Physiology and Pharmacology of BetiJ2 Adrenergic Agents

Transcription

1 Physiology and Pharmacology of BetiJ2 Adrenergic Agents Charles E. Reed, M.D., F.C.C.P. was the first truly effective drug available for the relief of asthma. Now, 60 years after its introduction, we are devoting a three-day symposium to a discussion of derivatives of epinephrine that have substantial practical advantages over the natural hormone. In this introduction it is my task to summarize our current understanding of the synthesis, release, and metabolism of catecholamines; how they act on vari OUS cells; and how chemical modification of the epinephrine molecule has led to new drugs that last longer, are effective by mouth, and act more selectivelyon the bronchi. SYNTHESIS, STORAGE, UPTAKE AND METABOLISM OF CATECHOLAMINES The three naturally occurring catecholamines, dopamine, norepinephrine, and epinephrine, are synthesized by decarboxylation and hydroxylation of tyrosine in specialized neuroendocrine cells widely distributed in the body and certain sites in the central nervous system. The sequence of reactions is: tyro sme OH dihydroxyphenylal amne. -COOH ~ dopamine~ norepinephrine~ epinephrine. Dopamine is an important neurotransmitter in the basal ganglia and in Peripheral autonomic ganglia. Norepinephrine is the neurotransmitter of the sympathetic nervous system. The enzyme for the synthesis of epinephrine from norepinephrine exists in man only in the adrenal medulla. Full activity of this enzyme, interestingly enough, depends upon the high concentrations of cortisol supplied by the portal venous system that drains from the adrenal cortex. After synthesis catecholamines are stored in vesicles located at the synaptic end of the neurons of adrenergic nerves or in the adrenal medulla. These vesicles can be readily identified histochemically by fluorescent techniques, so adrenergic innervation can be accurately mapped. Human bronchial smooth muscle does not appear to have sympathetic innervation, so that if the prominent beta receptors Supported by grant No. 2-Pl5-AI-I0404. Reprint requests: Dr. Reed, 504 North Walnut, Madison CHARLES Eo REED in human bronchi play any role in normal physiology their stimulation is the result of circulating catecholamines rather than sympathetic nerves. The impulse travelling down the post-ganglionic sympathetic nerve releases the catecholamine from these storage vesicles via a cholinergic mechanism that is blocked by drugs such as guanethidine. After release, catecholamines can enter into any of three reactions. They can react with the adrenergic receptor on target cells. They can re-enter the storage vesicles (uptake 1). Or, they can enter cells and be metabolized by catecholamine o-methyl transferase to an inactive compound (uptake 2). Most of the catecholamines, whether released locally from neurons or into the circulation from the adrenal medulla, are very rapidly cleared by the uptake 1 process. The uptake process is the main reason for the brevity of action of the catecholamines. This process is blocked by cocaine. After methylation at the 3-hydroxy position by catecholamine o-methyl transferase, these compounds are further oxidized by monoamine oxidase, an enzyme inhibited by some antidepressive drugs such as tranylcypromine (Parnate). This pathway accounts for only a fraction of elimination. Another important pathway of catecholamine metabolism, especially active in the gut and liver, is conjugation with sulfate also at the 3-OH position. The inactivity of this conjugate accounts for the fact that catecholamines are ineffective by the oral route. The synthesis of sympathomimetics with a prolonged action effective by mouth has been achieved by modifying the ring hydroxyls, so that the resulting compound is not cleared by the uptake 1 process or metabolized by catecholamine o-methyl transferase or sulfatase in the digestive tract. Ephedrine, a natural rather than a synthetic drug, was the first sympathomimetic drug that met these criteria. Introduced in 1924, it gained widespread use as a bronchodilator. Though it can activate adrenergic receptors, it acts largely indirectly like tyramine, releasing catecholamines from storage vesicles. Thus, it produces all the effects of sympathetic stimulation and lacks speci6city. Indeed, it has even more unwanted sympathetic effects than the

2 catecholamines because, lacking polar substituents on the benzene ring, it readily passes the bloodbrain barrier and exerts substantial central nervous system effects like amphetamines. CLASSIFICATION OF ADRENERGIC REcEProRS Infusion of epinephrine or norepinephrine produces a wide variety of cardiovascular and metabolic effects. These agents act on virtually every cell of the body ahecting organ function in complex ways. Alquist greatly advanced the understanding of these effects when he introduced the classification of adrenergic receptors into two types: alpha and beta. These different receptors are defined on the basis of the relative potency of a series of agonists and by the action of specific antagonists (Table 1). Many of these adrenergic effects are listed in Table 2. Two that especially interest us are the bronchodilating effect and the anti-inflammatory effect of inhibiting release of mediators from mast cells, lysosomal enzymes from neutrophils and monocyte-macrophages, and Iymphokines from lymphocytes. Adrenergic