Cardiac Excitation-contraction Coupling

Image credit: video of contracting cell provided by Luke Howlett, Leeds

Excitation-contraction coupling (ECC) refers to the mechanism by which the electrical and mechanical systems of the heart are linked. At the cellular level, the electrical excitation (the action potential, AP) and mechanical force are linked by the intracellular Ca²⁺ handling system; the intracellular Ca²⁺ transient is the trigger signal for cellular contraction.

Cellular ECC depends on cellular structure and dynamics from the nanometre to millimetre scales, and the heartbeat itself is ultimately determined by the coordination of cellular contraction. In my research, computational models of ECC at multiple scales are developed and combined with experimental investigation to tease apart the multi-scale mechanisms of ECC in health and its failure in disease.

Mechanisms of Excitation-Contraction Coupling

Excitation-contraction coupling in cardiac cells relies on complex spatio-temporal dynamics, dependent on both protein function and cellular structure. This system will be briefly described here. We must first cover the concept of the Action Potential and its relation to the intracellular Ca²⁺ handling cycle.

Concept: The Action Potential

Cardiac cells are encased by a cell membrane. Differences in the concentration of charged particles (called ions) inside and outside the cell result in a membrane potential - a difference in electric potential across the membrane. The Action Potential refers to the change in this membrane potential during excitation, illustrated below.

Animation: The Action Potential. Left panel shows a schematic of a cell with different types of ion and the channels which allow them to pass (coloured circles and channels), and the right panel shows the membrane potential at different stages of the Action Potential.

Ions can cross the cell membrane through specialised proteins (called ion channels), which can open or close to allow or prevent the passage of ions.

As ions flow through these ion channels, the movement of charge creates ionic currents which can change the membrane potential. Because the opening and closing of ion channels can also be controlled by the membrane potential, the currents can be coordinated to create the shape of the Action Potential.

There are multiple types of ion channel, each associated with specific ions (sodium, potassium, calcium and others).

"Inward" currents are those where positive charge enters the cell (yellow and red in the animation) - they make the membrane potential more positive, which we call “depolarisation”: Larger inward currents generally mean greater excitability and longer durations of the Action Potential.

"Outward" currents are those where positive charge leaves the cell (blue in the animation) – they make the membrane potential more negative, which we call "repolarisation": Larger outward currents generally mean lower excitability and shorter duration.

The balance of multiple different currents, controlled by the expression (amount of) each type of ion channel, can therefore control Action Potentials with different shapes and properties to match the varying requirements of cardiac function in different animals and under different conditions (e.g. rest versus exercise).

Concept: The intracellular Ca²⁺ handling cycle

The intracellular Ca²⁺ handling cycle is what couples the electrical and mechanical functions of a cell. Associated with each given cellular excitation (the Action Potential) is the intracellular Ca²⁺ transient (orange trace in the animation below), where the mechanical force follows this profile (with a delay). In order to maintain reproducible Ca²⁺ transients on every beat, cardiac cells require a sophisticated and regulated system to control the dynamics of the intracellular Ca²⁺ concentration. In broad terms, the system is determined by a process referred to as "calcium-induced-calcium-release" (described and illustrated below).

Animation: The intracellular Ca²⁺ handling cycle. Left panel shows a schematic of the cellular components involved in Ca²⁺ handling: the surface membrane and T-tubules (TT); the intracellular Ca²⁺ store called the Sarcoplasmic Reticulum (SR; blue-green); the contractile proteins (myofilaments); the Ca²⁺ carrying flux channels responsible for the initial influx (red), intracellular Ca²⁺ release (blue), SR refilling (yellow) and Ca²⁺ extrusion (green). Arrows indicate the timing and direction of fluxes. Green areas indicate high Ca²⁺ concentration. The right panel shows the Action Potential (blue) and whole-cell Ca²⁺ transient (orange) at each phase.

