In vitro activity of RO363, a beta1-adrenoceptor selective agonist

2-Phenethylamines in Medicinal Chemistry: A Review

A concise review covering updated presence and role of 2-phenethylamines in medicinal chemistry is presented. Open-chain, flexible alicyclic amine derivatives of this motif are enumerated in key therapeutic targets, listing medicinal chemistry hits and appealing screening compounds. Latest reports in discovering new bioactive 2-phenethylamines by research groups are covered too.

β1- and β1/β2-adrenergic receptor antagonists block 6-nitrodopamine-induced contractions of the rat isolated epididymal vas deferens

6-Nitrodopamine (6-ND) is an endogenous modulator of the contractility in the rat isolated epididymal vas deferens (RIEVD) and considered to be the main peripheral mediator of the emission process. Use of selective and unselective β-adrenergic receptor antagonists has been associated with ejaculatory failure. Here, the effects of selective β₁- and β₁/β₂-adrenergic receptor antagonists on RIEVD contractions induced by 6-ND, dopamine, noradrenaline, adrenaline, and electric-field stimulation (EFS) were investigated. The selective β₁-adrenergic receptor antagonists atenolol (0.1 and 1 µM), betaxolol (1 µM), and metoprolol (1 µM) and the unselective β₁/β₂-adrenergic receptor antagonists propranolol (1 and 10 µM) and pindolol (10 µM) caused significant rightward shifts of the concentration-response curve to 6-ND (pA₂ 6.41, 6.91, 6.75, 6.47, and 5.74; for atenolol, betaxolol, metoprolol, propranolol, and pindolol), but had no effect on dopamine-, noradrenaline-, and adrenaline-induced contractions. The effects of selective β₁- and β₁/β₂-adrenergic receptor antagonists at a higher concentration (atenolol 1 µM, betaxolol 1 µM, metoprolol 1 µM, propranolol 10 µM, and pindolol 10 µM) also reduced the EFS-induced RIEVD contractions in control, but not in RIEVD obtained from L-NAME-treated animals. The selective β₁-adrenoceptor agonist RO-363, the selective β₂-adrenoceptor agonist salbutamol, and the selective β₃-adrenoceptor agonist mirabegron, up to 300 µM, had no effect on the RIEVD tone. The results demonstrate that β₁- and β₁-/β₂-adrenoceptor receptor antagonists act as 6-ND receptor antagonists in RIEVD, further confirming the main role of 6-ND in the RIEVD contractility.

A new beta-adrenoceptor blocking agent derived from dehydrozingerone

1. Dehydrozingeronolol (DZPN; 0.1, 0.5, 1.0 mg/kg, i.v.) produced a dose-dependent bradycardia response and a sustained pressor action in urethane-anesthetized normotensive rats. DZPN inhibited the tachycardia effects by (-)isoproterenol, but had no blocking effect on the arterial pressor responses induced by phenylephrine. 2. In in vitro study, DZPN antagonized (-)isoproterenol-induced positive chronotropic effects in guinea-pig isolated right atria and relaxation responses in rat isolated uterus horns. 3. DZPN causes mild direct cardiac depression at high concentrations without intrinsic sympathomimetic activity (ISA). 4. The order of potency of beta-adrenoceptor antagonists in competing for the [3H]dihydroalprenolol binding sites was (-)propranolol >> DZPN > or = atenolol.

Effects of rolipram and siguazodan on the human isolated bronchus and their interaction with isoprenaline and sodium nitroprusside

