Cardiostimulatory effects of prenalterol, a beta-1 adrenoceptor partial agonist, in vivo and in vitro. Correlation between physiological effects and adenylate cyclase activity

Linking Aromatic Hydroxy Metabolic Functionalization of Drug Molecules to Structure and Pharmacologic Activity

Drug functionalization through the formation of hydrophilic groups is the norm in the phase I metabolism of drugs for the modification of drug action. The reactions involved are mainly oxidative, catalyzed mostly by cytochrome P450 (CYP) isoenzymes. The benzene ring, whether phenyl or fused with other rings, is the most common hydrophobic pharmacophoric moiety in drug molecules. On the other hand, the alkoxy group (mainly methoxy) bonded to the benzene ring assumes an important and sometimes essential pharmacophoric status in some drug classes. Upon metabolic oxidation, both moieties, i.e., the benzene ring and the alkoxy group, produce hydroxy groups; the products are arenolic in nature. Through a pharmacokinetic effect, the hydroxy group enhances the water solubility and elimination of the metabolite with the consequent termination of drug action. However, through hydrogen bonding, the hydroxy group may modify the pharmacodynamics of the interaction of the metabolite with the site of parent drug action (i.e., the receptor). Accordingly, the expected pharmacologic outcome will be enhancement, retention, attenuation, or loss of activity of the metabolite relative to the parent drug. All the above issues are presented and discussed in this review using selected members of different classes of drugs with inferences regarding mechanisms, drug design, and drug development.

The frequency of occurrence of autoantibodies against beta1-adrenoceptors and its clinical relevance in patients with hepatitis virus myocarditis

The aim of this study was to examine the frequency of occurrence of autoantibodies against beta1-adrenoceptors in patients with hepatitis virus myocarditis (HVM) and its possible correlation with clinical characteristics. A total of 103 patients with viral myocarditis were divided into a positive group (HVM group, n=29) and a negative group (Non-HVM group, n=74) according to the laboratory findings regarding their type of hepatitis virus. The study parameters included UCG, ECG, biochemical findings and screening of autoantibodies against beta1-adrenoceptor. It was shown that the positive rate of the hepatitis virus was 28.16% (29/103) in patients with viral myocarditis. The severity of myocardial or liver injuries and the frequency of occurrence of autoantibodies against beta1-adrenoceptors in patients with viral myocarditis were more pronounced, before treatment, in the HVM group than in the Non-HVM group. The positive rates of the antibodies against the hepatitis virus and the autoantibodies against beta1-adrenoceptors were highly consistent in patients with HVM (p<0.05). In conclusion, the frequency of occurrence of the autoantibodies against beta1-adrenoceptors may be one important marker of HVM and, thus, possibly involved in the pathogenesis of the HVM.

Modulation of L-type Ca current by denopamine, a nonparenteral partial beta 1 stimulant, in rabbit ventricular cells

The effects of denopamine, a nonparenteral partial beta agonist which is used clinically in Japan, on the L-type Ca2+ current (ICa) were examined in rabbit ventricular cells. Denopamine stimulated basal ICa with a maximum response of +33.2% and a concentration for half-maximal response (EC50) of 0.039 microM. The maximum response of ICa was only a quarter of that induced by isoprenaline (ISO), while 10 microM denopamine elicited 70-75% of the maximum inotropic response in the papillary muscle preparations. The denopamine stimulation of ICa was abolished by selective beta 1 antagonists (atenolol or bisoprolol). Pretreatment with forskolin or dialysis with cAMP also abolished the stimulation. Denopamine, in turn, inhibited ISO-stimulated ICa. This inhibition was not affected by pretreatment with pertussis toxin or prazosin. The presence of denopamine at various concentrations caused a rightward shift in the concentration/response curve for ISO stimulation of ICa. The Schild plot for this effect had a slope of 0.99 and Kp of 0.20 microM. In the presence of guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) (0.5 mM) in the pipette, denopamine (10 microM) stimulated the ICa to 86 +/- 5% of the maximum response induced by ISO. These findings indicate that denopamine modulates ICa exclusively through the beta 1 adrenoceptor-adenylate cyclase pathway, that the stimulatory GTP-binding protein regulates the agonistic potency of denopamine, and that the signal from the beta 1 adrenoceptors is amplified between ICa and the tension development, which would contribute to the spare capacity of beta adrenoceptors.

