Progestogens and mammary tumours in the beagle bitch

Progesterone administration induces specific cytosol receptors for progesterone in the mammary glands and uterus of adult beagle bitches but not in the mammary tissues or uterus of adult female rats. The specifiity of the progesterone receptor of dog myometrium is unlike that of human myometrium and is significantly different from progesterone receptors in tissues of th guinea pig. A spontaneous mammary benign mixed adenoma from an old beagle bitch was shown to contain specific progesterone receptors of similar affinity for progestogens as the dog uterine receptor. It is proposed that chronic toxicity tests of progestogens in the bitch cannot predict potential adverse human side effects because the regulatory mechanism for progesterone receptors is so different in the dog and major species differences exist in cytosol receptor affinity for particular progestational compounds.