Responses to catecholamines of the rat isolated uterus throughout the natural oestrous cycle

Summary The effects of noradrenaline (NA), adrenaline (Adr) and isoprenaline (Iso) on rat isolated uterus were studied, throughout the natural oestrous cycle. The -inhibitory effects of the catecholamines were measured as a percentage inhibition of a standard acetylcholine (ACh)-induced contraction. Iso produced approximately 80% maximum inhibition of the standard ACh-contraction in all 4 stages of the oestrous cycle. Adr and NA produced 80% maximum inhibition in oestrus only, and 50–60% maximum inhibition in proestrus, metoestrus and dioestrus. The differences in the degree of inhibition produced by the catecholamines were overcome (i.e. the maximum inhibition produced by Adr and NA was increased to become equal to that produced by Iso) when experiments were repeated in the presence of the uptake inhibitors desmethylimipramine (DMI) and normetanephrine (NMN), but not in the presence of an -adrenoceptor antagonist (azapetine 10^–7 M). Variations in the uptake of³H-Iso and³H-NA were observed in the different stages of the oestrous cycle. Small -adrenoceptor mediated motor responses to NA and Adr were observed only in the presence of a -antagonist (propranolol 10^–5 M), in uteri from rats in oestrus, metoestrus and dioestrus, but not proestrus. It is concluded that in the rat isolated uterus, -inhibitory receptors predominate throughout the natural oestrous cycle, although the existence of -excitatory receptors has been shown in 3 of the 4 stages.     

PHARMACOLOGICAL INVESTIGATION OF α-ADRENORECEPTORS IN GUINEA-PIG SPLENIC CAPSULE

• 1 A study has been made of α-adrenoreceptors in the smooth muscle of the guinea-pig splenic capsule.
• 2 In this preparation, noradrenaline (NA), adrenaline (Adr) and α-methyl-NA were full agonists, while phenylephrine, dopamine and the catechol-imidazolidine (3,4-dihydroxyphenylamino)-2-imidazolidine, (DPI) were partial agonists.
• 3 The imidazolidines clonidine and oxymetazoline showed no agonist activity, but both inhibited the agonist activity of NA. pA₂ values calculated from Arunlakshana and Schild plots were 6.88 and 6.95 for clonidine and oxymetazoline respectively. Slopes of the Schild plots were close to unity, indicating competitive antagonism.
• 4 The α-adrenoreceptor antagonists phentolamine, prazosin and yohimbine also inhibited the agonist activity of NA with mean pA₂ values of 8.32, 9.22 and 6.90 respectively. Slopes of Schild plots were significantly less than 1, suggesting that the inhibition was not truly competitive in nature.
• 5 The irreversible α-adrenoreceptor antagonist phenoxybenzamine at low concentrations (10^–9M) shifted the log concentration-response curve to NA to the right and greatly reduced the maximum response.
• 6 It is suggested that the adrenoreceptors in the guinea-pig splenic capsule are probably of the α₁-type, and that this tissue possesses relatively few spare receptors.

     

α-ADRENORECEPTOR-MEDIATED PHASIC AND TONIC ACTIVITY IN RAT PORTAL VEIN IN VITRO

• 1 The effects of α1- and α2-adrenoreceptor agonists on phasic or tonic activity have been examined in rat portal vein in vitro.
• 2 Noradrenaline (NA) and Phenylephrine (PE) increased phasic activity at low concentrations, these effects were superimposed on a sustained contracture at higher concentrations. The α2-adrenoreceptor agonists UK 14304 or TL 99 increased extracellular electrical activity and associated mechanical phasic activity without inducing sustained contracture. These α2-adrenoreceptor mediated effects were particularly sensitive to changes in Ca²⁺ concentration.
• 3 Prazosin was a potent, competitive antagonist of contracture to NA, or PE, but did not antagonise the phasic responses to NA, UK 14304 or TL 99, except at high concentrations. Prazosin was significantly less potent against PE-induced phasic responses than PE-induced contracture. The results suggest that α1-adrenoreceptors are predominantly involved in contracture.
• 4 Yohimbine was significantly more potent against UK 14304 or TL 99-induced phasic response than against PE-induced responses, however, phasic responses to NA were competitively antagonised only by combination of yohimbine and prazosin.
• 5 It is concluded that postsynaptic α2-adrenoreceptor mechanisms are involved in the phasic responses to selective α2-adrenoreceptor agonists, and to NA in rat portal vein. The response of contracture is mediated through α1-adrenoreceptors.

