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

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%M₂, 22%M₃ and 7%M₁. In the mucosa, 75%of sites were M₂ receptors, with 25%M₃/M₅.Using RT-PCR, expression of M₁, M₂, M₃ and M₅ mRNA was demonstrated in both detrusor and mucosa.The presence of a high density of mainly M₂ 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 M₃ but not M₂ receptors, in both male (P<0.0001) and female (P=0.019) detrusor. These findings correspond with reports of decreased detrusor contractility with ageing.British Journal of Pharmacology (2005) 144, 1089-1099. doi:10.1038/sj.bjp.0706147 [ABSTRACT FROM AUTHOR]