Certain beta-blockers can decrease beta-adrenergic receptor number: I. Acute reduction in receptor number by tertatolol and bopindolol

Slow receptor dissociation is not a key factor in the duration of action of inhaled long-acting β2-adrenoceptor agonists

BACKGROUND AND PURPOSE

β(2) -Adrenoceptor agonists are important bronchodilators used for the treatment of chronic obstructive pulmonary disease and asthma. Clinical data on β(2) -adrenoceptor agonists show a range of onset and duration of action. We have investigated whether the receptor binding kinetics of β(2) -adrenoceptor agonists can explain their observed onset of action and duration of effect in the clinic.

EXPERIMENTAL APPROACH

[(3) H]-DHA was used to label β(2) -adrenoceptors expressed in CHO-cell membranes (K(d) of 0.084 nM). Competition kinetic experiments were performed in the presence of unlabelled β(2) agonists at 37°C in HBSS containing GTP. To determine the kinetic parameters, three concentrations (10, 3 and 1 ×K(i) ) of the unlabelled compound were employed against a fixed concentration of [(3) H]-DHA (0.6 nM).

KEY RESULTS

The clinically used β(2) -adrenoceptor agonists exhibited a range of association and dissociation rates. The kinetic K(d) and the competition K(i) values of the eight β(2) -adrenoceptor agonists examined were strongly correlated, suggesting that the method had produced accurate k(off) and k(on) rates. The kinetic on-rate was highly correlated with equilibrium binding affinity.

CONCLUSIONS AND IMPLICATIONS

Although the β(2) -adrenoceptor agonists displayed a range of kinetic rate parameters, simulations at relevant drug concentrations suggest that receptor kinetics do not play an important role in determining onset of action in the clinic. In addition, it is unlikely that receptor kinetics exert an important influence on the duration of action of these agonists, as indacaterol (once daily dosing) had a shorter residency time at the receptor than salmeterol (twice daily dosing).

Bucindolol, a nonselective beta 1- and beta 2-adrenergic receptor antagonist, decreases beta-adrenergic receptor density in cultured embryonic chick cardiac myocyte membranes

Bucindolol and carvedilol, nonselective beta1- and beta2-adrenergic receptor antagonists, have been widely used in clinical therapeutic trials of congestive heart failure. The aim of the current study was to investigate long-term effects of bucindolol or carvedilol on beta-adrenergic receptor protein and gene expression in cardiac myocytes. Embryonic chick cardiac myocytes were cultured and incubated with bucindolol (1 microM), carvedilol (1 microM), or norepinephrine (1 microM) for 24 h. 125I-iodocyanopindolol binding assays demonstrated that incubation with norepinephrine or bucindolol, but not carvedilol, significantly decreased beta-adrenergic receptor density in crude membranes prepared from the myocytes. Neither bucindolol nor carvedilol significantly stimulated adenylyl cyclase activity in membranes from drug-untreated cells. Unlike by norepinephrine, the receptor density reduction by bucindolol incubation was not accompanied by a change in beta1-adrenergic receptor messenger RNA abundance. A decrease in membrane beta-adrenergic receptor density without a change in cognate messenger RNA abundance was also observed in hamster DDT1 MF2 cell line incubated with bucindolol (1 microM, 24 h). We conclude that incubation with bucindolol, but not carvedilol, results in true reduction of beta-adrenergic receptor density in chick cardiac myocyte membranes by mechanisms that are distinct from those responsible for receptor density reduction by the agonist norepinephrine.

