DIFFERENT MUSCARINIC RECEPTOR SUBTYPES MEDIATING THE PHASIC ACTIVITY AND BASAL TONE OF PIG ISOLATED INTRAVESICAL URETER

1 We have studied the effects of muscarinic cholinoceptor agonists and specific antagonists on both phasic activity and basal tone of the isolated intravesical ureter of the pig by means of isometric techniques in vitro. 2 Acetylcholine in the presence and absence of physostigmine increased both phasic activity and basal tone of ureteral strips in a concentration-dependent manner. Moreover carbachol, methacholine and oxotremorine-M increased both contractile parameters while bethanechol and McN-A-343 evoked only increases in tone without affecting the frequency of the phasic contractions. 3 The nicotinic receptor blocker, hexamethoniUm (10(-6)-10(-4) M), failed to modify the contractions evoked by a single dose of carbachol (10-5 M), whilst the muscarinic antagonist, atropine inhibited both phasic and tonic responses. 4 The muscarinic M1 (pirenzepine), M2 (AF-DX 116 and methoctramine), M3 (4-DAMP, HHSiD and p-F-HHSiD), and putative M4 receptor (tropicamide) antagonists significantly reversed increases in both frequency of phasic activity and baseline tone induced by a submaximal dose of carbachol (10(-5) M). The pIC50 values for inhibition of the induced phasic activity were: atropine (10.16)>4-DAMP (9.12) > HHSiD (8.22) = methoctramine (7.98) = p-F-HHSiD (7.88 > tropicamide (7.62) = pirenzepine (7.53)=AF-DX 116 (7.45) and for inhibition of basal tone were: atropine (10.73)>4-DAMP (9.32)>HHSiD (8.65)=pirenzepine (8.43)=p-F-HHSiD (8.38)>methoctramine (7.79)>tropicamide (7.53)>AF-DX 116 (7.04). 5 The antagonist profile indicates that an M1 receptor mediates the tonic response while the phasic activity could involve either both M2 and M3 or an M4 muscarinic receptor. These results suggest that different muscarinic receptor subtypes mediate the phasic and tonic contractile activity induced by a submaximal concentration of carbachol in the porcine intravesical ureter.