Prejunctional M1 facilitory and M2 inhibitory muscarinic receptors mediate rat bladder contractility

Subtype-selective muscarinic antagonists effects on carbachol-induced and electric field-stimulated contractility of rat bladder were compared in vitro. Schild plot analysis of cumulative carbachol dose-response curves in the presence of antagonists was consistent with M-3-mediated bladder contractions. However, nerve-evoked contractions were inhibited 15% at 30 Hz (P < 0.01) by 10 nM pirenzepine (M-1-selective antagonist), whereas 10 nM methoctramine (M-2-selective antagonist) increased these contractions by 17% at 30 Hz (P < 0.01). Identical doses had no effect on carbachol-induced contractions, indicating prejunctional M-1 facilitory and M-2 inhibitory receptors. m1 Receptors could not be identified by subtype-selective antibodies, nor could the m1 transcript be identified by Northern hybridization. However, m1, m2, m3, and m4 transcripts were identified in rat bladder using the reverse transcriptase-polymerase chain reaction, providing support for the existence of the m1 subtype. In conclusion, strong evidence is provided for the existence of prejunctional M-1 facilitory and M-2 inhibitory and postjunctional M-3 receptors modulating contractility in the rat urinary bladder.