ligands, molecular structure, and molecular pharmacology. Chapter ...... Rang HP, Dale MM, Pharmacology, Churchill Livingstone, Longman Group UK. Limited ...
SYNTHESIS AND IN VITRO PHARMACOLOGY OF A SERIES OF HISTAMINE H -AGONISTS WITH ADDITIONAL CARDIOVASCULAR ACTIVITIES 2
Johannes A.M. Christiaans
VRIJE UNIVERSITEIT
S Y N T H E S I S A N D IN V I T R O P H A R M A C O L O G Y O F A S E R I E S O F H I S T A M I N E H2-AGONISTS W I T H A D D I T I O N A L CARDIOVASCULAR ACTIVITIES
A C A D E M I S C H PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de Vrije Universiteit te Amsterdam, op gezag van de rector magnificus prof.dr E. Boeker, in het openbaar te verdedigen ten overstaan van de promotiecommissie van de faculteit der scheikunde op maandag 28 maart 1994 te 15.30 uur in het hoofdgebouw van de universiteit, D e Boelelaan 1105
door JOHANNES ANTONIUS MARIA CHRISTIAANS geboren te Boxmeer
Promotor
:
prof.dr H. Timmerman
Copromotor :
dr H. van der Goot
Referent
prof.dr W . Schunack
:
Aan mijn
ouders
T h e investigations described in this thesis w e r e financially supported by Byk Nederland B . V . and w e r e performed at the Department of P h a r m a c o c h e m i s t r y , Leiden/Amsterdam Center for Drug Research, Vrije Universiteit, Amsterdam, The Netherlands.
Contents Chapter 1
Pharmacotherapeutic treatment of cardiovascular diseases; drug effects on functions of the heart and the circulatory system
Chapter 2
Hybrid molecules: combination of more than one pharmacological property in one single molecule
33
Chapter 3
Organic nitrate esters as synthons for the synthesis of hybrid molecules combining histamine H2-agonistic properties and nitrovasodilation
81
Chapter 4
L-type voltage-operated C a - c h a n n e l s : molecular biology, ligands, molecular structure, and molecular pharmacology
Chapter 5
Synthesis and in vitro pharmacology of a series of new 1,4dihydropyridines. 1. Diethyl 2-(co-aminoalkylthio)methyl-2,6-dimethyl-4-[(substituted) phenyl]-l,4-dihydropyridine-3,5-dicarboxylates as potent calcium channel blockers
131
Chapter 6
Synthesis and in vitro pharmacology of a series of new 1,4dihydropyridines. 2. Diethyl 4-[2-(co-aminoalkoxy)phenyl]-2,6-dimethyl-l,4-dihydro pyridine-3,5-dicarboxylates and their corresponding isothioureas as tools for determining structure-activity relationships
149
Chapter 7
Synthesis and in vitro pharmacology of a series of hybrid molecules possessing 1,4-dihydropyridine calcium channel blocking activity and histamine H2-agonistic properties
163
Chapter 8
A new series of dimaprit analogues with histamine H2-agonistic and histamine Hi-antagonistic activities
191
2+
Summary Samenvatting List of Publications Curriculum Vitae Dankwoord I Acknowledgements
s
1
95
207 209 213 215 111
Chapter 1 Chapter 1 P h a r m a c o t h e r a p e u t i c treatment of cardiovascular diseases; drug effects on functions of the heart a n d the circulatory s y s t e m
1 Introduction T o understand causes of cardiovascular diseases and the effects of drugs, certain pathophysiological functions of the heart and the circulatory system have to be c o n s i d e r e d . A number of factors play an important role in the regulation of the circulatory system. These factors consist of the p u m p function of the heart, the vascular system, the blood, and the function of the kidney and the renal vascular system. In this chapter only aspects of the function of the heart and the vascular system will be described. T h e properties of the blood determining the blood flow, such as aggregation, coagulation, and the deformability of the blood cells, will not be considered. Besides, the function of the renal vascular system will not be discussed. In the first part, the function of the heart and the blood pressure regulation will be discussed. T h e following part deals with several drugs available for the treatment of c a r d i o v a s c u l a r d i s e a s e s . A distinction will b e m a d e u p o n a n t i a r r h y t h m i c s , antianginals, positive inotropic agents, and antihypertensives. Finally a summary will be presented about the prospects to achieve drugs with improved therapeutic value for the treatment of cardiovascular diseases. 1
