Regulation of intracellular [Ca⁺⁺] in cardiac muscle

As previously noted, the force of cardiac muscle contraction depends on the intracellular [Ca⁺⁺]. Opposite each Z line there is a tubular structure, the T tubule, running at right angles to the plasma membrane of the cell (Fig. 2.3). The T tubules help to spread electrical excitation rapidly into the cell and they run close to the sarcoplasmic reticulum (SR) in which Ca⁺⁺ ions are stored. Ca⁺⁺ is pumped into the stores by using Ca⁺⁺ ATPase pumps which are regulated by the inhibitory protein phospholamban. The Ca⁺⁺ used to trigger contraction of cardiac muscle therefore comes from two sources, the SR (about 75% of the total) and also transmembrane flux of Ca⁺⁺ from the extracellular fluid (about 25% of the total). This is in contrast to skeletal muscle which only uses SR stores of Ca⁺⁺ for contraction.

The resting intracellular [Ca⁺⁺] is about 0.1 μmol/L and when an action potential (see p. 21) occurs in a cardiac muscle cell it triggers an initial increase in the intracellular calcium ion [Ca⁺⁺_i] concentration. The action potential results in an inward flow of calcium from the extracellular fluid where the ionized calcium concentration is about 1.2 mmol/L. This takes place through L-type calcium channels located in the T tubules and in the plasma membrane. The initial small increase in [Ca⁺⁺_i] causes the release of further calcium ions from the SR stores—the so-called calcium-induced calcium release. This is mediated by a Ca⁺⁺ binding site on the SR which is part of a calcium channel protein often referred to as a ‘ryanodine-sensitive receptor’ or as a ‘foot protein’ (Fig. 2.3). As a result of calcium release from the SR the [Ca⁺⁺_i] increases, normally to about 0.5–2 μmol/L. In heart failure (see Chapter 6) there are significant alterations in how myocyte [Ca⁺⁺] is regulated.

During relaxation some Ca⁺⁺ has to be exported back out of the cell and some replaced into the SR. Ca⁺⁺ is predominantly expelled from the myocyte via a 3Na⁺−Ca⁺⁺ exchanger which uses the inward ‘downhill’ movement of the 3Na⁺ to move Ca⁺⁺ out of the cell (Fig. 2.3). This mechanism per se does not consume ATP although the Na⁺/K⁺-ATPase actively expels Na⁺ across the plasma membrane in order to maintain the electrochemical gradient for Na⁺. A portion of the Ca⁺⁺ is actively expelled from the cell across the plasma membrane by Ca ATPases. ATP is also used to pump Ca⁺⁺ back into the SR stores. Within the SR much of the calcium is stored as ionized Ca⁺⁺. However some is attached to calcium binding proteins of which calsequestrin is one of the most important.

Physiological stimulation of sympathetic nerves to the heart results in an increased force of contraction (see Chapter 4). The β₁-adrenoceptor activation leads to a rise in intracellular cyclic AMP (see Fig. 4.6), a second messenger which activates several protein kinases. Subsequent phosphorylation of the protein phospholamban accelerates transport of Ca⁺⁺ into the SR thus favouring retention of Ca⁺⁺ in the SR at the expense of efflux back across the plasma membrane. Contractility of the heart is therefore increased by raising the amount of Ca⁺⁺ stored in the SR. The rate of relaxation of cardiac muscle is also increased as the Ca⁺⁺ re-enters the SR more quickly. The effects of cAMP in these events can be manipulated by drugs such as milrinone and caffeine which act as phosphodiesterase inhibitors and hence prolong the half-life of cAMP.

Resting potential of ventricular muscle cells

The resting potential of a cardiac muscle cell is about −85 mV and, as in other excitable cells, this occurs as a result of the ionic concentration gradients maintained by the action of the Na⁺/K⁺-ATPase (see Chapter 1). The intracellular [K⁺_i] is about 140 mmol/L whilst the extracellular [K⁺_o] is about 4 mmol/L. We can consider a theoretical cell with such a concentration gradient for K⁺ and, initially, an equal number of positive and negative charges inside the cell. There is a diffusion gradient for positively charged potassium ions to move out of this theoretical cell and thus create a charge imbalance (potential difference) across the cell membrane with the inside of the cell negatively charged. The negative charge inside cells is mainly in the form of organic phosphates and ionizable groups on proteins, molecules which are too large to follow the K⁺ across the cell membrane. Eventually a situation is reached where the tendency for K⁺ ions to move out of the cell down the concentration gradient is balanced by the electrical gradient which will tend to move K⁺ ions back into the cell. This concept of the balance between the diffusive gradient and the electrical gradient is the basis for the derivation of the Nernst equation.