drugs are used in asthma for their bronchodilating effect. It is at present uncertain whether the anti-inhammatory effects demonstrated in vitro are achieved in the clinical setting. Drug concentrations that reduce histamine secretion by 50 percent in vitro are higher than that achieved with usual doses used therapeutically. There is also interest in the possibility of modulation of the immune response by adrenergic mechanisms. This subject is exceedingly complex because adrenergic stimulation has different effects on different lymphocyte subpopulations. Insufficient information is available for even tentative conclusions. It is worth a passing comment that beta stimulation inhibits gastric acid secretion, but theophylline increases it. This is a notable exception to the usual potentiation of beta adrenergic effects by theophylline. Occasionally, in patients with peptic ulcer, this antagonism can be a distinct advantage. The effects on bronchial mucous glands are discussed elsewhere in this symposium. In rats, chronic isoproterenol treatment leads to striking hypertrophy of the salivary glands. I have often wondered Table I-Delinition of.4'pluj and Bela Reeeplor. Sequence of Potency of agonist8 An~onist8 Alpha Phenylephrine { NOrepinePhrine} Phentolamine Dibenzyline Ergotamine Beta { } Norepinephrine Phenylephrine Alprenolol Tissue Heart Smooth muscle Skeletal muscle Glands Mito8is Alpha? weak inotropic Vasoeonstrietion Bronchoconstrlction Uterine contraction Bladder sphincter constriction GI inhibition Increased renal tubular resorption ofna+ Blood Eosinophilia. and blood Platelet aggregation cells (more "dopaminergic") Central Nervous System Metab- Efflux of K+ olism Glucagon release Growth hormone release Inhibition of insulin release Inhibition of lipolysis Funda- Influx of Ca++ mental Efflux of K+ biochem- Membrane depolarizaica1action tion Beta Inotropic Chronotropic Vasodilatation Bronchodilatation Uterine relaxation Detrusor relaxation GI inhibition Facilitates neuromuscular transmis8ion (tremor) Increased strength of contraction Inhibit gastric acid secretion Increase protein concentration of saliva Increase mitosis of salivary glands Decreased tubular resorption of Na + Renin production Eosinopenia, lymphopenia and granulocytosis Increase clotting factor VIII and fibrinogen Kininue activation Decreased mediator release, decreased lysosomal enzyme release Decreased lymphokine production Arousal Insomnia Nervousness Muscle glycogenolysis Hepatic glycogenolysis Inhibition of growth hormone release Insulin release Gluconeogenesis Lipolysis Activation of adenylate cycla8e Membrane stabilization Inhibits epidermis thymocyte8 Stimulates salivary glands No effect on hair follicle or GI mucosa PHYSIOlOGY AID PHARMACOlOCY 115

3 whether chronic administration of sympathomimeties might contribute to the bronchial gland hypertrophy so prominent in asthma and chronic bronchitis. The general outline of the biochemical basis of action of beta agonists is now quite clear. After binding of the agonist to the receptor in the cell membrane, adenylate cyclase is activated and the second messenger, cyclic AMP, is formed. The adrenergic receptor is closely linked to the insoluble membrane bound enzyme, probably by a phospholipid. Cyclic AMP is then bound to protein kinase which in turn, usually by phosphorylation, activates other enzymes which carry out the function of the cell. In the case of glycogenolysis these steps have been quite well worked out. For most other adrenergic functions the steps subsequent to formation of cyclic AMP remain fairly conjectural. The biochemical basis for alpha adrenergic activity is less well defined, but alpha stimulation has the end result of a rapid inbux of ionized calcium into the cell. Both alpha and beta stimulation may either initiate a response in the target cell such as smooth muscle contraction or relaxation, or modulate the intensity of a response to some other stimulus such as histamine release after antigen challenge. A third adrenergic receptor, the dopamine receptor, located on some neurons in the CNS and autonomic ganglia, resembles beta receptors in activating adenylate cyclase, but is blocked by alpha rather than beta antagonists. In vitro, after continued exposure to beta adrenergic agonists for an hour, cells lose responsiveness to the agonist. This phenomenon of receptor desensitization is specific since the cells remain responsive to other agents which stimulate adenylate cyclase such as prostaglandin E. After desensitization, the response can be restored by cortisol. Treatment of patients with beta agonists also causes reduction of some responses, particularly the rise in blood sugar and activation of lymphocyte adenylate cyclase. Modification of the molecule produces agonists which are several hundred fold more active on either alpha or beta than on the other receptor. Phenylephrine is the classic alpha agonist and isoproterenol the beta. As the size of the substituent group on the amine increases from hydrogen on norepinephrine through the methyl of epinephrine to the isopropyl of isoproterenol, alpha activity diminishes and beta increases. With further increase in the size of substituent the compound begins to have more activity on some beta functions than on others. This fact has led to the concept of subtypes of beta receptors designated beta, and beta, (Table 3). As