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

1 We investigated muscarinic receptors in the detrusor and mucosa of the human bladder body. Radioligand-binding studies with [H-3] 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. 2 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)). 3 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. 4 Using RT - PCR, expression of M-1, M-2, M-3 and M-5 mRNA was demonstrated in both detrusor and mucosa. 5 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. 6 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.