     

EVALUATION OF THE SELECTIVITY OF α-ADRENORECEPTOR BLOCKING DRUGS FOR POSTSYNAPTIC α1- AND α2-ADRENORECEPTORS IN A SIMPLE ANIMAL MODEL

• 1 The increase in diastolic pressure of pithed, normotensive rats was determined after i.v. administration of the α-adrenoreceptor agonists L-phenylephrine and B-HT 933.
• 2 α-Adrenoreceptor antagonists varied widely in their relative inhibitory effects towards either L-phenylephrine- or B-HT 933-induced vasoconstrictor responses. Prazosin displayed the highest affinity for the postsynaptic α-adrenoreceptor triggered by L-phenylephrine. The rank order of potency was further: phentolamine > dihydroergotamine > clozapine > corynanthine > azapetine > yohimbine > piperoxan > tolazoline > mianserin > rauwolscine. On the other hand, the rank order of potency towards B-HT 933 was: dihydroergotamine > rauwolscine > yohimbine > phentolamine > piperoxan > prazosin > tolazoline > mianserin > corynanthine > azapetine > clozapine. These data are in general agreement with the classification for α1-(triggered by L-phenylephrine) and α2-(triggered by B-HT 933) adrenoreceptors. Both populations are present postsynaptically in vascular smooth muscle of the pithed rat and are involved in vasoconstriction.
• 3 The ratio of K_B post α2/K_B post α1 was calculated as a measure of selectivity for either α-adrenoreceptor site. The α-adrenoreceptor antagonists used cover a 20,000-fold range of activity ratios. The antagonists most selective for either type were prazosin (α1) and rauwolscine (α2). The selectivity of the α-adrenoreceptor antagonists for postsynaptic α1- and α2-adrenoreceptors in the intact circulatory system of the pithed rat is comparable with the reported selectivity of these blocking agents for α1 (postsynaptic)- and α2 (presynaptic)-adrenoreceptors in the rabbit isolated pulmonary artery.
• 4 It is concluded that two distinct types of postsynaptic α-adrenoreceptors participate in vasoconstriction in the pithed rat. Apart from the classical α1-adrenoreceptor, vascular smooth muscle of the pithed rat contains postsynaptic α2-adrenoreceptors resembling those previously found mainly presynaptically. The presence of separate classes of postsynaptic α1- and α2-adrenoreceptors in the intact circulatory system of the pithed rat offers the possibility to use this relatively simple animal model as an in vivo test system for the pharmacological characterization of α-adrenoreceptor agonists and antagonists.

     

Effect of steroids on beta-adrenoceptor-mediated relaxation of pig bronchus.

1--Progesterone, testosterone (40 microM), cortisol and cortisol hemisuccinate (80 microM) caused 6-8 fold potentiations of (+/-)-isoprenaline (Iso)-induced relaxations of pig bronchus while several other steroids caused smaller potentiations or had no effect. 2--17 beta-Oestradiol (40 microM) increased the potency of Iso, (-)-adrenaline (Adr) and (-)-noradrenaline (NA) by 10.6, 2.3 and 2.6 fold respectively but had no significant effect on the potency of fenoterol (Fen). 3--Inhibition of catechol-O-methyl transferase (COMT) with U-0521 (30 microM) caused a 6 fold increase in the potency of Iso but failed to alter the potency of Adr, NA or Fen. The extraneuronal uptake inhibitor normetanephrine (50 microM) caused significant 2 fold increases in the potency of Iso and Adr but did not potentiate the responses to NA or Fen. 4--In preparations where the potency of Iso had already been increased by U-0521 (30 microM) or by normetanephrine, 17 beta-oestradiol produced no significant further increase in potency. These results indicate that steroid-induced increases in the potency of catecholamines in pig bronchus can be explained in terms of inhibition of COMT or extraneuronal uptake or both.     