The work which would have to be done to move a mole of K⁺ ions against an electrical gradient of E volts is:

where E = the potential difference in volts, Z = the valency of the ion (i.e. 1, in the case of Na⁺ and K⁺) and F = the number of charges in one mole of ions (the Faraday).

The work which would have to be done to move a mole of K⁺ ions against a chemical concentration gradient is:

where R = the gas constant, T = temperature in degrees Kelvin, K_o = potassium concentration outside the cell and K_i = potassium concentration inside the cell.

At equilibrium:

or

For a monovalent cation such as K⁺ at a typical body temperature of 37°C (310°K) this equation can be simplified to E_K = 61.5 log₁₀ (K_o/K_i) by multiplying all the constants together and converting from natural logarithms to base 10 logarithms.

E_K is called the equilibrium potential for potassium. This means it is the membrane potential at which the net of flow of potassium ions out of a cell down the diffusive gradient is exactly balanced by their net movement into a cell down the electrical gradient. Implicit in the concept of an equilibrium potential is that only a single ion is considered each time.

The equilibrium potential for K⁺ in cardiac muscle cells (−94 mV) is a little more negative than the actual resting potential (RP) of the cells which is about −85 mV.This is because the resting potential is also partly determined by the movement of ions other than K⁺. However because the membrane is relatively permeable to K⁺ and there is a substantial K⁺ gradient maintained by the Na⁺/K⁺-ATPase, potassium normally has the greatest influence on the magnitude of the resting potential. In practice the membrane is also a little permeable to Na⁺. It is possible to calculate the true resting potential by combining the concentration gradients of K⁺, Na⁺ and other ions weighted according to their relative permeabilities. Negatively charged ions have their concentration gradient reversed. The resulting equation is called the Goldman equation (see Box 2.1 for further explanation).

Cardiac action potential in ventricular muscle

The resting potential in cardiac myocytes is about −85 mV.When, as a result of a wave of excitation generated by pacemaker tissue, the cardiac muscle membrane potential has depolarized to the threshold potential (−60 to −65 mV) the opening of sodium gates in the membrane is triggered. As in other excitable cells, an action potential is then generated.

The ventricular muscle action potential has five phases (Fig. 2.4). In phase 0 the sodium gates open and the permeability to sodium (P_Na) increases about a hundred-fold which makes the sodium permeability much greater than that for other ions, including K⁺. As a result the membrane potential rises to between +20 and +30 mV, that is approaching the equilibrium potential for sodium (E_Na) which is about +40 mV. At this point the sodium gates are ‘inactivated’ by the electric charge distribution across the cell membrane and the sodium permeability falls.

In phase 1 the K⁺ permeability (P_K) begins to increase and K⁺ leaves the cell at an increased rate down both a favourable concentration and electrical gradient. However the membrane potential does not immediately fall to E_K because there is a simultaneous opening of L-type voltage-gated Ca⁺⁺ channels and an inward flow of Ca⁺⁺ ions from outside the cell (phase 2). This calcium current results in a plateau phase of the membrane potential which lasts for as long as the calcium current flows. As described earlier, these events cause the release of a larger quantity of Ca⁺⁺ from the sarcoplasmic reticulum which generates myocyte contraction (Fig. 2.3). Eventually the calcium channels are inactivated partly as a direct result of the rise in intracellular [Ca⁺⁺]. The membrane potential, under the influence of increased K⁺ channel opening, falls (phase 3) to a value close to the potassium equilibrium potential (E_K). At this stage the cycle recommences from the resting potential (phase 4).

As described, the Na⁺ channels in the muscle cell close at the peak of the action potential. They clearly have to be returned to a state where they can be stimulated to re-open before another action potential can be produced. The Na⁺ channels remain closed during the plateau phase of the action potential (Fig. 2.4) and stimulation of the muscle during this phase cannot produce a further action potential. This is the ‘absolute refractory’ period of the myocytes. During repolarization (phase 3) many, but not all, of the Na⁺ channels have re-opened by the time the membrane potential reaches −50 mV. Between −50 mV and complete repolarization a further action potential can be generated but this requires a greater than normal stimulation. This is the ‘relative refractory period’. This has an important consequence in clinical medicine. Hyperkalaemia, a rise in plasma [K⁺], is a frequent consequence of acidosis or inadequate excretion of K⁺ from the body, a process normally regulated by aldosterone effects on the kidney. Hyperkalaemia may become life threatening because it will lead to depolarization of cardiac myocytes, that is a rise in the resting potential towards zero. This will mean that return to a sufficiently low (negative) membrane potential to ensure opening of all populations of Na⁺ channels will not take place. Cardiac arrest may be the consequence.

The shape of the cardiac action potential is crucial to the functioning of the heart because the long plateau phase in the muscle cells outlasts the mechanical activity. This means that however hard the heart is stimulated individual contractions cannot fuse into a maintained tetanic contraction as happens in skeletal muscle. The heart is thus bound to beat rhythmically.

