Cardiovascular Pharmacology: The Mechanisms involved in Cardiac Cell Chloride Homeostasis - Essay Example

Running Head: CARDIOVASCULAR PHARMACOLOGY Cardiovascular Pharmacology:  Review the Mechanisms involved in Cardiac Cell Chloride Homeostasis. Discuss these Mechanisms as Potential Targets for Novel Therapeutic Agents Tony ------- University of ---- April 10, 2009 Cardiovascular Pharmacology:  Review the Mechanisms involved in Cardiac Cell Chloride Homeostasis. Discuss these Mechanisms as Potential Targets for Novel Therapeutic Agents Introduction The human body is continuously in search for steady state equilibrium in its internal milieu called homeostasis (Cosentino, 2004, ¶ 1). Together with a complicated interplay among the water, acid – base balance, and electrolytes, the physiological processes in the human body regulates and controls homeostasis (Cosentino, 2004, ¶ 1). Disturbance in the body fluid along with electrolyte imbalances result from acute or chronic diseases caused by disturbance in the homeostasis of the body fluids. Other illnesses that may cause imbalances in fluid and electrolyte include diarrhea, vomiting as well as nasogastric suctioning or administration of medication (Cosentino, 2004). The following are the electrolytes that can be found in the internal milieu of our body: sodium potassium, calcium, magnesium, chloride, bicarbonate, and phosphate (Cosentino, 2004, ¶ 3). Among these electrolytes, chloride, a major extra cellular anion, is important in acid – base balance; however, little has been known about chloride and not much study has been made. For life emergence, regulation of cell volume is a fundamental requirement considered from the evolutionary point of view (Sardini, 2004, ¶ 2). In all types of cells in the mammal, volume regulation remains to be the vital mechanism common to all. Although the intracellular osmotic pressure and transmembrane alkali metal cation movement are being influenced by Na+, K+, Ca2+, PO4−, Cl−, HCO3−, amino acids and peptides with low molecular weight, little was recognised that ion channels of chloride plays a significant role in the regulation of volume that takes place in just several minutes (Terashima, et al., 2009). Chloride, Chloride Channels and its Functions Kole, et al. (2003, ¶ 1) noted that chloride, a distinctively dominant diffusible anion inside most cells, is a critical component of all living cells. Normal physiological environment maintains cytoplasmic electroneutrality, and the total cellular content of solute accompanies changes in the cellular level of chloride (Kole, et al., 2003). On the other hand, changes in the cellular volume go along with the alteration in the content of cell solute because of high permeability of water in the membranes of the cell. The cellular traffic of chloride ion has been regulated by the voltage gated channels of chloride (Kole, et al., 2003, ¶ 1). Kole, et al. (2003, ¶ 2) and Jentsch (2002) added that cell volume regulation, stabilization of membrane potential, signal transudation, and transepithelial transport are but several of the functions performed by chloride channels. Moreover, Jentsch (2002) added that chloride channels, which can be found in the environment of plasma membrane and intracellular organelles is also recognized for its significance in the regulation of electrical excitability in the human body. Frédéric (2006) stated that the vital processes of cellular signalling are played by the chloride channels that contribute to the homeostasis of cells. In summary, the following are important physiological and cellular roles established in chloride channels (Kole, et al., 2003): (1) pH regulation (2) Volume homeostasis (3) Transport of organic solute (4) Migration of cell (5) Proliferation and differentiation of cell Woodley and Whelan (1994, ¶ 1) stated that the stable internal cellular milieu contributes critically to the cellular function, and any disturbance in the fluid and electrolyte would produce different abnormalities in the body. Kole, et al., (2003) pointed out that because of the aforementioned roles of chloride ions, different undesirable effects are noted when the activity of chloride channels is modulated. To date, studies in cardiovascular physiology and pathophysiology noted that the functional role of anion channels of chloride is much less known compared to that of cation channels of potassium, sodium, and calcium (Dy, et al., 2005). The fluid, electrolyte, and acid – base body composition is regulated tightly by the contribution of pulmonary, renal, and endocrine organ system (Woodley and Whelan, 1994, ¶ 1). Difficulties in the interpretation of data of voltage clamp in the past arose from the complexity of cardiac tissue as well as from various components of ionic currents’ unequivocal separation (Reuter, 1984). However, single cardiac cells can now be voltage clamped and dialyzed internally because of the evolution of science. As a result, patch clamp method can be used in the identification and analysis of single ion channels (Reuter, 1984). Dy, et al., (2005) noted that in cardiac cells, different types of currents of chloride have been recorded from different species inclusive that of human beings. To date, five different genes in the chloride channels encode all cardiac channels of chloride. These include the “PKA- and