Oestrogen enhancement of the myometrial response to exogenous parathyroid hormone-related protein (PTHrP), and tissue localization of endogenous PTHrP and its mRNA in the virgin rat uterus

Classical pharmacological studies have shown that oestrogen dominance in humans and other animals can increase the responsiveness of the uterus to many locally acting peptides. Parathyroid hormone-related protein (PTHrP) has been shown to be expressed in the pregnant and non-pregnant rat uterus and exogenous PTHrP is known to relax uterine contraction in vitro. We investigated whether oestrogen dominance can influence the responsiveness of the uterine horn to PTHrP, and further studied the localization of PTHrP mRNA and protein in the rat uterine horn using in-situ hybridization and immunohistochemistry. Exogenous PTHrP(1-34) inhibited spontaneous and electrically induced contractions in uteri isolated from non-cycling rats. Pretreatment of non-cycling rats with oestradiol-17 beta increased uterine sensitivity to PTHrP: EC50 values for inhibition of spontaneous contractions by PTHrP were 0.33 nmol/l, 1.1 nmol/l, 2.6 nmol/l and 7800 nmol/l in uteri from animals treated for 2 days with oestradiol-17 beta alone, 2 days with oestradiol-17 beta + 1 day progesterone, 1 day with oestradiol-17 beta alone and in untreated rats respectively. Similar EC50 values were obtained for electrically stimulated uteri. In agreement with these findings, uterine horns from cycling rats in pro-oestrous and oestrous phases of the cycle showed a higher responsiveness to PTHrP(1-34) when compared with uterine horns taken from rats in metoestrus and dioestrus. PTHrP mRNA and protein were detected in the endometrial epithelium lining of the lumen and the endometrial glands, as well as in the myometrium of rats which were either pretreated for 2 days with oestradiol-17 beta or untreated. This study suggests that PTHrP may act in an autocrine and/or paracrine manner to modulate uterine motility and function.

Effects of tachykinins on uterine smooth muscle

1. Sensory nerves supplying the mammalian uterus have been shown to contain substance P (SP) and neurokinin (NK)A. This review presents some of the advances that have led to a greater understanding of the effects of tachykinins on uterine smooth muscle. 2. The cell-surface peptidase neprilysin (EC.3 24.11, endopeptidase 24.11, enkephalinase, CALLA, CD10) has been shown to play a major role in regulating the actions of tachykinins on both rat and human myometrium. Because this peptidase is known to be regulated by steroids and pregnancy, its effects may be of physiological relevance. 3. Tachykinins produce contractions of isolated myometrial preparations from non-pregnant rats and mice. The NK2 receptor mediates these effects in rat uterus, while the NK1 receptor may mediate these effects in the mouse uterus. 4. The effects of tachykinins have been examined on myometrial preparations obtained at Caesarean section from near-term pregnant women. In the presence of the peptidase inhibitors (thiorphan, captopril and bestatin), the mammalian tachykinins SP, NKA and NKB produced concentration-dependent uterine contractions. 5. The order of agonist potency NKA > SP = NKB suggested that NK2 receptors mediate uterine contractions in the human. This was confirmed using the stable analogues [Sar9,Met(O2)11]SP, [Lys5MeLeu9Nle10]NKA(4-10) and [N-MePhe7]NKB, which are NK1, NK2 and NK3 receptor selective, respectively. Only [Lys5MeLeu9Nle10]NKA(4-10) produced concentration-related contractions of human uterine smooth muscle. 6. The experimental findings described in the present review, taken together with results published previously in the literature, indicate that tachykinin peptides may play a physiological or pathophysiological role in regulating uterine smooth muscle activity. However, more extensive research will be required to confirm such a role for these peptides.

Activation of neurokinin NK(2) receptors by tachykinin peptides causes contraction of uterus in pregnant women near term

The aim of this study was firstly to elucidate whether the mammalian tachykinins substance P (SP), neurokinin A (NKA) and neurokinin B (NKB)-regulated contractility of myometrium obtained from near-term pregnant women, and secondly to investigate the receptor subtype(s) responsible. In the presence of peptidase inhibitors, i.e. thiorphan (3 micromol/l; endopeptidase 24.11 inhibitor), captopril (10 micromol/l; angiotensin converting enzyme inhibitor) and bestatin (10 micromol/l; aminopeptidase inhibitor); all three mammalian tachykinins elicited concentration-related contractions of isolated myometrial preparations. The rank order of agonist potency of the mammalian tachykinins in the presence of the peptidase inhibitors was NKA > SP = NKB, indicating that the contractile effects were mediated by activation of an NK(2) receptor. The NK(2) receptor-selective agonist, [Lys(5), MeLeu(9), Nle(10)]NKA(4-10), produced concentration-related contractile responses, while the respective NK(1) and NK(3) receptor-selective agonists, [Sar(9), Met(O(2))(11)]SP and [N-MePhe(7)]NKB, had no effect either in the absence or presence of the peptidase inhibitors. The NK(2) receptor-selective antagonist, SR48968, produced concentration-related rightward shift in the log concentration curve to [Lys(5), MeLeu(9), Nle(10)]NKA(4-10). This study shows that tachykinins elicit contractile effects on human myometrium obtained from pregnant women near term, and that these effects are mediated by an NK(2) receptor. An excitatory effect of the tachykinins on these preparations could indicate a physiological role for these peptides in enhancing contractility of the uterus in women at term.

