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1 Neuromuscular blocking drugs (NMBD's) are known to produce cardiovascular side effects manifesting as brady/tachycardias. In this study we have examined the interaction of a range of steroidal NMBD's with recombinant human m1–m5 muscarinic receptors expressed in Chinese hamster ovary cells. Our main hypothesis is that NMBD's may interact with m2 (cardiac) muscarinic receptors. 2 All binding studies were performed with cell membranes prepared from CHO m1–m5 cells in 1 ml volumes of 20 mM HEPES, 1 mM MgCl2 at pH 7.4 for 1 h. Muscarinic receptors were labelled with [3H]-NMS and displacement studies were performed with pancuronium, vecuronium, pipecuronium, rocuronium and gallamine. In addition a range of muscarinic receptor subtype selective reference compounds were included. In order to determine the nature of any interaction the effects of pancuronium, rocuronium and vecuronium on methacholine inhibition of forskolin stimulated cyclic AMP formation in CHO m2 cells was examined. Cyclic AMP formation was assessed in whole cells using a radioreceptor assay. All data are mean±s.e.mean (n5). 3 The binding of [3H]-NMS was dose-dependent and saturable in all cells tested. Bmax and Kd values in m1–m5 cells were 2242±75, 165±13, 1877±33, 458±30, 127±2 fmol mg−1 protein and 0.11±0.02, 0.15±0.01, 0.12±0.01, 0.12±0.01, 0.22±0.01 nM respectively. 4 The binding of [3H]-NMS was displaced dose dependently (pK50) by pirenzepine in CHO m1 membranes (7.97±0.04), methoctramine in CHO m2 membranes (8.55±0.1), 4-diphenylacetoxy-N-methyl piperidine methiodide (4-DAMP) in CHO m3 membranes (9.38±0.03), tropicamide in CHO m4 membranes (6.98±0.01). 4-DAMP, pirenzepine, tropicamide and methoctramine displaced [3H]NMS in CHO m5 membranes with pK50 values of 9.20±0.14, 6.59±0.04, 6.89±0.05 and 7.22±0.01 respectively. These data confirm homogenous subtype expression in CHO m1–m5 cells. 5 [3H]NMS binding was displaced dose-dependently (pK50) by pancuronium (m1, 6.43±0.12; m2, 7.68±0.02; m3, 6.53±0.06; m4, 6.56±0.03; m5, 5.79±0.10), vecuronium (m1, 6.14±0.04; m2, 6.90±0.05; m3, 6.17±0.04; m4, 7.31±0.02; m5, 6.20±0.07), pipecuronium (m1, 6.34±0.11; m2, 6.58±0.03; m3, 5.94±0.01; m4, 6.60±0.06; m5, 4.80±0.03), rocuronium (m1, 5.42±0.01; m2, 5.40±0.02; m3, 4.34±0.02; m4, 5.02±0.04; m5, 5.10±0.03) and gallamine (m1, 6.83±0.05; m2, 7.67±0.04; m3, 6.06±0.06; m4, 6.20±0.03; m5, 5.34±0.03). 6 Cyclic AMP formation was inhibited dose dependently by methacholine in CHO m2 cells pEC50 for control and pancuronium (300 nM) treated cells were 6.18±0.34 and 3.57±0.36 respectively. Methacholine dose-response curves in the absence and presence of rocuronium (1 μM) and vecuronium (1 μM) did not differ significantly. Pancuronium, vecuronium and rocuronium did not inhibit cyclic AMP formation alone indicating no agonist activity. 7 With the exception of rocuronium there was a significant interaction with m2 muscarinic receptors with all NMBD's at clinically achievable concentrations suggesting that the brady/tachycardias associated with these agents may result from an interaction with cardiac muscarinic receptors. Furthermore pancuronium at clinically achievable concentrations antagonised methacholine inhibition of cyclic AMP formation in CHO m2 cells further suggesting that the tachycardia produced by this agent results from muscarinic antagonism. The mechanism of the bradycardia produced by vecuronium is unclear. British Journal of Pharmacology (1998) 125, 1088–1094; doi:10.1038/sj.bjp.0702166 |
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