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 cesarean 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.