Gender comparison of muscarinic receptor expression and function in rat and human urinary bladder: differential regulation of M2 and M3 receptors?

摘要

Since symptoms of bladder dysfunction occur more frequently in women than in men and since muscarinic receptors are the physiologically most important system to mediate bladder contraction, we have compared the number, subtype distribution and function of muscarinic receptors in bladders from male and female rats. Muscarinic receptor function was also assessed in bladder strips from male and female human bladder. Male and female rats expressed a similar number of muscarinic receptors (144±5 vs. 140±6 fmol/mg protein in saturation radioligand binding). While competition binding curves for the moderately M2-selective methoctramine were not consistently better fitted by a two-site model, most competition curves for the M3-selective darifenacin were biphasic and yielded 29±10% and 31±7% high affinity sites (corresponding to M3 receptors) in male and females, respectively. Immunoreactivity of α-subunits of the G-proteins Gq/11, Gi1/2, Gi3 and Gs did not significantly differ between both genders. The muscarinic receptor agonist carbachol similarly stimulated inositol phosphate accumulation in bladder slices from male and female rats with calculated maximum responses of 69±17 and 77±18% over basal and pEC50 values of 4.90±0.45 and 4.40±0.46, respectively. While darifenacin inhibited carbachol-stimulated inositol phosphate formation approximately 100-fold more potently than methoctramine, each antagonist was similarly potent in both genders. Carbachol concentration-dependently contracted bladder strips with a pEC50 of 5.66±0.05 and 5.72±0.06 and maximum effects of 4.3±0.1 and 4.2±0.2 mN/mg wet weight in male and female rats, respectively. The contractile effect of carbachol was concentration-dependently antagonised by the non-selective atropine (1–30 nM), the M1-selective pirenzepine (1–30 M), the M2-selective methoctramine (1–10 µM) and the M3-selective darifenacin (10–100 nM), with the latter exhibiting a partly unsurmountable antagonism. The overall potency of all four antagonists suggested that contraction was mediated predominantly if not exclusively by M3 receptors with no appreciable differences between both male and female rats. Similarly, the maximum effects (4.4±0.6 vs. 4.4±2.4 mN/mg) and pEC50 (6.07±0.05 vs. 6.32±0.14) of carbachol did not differ between genders in bladder samples from 25 consecutive patients. We conclude that number und function of muscarinic receptors and the relative roles of their M2 and M3 subtypes do not differ between urinary bladders of male and female rats; at least with regard to overall muscarinic responsiveness this situation appears to be similar in humans.