Calcium signaling in cardiac myocytes

Abstract

Calcium (Ca(2+)) is a critical regulator of cardiac myocyte function. Principally, Ca(2+) is the link between the electrical signals that pervade the heart and contraction of the myocytes to propel blood. In addition, Ca(2+) controls numerous other myocyte activities, including gene transcription. Cardiac Ca(2+) signaling essentially relies on a few critical molecular players--ryanodine receptors, voltage-operated Ca(2+) channels, and Ca(2+) pumps/transporters. These moieties are responsible for generating Ca(2+) signals upon cellular depolarization, recovery of Ca(2+) signals following cellular contraction, and setting basal conditions. Whereas these are the central players underlying cardiac Ca(2+) fluxes, networks of signaling mechanisms and accessory proteins impart complex regulation on cardiac Ca(2+) signals. Subtle changes in components of the cardiac Ca(2+) signaling machinery, albeit through mutation, disease, or chronic alteration of hemodynamic demand, can have profound consequences for the function and phenotype of myocytes. Here, we discuss mechanisms underlying Ca(2+) signaling in ventricular and atrial myocytes. In particular, we describe the roles and regulation of key participants involved in Ca(2+) signal generation and reversal.

Figure 1.

Excitation contraction coupling in ventricular myocytes. Panel A illustrates the distribution of L-type VOCCs (Ai) and type 2 RyRs (Aii) in a section of a ventricular myocyte. The distributions of these proteins are essentially overlapping at the level of the light microscope. Panel B is a cartoon sequence of events leading to the generation of a Ca²⁺ signal within a ventricular myocyte. A small section of a ventricular myocyte is depicted with two T-tubule projections (T-tubule spacing ∼1.8 µm). During the diastolic phase (Bi), the L-type VOCCs (red channels on the T-tubule membranes) and RyRs (blue channels on SR membrane) are silent. Arrival of the action potential causes depolarization of the sarcolemma and activation of the L-type VOCCs thereby generating “Ca²⁺ sparklets” (Bii). The Ca²⁺ sparklets trigger activation of the RyRs thereby producing “Ca²⁺ sparks” (Biii). Panel Ci depicts the consistent, global Ca²⁺ responses observed in an electrically paced ventricular myocyte. The black and gray traces indicate the Ca²⁺ concentration (measured with fluo4) at the center and edge of the myocyte. The profile of the Ca²⁺ signal was essentially the same in both locations. Panel Cii illustrates what happens in a ventricular myocyte following detubulation (using formamide treatment). Detubulation decreases the amplitude of systolic Ca²⁺ transients, and provokes spatial heterogeneity of the resultant Ca²⁺ signals. The black and gray traces indicate the Ca²⁺ concentration at the center and edge of the myocyte. Whereas the Ca²⁺ responses in the edge of the myocyte were reasonably consistent, the signals in the center of the cell showed beat-to-beat variation in amplitude. Such Ca²⁺ signal alternans are a potential cause of cardiac arrhythmia.

Figure 2.

Excitation contraction coupling in atrial myocytes. Panel A illustrates the distribution of L-type VOCCs (Ai) and type 2 RyRs (Aii) in a section of an atrial myocyte. The pattern of L-type VOCC expression is clearly different from that in ventricular cells. The distribution of RyRs is similar to that in ventricular cells, except that there is an evident population of peripheral RyRs around the edge of the myocyte. Solely these peripheral RyRs align with the L-type VOCCs to produce functional dyads. Panel B is a cartoon sequence of events leading to the generation of a Ca²⁺ signal within an atrial myocyte. A small section of an atrial myocyte is depicted. There are no T-tubules, but instead two prominent SR tubules with a spacing of ∼1.8 µm. Such SR tubules have previously denoted as “Z-tubules,” as they occupy the Z-line (just like T-tubules). During the diastolic phase (Bi), the L-type VOCCs (red channels on the T-tubule membranes) and RyRs (blue channels on SR membrane) are silent. Arrival of the action potential causes depolarization of the sarcolemma and activation of the L-type VOCCs thereby generating “Ca²⁺ sparklets” at the periphery of the cell (Bii). The Ca²⁺ sparklets trigger activation of nearby RyRs thereby producing “Ca²⁺ sparks” (Biii). Panel Ci depicts the gradient of Ca²⁺ typically observed during electrical pacing of atrial myocytes. The Ca²⁺ signal at the edge of the cell (black trace) is larger and more rapidly rising than the central response (gray trace). The extent to which the Ca²⁺ signal occurs in the center of the cell (and thereby causes contraction) is dependent on the inotropic status of the cell. Application of a β-adrenergic agonist can make atrial Ca²⁺ signals become homogenous.

Figure 3.

Excitation contraction coupling in healthy and decompensated hypertrophic cardiac muscle. The figure depicts the architecture and molecular composition of a cardiac dyad in a normal, healthy myocyte (upper panel), a compensated hypertrophic situation (middle panel), and a decompensated failing myocyte (lower panel). A key depicting the symbols used to represent the major players in EC-coupling is provided at the bottom of the figure. Depolarization of the plasma membrane results in Ca²⁺ influx through L-type voltage gated channels in the T-Tubule, which stimulates Ca²⁺ release via RyRs located on the juxtaposed SR. Following diffusion out of the dyadic cleft, Ca²⁺ encounters the contractile filaments causing myocyte contraction. Myocyte relaxation is then brought about by Ca²⁺ recycling back into the SR by the SERCA pump or extrusion across the plasma membrane via NCX. Neurohormonal activation of Gα_q-coupled receptors leads to the formation of InsP₃, which stimulates Ca²⁺ release via InsP₃Rs located in the dyad. This InsP₃-stimulated Ca²⁺ release sensitizes neighboring RyRs, causing enhanced Ca²⁺ fluxes and increasing the frequency of arrhythmic events. Activation of β-adrenergic receptors increases intracellular cAMP, which activates PKA leading to phosphorylation of PLB. On phosphorylation, PLB dissociates from SERCA thereby enhancing Ca²⁺ transport activity. Compensated/adaptive hypertrophy is associated with enhanced EC-coupling. Significantly contributing to this phenotype is an increase in SR store loading. This is brought about by an up-regulation of SERCA activity mediated by either an increase in SERCA or decrease in PLB expression. Alternatively, as a result of PKA-dependent phosphorylation, PLB interaction with SERCA and suppression of Ca²⁺ transport may also be decreased. Increased SERCA activity also serves to increase the rate of relaxation thereby allowing more rapid cycles of myocyte contraction. The width of the dyadic cleft may also marginally increase at this stage of hypertrophic remodeling. However, IP₃Rs are up-regulated in the dyad during hypertrophy supplementing the Ca²⁺ signal arising via RyRs to possibly further support EC-coupling. During decompensated hypertrophy, myocyte architecture and protein expression are remodeled. Specifically, the width of the dyadic cleft is increased making it harder for Ca²⁺ arising via VOCCs to activate Ca²⁺ release from RyRs. T-Tubules also atrophy resulting in orphaned RyRs. The SERCA-PLB ratio is also modified to favor decreased SERCA activity. Notably, InsP₃R expression in the dyad is increased. As a result, more of the RyRs that are located in this region are close enough to InsP₃Rs to be affected by Ca²⁺ arising from them. Overactivation of these InsP₃Rs, for example, by the elevated levels of circulating ET-1 present during heart failure, promotes arrhythmias thereby contributing to the pathology associated with heart failure.