2 The heart An optimal functioning of the heart is of vital interest for all mammals. The function of the heart is to p u m p blood throughout the body to supply all living cells with oxygen and nutrients. In a simplified view, the heart consists of two sections each acting as a p u m p . In the pulmonary circulation, blood deprived of oxygen and rich of carbon dioxide is pumped from the right part of the heart to the lungs. In the lungs carbon dioxide in the blood is removed and oxygen is taken up. The oxygenated blood is transported from the lungs to the left part of the heart. In the systemic circulation, the oxygenated blood is transported from the left part of the heart through the body to supply, for instance, muscles and organs with oxygen. T h e p u m p function of the heart is regulated in ^ n extremely sophisticated manner. This includes automatism, rhythmicity and contractility. 2.1 Automatism and rhythmicity A u t o m a t i s m and rhythmicity are two important functions to control an optimal performance of the heart. Cardiac muscle cells (myocytes) are electrically excitable like most other muscle cells but differ by having sino-atrial (SA) and atrio-ventricular (AV) nodes which generate
1
Chapter 1 spontaneous rhythm. This is called the intrinsic rhythm of the heart showing a frequency of about 7 0 beats per minute. A s a result of an electrical or chemical stimulus, the transmembrane action potential of a cardiac muscle cell changes. This cardiac t r a n s m e m b r a n e action potential, manifested as a p r o p a g a t i n g w a v e of transient depolarisation, can be divided into several phases (fig. 1). At resting potential, cardiac ventricular cells maintain a transmembrane potential varying from -80 to -95 m V . T h e resting potential is maintained by the concentration of intra- and extracellular potassium and is determined by K -permeability of the cell m e m b r a n e and a N a / K - A T P a s e which e x c h a n g e s three s o d i u m ions for two potassium ions. At p h a s e 0, a rapid depolarisation occurs by opening of N a - c h a n n e l s leading to a fast inward Na -current. At phase 1, a partial repolarisation takes place, k n o w n as the transient outward current caused almost exclusively by K -efflux ' . At phase 2, a plateau region exists as a result of reduced K -efflux and a slow influx of N a and C a leading to a net slow inward current. At phase 3 , a rapid repolarisation takes place because of the closure of N a - and C a channels and activation of one or more fast outward K - c h a n n e l s . This eventually leads to phase 4 in which the resting potential is reached. T h e K concentration is restored via the N a / K - p u m p and by K -permeability of the cell membrane. +
+
+
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2
3
+
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b +30 to +40 mV
•80 to -95 mV
0
100
200
300
400 ms
a) phase 0;fast inward Na+-current b) phase 1; partial repolarisation by K+-efflux c) phase 2; reduced K+-efflux and slow Na - and Ca * -influx d) phase 3; closure ofNa+- and Ca*+-channels, fast outward K -current e) phase 4; resting potential, K+-concentration is restored via the Na+IK+-pump and by the K+-permeability of the cell membrane +
2
+
Figure 1: Action potential curve This intrinsic rhythm, however, is not the only factor influencing heart rate. Also the s y m p a t h e t i c n e r v o u s s y s t e m , w h i c h accelerates heart b e a t i n g and the para sympathetic nervous system, which slows heart beating, play a role.
