Differentiation of receptor systems activated by sympathomimetic amines

Central beta-adrenergic receptors for blood pressure regulation in spontaneously hypertensive rats

A retrospective and prospective analysis of cerebral arteriograms in 28 patients with intracerebral arteriovenous malformation (AVM) was undertaken to identify those parameters that predict the efficacy of small silicone particle embolization as a method of treatment. It was noted that when the number and caliber of the arterial feeding vessels were greater than the number and caliber of the draining veins the embolization was successful (15 of 16 patients). On the contrary, when the number and caliber of the draining veins exceeded that of the arteries the embolization failed (seven of seven patients). In four of five patients with nearly equal ratios, the embolization reduced the flow but not the size of the malformation.

Selectivity of beta-adrenoceptor agonists and antagonists on bronchial, skeletal, vascular and cardiac muscle in the anaesthetized cat

1 The potencies of fifteen beta-adrenoceptor agonists of widely differing chemical structures were compared with that of (-)-isoprenaline on bronchial muscle, soleus muscle, blood pressure and heart rate in the anaesthetized cat. The beta-adrenoceptor antagonist potencies of propranolol and practolol were determined against (-)-isoprenaline in the same model. 2 (-)-Isoprenaline was the most potent agonist and its action was essentially unselective. Thus, on all four parameters the minimal effective dose was 0.003-0.01 mug/kg and maximal or near maximal responses were produced by 0.3-1 mug/kg. Trimetoquinol was also an essentially unselective agonist. 3 For thirteen of the remaining fourteen agonists, potency was similar on bronchial muscle, soleus muscle and blood pressure but significantly lower on heart rate. 4 The remaining agonist - AH 7616 (4-hydroxy-alpha1-[[(1-methyl-3,3-diphenyl-propyl)amino]-methyl]-m-xylene-alpha1, alpha3-diol, acetate) - was also significantly less potent on heart rate than on the other parameters; in addition, it was clearly less potent on soleus muscle and blood pressure than on bronchial muscle when 5-hydroxytryptamine (5-HT) was used to induce bronchospasm. However, when acetylcholine was used instead of 5-HT the potency of AH 7616 on induce bronchospasm. However, when acetylcholine was used instead of 5-HT the potency of AH 7616 on bronchial muscle, soleus muscle and blood pressure was very similar. AH 7616 may therefore possess a specific 5-HT antagonist action in addition to its beta-adrenoceptor agonist action. 5 The fifteen test agonists were longer acting than (-)-isoprenaline and this was particularly true of trimetoquinol and soterenol. 6 The beta-adrenoceptor antagonist potency of propranolol was almost identical on bronchial muscle, soleus muscle and blood pressure and very slightly lower on the heart. Practolol was 10-12 times more potent on the heart than on bronchial muscle, soleus muscle and blood pressure. 7 These findings suggest that it may not be possible to separate the bronchodilating and tremorenhancing properties of beta-adrenoceptor agonists. The results with agonists and antagonists are in accord with Lands' dual beta-adrenoceptor sub-classification.

Adrenergic factors influencing the mitotic rate in stratified squamous epithelium of the buccal mucosa of the rat

1. An in vivo stathmokinetic technique was used to determine the immediate effect of various adrenoceptor agonists and antagonists on the mitotic rate in the stratified squamous epithelium of the rat buccal mucosa. 2. The mitotic rate increased significantly in rats treated with propranolol and with practolol, whereas the mitotic rate decreased significantly in rats treated with metaraminol. In animals treated with isoprenaline and phentolamine the mitotic rate did not differ significantly from the control value. 3. The nature of the involvement of adrenergic mechanisms with cell proliferation is still uncertain but both alpha- and beta-adrenergic mechanisms appear to be associated with the control of cell proliferation.

Some adrenergic beta-blocking agents affecting lipolysis in human adipose tissue in vitro

Antagonistic actions of beta-blocking drugs on isoproterenol-induced lipolysis were studied in human omental adipose tissue. Competitive interaciton characterized by the following pA2 values was found: propranolol 8.7; trimepranol 8.7; practolol 7.1; H 35/25 6.0. The plot of pA2 values of these drugs for human versus rat adipose tissue is linear with slope 2.0 indicating a higher differentiation of beta-antagonist actions in human than in rat adipose tissue.

