Insights from preclinical ultra high-density electroanatomical sinus node mapping.

AIMS: Although sinus node modification by catheter ablation is an established therapy for the treatment of inappropriate sinus tachycardia, there is incomplete understanding of sinus node anatomy and function but also limited electroanatomical mapping data. Recently, an automatic, ultra high-resolution mapping system has been presented to accurately and quickly identify right atrial (RA) geometry and activation patterns but detailed assessment of sinus node activation has not been performed which was one aim of this study. Preclinical experiences are compared with previous sinus node mapping studies in animals and humans, and potential clinical implications for catheter ablation are discussed. METHODS AND RESULTS: In anaesthetized and ventilated 14 pigs, 30 endocardial and 2 eipcardial RA maps were generated using the Rhythmia™ mapping system without complications and earliest activation sites (EAS) and sinus break-out (SBO) were determined. At baseline, EAS and SBO were located anterior to the middle (n = 6) or lower third (n = 8) of the crista terminalis exhibiting a unicentric activation pattern in all cases. Conduction pathways were directed anterior, posterior, superior, or inferior with substantial inter-individual variation in direction, pathway distance, and conduction time. Orciprenaline, propranolol, or amiodarone shifted endocardial activation with considerable variation between animals with inconsistent patterns. Multicentric activation was found in one case after orciprenaline infusion. Sequential endocardial and epicardial high-density mapping of the RA was performed in two animals and showed a high congruence of the sinus node activation in the endo- and the epicardial map. CONCLUSION: Ultra high-density mapping allows fast, simple, and very detailed assessment of sinus node activation. Future studies are clearly needed to evaluate novel mapping and ablation strategies for an improved understanding of sinus node disease and better outcomes.

Activation of extracellular-signal regulated kinase by platelet-derived growth factor is potentiated by phenylephrine in primary cultures of adult rat hepatocytes.

We investigated the effects of the α(1)-adrenergic agonist phenylephrine on platelet-derived growth factor (PDGF)-stimulated extracellular signal-regulated kinase (ERK) in primary cultures of adult rat hepatocytes. Hepatocytes were isolated and cultured with PDGF (10 ng/ml) and/or α-adrenergic agonist. Phosphorylated ERK isoforms (ERK1 and ERK2) were detected by Western blotting analysis using anti-phospho mitogen-activated protein kinase (MAPK) antibody. PDGF stimulated phosphorylation of ERK2 (42 kDa MAPK) by 2.0-fold within 3-5 min. The PDGF-induced ERK activation was abolished by AG1296 (10(-7) M) or LY294002 (10(-7) M) treatment. MAPK kinase inhibitor, PD98059 (10(-6) M), completely inhibited the PDGF-induced increase in ERK activity. In addition, PDGF-induced mammalian target of rapamycin activity was completely inhibited by AG1296, LY294002, PD98059, or rapamycin treatment. Phenylephrine alone showed no effects on ERKs, but significantly increased phosphorylation of ERK2 induced by PDGF. Moreover, a synthetic analog of diacylglycerol (DG), phorbol 12-myristate 13 acetate (TPA; 10(-7) M), potentiated PDGF-induced ERK2 phosphorylation, while ionomycin had no effect (10(-6) M). The effects of phenylephrine and TPA were antagonized by the phospholipase C (PLC) inhibitor U73122 (10(-7) M), and the protein kinase C (PKC) inhibitor GF109203X (10(-7) M), respectively. Accordingly, PDGF-induced DNA synthesis and proliferation in the presence or absence of phenylephrine or TPA were completely inhibited by AG1296, LY294002, PD98059, or rapamycin treatment. These results suggest that activation of PLC/PKC by phenylephrine represent an indirect positive regulatory mechanism for stimulating ERK induced by 10 ng/ml PDGF.

Activation of extracellular-signal regulated kinase by epidermal growth factor is potentiated by cAMP-elevating agents in primary cultures of adult rat hepatocytes.