Effect of short-term and long-term treatments with sigma ligands on the N-methyl-D-aspartate response in the CA3 region of the rat dorsal hippocampus

1. Long-term treatments with the sigma ligand haloperidol decrease the density of sigma receptors in mammalian CNS. We have shown that sigma ligands, such as di(2-tolyl)guanidin (DTG), potentiate dose-dependently, with bell-shaped dose-response curves, the neuronal response of pyramidal neurones to N-methyl-D-aspartate (NMDA) in the CA3 region of the rat dorsal hippocampus. sigma Ligands producing such a potentiation were denoted 'agonists'. This potentiation was suppressed by low doses of other sigma ligands denoted 'antagonists'. High doses of DTG and JO-1784 did not modify the NMDA response but acted as 'antagonists' by suppressing the potentiation induced by sigma 'agonists'. 2. Following a 21-day treatment with haloperidol as well as with high doses of DTG or JO-1784, after a 48 h washout, the acute administration of sigma 'agonists' failed to induce any potentiation of the NMDA response. Following a 21 day treatment with a low dose of DTG or JO-1784, after a 48 h washout, the neuronal response to microiontophoretic applications of NMDA was markedly increased. A 21 day treatment with low or high doses of (+)-pentazocine, after a 48 h washout, did not produce any change. 3. Following a two day treatment with a high dose of haloperidol, DTG, JO-1784 and (+)-pentazocine, after a 24 h washout, the potentiation of the NMDA response induced by the acute administration of the sigma 'agonists' was unchanged. 4. With the minipumps on board, with DTG and JO-1784, a dose-dependent enhancement of the NMDA response was seen but no effect was observed in the groups of rats treated at the same doses with haloperidol or (+)-pentazocine. 5. The present data suggest that long-term treatments with sigma 'antagonists' induce a desensitization of the th receptors, whereas long-term treatments with th 'agonists' induce a supersensitivity of the th receptors.

Relationship between beta adrenoceptor occupancy and receptor down-regulation induced by beta antagonists with intrinsic sympathomimetic activity

The relationship between the Ki of the B2 adrenoceptor and EC50 values characterizing receptor down-regulation induced by isoproterenol and six beta antagonists classified as having weak to strong intrinsic sympathomimetic activity (ISA) was determined using L6 myoblasts. It was hypothesized that if receptor loss induced by beta antagonists with ISA was mediated through cAMP, EC50 = Ki. EC50/Ki ratios for (-)isoproterenol, (-) and (+) celiprolol were 0.006, 0.01 and 0.08, respectively (p < 0.05); ratios for (-)pindolol and dilevalol were 19 and 9.5, respectively (p < 0.05). EC50/Ki ratios for acebutalol and (-)alprenolol were not significantly different from 1.0. Isoproterenol and dilevalol maximally down-regulated receptor density 89 and 83%, respectively, followed by (+)celiprolol, 54%; (-)celiprolol, 53%; acebutalol, 41%; (-)pindolol, 36% and (-)alprenolol, 31%. Receptor loss was blocked in each case by ICI118,551 or sotalol. A sensitive radioimmunoassay failed to detect increased cAMP accumulation following pretreatment with concentrations of acebutalol, (-)alprenolol, celiprolol and (-)pindolol 100 times their respective Ki values. Isoproterenol and dilevalol stimulated cAMP accumulation 100- and 2-fold over basal, respectively. We conclude that receptor loss induced by beta antagonists with ISA is mediated through the beta 2 adrenoceptor and in at least some cases is cAMP-independent.

beta-Adrenoceptor regulation in rat heart, lung and skin after chronic treatment with (--)-tertatolol or (--)-propranolol