1 The effects of the selective inhibitors of cyclic AMP phosphodiesterase type IV (rolipram) and type III (siguazodan) and their interactions with isoprenaline and sodium nitroprusside have been studied in the human isolated bronchus. 2 On bronchi under resting tone rolipram was, in terms of potency (pD2 = 7.77 +/- 0.14, n = 8), very similar to isoprenaline (pD2 = 7.31 +/- 0.12, n = 12) and salbutamol (pD2 = 7.12 +/- 0.17, n = 10) and approximately 10 fold more potent than siguazodan (pD2 = 6.80 +/- 0.12, n = 6). In terms of efficacy (Emax, expressed as percentage of maximal effect induced by theophylline 3 mM), both rolipram and siguazodan were less efficient (Emax = 74 +/- 6.7%, n = 8 and 66 +/- 7.5%, n = 6, respectively) than isoprenaline (Emax = 98 +/- 0.4%, n = 12) and salbutamol (Emax = 83 +/- 2.4%, n = 10). 3 During precontraction induced by methacholine (3 x 10(-7) M) or acetylcholine (10(-3) M), concentration-response curves to rolipram and siguazodan were shifted to the right and maximal effects reduced. Rolipram was more potent than siguazodan and, in terms of efficacy, it was less active. 4. Rolipram 10(-8) and 10(-7) M but not siguazodan potentiated the effects of isoprenaline as shown by the shift to the left of the concentration-response curve to isoprenaline. Sodium nitroprusside-induced relaxation was not modified by either drug. 5. These results show that rolipram is a potent relaxant of the human isolated bronchus, potentiating the effects of beta-adrenoceptor stimulation and suggest that, as previously demonstrated in other species(guinea-pig, cow) (Tomkinson et al., 1993), there may be a connection between the beta2-adrenoceptor subtype, which predominate in human airway smooth muscle, and the cyclic AMP phosphodiesterase type IV.

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.

(+/-)-Pindolol has beta 2-adrenoceptor antagonist but not agonist action on the costo-uterine muscle of the rat

1. The effects of isoprenaline and (+/-)-pindolol on rat isolated costo-uterine muscle have been compared. 2. Isoprenaline produced reproducible concentration-dependent inhibition of contractions, and maximal doses (less than 0.1 mumol/l) produced mean inhibition of 87, 94 and 97% of field, carbachol and potassium-stimulated preparations, respectively. 3. (+/-)-Pindolol, when effective, produced inhibition only in concentrations greater than its pA2 value (9.87) as an antagonist of isoprenaline; mean maximal effects were less than 60% of those produced by isoprenaline. 4. It is concluded that (+/-)-pindolol is a potent antagonist, but has only variable agonist action, at the beta 2-adrenoceptors of the rat costo-uterine muscle.

The antinociceptive action of some beta-adrenoceptor agonists in mice

The antinociceptive actions of several beta-adrenoceptor agonist drugs have been studied in mice by use of a modified abdominal constriction test. All the drugs studied had high antinociceptive activity, with ID50 values in the nmol kg-1 range. (-)-Isoprenaline and (+/-)-isoxsuprine were the most potent, being about ten times more active than salbutamol, the least potent drug studied. All these drugs produced their action very rapidly and appear to act within the peritoneum. (-)-Isoprenaline had about six times the potency of the (+)-isomer. (+/-)-Propranolol caused rightward shifts, usually parallel, of the dose-response curves for (-)-isoprenaline. (+)-Propranolol was more than ten times less potent than the racemic drug. Practolol also caused parallel, rightward shifts of the dose-response curves for (-)-isoprenaline, and was about twice as potent as (+/-)-propranolol, whether given by subcutaneous or intraperitoneal injection. Atenolol and ICI 118551 had intermediate potencies. Propranolol, practolol and ICI 118551 were all considerably less potent in antagonizing the antinociceptive actions of fenoterol and RO363, than (-)-isoprenaline. None of these antagonist drugs showed more than a slight ability to discriminate between the beta 1- and beta 2-selective agonist drugs. No evidence was found for the involvement of opioid, dopamine, or alpha-adrenoceptors in the antinociceptive action of the beta-adrenoceptor agonist drugs. Evidence for and against the involvement of beta-adrenoceptors is discussed, and it is concluded that if these receptors do mediate the antinociceptive action they appear to be atypical.

Analysis of agonist dissociation constants as assessed by functional antagonism in guinea pig left atria

In electrically driven guinea pig left atria, positive inotropic responses to (-)-isoprenaline and the selective beta 1-adrenoceptor agonist RO363 were obtained in the absence and in the presence of the functional antagonists adenosine, carbachol, gallopamil, nifedipine, and Ro 03-7894. Each of the functional antagonists reduced the maximum response to both agonists and produced nonparallel rightward shifts in the cumulative concentration effect curves. For both agonists, dissociation constants (KA) were calculated using the equation described by Furchgott (1966) for irreversible antagonism. For RO363, which is a partial agonist with high agonist activity, the equations outlined for functional interaction by Mackay (1981) were also employed to calculate KA values. The KA values obtained by each method were compared with the dissociation constants (KD) for the two agonists determined from their ability to displace the radioligand (-)-[125I]iodocyanopindolol from beta 1-adrenoceptors in guinea pig left atrial membrane preparations. The estimates of KA varied substantially from KD values. The KD values were taken as more accurate estimates of the true values for the dissociation constants because a high degree of correlation exists between pKD and pD2 values for a number of other beta-adrenoceptor agonists that behave as partial agonists and between pKD and pKB values for a number of beta-adrenoceptor antagonists. Thus it appears that there are serious limitations in the current theory for using functional antagonism as a means of obtaining agonist dissociation constants.

