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24 PA R T I / Anatomy and Physiology

Outward Currents
FACTORS MODIFYING
A cell can experience outward current in two primary ways: (1) ELECTROPHYSIOLOGIC
potassium can flow out of the cell or (2) chloride can flow inward.
Both ways tend to repolarize the membrane that had been depo- FUNCTION
larized and to stabilize the resting membrane potential. There are
many types of potassium currents in cardiac muscle. Factors that alter cardiac cell depolarization and repolarization do
so by affecting the rates of voltage changes, the magnitudes of
voltage changes, or the timing of the phases of the cardiac action
Outward Rectifying Current I K
potential. Such changes affect cardiac impulse generation, impulse
The outward rectifying current causes repolarization after an ac-
conduction, or both, and reflect the effects of environmental al-
tion potential. It opens slowly after depolarization, so it is also
terations on transcellular ionic fluxes.
called the delayed rectifying current. It carries potassium, and it
Impulse generation, or automaticity, is influenced by a cardiac
closes with repolarization. It also may be labeled I x , and has been
cell’s maximal diastolic repolarization, threshold level, and rate of
subdivided into I x1 early rapid component and I x2 late slower
spontaneous depolarization to threshold (slope of phase 4). If
component.
maximal diastolic repolarization becomes more negative, if thresh-
old becomes less negative, or if the slope of phase 4 becomes less
Background Outward Current I K1 steep, the rate at which the entire cell is spontaneously depolarized
This potassium current flows through channels that close with de- can become slower; opposite effects can lead to a more rapid rate
polarization and open with repolarization. Thus, when the cell is of spontaneous depolarization.
depolarized during the plateau phase, the channel is closed. Were Cardiac impulse conduction velocity is influenced by the rate of
the channel open, potassium would flow outward, resulting in a depolarization (slope of phase 0), the magnitude of depolarization
repolarization. This repolarization would abort the plateau and (amplitude of phase 0), the distance from resting potential to
halt the calcium current, which activates contraction. Hence, it is threshold level, the action potential and refractory period dura-
efficient that this channel is closed during depolarization. It is tions, and the resistance to current flow. If the rate or amplitude
open with repolarization and serves to stabilize the membrane po- of phase 0 is decreased, the difference between resting potential
tential close to the potassium equilibrium potential. It is some- and threshold is increased, the action potential or refractory peri-
times called the inward rectifier because it is highly permeant to ods are lengthened, or the resistance to current flow is increased,
inward potassium currents but less permeant to outward currents. the rate of conduction can slow. For example, Purkinje cells have
When the membrane is depolarized and potassium can flow out- faster conduction velocities than nodal cells because the Purkinje
ward, the channel closes. It is sensitive to the extracellular potas- cells have rapid sodium channels that create fast and large depo-
sium concentration. larization.
The responsiveness of cardiac cells is described by the relation be-
tween the membrane potential before rapid depolarization and the
Transient Outward Current I to
This potassium current is linked with early (phase 1) rapid repo- maximal velocity of conduction during rapid depolarization. Car-
larization. It opens when a cell is depolarized after a period of hy- diac cell excitability is described by the current required to alter the
perpolarization, and it closes quickly. membrane potential from resting to threshold. 44 Although once
threshold is reached, the cell rapidly depolarizes, the amplitude of
Other Potassium Currents the action potential can be decreased if the distance between the rest-
ing potential and the threshold potential is less than usual. Stimuli
A nonvoltage-dependent potassium current, which is activated by
that are insufficient to depolarize a cell to threshold are not effective
an increase in the intracellular calcium concentration (I Kca ), may
in initiating action potentials, but such stimuli can have an effect on
participate in the maintenance of the plateau and in repolariza-
ionic movements; in pathophysiological situations, these stimuli
tion. This current may be the same as or similar to the transient
may influence cardiac arrhythmia generation and conduction.
outward current (I to ).
Cardiac impulse generation, conduction, or both can be al-
Of potential importance in the diseased heart is the ATP-
tered by the effects on cardiac cells of changes in the ratio of ex-
dependent potassium channel. This channel opens when the ATP
concentration falls to 10% to 20% of normal. 41 The action po- tracellular to intracellular ionic concentrations, acid–base changes,
sympathetic and parasympathetic stimulation, myocardial stretch,
tential becomes abbreviated during ischemia. This channel may
cooling, ischemia, and heart rate changes. These factors often af-
account for such a phenomenon. It opens when the ATP level
fect different cardiac cells in different ways; the following section
drops, shortens the action potential duration, and results in less
discusses general selected examples of some of these alterations.
contraction when the substrate needed for contraction is un-
available. 43 (The effects of alterations in extracellular ionic concentrations
on cardiac electrical and mechanical functions are discussed in
Acetylcholine activates potassium channels whose outward
Chapter 7.)
currents decrease during depolarization. Although this phenome-
non may be related to potentiation of the background outward
potassium current (I K1 ), there is evidence for a separate voltage- Adrenergic and Cholinergic Effects
responsive potassium current (I KACh ), whose channels are regulated
by muscarinic cholinergic receptors. Catecholamines
Other outward currents have been identified. The sodium– This broad class of biologically active compounds includes many
potassium–ATPase pump usually generates a small outward cur- endogenous hormones and neurotransmitters (epinephrine, nor-
rent (I p ). epinephrine, and dopamine) as well as pharmacological agents.