the end result of these developments in pharmacology and Agonists Antagonists Table 3-Be" 1 and 8e"2 Receptor.,..4.on.Ub,..4ntqoni..., "'"' Reapo... Norepin.iphrine Dichloroisoproterenol Practolol Responses Cardiac, inotropic and chronotropic Dilation of coronary arteries Relaxation of GI smooth muscle Lipolysis (1) medicinal chemistry, we now have beta adrenergic blocking agents such as practolol that act relatively selectively on the heart. Agonists such as albuterol, terbutaline, and fenoterol are relatively selective bronchodilators with little direct effect on the heart, though they may evoke sinus tachycardia, presumably as a reflex compensation to prevent a fall in blood pressure from vasodilatation. Given by mouth, these agents offer advantages over ephedrine. In doses that have equivalent bronchodilator effect they are much less likely to cause insomnia and nervousness, and to my knowledge, toxic psychosis has not been encountered. They are virtually devoid of alpha effect on the bladder sphincter or arterioles, and cause less urinary retention in patients with large prostates and are less likely to increase blood pressure in hypertension. The main side effect is tremor as a result of direct betas action on skeletal muscle. Occasionally there is an increase in blood sugar and a need to increase insulin in patients with diabetes. Given by injection they offer the advantages over epinephrine of longer action and less direct effect on the myocardium. Given by aerosol, they have the advantage over isoproterenol of a longer action (though the onset of action may be slightly slower) and a reduced cardiovascular effect. They have not been in widespread use long enough to know whether they will have the same or a reduced potential for abuse. BmLIOGRAPHY Betat Norepinephrine Isoetharine Albuterol Terbutaline Metaproterenol Fenoterol (If methyl dichloroiaoproterenol Butoxamine Relaxation of bronchial, uterine, and most arterial smooth muscle Glycogenolysis Insulin secretion Mast cell inhibition Skeletal muscle tremor Alquist RP: A study of the adrenotropic receptors. Am J Physioll53:586, CHARUS E. REED

4 Ariens EJ: The structure activity relationship of beta adrenergic drugs and beta adrenergic blocking drugs. Ann NY Acad Sci 139:606, 1967 Caron MG, Lefkowitz RJ: Beta adrenergic receptors: Basic studies and clinical implications. In New Directions in Asthma (Stein M, ed), Park Ridge, Ill., American College of Chest Physicians, 1975, p 85 Goodman LS, Gilman A, (eds): The Pharmacologic Basis of Therapeutics, (5th ed). New York, MacMillan, 1975 Lands AM, Arnold A, McAuleff JP, et al: Differentiation of receptor systems activated by sympathomimetic amines. Nature 214:597,1967 Robinson GA, Butcher RW, Sutherland EW: Adenyl cyclase as an adrenergic receptor. Ann NY Acad Sci 139:703, 1967 Richardson }B: The neural control of human tracheobronchial smooth muscle. In Asthma D: Physiology, Immunopharmacology and treabnent (Austen KF, Uchtenstein L, eds). New York, Academic Press, in press van As A: Beta adrenergic stimulant bronchodilators. In New Direction in Asthma (Stein M, ed). Park Ridge, Ill., American College of Chest Physicians, 1975, p 145. DISCUSSION Comment: Would you care to comment on the clinical significance of the inhibition of growth hormone release produced by beta adrenergic agents? Dr. Reed: I am not aware of any evidence that beta agonists actually suppress growth, although they do affect the release of growth hormone. Perhaps this is related to the duration of action. Brief interference with the release of growth hormone might not have any longterm effect on growth. Dr. MOf'Tis: I have seen data based on studies in rats or mice, I have forgotten which. These animals were given single daily injections of epinephrine and were dwarfed in comparison to their litter mates. However, I don't know of any studies that have been done to document this in humans. Dr. Shetter: We were taught that methyl-isoproterenol is a beta blocker. H that is true, perhaps you could comment on the time course of COMT metabolism versus MAO metabolism. For instance, if COMT metabolism is faster than MAO metabolism, one might have a build-up of methyl-isoproterenol with repeated use and potentially get a rebound blocking effect from thismetabolite. Dr. Reed: Patterson and Connolly observed that the administration of large doses of isoproterenol is associated with the urinary excretion of this metabolite which, unlike the similar metabolite from the natural catecholamine, has a weak beta adrenergic blocking effect. They postulated this mechanism as the basis for the loss of efficacy of isoproterenol. I think the idea is plausible, but quantitatively there is not enough of the metabolite produced to have a clinically relevant effect. The loss of efficacy with repeafed doses of isoproterenol is more likely related to the phenomenon of receptor desensitization which can be readily demonstrated in almost any vitro system. The receptor desensitization explanation does not account, however, for the observation that isoproterenol given by a different route, either by injection or sublingually, acts fully. I think there is still quite a bit to be learned here. Dr. Sackner: We have observed, as others have, that freon propellent causes bronchoconstriction in patients with reversible airway disease. It seems quite possible that patients inhaling