suggests a critical role for m-ATPase activity in the development of thermoregulation. Similarly, a stronger correlation between \102 and T3 than T4 within the period of study suggests a greater role for T3 than T4 in the development and maintenance of homeothermy. T3 and T4 also play an important role in the hatching process as indicated by their peak values then. The increasing T3fT4 ratio suggests increased conversion of T4 to T3 around the perinatal period.