We investigated the effects of α- and ß-adrenergic agonists on epidermal growth factor (EGF)-stimulated extracellular-signal regulated kinase (ERK) isoforms in primary cultures of adult rat hepatocytes. Hepatocytes were isolated and cultured with EGF (20 ng/ml) and/or α(1)-, α(2)- and ß(2)-adrenergic agonists. Phosphorylated ERK isoforms (ERK1; p44 mitogen-activated protein kinase (MAPK) and ERK2; p42 MAPK) were detected by Western blotting analysis using anti-phospho-ERK1/2 antibody. The results show that EGF induced a 2.5-fold increase in ERK2-, but not ERK1-, phosphorylation within 3 min. This EGF-induced ERK2 activation was abolished by treatment with the EGF-receptor kinase inhibitor AG1478 (10(-7) M) or the MEK (MAPK kinase) inhibitor PD98059 (10(-6) M). The α(2)-adrenergic and ß(2)-adrenergic agonists, UK14304 (10(-6) M) and metaproterenol (10(-6) M), respectively, had no effect in the absence of EGF, but metaproterenol significantly potentiated EGF-induced ERK2 phosphorylation. Moreover, the cell-permeable cAMP analog 8-bromo cAMP (10(-7) M), also potentiated EGF-induced ERK2 phosphorylation. The effects of these analogs were antagonized by the protein kinase A (PKA) inhibitor H-89 (10(-7) M). These results suggest that direct or indirect activation of PKA represents a positive regulatory mechanism for EGF stimulation of ERK2 induction.

The ß2-adrenoceptor agonist bronchodilators terbutaline and orciprenaline are also weak α1-adrenoceptor antagonists.

Recently, the ß2-adrenoceptor agonist terbutaline was shown to have α1-adrenolytic activity in mouse isolated pulmonary arteries in vitro and to lower pulmonary artery pressure in anaesthetised mice. The aim of our study was to determine the α1-adrenoceptor antagonist activity of terbutaline and its structurally close resorcinol, orciprenaline, in rat isolated small mesenteric arteries set up for myography. Their α1-adrenoceptor antagonist potency was then compared with their potency as ß2-adrenoceptor agonists. Concentration-response curves to methoxamine were competitively antagonised by terbutaline (30-300 µM) or orciprenaline (30-300 µM) with a pKB of 4.70 ± 0.09 or 4.79 ± 0.17, respectively. Both terbutaline and orciprenaline fulfilled the criteria for simple, silent competitive antagonism. Terbutaline (30-300 µM) had no effect on endothelin-1 concentration-contraction curves. Our findings suggest that after oral dosing of terbutaline, the maximum plasma levels would NOT reach levels to show α1-adrenoceptor antagonist activity. In conclusion, our work has provided additional quantitative evidence that terbutaline and orciprenaline are weak competitive α1-adrenoceptor antagonists, but this additional property is probably not therapeutically important in the clinical treatment of asthma or pulmonary artery hypertension with these more potent ß2-adrenoceptor agonists.

Prophylactic IL-12 treatment reduces postoperative metastasis: mediation by increased numbers but not cytotoxicity of NK cells.

Despite a promising potential, interleukin-12 immunotherapy has yielded limited clinical success while causing perilous toxicities. Here we study a context in which IL-12 may prove clinically beneficial--the removal of the primary tumor, when cell-mediated immunity (CMI) may eradicate minimal residual disease (MRD), but is inhibited by postoperative immunosuppression, potentially leading to enhanced malignant progression. F344 rats were preoperatively treated with IL-12 and inoculated postoperatively with syngeneic MADB106 tumor cells. An optimal regimen of eight-day sustained exposure to IL-12 was developed (1 microg/rat/day), which caused mild side effects, increased baseline resistance to experimental MADB106 metastasis, and abolished the promotion of metastasis by laparotomy and other immunosuppressive paradigms. Depletion of NK cells indicated their major role in controlling MADB106 metastasis in naïve and IL-12 treated rats. Studying NK cytotoxicity, we found that IL-12 did not potentiate activity per NK cell, nor protected it from suppression by surgery. However, IL-12 increased the numbers of NK cells in the circulation and marginating pulmonary pool of naïve and operated rats, and correspondingly increased total NK activity in these compartments. Therefore, this study indicates anti-tumor effects of IL-12 based on increased numbers of strategically located NK cells, and advocates a prophylactic approach against the potential metastasis-promoting effects of surgery.

Male--female differences in the impact of beta-adrenoceptor stimulation on resistance to experimental metastasis: exploring the effects of age and gonadal hormone involvement.

