Responses mediated by adrenoceptors in the separated layers of the myometrium and in the costo-uterine muscle of the guinea pig during the estrous cycle

The In Vitro Contractile Response of Canine Pregnant Myometrium to Oxytocin and Denaverine Hydrochloride

In pregnant bitches, the response to oxytocin and denaverine hydrochloride in dystocia management is usually poor. To better understand the effect of both drugs on myometrial contractility, the circular and longitudinal muscle layers were examined in an organ bath. For each layer, three myometrial strips were stimulated twice, each with one of three oxytocin concentrations. The effect of denaverine hydrochloride was studied once in direct combination with oxytocin and alone with subsequent oxytocin administration. Contractions were recorded and evaluated for average amplitude, mean force, area under the curve (AUC), and frequency. Effects of different treatments were analyzed and compared within and between layers. In the circular layer, oxytocin significantly increased amplitude and mean force compared to untreated controls regardless of stimulation cycles or concentrations. In both layers, high oxytocin concentrations caused tonic contractions, while the lowest concentration created regular rhythmic contractions. Longitudinal layer tissue responded to oxytocin with a significantly decreased contractility when stimulated twice, presumably a sign of desensitization. Denaverine hydrochloride neither affected oxytocin induced contractions nor showed a priming effect to subsequent oxytocin. Thus, no benefit of denaverine hydrochloride on myometrial contractility was found in the organ bath. Our results suggest a better efficiency of low-dose oxytocin in canine dystocia management.

Identification of functional alpha-adrenoceptor subtypes in the bovine female genital tract during different phases of the oestrous cycle

The concentration and functionality of the alpha-adrenoceptor (alpha-AR) subtypes in the genital tract of cyclic heifers were investigated. In each tissue sample, a single class of alpha1-ARs was observed, whereas two distinct classes of alpha2-ARs were discriminated: low-affinity (LA) and high-affinity (HA) alpha2-ARs. Statistical analysis showed the presence of significantly (p < 0.05) higher concentrations of all alpha-AR subtypes in the follicle than in the corpus luteum. No significant differences were found in the ovary or myometrium between the luteal and follicular phases. In the ovary, the density of alpha1-ARs was significantly (p < 0.05) higher than that of alpha2-ARs. By contrast, there were significantly (p < 0.05) more alpha2-ARs than alpha1-ARs in the myometrium. As far as alpha2-ARs are concerned, LA alpha2-ARs were significantly (p<0.05) higher than HA alpha2-ARs in all tested tissues. Competition studies suggested that the rank order of potency of antagonists for alpha1-ARs was prazosin > phentolamine > yohimbine, whereas for alpha2-ARs the order of potency was yohimbine > or = phentolamine>prazosin. Functional assays performed on myometrium showed that noradrenaline, phenylephrine and clonidine elicited concentration-dependent contractions only in dioestrus and pro-oestrus preparations and that clonidine was more effective than phenylephrine as a contractile agent. It appeared that there were no significant modifications in alpha-AR affinity or concentration during the different stages of bovine oestrous cycle, whereas the uterine spontaneous activity and the responsiveness to alpha-adrenergic stimulation was strongly influenced by hormonal levels. The modifications of uterine contractility observed during the oestrous cycle may be related to modifications induced in the transductional mechanisms of alpha-ARs.

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.

A1- and A2-purinoceptors in the guinea-pig uterus

1. Radioligand binding and functional studies were undertaken to investigate the P1-purinoceptors present in the separated myometrial layers and the endometrium of the guinea-pig uterus. 2. In preparations of endometrium-denuded circular myometrium, the A2-selective agonists (2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido-adeno sin e (CGS 21680, 100 mumol/L) and N-ethylcarboxamido adenosine (NECA, 1-10 mumol/L) inhibited contractile responses to phenylephrine. In preparations of endometrium-intact circular myometrium, NECA (10 mumol/L) enhanced responses to phenylephrine. NECA did not modulate the spontaneous contractions of longitudinal myometrium. 3. Homogenate binding studies with circular myometrium, longitudinal myometrium and endometrium revealed saturable high affinity [3H]-NECA binding sites. The mean maximal densities of binding sites (Bmax) were 2.08, 14.7 and 15.5 fmol/mg protein, and pKD (neg. log dissociation constant) values were 9.82, 9.19 and 7.44, respectively. 4. (R-) and (S-)-N6-(2-phenylisopropyl)adenosine (R- and S-PIA) both competed for two [3H]-NECA binding sites in preparations of circular myometrium. CGS 21680 competed for two [3H]-NECA binding sites in preparations of endometrium and longitudinal myometrium. All other agonist competition was for one site only. The rank orders of potency of high affinity binding were S-PIA > or =R-PIA > or = CGS 21680 (circular myometrium), R-PIA > CGS 21680 > or = S-PIA (longitudinal myometrium) and CGS 21680 > > S-PIA > or = R-PIA (endometrium). 5. In preparations of circular myometrium, longitudinal myometrium and endometrium the selective A1-purinoceptor antagonist, 1,3-dipropyl-8-(2-amino-4-chloro)-phenylxanthine (PACPX), competed for two [3H]-NECA binding sites, the non-selective antagonist 3,7-dimethyl-1-propargylxanthine (DMPX), competed for one site only.(ABSTRACT TRUNCATED AT 250 WORDS)