1. The effect of long-term treatment with the beta-adrenoceptor antagonists (--)-tertatolol and (--)-propranolol was studied. Sprague-Dawley rats were treated with either (--)-tertatolol (50 micrograms kg-1 hr-1), (--)-propranolol (250 micrograms kg-1 hr-1) or vehicle (1 mM HCl) for 14 days with osmotic minipumps implanted subcutaneously. 2. The mean daily systolic blood pressure and heart rate of rats treated with either (--)-tertatolol (108 +/- 1 mmHg/330 +/- 3 bpm) or (--)-propranolol (103 +/- 1 mmHg/330 +/- 2 bpm) were lower than in the control (126 +/- 1 mmHg/405 +/- 3 bpm, P < 0.001, n = 8-10) indicating the effectiveness of drug delivery. 3. Autoradiographic studies in areas of heart, lung and skin showed that beta-adrenoceptor populations were not significantly affected by the drug treatment (all regions P > 0.05). Nevertheless, the receptor population in the homogenates of (--)-tertatolol treated lung were halved (194 +/- 28 fmol mg protein-1 compared with a control value of 388 +/- 54 fmol mg protein-1, P < 0.01, n = 6). 4. In the presence of CGP 20712A, the left atrial inotropic and right atrial chronotropic responsiveness to (--)-isoprenaline were hypersensitive in both (--)-tertatolol and (--)-propranolol-treated groups (P < 0.005, ANCOVA). 5. (--)-Propranolol treated left ventricular free wall had lower basal [3H]-forskolin binding to adenylate cyclase (14.45 +/- 1.20 fmol mg protein-1 compared with a control value of 18.91 +/- 0.78 fmol mg protein-1, P = 0.01, n = 6). (--)-Tertatolol treatment had no effect on the basal binding. In the presence of the G-protein activators NaF and Gpp(NH)p, the enhancement of [3H]-forskolin binding did not differ between control and the drug treated groups. 6. Chronic (--)-tertatolol or (--)-propranolol treatment therefore did not produce an increase in receptors in heart, lung or skin but the beta-adrenoceptor-mediated responses were enhanced. In addition, [3H]-forskolin binding did not increase suggesting that the hypersensitivity was not due to changes in the number of receptors or adenylate cyclase. Hypersensitivity following beta-adrenoreceptor antagonist administration may therefore involve enhanced coupling of receptors to G-proteins.

Propranolol therapy in experimental heart failure in rabbits improves cardiac response to catecholamines without beta-adrenoceptor up-regulation

Beta-blockade has been shown to improve cardiac response to catecholamines in heart failure but cellular mechanisms of the improvement are unknown. The effect on left ventricular function of a 14 day propranolol treatment was studied in seven treated and eight non-treated rabbits with experimental heart failure. All animals were subjected to a volume (aortic insufficiency) plus pressure (aortic constriction) overload and were instrumented with a left ventricular catheter and ultrasonic crystals measuring anteroposterior left ventricular diameter. Beta-adrenoceptors were measured using 125I-Cyanopindolol in crude membranes. With isoproterenol, the heart rate was slower in treated rabbits than in non-treated rabbits (p < 0.005) and isoproterenol increased more systolic diameter shortening in treated than in non-treated rabbits (p < 0.05). With norepinephrine, for matched pressures, % delta D increased in the treated group but it did not change in the non-treated group. This improvement of ventricular function was due, in a large part, to an increased diastolic response to norepinephrine: end-diastolic diameter increased in the treated group but not in the non-treated group. In contrast with the improved ventricular response to catecholamines, beta-adrenergic receptor density in the treated group was identical to that of the non-treated group (27.8 fmoles/mg/proteins) and was significantly lower than that of normal rabbits (58.2 fmoles/mg, p < 0.01). The improvement of ventricular response to catecholamines appears to be due to a myocardial protection by propranolol against the toxic effect of catecholamines in heart failure and not, at least in this model, to an up-regulation of beta-adrenoceptors.

Effect of captopril on lymphocytic beta-adrenergic receptors in normal and hypoxic conditions

BACKGROUND

During heart failure, due to increased level of circulating norepinephrine, the number of beta-adrenergic receptors (beta-AR), both at the cardiac and the lymphocytic level, is reduced (down-regulation). Captopril, an ACE-inhibitor containing an SH group appears capable of resetting beta-AR when used in patients with heart failure. Our study was aimed at checking whether captopril exerts a direct effect upon the beta-AR, possibly through its SH group by disulphur binding with cysteine residues located at the binding sites for catecholamines.