Pacemaker tissue

Cardiac muscle differs from skeletal muscle and most neurones in that in some areas of the heart the ‘resting potential’ is particularly unstable. After an action potential in these areas the membrane potential gradually drifts upwards (depolarizes) until a threshold potential is reached where the opening of sodium ion channel gates is triggered and ion permeability rises rapidly producing another action potential. Cardiac muscle therefore has the property of producing rhythmic depolarizations which, in turn, result in rhythmic contractions. The mammalian heart is said to be capable of myogenic activity. This can be seen when a piece of cardiac muscle is removed from a living heart. If it is kept warm and oxygenated in an artificial extracellular fluid environment it will continue to contract and relax spontaneously for some time. The resting and action potentials generated in the sinoatrial node are illustrated in Fig. 2.5.

The ability of a piece of cardiac muscle tissue to undergo spontaneous depolarization and hence generate action potentials is called automaticity. The parts of the heart which display automaticity are the sinoatrial node, the atrioventricular node and the bundle of His together with its Purkinje fibres. Ordinary ventricular muscle cells do not normally have automaticity.

The rate of contraction of any given piece of muscle will depend upon the rate of depolarization of the resting potential in the muscle cells. The part of the heart with the fastest rate of drift of the resting potential will have the fastest intrinsic rhythm. In the human heart this is the sinoatrial node (SAN), which is a band of tissue in the right atrium close to the junction with the superior vena cava (Fig. 2.6). In the SAN the K⁺ permeability is lower and hence the initial resting potential (−60 mV) is less negative than elsewhere in the heart mainly because of the absence of one type of K⁺ channel (the ‘inward rectifier’ potassium channel). The membrane potential drifts upwards (depolarizes) faster than in other parts of the heart and is called a pacemaker potential. The ionic events which contribute to the pacemaker potential are complex and include inward movement of Na⁺ ions, an outward movement of K⁺ ions which decays with time and, once the pacemaker potential has depolarized past −55 mV, there is an inward Ca⁺⁺ current. The latter ion movement accelerates the rate of depolarization towards the threshold potential of between −55 mV and −40 mV at which an action potential is triggered (Fig. 2.5). In the normal heart the SAN serves as the cardiac primary pacemaker and determines the rate at which the whole heart beats. The intrinsic rate of the human SAN is about 110–120 beats per minute (bpm) but at rest it is normally under tonic parasympathetic inhibition so that the resting heart rate is typically about 70 bpm.

If, for some reason, the SAN fails to function or becomes electrically isolated from the rest of the heart then the area with the next fastest intrinsic rhythm is the atrioventricular node (AVN) which has an intrinsic rate of about 50 bpm. If the conduction of activity to the ventricle through the AVN is interrupted (‘complete heart block’) then the ventricles will beat at their own rate of about 30–40 bpm driven by the intrinsic depolarization rate of the Purkinje fibres.

The control of heart rate is discussed in Chapter 4. Sympathetic nerve stimulation at the SAN increases the rate of phase 4 depolarization and therefore the threshold potential is reached more quickly and heart rate increases. Parasympathetic nerve stimulation via the vagus nerve slows the heart rate by a combination of two mechanisms. The SAN cells are hyperpolarized and the rate of rise of the phase 4 resting potential is also reduced. Both of these effects mean that it takes longer for the resting potential to reach the threshold potential.

An outline of a case history of a child with a disturbance of the cardiac conduction system is described in Case 2.1:1.

The transmission of the cardiac action potential

The cardiac action potential normally originates at the SAN because it is the region of the heart with the fastest intrinsic contractile rate. The action potential then spreads across the left and right atria. The atrial action potential is similar in shape to that of the ventricle although the plateau phase is shorter. There is a region of fibrous tissue called the annulus fibrosis, part of the fibrous skeleton of the heart, which effectively provides an area of insulation between the atria and the ventricles so that excitation must normally pass through the AVN in order to reach the ventricles. If there is an electrical ‘leak’ in the ring of fibrous tissue separating the atria and the ventricles this will be an alternative, direct route for excitation to spread from atria to ventricles. When this occurs it is called the Wolff–Parkinson–White (WPW) or pre-excitation syndrome and it generates a characteristic arrhythmia (see Chapter 7).

The cells within the AVN structurally resemble those of the SAN and the shape of their action potentials is similar except that the initial resting potential is about −80 mV and the depolarization ‘overshoot’ does not normally exceed +5 to +10 mV. The depolarization drift in phase 4 is slower because of the absence of a population of sodium channels, hence the slower intrinsic rate of firing. The AVN provides a junction between the atria and the ventricles but it also inserts a small delay of approximately 0.1 second (AVN delay) into the onward passage of the action potential. This is reflected in the rather gentle slope of phase 0 of the action potential in these cells and is caused by the small diameter of the nodal cells and their complex morphology. The delay allows time for the contraction of the atria to complete ventricular filling before the ventricles contract and expel blood from the heart.