Comparison of propranolol and practolol in the management of hyperthyroidism

Twenty-one hyperthyroid patients participated in an 8-week double-blind crossover trial of propranolol and practolol, and the effecte of these drugs on the clinical and metabolic features of the disease were studied. Propranolol was marginally more effective than practolol, as measured by the hyperthyroid diagnostic index and anxiety scale. Propranolol produced a significant reduction in the serum concentration ratio of tri-iodothyronine to thyroxine, compatible with partial inhibition of peripheral deiodination of thyroxine. Adverse reactions occurred more frequently with propranolol than with practolol. In veiw of the efficacy of practoloo, further trials in hyperthyroid patients of newer beta1-adrenoceptor antagonists, preferably without partial agonist activity, are indicated.

Comparison of salmefamol with salbutamol aerosols in asthamatics

In twelve asthmatic patients 200 mug of salmefamol and salbutamol given by metered aerosol produced a similar initial effect on FEV1, FVC and PEFR without significant effect on heart rate or blood pressure. The duration of effect of salbutamol was approximately 4 hours; there was still an appreciable effect from salmefamol at 8 hours.

Comparative effects of acebutolol and practolol on the lipolytic response to isoprenaline

1 The effects of beta-adrenoceptor blockade on the metabolic responses to isoprenaline have been studied in an in vitro system of isolated fat cells and in six normal subjects. 2 The inhibitory effects of varying concentrations of acebutolol, practolol and propranolol on free fatty acid (FFA) release produced by isoprenaline (10(-7) M) were compared in isolated fat cells prepared from rat epididymal adipose tissue. Acebutolol and practolol, at equimolar concentrations, showed a similar inhibitory effect whilst propranolol was approximately 100 times more potent then either drug. At 10(-5)M concentration of propranolol, lipolysis was virtually abolished whilst at the same molar concentration, acebutolol and practolol halved the response. 3 Six healthy volunteers received three successive 15 min intravenous isoprenaline challenges (0.03 mug kg-1 min-1) per individual experiment. The first acted as a control whilst the following two were given either after single oral doses of placebo, acebutolol or practolol. The mean (+/- s.e. mean) basal FFA level was 0.77 +/- 0.06 mE1/1 and subsequent resting values after the administration of placebo or beta-adrenoceptor blocker were not significantly different. 4 Acebutolol inhibited the respective mean rises in FFA, produced by both post-control isoprenaline challenges, by (mean +/- s.e. mean) 70 +/- 4% and 84% +/- 5%. The comparable figures for practolol were 33 +/- 15% and 24 +/- 20%. The higher serum concentration of acebutolol produced greater inhibition but correlation of log serum concentration of the drug with percentage inhibition of FFA rise did not achieve significance. 5 Administration of isoprenaline, acebutolol or practolol did not significantly alter serum glucose, triglyceride or cholesterol levels. 6 Acebutolol and practolol effectively blocked the isoprenaline-induced tachycardia. The degree of blockade produced by practolol was greater than its inhibitory effect on FFA release. The diatolic fall in blood pressure in response to isoprenaline was abolished by acebutolol suggesting that its beta-adrenoceptor blocking action encompasses peripheral vascular sites. The comparable effect with practolol was a partial inhibition of the diastolic fall.

Comparison between ibuterol hydrochloride and terbutaline in asthma

The bronchodilating effect, circulatory effects, and subjective side effects of ibuterol hydrochloride, the di-isobutyric acid ester of terbutaline, at two dose levels (2 and 4 mg) were compared with those of 5 mg terbutaline sulphate in a double-blind cross-over study on 12 patients with asthma. Both drugs were given by mouth. The 2-mg dose of ibuterol had the same bronchodilating effect during the first three hours as 5 mg terbutaline. The 4-mg dose, however, produced a significantly greater increase in the peak expiratory flow rate between the 30th and 120th minutes than terbutaline. No significant changes in heart rate or pulse amplitude were noted, and there was no difference in the incidence of subjective side effects between ibuterol at either dose level and terbutaline.

