Muscarinic binding sites of the pig intravesical ureter.

1. Muscarinic receptors in the pig intravesical ureter were characterized by binding assays in which the muscarinic receptor antagonist, (-)-[3H]-quinuclidinyl benzylate ([3H]QNB) was used as radioligand. 2. The specific binding of [3H]-QNB (about 90% of the total binding, as defined with 10(-5) M unlabelled atropine) was dependent on protein concentration, saturable, and of high affinity (KD = 0.13 +/- 0.02 nM). 3. Displacement of [3H]-QNB specific binding by the M1-selective antagonist, pirenzepine, described a two-component curve, with a minor (17%) high affinity component (pKiH = 8.75), and a major (83%) low affinity one (pKiL = 6.34). The M3-preferential antagonists, hexa-hydro-sila-difenidol (HHSid) and p-fluoro-HHSiD (p-F-HHSiD) delineated also two sites, with pKiH of 8.91 and 8.57 and pKiL of 6.94 and 7.05, respectively. However, the M2-selective antagonists, 11-(2-(diethyl-amino)methyl-1-piperidinylacetyl)-5,11-dihydro-6H-p yrido-(2,3-b)- (1,4)-benzodiazepin-6-one (AF-DX 116, pKi = 6.72) and methoctramine (pKi = 8.34), as well as the M4-selective antagonists, tropicamide (pKi = 7.15) and himbacine (pKi = 8.65) fitted best to a single population of sites. Moreover, 4-diphenyl-acetoxy-N-methyl-piperidine methiodide (4-DAMP), a muscarinic antagonist that discriminates the M1 and M3 versus the M2 subtypes, also delineated one site (pKi = 8.36). 4. The antagonist profile clearly indicates the existence of an M2 population in the porcine intravesical ureter. In addition, the presence of a minor non-M2 population, which may be formed by a mixture of several muscarinic subtypes (i.e. M1, M3 and/or M4) can not be discounted. 5. The present work confirms the results obtained in previous functional studies where the stimulation of muscarinic receptors by carbachol evoked the contraction of the pig isolated intravesical ureter.