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.