2
Chapter 1 2.2 Role of calcium Calcium is involved in all processes of excitation resulting in biological effects. These excitation processes can be initiated by either an electrical or chemical stimulus. A calcium concentration gradient across the cell m e m b r a n e contributes to generation and maintenance of a potential gradient. Calcium homeostasis in the cell is regulated by several mechanisms. Calcium influx takes place via calcium channels. Calcium efflux proceeds via a N a / C a - e x c h a n g e process driven by the N a electrochemical potential and a C a - A T P a s e which relies on the utilization of A T P . Calmodulin-calcium-sensitive C a - A T P a s e s with different properties are present in the plasma membrane and in the endoplasmic reticulum of smooth muscles. Activation of calcium channels, leading to C a - i n f l u x , can occur by m e m b r a n e depolarization or by receptor stimulation. Calcium channels which are primarily regulated by electrical signals are called voltage-operated channels ( V O C ) , potentialdependent channels (PDC), or voltage-dependent calcium channels ( V D C C ) . Calcium channels which respond to chemical signals are called receptor-operated channels (ROC) or ligand-gated channels. The cellular system is provided with mitochondria and sarcoplasmic reticulum, which prevent intracellular calcium overload. The mitochondria and sarcoplasmic reticulum are intracellular pools responsible for the calcium sequestration and calcium liberation after stimulation. Influx of C a through V O C s or R O C s triggers the release of a relatively large amount of calcium from intracellular stores to reach an intracellular calcium concentration which is above a threshold resulting in a contractile response. In the heart, the intracellular C a - r e l e a s e occurs via a calcium release channel which is thought to be part of a large protein called the ryanodine receptor and is located in the membrane of the sarcoplasmic reticulum . T h e alkaloid ryanodine exhibits two opposing effects on the calcium release channels in the sarcoplasmic reticulum, which are concentration dependent. At high concentrations (> 30 | i M ) the channel is deactivated (closed), and at lower concentrations the channel is activated (opened). Gerzon et al. reported synthetically modified ryanodine analogues which are able to open the channels and lack the ability to close t h e m . +
+
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2+
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4
5
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In the skeletal muscle, the C a - r e l e a s e from the sarcoplasmic reticulum is initiated by "voltage sensors", located in specialized regions of the sarcoplasmic m e m b r a n e , instead of activation of the sarcoplasmic C a - r e l e a s e channels by calcium, as in heart cells . In addition also in smooth muscles evidence exists Tor calcium being released from the sarcoplasmic reticulum either through C a - i n d u c e d C a - r e l e a s e m e c h a n i s m s ' or through a mechanism linked to phosphatidylinositol m e t a b o l i s m ' . 2+
6
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7
9
8
10
The reaction pathway leading to contraction in heart and skeletal muscle differs from that in smooth muscle. Therefore, in the next section, these contractile processes will be discussed in more detail.
3
Chapter 1 Calcium homeostasis in the cell is influenced by modification of cellular calciumbinding proteins that regulate calcium metabolism. There are two types of calcium-binding p r o t e i n s : - calcium-transporting proteins integrated in the cell wall or integrated in membranes of intracellular organelles. - calcium-modulated proteins which are freely dissolved in the cell or are part of a nonmembranous intracellular structure like troponin or calmodulin. Drugs affecting cardiac calcium-binding proteins can therefore act on calciumtransporting proteins or calcium-modulating proteins by direct binding to the protein or by modification of the binding through drug-receptor interaction. As mentioned before, calcium homeostasis is regulated by C a - i n f l u x and efflux processes. Indirectly, the intracellular calcium concentration is also influenced by the N a / K - e x c h a n g e process. Interferences of N a / K - p u m p disturbs the steady state concentration of sodium and hence via the N a / C a - p u m p also the intracellular calcium concentration. 11
2+
+
+
+
+
+