Actions of forskolin and isoprenaline on the separated myometrial layers of the guinea-pig uterus: influence of ovarian steroids and pregnancy

Responses to forskolin and isoprenaline of electrically stimulated preparations of longitudinal and circular myometrium were compared. The guinea-pigs used were: untreated (dioestrous), ovarian steroid-treated, pregnant and post-partum. Forskolin inhibited electrically evoked contractions of both myometrial layers obtained from all groups; there was no change in its potency or its maximum effect (90-100% inhibition). The inhibition of contractions of longitudinal myometrium by isoprenaline was enhanced with steroid treatment, attaining a maximum in late pregnancy and immediately post-partum. In circular myometrium, there was no significant change in the potency of isoprenaline on tissues from ovarian steroid-treated animals. However, maximum potency as well as maximum response was observed in late pregnancy. Since the effects of isoprenaline but not of forskolin were modulated, we suggest that ovarian steroids and pregnancy may alter the coupling of beta-adrenoceptors to the adenylate cyclase system.

Actions of some autacoids and peptides, including relaxin, on costo-uterine muscle from rats

1. The actions of angiotensin II, bradykinin, oxytocin, arginine vasopressin, relaxin, serotonin and the prostaglandins E2 and F2 alpha were examined on preparations of costo-uterine muscle from stilboestrol-treated rats. 2. All the agonists, except relaxin, when used in concentrations which contract the rat uterus, also produced contractions of costo-uterine muscles. Concentration-response curves were steep and maximal responses to the agonists were comparable. The negative log molar EC50 values were: serotonin, 6.5; angiotensin II, 8.8; bradykinin, 8.4; PGE2, 8.3; PGF2 alpha, 7.1. The EC50 values (units/L) for oxytocin and vasopressin were 4.4 and 2.7 respectively. 3. Indomethacin (2.8 or 5 mumol/L) did not decrease the contractile effects of the peptides or serotonin. The effects of serotonin were reduced, but not reversed, by methysergide (0.94 mumol/L). 4. Porcine relaxin inhibited field stimulation-induced contractions of costo-uterine muscle and uterine horns from immature rats pretreated with oestradiol cypionate and from stilboestrol-treated mature rats. It was much less potent, and its effects were less clearly concentration-related, on costo-uterine muscle. 5. The inhibitory effects of relaxin on the uterus were unaffected by propranolol (1 mumol/L), confirming that on this tissue relaxin acts independently of the release of catecholamines. Progesterone (30 mumol/L) was also without effect on the action of relaxin on the uterus. 6. These results taken together indicate that the costo-uterine muscle of the rat: (i) contracts in response to serotonin and the peptides angiotensin II, arginine vasopressin, bradykinin and oxytocin independently of the release of the contractile prostaglandins F2 alpha and E2; and (ii) in contrast to the uterus, may lack a significant population of receptors for relaxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Fenoterol and salbutamol actions on guinea-pig myometrium: effects of ovarian steroids

Responses of isolated preparations of longitudinal myometrium from the guinea-pig to fenoterol, salbutamol and isoprenaline were examined. Virgin adult guinea-pigs were treated (i) with oestradiol cypionate 20 micrograms/kg s.c. thrice weekly for two weeks; or (ii) as in (i) plus progesterone 3 mg/animal for the last four days. Other animals (diestrous; cycle days 6-10) were untreated. The order of potency of the agonists in inhibiting field stimulation-induced contractions of the preparations (isoprenaline greater than or equal to fenoterol greater than salbutamol) indicated interaction with beta 2-adrenoceptors. Oestrogen treatment enhanced the potency of isoprenaline 5-fold. Treatment with oestrogen plus progesterone enhanced the potency of all agonists 10- to 30-fold, and increased the mean maximum response to salbutamol. The mean pKD values for displacement by fenoterol of binding of (-)-[125I]cyanopindolol to membrane preparations from animals in both treatment groups were 5.57-5.92. These results raise the possibility that progesterone may enhance the coupling of beta 2-adrenoceptors to transduction mechanisms in the longitudinal myometrium from oestrogen-primed guinea-pigs.

