Presence of a temperature gradient in the female genital tract and a new hypothesis for the thermogenic action of progesterone in the cow

The temperature of the deeper portions of the female genital tract and its relation to the temperature measured in vagina (VT) or rectum (RT) has not been revealed yet. In the cow, VT shows a significant increase in the mid-luteal phase of the estrous cycle. This pattern was attributed to the systematic thermogenic action of progesterone (P₄). However, it has not yet been elucidated whether the RT and the temperature of the cervix (CT), uterine body (UBT) and uterine horns (UHT) are exposed to the same effect.  In the present study, the RT, VT, CT, UBT and UHT were recorded daily over two successive estrous cycles in four Japanese black cows using a specially designed digital thermometer. Blood samples were collected daily for assaying of the peripheral P₄ and estradiol-17β (E₂) concentrations. We found that, the temperature within the genital tracts significantly increased towards the uterine horns from vagina (difference; up to 0.7 C), resulting in a temperature gradient within the female genital tract. Both RT and VT were strongly correlated (r=0.79; P˂0.001). Moreover, VT was more significantly (P˂0.01; Steiger’s Z-test) correlated to CT, UBT and UHT than RT does. Thus, VT monitoring could be an efficient tool for better understanding of the uterine thermal environment in heat stressed cows. We found that VT, CT, UBT and UHT were significantly increased in association with peripheral P₄>4 ng/ml when compared to lower P₄ ranges. While, the RT did not show any significant difference over the different P₄ ranges, suggesting that the P₄ have a localized thermogenic action on the genital tract rather than a systematic one. <br>