A comparative study of beta-adrenoceptors in human and porcine lung parenchyma strip

1. Responses to (+/-)-isoprenaline (Iso), (-)-adrenaline (Adr) and (-)-noradrenaline (NA) were compared in isolated preparations of human and porcine lung parenchyma strip. 2. The order of relaxant potencies of these catecholamines in both human and porcine lung parenchyma was Iso greater than Adr greater than NA (1:0.24:0.01, human; 1:0.21:0.01.pig). These results suggest that beta 2-adrenoceptors predominate in both types of lung parenchyma strip. 3. pA2 values for the beta-adrenoceptor antagonist, propranolol (non-selective), with Iso as the agonist, in human and porcine lung strips were 7.84 and 7.83 respectively and for atenolol were 6.50 and 5.35 respectively. Taken as a whole results indicate the existence of an apparently homogeneous population of beta 2-adrenoceptors in porcine parenchyma strip, while both beta 1 and beta 2-adrenoceptors were revealed in human lung parenchyma.     

INCREASED α2-ADRENORECEPTOR MEDIATED VASCULAR CONTRACTION IN DIABETIC RATS

• 1 Contractile responses of aortic ring preparations from control and diabetic rats to nonselective (noradrenaline) and selective α₁-(phenylephrine) and α₂- (clonidine) adrenoreceptor agonists were examined to determine the contribution of these receptor subtypes to the response.
• 2 Aortae from diabetic rats were more responsive to noradrenaline compared to age-matched control rats. There was no difference between contractile responses of aortae from control and diabetic rats to phenylephrine, whereas clonidine-induced contractions were enhanced in aortae from diabetic rats.
• 3 Dependence of clonidine-induced contractions on extracellular calcium availability was determined. Each of the methods used indicated increased dependency of aortae from diabetic rats on extracellular calcium availability for clonidine-induced contractile force generation.
• 4 These data indicate that the increased noradrenaline-induced responsiveness of aortae from diabetic rats may be attributed to an increased α₂-adrenoreceptor component of the contractile response. It is also concluded that the α₂-adrenoreceptor mediated contraction is primarily dependent upon extracellular calcium. From this we suggest that the increased contractile responsiveness of aortae from diabetic rats to α₂-adrenoreceptor stimulating drugs may be related to an altered coupling mechanism between the α₂-adrenoreceptors and the calcium ion channels or due to a change in membrane permeability as a result of the disease process.

     

SELECTIVITIES OF SOME AGONISTS ACTING AT α1- AND α2-ADRENORECEPTORS IN THE RAT VAS DEFERENS

• 1 In this paper, estimates of the selectivities of a series of twelve sympathomimetic agents acting at postjunctional α₁- and prejunctional α₂-adrenoreceptors were investigated, using epididymal and prostatic segments of the rat vas deferens.
• 2 The relative order of potency for the twelve agonists at prejunctional α₂-adrenoreceptors mediating inhibition of field-stimulation-induced contractions in the prostatic segment of the vas deferens was: clonidine > (?)-adrenaline > xylazine > (?)-noradrenaline > (+)-adrenaline > dopamine phenylephrine ≥ metaraminol ≥ (+)-noradrenaline > (?)-isoprenaline > methoxamine > (+)-isoprenaline.
• 3 The relative order of potency for the agonists at postjunctional α₁-adrenoreceptors mediating contraction of smooth muscle in epididymal segments of the vas deferens was: (?)-adrenaline ≥ (?)-noradrenaline > phenylephrine > clonidine ≥ (+)-adrenaline ≥ methoxamine ≥ (+)-noradrenaline ≥ metaraminol ≥ dopamine ≥ (?)-isoprenaline ≥ xylazine; (+)-isoprenaline was inactive.
• 4 (+)-Noradrenaline, the stereoisomers of adrenaline and isoprenaline, dopamine, clonidine, xylazine and metaraminol displayed α₂-selectivity whereas phenylephrine and methoxamine displayed α₁-adrenoreceptor selectivity. (?)-Noradrenaline possessed a similar potency at both α₁- and α₁-adrenoreceptors thus making it non-selective by the criteria used in this study.
• 5 Estimates of the potencies of optical isomers of catecholamines and of dopamine on epididymal and prostatic segments of the vas deferens indicated the steric orientation of the β-hydroxyl substituent is equally important at both subtypes in this tissue.