The cardiovascular effects of centrally administered 5-hydroxytryptamine in the conscious normotensive and hypertensive rat

The cardiovascular effects of centrally administered 5-hydroxytryptamine (5-HT) have been analysed in conscious normotensive and hypertensive rats. In conscious normotensive rats, 5-HT, (1-30 micrograms) administered intracerebroventricularly (i.c.v.) produced profound and immediate dose-related decreases in heart rate and small increases in blood pressure. The initial pressor responses were followed by secondary secondary depressor responses at high doses of 5-HT. Similar effects were produced by 5-HT i.c.v. in conscious DOCA-salt and spontaneously hypertensive rats, although the magnitude of the pressor responses was substantially greater in hypertensive than normotensive rats. Pretreatment with either N-methylatropine or atenolol intra-arterially reduced the 5-HT-induced bradycardia in normotensive rats; the reduction was enhanced when both antagonists were given in combination. The 5-HT2 antagonist, cyproheptadine (10 micrograms i.c.v.) increased basal blood pressure and heart rate in normotensive rats. Subsequent administration of 5-HT i.c.v. produced biphasic effects on heart rate consisting of an initial tachycardia followed by a marked bradycardia. Methysergide (10 micrograms i.c.v.) pretreatment did not alter resting heart rate, but attenuated the 5-HT induced bradycardia. A higher dose of methysergide, (30 micrograms i.c.v.), decreased resting blood pressure and heart rate. This study has demonstrated, therefore, that the 5-HT induced bradycardia is produced by not only a centrally mediated decrease in sympathetic tone, but also an increase in vagal drive to the heart. The bradycardia is antagonised by centrally administered methysergide, but not by cyproheptadine, which suggests that it is probably mediated through a '5-HT1-like' receptor mechanism.

Influence of assay buffer on dissociation constants of drugs at beta-adrenoceptor subtypes

The present investigation has assessed the influence of assay buffer (Tris versus Krebs) on the abilities of several beta-adrenoceptor agonists and antagonists to displace the radioligand (-)-[125I]iodocyanopindolol [( 125I]CYP) from beta 1-(left atrial) and beta 2- (uterine) adrenoceptor sites. Saturation studies indicated that the dissociation constant (KD) for [125I]CYP at both beta-receptor sites was approximately 2-fold greater in Krebs as opposed to Tris buffer, and that the maximal density of binding sites (Bmax) in atria (but not uterus) was also reduced 2-fold. In general, the KD values for beta-adrenoceptor agonists were more influenced by the type of buffer used than were KD values for antagonists. Agonist KD values at beta 2-adrenoceptor sites were higher in Krebs than in Tris buffer, while at beta 1-adrenoceptor sites, variable changes resulted. The selective affinity of agonists at beta 1- and beta 2-adrenoceptor sites was therefore markedly influenced by the buffer used and could not be predicted from the established beta 1-/beta 2-adrenoceptor selectivity of the agonists as found in organ bath studies.

The influence of molecular structure on the affinity and efficacy of some beta-adrenoceptor agonists