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2.3 Contractility The heart exerts its work by rhythmic contraction (systole) and relaxation (diastole). Contraction of the cardiac muscle at constant ventricular v o l u m e causes a rise in ventricular p r e s s u r e . W h e n this ventricular pressure e x c e e d s the actual aortic pressure, the ventricular valve will be opened and the blood flows into the aorta. The stroke v o l u m e is about 7 0 ml and a rest volume of about 7 0 ml remains in the ventricle. With the decrease in ventricular pressure, the aortic valve is closed and the atrio-ventricular valve is opened when the aortic pressure is higher then the ventricular pressure. The influx of blood into the atrium and ventricle takes place via a passive mechanism. The contractile process of the cardiac myocytes is regulated by biochemical factors such as A T P and intracellular calcium and hence depends on C a - e n t r y across the cell membrane. The slow inward C a - c u r r e n t promotes the release of a much larger amount of C a from the sarcoplasmic reticulum. The increased intracellular C a concentration unblocks the effects of troponin and tropomyosin, which then in turn allows actin and myosin to interact with the muscle fiber filaments to contract. The contractile activity in smooth muscle is also regulated by the free intracellular Ca -concentration and Ca -sensitivity of the contractile proteins. Upon stimulation, the intracellular C a - c o n c e n t r a t i o n is increased from about 10~ M to 10~ M, by influx of extracellular C a and from release M C a from intracellular stores. In the cytosol, the increased C a - c o n c e n t r a t i o n allows calcium to interact with specific binding sites on calmodulin. Calmodulin is an intracellular protein involved in several activation processes of target enzymes. This protein is the mediator of the excitationcontraction coupling in smooth muscle. The calcium-calmodulin complex interacts with another protein, the myosin light chain kinase. This newly formed complex then phosphorylates a smooth muscle myosin light chain which, in its phosphorylated form, interacts with actin leading to shortening (contraction) of the myofilaments. 2+
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7
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4
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Chapter 1 T h e excitation-contraction process is reversible and therefore dephosphorylation of myosin light chain causes the muscle to relax. In these processes, calmodulin plays a complex regulatory role. However, the consideration of the several distinct classes of so-called calmodulin antagonists and their putative applications in therapy, is beyond this scope, they are reviewed e l s e w h e r e . 12,13
2.4 Cardiac performance Cardiac performance and the mean arterial pressure are increased when the stroke volume of the heart is increased at constant peripheral resistance. The stroke volume depends on the central venous pressure (pre-load) which influences the cardiac filling pressure and hence the end-diastolic volume. By increasing the pre-load, the cardiac work is increased and the oxygen consumption of the heart is enhanced. W h e n the peripheral resistance is increased without changes in the central venous pressure, the after-load is increased. To achieve the same cardiac output, the cardiac work and hence the oxygen consumption are increased. At exercise, cardiac output is influenced by changes in stroke v o l u m e and heart frequency. Sympathetic activation gives an increased positive inotropic effect so that more of the rest volume can be used and hence more stroke volume is available or it can overcome a higher peripheral resistance. Increasing of the heart frequency gives a shortening of the diastole and thus a shorter time for ventricle filling and less coronary perfusion of the ventricle if it is not a c c o m p a n i e d with coronary dilation. A fast increase in frequency is thereby economically unfavourable. 2.5 Myocardial oxygen supply, angina, and myocardial infarction The heart consumes energy for all the processes executed. This energy is mainly supplied by glucose and by oxygen delivered by the blood in the cardiac chamber and, especially, from blood in the coronary artery system. The heart is particularly sensitive to disturbances in blood supply. If the heart is deprived of blood (becomes ischemic), the p u m p function of the heart is reduced within seconds. Atherosclerosis is often the underlying mechanism which leads to ischemic heart disease. Partial occlusion of coronary arteries by formation of atherosclerotic plaques leads to angina, because of oxygen depletion of the heart. Angina is felt as a severe pain in the chest and occurs upon exercise or excitement. Total occlusion of a coronary artery, also called myocardial infarction, can lead to necrosis of certain parts of the heart. Antianginal drugs reduce the oxygen demand of the heart and increase myocardial oxygen supply by improving myocardial perfusion. Decreased cardiac p-adrenoceptor density and tissue reactivity, as a result of prolonged treatment with P-adrenergics, are reported in heart failure in both clinical and animal s t u d i e s . Also, evidence for reduction of both ATP-sensitive K channel and dihydropyridine-sensitive C a - c h a n n e l densities are shown in left ventricular tissue homogenates from rats with heart failure . 1 4 , 1 5
+
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5