The Intracellular Ca²⁺ handling cycle:

Influx: Ca²⁺ ions enter the cell during the AP, through openings of channels in the cell membrane which are primarily selectively permeable to Ca²⁺ ions (these are represented by the red channel in the illustration above and AP illustration previous).
Release: Local elevation of Ca²⁺ concentration promotes binding with Ca²⁺-release channels (blue channels in the animation) on the surface of the intracellular Ca²⁺ store, the "SR"; these channels open, releasing a larger amount of Ca²⁺ from the store into the bulk intracellular space.
Diffusion: This Ca²⁺ then propagates throughout the cell, filling the volume of the intracellular space.
Contraction: This global elevation of Ca²⁺ concentration promotes binding with contractile proteins, called myofilaments, which hinge together to cause cellular contraction.
Relaxation: During relaxation, Ca²⁺ is released from its binding sites with the myofilaments.
And the resting levels of Ca²⁺ are restored through energy dependent ion-pumps which refill the intracellular Ca²⁺ store (yellow channels) and remove Ca²⁺ into the extracellular space (green channels).

All of these various fluxes must balance in order to maintain homeostasis; imbalance in these fluxes as well as degradation of the various protein function can result in failed coupling between the electrical and mechanical function and/or pro-arrhythmic behaviour (abnormal cellular dynamics which promote a loss of the regular rhythm of the heart).

Researching ECC through computational modelling

Modelling the intracellular Ca²⁺ handling system

Because the intracellular Ca²⁺ handling system depends on both channel dynamics and intracellular structure, sophisticated and accurate computational models of the system require consideration of both of these factors - "spatio-temporal" Ca²⁺ handling models. At these microscopic scales, random transitions of the channel proteins are a critical property of successful calcium-induced-calcium-release, and so the models must employ non-deterministic components (deterministic referring to equations which give the exact same answer with the same inputs) to reproduce these stochastic dynamics (referring to behaviour which depends on randomness, so the exact same answer is not given for repeated solutions using the same inputs). Due to the inherent complexity, and the relatively limited available experimental data, these models are less well developed than traditional cell models which treat the cell as a "point-source", such as those discussed in the Atrial Fibrillation section, and significant development is still required.

Incorporating realistic structure into whole-cell Ca²⁺ handling models

Figure: Electron-microscope images of intracellular cardiac structure (left), with 3D reconstructions of the intracellular Ca²⁺ store (the network like structure; lower panels) and locations of the sites of calcium-induced-calcium-release (green dots; lower panels). Reconstructions of these structures in a portion of the cell for computational simulation are shown on the right. The upper panels show the surface membrane and its protrusions into the cell interior, with the locations of the sites of calcium-induced-calcium release (blue dots); the lower panel shows the reconstruction of the intracellular Ca²⁺store. Data and images from Colman et al. 2017. PLOS Comp. Biol.

In collaboration with Ashraf Kitmitto at the University of Manchester, whom collected high-resolution imaging data of cardiac sub-cellular structure, an approach was developed to incorporate the realistic structure of multiple aspects of the Ca²⁺ handling system into a single computational model. These structures were imaged by my collaborators using an electron microscope, allowing resolutions at the 10s of nanometre scale (that's a millionth of a millimetre). Incorporating all of these structures involved processing of these imaging data and reconciliation of features at multiple scales into an efficient cell model. The model revealed the importance of these structures and fluxes in determining the spatial profile of Ca²⁺ during both normal and abnormal excitation.

Animation: Simulation of normal cardiac exciation in the 3D cell model. The traces on the left show the membrane potential (upper) and whole-cell average Ca²⁺ in the bulk intracellular space (middle) and the intracellular store (lower). Spatial dynamics are shown for the bulk intracellular space (centre panels; purple-green-yellow indicates low-high concentrations of Ca²⁺) and intracellullar store (right panels; purple-orange-yellow indicates low-high concentrations). Data and video from Colman et al. 2017. PLOS Comp. Biol.