|Budget Amount *help
¥2,100,000 (Direct Cost : ¥2,100,000)
Fiscal Year 1994 : ¥800,000 (Direct Cost : ¥800,000)
Fiscal Year 1993 : ¥1,300,000 (Direct Cost : ¥1,300,000)
Heat stress suppresses the reproductive performance of animals and thereby reduces productivity in the tropics. In the present study, 1) the mechanisms of hypogonadism under heat stress conditions were analyzed in relation to neuroendocrine functions of the hypothalamo-pituitary-gonadal axis, and 2) heat tolerance of buffaloes was compared with cattle in terms of thermo-regulatory responses aiming at the efficient use of buffaloes in future. The results obtained were summarized as follows :
1. Pituitary gonadotropin secretions were not suppressed by heat stress in ovariectomized goats in which ovarian steroidal milieu were maintained at constant levels by steroid implants. Hypothalamic GnRH pulse generator activity, as recorded by a electrophysiological method, also showed no apparent influence of heat stress, although a number of animals used was small. From these results, it was assumed that hypogonadism under heat stress conditions is not attributed to the decrease of neuroendocrine
functions of hypothalamus and pituitary.
2. Ovarian responses, as determined by the increase of plasma estradiol concentration and ovulation, were significantly suppressed under heat stress conditions, when the pulsatile LH secretion was maintained by hourly injection of GnRH in anestrous animals. From these results, a poor responsiveness of the ovary was considered as a possible cause of hypogonadism under heat stress conditions.
3. Ovarian blood flow, as measured by the ultrasound transit-time flow meter, was not influenced by heat stress and therefore a possibility of the reduced blood supply to the ovary was ruled out.
4. In the comparative study of heat tolerance of buffaloes and cattle, it was shown that heat dissipation in buffaloes was much depended on sensitive heat loss from body surface rather than insensible heat loss by evaporation through sweating and respiration compared with cattle.
5. In buffaloes, a rapid increase of blood volume occurred under hot conditions, and this resulted in an increase of blood flow to the skin and contributed to an increased heat dissipation from body surface. Less