The affinity and efficacy of a number of sympathomimetic amines structurally related to prenalterol and the selective beta 1-adrenoceptor agonist RO363 were determined using a combination of radioligand binding and organ bath techniques. Affinity of the molecules (pKD) was calculated from their ability to displace the radioligand [125I]iodocyanopindolol ([125I]CYP) from beta-adrenoceptor sites in left atrial (beta 1) and uterine (beta 2) membrane homogenates. These pKD values were used to calculate efficacy from the positive inotropic and uterine relaxant responses elicited by the drugs in organ bath experiments. The drugs studied were either arylethanolamines i.e., (-)-isoprenaline (ISO), p-hydroxyisoprenaline (pOH-ISO), compounds XIV and XVI or aryloxypropanolamine-derivatives, i.e., oxymethylene-isoprenaline (OM-ISO), prenalterol and Compound XI which possessed a p-phenol or catechol ring and an isopropyl or a homoveratryl amine substituent. Only ISO, OM-ISO, pOH-ISO and Compound XVI were active as agonists in both tissue preparations. These drugs were partial agonists which exhibited a wide range of pD2 values and did not display any marked selectivity for either beta-adrenoceptor subtype. Compound XI and prenalterol were inactive as agonists and together with the partial agonists behaved as competitive antagonists to ISO in the two preparations. All drugs tested displaced [125I]CYP from beta-adrenoceptor sites, however, there was also a wide range of potency amongst the drugs. Analysis of the structure-affinity and structure-efficacy relationships indicated that removal of the 3-hydroxyl group from the catechol ring reduces both affinity and efficacy without altering the selectivity of the drug for either beta-adrenoceptor subtype.(ABSTRACT TRUNCATED AT 250 WORDS)

The guinea-pig isolated bronchus for the in vitro study of small calibre airway reactivity

Small calibre airway reactivity to different contractile and relaxant agents was tested in vitro using small segments (about 1 mm long and 0.2 mm in internal diameter) of guinea-pig isolated intralobular bronchi. EC50 values of, and maximal contractile responses to contractile agents were as follows (mean +/- s.e.mean, n = 6): acetylcholine 13.6 +/- 2.6 microM and 1140 +/- 80 mg; histamine 5.2 +/- 0.7 microM and 1094 +/- 95 mg; 5-hydroxytryptamine (5-HT) 0.7 +/- 0.1 microM and 595 +/- 61 mg; prostaglandin F2 alpha (PGF2 alpha) 8.8 +/- 1.2 microM and 1100 +/- 88 mg; tetraethylammonium 2.9 +/- 0.3 mM and 1055 +/- 94 mg; KC1 14.6 +/- 0.5 mM and 965 +/- 81 mg. EC50 values of, and maximal relaxant responses to beta-adrenoceptor stimulants on preparations precontracted with acetylcholine (1.4 X 10(-4)M) were: isoprenaline 0.40 +/- 0.5 microM and 782 +/- 65 mg, n = 18; salbutamol 0.19 +/- 0.02 microM and 494 +/- 55 mg, n = 5; terbutaline 0.87 +/- 0.18 microM and 263 +/- 40 mg n = 5; fenoterol 0.06 +/- 0.02 microM and 722 +/- 47 mg, n = 5; adrenaline 0.71 +/- 0.13 microM and 653 +/- 62 mg, n = 5; noradrenaline 10.8 +/- 0.9 microM and 566 +/- 97 mg, n = 5. Differences in the maximal relaxant effects between the beta-adrenoceptor stimulants showed that the preparation utilized is a relevant model for assessment of the intrinsic activity of these drugs. 5 The high ratio (about 180) of the ECm for noradrenaline (beta-adrenoceptor agonist) to that for fenoterol (beta 2-adrenoceptor agonist), and the lack of effect of prenalterol (beta 1-adrenoceptor agonist) suggested that beta 2-adrenoceptors are preferentially involved in the relaxant activity of beta-adrenoceptor stimulants in this preparation.

The in vitro pharmacology of xamoterol (ICI 118,587)

The effect of xamoterol and (-)-isoprenaline have been compared for their activity at beta-adrenoceptor sites in a number of in vitro cardiac and smooth muscle preparations. Xamoterol produced weak positive chronotropic effects in guinea-pig, rat and cat atria (intrinsic activity less than 0.55, (-)-isoprenaline = 1). Positive inotropic effects were obtained in driven left atria of the cat but were absent in guinea-pig left atrial and right ventricular strip preparations. Agonistic effects were due to beta 1-adrenoceptor stimulation. Xamoterol was without beta-adrenoceptor-mediated inhibitory effects in guinea-pig ileal, tracheal and uterine preparations and in the rat vas deferens and oestrogen-primed uterus. Weak beta 2-adrenoceptor-mediated relaxation was obtained in progesterone-primed rat uteri. Xamoterol produced non-specific inhibitory effects in guinea-pig ileal and tracheal preparations. Xamoterol acted as a competitive antagonist at beta 1-(pA2 range = 7.4 to 7.8) and beta 2-adrenoceptors (pA2 range 5.2 to 6.2) and displaced [125I]-iodocyanopindolol from guinea-pig left atrial (pKD = 7.25) and uterine (pKD 5.24) membrane preparations. It is concluded that xamoterol displays a selective affinity for beta 1-adrenoceptors. Although its partial agonistic actions are more evident at beta 1-adrenoceptor sites, like prenalterol, xamoterol displays a degree of tissue rather than receptor-dependent selectivity.