     

DUAL PRESYNAPTIC EFFECTS OF 5-HYDROXYTRYPTAMINE ON PERIPHERAL NORADRENERGIC SYNAPSES

• 1 The aim of the present experiments was to study the effects of 5-hydroxytryptamine (5-HT) on the responses to postganglionic stimulation of two models of the peripheral sympathetic nervous system: the isolated nictitating membrane of the cat and the guinea-pig isolated atria.
• 2 In the nictitating membrane of the cat, 5-HT (0.1 μM) shifted to the left the frequency-response curve to nerve stimulation. This potentiating effect was prevented by 5-HT receptor antagonists (0.1 μM. methysergide, 0.1 μM pizotifen and 0.1 μM morphine) and also by the β-adrenoreceptor blocker propranolol (0.1 μM). The α₂-adrenoreceptor antagonist yohimbine (0.1 μM) had no effect on the 5-HT-induced potentiation.
• 3 In the guinea-pig isolated atria the responses to cardioaccelerans nerve stimulation were diminished by 5-HT (0.1 to 1.0 μM). The shift to the right in the frequency-response curve induced by 5-HT (1.0 μM) was additive to the antagonism caused in the atria by propranolol (0.1 μM). The inhibitory effect of 5-HT on the pacemaker responses to nerve stimulation was prevented by the 5-HT receptor antagonists methysergide (1.0 and pizotifen (1.0 μM) and also by the α-adrenoreceptor antagonist phentolamine (0.1 μM).
• 4 The selective α₂-adrenoreceptor agonist clonidine (0.01 μM) reduced to the same extent as 5-HT (1.0 μM) the responses of the guinea-pig atria to nerve stimulation. The inhibitory effect of clonidine was prevented by the α-adrenoreceptor blocker phentolamine (0.1 μM) but not by the 5-HT receptor blocker pizotifen (1.0 μM).
• 5 With the exception of propranolol, which in the atria shifted to the right the concentration-response curve to exogenous noradrenaline (NA), neither 5-HT nor the different antagonists employed modified the sensitivity to NA in the tissues studied.
• 6 The present observations show that 5-HT can produce a dual effect on the sympathetic neurotransmission. It is proposed that a modification in the overflow of NA in response to nerve stimulation is caused by 5-HT and results from the interaction of 5-HT with specific receptors located on the sympathetic fibres. These presynaptic 5-HT receptors behave as excitatory (cat nictitating membrane) or inhibitory (guinea-pig atria) depending on the tissue studied.

     

A STUDY ON THE POSTJUNCTIONAL EXCITATORY α-ADRENORECEPTOR SUBTYPES IN THE MESENTERIC ARTERIAL BED OF THE RAT

• 1 The nature of the postsynaptic adrenoreceptor subtypes which mediate vasoconstriction in the mesenteric arterial bed of the rat was investigated using mixed and selective α₁, α₂ and β-agonists and antagonists.
• 2 Phenylephrine (PE) an α₁selective agonist and noradrenaline (NA) a mixed α₁and α₂-agonist, produced a rise in perfusion pressure (vasoconstriction). The responses to NA remained stable with time whereas responses to PE considerably increased.
• 3 UK14304 an α₂-selective agonist at low doses (10^?8 ?10^?7 moles), caused small, slow contractions in most preparations. Repeated administration of these doses or slightly higher ones, desensitized the tissue to this compound but not to NA or PE. Finally, UK14304 given simultaneously with NA or PE, at doses higher than 5 × 10^?7 moles, reduced contractions to the latter compounds and this effect was not altered by 10^?7 M rauwolscine, an α₂-selective antagonist.
• 4 Prazosin, an α₁-selective antagonist, as expected, reduced contractions to NA considerably at 10^?10 ?10^?8 M and abolished contractions to UK14304 at 2 × 10^?9 M.
• 5 Rauwolscine, at 10^?8 M, potentiated contractions to NA and at 10^?6 M reduced contractions to both NA and PE (when compared to time controls).
• 6 When propranolol (10^?6 M), a β-antagonist was included in the perfusion fluid, rauwolscine no longer potentiated responses to NA but reduced them at all concentrations. Under the same conditions rauwolscine affected the responses to PE in a similar direction to that observed in the absence of propranolol.
• 7 These results suggest that in the rat mesenteric arterial bed:

