Muscarinic Receptors And Genitourinary Smooth Muscle Function
The biological effects resulting from parasympathetic nerve stimulation in genitourinary tissues are mediated chiefly through muscarinic cholinoceptors of which there are five known subtypes (M1-M5). With the exception of the corpus cavernosum, which relaxes in response to muscarinic receptor agonism, most genitourinary tissues contract, either directly or indirectly, when exposed to muscarinic receptor agonists. In recent years, the availability of selective pharmacological tools has allowed us to make considerable strides in our understanding of the functional role of individual muscarinic receptor subtype(s) in genitourinary tissues. The muscarinic receptor subtype mediating the contractile response is dependent on the species, tissue, and in some cases, region of tissue, under study. Recent studies have unmasked the functional role of M2 receptors in the urinary bladder and these findings may set the stage for the development of safer and more efficacious subtype selective muscarinic antagonists for the treatment of overactive bladder. Unambiguous elucidation of muscarinic receptor pharmacology in the urethra, prostate, vas deferens and corpus cavernosum requires additional rigorous studies. Lastly, the identity of the muscarinic receptor mediating uterine contraction continues to generate controversy.