Secretion patterns of plasma prolactin and progesterone in pregnant compared with nonpregnant dioestrous beagle bitches.

The plasma concentration of prolactin has been poorly described in dogs. Published data are still controversial. In this study, plasma prolactin and progesterone concentration patterns were determined in groups of ten pregnant and ten nonpregnant beagle bitches. Blood samples were collected daily between 08:30 h and 09:30 h from day-15 to day 135 after the LH peak. Plasma concentrations of prolactin increased from day 25 after the LH surge in the pregnant animals to a peak value on day 65, the mean day of parturition. After parturition, they fell for 24-48 h, and then increased again and remained high during lactation. Variations observed in lactation were great and probably due to suckling. In the nonpregnant bitches, plasma prolactin remained constant throughout the observation period. An increase was observed at about day 70, but was not significant. Plasma prolactin patterns differed greatly, depending on whether the bitch was pregnant or not, even though the luteal phases had the same apparent duration in both cases. No significant difference was observed between progesterone concentrations of pregnant and nonpregnant bitches, even if plasma progesterone was higher from day 40 in pregnant animals. The luteal period ended abruptly in pregnant bitches on day 64, that is 1 day before parturition, and was more progressive in nonpregnant animals. Significantly higher prolactin secretion in pregnancy suggests specific luteal regulation, whereas the lower concentrations observed in nonpregnant dioestrous bitches make its role in nonpregnant luteal phases questionable.