• a. rauwolscine exerts an effect additional to α₂-adrenoreceptor antagonism. Modification of this effect by propranolol indicates an interaction between this effect of rauwolscine and the β-adrenoreceptor.
• b. vasoconstriction in the mesenteric arterial bed of the rat is mainly mediated postsynaptically by α₁-adrenoreceptors although the contribution of an α₂-mediated component cannot be excluded.
• c. UK14304 is an α₁-partial agonist as well as an α₂-agonist.

     

β-ADRENORECEPTOR-MEDIATED MODULATION OF VASOCONSTRICTION IN RAT ISOLATED PERFUSED MESENTERIC ARTERIES

• 1 Pressor responses to bolus injections of noradrenaline (NA) were studied, in the isolated perfused rat mesenteric arterial bed, in the presence of β-adrenoreceptor agonists and antagonists, in an attempt to identify a possible β-adrenoreceptor mediated modulation of catecholamine-induced vasoconstrictor effects.
• 2 NA-induced responses were potentiated in the presence of timolol and (-)propranolol and suppressed in the presence of (-)isoprenaline; (+)isoprenaline was less effective against the NA-induced responses.
• 3 Timolol attenuated the effects of (-)isoprenaline on NA-induced responses but not those of the stereoisomer (+)isoprenaline.
• 4 It is concluded that the NA-induced pressor effect in the rat mesenteric vasculature is the net result of vasoconstrictor α- and vasodilator β-adrenoreceptor activation and that the interaction of the two opposing adrenoreceptor-mediated effects represents a ‘physiological antagonism’.

     

PHARMACOLOGY OF POSTSYNAPTIC α-ADRENORECEPTORS IN THE MOUSE ANOCOCCYGEUS MUSCLE

1 The α-adrenoreceptor agonists noradrenaline (NA), methoxamine, phenylephrine, naphazoline, oxymetazoline, and xylazine produced contractions of the mouse anococcygeus muscle. All were full agonists. 2 Cocaine (2 μM) increased the _pD₂ values of NA and phenylephrine. 6-hydroxydopamine pretreatment increased the _pD₂ values of NA, phenylephrine, and xylazine. After both procedures the order of potency of the agonists was oxymetazoline > naphazoline > NA > phenylephrine > methoxamine > xylazine. 3 The order of potency of antagonists against all six agonists was prazosin > phentolamine > yohimbine. However, from differences in the pA₂ values and slopes of the Schild plots of the antagonists it was possible to distinguish three distinct groups of agonists: the phenylethylamines; the imidazolines; and the thiazine derivative xylazine. 4 The results suggest that the postsynaptic α-adrenoreceptor of the mouse anococcygeus muscle may be broadly classified as α₁. However, there may be at least three drug recognition sites on the receptor which interact with agonists of differing chemical structure.     