Comparison of respiratory and cardiovascular effects of isoproterenol, propranolol, and practolol in asthmatic and normal subjects

Pulmonary functions, by means of FEV1, FVC, and airway resistance, and cardiovascular responses, i.e., ECG, blood pressure, and pulse contour, were measured in 10 control and 15 symptom-free asthmatic subjects during and after the infusions of isoproterenol, with or without previous administration of propranolol or practolol. Bronchial sensitivity to methacholine and response to isoproterenol after methacholine were also measured. Bronchial sensitivity to methacholine challenge was markedly enhanced by propranolol and the effect of isoproterenol infusion on the pulmonary function was also significantly diminished, whereas practolol did not reveal any effect on bronchial sensitivity to methacholine or isoproterenol. The above changes were much more profound in symptom-free asthmatic subjects than in control subjects. The results suggest that practolol does not significantly affect the beta-2 bronchial receptors, is safe to use in asthmatic subjects, and is consistent with an abnormality of these receptors in bronchial asthma.

Comparison of the effects of (--)-isoprenaline, orciprenaline terbutaline, and Me506 on heart rate, soleus muscle contractility and pulmonary resistance of anaesthetized cats

1. Three resorcinol derivatives with N-isopropyl (orciprenaline), N-t-butyl (terbutaline) and N-p-hydroxypheny-t-butyl (Me506) amine substituents have been compared with (--)-isoprenaline for their ability to produce beta-receptor mediated reductions in serotonin-induced increases in pulmonary resistance, decreases in soleus muscle contractility and increases in heart rate in anaesthetized cats. 2. For all parameters studied the four compounds produced similar maximal responses and dose-response curves were close to parallel. From the graphs doses of the compounds producing 50% of the maximal response (ED50) were interpolated, and from these dose-ratios with respect to (--)-isoprenaline [drug ED50:(--)-isoprenaline ED50] were calculated on a molar basis. 3. Increasing the size of the amine substituent from N-isopropyl to N-t-butyl led to an increase in beta-receptor stimulant activity in bronchial and skeletal muscle, but not in the heart. The change from N-t-butyl to N-p-hydroxyphenyl-t-butyl did not further affect stimulant activity in any of the parameters studied. 4. Calculation of selectivity ratios [molar dose-ratio (heart): molar dose-ratio (pulmonary resistance)] showed that orciprenaline was non-selective, and that terbutaline and Me506 showed a similar degree of selectivity for beta2- as opposed to beta1-receptor mediated actions.

Clinical study of broncholytic effect and side-effects of isoetharine ('Numotac')

In thirty-nine adult asthmatics a clinical trial was carried out with 10 mg slow-release tablets of isoetharine ('Numotac', 3M Riker) at two dose levels: 10 mg and 20 mg four times a day. The trial was double-blind with crossover after six weeks. Twenty-three patients reported a positive effect on their respiratory symptoms when isoetharine had replaced their previous treatment; negative effect was reported by one patient while twelve patients were undecided. Tremor was a common side-effect but except in three cases it was slight. There was no difference in side-effects between the high and the low doses if the initial dose was low. However, there were significantly more side-effects when the trial was started with the high dose.

Hypercalcemic effect of catecholamines and its prevention by thyrocalcitonin

Earlier work by others has shown that the catecholamines, epinephrine and isoproterenol, can raise blood calcium levels in parathyroidectomized but not intact rats, and can restrict the hypocalcemic effect of injected thyrocalcitonin (TCT). The present findings support this earlier work, further showing that such catecholamines can produce hypercalcemia in rats after removal of the thyroid gland by acute thyroparathyroidectomy (TPTX) and indicating that these drugs may raise blood calcium by mobilizing calcium from bone. Rats were fasted overnight, subjected to TPTX and concurrently injected with adrenergic agonist or antagonist drugs alone or in combination. Epinephrine, isoproterenol, and the beta-2 adrenergic agonist, salbutamol, in doses greater than or equal to 1 mg/kg raised blood calcium from low normal levels (approximately 9-10 mg/100 ml) by 1.5 to 2 mg/100 ml (p less than 0.01). Hypercalcemia was apparent by 1 hour after injection and lasted for 1-4 hours. The extent of Ca elevation was dose-related. Pretreatment of rats with the alpha-adrenergic antagonist, phenoxybenzamine, enhanced the effect of epinephrine while pretreatment with the beta-antagonist, propranolol, reduced the effect of isoproterenol. The more selective beta-2 antagonist, butoxamine, but not the beta-1 antagonist, practolol, also reduced the hypercalcemic effect of isoproterenol in TPTX rats. These results suggest that catecholamine-induced hypercalcemia in TPTX rats is mediated by beta-2 adrenergic receptors. Related studies using rats prelabeled with 45Ca further suggest that the catecholamines, like parathyroid hormone, may act to raise blood calcium by mobilizing calcium from bone. The fact that these catecholamines could induce marked hypercalcemia in acutely TPTX rats but not in intact rats indicated that endogenous TCT protects the thyroid intact rat against hypercalcemia. The present findings support this idea in showing that isoproterenol and salbutamol raised levels of immunoreactive rat TCT in both thyroid venous and peripheral blood. Catecholamines apparently can promote TCT secretion, either directly or by a small transient increase in blood calcium. This, in turem, acts to combat hypercalcemia in thhroid-intact rats.

