Isolated lung strips of guinea pigs: responses to beta-adrenergic agonists and antagonists

Ritodrine: a beta-adrenergic receptor antagonist in human amnion

OBJECTIVE

Human amnion is important in the initiation of labor. Ritodrine, when administered as a tocolytic, is found unchanged in amniotic fluid. We characterized effects of ritodrine binding to beta-adrenergic receptors in amnion and amniocytes.

STUDY DESIGN

Iodine 125-iodopindolol, beta-adrenergic receptor agonists, and beta-adrenergic receptor antagonists were used to describe binding characteristics. Experiments were designed with and without isoproterenol and ritodrine to study intracellular cyclic adenosine 3'5'-monophosphate and prostaglandin E2 release.

RESULTS

Scatchard analysis revealed a single class of saturable binding sites, with maximum binding capacity of 70.0 +/- 17.2 fmol/mg protein (n = 12) and with high-affinity dissociation constant of 458.9 +/- 72.1 pmol/L. Agonists and antagonists competed for the 125I-iodopindolol binding site consistent with a beta 2-adrenergic receptor. Hill coefficients were 0.6 to 0.8 for agonist competition and 1.0 for antagonist competition and ritodrine. Stimulation with isoproterenol resulted in dose-dependent increases in cyclic adenosine 3'5'-monophosphate and prostaglandin E2. Ritodrine failed to stimulate cyclic adenosine 3'5'-monophosphate and inhibited isoproterenol-stimulated cyclic adenosine 3'5'-monophosphate and prostaglandin E2 production.

CONCLUSION

In human amnion binding of ritodrine to beta 2-adrenergic receptors and lack of ritodrine-mediated postreceptor effects are characteristic of a beta 2-adrenergic antagonist.

Alpha-adrenoceptors do not contribute to the chronotropic or inotropic responses of the avian heart to noradrenaline

1. The chronotropic and inotropic responses of the young chick heart to noradrenaline have been investigated in isolated right atria, left atria and ventricular strips from 14-day-old chicks. 2. In the presence of desipramine and metanephrine to inhibit amine uptake, concentration-response curves to noradrenaline in all three tissues were shifted to the right by propranolol (1 microM) but were not altered by the presence of either phentolamine (5 microM) or prazosin (10 microM). 3. Similar results were obtained in the presence of cocaine (10 microM) and corticosterone (10 microM) to inhibit amine uptake. Propranolol (0.3-3.0 microM) produced rightward shifts of noradrenaline concentration-response curves which gave pA2 values of 8.1-8.4. Phentolamine (5 microM), in contrast, did not affect responses to noradrenaline in any tissue, either in the absence or presence of propranolol (1 microM). 4. Isoprenaline produced positive chronotropic responses in right atria and positive inotropic responses in left atria and ventricular strips. Methoxamine elicited positive inotropic responses in left atria but only negative chronotropic responses in right atria and negative inotropic responses in ventricular strips. 5. These results demonstrate that cardiac responses to noradrenaline in the chick heart are mediated via beta-adrenoceptors only, and that cardiac alpha-adrenoceptors are not involved in these responses.

A study of beta-adrenoceptors in rat lung parenchymal strip

The aim of the present study was to characterize the beta-adrenoceptor population in rat lung strip. For this purpose, Schild plots were obtained for the beta-adrenoceptor antagonists atenolol (beta 1-selective), butoxamine (beta 2-selective) and propranolol (non-selective), using three different agonists:isoprenaline (non-selective), salbutamol (beta 2-selective) and noradrenaline (beta 1-selective). The slopes of these Schild plots were close to the theoretical value of unity, and pA2 values determined with isoprenaline, salbutamol and noradrenaline as agonists were: for propranolol, 7.86 +/- 0.22, 7.72 +/- 0.15 and 7.89 +/- 0.23; for atenolol, 5.19 +/- 0.05, 5.33 +/- 0.07 and 5.47 +/- 0.22 and for butoxamine, 6.31 +/- 0.11, 6.34 +/- 0.03 and 5.99 +/- 0.23, respectively. These data suggest that pharmacological responses of rat isolated lung strip to beta-adrenoceptor agents are mediated by a homogeneous population of beta 2-adrenoceptors, although the presence of a minor population of beta 1-adrenoceptors undetected by the agonists used cannot be excluded.

