Changes in Intracellular Ca2+ Mobilization and Ca2+ Sensitization as Mechanisms of Action of Physiological Interventions and Inotropic Agents in Intact Myocardial Cells.

Physiological and pharmacological interventions are used to regulate cardiac contractile functions via modulation of Ca2+ signaling. The relevant regulatory mechanisms have recently been assessed in detail by use of novel experimental procedures, which include simultaneous measurements of intracellular levels of Ca2+ ions and contractile force in intact myocardial preparations loaded with the intracellular Ca2+ indicator aequorin and fluorescent dyes, namely, fura-2, indo-1 and fluo-3. Association with or dissociation from intracellular Ca2+ transients of contractile activity is taken as evidence that reflects the primary mechanism of action of individual inotropic interventions. In addition, motility assays of actin-myosin interactions in vitro have made it possible to define the site of action of Ca2+ sensitizers as troponin C and the interaction of the troponin-tropomyosin complex with actin or the actin-myosin interface at crossbridges. Frank-Starling mechanism operates at the level of the binding of Ca2+ ions to troponin C and subsequent regulatory processes, while the force-frequency relationship is mainly ascribed to an alteration in the intracellular mobilization of Ca2+ ions. Cardiotonic agents can be classified as follows: 1) agents that act via a cyclic AMP-dependent or a cyclic AMP-independent mechanism; and 2) agents that facilitate the intracellular mobilization of Ca2+ ions or increase in myofibrillar sensitivity to Ca2+ ions. Regulatory mechanisms mediated via the phosphorylation of functional proteins induced by cyclic AMP, which is responsible for the actions of novel cardiotonic agents, beta 1-adrenoceptor partial agonist and selective inhibitors of phosphodiesterase (PDE) III, have currently been clarified in more detail. Ca2+ sensitizers are of extreme therapeutic interest because of their ability to increase myocardial contractility without an increase in activation energy; they are devoid of risks of arrhythmogenicity and myocardial cell death from intracellular Ca2+ overload; and they effectively reverse contractile dysfunction under pathophysiological situations, such as acidosis or myocardial stunning.