Alpha 1 , Adrenergic Receptors in Canine Lower Genitourinary Tissues: Insight into Development and Function

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Abstract
Radioligand receptor binding methods were used to characterize the alpha1-adrenergic receptor in the bladder body, bladder base, prostate and urethra of the male dog. Saturation experiments were performed in tissue homogenates using [125iodine]-Heat, an alpha1-adrenergic antagonist of high specific activity (2,200 Ci. per mmol). The equilibrium dissociation constant Kd for [125iodine]-Heat binding in the bladder body (0.56 pM.), bladder base (0.81 ± 0.11 pM.), prostate (0.86 ± 0.19 pM.) and urethra (0.55 pM.) was similar, suggesting homogeneity of alpha1-adrenergic binding sites in lower genitourinary tissues. The receptor density in the bladder body, bladder base, prostate and urethra, expressed as fmol. per mg. wet weight, was 0.22 ± 0.02, 0.82 ± 0.09, 0.55 ± 0.06 and 0.27 ± 0.06, respectively (mean ± standard error of mean). Competitive binding experiments with [125iodine]-Heat and unlabeled prazosin and clonidine confirmed the selectivity of Heat for alpha1-adrenergic binding sites. Anatomical dissections have revealed that a major component of the smooth muscle of the bladder base and prostate originates from the ureter, whereas a major component of the smooth muscle of the urethra originates from the bladder. The measured alpha1-adrenergic receptor densities support these developmental theories.