METHODS

The study was carried out in vitro on human lymphocytes obtained from healthy volunteers: 10 males (mean age, 34 years; range, 25-45) and 10 females (mean age, 34 years; range 26-48). Lymphocytes were randomly divided in two groups of equal size. Group I were controls; in Group II cells were incubated with three different doses of captopril: 1, 10, and 100 microM. Control lymphocytes and those treated with 10 microM of captopril were exposed to 1 microM isoproterenol. The number of total and surface beta-AR, and the sequestration of beta-AR from isoproterenol under normoxic conditions and after 20 h of hypoxia were checked. Furthermore, the content of cAMP was assayed both in basal conditions and after stimulation with 10 microM and 100 microM isoproterenol and forskolin, respectively.

RESULTS

Total beta-AR: 1082 +/- 133 (controls) vs. 1174 +/- 94 (treatment with 1 microM captopril), vs. 1237 +/- 88 (10 microM captopril), vs. 1092 +/- 105 (100 microM captopril). Surface beta-AR: 84 +/- 4.41% (controls) vs. 90.5 +/- 2.1% (10 microM captopril). Basal cAMP: 1.21 +/- 0.4 (controls) vs. 1.23 +/- 0.5 pmol/10 cells (1 microM captopril), 1.05 +/- 0.6 pmol/10 cells (10 microM captopril), 1.15 +/- 0.4 pmol/10 cells (100 microM captopril). After 10 microM isoproterenol: controls 4.10 +/- 0.8 vs. 4.30 +/- 0.9 pmol/10 cells (1 microM captopril), 4.15 +/- 0.7 pmol/10 cells (10 microM captopril), 3.50 +/- 1.0 pmol/10 cells (100 microM captopril). After 100 microM forskolin: controls 13.2 +/- 3.1 vs. 11.2 +/- 3.1 pmol/10 cells (1 microM captopril), 13.1 +/- 4.2 pmol/10 cells (10 microM captopril), 12.6 +/- 2.9 pmol/10 cells (100 microM captopril). Neither of these differences were significant. Lymphocytic beta-AR exposed to hypoxia did not show any significant difference. Exposure to captopril did not cause any further alteration on beta-AR sequestration.

CONCLUSIONS

Captopril does not seem to exert any direct action upon lymphocyte beta-AR from healthy volunteers. Moreover, captopril does not modify cAMP storage either in basal conditions or after stimulation with isoproterenol or forskolin. Therefore our data suggest that action of captopril on beta-AR is probably due to the inhibition of both systemic and tissue ACE-system.

Regulation of lens beta-adrenergic receptors by receptor occupancy and dexamethasone

Beta-adrenergic binding sites in primary cultures of chick lens annular pad (CLAP) cells were characterized with dihydroalprenolol (DHAP). Binding site affinities and densities were similar to beta-adrenergic receptors (BARs) previously characterized on crude membranes from freshly isolated cells. In competitive displacement studies, the beta-blocker propranolol was shown to increase the number of available binding sites in a concentration dependent manner. Acute exposure of CLAP cells to propranolol prior to DHAP binding also resulted in an increase in the number of available binding sites. Finally, lens beta-adrenergic binding site levels could be modulated by dexamethasone treatment. These results indicate that lens BARs are subject to common regulatory mechanisms and further implicate ophthalmic pharmaceuticals as possible cataractogenic agents.

