Dopamine reduces the receptor binding activity and not the secretion rate of placental lactogen in vitro.

Incubation with high concentrations of dopamine (greater than or equal to 10(-3) mol/l) depressed the ability of goat placental lactogen, human placental lactogen, ovine prolactin and human growth hormone to compete with labelled hormone in radioreceptor assays for lactogenic activity, with the greatest effects on placental lactogen. The activity of ovine prolactin in a radioimmunoassay was also depressed. The effects on human placental lactogen depended on the concentration of dopamine and on the time and temperature of incubation. Therefore, effects on activity, rather than on secretion rate, appear to explain the inhibition by dopamine of placental lactogen in the medium of placental explant cultures.