Cardiac Muscle Excitation–Contraction Coupling
Cardiac muscle contracts when cytosolic [Ca2+] rises above about 100 nmol/L. This rise in [Ca2+] couples the action potential (AP) to contraction, and the mechanisms involved are referred to as excitation–contraction coupling. The relationship between cardiac muscle force and stretch is discussed in Chapter 17. The ability of cardiac muscle to generate force for any given fibre length is described as its contractility. This depends on cytosolic [Ca2+], and to a lesser extent on factors that affect Ca2+ sensitivity of the contractile apparatus. The contractility of cardiac muscle is primarily dependent on the way that the cell handles Ca2+.
During the plateau phase of the AP, Ca2+ enters the cell through L-type voltage-gated Ca2+ channels (Figure 12). L-type channels are specifically blocked by dihydropyridines (e.g. nifedipine) and vera- pamil. However, the amount of Ca2+ that enters the cell is less than 20% of that required for the observed rise in cytosolic [Ca2+] ([Ca2+]i). The rest is released from the sarcoplasmic reticulum (SR), where Ca2+ is stored in high concentrations associated with calsequestrin. APs travels down the T tubules which are close to, but do not touch, the terminal cisternae of the SR (Figure 12a). During the first 1–2 ms of the plateau Ca2+ enters and causes a rise in [Ca2+] in the gap between the T tubule sarcolemma and SR. This rise in [Ca2+] activates Ca2+-sensitive Ca2+ release channels in the SR, through which stored Ca2+ floods into the cytoplasm. This is called calcium-induced calcium release (CICR) (Figure 12a). The amount of Ca2+ released depends both on the content of the SR and size of the activating Ca2+ entry, and modulation of the latter is the major way by which cardiac function is regulated (see Regulation of contractility below). Ca2+ release and entry combine to cause a rapid increase in [Ca2+]i, which initiates contraction. Peak [Ca2+]i normally rises to ∼2 µmol/L, although maximum contraction occurs when [Ca2+]i rises above 10 µmol/L.
The arrangement of actin and myosin filaments is discussed in Chapter 2. Force is generated when myosin heads protruding from thick filaments bind to actin thin filaments to form crossbridges, and drag the actin past in a ratchet fashion, using ATP bound to myosin as an energy source. This is the sliding filament or cross-bridge mechanism of muscle contraction. In cardiac muscle [Ca2+]i controls crossbridge formation via the regulatory proteins tropomyosin and troponin. Tropomyosin is a coiled strand which, at rest, lies in the cleft between the two actin chains that form the thin filament helix, and covers the myosin binding sites. Myosin therefore cannot bind, and there is no tension. Troponin is a complex of three globular proteins (troponin C, I and T), bound to tropomyosin by troponin T at intervals of 40 nm. When [Ca2+]i rises above 100 nmol/L, Ca2+ binds to troponin C causing a conformational change which allows tropomyosin to shift out of the actin cleft. Myosin binding sites are uncovered, myosin crossbridges form and tension develops. Tension is related to the number of active cross bridges, and will increase until all troponin C is bound to Ca2+ ([Ca2+]i >10 µmol/L).
When [Ca2+]i rises above resting levels (∼100 nmol/L), ATP- dependent Ca2+ pumps in the SR (sarcoendoplasmic reticulum Ca2+-ATPase; SERCA) are activated, and start to pump (sequester) Ca2+ from the cytosol back into the SR (Figure 12b). As the
AP repolarizes and L-type Ca2+ channels inactivate, this mechanism reduces [Ca2+]i towards resting levels, so Ca2+ dissociates from troponin C and the muscle relaxes. However, the Ca2+ originally entering the cell must now be expelled. Ca2+ is transported out of the cell by the membrane Na+–Ca2+ exchanger (NCX) (see Chapters 10 and 11). This uses the inward Na+ electrochemical gradient as an energy source to pump Ca2+ out, and three Na+ enter the cell for each Ca2+ removed (Figure 12b). Sarcolemmal Ca2+-ATPase pumps are present but less important. At the end of the AP about 80% of the Ca2+ will have been resequestered into the SR, and most of the rest ejected from the cell. The remainder is slowly pumped out between beats.
Regulation of contractility
Inotropic agents alter the contractility of cardiac muscle; a positive inotrope increases contractility, while a negative decreases it. Most inotropes act by modulating cell Ca2+ handling, although some may alter Ca2+ binding to troponin C. A high plasma [Ca2+] increases contractility by increasing Ca2+ entry during the AP.
Noradrenaline from sympathetic nerve endings, and to a lesser extent circulating adrenaline, are the most important physiological inotropic agents. They also increase heart rate (positive chronotropes; see Chapter 11). Noradrenaline binds to β1-adrenoceptors on the sarcolemma and activates adenylate cyclase (AC), causing production of the second messenger cAMP. This activates protein kinase A (PKA), which phosphorylates L-type Ca2+ channels so that they allow more Ca2+ to enter during the AP (Figure 12c; see Chapter 11). The elevation of [Ca2+]i is thus potentiated and more force develops. Any agent that increases cAMP will act as a positive inotrope, for example milrinone, an inhibitor of the phosphodiesterase that breaks down cAMP. Noradrenaline (and cAMP) also increase the rate of Ca2+ reuptake into the SR, mediated by PKA and phosphorylation of phospholamban, a SERCA regulatory protein. While not affecting contractility, this assists removal of the additional Ca2+ and shortens contraction, which is useful for high heart rates.
The classic positive inotropic drug is digoxin, a cardiac glycoside. Digoxin inhibits the Na+ pump (Na+-K+ ATPase) which removes [Na+] from cells. Intracellular [Na+] therefore increases, so reducing the Na+ gradient that drives NCX (see Chapter 11). Consequently, less Ca2+ is removed from the cell by the NCX (Figure 12c) and peak [Ca2+]i and force increase.
Overstimulation by positive inotropes can lead to Ca2+ overload, and damage due to excessive uptake of Ca2+ by the SR and mitochondria. This can contribute to the progressive decline in myocardial function in chronic heart failure (see Chapter 46), when sympathetic stimulation is high.
Acidosis is negatively inotropic, largely by interfering with the actions of Ca2+. This is important in myocardial ischaemia and heart failure, where poor perfusion can lead to lactic acidosis and so depress cardiac function.
Influence of heart rate
When heart rate increases there is a proportional rise in cardiac muscle force. This phenomenon is known as the staircase, Treppe or Bowditch effect. It can be attributed both to an increase in cytosolic [Na+] due to the greater frequency of APs, with a consequent inhibition of NCX (see above), and to a decreased diastolic interval, which limits the time between beats for Ca2+ to be extruded m the cell.