Hexoprenaline: beta-adrenoreceptor selectivity in isolated tissues from the guinea-pig

1. A catecholamine beta-adrenoreceptor agonist, hexoprenaline, was examined in vitro on five guinea-pig tissues and its potency relative to isoprenaline (as 100) obtained. 2. Hexoprenaline clearly delineated between those tissues classified as containing beta2-adrenoreceptors (trachea, hind limb blood vessels and uterus; relative potencies 219, 110 and 76 respectively) and those classified as containing beta1-adrenoreceptors (atria and ileum; relative potencies 3.3 and 1.0 respectively). 3. Hexoprenaline differed from some previously studied noncatecholamine beta-adrenoreceptor agonists in being only two-fold less potent, relative to isoprenaline, as a vasodilator in perfused hind limb than as a tracheal relaxant.

Differential blockade of guinea-pig atrial rate and force responses to (--)-noradrenaline by practolol - an uptake phenomenon

The relative blockade of rate and force responses of guinea-pig isolated atria to catecholamines by practolol was compared. pA2 determinations revealed that with (--)-noradrenaline as the agonist, practolol was more effective in antagonizing rate (pA2, 6.99) than force (pA2, 6.44). This difference was absent in either atria from reserpinized animals incubated with phenoxybenzamine and tropolone or atria incubated with cocaine. Similarly, when (--)-isoprenaline was the agonist, force and rate pA2 values were identical, suggesting that rate and force beta1-adrenoceptors do not differ. The preferential blockade of (--)-noradrenaline-induced rate responses in untreated atria was attributed to the operation of a saturable neuronal uptake mechanism which differs between left and right atria. To avoid high (--)-noradrenaline concentrations encountered during pA2 determinations, the % reduction of responses to single doses of (--)-noradrenaline by practolol were measured in untreated atria. Far from confirming the lack of receptor differentiation indicated by the pA2 determinations, this study revealed force to be more practolol sensitive. The relative merits of the two methods are discussed in an attempt to resolve the question whether rate and force beta1-adrenoceptors differ.

A double-blind trial of salmefanol, a new bronchodilator

Salmefamol is a new sympathomimetic drug with predominantly β₂ actions. In a double-blind trial in a group of twenty-four patients with airway obstruction, we have compared the effect of 100 μg of salmefamol given by metered aerosol with that of a placebo aerosol. In the same patients we have also compared the aerosols with subcutaneous adrenaline and subcutaneous atropine, the sequence of treatments being determined by a latin square design.

At all times salmefamol had a significantly greater bronchodilator effect than the placebo. Although the effects of atropine and adrenaline were greater at the earlier times of measurement, that of salmefamol was greater at 4 hr.

The metabolic consequences of adrenergic blockade: a reveiw

The effects in man of adrenergic blocking agents on plasma insulin, glucagon, growth hormone, and lipid metabolism are reviewed. Whereas basal insulin may be slightly inhibited by beta- and enhanced by alpha-adrenergic blockade, more marked suppression may be achieved under circumstances of high exogenous or endogenous catecholamine stimulation. The relative effects of beta1 or combined beta 1 and beta2 blockers in man are unknown. Glucagon release is probably provoked by beta- and inhibited by alpha-stimulation in man. Muscle glycogenolysis is inhibited by propranolol, and under situations of hepatic glycogen depletion, clinical hypopglycemia may occur. This may also account for the failure of significant hyperglycemia to be observed in short-term experiments on fasting subjects in whom insulin release may be suppressed and glucagon release enhanced. Growth-hormone release is enhanced by beta-adrenergic blockade. Free fatty acid formation in vivo is inhibited by intravenous beta blockade, but the effects of oral administration on triglyceride production and lipoprotein profiles remain uncertain. The inter-relationships between the effects of adrenergic blockade at different sites of hormone and substrate release are unclear but may have important consequences in alteration in carbohydrate tolerance and lipid metabolism. The relative effects of beta-blocking drugs with differing specificity must be determined.