Haloperidol treatment differentially regulates [3H]DTG and [3H](+)-3-PPP labeled sigma binding sites

The effect of repeated haloperidol administration on sigma binding sites in brain membranes was assessed using [3H](+)-3-(3-hydroxyphenyl)-N-(1- propyl)piperidine ((+)-3-PPP) and [3H]1,3-di-o-tolylguanidine (DTG). Administration of haloperidol (1 mg/kg, i.p.) to guinea pigs for 14 consecutive days followed by a 4 day drug-free period prior to sacrifice resulted in 75% and 6% decreases in the specific binding of [3H](+)-3-(3-hydroxyphenyl)-N-(1- propyl)piperidine and [3H]1,3-di-o-tolylguanidine, respectively, when measured using a single concentration (2 nM) of radioligand. Scatchard analysis revealed a reduction in both the maximum number of [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine binding sites and the affinity of these sites for the radioligand; the potency of 1,3-di-o-tolylguanidine to inhibit [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine binding was also reduced. In parallel studies, the potency of 1,3-di-o-tolylguanidine to inhibit [3H]1,3-di-o-tolylguanidine binding was unaffected by haloperidol treatment, but the potency of (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine against [3H]1,3-di-o-tolylguanidine was reduced 3-fold. Phenytoin, which increased (10-fold) the potency of dextromethorphan to inhibit [3H]1,3-di-o-tolylguanidine binding in control membranes, had no effect in membranes obtained from haloperidol-treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of propranolol-resistant (-)-[125I]-cyanopindolol binding sites in rat soleus muscle

1. The characteristics of a propranolol-resistant (-)-[125I]-cyanopindolol (CYP) binding site in rat soleus muscle were determined. 2. Saturation studies performed on homogenates of rat soleus muscle showed two phases of (-)-[125I]-CYP binding, a high affinity site (KD1 30.5 +/- 16.3 pM, Bmax 9.4 +/- 1.38 fmol mg-1 protein) and a lower affinity site (KD2 522.5 +/- 29.1 pM, Bmax 62.19 +/- 11.76 fmol mg-1 protein, n = 4). 3. In rat soleus muscle homogenates labelled with (-)-[125I]-CYP (500 pM), (-)-propranolol competition curves were biphasic with pKD values of 8.30 +/- 0.19, and 5.33 +/- 0.08, n = 7. 4. Competition between (-)-[125I]-CYP (500 pM) and (+/-)-tertatolol, (+/-)-nadolol, (+/-)-alprenolol, (+/-)-CYP, and (-) and (+)-pindolol showed that these compounds competed for binding at the propranolol-resistant site with affinities lower than those displayed at typical beta-adrenoceptors. The atypical beta-adrenoceptor agonists BRL 37344, SR58611A and ICI D7114 and the partial agonist (+/-)-CGP 12177 also competed for (-)-[125I]-CYP binding. 5. Stereoselectivity was demonstrated for the stereoisomers of alprenolol and tertalolol. The (-)-isomers of alprenolol and tertalolol had higher affinity than their corresponding (+)-isomers (3.1 and 2.6 fold respectively). These low stereoselectivity values are a characteristic of atypical beta-adrenoceptors. 6. The beta-adrenoceptor agonists, (-)-adrenaline, (-)-isoprenaline and (-)-noradrenaline, all showed lower affinity than the atypical beta-adrenoceptor agonists and competition curves appeared biphasic in nature. 7. These results confirm the presence of a propranolol-resistant (- )-[125I]-CYP binding site in rat soleus muscle. The affinities of the tested compounds at the propranolol-resistant (- )-[125I]-CYP binding site show similarities to their affinities at 'atypical' beta-adrenoceptors in adipocytes and gastrointestinal tissues and at the cloned beta 3-adrenoceptor.

Interaction of tertatolol at the “atypical” or beta3-adrenoceptor in guinea-pig ileum

1. Experiments were performed to investigate the possible interaction between tertatolol and the "atypical" beta-adrenoceptor present in guinea-pig ileum. 2. In the electrically field stimulated guinea-pig ileum, both isoprenaline and BRL-37344 produce concentration-dependent inhibition of the "twitch" response. 3. Schild regression analysis of the interaction between isoprenaline and tertatolol indicated two sites of action and yielded an apparent pA2 value for tertatolol of 6.8 at the "atypical" beta-adrenoceptor. 4. BRL-37344 selectively activated the "atypical" beta-adrenoceptor and tertatolol again yielded an apparent pA2 value of 6.7 and 6.8.

