Cyclic nucleotides do not mediate excitability-suppressing cellular action of progesterone in the rabbit myometrium

Uterine cAMP and cGMP levels were measured to radioimmunoassay in progesterone (P) dominated 25-day pregnant (Pr) and estrogen dominated post partum (Pp) rabbits, and 1, 3, 6, 12 and 24 hours after P treatment (10 mg i.m.) of Pp animals, simultaneously with changes in uterine smooth muscle excitability. Uterine cAMP level was similar in the Pr and Pp rabbits while cGMP concentration was significantly higher in the Pr uterus. P treatment significantly increased superficial Ca2+-binding and decreased electrical (field stimulation) and pharmacological (oxytocine, prostaglandin F2 alfa) excitability of Pp uterus after a latency period of 8--12 h. Uterine cAMP level did not change while cGMP level increased following P treatment. However, the significant elevation of cGMP was preceded by suppression of smooth muscle excitability. The data suggest that cyclic nucleotides are not involved in the excitability-suppressing cellular action of P in the rabbit uterus.