Catechol-substituted phenoxypropanolamines: adrenoceptor activity in the anaesthetized cat

Comparison of (β-adrenoceptor populations in cat and guinea-pig left atria

The subtypes of β-adrenoceptor present in left atrial preparations from the guinea-pig and cat have been assessed using both responses obtained in organ bath experiments and radioligand binding studies. From the positive inotropic responses to procaterol, the pKB values for practolol using a variety of agonists, and from displacement of [125 I]cyanopindolol from left atrial membrane homogenates by the selective β1 and β2-adrenoceptor antagonists L643, 717-01J10 and ICI 118, 551, it was concluded that guinea-pig left atria possess only β1-adrenoceptors, whilst cat left atria possess both β1- and β2-adrenoceptor subtypes.

Organ-bath studies using the irreversible beta-adrenoreceptor antagonist bromoacetylalprenololmenthane (BAAM)

1. The effects of the irreversible beta-adrenoreceptor antagonist bromoacetylalprenololmenthane (BAAM) were studied in isolated cardiac and uterine preparations from guinea-pigs and rats and in guinea-pig ileal preparations. 2. In the presence of BAAM (0.1-10 microM) concentration-effect curves to (-)-isoprenaline were shifted to the right in a concentration-dependent manner in all cardiac and uterine tissues. Maximum responses to (-)-isoprenaline were unaffected by BAAM except in guinea-pig left atrial and in some guinea-pig uterine preparations; however, the reductions in the maximum responses were not concentration-dependent. The mean pKB values for BAAM in guinea-pig left atria, right atria, rat whole atria and rat uterus were 7.26, 7.24, 6.84 and 7.90 respectively. 3. In guinea-pig ileal preparations, BAAM (0.1-30 microM) relaxed contractions induced by K+, histamine and acetylcholine in a non-beta-adrenoreceptor-related manner since relaxant responses were unaffected by propranolol (0.5 microM). In other tissues higher concentrations of BAAM (30-100 microM) elicited atrial standstill and depressed K+-induced contractions in uterine smooth muscle. 4. Treatment of tissues with BAAM (10-100 microM) followed by extensive wash-out increased the EC50 values for (-)-isoprenaline 21- to 83-fold. The maximum response to the catecholamine was unaffected by BAAM except in guinea-pig left atrial preparations following treatment with 100 microM BAAM. At these concentrations BAAM markedly increased the effective refractory period. 5. Concentration-effect curves for the partial agonist, oxymethylene-isoprenaline (OM-ISO) were shifted to the right 12- to 355-fold after pretreatment of tissues with BAAM (10-30 microM) followed by wash-out. The maximum response to OM-ISO was unaltered in guinea-pig and rat uteri and was reduced to a similar degree as observed with (-)-isoprenaline in guinea-pig left atria. 6. In general, the non-selective beta-adrenoreceptor antagonist BAAM depressed maximum responses to beta-adrenoreceptor agonists only in cardiac preparations and at concentrations which elicited depressant activity. On the basis of the present study, BAAM does not appear to be a suitable irreversible beta-adrenoreceptor antagonist for use in organ bath experiments.

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.

Effect of cortisol-induced inhibition of extraneuronal uptake on responses to isoprenaline in the kitten

1. In anaesthetized kittens, cortisol (16 mg/kg) potentiated peak chronotropic responses to isoprenaline but did not affect peak bronchodilator responses to this amine. 2. In contrast, the duration of both chronotropic and bronchodilator effects of isoprenaline were clearly potentiated by cortisol. 3. These results have been linked with cortisol-induced inhibition of 3H-isoprenaline uptake in the perfused kitten heart and isolated tracheal smooth muscle.

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

1 The beta-adrenoceptor stimulant effects of RO363 and (--)-isoprenaline have been compared in a variety of isolated tissue preparations. 2 RO363 is approximately half as potent as (--)-isoprenaline in tissues where actions are due to beta1-receptor activation (guinea-pig atrial and ileal preparations and ventricular strips from the rabbit, rat and guinea-pig. 3 In uterine and lung strip preparations from the guinea-pig, where responses are due to beta2-receptor stimulation. RO363 is 100 to 350 times less active than (--)-isoprenaline and has a low intrinsic activity. 4 In spontaneously contracted tracheal preparations from the guinea-pig, RO363 is a full agonist and is approximately half as potent as (--)-isoprenaline. These effects of RO363 are due to the activation of a population of beta1-receptors in the tissue since RO363 and (--)-isoprenaline have the same relative potencies in trachea, cardiac and ileal preparations. In addition the Kb values for practolol are similar in all these preparations when RO363 is used as the agonist. 5 The results show that RO363 is a potent and highly selective beta1-receptor agonist.

Agonistic and antagonistic actions of 3,4-dihydroxy-substituted phenoxypropanolamines in guinea-pig atria and trachea

1. Racemic mixtures of noradrenaline, adrenaline, isoprenaline, N-t-butylnor-adrenaline and their corresponding derivatives containing an oxymethylene (OM) link between the phenyl ring and ethanolamine side-chain have been tested for their effects on beta-adrenoceptors in isolated guinea-pig atrial and tracheal preparations. 2. In atrial and in spontaneously contracted tracheal preparations both the parent catecholamines and their corresponding OM-derivatives had a similar order of potency as beta-receptor agonists. 3. In carbachol-stimulated tracheal preparations the OM-derivatives were shown to have partial agonistic actions. 4. As in other phenylethanolamines and phenoxypropanolamines, both the agonistic and antagonistic potency of the OM-derivatives increased with increasing amine substitution.