14
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
The second mechanism is the sodium-cal-
cium exchanger, through which Na+ and
Ca++ are transported in opposite directions.
The exchanger can operate in either direc-
tion across the sarcolemma depending on the
Em. In resting cells, the negative Em causes
Na+ to enter the cell in exchange for Ca++,
which leaves the cell. Three sodium ions are
exchanged for each calcium ion; therefore,
the exchanger generates a small (few milli-
volts) electrogenic potential that follows the
direction of Na+. The opposite occurs in depo-
larized cells. This exchanger is also strongly
influenced by changes in intracellular Na+
concentration. For example, when the activ-
ity of the Na+/K+-ATPase pump is decreased by
drugs such as digoxin, the increase in intra-
cellular Na+ concentration reduces the gradi-
ent for Na+ movement into the cell through
this exchanger, which results in less Ca++
efflux, thereby increasing intracellular Ca++.
As described in Chapter 3, this can lead to an
increase in the force of myocyte contraction.
Ion Channels
Ions move across the sarcolemma through
specialized ion channels in the phospholipid
bilayer of the cell membrane. These channels
are made up of large polypeptide chains that
span the membrane and create an opening in
the membrane. Conformational changes in the
ion channel proteins alter the shape of the chan-
nel, thereby permitting ions to transverse the
membrane channel or blocking ion movement.
Ion channels are selective for different cati-
ons and anions. For example, there are ion
channels selective
for sodium, potassium,
and calcium ions (Table 2-1). Furthermore,
a given ion may have several different types
TABLE 2-1
CARDIAC ION CHANNELS AND CURRENTS
C H A N N E L S
j
G A T IN G
j
C H A R A C T E R IS T IC S
Sodium
Fast Na+ (INa)
Voltage
Phase 0 of myocytes
Slow Na+ (If)
Voltage and
receptor
Contributes to phase 4 pacemaker
current in SA and AV nodal cells
Calcium
L-typ e (ICa)
Voltage
Slow inward, long-lasting current; phase
2 of myocytes and phases 4 and 0 of SA
and AV nodal cells
T-tyPe (ICa)
Voltage
Transient current; contributes to phase 4
pacemaker current in SA and AV nodal
cells
Potassium
Inward rectifier (IK1)
Voltage
Maintains negative potential in phase 4;
closes w ith depolarization
Transient outward (Ito)
Voltage
Contributes to phase 1
in myocytes
Delayed rectifier (IKr)
Voltage
Phase 3 repolarization
ATP-sensitive (IK, ATP)
Receptor
Inhibited by ATP; opens when ATP
decreases during cellular hypoxia
Acetylcholine activated (IK ACh)
Receptor
Activated by acetylcholine and adenos-
ine; Gi-protein coupled; slows SA nodal
firing
Calcium activated (IKCa)
Receptor
Activated by high cytosolic calcium;
accelerates repolarization
Ix, name of specific current.
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