Autoimmunity in idiopathic dilated cardiomyopathy. Characterization of antibodies against the beta 1-adrenoceptor with positive chronotropic effect

BACKGROUND

Autoantibodies against the beta 1-adrenoceptor have been detected in the sera of patients with idiopathic dilated cardiomyopathy (DCM). The mechanisms by which these autoantibodies can alter normal receptor function are investigated, and the results are interpreted in the light of the beneficial effects of beta 1-blockade in some of these patients.

METHODS AND RESULTS

Autoantibodies against the beta 1-adrenoceptor, affinity purified from sera of patients with idiopathic DCM, were analyzed in a functional test system of spontaneously beating neonatal rat heart myocytes. Antibodies from rabbits immunized with peptides derived from the amino acid sequence of this receptor were also analyzed. Autoantibodies, against the second extracellular loop increased the beating frequency of isolated myocytes in a concentration-dependent manner, to approximately 80% of maximal isoproterenol stimulation. Rabbit anti-peptide antibodies against the second extracellular loop increased the beating frequency correspondingly. Autoantibodies and rabbit anti-peptide antibodies against the second extracellular loop were able to immunoprecipitate the unliganded receptor but not the antagonist-occupied receptor. In contrast, rabbit antibodies against the extracellular N-terminal sequence 34-57 of the beta 1-adrenoceptor were able to immunoprecipitate both the unliganded and the antagonist-occupied receptor although with no effect on the beating frequency of myocytes. The positive chronotropic effect of the antibodies was completely neutralized both by the addition of increasing concentrations of the beta 1-selective antagonist bisoprolol and by preincubation with the peptide corresponding to the second extracellular loop. The antibody-induced increase in beating frequency remained unchanged for more than 6 hours. This should be compared with the isoproterenol-stimulated beating frequency, which undergoes desensitization within 60 minutes. Addition of isoproterenol to autoantibody-stimulated myocytes resulted in only a small increase in beating frequency and did not cause desensitization. Antibodies had only a marginal effect on cyclic AMP production of stimulated cardiomyocytes compared with the 10-fold increase obtained after stimulation with isoproterenol.

CONCLUSIONS

The second extracellular loop of the beta 1-adrenoceptor is a specific target for antibodies with stimulatory activity detected in patients with idiopathic DCM. The antibodies have a positive chronotropic effect on isolated rat heart myocytes. Autoantibody stimulation does not cause the normal agonist-induced desensitization phenomena of the effector system. These findings could contribute to our understanding of the pathophysiological mechanisms of the autoantibodies and of the beneficial effect of beta 1-blocking agents in the treatment of patients with idiopathic DCM.

The inhibitory GTP-binding protein (Gi) regulates the agonistic property of beta-adrenergic ligands in isolated rat adipocytes. Evidence for a priming effect of cyclic AMP

Prenalterol, an allegedly beta 1-selective adrenergic agonist with high intrinsic sympathomimetic activity (ISA), was shown to be weakly lipolytic in rat adipocytes. However, in pertussis-toxin-treated adipocytes, the ISA of prenalterol was markedly increased (from 10-20% to approx. 100% of that of isoprenaline). The cellular sensitivity was also increased (EC50 approx. 60 nM and approx. 3 microM in pertussis-toxin-treated and control cells respectively). A similar effect was seen for other partial agonists such as the beta 2-selective agonist terbutaline and for beta-adrenergic antagonists with some intrinsic activity (metoprolol, pindolol). There was no clear change in sensitivity to isoprenaline's ability to stimulate adenylate cyclase in adipocyte membranes from pertussis-toxin-treated animals but the cyclase activity was increased approx. 4-fold in the presence of 1 microM-GTP. Prenalterol stimulated lipolysis by only small increases in intracellular cyclic AMP (cAMP) levels (less than 10% of that seen with isoprenaline). Basal lipolysis was increased in cells from pertussis-toxin-treated rats and the cellular sensitivity to the non-degradable cAMP analogue, N6-monobutyryl-cAMP, was increased. In control cells, a submaximal concentration of prenalterol (0.1 microM) increased the sensitivity to the cAMP analogues, N6-monobutyryl-cAMP and 8-bromo-cAMP. A low concentration (1 mM) of 8-bromo-cAMP also increased the effect of prenalterol. Similar effects were seen when the phosphodiesterase was inhibited. Thus (1) lipolysis is extremely sensitive to small increases in intracellular cAMP; (2) the degree of activation of adenylate cyclase and thus cAMP formation is the rate-limiting step for the biological response of partial agonists; (3) the inhibitory GTP-binding protein, Gi, is an important modulator ('tissue factor') of the beta-adrenergic agonistic property; (4) low levels of cAMP exert a priming effect on protein kinase A.

