Heterogeneity of α1-adrenoceptors in the guinea-pig uterus

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

To compare the inhibition of uterine contractility mediated by beta-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 beta-adrenoceptors between muscle layers was characterized by means of adenylate cyclase activity assay, cyclic AMP assay and [(3)H]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-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol (ICI 118,551; 100 nM). The rank order of potency was isoprenaline > or =adrenaline > noradrenaline. The beta(2)-adrenoceptor-selective agonist, clenbuterol, was more potent than xamoterol (beta(1)-selective) and (+/-)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phenoxyacetic acid (BRL 37344; beta(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(1H)-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, beta(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: beta(2)-adrenoceptors, circular muscle: 5-HT(7) receptors).

The mechanisms of alpha(2)-adrenoceptor agonist-induced contraction in longitudinal muscle of the porcine uterus

The aim of the present study was to clarify the cellular mechanisms underlying the alpha(2)-adrenoceptor-mediated contraction of porcine myometrium (nonvascular smooth muscle). Acetylcholine (3 nM-1 microM), clonidine (1 nM-10 microM) and 5-bromo-N-[2-imidazolin-2-yl]-6-quinoxalinamine (UK14304) (1 nM-10 microM) in Krebs solution caused a concentration-dependent contraction in the longitudinal muscles of the porcine uterus with similar EC(50) values and maximum responses. A lowered external Ca(2+) concentration and verapamil (10 nM-10 microM) decreased the contractile response to clonidine and UK14304 more markedly than the response to acetylcholine. However, in Kumagai solution, neither clonidine nor UK14304 caused contractile responses, but acetylcholine remained effective. The effects of alpha(2)-adrenoceptor agonists on intracellular Ca(2+) concentration ([Ca(2+)](i)) and smooth muscle force were measured simultaneously using fura-PE3-loaded muscle preparations. Clonidine and UK14304 caused increases in [Ca(2+)](i) and force of the longitudinal muscle. The increases in [Ca(2+)](i) and muscle force were markedly inhibited by verapamil and in Ca(2+)-free solution (EGTA, 1 mM). In the absence of external Ca(2+), clonidine caused only a small increase in [Ca(2+)](i) in Ca(2+)-loaded preparations compared with those increases caused by carbachol, histamine, and oxytocin. Ca(2+) (2.5 mM) caused increases in [Ca(2+)](i) and force of the longitudinal muscles in a Ca(2+)-free high K(+) solution. Clonidine concentration dependently potentiated the Ca(2+)-induced contraction without significantly changing the increase in [Ca(2+)](i), and this potentiation was inhibited by yohimbine. These results suggested that clonidine increases the Ca(2+) sensitivity of the contractile elements through activation of alpha(2)-adrenoceptors. During the development of the contractile response to clonidine (1 microM, 0-5 min), tissue cyclic AMP levels did not change significantly. In vitro treatment with pertussis toxin (1 microg/ml for 2 h) significantly decreased the contraction induced by clonidine without affecting the responses to carbachol and high K(+). The present results indicate that in porcine myometrium, alpha(2)-adrenoceptor stimulation caused contraction of the longitudinal muscles by mechanisms largely dependent on the influx of extracellular Ca(2+), probably through voltage-dependent Ca(2+) channels (VDCCs), and that the potentiation of the Ca(2+) sensitivity of the contractile elements is another mechanism of the contractile responses. These actions involve a pertussis-toxin-sensitive G protein (probably G(i) type) in the signal transduction pathway.

Smooth muscle layer-specific variations in the autonomic innervation of bovine myometrium