The costo-uterine muscle of the rat contains a homogeneous population of beta-adrenoceptors

The effects of two selective beta-adrenoceptor antagonists on the inhibitory responses to some sympathomimetic amines of electrically-stimulated preparations of costo-uterine muscle, taken from virgin rats, have been examined quantitatively. pA2 values for the antagonist, atenolol (beta 1-selective) and ICI 118,551 (beta 2-selective) were obtained using as agonists, fenoterol (beta 2-selective agonist) and noradrenaline (alpha- and beta-adrenoceptor agonist, beta 1-selective); and in addition, with ICI 118,551 only, isoprenaline (beta-agonist, non-selective) and adrenaline (alpha- and beta-adrenoceptor agonist, beta 2-selective). Catecholamine uptake mechanisms and alpha-adrenoceptors were not blocked in any of these experiments. Atenolol competitively antagonized the effects of fenoterol and noradrenaline to a similar extent, the pA2 values being 5.4 and 5.7, respectively. ICI 118,551 competitively antagonized the effects of fenoterol, isoprenaline, adrenaline and noradrenaline to a similar extent; pA2 values ranged from 8.7 with noradrenaline to 9.1 with isoprenaline. These results extend our previous observations which indicated that the adrenoceptors mediating inhibition of electrically-evoked contractions of costo-uterine muscle of the virgin rat are homogeneous and of the beta 2-subtype. The potency of the beta 1-selective agonist RO 363 in producing inhibition of electrically-evoked contractions of this tissue was also examined. RO 363 was 200 times less potent than isoprenaline but was a full agonist. This indicates that there is efficient coupling between beta 2-adrenoceptor activation and tissue response in this non-innervated preparation.

An homogeneous population of beta 1-adrenoceptors subserves inhibitory responses in guinea-pig ileal preparations

In transmurally stimulated guinea-pig ileal preparations, the pA2 values obtained with the selective beta 1- and beta 2-adrenoceptor antagonists atenolol and ICI 118,551 were independent of the beta-adrenoceptor agonist used [-)-isoprenaline or fenoterol) and were similar to values reported in isolated cardiac tissue. On the basis of these results, the relaxant responses to beta-adrenoceptor agonists in guinea-pig ileal preparations appear to be mediated by an homogeneous population of beta 1-adrenoceptors.

The affinity and efficacy of the selective beta 1-adrenoceptor stimulant RO363 at beta 1- and beta 2-adrenoceptor sites

(-)-Isoprenaline and the selective beta 1-adrenoceptor agonist RO363 were tested for their inotropic effects in left atrial (beta 1) and relaxant effects in K+-depolarized uterine (beta 2) preparations from the guinea-pig. The drugs had similar activities as positive inotropic agents but RO363 was approximately 400 times less active than (-)-isoprenaline as a uterine relaxant. RO363 had intrinsic activities of 0.8 and 0.25 ((-)-isoprenaline = 1) in atrial and uterine preparations, respectively. Apparent dissociation constants (KD values) determined from the ability of the agonists to displace (-)-[125I]-iodocyanopindolol ([125I]-CYP) bound to membranes prepared from both tissues were used as a measure of affinity. The [125I]-CYP binding sites possessed the characteristics of homogeneous populations of beta 1-adrenoceptors in atrial and beta 2-adrenoceptors in uterine membrane preparations. The pKD values for (-)-isoprenaline were similar in the two tissues (left atria 6.4, uterus 6.0) whilst for RO363 the atrial value (7.8) was considerably greater than that for the uterus (6.0). The latter value is very similar to the pKB value determined from shifts in (-)-isoprenaline curves produced by RO363 in uterine preparations. Graphical plots of the fraction of receptors occupied vs response were constructed. The relative efficacy of (-)-isoprenaline with respect to RO363 was calculated to be 25 in atrial and 2633 in uterine preparations. The selective beta 1-adrenoceptor stimulant actions of RO363 are a reflection of both its greater affinity and efficacy for beta 1- as opposed to beta 2-adrenoceptor sites. The potent actions of (-)- isoprenaline in both tissues are largely dependent on efficacy.