Effects of prenalterol on beta adrenergic responsiveness and receptors in the cerebral cortex of the rat

The activity of the beta-adrenoceptor agonist, prenalterol, at beta adrenoceptors in the cerebral cortex of the rat and the effect of chronic intraperitoneal infusion of prenalterol on the biochemical responsiveness and density of cerebral cortical beta adrenoceptors was studied. Whereas isoproterenol caused a four-fold rise in the content of cyclic AMP in slices of cerebral cortex, prenalterol did not produce a significant increase in cyclic AMP. However, prenalterol inhibited the isoproterenol-stimulated increase in cyclic AMP in cortical slices in a concentration-dependent manner. Using an in vivo binding technique, prenalterol (3.8 mg/kg/hr) infused intraperitoneally through osmotic minipumps, penetrated the brain and significantly inhibited the binding of the beta adrenoceptor antagonist, [125I]iodopindolol (125I-IPIN), to cortical beta adrenoceptors. Infusion of prenalterol (3.8 mg/kg/hr) for 7 days resulted in a small (20%), but significant, reduction in the ability of isoproterenol to stimulate maximally the accumulation of cyclic AMP in slices of cerebral cortex. No alteration in the Bmax or KD of the binding of [125I]iodopindolol was observed in homogenates of cortex obtained from prenalterol-treated rats. Furthermore, no change was observed in the binding of the hydrophilic ligand [3H]CGP-12177 in homogenates of cortex. In this study, then, prenalterol exhibited properties in vitro of a beta adrenoceptor antagonist, but did cause modest desensitization of beta adrenoceptor responsiveness when administered continuously in vivo.

Effect of denopamine on the phosphorylation of cardiac muscle proteins in the perfused guinea-pig heart. Comparison with isoproterenol

Effects of the new selectively beta 1-adrenergic cardiotonic drug denopamine (TA-064) on the phosphorylation of cardiac muscle proteins in the perfused guinea-pig heart were investigated in comparison with isoproterenol. Denopamine at 3 X 10(-6) M and isoproterenol at 10(-7) M were equipotent in their effects on the contractile force and + (dF/dt). Under these conditions, the increases in heart rate and tissue c-AMP levels by denopamine were significantly less than those by isoproterenol. Isoproterenol exerted a greater effect on -(dF/dt) than on +(dF/dt), whereas denopamine influenced both to the same extent. Denopamine (3 X 10(-6) M) and isoproterenol (10(-7) M) both stimulated 32P incorporation into the proteins of molecular weights of 150,000, 30,000, 19,000, 15,000 and 11,000 daltons. Among these proteins, the 30,000 and 11,000 dalton proteins, probably troponin-I and phospholamban, were phosphorylated to significantly lesser extents by denopamine than by isoproterenol. The above differences in the effects on c-AMP levels and protein phosphorylation between denopamine and isoproterenol may be causally related to the differences in their pharmacological properties such as the weaker arrhythmogenicity and comparatively less marked relaxation effect of denopamine compared with isoproterenol in the presence of similar cardiotonic effects.

Relative potencies of various beta-adrenoceptor antagonists (BAA) at the level of the human myocardial beta-adrenoceptor-adenylate cyclase (AC) complex. Is intrinsic sympathomimetic activity (ISA) due to AC activation?