Localization of (-)-[125I]cyanopindolol binding in guinea-pig heart: characteristics of non-beta-adrenoceptor related binding in cardiac pacemaker and conducting regions

Receptor autoradiography was used in guinea-pig heart to locate binding sites for the beta-adrenoceptor ligand (-)[125I]cyanopindolol (CYP) resistant to blockade by the beta-adrenoceptor antagonist (-)-propranolol (1 microM). Highly localized binding was observed to regions closely associated with the sinoatrial node, atrioventricular node and bundle of His but was not observed on myocardial, pacemaker, conducting cells or adipose tissue. Free [125I] also bound to identical sites. Binding was enhanced in the presence of ascorbic acid but was completely inhibited by (-)-isoprenaline (100 microM), serotonin (5-HT) (10 microM) and phentolamine (10 microM).

Localization and characterization of two propranolol resistant (-) [125I]cyanopindolol binding sites in rat skeletal muscle

Autoradiographic studies were performed in sections of rat gastrocnemius, plantaris and soleus muscle bundles with (-)-[125I]cyanopindolol (59-69 pM) in the presence of (-)-propranolol (1 microM) to block beta 1- and beta 2-adrenoceptors. Two distinct populations of binding sites remained, one evenly distributed over the muscle bundles and the other localized in discrete patches. Evenly distributed binding was highest in the soleus muscle and inhibited by (+/-)-, (-)- and (+)-alprenolol (20 microM), tertatolol (1 microM), BRL 37344 (2-20 microM), (-)-isoprenaline (100 microM), phentolamine (10 microM) and haloperidol (250 microM) but not ICI 118,551 (70 nM), CGP 20712A (100 nM), (+)-isoprenaline (100 microM), pindolol (2 microM), cimaterol (100 microM) or serotonin (10 microM). Stereoselectivity for the optical isomers of alprenolol was displayed in the soleus muscle only. Highly localized binding was inhibited by serotonin (10 microM), (-)- and (+)-isoprenaline (100 microM) and phentolamine (10 microM).

Alterations in beta adrenergic and muscarinic receptors in aged rat heart. Effects of chronic administration of propranolol and atropine

The cardiac responses to sympathetic and vagal stimulations are attenuated with ageing. To understand these findings, the densities of beta adrenergic (beta R) and muscarinic (MR) receptors in the left ventricles have been quantitated in parallel in male Wistar rats (4- and 24-month-old) using [125I]iodocyanopindolol and [3H]quinuclidinyl benzilate as specific radioligands. The homologous regulation of these receptor densities was also explored after a 7-day continuous infusion of propranolol or atropine. As compared to young rats, the beta R and MR densities in aged animals were decreased (from 31 +/- 2 to 23 +/- 2 fmol/mg protein, P less than 0.05 for beta R; from 104 +/- 7 to 54 +/- 3 fmol/mg protein, P less than 0.001 for MR) but the diminution in MR was more pronounced (-48%) than that in beta R (-26%), resulting in a drop in the beta R/MR ratio. Continuous infusion of propranolol or atropine up-regulated the beta R and MR densities (respectively +50%, P less than 0.01 and +33%, P less than 0.05) in aged but not in young adult rats. We therefore conclude: (i) that the diminution of the cardiac response to the sympathetic and vagal stimulations during ageing may be partly explained by a decrease in the corresponding receptor density; (ii) these changes are reversible and the density of these two groups of receptors can return to adult control values by chronic administration of the appropriate antagonist.

Newer beta blockers and the treatment of hypertension

Beta-adrenoceptor blocking agents are established as one of the principal classes of antihypertensive agents. Despite progressive refinements over the years, they still possess some unwanted effects, which limit their considerable value. In recent years a wide range of variations upon the beta-blocker theme has been developed. The full clinical advantages of the newer agents remain to be defined.

