5-HT 7 receptor and β 2-adrenoceptor share in the inhibition of porcine uterine contractility in a muscle layer-dependent manner

Abstract

To compare the inhibition of uterine contractility mediated by β-adrenoceptors and 5-HT 7 receptors, the effects of catecholamines and 5-HT on spontaneous contractions were examined in longitudinal and circular muscles isolated from three different regions (cornu, corpus and cervix) of the non-pregnant proestrus porcine uterus. In addition, the distribution of β-adrenoceptors between muscle layers was characterized by means of adenylate cyclase activity assay, cyclic AMP assay and [ 3H]dihydroalprenolol binding studies. In the cornu, isoprenaline, adrenaline and noradrenaline inhibited the spontaneous contraction of longitudinal and circular muscles but longitudinal muscle was more sensitive to catecholamines than was circular muscle. The inhibitory response to isoprenaline was antagonized by propranolol (300 nM) or (±)-1-[2,3-(dihydro-7-methyl-1 H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol (ICI 118,551; 100 nM). The rank order of potency was isoprenaline≧adrenaline>noradrenaline. The β 2-adrenoceptor-selective agonist, clenbuterol, was more potent than xamoterol (β 1-selective) and (±)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phenoxyacetic acid (BRL 37344; β 3-selective) to inhibit the spontaneous contraction of longitudinal muscles. Isoprenaline increased adenylate cyclase activity in both muscle layers, but the activity in the longitudinal muscle was greater than that in the circular muscle. Cyclic AMP production by isoprenaline was also more conspicuous in the longitudinal muscle than in the circular muscle. Although both muscle layers contained a single class of [ 3H]dihydroalprenolol binding sites with similar K d values (longitudinal muscle, 3.1±0.94 nM, n=4; circular muscle, 2.4±0.73 nM, n=4), B max in the longitudinal muscle (175.7±32.8 fmol/mg protein, n=4) was significantly higher than that in the circular muscle (53.1±5.1 fmol/mg protein, n=4). As previously reported [Br. J. Pharmacol. 130 (2000) 79], 5-HT also inhibited the spontaneous contraction of both muscle layers from the cornu and the 5-HT 7 receptor antagonist, 2a-[4-(4-phenyl-1,2,3,6-tetrahydropyridyl)butyl]-2a,3,4,5-tetrahydro-benzo[cd]indol-2(1 H)-one (DR4004; 100 nM, n=4) blocked the 5-HT-induced inhibition of spontaneous contractions in the circular muscles, and reversed the less marked inhibition in the longitudinal muscles. In other regions of the uterus (corpus and cervix), 5-HT inhibited the spontaneous contraction of the circular muscles but contracted the longitudinal muscle strips. On the other hand, isoprenaline caused muscle layer-dependent inhibition (longitudinal muscle>circular muscle) in both regions, and the responsiveness tended to increase toward the cervix. In conclusion, β 2-adrenoceptors are present heterogeneously in the porcine uterus (longitudinal muscle>circular muscle) and share the inhibition of uterine contractility with 5-HT 7 receptors in a layer-dependent manner (longitudinal muscle: β 2-adrenoceptors, circular muscle: 5-HT 7 receptors).