Relaxant actions of viprostol and prostaglandin E2 on tracheal muscle and their effects on calcium influx

The relaxant effect of viprostol was studied in monkey and guinea-pig tracheal muscle rings in vitro and compared to that of prostaglandin E2 (PGE2), isoproterenol (ISO) and verapamil (guinea-pig trachea only). Viprostol, PGE2, ISO and verapamil produced a concentration-dependent relaxation of carbachol-contracted tracheal preparations. The rank order of potency in monkey trachea was viprostol = ISO greater than PGE2, while in guinea-pig treachea it was ISO greater than viprostol greater than PGE2 greater than verapamil. The relaxant effect of viprostol or PGE2 was not antagonized by propranolol, suggesting that beta-adrenoceptors are not involved. Epithelium removal did not affect the bronchorelaxant effects of viprostol, PGE2 or ISO. In K+-rich, Ca++-free Krebs solution, preincubation with an IC30 of verapamil antagonized CaCl2-induced contractions while an IC30 of viprostol, PGE2 or ISO did not. Preincubation with an IC90 of viprostol, PGE2 or ISO produced 0.5, 0.5 and 1.0 log unit shifts to the right of the CaCl2 concentration response curves, respectively. At this concentration, viprostol did not reduce the maximum effect of CaCl2, but PGE2 and ISO reduced it approximately 20%. However, preincubation with an IC90 of verapamil completely abolished the CaCl2 contraction. In conclusion, viprostol is a potent bronchodilator whose effect does not depend on the epithelium, beta-adrenoceptors or antagonism of Ca++ influx. Whether the bronchodilator effect of viprostol is via intracellular sequestration of calcium as that of PGE2 remains to be studied.

Partial agonists at guinea-pig atrial beta-adrenoceptors display relaxation responses in the guinea-pig ileum independent of beta-adrenoceptor stimulation

The beta-adrenoceptor mediated responses of oxyfedrine, ritodrine, tazolol, prenalterol, salbutamol and carteolol were examined on guinea-pig left and right atrial and ileal preparations. All agonists tested in left and right atrial preparations were partial agonists relative to isoprenaline. All agonists with the exception of salbutamol, which appeared a full agonist, produced relaxation responses significantly greater than isoprenaline in ileal preparations. The response to ritodrine in the ileum was not influenced by practolol, in a concentration which antagonized the responses of ritodrine in the right atria. The response of the ileum to beta-adrenoceptor antagonists of varying lipophyllicity was examined. Propranolol and pindolol both produced relaxation responses relative to their lipophyllicities. No relaxation was observed to atenolol, which exhibits very low lipophyllicity. It is concluded that beta-adrenoceptor agonists exhibit a substantial relaxation of guinea-pig ileum that is independent of beta-adrenoceptor stimulation.

Pharmacological characterization of the effects of verapamil and nifedipine on isolated guinea-pig lung parenchymal strip

Verapamil and nifedipine elicited dose-related relaxations and produced a right downward shift of the concentration-response curves to CaCl2, KCl, acetylcholine and histamine in the isolated guinea-pig lung parenchymal strip. These Ca-entry blockers had greater inhibitory effects on contractions evoked by CaCl2 and KCl than against those elicited by acetylcholine and histamine.

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.

Temperature dependence of beta 1-adrenoceptor-mediated responses examined by use of partial agonists

The inotropic and chronotropic responses of guinea pig atria, and the relaxation responses of guinea pig intestine, trachea, lung, uterus and vas deferens to catecholamines have been examined at bath temperatures of 38 degrees C and 30 degrees C. Hypothermia resulted in a supersensitivity of cardiac tissues with a decrease in isoprenaline EC50 and an increase in the maximum response to the partial agonist, salbutamol. Ileum responses to isoprenaline were potentiated at 30 degrees C but no partial agonist could be found on this tissue. Responses of the lung and vas deferens to partial agonists were not affected by temperature, while uterine responses were inhibited by hypothermia. The trachea was supersensitive to isoprenaline at 30 degrees C, however this was not due to a change in beta-adrenoceptor sensitivity but an inhibition of COMT. Partial agonist responses of trachea were similar at both temperatures. beta-Adrenoceptor supersensitivity was therefore observed only where responses are mediated primarily by beta 1-adrenoceptors and supports the concept that beta 1- but not beta 2-adrenoceptors exhibit hypothermia-induced supersensitivity.

