In the European eel, Anguilla anguilla L., neurohypophysial peptides caused prolonged, dose-dependent, increases in blood pressure in the ventral aorta but decreased that in the dorsal aorta. In order of potency they were: isotocin, oxytocin, arginine vasotocin and arginine vasopressin. Lysine vasopressin was without appreciable effects.
Depletion of adrenergic stores in the animal by pharmacological means or blockade of α-adrenergic receptors reduced or abolished the pressor response in the ventral aorta. Partial tachyphylaxis to the peptides could be restored by injection of noradrenaline. Blockade of cholinergic receptors abolished the postbranchial vasodepressor effect. Thioglycerol attenuated the pressor response. The possible mechanism of action of neurohypophysial peptides with special reference to the unifying concept of increases in membrane permeability is discussed.