large amounts of aerosolized adrenergic agonists might become resistant to isoproterenol, while the bronchoconstrictor effect of the freon propellent takes over. I would like to hear your comments about this. Dr. Reed: I think that is very interesting. However, I don't think the freon propellent fully accounts for this particular aspect of isoproterenol that we have just been discussing, since we see this in patients who use an aerosol generated by compressed air rather than freon. Dr. Trautlein: I would like to add that we have found a few cases in which there was no response or a paradoxic response after a bronchodilator and this phenomenon seemed to be peculiar to isoproterenol. It did not occur with metaproterenol, nor did it occur with terbutaline using the same gas as a propellent. Dr. Reed: Eosinophilia is an alpha adrenergic effect, and this is presumably the result of constriction of smooth muscle in the spleen with the release of eosinophils from storage sites. The eosinopenia produced by epinephrine has to do with the trafficking of these cells, but the details are not clearly understood. The reason for classifying it as a beta effect is that the eosinopenia is prevented with propranolol. It probably represents a shutting-off of the entry of eosinophils into the circulation from the bone marrow rather than an increase in the rate at which they leave the circulation to storage sites. As I understand the dynamics of the distribution of leukocytes, the neutrophilic granulocyte comes and goes in the circulation, being stored away in what is called the marginal pool The action of epinephrine is to release the neutrophil from storage sites in the marginal pool, and thus they appear in the circulation. I understand that this is not the case with eosinophil. The eosinophil rarely recirculates. Dr. Bierman: With regard to the tremor associated with beta, drugs, has anyone looked at the possibility that this might be related to hypoglycemia as a result of increased insulin secretion? Comment: We have noted a marked hypokalemia associated with the use of intravenous dibutamol and presumablythis is also due to the effect of insulin. Dr". Toppell: In reference to the tremor, I have found that approximately one-third of the patients in our clinic develop tremor whether I use metaproterenol or terbutaline. Generally, the tremor is of such a degree that we have to take the patients off the medication unless we switch them to aerosolized forms of the drugs, in which case less than 10 percent of them get the tremor again. I~ this simply a dose effect? Dr. Balchum: I have been quite interested in this because I have had the same experience in about the same proportion of patients. There have been several studies PHYSIOLOGY AND PHARMACOLOGY 917

5 in which the drugs were given by regional intra-arterial injection and the tremor was observed under those conditions. It appears then that this is a direct muscular effect and this might explain why a lesser effect is observed with more localized therapy by inhalation. Dr. Chervinsky: I am sure that the tremor effects are dose-related. Most patients will have lessening of the tremor if the dose is decreased. As far as the cause of the tremor is concerned, there have been studies showing that propranolol can block the tremor produced by intraarterial infusions of isoproterenol It was felt that this was good evidence that the tremor was primarily a beta, effect on receptors in skeletal muscle. One of the previous discussants mentioned the effect of beta agonists on the blood glucose and whether that could cause a tremor. In my observations the most marked decrease in blood glucose is found in the pediatric age group, and these patients for some reason have the least amount of tremor. Dr. Sherter: We recently studied the effects of terbutaline taken over a four-to-six-week period, and one of the things we evaluated was the tremor. Although the study is not complete, it appears that the tremor disappears in conjunction with an attenuation of bronchodilator effect over this time interval. We are not certain of the mech- anism, but it seems that with long term therapy tolerance develops not only to the bronchodilator effects but also to the tremor producing effects. Dr. Rosenthal: We have had the opportunity to look at a group of patients on terbutaline and, while I have not noticed changes in blood glucose levels, I have noticed elevations in CPK. I would be interested in knowing if anyone else hashad this experience. Dr. Balchum: We have been studying a derivative of terbutaline and have also found changes in CPK which I am unable to explain. Our patients were studied monthly for three years so we have quite a few observations. Generally, the CPK elevations are mild and they vary above normal only -on occasion. In addition, these patients appear to be under good control as far as their asthma is concerned. Dr. Burki: We have been following patients with chronic asthma for the last three years, and we have been able to relate rises in CPK to the severity of airway obstruction. We found no correlation between CPK rises and drug therapy. All of the CPK appears to be derived from muscle. On further investigation we found that if you take normal subjects and make them hyperventilate voluntarily, you will find CPK rises which in several instances was as high as 1,000 units. 918 CHARLES E. REED