Properties of [³H]Prazosin‐Labeled α₁Adrenergic Receptors in Rat Brain and Porcine Neurointermediate Lobe Tissue

[3H]Prazosin binding to alpha 1 receptors in homogenates of rat prefrontal cortical tissue and porcine pituitary neurointermediate lobe tissue was investigated. Competition curves produced by coincubating adrenergic agonists and antagonists with 0.5 nM [3H]prazosin and tissue revealed some anomalous binding properties. In the brain and pituitary tissue, agonist competition curves produced "shallow" slopes, with Hill coefficients significantly lower than unity. The IC50 of the agonists epinephrine, norepinephrine, and clonidine for inhibition of 0.5 nM [3H]prazosin binding were significantly lower in the porcine pituitary than in the rat brain. Most antagonists, such as prazosin, chlorpromazine, and piperoxan, produced "steep" competition curves with Hill coefficients close to unity, with two notable exceptions. WB-4101 and phentolamine produced competition curves with Hill coefficients significantly less than unity in the rat brain preparation. Ketanserin, an antagonist, displayed a sevenfold higher affinity for the alpha 1 sites in the pituitary tissue than in the brain tissue. These anomalies in the binding results may indicate the presence of an endogenous modulatory factor affecting agonist and antagonist affinities for the alpha 1 receptor.