PKC-activated cystic fibrosis tansmembrane conductance regulator (CFTR), the volume-regulated ClC-2 and ClC-3, and the Ca2+-activated CLCA or Bestrophin,” (Dy, et al., 2005). In the context of health and disease, the functional role of chloride channels was examined using several approaches which demonstrated that chloride channels might contribute to the following (Dy, et al., 2005): (1) Arrhythmogenesis in the injury of myocardium (2) Cardiac ischemic preconditioning (3) During myocardial hypertrophy and heart failure, chloride channels contribute to the adaptive remodelling of the heart. West (1991, ¶ 3) noted that studies of chloride channels are not considerably investigated and the search for the chloride channels’ potent and specific blocker remains to be on progress. Fong (2002) stated that several examples of chloride channel dysfunction have been offered by the CLC family of voltage gated channels of chloride (cited from Jentsch, et al, 2002, ¶ 2). Zhang, et al. (1997) reported that like any other types of cells, the cardiac cells regulate the volume when challenged by extracellular osmolarity reduction. A net loss of intracellular inorganic and organic osmolytes together with osmotically obliged water when the regulatory volume decrease is activated under hyposmotic conditions (Zhang, et al., 1997). In response to swelling of the cardiac cells, the activation of conductance that is selective with chloride has been associated with processes of volume regulation. However, not much information is available that would explain the mechanism responsible for signalling chloride channel activation (Zhang, et al., 1997). Zhang, et al. stated that in the regulation of cell volume, membrane transporters are activated by a number of secondary messenger systems. Zhang, et al. added that in their earlier studies, the activation of swelling induced chloride current is determined by levels of intracellular calcium and adenosine 3,5-cyclic monophosphate (cAMP); however, the mechanism sensing cellular volume changes and intracellular secondary messenger initiation remains unknown at large. Zhang, et al. also noted that the regulation of cell volume implicated the cytoskeleton in several lines of evidence, and in a variety of types of cells, swelling of the cells is associated with F – actin conformational changes. Zhang, et al (1997, ¶ 3) added that F – actin disruption, together with cytochalasin B (action polymerization inhibitor) was shown to abolish regulatory volume decrease. For that reason, in order to have a normal response of volume regulation, the network of F – actin that is intact must be considered to be essential. Cardiovascular Physiology Overview of Cardiac Electrophysiology Rogers (1999) stated that spontaneous contraction of the cardiac muscles resulted from voltage change across the membrane of the cells that conducts an action potential. Rogers noted that myocardial contraction is caused by an electrical impulse that depolarizes from the sinoatrial node going to the two ventricles. A large increase of intracellular calcium concentration is caused by a depolarisation of the membrane of the myocardial cells causing contraction as a result of temporary binding of actin and myosin fibrils (Rogers, 1999). Rogers noted that the action potential that takes place in the myocardium is much prolonged than that of the skeletal muscles. Figure 1: Action Potential of the Heart (Source: Kirk, 2005) Action Potential of the Heart The cardiovascular sequence of action potential is a consequent event that was caused by activating at least nine channels of ions. Opening of fast channels of sodium causes initial depolarization that allows Na+ ionic influx into the cells and is subsequently reversed by opening of potassium channel that allows potassium influx (Guyton and Hall, 1996; Kirk, 2005). Influx of chloride ions is caused by the opening of chloride channel triggered by calcium ligand (Kirk, 2005). Movement of all ions is reduced by closing of channels, which therefore results to a plateau phase. These channels of slow calcium remain open, and finally, potassium efflux further results to repolarization (Guyton and Hall, 1996; Kirk, 2005). The slow movement of calcium ions inwardly causes “leaky” ionic barrier making the cell membranes of SA and AV nodes special. The electrical potential of the cell drift down to the critical level once the resting potential is achieved, which eventually leads to depolarization that triggers the action potential (Guyton and Hall, 1996; Kirk, 2005). Such characteristic explains the automaticity property of the heart which can only be altered through vagal stimulation and catecholamines. Discussion Hervé (2006) stated that a better understanding of structures of membrane channels and its functions allowed the investigation and innovation of approaches on the pharmacologic effects of traditional drugs such as antiarrhythmic and antiepileptic medications. To date, ion channels continued to be exploited in the market as target drugs (Hervé, 2006). The application of large scale format screening in measuring functions of ion channels offers enormous opportunities in discovering drugs and scholastic research. Hervé added that in order to find novel drugs, screening of large numbers of compounds and natural products on the function of ion channels has been