Muscarinic receptor subtypes in human bladder detrusor and mucosa, studied by radioligand binding and quantitative competitive RT-PCR: Changes in ageing

Abstract

We investigated muscarinic receptors in the detrusor and mucosa of the human bladder body. Radioligand-binding studies with [ 3H]QNB were conducted using specimens collected from patients (36-77 years) with normal bladder function, undergoing surgery. For RT-PCR, biopsies of normal bladder were obtained from patients (30-88 years) undergoing check cystoscopy. Binding of [ 3H]QNB in detrusor (n = 20) was of high affinity (K D 77.1 (55.2-99.0) pM) and capacity (B max 181±7 fmol mg protein -1). Similar values were obtained in mucosa (n = 6) (K D 100.5 (41.2-159.9) pM; B max 145±9 fmol mg protein -1). Competition-binding experiments in detrusor membranes with muscarinic receptor antagonists including trospium, darifenacin, 4-DAMP, methoctramine, AQ-RA 741, AF-DX 116 and pirenzepine indicated a receptor population of 71% M 2, 22% M 3 and 7% M 1. In the mucosa, 75% of sites were M 2 receptors, with 25% M 3/M 5. Using RT-PCR, expression of M 1, M 2, M 3 and M 5 mRNA was demonstrated in both detrusor and mucosa. The presence of a high density of mainly M 2 muscarinic receptors in the mucosa appears to be a novel finding and raises the question of their physiological significance and the source of their endogenous ligand. There was a negative correlation of receptor number (B max) with age in detrusor muscle from male patients (P = 0.02). Quantitative competitive RT-PCR demonstrated a selective age-related decrease in mRNA for muscarinic M 3 but not M 2 receptors, in both male (P < 0.0001) and female (P = 0.019) detrusor. These findings correspond with reports of decreased detrusor contractility with ageing. © 2005 Nature Publishing Group All rights reserved.