Reserpine-induced supersensitivity occurs for beta-adrenoceptor-mediated responses of heart and trachea but not of the uterus and lung

This study was undertaken to determine whether reserpine-induced supersensitivity occurs in tissues containing beta1-adrenoceptors and in those with beta 2-adrenoceptors. Guinea-pigs and rats were pretreated with reserpine for either 3 days (5 mg kg-1 i.p. at 72 h, 3 mg kg-1 at 48 h and 3 mg kg-1 at 24 h before use) or 7 days (1 mg kg-1 daily). The sensitivities of left and right atria, papillary muscles, tracheal spirals, lung strips and uteri to isoprenaline were compared with those from untreated animals. The positive inotropic responses of left atria and papillary muscles and chronotropic responses of right atria from reserpine-pretreated animals were supersensitive to isoprenaline, the concentration-response curves being to the left. The relaxation response of the carbachol-contracted trachea also exhibited supersensitivity, but to a lesser extent. However, no supersensitivity occurred for the relaxation of carbachol-contracted lungs, K+-depolarized guinea-pig uteri or electrically stimulated rat uteri. As a pharmacological index of the presence of releasable noradrenergic stores, tyramine was added cumulatively to each tissue. Only cardiac and tracheal preparations yielded substantial responses, indicating the presence of sympathetic innervation. A relaxation of the rat uterus by tyramine was not attributable to releasable noradrenaline stores. The supersensitivity of the heart and trachea could therefore be associated with their sympathetic innervation and with the fact that their responses are mediated via beta 1-adrenoceptors; the trachea containing a small proportion of beta 1-adrenoceptors. The responses of the lung and uterus, however, are beta 2-adrenoceptor-mediated and failed to exhibit supersensitivity. Since the supersensitivity is a consequence of the neuronal depleting action of reserpine, these results are compatible with the concept that beta 1-adrenoceptors are associated with sympathetic innervation whereas beta 2-adrenoceptors are not.

Pharmacological analysis of the responsiveness of guinea-pig lung parenchymal strip to dopamine

Responses to dopamine were examined in the guinea-pig isolated lung parenchymal strip. Complete cumulative concentration-response curves to dopamine exhibited a biphasic pattern with a small initial contraction at concentrations below 10(-5) M followed by a dose-dependent relaxation at higher concentrations. Phentolamine (10(-5) M) completely abolished the contractile component and enhanced sensitivity and maximal relaxation to dopamine. In the presence of phentolamine, propranolol antagonized the dopamine-induced relaxation (pA2 = 8.54 +/- 0.07). In the presence of propranolol (10(-6) M), dopamine produced a dose-related contraction displaced to the right by phentolamine. Incubation with haloperidol (10(-5) M) did not modify the characteristics of the concentration-response curve to dopamine. Pretreatment with reserpine abolished the contraction to dopamine without affecting its relaxant response. Cocaine significantly increased the pD2 value of dopamine in the presence of propranolol. It is concluded that dopamine produced both relaxation of lung parenchymal strip due to direct activation of beta-adrenoceptors and contraction mediated through direct and indirect (catecholamine release) actions at alpha-adrenoceptors. There is no evidence in favour of the existence of specific dopamine-receptors in this preparation.

Responsiveness to tyramine in isolated lung parenchymal strip of guinea-pig and rat

The effects of alpha/beta-adrenoceptor agonists in isolated lung parenchymal strip are well characterized but information related to indirectly acting sympathomimetic agents is scarce. In the present study the response to tyramine was analyzed in lung strips from guinea-pig and rat. Tyramine elicited a dose-related contraction of the lung strip with an EC50 of 1.12 X 10(-4) M in the guinea-pig and 4.28 X 10(-4) M in the rat. Incubation with propranolol did not modify the tyramine-induced contraction but blocked the relaxation which occasionally appeared at low concentrations of tyramine. No tachyphylaxis to tyramine was detected. Cocaine (3 X 10(-5) M) and reserpine significantly increased the EC50 values of tyramine. In reserpine-treated animals, phentolamine (both animal species), clemizole (guinea-pig) and methysergide (rat) displaced to the right the concentration-response curve to tyramine but the dose ratios were significantly lower than those obtained for their specific agonists. These results suggest that in addition to the adrenergic component (catecholamine release and direct alpha-adrenoceptor activation) there is an important contribution of other receptor systems such as histamine (guinea-pig) and 5-hydroxytryptamine (rat) to the contractile response to tyramine in lung tissue.