Nine different beta-adrenoceptor antagonists (BAA), five with intrinsic sympathomimetic activity (ISA), were examined for their ability to inhibit isoproterenol-stimulated adenylate cyclase (AC) activity and specific 125I-cyanopindolol (CYP) binding in crude membrane particles from human myocardium. The BAA's were: propranolol, pindolol, timolol, alprenolol, metoprolol, atenolol, prenalterol, ICI 141.292 'Visacor', and ICI 118.587 'Corwin'. Whether BAAs with strong ISA were able to stimulate AC activity by themselves were examined in separate experiments and compared to the AC stimulation by full agonists. All the BAAs caused a concentration dependent, and at high doses apparently complete, inhibition of both isoproterenol-stimulated AC activity and 125I-CYP binding. Both assays made possible a 'potency-ranking' of the different BAAs (pindolol greater than or equal to propranolol and timolol greater than ICI 142.292 and alprenolol greater than ICI 118.587, prenalterol and metoprolol greater than atenolol). Corrected IC50-values, derived from inhibition curves with both techniques, show that receptor binding and inhibition of receptor function follow each other closely. Prenalterol caused a very weak AC activation (5.4% of maximum), whereas the 'ISA-blockers', pindolol, ICI 141.292, and ICI 118.587 were unable to stimulate AC activity at concentrations which completely displaced 125I-CYP binding. In comparison, norepinephrine stimulated AC activity to the same level as isoproterenol (three to four times basal activity) and the beta 2-selective agonist terbutaline caused some 50% of maximal AC stimulation. This raises the question whether ISA is due to AC activation. The effect upon AC activation and 125I-CYP binding of drugs with beta-selectivity shows that both beta 1- and beta 2-receptors are coupled to the AC.

The quantification of relative efficacy of agonists

While much pharmacological effort has been expended in the measurement, quantification, and comparison of agonist affinity for the classification of drugs and drug receptors, inordinately less emphasis has been placed on the quantification of the other property of agonists, namely intrinsic efficacy. This is unfortunate as the existing studies of the relative intrinsic efficacy of agonists show this to be a most useful scale for the classification of agonists and the prediction of tissue responses. This paper will review some of the theories that describe efficacy on a molecular level, the methods of measuring relative efficacy, and the factors in these procedures which can lead to artifacts and misleading information for the classification of drug receptors. Lastly, the value of the quantification of efficacy will be discussed in terms of the design of agonists for therapeutic advantage.

Beta adrenergic influence on esophageal and colonic motility in man

Gastrointestinal (GI) motility is centrally controlled through the sympathetic and parasympathetic nerves, sympathetic effects being partly mediated by beta adrenoceptors. Although beta adrenoceptor agonists and antagonists are widely used for different disorders, little is known about the influence of these agents on GI motility. The present study was initiated to investigate whether there is a physiological, beta adrenergic influence on human GI motility and to describe the effects of selective beta adrenoceptor stimulation on motility in the proximal and distal parts of the GI tract. Esophageal peristalsis was measured in healthy subjects using electronic catheters. Distal colonic motility was measured with an open-tipped, water-perfused catheter in the sigmoid colon and from an air-filled balloon in the rectum in healthy subjects and in patients with the irritable bowel syndrome (IBS). In one study, colonic motility was stimulated with continuous infusion of the octapeptide of cholecystokinin (CCK-OP). Esophagus: Peristaltic amplitude was increased in the distal smooth muscle part of the esophageal body after infusion of both the nonselective beta blocker propranolol and the beta-1 selective blocker metoprolol. After infusion of the beta-1 agonist prenalterol and the beta-2 selective agonist terbutaline, a profound decrease in esophageal peristaltic amplitude was seen. Pretreatment with metoprolol selectively blocked the response to a moderate dose of prenalterol but did not block the response to terbutaline. The latter response was blocked by propranolol. Peristaltic velocity in the proximal part of the esophagus was decreased by beta-1 stimulation and in the distal part by beta-2 stimulation. Distal colon: In healthy subjects the sigmoid motility index showed a dose-dependent increase after metoprolol and propranolol, respectively. The increase was more marked after propranolol infusion. Terbutaline decreased the sigmoid motility index both in healthy subjects and in patients with the IBS. Furthermore, the rectal motility index was decreased in the group of healthy subjects. The effects of prenalterol on rectal and sigmoid motility did not differ from those of placebo. The IBS patient group showed larger intraindividual variations in sigmoid motility from day to day and also lower rectal motility indices than the healthy subjects. Infusion of CCK-OP increased the sigmoid motility index compared to non-stimulated conditions. No effects on CCK-OP stimulated motility were seen after either terbutaline, prenalterol or placebo.(ABSTRACT TRUNCATED AT 400 WORDS)