allowed by several technologies. This improved further the capabilities of drug discovery of ion channel. In effect, the possibility of accelerating discoveries of biology of ion channel has been furnished (Hervé, 2006). In the pharmaceutical industry, a significant difficulty has been confronted during the macromolecular targets’ initial identification and selection upon which programs are initiated in discovering drugs de novo (Hervé, 2006). Several criteria such as biological functions, assay system for characterisation in vitro, and high output screening, has to be answered by a drug target and in vivo, this has to be modified specifically and has to be made accessible to compounds that are small in molecular weight (Hervé, 2006). Discovering novel drugs are attributed by membrane channels and can be viewed as targets that are suitable for drugs with small molecule. Hervé (2006) noted that in the lipid bilayer, macromolecular complexes of membrane channels contain aqueous central pores that allow ion passage where small molecules are embedded. These macromolecular complexes plays a critical role in various physiological processes such as transduction of electrical signal, chemical signals, transepithelial transport, regulation of ion and pH in the cytoplasm and vesicles, as well as regulation of volume in the cell. As a result, drugs that modulate the activities of ion channel are later on discovered that it can treat primarily various common diseases such as arrhythmia, type II diabetes, and many others (Hervé, 2006). On the account of the aforementioned statement, Dy, et al., (2005) concluded that channels of anion had represented novel targets that are very attractive for therapeutic approaches in treating diseases of the heart. Latest data suggested that like channels of cations, chloride channels have the capacity to function as complexes of multiprotein or functional module. The assessment of the structures and functions of integrated chloride channels’ cardiac multiprotein complexes that emerged in the post – genome era necessitated a new paradigm shift that provides a new insight responsible for the underlying mechanisms of the abnormalities and protection of the heart (Dy, et al., 2005). Frédéric (2006) and Becq (2006) stated that since year 1987, more than ten genes of ion channel were identified to cause hereditary diseases in the human. “Voltage-dependent chloride channel ClC-1 (myotonia) and the cystic fibrosis transmembrane conductance regulator (CFTR) protein (cystic fibrosis)” were among of these genes (Frédéric, 2006; Becq, 2006). In 1989, a gene for CFTR was cloned and an ATP - gated and phosphorylation - regulated channel of chloride, a protein product, were identified two years later. Frédéric and Becq reported that from that time onwards, the evolution of CFTR pharmacology developed to identify novel molecules, and constituted a reservoir of future agents of therapy for cystic fibrosis. Parallel to the claim of Frédéric, in 2000, Mulvaney, et al. reported that drugs that have the capacity to block cation channels in the heart were marketed to solve cardiac arrhythmia; however, improving the survival rate amongst these cardiac patients was only moderately successful. As a consequence, a newer target for future agents that treats arrhythmia has been needed. The explanation of the identity and physiological roles of chloride channels were left behind by many other drug targets because these ions represents the under - explored class target of medicine breakthrough (Verkman and Galietta, 2009). Little did they know that in a wide range of biological functions, chloride channels is involved in the secretion of epithelial fluid, regulation of cell – volume, neuroexcitation, contraction of smooth muscle, and intracellular organelle acidification. In line with these, human diseases that include cystic fibrosis, degeneration of macula, myotonia, kidney stones, renal salt wasting, and “hyperekplexia” are caused by mutations in several channels of chloride (Verkman and Galietta, 2009). Verkman and Galietta added that modulators of chloride channel, namely, GABAA (-aminobutyric acid A) receptors have been used in the clinical setting, and several modulators of small molecule channels of chloride are now being tried in the clinics. Fong (2004) stated that chloride channels have been referred not long ago by physiologists and biophysicists as the “dark side.” Nevertheless, having these molecular and structural tools, chloride channels became an interesting subjects with increasingly sophisticated physiological techniques. Consequently, the role of chloride channels in the regulation of cell – volume, membrane potential stabilization, vesicular acidification, and epithelial transport is now much appreciated in the research and pharmaceutical industry. Fong (2004) also pointed out that physiological disorders are usually observed when these chloride channels go wrong. Hence, the disorders that have been observed were used as an opportunity to learn the properties that are fundamental to these channels that led to such state, and consequently, improve ones understanding towards the development of possible therapies. However, Fong (2004) pointed out in her study that lack of specific pharmacological tool is one of the limitations of studying channels of chloride. To further