We studied the development of sexual dimorphism in resistance to NK-sensitive experimental metastasis under baseline conditions and following adrenoceptor stimulation. With increasing age, baseline resistance to MADB106 lung tumor retention (LTR) increased in both sexes, but also the susceptibility to the tumor-enhancing effects of a beta-adrenergic agonist, metaproterenol. Beginning at 13 weeks, males exhibited a 2- to 3-fold greater increase in LTR than females following adrenoceptor stimulation. This adult dimorphism was robust to ovariectomy, and questionably related to androgens. The findings are consistent with reduced female responsiveness to sympathetic activation, and substantiate the importance of including both sexes when studying neuroimmunomodulation.

Egr-1 negatively regulates expression of the sodium-calcium exchanger-1 in cardiomyocytes in vitro and in vivo.

OBJECTIVE: Increased expression of the transcription factor early growth response gene-1 (Egr-1) accompanies catecholamine infusion. Catecholamine-treated, Egr-1-deficient (-/-) mice show exacerbated cardiac damage when compared to similarly treated wild-type (+/+) mice, suggesting that Egr-1 reduces heart damage. We sought to identify Egr-1-mediated cardiac sparing genes. METHODS: Microarray analyses identified increased sodium calcium exchanger-1 (NCX1) expression in catecholamine-treated -/- mice. Immunoblots assessed NCX1 expression in +/+, -/-, and transgenic mice overexpressing Egr-1 in heart and cardiac differentiated H9c2 cells harboring wild-type Egr-1 (wtEgr-1) or NAB-binding ablating mutations. Chromatin immunoprecipitation (ChIP) used anti-Egr-1 antibody coupled to amplification of purified Egr-1/associated DNA. RESULTS: Immunoblots revealed a two- to threefold increase in NCX1 in catecholamine-stimulated and naive -/- versus +/+ mice. In contrast, transgenic mice overexpressing Egr-1 in heart had 30% of normal NCX1 protein. Thus, the in vivo data indicate that Egr-1 negatively controls NCX1 expression. In vitro cardiac differentiated H9c2 cells overexpressing wtEgr-1 also showed 30% NCX1 expression. However, cells overexpressing NAB-ablating Egr-1 mutations showed four- to fivefold increased NCX1 expression. NCX1 promoter DNA was specifically amplified from Egr-1/associated DNA. Thus, the in vitro results indicate that Egr-1/NAB interactions are critical for NCX1 repression at the NCX1 promoter. CONCLUSIONS: NCX1 is responsible for calcium exit from cardiomyocytes, and continued overexpression is thought to be detrimental. We propose that one way Egr-1 action is cardiac sparing is by promoting a reduction in NCX1 expression.

Serial pulmonary function in patients with acute heart failure.

This study delineates the effects of congestive heart failure on routine pulmonary function tests and assesses the changes in pulmonary function as congestive heart failure was treated. Twenty-eight patients had spirometry, lung volumes, and diffusing capacity measurements initially and at frequent intervals after their initial hospitalization for congestive heart failure. Initially the patients had both obstructive (mean forced expiratory volume in 1 s [FEV1], 48.2% +/- 13% predicted) and restrictive (mean forced vital capacity [FVC], 5.6% +/- 15.7% predicted) ventilatory dysfunction, but a normal carbon monoxide diffusing capacity. With treatment, pulmonary function rapidly improved initially and there was no further significant improvements in the mean pulmonary function after two weeks of treatment. However, there was marked interindividual variability and several patients took months to reach their best level of pulmonary function. Even with treatment, the patients retained evidence of obstructive ventilatory dysfunction and at the time of their best spirometry 53% (8/15) of nonsmokers still had an abnormally low FEV1/FVC ratio.

Concentration-response curves of positive inotropic agents before and after ouabain pretreatment.