1. To clarify the autonomic innervation regulating longitudinal muscle (LM) and circular muscle (CM) motility in the bovine uterus, functional (nerve stimulation, adrenergic drug responsiveness) and biochemical studies (catecholamine content, radioligand binding) were conducted on parous luteal-phase myometrium. 2. Electrical field stimulation (EFS; 60 V, 0.5-msec duration) caused tetrodotoxin (1 microM)-sensitive contractions in a frequency-dependent manner (0.5-20 Hz) in both LM and CM layers. 3. The EFS-induced LM contractions were potentiated by propranolol and conspicuously decreased by phentolamine, yohimbine, idazoxan or guanethidine, but were unaffected by prazosin or atropine. 4. On the other hand, CM contractions were only slightly decreased by phentolamine, idazoxan, yohimbine and guanethidine, but were insensitive to propranolol, prazosin or atropine. 5. The noradrenaline content in LM was about five times higher than that in CM. 6. Noradrenaline, adrenaline, clonidine, xylazine, UK14,304 and phenylephrine caused concentration-dependent contractions of both smooth muscle layers. 7. Clonidine, UK14,304 and xylazine were more potent contractile agents than noradrenaline and phenylephrine. 8. The contractile response to noradrenaline was competitively antagonized by yohimbine, but not by prazosin. 9. Binding studies using [3H]-prazosin and [3H]-rauwolscine revealed that the bovine myometrium contained both alpha1- and alpha2-adrenoceptors, but the alpha2-type receptor was dominant in both LM (94% of alpha-adrenoceptors) and CM (88%) layers. 10. The distribution of alpha-adrenoceptors was muscle layer-specific; that is, the concentration of alpha1-receptors in LM was the same as in CM, but the concentration of alpha2-receptors in LM was 2.6 times higher than that in CM. 11. The results of the present study indicate that there are layer-specific variations in the functional innervation of the parous bovine myometrium (exclusive adrenergic innervation in LM and adrenergic [minor] plus nonadrenergic, noncholinergic innervation [major] in CM), and that alpha2-adrenoceptors, which were responsive to the excitatory response of endogenous and exogenous noradrenaline, were dominant in both muscle layers of the bovine myometrium.

Binding and functional characterization of alpha-2 adrenoceptors in isolated swine myometrium

1. Alpha-adrenoceptors in the uterus of cyclic mature pigs were investigated for their binding characteristics as shown by a radioligand study employing [3H]-prazosin and [3H]-rauwolscine, and the alpha-adrenoceptors were studied for their functional roles on motility as shown mechanically in vitro. 2. Noradrenaline (10 nM-1 microM) caused a tetrodotoxin-resistant contraction in the cornual longitudinal muscle. The response was decreased by yohimbine (10 nM-1 microM) but not by prazosin (1 microM). 3. Clonidine, xylazine, phenylephrine and methoxamine also elicited a concentration-dependent contraction of the cornual longitudinal muscle. The ranking according to the potency was clonidine > xylazine >> phenylephrine > or = methoxamine. On the other hand, these alpha-adrenoceptor agonists did not elicit contraction of the cornual circular muscle even at 100 microM. 4. Noradrenergic contraction induced by transmural stimulation was significantly decreased by yohimbine (10-100 nM) but not by prazosin (1 microM). 5. The swine myometrium contained both alpha-1 (13%) and alpha-2 (87%) adrenoceptors. The distribution of alpha-adrenoceptors was muscle layer-specific, i.e. the number of both types of adrenoceptors in the longitudinal muscle was four times higher than that in the circular muscle. 6. These results show that although swine myometrium contains both alpha-1 and alpha-2 adrenoceptors, the alpha-2 type is predominant and largely responsible for mediating the excitatory response to both exogenous and endogenous noradrenaline in the cornual longitudinal muscle.

Alpha 2-adrenoceptors in the guinea-pig uterus: heterogeneity in the circular and longitudinal smooth muscle layers

Homogenate binding and functional studies have been undertaken to investigate the role of alpha 2-adrenoceptors on the circular and longitudinal myometrial layers of the dioestrous guinea-pig uterus. Each myometrial layer contained a single population of [3H]rauwolscine binding sites (KD values approximately 3 nM) for which yohimbine exhibited a higher affinity than prazosin, and xylazine a higher affinity than phenylephrine, indicating the presence of alpha 2-adrenoceptor binding sites. In circular myometrium, xylazine enhanced contractile responses, was more potent than either noradrenaline or phenylephrine in inhibiting forskolin-stimulated accumulation of cyclic AMP, and reduced the inhibitory effect of forskolin on phenylephrine-induced phosphatidyl inositol hydrolysis. In longitudinal myometrium xylazine enhanced contractile responses to phenylephrine but did not inhibit forskolin-stimulated accumulation of cyclic AMP. We conclude that alpha 2-adrenoceptor binding sites are present in both uterine layers and mediate uterine contractility possibly through different mechanisms.