Inotropic drugs and their mechanisms of action

This report describes various old and new positive inotropic drugs with respect to their mechanisms of action. Drugs with established cardiotonic effects include cardiac glycosides, beta 1-adrenergic agents, glucagon, histamine and the methylxanthines. New agents discussed are prenalterol, beta 2- and alpha-adrenergic drugs, amrinone and sulmazole. Prenalterol is a beta 1-adrenergic agent. Beta 2-adrenergic drugs, amrinone and sulmazole, combine a positive inotropic and a vasodilator effect. The latter resemble theophylline and other methylxanthines in that they appear to act mainly as phosphodiesterase inhibitors with a subsequent increase in cyclic adenosine monophosphate (cAMP). The mechanism of the positive inotropic effect of alpha-adrenergic stimulating agents (for example, phenylephrine) is unknown. It is independent of the cAMP system and is not accompanied by changes in frequency.

Effects of in vivo treatment with isoprenaline or prenalterol on beta-adrenoceptor mechanisms in the heart and soleus muscle of the cat

The full agonist isoprenaline (5.3-6.6 nmol/kg . min) and the partial beta-adrenoceptor agonist prenalterol (10.6-13.3 nmol/kg . min) were administered to cats continuously via osmotic minipumps (i.p.). After seven days the functional and adenylate cyclase responsiveness to the agonists, as well as the beta-adrenoceptor-binding characteristics, were studied in cardiac and soleus muscle preparations in vitro. After isoprenaline pretreatment, the papillary muscles and soleus muscle strips wer 15-18 times less sensitive to isoprenaline compared with muscles from control cats. The stimulatory potency (pD2) of prenalterol in the papillary muscle was not changed significantly. The affinity of the agonists to the beta-adrenoceptors was unaffected in both tissues by the pretreatment, but the densities of beta-adrenoceptors were significantly reduced, by 36% (myocardium) and 47% (soleus) respectively. In the cat papillary muscle the intrinsic sympathomimetic activity (ISA) of prenalterol on contractile parameters was reduced from 84 (Tmax), 69 (dT/dtmax) and 71% (dT/ dtmin ) in control animals, to 33, 22 and 28%, respectively in the animals pretreated with isoprenaline. Prenalterol pretreatment did not induce any marked changes, either in the stimulatory potency or affinity of the agonists in the two tissues or in the maximal response (ISA) of prenalterol in the papillary muscle. The marked reduction in the stimulatory potency of isoprenaline and the reduced ISA of prenalterol in the myocardium after isoprenaline pretreatment can not be explained by the reduction in beta- adrenoceptor density alone. Since the affinity to the beta- adrenoceptors is unaffected, a reduced efficiency in the signal transmission must be the main cause.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of long-term beta receptor stimulation with prenalterol on intrinsic heart rate in rats

Previous studies have shown that the intrinsic heart rate (IHR) may undergo changes, e.g., decrease after long-term endurance training. The mechanism for this adaptation is not known. In this study, rats were subjected to long-term oral treatment with the beta receptor stimulating drug prenalterol. During the treatment period heart rates at rest and during submaximal exercise were measured. Heart rate after 30 min rest and also 2 min after exercise was higher in the treated animals, due to the beta stimulation. The treated rats had a significantly lower heart rate increase during exercise than untreated controls, consistent with a partial beta-blocking effect of the drug in states with a high endogenous sympathetic activity. Therefore, the animals were not trained but only exposed to the increased stimulation of cardiac beta receptors accomplished by the drug while at rest. After 25 weeks, prenalterol was withdrawn and the IHR was measured in situ after a denervation procedure. The treatment with prenalterol had not altered the IHR. Our previous results from training studies indicate that a heart rate increase above a certain level or the stimulation for a lower setting of the IHR as seen after endurance training. In this study chronic beta receptor stimulation with prenalterol did not influence the IHR, which supports that hypothesis.