support the claim of Fong (2004), Xie, et al., (2004), in their study noted that in recent developments, ion channels are becoming more promising significant target for discovering drugs. These novel channels of ions were identified and were being targeted in treating various diseases including cancers, immune disorders, and other diseases such as CNS and cardiovascular diseases (Xie, et al., 2004). Xie, et al. added that multiple normal physiological processes that include contraction of the muscle and signal transmission of the neurons require ion channels, and ion channel function defects results to various disorders such as cystic fibrosis and cardiac arrhythmia. However, the extent of alteration in the subcellular element distribution of sodium, potassium, chloride, calcium, magnesium, and phosphorus remains to be unknown, and a gap in the commercialization of “novel ion channel drugs” is witnessed in the last few years (Xie, et al., 2004; Akal, et al., 2003). Xie, et al. also noted that a promising new generation of drugs that may result to the screening methods and development of discovering newer drugs that led to blockbuster treatments like Norvasc and Ambien can be enforced to new target ion channel and are being anticipated among scientist and other developers of drug. Fortunately, numerous novel ion channels were discovered and characterised in the recent development of researches in the academe and industry alike. Moreover, the identification of future drugs for ion channel facilitated the technical advances in screening and structural elucidation (Xie, et al., 2004). It was also reported by Xie, et al. that an extremely successful class of drugs that has generated revenues of over 12 billion USD is formed by modulators of ion channel. To name a few, Norvasc, an antihypertensive agent that decreases cardiac muscle contractility, manufactured by Pfizer, generated 4 billion USD in 2003. Other drugs that modulate ion channels are Ambien and Xanax, which acts by increasing chloride flux through anion channels in the neurons (Xie, et al., 2004). Xie, et al. concluded that researchers are on the edge of producing a novel targeted drug with the recent characterization of functional TRP and novel channels of ion. The drug company that discovers, and exploited successfully these new channels possesses the following diverse assets and capabilities: (1) Novel chemistry access (2) Technology in electrophysiology and image – based screening (3) Medicinal chemistry (4) To protect downstream successes, intellectual property For the treatment of cancer, disorders in the immune system, osteoporosis, and other conditions that not typical for drug discovery on ion channel, Xie, et al. (2004) reported that in most recalcitrant diseases, program advancement allows investigators to test novel approaches. Sardini (2007) noted that for the longest time period, the volume – activated channel has been the most sought after, and a considerable investment of resources has been made by many laboratories for this scientific venture that results in several protein candidate proposals intended for scientific community’s extreme scrutiny. Sardini (2004) added that information on volume sensitive outwardly rectifying channel of chloride (VSOR) has not been acknowledged in the market. Most importantly, in cellular physiology, the molecular nature identification of VSOR will not only be a significant breakthrough, but offers a useful target in the cure of several pathological conditions routinely encountered by the medical practitioners (Sardini, 2007). Terashima, et al., (2009) stated that in 2003, Armstrong successfully completed a model study in the skeletal muscle on the regulation of osmotic cell volume induced by ion fluxes membrane because of the concept that cations and chloride are neither synthesized nor degraded and therefore, can be theoretically analysed. In his study, Armstrong used a simple cellular mode to reveal the role of sodium potassium pump and Vm in lowering chloride using hypothetical channels of sodium, potassium, and chloride as well as the standard sodium – potassium pump. Previous studies discussed had never mentioned the mechanism by which chloride ion channels caused several diseases and became the target for treating various diseases especially in the cardiovascular system. A study made by Akar, et al. (2003) noted that intracellular accumulation of chloride induced by “rapid pacing is a novel finding” and probably plays a role in the pathogenesis of atrial fibrillation by causing depolarization of resting membrane and reduction of ERP. Akar, et al. (2003) added that further evaluation in the exact mechanism of pacing - induced accumulation of chloride warrants further evaluation leading to novel therapeutic agent development for atrial fibrillation treatment. References Akar, J., Everett, T., Ho, R., Craft, J., Haines, D., Somlyo, A., and Somlyo, A. (2003). ‘Intracellular Chloride Accumulation and Subcellular Elemental Distribution during Atrial Fibrillation’, 107(2003), 1810. Becq, F. (2006). On the discovery and development of CFTR chloride channel activators. Retrieved April 5, 2009, from http://www.ionchannels.org/showabstract.php?pmid=16472140 Cosentino, B. (2004). Electrolyte Imbalance: A Matter of Equilibrium. 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