Alpha1 Adrenergic Receptors in the Porcine Pituitary Neurointermediate Lobe: Detection with [3H]Ketanserin

Abstract

³H]Ketanserin, a serotonin receptor antagonist, labelled high affinity, saturable sites in homogenates of porcine neurointermediate lobe tissue. Cinanserin, a potent and selective serotonin receptor antagonist, inhibited the specific binding of 5 × 10^-10M ³H]ketanserin with a high affinity component representing 20% of the total binding. Prazosin, a potent and selective alpha₁ adrenergic antagonist, inhibited ³H]ketanserin binding with a high affinity component representing 60% of total binding. The prazosin-specific component was demonstrated to be distinct from the cinanserin-specific component. 10^-7M cinanserin was co-incubated with ³H]ketanserin to eliminate the serotonergic component of the binding and allow pharmacological characterization of the remaining prazosin-specific component. The prazosin-specific binding of ³H]ketanserin binding closely resembled the results of experiments using ³H]prazosin to label alpha₁ receptors in neurointermediate lobe tissue homogenates. Ketanserin was found to be sevenfold more potent in inhibiting ³H]prazosin binding to alpha₁ adrenergic receptors in the neurointermediate lobe tissue than in brain tissue. This observation explains why low concentrations of ³H]ketanserin can selectively label serotonin receptors in the brain but will label both adrenergic and serotonin receptors in the neurointermediate lobe.