The interaction of methoctramine and himbacine at atrial, smooth muscle and endothelial muscarinic receptors in vitro

Abstract

1 The action of methoctramine and himbacine at muscarinic receptors has been studied using guinea-pig isolated trachea, oesophageal muscularis mucosae, paced left atria, and rat aortic preparations. 2 Methoctramine (1 × 10⁻⁶-3.2 × 10⁻⁴ m), but not himbacine, elicited positive inotropic responses. These responses were enhanced by pretreating the animals with reserpine. The responses in reserpine-treated animals were not antagonized by phentolamine (1 × 10⁻⁶ m) but were antagonized by propranolol (1 × 10⁻⁶ m). 3 Methoctramine, but not himbacine, exhibited allosteric inhibitory effects at cardiac muscarinic receptors, resulting in a curvilinear Schild plot. Deviations from competitive antagonism were also observed in combination dose-ratio experiments using atropine and methoctramine. At 1 × 10⁻⁶ m, the pK_B value for methoctramine was 7.88 ±0.15 (mean ± s.e.mean, n = 5). The pA₂ value for himbacine at cardiac muscarinic receptors was 8.52 ± 0.06 (n = 3). 4 At tracheal and oesophageal muscularis mucosal smooth muscle receptors, the Schild plots for both antagonists were linear. The pA₂ values for methoctramine at receptors in these two preparations were similar (6.08 ± 0.05 and 6.03 ± 0.09 respectively, n= 4) and were approximately 60 fold less than those values observed at atrial receptors. Himbacine, also exhibited similar values at muscarinic receptors in the trachea and oesophageal muscularis mucosae (7.61 ± 0.05 and 7.57 ± 0.04 respectively, n= 4). 5 Muscarinic receptors mediating relaxation of the rat aortic endothelium exhibited pA₂ values for methoctramine (5.87 ± 0.12, n= 6) which were similar to those observed in the smooth muscle, but not the atria. The pA₂ values for himbacine at endothelial muscarinic receptors were approximately 0.5 pA₂ units lower than those observed at muscarinic receptors in smooth muscle (6.92 ± 0.80, n= 6). In addition, the Schild slopes for methoctramine and himbacine at these receptors were significantly (P < 0.05) less than unity. 6 Methoctramine, and to a lesser extent himbacine, are potent and selective antagonists for cardiac muscarinic receptors. However, caution should be used in interpretation of the data with methoctramine in view of the inhibitory allosteric properties and direct inotropic actions of this compound.