Chapter 1 In the majority of patients with heart failure an enlargement of the left ventricle is the initial process leading to progressive cardiac dysfunction. Congestive heart failure (CHF) results when the left ventricle is unable to adequately perfuse the peripheral tissue and thereby c a n n o t p r o v i d e the cardiac output d e m a n d e d by exercise. Insufficient cardiac output initiates a cascade of responses which gives a reduction of peripheral circulation eventually leading to a worsening of the effect. Congestive heart failure (CHF) can be therapeutically attended by: -improving the action of the left ventricle with positive inotropic agents also called cardiotonics -lowering of the peripheral resistance with antihypertensive agents as vasodilators 2.6 Blood pressure H y p e r t e n s i o n is a health p r o b l e m of wide dimensions and is correlated with cardiovascular disease and results from an increase in peripheral vascular resistance . Initially, hypertension is caused by vasoconstriction; prolonged hypertension leads to structural changes of the vessel walls by hypertrophy of the arteries and increase of left ventricular m a s s . Increased peripheral vascular resistance remains a major risk for cardiovascular disease, especially for stroke, myocardial infarction, congestive heart failure, and renal failure. The electrochemical, mechanical, and metabolic activities of the heart are mediated by transmembrane movements of ions and the intracellular calcium concentration. The mechanical output of the heart is not only determined by the force of contraction but also d e p e n d e n t on the state of the circulation system, such as the peripheral resistance of the blood vessels and the viscosity of the blood. T h e peripheral resistance of the blood vessels affects the performance of the heart by influencing the systole-diastole mechanism. Vasodilators increase blood flow by lowering the peripheral resistance. Depending on the class to which vasodilators belong, they are used as antianginal drugs or for the treatment of congestive heart failure or in peripheral vasoconstrictive conditions. 3 Cardiovascular drugs Cardiovascular drugs are used for the treatment or prevention of cardiovascular diseases and can be divided in: - antiarrhythmics - antianginal drugs - cardiotonics, also called positive inotropic agents - antihypertensives 17
18
3.1 Antiarrhythmics Cardiac arrhythmias result from disorders in impulse generation and/or conduction [for details see ref. 19].
6
Chapter 1 Antiarrhythmic drugs modify or restore an abnormal cardiac rhythm. Therefore, drugs used for the treatment of arrhythmia can be directed towards several phases of the cardiac action potential. There are at least four different groups of antiarrhythmic drugs originally defined by Vaughan W i l l i a m s : 20
- Class I The class I antiarrhythmic agents exists of three subtypes I , I and I (e.g., I agents: quinidine 1, procainamide 2, disopyramide 3 ; I agents: tocainide 4, mexiletine 5; I agents flecainide 6, and encainide 7; fig. 2a). This subclassification is based on their effect on action potential duration. All class I agents are sodium channel blockers and inhibit the propagation of the action potential by reducing the rate of depolarisation during phase 0 (reduction of the fast inward Na -current). 2 1
a
b
c
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+
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Disopyramide
4
Tocainide
Figure 2a: Class I antiarrhythmics; sodium channel blockers
7
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Mexiletine
a
Chapter 1 - Class n A r r h y t h m i a s occurring after myocardial infarction are partly a result of increased sympathetic activity. Adrenaline can cause ventricular extrasystoles by affecting the resting potential of the action potential curve. T h e AV conduction is disturbed and the refractory period is shortened by an increased s y m p a t h e t i c a c t i v i t y . p a d r e n e r g i c a n t a g o n i s t s [like p r o p r a n o l o l 8 (nonselective), metoprolol 9 and atenolol 10; fig. 2b] increase the refractory period of the A V n o d e , interfere with A V conduction, and slow down the ventricular rate. T h e most important adverse effect of p b l o c k e r s is the negative inotropic activity and reduced cardiac output. r
r
Figure 2b: Class II antiarrhythmics; p a d r e n e r g i c blockers r
- Class IH The primary antiarrhythmic activity of agents belonging to this class rely on the fact that they prolong the cardiac action potential and so increase the refractory period of the cardiac muscle. These electrophysiological changes are caused by cardiac potassium channel blockade. Amiodarone 11 (fig. 2c) was one of the first drugs showing K -channel blockade. Since then major interest was shown in this field of drugs. Nowadays sotalol 12, originally developed as a p-adrenergic receptor antagonist, is the typical class III antiarrhythmic drug. T h e majority of potent class III anti-arrhythmic drugs recently developed (E4031 13, UK-68,798 14, L-691,121 1 5 ; fig. 2c) all possess the methanesulfonamide group as present in sotalol. +
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N
- Class IV C a l c i u m channel blockers reduce the slow calcium influx and shorten the plateau phase of the cardiac action potential and restore the intrinsic rhythm. The disadvantageous effect is the reduction in myocardial contractility. Calcium channel blockers are discussed in more detail as antihypertensives.