Effects of several beta-blockers on blood pressure in the rat

Effects of practolol, alprenolol and pindolol on blood pressure in the rat were studied. Also effects of these three beta-blocking agents on blood pressure and heart rate in spinal rats during adrenaline infusion were studied and compared with those of propranolol. The beta-blocking agents produced a sustained pressor action in the rat, and in the spinal rat infused with adrenaline. The magnitude of the pressor action induced by the beta-blockers was in the following order: pindolol larger than or equal to propranolol larger than or equal to alprenolol greater than practolol. Minimum doses of these beta-blockers required to cause a pressor action in the spinal rat infused with adrenaline were in the following order; practolol greater than alprenolol larger than or equal to propranolol larger than or equal to pindolol. The magnitude of the pressor action produced by the same dose of these beta-blockers and minimum doses of these beta-blockers required to cause a pressor action in the spinal rat infused with adrenaline seemed to be roughly proportional to their beta-receptor blocking activities. It was concluded that the minimum doses of these beta-blockers required to cause a pressor action and the magnitude of the pressor action induced by the beta-blockers in the spinal rat infused with adrenaline could be used to compare their beta-blocking activities and that practolol, a cardioselective beta-blocker, seems to block not only cardiac beta-receptor but to some extent also peripheral vascular beta-receptors.

Isoprenaline antagonism of cardioselective beta-adrenergic receptor blocking agents on human and rat adipocytes

1. The beta-adrenergic blocking potencies of practolol, ICI 66082, tolamolol, acebutolol, H 93/26, H 87/07, pindolol and Ro 3-4787 were compared with that of propranolol, on human and rat adipocytes. 2. A good correlation was found between the potencies on adipocytes of the two species but not between our results and literature data on antagonism of isopernaline tachycardia in the anaesthetized cat. 3. The results indicate that differences between adrenergic beta-receptors in heart and adipose tissue may be detected using cardioselective beta-adrenergic receptor blocking agents.

Antihypertensive effect of beta-1 receptor blockade and beta-2-receptor stimulation in essential hypertension

The antihypertensive effect of practolol, a beta-1-blocker, alone and in combination with a beta-2-stimulating drug, salbutamol, has been studied in a double-blind cross-over trial on 19 patients. Practolol treatment induced a significant BP reduction, while the addition of salbutamol failed to give any further decrease.

Ocular hypotensive effect of atenolol (Tenormin, I.C.I.). A new beta-adrenergic blocker

Atenolol (Tenormin or I.C.I. 66082) is a new beta-adrenergic blocking drug, unique in being cardio-selective and in having no intrinsic sympathomimetic or membrane activity. In a controlled double-blind study, a single 50 mg oral dose produced a significant fall in ocular tension for about 7 hours in five patients with definite or suspected glaucoma. The average maximum fall was 35 per cent of the initial pressure; it occurred at 5 hours after oral ingestion. Accordingly neither intrinsic sympathomimetic nor membrane activity can account for all the ocular hypotensive effect of beta blockers in humans. The practical implications for treatment of glaucoma require longer-term investigations some of which are in progress.

Beta-adrenoreceptors in denervated skeletal muscles of the cat

1. A number of sympathomimetic amines have been compared with (-)-isoprenaline for their ability to produce contractions in chronically denervated soleus and tibialis anterior muscles and to increase heart rate in bilaterally vagotomized anaesthetized cats. 2. The mean dose-ratios ((-)-isoprenaline equals 1) for adrenaline, orciprenaline and salbutamol were 2.9, 29.7 and 11.5 respectively in the tibialis anterior muscle and 3.5, 24.1 and 14.3 in the soleus muscle. The dose-ratios for noradrenaline were 21.1 in the tibialis anterior and 57.6 in the soleus muscle. 3. The dose-ratios ((-)-isoprenaline equals 1) obtained for the positive chronotropic effects of the drugs in bilaterally vagotomized cats were 14.5, 14.6, 29.6 and 24.2 for adrenaline, noradrenaline, orciprenaline and salbutamol respectively. 4. The beta2-receptor antagonists butoxamine and H35/25 antagonized the vasodepressor and skeletal muscle responses to (-)-isoprenaline to a greater extent than the cardiac responses, whereas the beta1-receptor selective antagonists practolol and H93/26 antagonized cardiac to a greater extent than vascular and skeletal muscle responses. 5. The results obtained suggest that the contractions of denervated skeletal muscle to sympathomimetic drugs result from stimulation of beta2-adrenoreceptors.

