Uterine Norepinephrine Levels as Related to Plasma and Tissue Progesterone in Pseudopregnant Rabbits

Pseudopregnancy was induced in rabbits by a single iv injection of 100 IU of human chorionic gonadotrophin (hCG). Uterine norepinephrine, located exclusively in adrenergic nerves, was markedly reduced (fluorometric determinations) during the first 6 days of pseudopregnancy and then returned towards pre-injection values. Plasma progesterone (measured by radioimmunoassay) was found to be elevated at days 3,6, and 12 of pseudopregnancy as compared to the very low values of the untreated controls. Although tissue progesterone also increased during pseudopregnancy (fourfold at day 6), there was a somewhat paradoxical relationship between plasma and tissue progesterone. In spite of unchanged plasma levels, tissue progesterone decreased considerably between days 6 and 12. The uterine/plasma progesterone ratio was very high (approximately 28) in non-pregnant (control) animals and ranged between 1 and 3 during pseudopregnancy. These results suggest that there is a causal relationship between the high levels of progesterone in the uterus and the reduced amount of norepinephrine transmitter in the adrenergic nerves in this organ, and that plasma progesterone concentration and the changing progesterone receptor levels regulate uterine progesterone concentration.