Uterine contractility and actions of catecholamines in longitudinal and circular uterine layers from ovariectomised guinea-pigs: the effects of ovarian steroids

The influence of ovarian steroids upon responses to electrical stimulation and to activation of adrenoreceptors in field-stimulated preparations of longitudinal and circular myometrium from ovariectomised guinea-pigs has been investigated. Adult virgin guinea-pigs were bilaterally ovariectomised and were treated two weeks later with thrice-weekly injections of oestradiol cypionate for two weeks, or treated as in then given oestradiol cypionate and progesterone for a further four days. Control groups of bilaterally ovariectomised and sham ovariectomised animals remained untreated. Both myometrial layers from untreated ovariectomised guinea-pigs were atrophied. Responses to field stimulation in the circular myometrium were much smaller than those in the longitudinal layer. Steroid pretreatment, most notably treatment with oestradiol and progesterone, were associated with decreased and increased responsiveness to electrical stimulation in the circular and longitudinal myometrial layers respectively. Adrenaline and noradrenaline were consistently excitatory on preparations of circular myometrium from ovariectomised animals. Responses comprised either enhancement of electrically-evoked contractions, or, with the higher concentrations, the appearance of rapid contractions superimposed upon an increase in basal tone. The latter effects were also evident in preparations of circular myometrium from sham operated animals. In preparations of longitudinal myometrium from untreated ovariectomised animals noradrenaline consistently and adrenaline usually caused a simple enhancement of the magnitude of the evoked contractions. Phentolamine reduced the excitatory effects of both amines in both layers. In circular myometrium from the oestrogen-treated group both catecholamines produced phentolamine-sensitive enhancement of electrically-evoked contractions, but did not cause high frequency contractions or increased tonus. Noradrenaline and adrenaline produced qualitatively similar phentolamine-sensitive effects in preparations of longitudinal myometrium from this group.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of adrenaline and noradrenaline in separated longitudinal and circular myometrial preparations from the guinea-pig: selective modulation by ovarian steroids

Field-stimulated strips of circular and longitudinal myometrium from virgin adult guinea-pigs were used to study the influence of ovarian steroids upon uterine responses to adrenaline and noradrenaline. Preparations were taken from animals (i) untreated, on day 9 or 10 of the oestrous cycle; (ii) treated for 14 days with oestradiol cypionate, beginning on day 9 or 10; and (iii) treated as in (ii) then given both oestradiol cypionate and progesterone for a further four days. Oestradiol treatment led to a 2-fold increase in the circulating level of progesterone; subsequent treatment of oestradiol-primed animals with progesterone resulted in even higher circulating progesterone levels, comparable with those occurring during mid-pregnancy in this species. Both adrenaline and noradrenaline produced phentolamine-sensitive motor responses of circular myometrial preparations from each treatment group. Steroid treatment was without significant effect on the potencies or maximal effects of the two amines on the circular muscle layer. Both catecholamines effected phentolamine-sensitive excitatory responses of longitudinal myometrial preparations from untreated guinea-pigs. Adrenaline usually produced propranolol-sensitive inhibitory responses of longitudinal preparations from oestradiol-treated animals, while noradrenaline did so only in a minority of cases. The potencies of adrenaline as an inhibitory agonist and noradrenaline as an excitatory agonist were markedly increased in preparations from oestradiol-primed animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ovarian steroids upon responses mediated by adrenoceptors in separated layers of the myometrium and in the costo-uterine muscle of the guinea-pig

1 This study describes the effects of ovarian steroid hormones upon the responses to adrenoceptor agonists of isolated myometrium, separated into its longitudinal and circular layers, and of costo-uterine muscle from guinea-pigs. The preparations were field-stimulated at 100 s intervals, and the adrenoceptor agonists phenylephrine and isoprenaline produced enhancement or inhibition of the evoked contractions.2 Isoprenaline produced propranolol-sensitive inhibitory effects in longitudinal and circular myometrium and costo-uterine muscle preparations from animals from all experimental groups: i.e. from nonsteroid-treated animals (ovariectomized and intact); intact animals treated with either oestrogen or progesterone alone; ovariectomized animals treated with oestrogen; ovariectomized and intact animals treated with progesterone following oestrogen priming; and from animals 1-4 days post-partum. Longitudinal myometrial preparations from progesterone-treated oestrogen-primed and from post-partum animals were most sensitive to this agonist.3 Phenylephrine produced phentolamine-sensitive excitatory effects in circular myometrial and costo-uterine muscle preparations from animals from all the experimental groups. In contrast, propranolol-sensitive inhibitory responses to phenylephrine occurred in longitudinal myometrial preparations taken from animals treated with progesterone following oestrogen priming, and from post-partum animals. Longitudinal myometrium from animals from the remaining experimental groups exhibited phentolamine-sensitive excitatory responses to phenylephrine.4 The basis for the selective effect upon the longitudinal myometrium of exposure to progesterone following a period of oestrogen priming, is discussed. The results described are consistent with the possibility that in the longitudinal layer of guinea-pig uterus exposed to progesterone following oestrogen priming there is an increase in the proportion of beta-adrenoceptors in this layer. This increase may reduce the likelihood of contractions arising via direct stimulation of alpha-adrenoceptors in this layer in response to sympathetic activation during pregnancy.