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18 PA R T I / Anatomy and Physiology

is halted. To summarize, the sodium channel is conceptualized as calcium ion transported out of the cell. In this situation, the
having two gates. At resting membrane potential, the channel is pump is electrogenic, but the direction or ratios of transmem-
closed because the activation gate is closed. Depolarization opens brane ion exchanges may be reversed or changed. When the con-
that gate but, after a brief lag, the inactivation gate closes, again centration of intracellular sodium ion is increased (e.g., when the
closing the channel. Repolarization opens the inactivation gate use of digoxin-like drugs has partially blocked the sodium–
but closes the activation gate. potassium–ATPase pump), there is less energy stored in the
Scores of channels have been described, each with character- sodium gradient. This exchange mechanism does not promote as
istic gating and selectivity profiles. The mixing of channel types great a sodium influx and calcium efflux. There is then more cal-
in various membranes can produce a rich repertoire of biologic cium ion stored in the SR and more calcium ion released during
operating characteristics. The membrane of vertebrate cardiac activation, with net positive inotropic effects.
muscle is especially complex, with a diverse mix of channels.
The result is a dynamic, responsive membrane that can be finely Calcium ATPase Pumps. The cardiac SR actively pumps
tuned to varying operating conditions. Some of the other ma- calcium ion uphill into its core in a process that hydrolyzes ATP
jor channels of the vertebrate heart are described later in this as an energy source. An active calcium pump in the cardiac sar-
chapter. colemma also extrudes calcium ion from the cell. The latter may
be more important in vascular tissue than in cardiac muscle.
Active Ion Transport. Any movement of ion against its elec-
trochemical gradient is said to be active movement or active trans-
port. To move any ion against its electrochemical gradient requires CARDIAC ACTION POTENTIAL
energy. The energy may be stored in ATP. In some cases, the en-
ergy stored in one ion’s electrochemical gradient can be expended Each structural cardiac cell type (e.g., working myocardial, nodal,
to power the movement of another ion against its electrochemical Purkinje cells) has characteristic action potential features. Electri-
gradient. The former ion is said to be moving “downhill” or in the cally, there are two general types of cardiac cells: fast- and slow-
direction of a lower energy state. The ion that is moved against the response cells. Fast-response cells (e.g., Purkinje and working my-
gradient is said to be transported “uphill.” ocardial cells) have a fairly constant resting membrane potential, a
rapid depolarization, and then a period of sustained depolarization
Sodium–Potassium–Adenosine Triphosphatase Pump. (called plateau phase) before repolarizing to resting potential. Im-
At resting potential, there is a slight inward trickle of sodium pulse conduction to adjacent cells is rapid. Slow-response cells
ions. During activation, there is transient inward sodium cur- (e.g., sinus and AV nodal cells) slowly and spontaneously depolar-
rent. Sodium–potassium pumps on the cardiac muscle mem- ize during the interim prior to the action potential, and have a
brane (as well as on many other types of membranes) moves shorter, nonprominent plateau phase that merges into a slow repo-
sodium ion back out of the cell in exchange for an inward move- larization period. These cells conduct more slowly (Fig. 1-17).
ment of potassium ions. Both ions are moving against a concen- Ionic current differences account for varying action potential
tration gradient. The pump is powered by the energy stored in shape.
ATP; hence, the pump is known as the sodium–potassium pump In the following sections, the cardiac action potential is de-
or sodium—potassium–ATPase. This pump helps to re-establish scribed. Table 1-3 summarizes the electrophysiological properties
the resting concentrations of intracellular sodium and potassium of the various tissue types.
after cardiac depolarization. The ratio of sodium ions pumped
out to potassium ions pumped in is usually 3:2. This ratio of 3:2
results in a net outward charge movement, hyperpolarizing the Fast-Type Myocardial Action
membrane. A primary regulator of this pump is the intracellular Potentials
sodium ion concentration. Other factors influencing pump ac-
tivity include extracellular sodium concentration and intracellu- The fast response type cell has a five-phase action potential (Fig.
lar and extracellular potassium concentration. Digoxin-like 1-18). Phase 0 is the initial period of rapid depolarization, the ac-
drugs block the sodium–potassium pump. 39 Epinephrine and tion potential upstroke. Membrane potential changes from resting
insulin both stimulate the sodium–potassium pump, causing potential (approximately 90 mV) to a value positive to 0 mV
uptake of potassium into cells. Clinicians capitalize on this fea- (e.g., 30 mV). After this brief ( 1 to 2 milliseconds) phase, the
ture when they administer insulin and glucose to the hyper- cell repolarizes slightly (phase 1) and then there is a period of sus-
kalemic patient. Epinephrine and insulin can be associated with tained depolarization called the plateau phase (phase 2). In phase 3,
hypokalemia. repolarization becomes rapid, returning the membrane to resting
potential. Phase 4 is the interval between action potentials; the
Sodium–Calcium Exchange. Another important cardiac resting potential is fairly constant. The cardiac action potential
membrane pump is the sodium–calcium pump. Calcium ion may take hundreds of milliseconds. Duration and amplitude of
moves across the sarcolemma into the cell to activate contraction. each phase depends on the opening and closing of various ion
It must be removed. Although there is some harvesting of calcium channels, which in turn depends on the ionic and neurohormonal
ion into the intracellular sequestering sites such as SR, the inward milieu. Conduction to adjacent cells is rapid.
movement and storage cannot go on unopposed. Calcium ion is
moved back into the extracellular space by means of an exchange Phase 0: Action Potential Upstroke
pump. The energy stored in the sodium gradient powers the The working myocardial cell action potential is initiated by an in-
movement of calcium ion. In other words, sodium ion is moved ward current flowing primarily by way of the low-resistance nexus.
downhill to pump calcium ion uphill. 40 Usually, this exchange This small current depolarizes the cell to threshold (approximately
mechanism transports three sodium ions into the cell for one 70 mV; Fig. 1-19). Once threshold voltage is reached, the