Sympathoadrenal and adrenocortical activity in parturient mares

In the mare, parturition only takes place if the environment is perceived as safe, and neither external nor internal stressors impair rapid delivery of the foal. On one hand parturition is assumed to be a very strenuous process but on the other hand sympathoadrenal activation during labor may inhibit uterine contractions and thus delivery. We hypothesized that normal parturition in horses can only take place in the absence of any major acute stress response. To determine changes in physiological stress parameters during parturitionwe analyzed heart rate (HR), heart rate variability (HRV), catecholamine (adrenaline and noradrenaline) and cortisol release in Warmblood brood mares (n1⁄417) throughout spontaneous foaling. A continuous ECG was recorded from 2 h before to 2 h after birth of the foal. Saliva samples for cortisol analysis were taken during second stage labor and 2 h thereafter. Salivary cortisol represents the free, i.e. biologically active fraction of total cortisol and analysis has been validated for the horse. In a subgroup of mares (n1⁄410) blood for catecholamine determination was taken during second and third stage of labor. During the observation period heart rate decreased significantly from of 51 1 beats/min before to 45 0 beats/min after birth of the foal (P<0.05) and thus was not elevated versus baseline heart rate in pregnant mares. At the same time both heart rate variables determined standard deviation of the RR interval (SDRR) and root mean square of successive RR differences (RMSSD) – increased significantly (P<0.05), indicating decreased