EFFECTS OF DILTIAZEM ON PHASIC AND TONIC ACTIVITY IN RAT PORTAL VEIN

• 1 The effects of the Ca²⁺ entry blocking drug, diltiazem, have been evaluated in the rat isolated portal vein, against phasic or tonic responses induced by a range of agonists.
• 2 Diltiazem was a potent antagonist of phasic responses induced by low concentrations of K⁺, tetraethylammonium (TEA), the selective α₂-adrenoreceptor agonists UK14304 or TL99 and angiotensin II (AII).
• 3 Diltiazem was significantly less potent as an antagonist of phasic responses induced by the selective α₁-adrenoreceptor agonists phenylephrine (PE) or methoxamine (ME) or the non-selective α-adrenoreceptor agonist (NA), or of tonic responses evoked by high concentrations of K⁺, or PE. The non-stimulated phasic activity of the portal vein was antagonised by diltiazem at higher concentrations only.
• 4 It is concluded that in the rat portal vein, phasic or tonic activity are associated with different Ca²⁺-gating mechanisms. It is considered that these differences could represent different Ca²⁺-channels, different rates of activation or deactivation of the channels, or involve other sources of activator Ca²⁺ than extracellular Ca²⁺. The α₂-adrenoreceptor subtype may be functionally linked with a voltage dependent Ca²⁺-channel to cause phasic responses in this preparation.

     

MODULATION OF ENDOGENOUS NORADRENALINE RELEASE BY PREJUNCTIONAL α-ADRENORECEPTORS: COMPARISON OF A CEREBRAL AND PERIPHERAL ARTERY

• 1 Stimulation-evoked fractional release/pulse of noradrenaline (NA) is markedly different in the rabbit basilar artery as compared to the rabbit ear artery. Therefore, using both agonists and antagonists of prejunctional α-adrenoreceptors, the hypothesis that there are differences in prejunctional mechanisms between these functionally different arteries was tested.
• 2 Clonidine (10^?6M) had similar effects on NA release in both vessels, decreasing release at 2 Hz, but increasing release at 8 Hz. However, 10^?5M clonidine had different effects in the two vessels.
• 3 α-Adrenoreceptor antagonists, phentolamine (10^?6M), yohimbine (10^?5M), and phenoxybenzamine (10^?5M), all increased NA release to a similar extent in both vessels. However, the effect of phenoxybenzamine was considerably greater than that of the other two antagonists, suggesting that it may have additional sites of action.
• 4 When the effects of modest drug concentrations were considered, no major qualitative or quantitative differences in the effects of agonists and antagonists on NA release from the two vessels were seen. Thus, the difference in fractional NA release between the basilar and ear arteries cannot be accounted for by differences in prejunctional α-adrenoreceptor modulation.

     

THE EFFECTS OF INTRAVENOUS 6-HYDROXYDOPAMINE ON PERIPHERAL α-ADRENORECEPTORS

• 1 Peripheral α-adrenoreceptor-mediated responses were studied in conscious intact rabbits and in animals pretreated with intravenous 6-hydroxydopamine (50 mg/kg).
• 2 Treatment caused a large decrease in noradrenaline in plasma and peripheral tissue but only small falls in blood pressure.
• 3 There was a shift to the left in pressor dose-response curves to the α-adrenoreceptor agonist phenylephrine and the mixed α₁/α₂-adrenoreceptor agonist noradrenaline in 6-hydroxydopamine-pretreated animals. The α1-adrenoreceptor antagonist prazosin was less effective at blocking the phenylephrine pressor response in these animals.
• 4 There was no change in pressor responses to the α₂-adrenoreceptor selective agonists clonidine and guanabenz and α-adrenoreceptor antagonists were equally effective at blocking this pressor response in 6-hydroxydopamine-pretreated and in intact rabbits.
• 5 In conscious animals 6-hydroxydopamine pretreatment causes alteration in responses mediated by postsynaptic α₁-but not postsynaptic α₂-adrenoreceptors. These changes may help maintain blood pressure in the 6-hydroxydopamine treated rabbits.

