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 t he functional muscarinic receptors implicated in rabbit detrusor contractio n and coupled to inositol phospholipid turnover in rabbit detrusor and paro tid gland. The selectivity of several muscarinic antagonists for detrusor v s. salivary gland muscarinic receptors was also examined. The affinities fo r the muscarinic m(1)-, m(2)- and ms-receptor subtypes were determined usin g membranes from human cloned receptors expressed in CHO-K1 cells using [H- 3]-N-methyl scopolamine as a radioligand. Anti-muscarinic activity was dete rmined in isolated rabbit detrusor by measuring the displacement of the con tractile response to carbachol and in rabbit detrusor and rabbit parotid by measuring the displacement of inositol phospholipid hydrolysis (total inos itol phosphate accumulation) to carbachol. A significant correlation was fo und between the potencies to antagonize carbachol-induced rabbit detrusor c ontraction (pK(B)) and the affinities (pKi) for the m(3)-receptor subtype ( r = 0.93, P = 5 x 10(-6)). Lower, but significant, correlations [0.88 (P = 6.3 x 10(-5)), 0.72 (P = 4.6 x 10(-3))] were obtained with m(1)- or m(2)-re ceptor subtypes, respectively. Each muscarinic antagonist tested displayed similar potency to antagonize carbachol-stimulated inositol phospholipid hy drolysis in rabbit detrusor and parotid (r = 0.96, P = 8 x 10(-3)). A signi ficant correlation was found between the potencies to antagonize carbachol- stimulated inositol phospholipid hydrolysis (pK(B)), determined in rabbit d etrusor and rabbit parotid, and the affinities (pKi) for the m(3)-receptor subtype [r = 0.96 (P = 0.01), 0.99 (P = 5 x 10(-5)), respectively] and for the mi-receptor subtype [r = 0.98 (P = 3.5 x 10(-3)), 0.94 (P = 0.02), resp ectively] but not for the m(3)-receptor subtype [r = 0.33, 0.57, ns, respec tively]. In each in vitro assay, methoctramine (preferential M2 selective a ntagonist) and pirenzepine (preferential M1 selective antagonist) were slig htly potent. We suggest that the muscarinic receptor implicated in the resp onse to carbachol in rabbit detrusor and parotid gland corresponds to the M 3-subtype. None of the muscarinic antagonists studied in rabbit tissues dis played preferential affinity for the detrusor, (C) 1999 Editions scientifiq ues et medicales Elsevier SAS.