NK2 receptors mediate tachykinin-induced contractions of rat uterus during the oestrous cycle

We examined tachykinin-induced contractions of uteri from rats during the oestrous cycle. The potencies of substance P, neurokinin A, neurokinin B and the tachykinin NK2 receptor-selective agonist, [Lys5, MeLeu9, Nle10] neurokinin A-(4-10), and of the non-peptide tachykinin NK1, NK2 and NK3 receptor antagonists (S)1-[2-[3-(3,4-dichlorophenyl)-1-(3-isopropoxyphenylacetyl)pip eridin-3-yl]ethyl]-4phenyl-1-azonia-bicyclo[2.2.2]octane (SR 140333), (S)-N-methyl-N [4-(4-acetylamino-4-phenylpiperidino)-2-(3,4-dichlorophenyl)butyl]benzam ide (SR 48968) and (S)-(N)-(1-(3-(1-benzoyl-3-(3,4-dichlorophenyl)piperidin-3-yl)prop yl)-4-phenylpiperidin-4-yl)-N-methylacetamide (SR 142801), were examined. The relative agonist potencies, i.e., [Lys5, MeLeu9, Nle10] neurokinin A-(4-10) > or = neurokinin A > neurokinin B > or = substance P were similar in preparations from rats in dioestrus/metoestrus and those in proestrus/oestrus. Apparent pK(B) values for SR 48968 versus neurokinin A and [Lys5, MeLeu9, Nle10] neurokinin A-(4-10), were 9.9 and 9.2, respectively, indicating activation of an NK2 receptor. SR 140333 (10 nM) produced only a small rightward shift of the log concentration-response curve to substance P. SR 48968 (3 nM), but not SR 142801 (100-300 nM) reduced the effect of neurokinin B. These data indicate that in the rat tachykinin-induced contractions of the uteri during the oestrous cycle are mediated primarily by tachykinin NK2 receptors, and that fluctuations in ovarian hormonal levels during the oestrous cycle have little influence on the uterine response to tachykinins.

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.

The effects of P2 purinoceptor agonists on the isolated portal vein of the guinea pig

UTP, ATP and several of its analogues enhanced contractions of the longitudinal smooth muscle layer of the guinea-pig portal vein. The rank order of potency was 2-methylthioATP > alpha, beta-methyleneATP > adenosine tetraphosphate > or = beta, gamma-methyleneATP > or = ATP = UTP > > adenosine. Suramin (100 microM) blocked the contractile effects of 2-methylthioATP and alpha,beta-methyleneATP, but not those of ATP and adenosine tetraphosphate. The P1 purinoceptor antagonist, 8-phenyltheophylline (10 microM), was without effect on the response to ATP. Field stimulation (5 s trains every 100 s, 1 ms, 55 V) caused frequency-dependent contractions that were partially reduced by the noradrenergic neurone blocking drug; BW 172C58 (4-benzoyl-xylocholine, 10 microM), but not by suramin. alpha,beta-MethyleneATP was more potent than beta,gamma-methyleneATP, UTP and adenosine tetraphosphate in partially inhibiting field stimulation-induced contractions of the portal vein; its effects, but not those of adenosine tetraphosphate, were reduced by suramin. These results indicate that the guinea-pig portal vein contains P2 purinoceptors; these include a P2x subtype, mediating contraction.

alpha(1L)-adrenoceptors mediate noradrenaline-induced contractions of the guinea-pig prostate stroma

The alpha(1)-adrenoceptor subtype mediating noradrenaline-induced contractions of the guinea-pig isolated prostatic smooth muscle was investigated. Noradrenaline produced concentration-dependent contractions of the tissue with a mean pD(2) value of 5.26+/-0.03 (n=20). These contractions were antagonised by prazosin, 2-(2, 6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane hydrochloride (WB-4101), N-[2-(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-alpha, alpha-dimethyl-1H-indole-3-ethanamine hydrochloride (RS-17053) and (R)-5-[2-[[2-(2-ethoxyphenoxy)ethyl]amino]-2-methylethyl]-2- methoxybenzensulfonamide methanesulphonate hydrate (tamsulosin). Mean pA(2) or apparent pK(B) estimates for the antagonism of noradrenaline were 8.15+/-0.05 for prazosin; 8.83+/-0.11 for WB-4101, 7.18+/-0.14 for RS-17053 and 10.11+/-0.12 for tamsulosin. The relatively low estimates of the apparent dissociation constant for all antagonists except tamsulosin indicate that an alpha(1L)-adrenoceptor mediates noradrenaline-induced prostatic smooth muscle contraction in the guinea-pig prostate gland.

