Initial detection of [3H]prazosin-labeled alpha 1-receptors in the porcine pituitary neurointermediate lobe.

[3H]Prazosin binding to alpha 1-adrenergic receptors was investigated in homogenates of tissue from the porcine pituitary neurointermediate lobe. Potent alpha 1-adrenergic antagonists such as prazosin, WB-4101, and phentolamine displayed high potency in competing for [3H]prazosin specific binding in pituitary tissue. Epinephrine and norepinephrine were the only neurotransmitters which displayed high potency in competing for [3H]prazosin binding sites. There was a strong correlation in the pharmacological properties of the alpha 1 sites in pituitary tissue and the alpha 1 sites detected by [3H]prazosin in rat brain tissue. Alpha-adrenergic agonists demonstrated significantly higher potency for the alpha 1-receptor sites in pituitary tissue than in brain tissue preparations. The antagonist ketanserin demonstrated significantly higher potency at alpha 1 sites in pituitary tissue than in brain tissue. This latter observation has important implications for investigators attempting to use [3H]ketanserin as a selective serotonin receptor ligand in pituitary tissue. The results of this study represent the first demonstration of alpha 1-receptors in mammalian neurointermediate lobe tissue.