Current therapy for congestive heart failure (diuretics, digitalis, vasodilators) may be insufficient. Addition of a second positive inotropic substance to digitalised patients has been previously shown to increase cardiac index and decrease vascular resistance. To test the hypothesis that the positive inotropy of ouabain can be increased by other inotropic agents, the following studies were performed. Firstly, concentration-response curves of positive inotropic agents (ouabain, dobutamine, dopamine, orciprenaline, phenylephrine, theophylline, amrinone, sulmazole and histamine) were measured in contracting left atria and papillary muscles from cat and guinea pig hearts. The maximal increase in force of contraction was similar for all compounds except histamine and phenylephrine which gave decreased effects in guinea pig heart muscle. Secondly, these positive inotropic agents were added to the contracting heart muscles after maximal inotropy without toxicity of a ouabain concentration which gave more than 90% of the maximal increase in force of contraction. In guinea pig left atria, dobutamine was the only compound to give a significant, although transient, increase in force of contraction above the maximal ouabain response. Theophylline (2 X 10(-4) mol X litre-1, EC25) produced significant decreases in force of contraction. In papillary muscles, low concentrations of all positive inotropic compounds, except amrinone, significantly increased force of contraction after a submaximal ouabain concentration. However, the maximal increase in force of contraction after combined addition of ouabain and a second inotropic agent was not different from the maximal increase with ouabain, dobutamine or dopamine alone. Addition of higher concentrations of the second inotropic agents after ouabain pretreatment led to a markedly increased incidence of toxicity with only transient positive inotropic effects. These results indicate that any haemodynamic improvement observed in adequately digitalised patients after combined positive inotropic therapy is unlikely to result from directly additive inotropic effects but is probably a result of other cardiovascular effects such as vasodilatation.

Somatostatin modulation of pancreatic glucagon, insulin, glucose and free fatty acids following beta-adrenergic stimulation.

This study was undertaken to determine the effect of somatostatin on acute, orciprenaline mediated, beta-adrenergic stimulation of free fatty acids, blood glucose, insulin, and glucagon in healthy subjects. After orciprenaline and somatostatin insulin and glucagon decreased, whereas blood glucose and free fatty acids increased, probably in part as a result of the lesser inhibition of glucagon (50%) than of insulin (83%). From these observations it is tentatively concluded that the inhibitory effects of somatostatin on insulin and glucagon release in man are a consequence of beta-adrenergic receptor involvement. These effects are possibly mediated through increased destruction of cAMP, blocking of camp dependent secretion or impairment of calcium uptake.

Bronchodilating effect of terbutaline aerosol.

We compared the efficacy of terbutaline with that of metaproterenol, isoproterenol, and placebo aerosols in 16 asthmatic patients. Terbutaline, metaproterenol, and isoproterenol produced equivalent improvements in flow rates. At 5 hr, the effect of terbutaline on tests of small airways, FEF25%--75%, and FEF50%, was greater (p less than 0.05) than that of metaproterenol and isoproterenol. Terbutaline produced no significant change of heart rate or blood pressure.

Beta-adrenergic agonist-mediated desensitization in senescent rats.

The capacity of senescent rats to develop the catecholamine refractory state was investigated in CDF (F-344) rats of 3 and 24 months of age. Beta-adrenergic receptor number, receptor-agonist affinity, and adenylate cyclase activity in heart membranes were assessed, following the chronic in vivo administration of the beta-adrenergic agonist, metaproterenol. Drug treatment leads to marked myocardial hypertrophy, receptor down-regulation, and reduced isoproterenol-stimulated adenylate cyclase activity. The extent of catecholamine-refractoriness was not different in the older rats, indicating the catecholamine desensitization of myocardial beta-adrenergic responsiveness is not impaired in senescence. Receptor agonist affinity and the percent of receptors in the high-affinity state decrease with age. These parameters are further reduced by agonist treatment but to a lesser extent in the older animals. Thus, the effects of age and agonist desensitization are not additive and suggest that aged animals may already be partially desensitized.

Tissue protein turnover in animals treated with the mixed beta-agonist metaproterenol: influence of dose, route and pattern of administration.

beta-Adrenergic agonists have been shown to increase protein deposition as a result of changes in the balance between protein synthesis and degradation rates. The aim of this study is to investigate the effect of the treatment with the non-selective beta-adrenergic agonist, metaproterenol, on protein metabolism in rats as well as the influence of the route and pattern of administration. A short- and long-term experimental trial were carried out. After the short-term treatment with the beta-agonist (1 mg/kg), neither protein nor nucleic acids were affected in liver or gastrocnemious muscle, while cathepsin A activity, an index of protein degradation, significantly increased in muscle. However, cathepsin A activity was reduced in muscle by the oral administration during 21 days of metaproterenol (2 ppm/day), but not by the subcutaneous injections (0.1 mg/kg/day). On the other hand, RNA/DNA, an index of protein synthesis capacity, and protein/DNA, an indicator of cell size, significantly diminished in muscle after the subcutaneous long-term treatment but did not change in the liver of treated rats. Our study has demonstrated a different outcome of a mixed beta-adrenergic agonist on protein metabolism depending on the duration of the treatment and the route of administration.