Correlations between pharmacological responses and structure of human lung parenchyma strips

Correlations were sought between responses of human lung parenchyma strip to 5-hydroxytryptamine (5-HT) and (-)-noradrenaline (NA) and the proportions of the three major, potentially contractile components within the strip, namely smooth muscle in airways proximal to alveolar ducts, vascular smooth muscle and contractile cells in alveolar septa. After the isometric measurement of responses to 5-HT or to NA, lung strips were processed for stereological examination at the light microscopic level. On average, approximately 46% of the total volume of the lung strip was tissue and the remainder was air space. Tissue contained blood vessels (16.8%), airways proximal to alveolar ducts (4.8%) and alveolar parenchyma (78.4%). Human lung parenchyma strips relaxed, contracted or failed to respond to 5-HT or NA. Results indicated that these agonists caused simultaneous contraction of blood vessels and relaxation of airways proximal to alveolar ducts. The size and type of responses to 5-HT or NA was significantly correlated with the ratio of the volume of blood vessels and larger airways. Conversely, the proportion of alveolar tissue in lung strips was not significantly correlated with responses to 5-HT or NA.

Classification of beta-adrenoceptors in isolated bronchus of the pig

1 (+/-)-Isoprenaline (Iso), (-)-adrenaline (Ad), (-)-noradrenaline (NA), the beta 2-selective adrenoceptor agonist (+/-)-fenoterol (Fen) and the beta 1-selective adrenoceptor agonist (+/-)-RO363 caused concentration-dependent relaxation of preparations of pig bronchus pre-contracted with carbachol 40-ng/ml (0.22 microM). Iso, Ad, NA and Fen caused complete relaxation of carbachol-induced tone, but RO363 caused relaxation equivalent to only 59% of the maximal response to Iso. 2 When relaxation responses to these amines were plotted as a % of their maximal effects, comparison of EC50 values showed that the order of potency was RO363 greater than Iso greater than NA greater than Fen greater than Ad (14.4:4.6:1:0.4:0.3). 3 pA2 values determined for the beta-adrenoceptor antagonists propranolol (non-selective) and atenolol (beta 1-selective), or the partial agonist salbutamol (beta 2-selective) using Iso as agonist were 8.3, 7.3 and 4.4 respectively. The pA2 value for atenolol using RO363 as the agonist was 7.6. 4 These results indicate that porcine bronchus contains a homogeneous population of beta 1-adrenoceptors.

Comparison of guinea pig uterine and rat vas deferens preparations for assessment of beta 2-adrenoceptor-mediated activity

In the present study, investigations were performed to determine the subtype(s) of beta-receptor-mediating relaxation in K+-depolarized guinea pig uterine preparations, and inhibition of twitches in field-stimulated rat vas deferens preparations. The relative activities of (-)-isoprenaline, salbutamol, terbutaline, noradrenaline, and RO363, and the affinity constants determined for the beta 1-receptor antagonist atenolol, and the beta 2-receptor antagonist lCl 118551, indicated that all the inhibitory responses in both preparations were mediated solely through beta 2-adrenoceptor stimulation. In the uterine preparations there was a small population of alpha-receptors which were of little consequence when assessing beta 2-receptor-mediated activity. During K+-induced contractures, both neuronal and extraneuronal uptake systems were inactivated. In rat vas deferens preparations, it was necessary to pretreat the tissues with phenoxybenzamine in order to prevent alpha-receptor actions and neuronal and extraneuronal uptake systems interfering with beta-receptor-mediated activity. In both tissues, 6-8 highly reproducible concentration-effect curves to (-)-isoprenaline could be established. However, the uterine preparation responded more robustly to repeated washings, required less sensitive recording apparatus, and showed responses to individual concentrations of drugs that were more readily quantified. However, in general, both preparations appear to be suitable as tissues for assessing the beta 2-adrenoceptor actions of drugs.

