Pharmacological characterization of muscarinic receptors implicated in rabbit detrusor muscle contraction and activation of inositol phospholipid hydrolysis in rabbit detrusor and parotid gland

— In the present study, we evaluated the pharmacological characteristics of the functional muscarinic receptors implicated in rabbit detrusor contraction and coupled to inositol phospholipid turnover in rabbit detrusor and parotid gland. The selectivity of several muscarinic antagonists for detrusor vs. salivary gland muscarinic receptors was also examined. The affinities for the muscarinic m1‐, m2‐ and m3‐receptor subtypes were determined using membranes from human cloned receptors expressed in CHO‐K1 cells using [3H]‐N‐methyl scopolamine as a radioligand. Anti‐muscarinic activity was determined in isolated rabbit detrusor by measuring the displacement of the contractile response to carbachol, and in rabbit detrusor and rabbit parotid by measuring the displacement of inositol phospholipid hydrolysis (total inositol phosphate accumulation) to carbachol. A significant correlation was found between the potencies to antagonize carbachol‐induced rabbit detrusor contraction (pKB) and the affinities (pKi) for the m3‐receptor subtype (r = 0.93, P = 5 × 10−6). Lower, but significant, correlations [0.88 (P = 6.3 × 10−5), 0.72 (P = 4.6 × 10−3)] were obtained with m1‐ or m2‐receptor subtypes, respectively. Each muscarinic antagonist tested displayed similar potency to antagonize carbachol‐stimulated inositol phospholipid hydrolysis in rabbit detrusor and parotid (r = 0.96, P = 8 × 10−3). A significant correlation was found between the potencies to antagonize carbachol‐stimulated inositol phospholipid hydrolysis (pKB), determined in rabbit detrusor and rabbit parotid, and the affinities (pKi) for the m3‐receptor subtype [r = 0.96 (P = 0.01), 0.99 (P = 5 × 10−5), respectively] and for the m1‐receptor subtype [r = 0.98 (P = 3.5 × 10−3), 0.94 (P = 0.02), respectively] but not for the m2‐receptor subtype [r = 0.33, 0.57, ns, respectively]. In each in vitro assay, methoctramine (preferential M2 selective antagonist) and pirenzepine (preferential M1 selective antagonist) were slightly potent. We suggest that the muscarinic receptor implicated in the response to carbachol in rabbit detrusor and parotid gland corresponds to the M3‐subtype. None of the muscarinic antagonists studied in rabbit tissues displayed preferential affinity for the detrusor.