Identification of receptor subtypes in the rabbit and human urinary bladder by selective radio-ligand binding

Recent advances in receptor technology have demonstrated that subtypes of each autonomic receptor exist. Using both direct radio-ligand studies and the inhibition of receptor binding by subtype-selective pharmacological antagonists, we have studied the distribution of subtypes of alpha and beta adrenergic receptors and muscarinic cholinergic receptors in the urinary bladder of the rabbit and man. Alpha adrenergic receptors were quantified by direct binding of tritiated prazosin (alpha-1), yohimbine (alpha-2), and the non-selective alpha adrenergic ligand dihydroergocriptine (DHE). These studies demonstrated that the distribution of alpha receptor subtypes in the bladder base (for both rabbit and human) is approximately 80% alpha-1 and 20% alpha-2. Beta receptor subtypes were identified by the inhibition of the non-selective ligand 3H-dihydroalprenalol (DHA) by the beta-1 selective inhibitor ICI-89 and the beta-2 selective inhibitor ICI-118. Initial studies demonstrated that the beta adrenergic density of the bladder body was 92 fmol per mg. protein for the rabbit and 32 fmol per mg. protein for human bladder body. Inhibition of DHA binding by ICI-118 demonstrated a single class of receptor with an IC50 of approximately 0.013 microM for both rabbit and human. Inhibition of DHA binding by ICI-89 also demonstrated one class of receptors with an IC50 of approximately 9.0 microM for both species. These results indicate that there are primarily beta-2 receptors in the rabbit and human bladder body. Although the number of muscarinic subtypes in existence is currently being re-evaluated, there are at least two which can be identified by the selective muscarinic agent pirenzepine (PZP). The brain has been shown to contain both high and low affinity PZP sites. Using both direct PZP binding to the bladder body, and the inhibition by PZP of the non-selective radio-ligand quinuclidinyl benzylate (QNB), we have demonstrated that both the rabbit and human bladder body have no observable high affinity PZP-selective binding and the inhibition of 3H-QNB by PZP demonstrated that there was only the low-affinity PZP binding site. Although receptor subtypes in the bladder have been the subject of numerous investigations, this is the first study describing the distribution of both adrenergic and cholinergic receptor subtypes in both the rabbit and human.