A multicenter study of nebulized bronchodilator solutions in chronic obstructive pulmonary disease.

A multicenter, 85-day, double-blind, randomized study was conducted to compare the effects of a single-dose and chronic inhalation of ipratropium bromide solution (500 micrograms) to the beta-adrenergic agonist metaproterenol (5% solution, 15 mg) in patients with chronic obstructive pulmonary disease (COPD). Patients were required to have a relatively stable, moderately severe COPD, forced expiratory volume in 1 second (FEV1) < 65% of predicted normal, FEV1 <70% of forced vital capacity (FVC), and a smoking history of > 10 pack-years. Following a 2-week baseline period, patients were randomized into either the ipratropium bromide (106 patients) or metaproterenol (107 patients) study groups. Pulmonary function testing was performed on days 1, 43, and 85. Secondary efficacy variables examined included peak expiratory flow rates, physician's global evaluation, quality of life, COPD symptom score, and use of concomitant medications. FEV1 was comparable between the two groups on day 1 (1.00 and 1.02 liters, ipratropium bromide vs metaproterenol, respectively; p = nonsignificant). The baseline FEV1 increased significantly in the ipratropium bromide group between day 1 and 43 by 10% (from 1.00 to 1.10 liters, p < 0.002) and remained 7% elevated on day 85 (1.07 liters) compared to day 1 (p < 0.02); it did not change in the metaproterenol group across all three test days. A clinically significant (> 15%) mean FEV1 response was observed on each of the 3 test days following drug inhalation in both treatment groups. The median duration of action was similar between groups (5 hours) on test day 1, but on day 85 the median duration for ipratropium bromide was r.5 hours compared to 3.0 hours for metaproterenol (p < 0.04). The secondary efficacy variables were uniformly better in the ipratropium bromide than the metaproterenol group. Side effects were infrequent and generally mild in both groups. These data suggest that the availability of a high-dose ipratropium bromide solution offers a safe and effective means of producing prolonged bronchodilation in patients with COPD.

Interaction of glutamatergic and adrenergic inputs of cortical neurons during conditioning.

Background and evoked activities of sensorimotor cortex neurons have been examined on learning cats with conditioned placing reaction before, during and after iontophoretic application of synaptically active drugs. It was shown that glutamate exerted not only a direct excitatory effect on the cortical neurons during its application, but also developed modulatory influences on background and evoked impulse activity after cessation of application in the subsequent 10-20 min. Adrenergic influences on the activity of neocortical neurons evoked by application of adrenomimetic drugs were complex and consisted of at least two different types. Noradrenaline depressed background and particularly evoked activity of many neurons through beta1-adrenoreceptors. At the same time, activation of beta2-adrenoreceptors was accompanied by facilitation of background and evoked activity during application and 10-20 min after its cessation, as was shown in experiments with alupent. Co-application of glutamate and alupent improved facilitation of impulse response evoked by conditioned stimuli. It was concluded that beta1- and beta2-adrenergic inputs to neocortical neurons are involved in plasticity changes of glutamate inputs of some cortical neurons.

Effect of beta-adrenoreceptor stimulants on exercise-induced asthma.

Exercise-induced asthma can be prevented or minimized by beta-adrenoreceptor stimulating drugs, which may elicit bronchodilation before exercise has begun or may enhance thebronchodilating effect of more prolonged exercise. Recently introduced beta-adrenoreceptor stimulants have longer durations of action than isoproterenol, are effective following oral administration, and are selective for beta-adrenoreceptors.

Temperature-induced changes in dissociation constants (KA) of agonists at cardiac beta-adrenoceptors determined by use of the irreversible antagonist Ro 03-7894.