8
Chapter 1 Until now, no selective class III antiarrhythmic drug has reached the market and, therefore, the effectiveness of these type of agents in therapy is still u n p r o v e d . In clinical use, the choice of the best antiarrhythmic drug is often a matter of trial and error. A special p r o b l e m is that some antiarrhythmic drugs used to suppress arrhythmias actually turned out to act pro-arrhythmogenic . In practice the dihydropyridine calcium channel blockers carry a serious risk for proischemia, a term defined by W a t e r s . T h e most likely possibility to cause proischemia is that dihydropyridineinduced peripheral vasodilation simultaneously reduces coronary perfusion pressure and, as a reflex, increases heart rate. 23
24
25
i
11
Amiodarone
14
12
Sotalol
C H 3
UK-68,798
Figure 2c: Class III antiarrhythmics; potassium channel blockers T h e most promising drugs for the treatment of arrhythmias seem to be the sodium channel blockers. However, much is still unclear about the exact mechanisms of these drugs. All antiarrhythmic drugs exert negative inotropic effects which may induce or worsen congestive heart failure in up to 5 % of all treated p a t i e n t s . Treatment of arrhythmia with single drugs is often ineffective. Therefore, combinations of antiarrhythmic drugs are used. In general, drugs belonging to the same electrophysiological class are not applied in c o m b i n a t i o n t h e r a p y . H o w e v e r , also d r u g s with p h a r m a c o k i n e t i c interactions, such as quinidine and amiodarone, should be a v o i d e d . 26
27
9
Chapter 1 3.2 Antianginal drugs As already discussed, angina is caused by a disturbance in balance between oxygen supply by the coronary blood flow and the oxygen demand of the heart. Relief of anginal pain can be achieved by drugs increasing the oxygen supply or by drugs which decrease the oxygen demand of the heart by reducing contractility (inotropy) and frequency (chronotropy). Drugs used for the treatment of angina are vasodilators as the organic nitrate esters and the calcium channel blockers, which both reduce the oxygen demand and simultaneously improve myocardial perfusion, p-adrenoceptor antagonists only affect the oxygen demand of the heart. The antianginal drugs can be divided into three classes: - nitrate and nitrites - calcium channel blockers - P a d r e n o c e p t o r antagonists r
3.2.1 Nitrates and nitrites Nitrates and nitrites exert their effects on the systemic circulation by relaxation of small bloodvessels, arterioles, capillaries and the venous capacitance v e s s e l s . The secondary effect is the reduction of the pre-load. Reduction of the arterial pressure and cardiac output leads to a lowering of the cardiac o x y g e n c o n s u m p t i o n . Continuous treatment with nitrates, however, results in a diminishing relaxant effect. This tolerance especially occurs with long-acting organic nitrates, like isosorbide dinitrate 16 and to a lesser extent with nitroglycerin 17. Organic nitrates and nitrites most likely produce their vasodilation by releasing nitric oxide. Organic nitrites (like amylnitrite 18) release nitric oxide via a simple one-electron reduction. T h e organic nitrates require a three-electron reduction to release nitric oxide, proceeding via an enzyme system attached to the cellular surface of vascular smooth muscle membranes. The nitric oxide releasing activity mimics the effect of the endothelium-derived relaxing factor (EDRF), originating from endothelial cells present in bloodvessels, which also turned out to be nitric o x i d e . 28