     

ANALYSIS OF THE ANTAGONISM BY PRAZOSIN OF NORADRENALINE AND PHENYLEPHRINE INDUCED CONTRACTIONS OF THE RAT ANOCOCCYGEUS MUSCLE*

• 1 Blockade of α-adrenoreceptors was analyzed with prazosin in the rat anococcygeus muscle contracted with noradrenaline (NA) and phenylephrine (PHE).
• 2 Prazosin (1.1 times 10^–8– 8.8 times 10^–7M) competitively blocked phenylephrine-induced contractions (pA₂= 8.93 ± 0.04, slope = 0.85 ± 0.07) whilst its antagonism of NA was non-competitive (pA₂= 8.45 ± 0.04, slope = 0.46 ± 0.03).
• 3 At higher concentrations, (1.76 times 10^–5– 5.28 times 10^–6M) prazosin competitively antagonized NA-induced contractions (pA₂= 7.58 ± 0.11, slope = 0.93 ± 0.09).
• 4 Phentolamine also reduced the effect of NA competitively (pA₂= 8.11 ± 0.09, slope = 0.82 ± 0.26).
• 5 It is concluded that low concentrations of prazosin selectively blocked an α₁-adrenoreceptor component of NA-mediated response whilst higher concentrations of prazosin non-selectively blocked both α₁- and α₂-adrenoreceptors.

     

β2-ADRENORECEPTOR BINDING SITES IN CIRCULAR AND LONGITUDINAL MYOMETRIAL LAYERS OF THE VIRGIN GUINEA-PIG: THE INFLUENCE OF OVARIAN STEROIDS

• 1 Membranes prepared from both circular and longitudinal muscle layers of the uteri of two groups of virgin adult guinea-pigs were used to study the influence of ovarian steroids upon β-adrenoreceptor binding sites. Animals were (i) treated for 14 days with oestradiol cypionate, beginning on day 9–10 of the oestrous cycle; or (ii) treated as in (i) and in addition, with progesterone for the last four days of oestradiol administration.
• 2 (-)-[¹²⁵I]-iodocyanopindolol ([¹²⁵I]-CYP) was used to determine the numbers and characteristics of β-adrenoreceptor bnding sites in the four membrane preparations. In all cases, binding displayed characteristics of a saturable bimolecular reaction; the estimates of receptor site density (B_Max; 0.26-0.33 pmol g^?1 wet weight) were similar in all four preparations, as were those of binding affinity K_D; 18–31 pmol 1^?1).
• 3 The mean negative logarithms of apparent dissociation constants (pK_D) for the inhibition of specific [¹²⁵I]-CYP binding by ICI118, 551 (β₂-adrenoreceptor selective antagonist) ranged from 8.4 to 8.6; and those for L 643, 717-01J10 (pradrenoreceptor selective antagonist) were 5.7-6.2. Thus the [¹²⁵I]-CYP binding sites in all four membrane preparations displayed the characteristics expected of homogeneous populations of adrenoreceptors of the β₂-subtype. The pK_D values for isoprenaline were also similar in each type of membrane preparation (5.8-6.0).
• 4 It is concluded that the clearcut differences in the contractile responsiveness, to adrenoreceptor agonists, of the circular and longitudinal myometrial preparations from oestrogen-treated guinea-pigs are not due to differences in the numbers, subtype or binding affinities of β-adrenoreceptor binding sites. Moreover, enhancement of the inhibitory potencies of β-adrenoreceptor agonists, in preparations of longitudinal myometrium from oestrogen-primed guinea-pigs treated with progesterone, is not due to an effect of progesterone upon β-adrenoreceptor binding sites.

     

Pregnancy-associated changes in responsiveness of the porcine myometrium to bioactive substances

