Physiological and pharmacological modulation of cardiac contractile proteins

The contractile-relaxation cycle of cardiac muscle is dependent upon the coordination of a variety of membrane and cytosolic events. The functional activities of the structures responsible for these events can be altered by endogenous modulators, on both a beat-to-beat or a long-term basis, in order to optimize cardiac performance and meet the circulatory and nutritional needs of the body. This review focuses on the force-generating structures, or myofibrils, of cardiac muscle, its activation by intracellular calcium, and the ways that activation of the myofibrillar proteins can be modulated by physiological and pharmacological mechanisms. Activation of the myofibrillar proteins is regulated by the troponin-tropomyosin system of the actin (thin) filament; when troponin C binds calcium, sites along the thin filament are made available for myosin (thick) filament interaction. There are a number of physiological mechanisms that influence the affinity of troponin C for calcium, three of which include troponin I phosphorylation, intracellular pH, and muscle length. The relative affinity of troponin C for calcium decreases in parallel with a reduction in muscle length and intracellular pH and in parallel with an increase in troponin I phosphorylation. The type of myosin (isozyme) present in cardiac tissue also regulates myofibrillar activity. Ventricular muscle can contain up to three isozymes of myosin (V1, V2, and V3). Myofibrils containing only the V1 myosin isozyme can have an ATPase activity two to threefold hight than myofibrils containing only the V3 myosin isozyme. The type of myosin isozyme present in cardiac tissue is determined by a variety of factors; thyrotoxicosis and strenuous exercise tend to increase the relative amount of the V1 isozyme, whereas insulin and thyroxine insufficiency as well as aortic and pulmonary stenosis tend to decrease the relative amount of the V1 isozyme. Besides these physiologicall mechanisms, there are a variety of pharmacological agents that have been shown to alter the calcium activation (sensitize or desensitize) and/or maximal activity (potentiate or attenuate) of the cardiac contractile proteins. Some cardiotonic agents which increase the calcium sensitivity of the contractile proteins are AR-L 115BS (Sulmazole), AR-L 57, pimobendan, APP 201-533, DPI 201-106, and MCI-154. However, some of these compounds are not selective and have ancillary activities, such as inhibiting cyclic nucleotide phosphodiesterase, so a single direct link to cardiotonic activity cannot be made. In addition to this class of agents, some calmodulin antagonists also can modulate cardiac contractile protein activity via modulation of calcium binding to troponin C. Some of the calcium-binding protein modulators, such as calmidazolium, trifluoperazine, perhexiline, and bepridil are several times more potent in increasing myofibrillar ATPase activity than either AR-L 115BS or APP 201-533. The effect of these agents on altering calcium activation of the contractile proteins is also dependent upon the species and cardiovascular condition of the animal from which myofibrils are obtained. For example, calmidazolium increased basal and decreased maximal myofibrillar ATPase activity in both normal and myopathic hamster hearts but only increased the calcium sensitivity of myofibrils prepared from normal canine hearts. Studies are ongoing to evaluate the effect of these types of agents on ATPase activity of myofibrils from normal human hearts and hearts of patients with end-stage failure. The relative advantages/disadvantages of using these types of agents to alter cardiac performance await future synthesis and identification of nanomolar potent compounds that selectively and specificaally alter the activity of cardiac contractile proteins.