29
ON0
ON0
2
2
16 I s o s o r b i d e dinitrate 17 Nitroglycerin The nitrate esters are extensively reviewed in chapter 3.
18
Amyl nitrite
3.2.2 Calcium channel blockers Reduction of myocardial oxygen consumption is thought to result from both an increase in coronary blood flow and a reduction of total heart work. F r o m these effects and from the afterload reduction due to their hypotensive action, dihydro-
10
Chapter 1 pyridines are expected to be beneficial for the treatment of angina. T h e calcium channel blockers are discussed in a following section (Antihypertensives). 3.2.3 ^-Adrenoceptor
antagonists
Pi-antagonists are used to reduce the frequency of anginal attacks. They reduce the cardiac work by inhibition of the sympathetic nervous system and inhibit the action of circulating catecholamines on the heart. T h e catecholamines exert a positive inotropic and chronotropic action on the heart. By blockade of the p a d r e n o c e p t o r s the frequency of the heart is reduced during exercise and at rest. p a d r e n o c e p t o r antagonists have already been discussed in the section of class II r
r
antiarrhythmics. 3.3 Cardiotonics (inotropic agents) Cardiotonic drugs increase the contractile force (inotropy) of the heart and exert important actions on cardiac excitability, automaticity, conduction velocity and refractory periods and are mainly indicated for congestive heart failure. A number of agents have effects on cardiac function and can be classified as follows: - cardiac glycosides - p a d r e n o c e p t o r agonists and histamine H -agonists - phosphodiesterase (III) inhibitors - calcium channel activators r
2
Figure 3 shows the sites of action of the different positive inotropic agents in which they interfere in the intracellular calcium concentration. Positive inotropic agents act by increasing the intracellular Ca -concentration which then becomes available for the contractile proteins (troponin, myosin, actin). Stimulation of G-protein c o u p l e d receptors ( p - a d r e n o c e p t o r s , h i s t a m i n e H receptors) activates the adenylate cyclase system, converting A T P into c A M P . In cardiac tissue a special form of adenylate cyclase has been found recently, which is inhibited by C a . It is proposed that the physiological significance of this type of adenylate cyclase provides a negative feedback control on elevation of c A M P and hence on cardiac contractility and r h y t h m i c i t y . c A M P is capable to phosphorylate certain protein kinases. T h e s e kinases, in turn, p h o s p h o r y l a t e proteins in the sarcolemma and proteins in the sarcoplasmic reticulum . Although the primary signal to which voltage-operated calcium channels respond is constituted by the membrane potential, also a variety of ligand-receptor initiated modulations of these channels occur. This happens by phosphorylation of certain proteins in the sarcolemma, which are part of voltage dependent L-type C a - c h a n n e l s . W h e n phosphorylated, these proteins allow the ion channel configuration to switch in the open, also called activated, state. This leads to a slow inward C a - c u r r e n t . p A d r e n o c e p t o r stimulation increases the slow inward C a - c u r r e n t through cardiac L-type C a - c h a n n e l s . Guanidine nucleotide binding proteins (G-proteins) provide the link between p a d r e n o c e p t o r s and K - or C a - c h a n n e l s . This link can be either 2+
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Chapter 1 indirect (also called cytoplasmic pathway) or direct (also called the m e m b r a n e delimited p a t h w a y ) . Indirectly activation takes place via activation of cytoplasmic kinases w h i c h p h o s p h o r y l a t e the ion c h a n n e l s ' . T h e stimulating protein G directly activates K - and C a - c h a n n e l s . Although a direct effect of G-proteins on neuronal C a - c h a n n e l s have been established, it is still debated if such a mechanism also regulates cardiac C a - c h a n n e l s ' . 3 3
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Calcium channel blockers Cardiac glycosides Ca Na
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u
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ion channel +
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ATP^AMP K
ss-*-® — adenylate cyclase
A T P ^ DAMP \
+
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\ . protein
/
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/ ^
PDE III inhibitors
°C-*5'-AMP \ troponin^^ 2 +
C a - calmodulin electromechanical coupling via the troponin-myosin-actin complex
MLCK-0 2 +
C a - calmodulin - MLCK SMOOTH MUSCLE
myosin • LC
ATP m
y s i n - LC