Comparison of the cardiopulmonary effects of subcutaneously administered epinephrine and terbutaline in patients with reversible airway obstruction

The cardiopulmonary effects of epinephrine and terbutaline were compared in a doubleblind crossover study in 23 subjects with chronic obstructive airway disease. On each of three days each subject received a single subcutaneous dose of saline, 0.25 mg of epinephrine or 0.5 mg of terbutaline. Treatment with epinephrine produced significant increases in forced vital capacity (FVC), forced expiratory volume in one second (FEV-1), maximal expiratory flow rate (MEFR) and maximal mid-expiratory flow (MMEF). Terbutaline caused even more pronounced increases in all four parameters and exhibited a longer duration of action. Neither drug altered arterial pH, arterial oxygen pressure (PaO-2), or arterial carbon dioxide pressure (PaCO-2). With regard to cardiovascular effects, no alterations in either systolic or diastolic pressure were observed. Administration of epinephrine and terbutaline caused statistically significant increases in heart rate. The effect of terbutaline was more pronounced that that of epinephrine. In addition, terbutaline caused a heart rate-related depression of the T-wave of the lead 2 ECG. Neither drug altered any of the hematologic, hemochemical or urinary parameters monitored before and after treatment. Side effects were seen in eight subjects after administration of saline solution, in 13 subjects after epinephrine and in 19 subjects after terbutaline. None of these side effects was considered clinically serious and none required treatment. It is concluded from this study that subcutaneously administered terbutaline is a more effective bronchodilator than epinephrine.

Metabolic responses to selective beta-adrenergic stimulation in man

The responses of plasma free fatty acid, glucose, lactate, insulin and growth hormone to intravenous administration of the predominantly β-2 stimulant, salbutamol, were studied in nine normal subjects. Four subjects received the predominantly β-1 blocking agent, practolol, together with salbutamol.

Salbutamol produced a marked rise in free fatty acid levels and there was also an appreciable increase in insulin values; lactate levels rose moderately and glucose values increased slightly; growth hormone levels were not affected. Practolol did not alter the free fatty acid response; it diminished but did not appear to abolish the insulin response, and it appeared to suppress the lactate and glucose rises.

Tentative proposals are made regarding the nature of the β-receptor subtypes responsible for mediating these effects; most definite is the suggestion that lipolysis is subserved by β-2 receptors. Possible clinical implications are briefly discussed.

Metabolic and cardiovascular effects of isoprenaline and salbutamol in the dog

1 The intravenous infusion of isoprenaline and salbutamol into the greyhound increased heart rate and levels of free fatty acids, lactic acid and glucose. 2 On terminating the infusions of isoprenaline the changes produced declined rapidly but the effects produced by salbutamol were more persistent. When high doses of salbutamol had been infused, glucose and lactic acid levels in fact increased during the 20 min following the infusions. 3 These results support suggestions that, in the dog, lipolysis is mediated by beta1-adrenoceptors and liver glycogenolysis by beta2-adrenoceptors. The beta-adrenoceptors mediating muscle glycogenolysis could not be assigned inequivocally to either subtype. 4 The differences in the behaviour of isoprenaline and salbutamol in the period following the infusions are considered to be due partly to slower removal of salbutamol. Increases in lactic acid levels after infusion of large amounts of salbutamol may be secondary to the persistence of high glucose levels.

The selectivity of beta-adrenoceptor antagonists on cardiovascular and bronchodilator responses to isoprenaline in the anaesthetized dog

1 The actions of five beta-adrenoceptor antagonists, chosen because of reported differences in their selectivities, were compared using the positive chronotropic, vasodepressor and bronchodilator responses to isoprenaline in anesthetized dogs. 2 Propranolol was a potent antagonist of the isoprenaline responses in all three systems. 3 Practolol and acebutolol (M & B 17,803) blocked the positive chronotropic responses to isoprenaline to a greater extent than the vasodepressor or bronchodilator responses. 4 Butoxamine and alpha-methyl dichloroisoprenaline showed the opposite selectivity, blocking the vasodepressor and bronchodilator responses to isoprenaline to a greater extent than positive chronotropic responses. However, both drugs were considerably less potent than the other antagonists studied and their selectivities were less clear-cut than those of practolol or acebutolol. 5 All the antagonists lowered the resting heart rate and to a lesser extent the diastolic blood pressure. The effects of propranolol, practolol and acebutolol on heart rate probably result from cardiac beta-adrenoceptor blockade. With butoxamine and alpha-methyl dichloro isoprenaline, however, the effects on heart rate probably result from a direct cardiodepressant action. 6 The relevance of the results to the problem of the sub-classification of beta-adrenoceptors is discussed.