Effect of prenalterol in asthmatic patients

The bronchial effects of prenalterol, a selective inotropic beta-adrenoceptor agonist, were examined to see whether beta 1-adrenoceptor stimulation would produce bronchodilatation, or whether beta 2-adrenoceptor blockade could be demonstrated. In a first investigation 8 asthmatic patients were studied under standardized conditions in a double-blind, randomized, crossover comparison between i.v. doses of prenalterol and saline infusions. After the fourth dose of prenalterol/saline, 4 intravenous doses of terbutaline were given. All medication was given at 30 min intervals. In a second study in 7 of the previously studied patients, 5 increasing doses of prenalterol or saline were inhaled from a bird ventilator, and after the last dose 4 i.v. doses of terbutaline were given as before. Prenalterol and saline caused no change in FEV1 during the intravenous study. Terbutaline infusion produced a dose dependent increase in FEV1 which was not significantly altered after prenalterol treatment. In the inhalation study, in which a much larger dose was applied to the bronchial tree, there was no significant difference in FEV1, between the saline and prenalterol inhalations. However, prenalterol did show beta 2-blocking property, as the dose response curve of terbutaline was shifted to the right. The heart rate increased significantly and to the same degree during both the i.v. and inhaled prenalterol treatments. Thus, this study revealed no significant beta 2-blocking effect of therapeutic doses of i.v. prenalterol. However, when the drug was inhaled in a much higher dose, a beta 2-blocking effect on FEV1 was recorded. No significant beta 1-adrenoceptor-mediated bronchodilator effect of prenalterol could be demonstrated.

The use of the rat right ventricle strip in studies on noradrenergic transmission

1. The possibility of using contractility studies with the rat right ventricle strip to assess the effects of drugs on all aspects of noradrenergic transmission has been examined. 2. The force of the contractile responses to field stimulation at 0.5 and 2, but not 5 Hz, markedly decreased with time. About 55% of the tissues responded directly to (-)-isoprenaline, 1 X 10(-6)M (123 tissues from 220 tested) in the absence of other stimuli. In these tissues the force of the contractile responses remained constant and the rate increased with time. Thus it is essential to provide time controls in studies with rat right ventricle. 3. Under conditions in which the responses to (-)-isoprenaline, 1 X 10(-6)M, alone were unaffected the responses to field stimulation at 5 Hz were inhibited by 66 and 86% by 9 X 10(-6)M tetrodotoxin and 1 X 10(-5)M guanethidine, respectively. (+/-)-Propranolol and timolol (both at 1 X 10(-6)M), but not phentolamine, 1 X 10(-6)M, inhibited responses to (-)-isoprenaline, 1 X 10(-6)M alone and responses to field stimulation at 5 Hz. This demonstrates that the responses to field stimulation are largely due to activation of noradrenergic nerves, the released noradrenaline acting at postsynaptic beta-adrenoreceptors. 4. Although nortriptyline is a potent inhibitor of the neuronal uptake of noradrenaline, at 1 X 10(-6)M it had no effect on the contractile responses to field stimulation at 5 Hz and inhibited responses to (-)-isoprenaline, 1 X 10(-6)M, alone and at a higher concentration (1 X 10(-5)M) nortriptyline abolished both responses. It is suggested that the rat ventricle preparation may be useful in examining the effects of drugs on noradrenergic transmission in the heart.

beta 1-Adrenoceptors mediating relaxation of the guinea-pig trachea: experiments with prenalterol, a beta 1-selective adrenoceptor agonist

In the presence of 17 beta-oestradiol, prenalterol, a beta-selective adrenoceptor agonist, caused a dose-dependent relaxation of the isolated, pilocarpine-contracted guinea-pig trachea. This effect was blocked by the antagonists propranolol (non-selective) and practolol (beta 1-selective) but not by IPS 339 [(t-butylamino-3-ol-2-propyl)oximino-9-fluorene HCl] (beta 2-selective). The relaxing effect of terbutaline, a beta 2-selective adrenoceptor agonist, was more efficiently blocked by IPS 339 than by practolol. These data support the hypothesis that the guinea-pig trachea contains both beta 1- and beta 2-adrenoceptors mediating relaxation and that the beta 1-adrenoceptors are selectively stimulated by prenalterol. The efficacy of prenalterol was less than that of terbutaline, thus confirming its partial agonistic activity. In the absence of 17 beta-oestradiol, the ability of prenalterol to relax the pilocarpine-contracted trachea was lost. It is suggested that 17 beta-oestradiol may act as a functional antagonist to pilocarpine as it caused a partial relaxation itself.