The depolarization passes from the AVN into the ‘bundle of His’ (Fig. 2.6) which consists of specialized conducting tissue known as Purkinje fibres. These fibres are modified ventricular muscle cells and are grouped together in left and right bundles. They have a relatively large diameter and they are the largest of all the cells in the heart. This means they have a high conduction velocity. Purkinje fibres, together with the AVN tissue, have the longest refractory period of any cardiac cells. The functional importance of this is that is protects the heart against ‘re-entry’ excitation from adjacent myocytes back into the conducting tissue which could potentially spread back to the atria and cause dangerous arrhythmias. Other characteristics of the cells which comprise the conducting system are that although they are modified cardiac myocytes they have few myofibrils and so do not contract significantly when they are depolarized.

The Purkinje fibres conduct excitation quickly down each side of the septum before spreading out over the ventricles. As a result, depolarization of the ventricles occurs in a prescribed sequence starting with the papillary muscles and the septum and then spreading to the endocardial (inner) part of the ventricular muscle and out towards the epicardial (outer) surface. This coordinated spread of the electrical activity through the heart is responsible for the shape of the ECG (see Chapter 7). Repolarization of the muscle proceeds from epicardial to endocardial surface, the opposite direction to depolarization. This is the basis for the fact that the QRS complex (depolarization) and the T wave (repolarization) are both upwards deflections in a normal ECG, opposite polarity currents moving in opposite directions.

Arrhythmias

The beating of the heart is normally driven by the spontaneous activity of the SAN and the heart is thus said to be in sinus rhythm. However there are occasions when this is not the case and an arrhythmia (the more scientifically rigorous term dysrhythmia is preferable but sadly it is not in widespread use) may be present. Arrhythmias may result from abnormal depolarization of cardiac tissue such that the cardiac rhythm is not being generated by the SAN or they may occur because cardiac excitation originates at the SAN but its conduction through the heart is abnormal (see Chapter 7).

If a piece of cardiac muscle is damaged and unable to conduct activity then the wave of excitation must go around it much like a crowd of people leaving a football match might move to either side of a tree. If one pathway is much longer than the other then, when the activity rejoins the healthy tissue, it may do so after the impulse which took the shorter route has gone and the cells are past their refractory period and capable of being stimulated again. As a result an action potential will be set up which will be conducted in the conventional direction but also backwards (retrogradely) along the faster route (Fig. 2.7). This activity may collide with the next impulse passing down the faster route and negate it, or it may carry on around the loop producing what is called a re-entry arrhythmia. In circumstances where the ventricular rate is irregular, not all ventricular contractions may result in sufficient ejection of blood to allow a pulse to be palpated at, for example, the radial or brachial artery. As a result the pulse rate may not be the same as the ventricular rate recorded on an ECG.

On occasions, the ability of the wave of depolarization originating from the SAN to successfully pass through the AVN is impaired and this will result in various degrees of ‘heart block’. In first-degree heart block there is a prolongation of the P–R interval of the ECG to greater than 0.2 s (see Chapter 7) as transmission through the AVN is slowed. In second-degree heart block one may see a situation where not every wave originating at the SAN is able to pass through the AVN. This usually occurs in a repetitive manner such that perhaps every third or fourth atrial depolarization wave fails to produce ventricular activity. Missed beats are often accompanied by a progressive lengthening of the P–R interval in the two or three preceding beats. Finally, in complete heart block there is no synchronicity between atrial and ventricular activity at all. In patients with complete heart block atrial systole will occasionally coincide with ventricular systole and thus contraction of the right atrium is attempting to move blood against a closed tricuspid valve (see Chapter 3). When this occurs blood from the right atrium flows back up the neck in the jugular vein producing what are known as cannon waves.

The bundle of His has two main branches, right and left. Blockade of conduction through only one branch is called bundle branch block. This is further discussed in Chapter 7 in the context of the characteristic ECG patterns.

Occasionally the phase 4 depolarization in cardiac cells may be more rapid than usual. This is likely to occur in pacemaker cells or in cells where the resting potential is normally stable but their ability to maintain a stable plateau potential is impaired, for example by ischaemia. This may result in the establishment of an ectopic pacemaker in areas of the heart where this does not normally occur. The ability of heart muscle to initiate its own activity, be it at a normal pacemaker site or at an ectopic site, is referred to as ‘automaticity’.

Antiarrhythmic drugs

There is a widely used classification of antiarrhythmic drugs—the Vaughan Williams classification.

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