To determine the pregnancy-associated changes in the porcine uterine contractility, the spontaneous contraction and the mechanical responses to bioactive substances of uteri in nonpregnant proestrus and pregnant pigs (25-60 days of gestation) were compared in vitro. Longitudinal muscle (LM) and circular muscle (CM) of the uterus exhibited spontaneous contraction, but the frequency in pregnant pigs was lower than that in the nonpregnant pigs. The duration and force of spontaneous contraction in the pregnant pigs were long and large compared with both in the nonpregnant pigs. L-Nitroarginine methylester (L-NAME) and 2a-[4-(4-phenyl-1,2,3,6-tetrahydropyridyl)butyl]-2a,3,4,5-tetrahydro-benzo[cd]indol-2(1H)-one (DR4004) did not change the spontaneous contraction in the uteri of nonpregnant pigs but increased its amplitude in the uteri of pregnant pigs. Isoprenaline inhibited the uterine spontaneous contraction of the nonpregnant and pregnant pigs, and the inhibition was stronger in the pregnant than in the nonpregnant pigs. 5-Hydroxytryptamine also caused inhibition of spontaneous contraction in the uteri of nonpregnant pigs (CM>LM). In the pregnant pigs, sensitivity to 5-hydroxytryptamine increased in a muscle layer-dependent manner (LM>CM) and difference in the responsiveness between LM and CM decreased. Acetylcholine contracted the uterine LM and CM strips of the pregnant and nonpregnant pigs. The responsiveness of CM increased slightly during pregnancy, but that of the LM did not change. 5-Bromo-N-(2-imidazolin-2-yl)-6-quinoxalinamine (UK14304) caused contraction of only LM in the uteri of nonpregnant pigs, but contracted both LM and CM strips in the pregnant pigs. Oxytocin and prostaglandin F(2 alpha) also contracted the uteri of nonpregnant pigs (LM>CM). Pregnancy increased the contraction of both agents in the LM and CM, but the increment was marked in the CM. The contractile forces induced by all stimulants were increased (by 1.7- to 2.5-fold) in the LM and CM of pregnant pigs. In conclusion, (1) low frequency, slow kinetics and large force of spontaneous contraction are characteristics of the pregnant porcine uteri, and nitric oxide and 5-hydroxytryptamine are supposed to be partially involved in the regulation of spontaneous contraction, and (2) responses to both contractile and inhibitory agents are increased in the pregnant pigs. Increment of the responsiveness is conspicuous in the muscle layer that is less sensitive to each agonist in the uteri of nonpregnant pigs. According to the pregnancy-associated changes, muscle layer-related differences of responsiveness to bioactive substances in the nonpregnant pigs tend to decrease in the pregnant pigs.     

THE INHIBITORY EFFECT OF CLONIDINE ON THE OESTROGEN-PRIMED RAT ISOLATED UTERUS

• 1 The investigation was undertaken to study the mechanism of the inhibitory effects of clonidine on the oestrogen-primed rat isolated uterus.
• 2 Clonidine produced dose-dependent relaxations of the rat isolated uterus which were competitively blocked by yohimbine but not by prazosin.
• 3 Metiamide, a specific H₂-histamine receptor antagonist, inhibited clonidine-induced responses only at a high concentration (1.0 times 10^-6M).
• 4 Propranolol, a β-adrenoreceptor antagonist produced competitive antagonism of the clonidine responses.
• 5 Like clonidine, noradrenaline also produced dose dependent relaxations of the rat isolated uterus. The responses to noradrenaline were not antagonized by either prazosin or yohimbine but were competitively antagonized by propranolol. 7 It is concluded that clonidine inhibits the uterus via an action on α₂-adrenoreceptors and possibly also to a lesser extent on H₂-histamine receptors, leading ultimately to release of noradrenaline from endogenous stores which causes relaxation by acting on β-adrenoreceptors.

     

FUNCTIONAL AND BIOCHEMICAL MODIFICATIONS OF LUNG β-ADRENORECEPTORS AFTER IN VIVO DESENSITIZATION: PREVENTION BY INDOMETHACIN

• 1 Desensitization of lung β-adrenoreceptors induced by 4 day in vivo isoprenaline administration to rats has been investigated both from a functional and a biochemical viewpoint.
• 2 Chronic isoprenaline treatment significantly reduced the relaxing activity of the β-agonist when tested ex vivo in lung parenchymal strips and also impaired the adenylate-cyclase system. Moreover, the desensitization procedure decreased by about 30% β-adrenoreceptor number.
• 3 In vivo indomethacin treatment prevented the loss of pharmacological responsiveness of the tissue to isoprenaline and restored basal adenylate-cyclase activity.
• 4 These data indicate that in vivo isoprenaline administration actually leads to pulmonary β-adrenoreceptor desensitization. The involvement of arachidonic acid metabolites in this phenomenon is also discussed.