The beta-adrenoceptors of the lung mediating inhibition of antigen-induced histamine release

The beta-adrenoceptor stimulants, isoprenaline (IPR), orciprenaline (OPR), terbutaline (TRB), and ITP were studied for effects on antigen-induced release of histamine from guinea-pig lung tissue and for effects on guinea-pig isolated trachea and heart. The order of potency for the agents in the four funct-ons studied were: (a) inhibition of histamine release, IPR greater than OPR approximately equal to TRB greater than ITP equal 0; (b) heart stimulation, chronotropic effect, IPR greater than OPR greater than ITP approximately equal to TRB; (c) heart stimulation, inotropic effect, IPR greater than OPR greater than ITP greater than TRB; (d) trachea relaxation: IPR greater than TRB greater than OPR greater than ITP. These findings suggest that the beta-adrenoceptors mediating inhibition of antigen-induced release of histamine are more related to those mediating trachea relaxation (beta2) than those mediating cardiac stimulation (beta1).

Characterization of pharmacological receptors

The isolation of macromolecules that bind drugs and neurohumoral mediators at receptor sites will require analytical methods adequate to detect these substances in homogenates and crude mixtures. Although the binding of isotopically labeled agonists has been useful in a few instances, equilibrium binding studies of radioactive, lipophilic antagonists promises to be more useful with beta-adrenergic receptors. Alkylation by radioactive reagents that seek out specific receptors or specific functional groups on protein have been used in attempts to label other receptors. Conformationally directed labeling of receptor and receptor-related proteins by the latter group of alkylating agents has been used with receptors for cardiac glycosides and may be applicable to other systems.

Dobutamine: development of a new catecholamine to selectively increase cardiac contractility

We systematically modified isoproterenol's chemical structure to reduce chronotropic, arrhythmogenic, and vascular side effects. Experiments on dogs showed that the resulting drug, dobutamine, had an inotropic efficacy as great as that of epinephrine due to a direct action on beta1 cardiac receptors. However, unlike epinephrine, dobutamine's effect on alpha and beta2 vascular receptors was slight. At equivalent inotropic doses, dobutamine had less than a fourth of the chronotropic effect of isoproterenol. Desmethylimipramine (DMI), which blocks the sympathetic nerve fiber uptake mechanism, had no effect on dobutamine's actions. In contrast, DMI antagonized dopamine's inotropic effect, and marked chronotropic and pressor responses occurred when we used doses of dopamine large enough to elicit a direct inotropic effect. Dobutamine increased the contractility of isolated cat papillary muscles more but the automaticity less than did isoproterenol. In ischemic dog hearts, dobutamine lacked significant arrhythmic activity, whereas dopamine, norepinephrine, and isoproterenol caused severe ectopic activity. In dogs with experimentally induced low cardiac contractility, low cardiac output, and hypotension, dobutamine produced dose-related increases in cardiac contractility and output, restored arterial blood pressure, and reduced total peripheral resistance slightly. In contrast, isoproterenol failed to restore blood pressure, had only a meager effect on cardiac contractility and output, cuased extreme tachycardia, and lowered peripheral resistance more than did dobutamine. Norepinephrine, which did not increase cardiac contractility or output as much as dobutamine, excessively elevated peripheral resistance and arterial blood pressure.

Pressure-flow relationships in the peripheral circulation of the dog with practolol

1. The influence of practolol (a proposed beta1-adrenoreceptor antagonist) upon the pressure-flow relationships in the peripheral circulation was studied in eight anaesthetized dogs during right heart by-pass procedures. 2. Practool (1 mg/kg) produced a significant increase in the resistance to venous return which resulted in a significant fall in venous return. 3. There was no significant change in arterial resistance. 4. This study suggests that practolol should not be classified as an exclusive cardioselective beta-adrenoreceptor antagonist.