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 betwee n 57-71 days due to the long period of receptivity at oestrus and the exten ded period of sperm survival in the female genital tract. In contrast to ot her domestic animals progesterone- and oestrogen concentrations are almost identical in pregnant and non pregnant bitches, except for their rapid decl ine immediately prior to parturition. Control of luteolysis still poorly un derstood. Experiments with indomethacin leading to a blockade of the prepar tal PgF(2 alpha) increase, which commences with the decrease of progesteron e, point toward a role of PgF(2 alpha) at this stage of pregnancy, which wa s extended for several days. At physiological conditions first visible sign s of parturition were observed at peak-PgF(2 alpha) levels, 33.6 +/- 17.6 h ours 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 bi tches were treated with the antiprogestin RU 38486 which inhibits the activ ity 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. H owever, the process of parturition came to an end after cervical opening an d totally only one puppy was born. Different to a normal parturition no inc rease of PgF(2 alpha) was observed. Relaxin levels were not influenced by t reatment. These observations suggested that treatment with antiprogestin fo llowed by PgF(2 alpha) might be an adequate method to induce parturition in the dog; first experiences seem to confirm this conclusion.