Effect of hypothermia on beta 1-adrenoceptor-mediated relaxation of pig bronchus

The relaxant potencies of (+/-)-isoprenaline and of (-)-noradrenaline (NA) in the pig isolated bronchus were increased 5.4 and 3.1 fold respectively by lowering the organ bath temperature from 37 degrees C to 27 degrees C, whereas the potencies of the non-catecholamine beta-adrenoceptor agonists fenoterol and orciprenaline were not significantly changed. At 37 degrees C, the catechol-O-methyl transferase (COMT) inhibitor U-0521 (30 microM), caused a 7.2 fold increase in the potency of isoprenaline but had no effect on the potency of fenoterol. At 27 degrees C the potency of isoprenaline was similar in the absence or presence of U-0521 (30 microM). Furthermore, in bronchi where extraneuronal uptake was inhibited by phenoxybenzamine, the potency of NA was not significantly altered by reducing bathing temperature from 37 degrees C to 27 degrees C. These results suggest that the hypothermic potentiation of isoprenaline in pig bronchus resulted from inhibition of COMT or of access to COMT, rather than from sensitization of beta 1-adrenoceptors.

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.

Responses mediated via beta 1, but not beta 2-adrenoceptors, exhibit hypothermia-induced supersensitivity

The beta-adrenoceptor-mediated responses of various isolated tissues from the guinea-pig were examined at bath temperatures of 38 and 30 degrees C. The positive inotropic responses to orciprenaline of paced left atria and papillary muscles were potentiated at the lower bath temperature, as indicated by a significant shift of the dose-response curve to the left. A similar hypothermia-induced supersensitivity was observed for the positive chronotropic response of right atria. The beta-adrenoceptor-mediated inhibition of coaxially stimulated ileum in response to orciprenaline also exhibited supersensitivity at 30 degrees C. In contrast, the relaxation of lung strips was not altered at the lower bath temperature. The relaxant responses of potassium-contracted uterine strips from untreated guinea-pigs or progesterone-dominated guinea-pigs and rats also failed to reveal any hypothermia-induced supersensitivity. The responses of lung and uterine strips are mediated via beta-adrenoceptors of the beta 2-type, whereas they are of the beta 1-type in the cardiac preparations and ileum. Therefore, these results suggest that hypothermia-induced supersensitivity occurs only at the beta 1-adrenoceptors, indicating a fundamental temperature-dependent difference between the two receptor types.

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.

Autacoid and autonomic reactivity of sheep lung parenchymal strip and its modification by antigenic sensitization

Lung parenchymal strips (LPS) of horse plasma-sensitized and control sheep were studied isometrically in isolated organ baths and their responses to autacoid and autonomic agents were compared. Control LPS responded with contractions to histamine greater than carbachol greater than 5HT and relaxed to isoproterenol greater than adrenaline greater than phenylephrine. In sensitized LPS, adrenaline and phenylephrine-induced relaxations were converted to contractions and a new order of potency and efficacy for spasmogens was observed i.e. histamine greater than carbachol greater than adrenaline greater than phenylephrine greater than 5HT. Isoproterenol was also significantly (P less than 0.05) less potent and less effective in relaxing sensitized LPS compared to controls. Mepyramine, atropine and propranolol competitively antagonized their respective agonists (i.e. histamine, carbachol and isoproterenol) confirming the presence of H1-histaminergic, muscarinic-cholinergic and beta-adrenergic receptors respectively. The conversion of the relaxant effect of adrenaline and phenylephrine to a contraction supplemented by a significant reduction in isoproterenol activity suggests an impairment in the effective ratio of beta: alpha adrenoceptors in ovine LPS subsequent to antigenic sensitization.

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.