Effects of prolonged infusion of dopexamine on beta 1- and beta 2-adrenoceptors in guinea-pig myocardium

1. The effects of 7-day infusion of dopexamine (50 and 200 micrograms kg-1h-1) were examined on beta 1- and beta 2-adrenoceptors in guinea-pig left ventricular membranes. 2. Receptor binding performed using the high affinity radioligand (-)-[125I]-cyanopindolol (CYP) and the beta 1-adrenoceptor antagonist CGP 20712A showed that treatment with dopexamine 200 micrograms kg-1h-1 caused a 45% reduction in beta 2-adrenoceptors and a small but not significant increase in beta 1-adrenoceptors. 3. Functional effects of dopexamine were examined in the guinea-pig isolated electrically driven left atria and K(+)-depolarized uterus. Dopexamine was an antagonist at beta 1-adrenoceptors in left atria (pKB = 4.49), and a partial agonist at beta 2-adrenoceptors in the uterus (alpha = 0.78, pD2 = 6.59). 4. The effects of dopexamine on beta 1- and beta 2-adrenoceptor density in guinea-pig ventricular myocardial membranes may be related to agonist activity at beta 2-adrenoceptors and antagonist activity at beta 1-adrenoceptors.

Intrinsic sympathomimetic activity of beta-adrenoceptor antagonists: down-regulation of cardiac beta 1- and beta 2-adrenoceptors

Prolonged treatment of cultured rat heart muscle cells containing beta 1- and non-muscle cells containing beta 2-adrenoceptors with beta-adrenoceptor antagonists devoid of intrinsic sympathomimetic activity had no effect on beta-adrenoceptor density. In contrast, antagonists with intrinsic sympathomimetic activity decreased beta-adrenoceptor density and response (adenylate cyclase stimulation) in both heart muscle (beta 1) and non-muscle cells (beta 2) by a maximum of about 50%. An even larger down-regulation of beta-adrenoceptors and loss of receptor-stimulated adenylate cyclase activity was induced by the full endogenous agonist, noradrenaline, with the beta-adrenoceptors of heart muscle cells (beta 1) being much more sensitive to the beta 1-selective noradrenaline than the heart non-muscle cell beta 2-adrenoceptors. When combined with noradrenaline, the antagonists with intrinsic sympathomimetic activity prevented the action of noradrenaline at both beta 1- and beta 2-adrenoceptors, thereby leading to an apparent up-regulation of receptor density and response. This apparent reversal from an agonist to an antagonist action was observed at much lower concentrations of noradrenaline at beta 1- than at beta 2-adrenoceptors. The data presented indicate that the beta-adrenoceptor antagonists with intrinsic sympathomimetic activity, but not those without, upon prolonged treatment decrease the density and responsiveness of both beta 1- and beta 2-adrenoceptors in cultured rat heart cells. This suggests that the intrinsic sympathomimetic activity of these agents is not a subtype-selective component. Furthermore, the agonist and antagonist activity of these agents apparently depends on the concomitant presence of an endogenous full agonist and an its own affinity and that of the partial agonist for the beta-adrenoceptor subtype.

Short-term delay in blood processing does not alter basal or isoproterenol-stimulated cyclic AMP accumulation in human lymphocytes

Isoproterenol-stimulated cyclic AMP (cAMP) accumulation in lymphocytes is an index of beta-adrenergic receptor functioning. It is not known if a delay between blood collection and subsequent cell preparation influences cAMP accumulation. We compared cAMP levels in lymphocytes processed from whole blood immediately after collection and following a 3.5 hour delay. There were no significant changes in either basal or stimulated levels of cAMP. This information may alleviate time constraints in certain experimental designs where immediate isolation of lymphocytes from whole blood may not be practical.