Oxytocin receptor binding in rat and human heart

To determine whether cardiac oxytocin receptors are detectable by radioligand binding assay. Results from in vitro and in vivo animal studies suggest a possible direct effect of oxytocin on the cardiovascular system.

A radioligand binding assay was used to characterize and quantify specific binding of [3H] oxytocin to standardized micropunches (thickness 400 microm; diameter 2 mm) obtained from rat left ventricle and human right atrium. Tissues from fetal and newborn rat heart were also studied.

Saturable, high affinity binding of [3H] oxytocin to rat left ventricle was observed, consisting of a high affinity site (affinity [KD] approximately 1 nM; receptor density [Bmax] approximately 1480 fmol/mg protein) and a higher capacity, lower affinity site (KD approximately 75 nM; Bmax 3730 fmol/mg protein). Binding was displaceable by oxytocin, vasopressin and the agonist [Thr4, Gly7] oxytocin but not by the antagonist atosiban. Cardiac binding was reduced in ovariectomized (ie, estrogen-free) rats and increased in late gestation rats, when blood levels of ovarian steroids are maximal. Cardiac oxytocin receptors were undetectable in fetal and newborn rat heart. High concentrations of specific [3H] oxytocin binding were also found in samples of human right atrial appendage.

These data confirm the presence of a specific oxytocin binding site in rat left ventricle and in human atrium. Binding density is regulated by ovarian steroids and is especially marked in the late stage of pregnancy. These observations provide an explanation for the putative direct effect of oxytocin on cardiac function.