Effects of ageing on muscarinic receptor subtypes and function in rat urinary bladder

We compared the density and function of M-2 and M-3 muscarinic acetylcholine receptor subtypes in the urinary bladder of young adult (3 months) and old (23 months) male Wistar rats. Old rats had a reduced density of muscarinic receptors (96 +/- 10 vs. 156 +/- 21 fmol/mg protein), but competition experiments with the M-3-selective darifenacin did not indicate alterations in the relative roles of M-2 and M-3 receptors, with the former being more abundant. The amount of immunodetectable alpha-subunits of various G-proteins potentially linked to muscarinic receptor function was unchanged. The potency of carbachol to contract bladder strips was also unaltered; its maximum effects as well as those of a single KCl concentration were unchanged if raw data or those corrected for strip length were analysed, but somewhat reduced when those corrected for strip weight were analysed. Antagonistic effects of atropine, the M-2-selective Ro 320-6206 and the M-3-selective darifenacin were unchanged. Agonistic effects of the M-3-sparing agonist 4-(2-oxo-2,3-dihydro-benzoimidazol-1-yl)-[1,4']bipiperidinyl-1'-carboxyl ic acid ethyl ester were similarly poor in young and old rats. Additional experiments were concomitantly performed in submandibular glands from the same animals. While total muscarinic receptor density in submandibular glands was not significantly affected by age (56 +/- 5 vs. 61 +/- 4 fmol/mg protein), the relative contribution of M-3 receptors significantly declined from 68 +/- 3% to 57 +/- 2% based upon darifenacin competition curves. We conclude that aged Wistar rats express fewer muscarinic receptors in their urinary bladder, but there is no change in the relative abundance of M-2 and M-3 receptors; this is accompanied by only minor if any alterations in receptor responsiveness. In contrast, submandibular gland expresses similar receptor numbers in young and old rats, but slightly fewer M-3 receptors in old animals