The positive inotropic responses of guinea-pig left atria and papillary muscles and positive chronotropic responses of right atria to sympathomimetic amines were examined at 38 degrees and 30 degrees C. At the lower temperature, supersensitivity to orciprenaline and isoprenaline was exhibited as shifts of the dose-response curves to the left and significant reductions in EC50 values. This supersensitivity could not be attributed to reduced metabolism since the experiments were performed in the presence of metanephrine (10(-5)M) and U-0521 (3',4'-dihydroxy-2-methylpropiophenone) (10(-4)M) as inhibitors of extraneuronal uptake and catechol-O-methyltransferase (COMT) respectively, and the agonists are not susceptible to neuronal uptake. After incubation of the tissues with Ro 03-7894 (1-(5-chloracetylaminobenzfuran-2-yl)-2-isopropylaminoethanol), followed by its prolonged washout (greater than 2h), the maximum responses to isoprenaline and orciprenaline were depressed, confirming the apparently irreversible beta-adrenoceptor antagonism. Dissociation constants (KA) for isoprenaline and orciprenaline were determined from the equiactive concentrations obtained before (A) and after (A') incubation with Ro 03-7894, plotted as 1/A against 1/A' (KA = (slope-1)/intercept). KA values were the same for orciprenaline in the three cardiac preparations and for isoprenaline in the atria. This applied at 38 degrees and 30 degrees C and indicates that the beta-adrenoceptors mediating the inotropic and chronotropic responses of the guinea-pig heart do not differ. The KA values of both agonists were, however, consistently and significantly lower at 30 degrees than at 38 degrees C, indicating an increase in affinity. 8 It is concluded that hypothermia-induced supersensitivity of cardiac tissue to sympathomimetic amines is associated with an increase in their affinity for the B-adrenoceptors.

Construction of antagonist dose-response curves for estimation of pA2-values by Schild-plot analysis and detection of allosteric interactions.

1. One aim of this paper is to show an alternative approach for the determination of antagonist affinity estimates, KB and pA2, by construction and evaluation of antagonist dose-response curves (DRCs), using the curve-fitting programme, ALLFIT. 2. Parallel antagonist DRCs were derived by vertical analysis of families of conventional agonist DRCs in the presence and absence of an antagonist at a certain agonist concentration above its ED50. The latter represents a chosen, i.e. fixed dose-ratio (DR). The antagonist concentration that reduces an agonist effect to its Emax/2 was termed Bx. It corresponds to B, the fixed antagonist concentration, tested to obtain DR-1, conventionally. 3. The dissociation constant was calculated as KB = Bx/DR-1, analogous to the conventional approach (KB = B/DR-1). Likewise, pA2-values were estimated by plotting log Bx, obtained by the alternative approach, vs log (DR-1) in an 'alternative Schild plot'. 4. Experimental agonist DRCs from our laboratory and from the literature were analysed and KB- and pA2-values obtained by the alternative approach were compared with those obtained by the conventional method. The results showed a very good agreement (correlation) between the pA2-values obtained by either method (slope = 1.02, r = 0.99, n = 9), in agreement with theoretical DRCs. 5. Besides estimation of KB and pA2, antagonist DRCs were also evaluated qualitatively. The most important finding was that allosteric antagonists or competitive antagonists with an allosteric component, such as gallamine, showed a significant reduction in the maximum of the antagonist DRCs (Imax). The evaluation of antagonist DRCs appears to be a sensitive procedure to detect allosteric interactions.6. This alternative approach can supplement or replace the conventional approach for the evaluation of antagonists on a quantitative and qualitative basis. The alternative approach appears of special advantage where the supply and/or the solubility of the agonist is limited, resulting in incomplete agonist DRCs.7. For rapid screening of potential antagonists, a single antagonist DRC at the maximum effective agonist concentration may be constructed to calculate KB reliably.

Potency and selectivity in vitro of compounds related to isoprenaline and orciprenaline on beta-adrenoceptors in the guinea-pig.

1 Amines related in structure to either isoprenaline (catechol series) or orciprenaline (resorcinol series) were examined for activity on isolated trachea (relaxation), atria (chronotropic action) and perfused hind-limb (vasodilatation) of the guinea-pig.2 Compounds with a resorcinol nucleus were less potent on all three preparations but more selective for trachea than were compounds with a catechol nucleus.3 In both catechol and resorcinol compounds potency on trachea was enhanced by and selectivity for trachea was favoured by substitution of a p-OH phenyl group in the N-iso propyl, or by replacement of the N-iso propyl with an N-t-butyl, with or without a p-OH phenyl group.4 Most of the compounds, particularly the resorcinols, had much lower potencies, relative to isoprenaline, on hind-limb than on trachea.5 Some of the problems associated with the quantitative measurement of selectivity and with sub-classification of beta-adrenoceptors are discussed.