Lack of functional influence by prenalterol through alpha 1-adrenoceptors in rat myocardium

Some of the cardiac properties of the partial beta-agonist prenalterol may indicate a contribution from alpha 1-adrenergic stimulation. We therefore studied whether prenalterol interacted with alpha 1-adrenoceptors in rat myocardium. Combination with propranolol did not reveal an alpha 1-adrenergic inotropic effect of prenalterol in papillary muscles. Neither did prenalterol block the alpha 1-adrenergic response to phenylephrine. In myocardial cells, prenalterol inhibited 3H-prazosin binding to alpha 1-adrenoceptors only at very high concentrations. Prenalterol thus exhibited no functionally important interactions with alpha 1-adrenoceptors in myocardium, and its properties cannot be accounted for in terms of contribution from alpha 1-adrenergic effects.

Comparison of the chronotropic effect and the cyclic AMP accumulation induced by beta 2-agonists in rat heart cell culture

1 The chronotropic response and the variation in cyclic adenosine 3',5'-monophosphate (cyclic AMP) accumulation induced by isoprenaline and six beta 2-selective agonists (fenoterol, salmefamol, soterenol, zinterol, salbutamol and formoterol) were analyzed on cultured heart cells of the rat. 2 The compounds elicited an enhancement of the frequency, but the time course of the variation of the beating rate was not identical for all of them. A rapid onset was observed for isoprenaline, zinterol and formoterol while it was slower for fenoterol, salmefamol and salbutamol. 3 In contrast with isoprenaline, the beta 2-selective agonists gave concentration-beating frequency curves which were not sigmoidal. Their effects extended up to a concentration of 5 to 6 orders of magnitude. Nevertheless, the concentration at which the maximal effect occurred and the intrinsic activities of the various compounds agrees better with the responses observed on guinea-pig atria than with those on trachea. 4 All the beta 2-selective agonists increased the accumulation of cyclic AMP in rat heart cells with a maximal effect at 10(-5)M or less. The effects of beta 2-agonists on cyclic AMP production showed some analogies with those on beating frequency of the heart cells. The increase in cyclic AMP accumulation induced by beta 2-agonists also corresponded to their chronotropic effects on guinea-pig atria. Thus, the correlation coefficient between the inverse of the log of the concentration producing the half maximal cyclic AMP accumulation in cultured heart cells and the pD2 values on guinea-pig atria was 0.93. 5 It is concluded that, in contrast to what was observed in other models, the beta 2-selective agonists induce an increase in the production of cyclic AMP in rat heart cells. Furthermore, the effects of the beta 2-agonists on cyclic AMP accumulation and on beating rate in the heart cells may correspond with their beta 1-adrenoceptor potencies.

beta 1-and beta 2-adrenoceptor stimulatory effects of prenalterol

Prenalterol, previously characterized as a functionally cardioselective partial beta-adrenoceptor agonist, was shown to relax K+ -elicited contractures in the uterine muscle from progesterone pretreated rats (pD2 7.7) and to increase beating rate in the rat right atrium (pD2 8.0) at about the same concentrations with maximal effects corresponding to 94 and 82% respectively of those of isoproterenol. Terbutaline, with equal maximal effects as isoproterenol, was 50 times more potent in the uterus (pD2 7.8) than in the right atrium (pD2 6.1). Both tissues displayed a high sensitivity to isoproterenol (pD2 9.1 in both tissues) indicating large receptor reserves for the full agonist. The maximal relaxing effect of prenalterol in the uterus was obtained at about a three-fold increase of the cyclic AMP content, which is similar to that obtained with isoproterenol at a corresponding relaxation. The effects in the uterine muscle of all three agonists were mediated through beta 2-adrenoceptors since beta 2-adrenoceptor blockers (ICI 118, 551 and IPS 339) antagonized the effects in concentrations which had only marginal effects on the atrial responses of the agonists. The beta 1-antagonists pafenolol and pamatolol in concentrations higher than those, which blocked the effects of the agonists on beating rate, were devoid of inhibitory effects in the uterus. These results indicate that prenalterol possesses the ability to elicit a functional response by stimulation of either beta 1-or beta 2-adrenoceptors provided that the tissue has a large spare receptor reserve for full agonists.