Effect of Certain Drugs on Perfused Human Placenta IV: Detection of Specific Receptor Sites

The effects of three adrenergic blocking agents (phentolamine, tolazoline, and hydralazine) on the action of epinephrine, norepinephrine, l-epinephrine, histamine, and 5-hydroxytryptamine in placental vessels were investigated. The results indicated that phentolamine not only diminished and/or blocked the vasoconstricting action of epinephrine, norepinephrine, and histamine but also caused a reversal of their effects. However, phentolamine could only diminish the vasopressor action of 5-hydroxytryptamine. From the results obtained in this investigation, it appears that the placental vessels have both alpha and beta receptors.