Aspects on hormonal control of normal and induced parturition in the dog

Average length of gestation on the dog is 63 +/- 2 days but may vary between 57-71 days due to the long period of receptivity at oestrus and the extended period of sperm survival in the female genital tract. In contrast to other domestic animals progesterone- and oestrogen concentrations are almost identical in pregnant and non pregnant bitches, except for their rapid decline immediately prior to parturition. Control of luteolysis still poorly understood. Experiments with indomethacin leading to a blockade of the prepartal PgF(2 alpha) increase, which commences with the decrease of progesterone, point toward a role of PgF(2 alpha) at this stage of pregnancy, which was extended for several days. At physiological conditions first visible signs of parturition were observed at peak-PgF(2 alpha) levels, 33.6 +/- 17.6 hours after onset of luteolysis, which lasted over 16.8 +/- 3.4 hours. Pulse-type releases of oxytocin were only observed after this point of time. To test for the effect of progesterone-withdrawal, four 51-57 days pregnant bitches were treated with the antiprogestin RU 38486 which inhibits the activity of progesterone at the receptor level. In all dogs first visible signs of parturition were observed 33.5 +/- 7.5 hours after onset of treatment. However, the process of parturition came to an end after cervical opening and totally only one puppy was born. Different to a normal parturition no increase of PgF(2 alpha) was observed. Relaxin levels were not influenced by treatment. These observations suggested that treatment with antiprogestin followed by PgF(2 alpha) might be an adequate method to induce parturition in the dog; first experiences seem to confirm this conclusion.