ALPHA-ADRENERGIC RECEPTORS IN HUMAN MYOMETRIUM DURING PREGNANCY

The distribution and mechanisms of .alpha.-adrenergic receptors have been studied in myometrial preparations from women delivered by ceasarean section at term. With the use of the radioligand tritiated dihydroergocryptine, the number of .alpha.-adrenergic receptors was 210 fmol/mg of protein in the uterine fundus and 195 fmol/mg of protein in the lower uterine segment. Competition experiments showed that 60% of the .alpha.-adrenergic receptors had properties as .alpha.1-adrenergic receptors and 40% as .alpha.2-adrenergic receptors. In vitro tension studies verified the existence of physiologically active .alpha.1- and .alpha.2-adrenergic receptors in the myometrial preparations. .alpha.2-Adrenergic receptor stimulation resulted in lowered levels of intracellular cyclic adenosine monophosphate. The intracellular cyclic adenosine monophosphate was further reduced by additional .alpha.1-adrenergic receptor stimulation, probably secondary to an activation of calcium-calmodulin-dependent phosphodiesterase.