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 mechanical responses of field-stimulated preparations of the longitudinal and circular myometrium, and of the costo-uterine muscle of the guinea pig to adrenoceptor agonists have been examined on Days 1, 4, 10 and 15-16 of the estrous cycle. The alpha-adrenoceptor agonist, phenylephrine, was exclusively excitatory in all three preparations taken from animals at each of the cycle days studied. Concentration-response curves for phenylephrine in all three tissues were unaffected by the fluctuations in ovarian hormones occurring during the estrous cycle. Isoproterenol (1 nmol/1-5 mumol/1) usually inhibited field-stimulation-induced contractions of the longitudinal uterine layer and the costo-uterine muscle; higher doses usually caused enhancement of contractions. In neither tissue did the potency or maximum effect of this inhibitory agonist differ at differing cycle stages. In the circular myometrium, the inhibitory effects of isoproterenol were less consistently observed, particularly in preparations taken at proestrus (Days 15-16). It is concluded that, except perhaps in the circular myometrium at proestrus, the fluctuations in ovarian hormones occurring during the estrous cycle in this species are insufficient to modify uterine responses mediated via alpha- and beta-adrenoceptors. It remains to be established whether the steroid hormones are capable of modulating responses mediated by adrenoceptors in the costo-uterine muscle.

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 adrenaline, isoprenaline and forskolin on pregnant human myometrial preparations

1. The effects of adrenaline, isoprenaline and forskolin upon the evoked contractions of field-stimulated preparations of human, pregnant, isolated myometrium have been examined. Specimens were obtained at lower segment Caesarean section from patients at 31 (n = 1) and 36-40 (n = 10) weeks of gestation. 2. Adrenaline enhanced the electrically evoked contractions of all preparations studied, indicating that its predominant action on these pregnant myometrial tissues was at alpha- and not beta-adrenoceptors. 3. Isoprenaline in concentrations at and below 10 mumol/l produced inhibitory effects in eight of 11 experiments. In the remaining three experiments, tissues were not responsive to the inhibitory effects of isoprenaline. 4. In all preparations exposed to higher concentrations of isoprenaline (30 or 100 mumol/l), its effects were excitatory. 5. Forskolin produced inhibitory effects on preparations from all uteri, including those from which tissues unresponsive to isoprenaline had been obtained. 6. It is suggested that forskolin in the concentrations which were effective in this study produced its inhibitory effects largely through activation of adenylate cyclase. This implies that the lack of an inhibitory response of some preparations to isoprenaline was not due to reduced activity of the adenylate cyclase system, but that the failure of isoprenaline to produce an inhibitory effect could be due to diminished numbers of beta-adrenoceptors and/or increased numbers of alpha-adrenoceptors, or to a defect in the coupling of the receptors to the adenylate cyclase system. Alternatively, the presence of an endogenous antagonist of the effects of isoprenaline (for example, an eicosanoid), could mask its inhibitory effects. 7. The absence of an inhibitory effect of isoprenaline on some specimens of human gravid myometrium could have clinical implications, in view of the widespread use of beta 2-adrenoceptor agonists as uterine relaxants.

Adrenoreceptor-mediated responses in the isolated portal vein of the hypothyroid rat

1 The effect of hypothyroidism, induced by methimazole, upon the response of the isolated portal vein of the rat to adrenoceptor agonists and antagonists, to angiotensin II and to papaverine has been investigated. 2 The positions and maxima of log doses-response curves, to the vasoconstrictor agonists, noradrenaline and angiotensin II, and to the vasodilator agonists isoprenaline and papaverine, were unaffected by methimazole treatment. 3 The alpha-adrenoreceptor antagonist phentolamine competitively inhibited the effects of noradrenaline to a similar extent in preparations from control and hypothyroid animals. The competitive antagonism of isoprenaline by the beta-adrenoreceptor antagonist propranolol was similarly unaffected by hypothyroidism. 4 These results taken together indicate that hypothyroidism is without significant effect upon either the properties of postjunctional alpha- and beta-adrenoreceptors in the rat portal vein, or upon the reactivity of this blood vessel to the vasoactive agonists studied.

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)

Supersensitivity to the stimulant action of noradrenaline on human myometrium near term

Noradrenaline (10-50 nM) and tyramine (0.05-1 mM) enhanced contractile force elicited by field stimulation of strips of myometrium from non-pregnant and pregnant women. In higher concentrations, noradrenaline produced sustained contractions. The EC50 values for noradrenaline were 0.4 microM in tissues from pregnant women and 3.1 microM in tissues from non-pregnant women; maximum responses were greater in the former tissues. In addition, the effects of noradrenaline on myometrium from pregnant women were more marked on the inner layer than on the outer layer, antagonized by the alpha 1-adrenoceptor antagonist prazosin (0.1 and 1.0 microM), and unaffected by the inhibitor of neuronal uptake, nisoxetine (0.1 microM). Taken together, these observations confirm that supersensitivity to noradrenaline develops during pregnancy and is present near term. The supersensitivity to noradrenaline at term can be attributed only in part to a decrease in its removal by the sympathetic innervation, which declines towards term, because responses to tyramine were also enhanced in tissues from pregnant women. It is possible that gap junction formation may also contribute to this supersensitivity.

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.

Cholinoreceptors in the isolated costo-uterine muscle of the rat

1 The aim of this study was to examine the cholinoreceptor population in the rat costo-uterine muscle. 2 The nicotinic cholinoreceptor agonists nicotine and DMPP, and the ganglionic muscarinic cholinoreceptor stimulant McNeil A-343, had no effects upon isolated preparations of this tissue. 3 Acetylcholine was more potent than carbachol and approximately equipotent with methacholine (the mean EC50 values were 7.0, 6.3 and 6.7 respectively) in producing contractions of the preparation; each was a full agonist. The potencies of carbachol and methacholine were similar in preparations taken from animals in oestrus and in dioestrus. 4 Atropine competitively antagonised the effects of carbachol and methacholine, the pA2 values were 9.37 and 9.41 respectively. The pA2 value for pirenzepine with carbachol as the agonist was 6.69. 5 Pilocarpine produced phasic contractions of the tissue (EC50 value = 4.17), and competitively antagonised the effects of carbachol with a pA2 value of 5.26. The anticholinesterase, physostigmine, produced only a small potentiation of the effects of acetylcholine. 6 It is concluded that the cholinoreceptors which mediate contraction of the rat costo-uterine muscle are muscarinic, homogeneous in nature and unaffected by fluctuating levels of ovarian hormones occurring during the oestrous cycle. The consequences of inhibition of cholinesterase activity in isolated preparations of the tissue are minimal.

Endopeptidase 24.11 inhibition does not modify uterotonic effects of endothelins in rat uterus

We investigated effects of the endopeptidase 24.11 inhibitor, SCH 39370, on uterotonic effects of endothelins (ETs) and sarafotoxin S6b. Responses of uteri from non-pregnant rats were inhibited by the ETA receptor antagonist, BQ123 (1 microM) but not the ETB receptor antagonist, BQ 788 (1 microM). ET-1, sarafotoxin S6b and ET-2 were more potent than ET-3 in tissues from non-pregnant and pregnant rats. SCH 39370 (10 microM) did not affect uterotonic responses to these peptides in either group, but inhibited those of big ET-1 in non-pregnant rat tissues, indicating inhibition of conversion of big ET-1 to ET-1. These data indicate that endopeptidase 24.11 does not inactivate the endothelin peptides in the rat uterus.

Inhibitory potency of isoprenaline on guinea-pig and gravid human myometrium following extraneuronal uptake blockade

The hypothesis that inhibitory effects of isoprenaline on myometrial contractility may be constrained by activation of putative intracellular beta-adrenoceptors negatively-coupled to adenylate cyclase was examined. Field-stimulated preparations of guinea-pig and human myometrium were used to examine the influence of the catecholamine extraneuronal uptake2 inhibitors, corticosterone and beta-oestradiol, on the inhibitory effects of the beta-adrenoceptor agonist, isoprenaline, on uterine contraction. Longitudinal and circular myometrial layers were obtained from guinea-pigs in dioestrus, primed with oestrogen before progesterone, or pregnant (Days 62-65). In the guinea-pig myometrium, corticosterone (30 microM) did not affect responses to isoprenaline. beta-oestradiol (10 microM) induced a small potentiation of the effects of isoprenaline on longitudinal myometrium from dioestrus guinea-pigs. Myometrial preparations were obtained from pregnant women (36-40 weeks gestation) undergoing caesarean section. Isoprenaline inhibited stimulation-evoked contractions in 7 of 10 preparations of the inner myometrial layer and in 5 of 8 preparations of outer myometrial layer. Corticosterone (30 microM) reduced the effects of isoprenaline on the inner layer and did not affect the outer layer. These results do not support the existence of mechanism involving isoprenaline-sensitive intracellular receptors which constrain responses to beta-adrenoceptor agonists.