CHAPTER 3 • CELLULAR STRUCTURE AND FUNCTION
43
which serve as the site of myosin adenosine
triphosphatase (myosin ATPase), an enzyme
that
hydrolyzes
adenosine
triphosphate
(ATP). ATP is required for the cross-bridge
formation between the thick and thin fila-
ments. The molecule’s heads interact with a
binding site on actin (Fig. 3.2). Regulatory
subunits (myosin light chains) that can alter
the ATPase activity when phosphorylated are
associated with each myosin head.
Each thick filament is surrounded by a
hexagonal arrangement of six thin filaments.
The thin filaments are composed of actin,
tropomyosin, and troponin (Fig. 3.2). Actin
is a globular protein arranged as a chain of
repeating globular units, forming two heli-
cal strands. Interdigitated between the actin
strands are rod-shaped proteins called tropo-
myosin. Each tropomyosin molecule is asso-
ciated with seven actin molecules. Attached to
the tropomyosin at regular intervals is the tro-
ponin regulatory complex, made up of three
■ FIGURE 3.2 Com position of cardiac thick and
thin myofilaments. The thick filaments are com -
posed of myosin molecules, w ith each molecule
having two myosin heads, which serve as the
site of the myosin ATPase. Thin filaments are
composed of actin, tropomyosin, and regulatory
proteins (troponin complex, TN) having three sub-
units: TN-T (binds to tropom yosin), TN-C (binds to
calcium ions), and TN-I (inhibitory troponin, which
inhibits myosin binding to actin). Calcium binding
to TN-C produces a conform ation change in the
troponin-tropom yosin complex that exposes a
m yosin-binding site on the actin, leading to ATP
hydrolysis. For simplicity, this figure shows only
one actin strand and its associated tropom yosin
filament.
subunits: troponin-T (TN-T), which attaches
to
the
tropomyosin;
troponin-C
(TN-C),
which serves as a binding site for Ca++
dur-
ing
excitation-contraction
coupling;
and
troponin-I
(TN-I), which inhibits myosin
binding to actin. The troponin complex holds
tropomyosin in position to prevent binding
of myosin heads to actin. When Ca++
binds to
TN-C, a conformational change occurs in the
troponin complex such that the troponin-
tropomyosin complex moves away from the
myosin-binding site on the actin, thereby
making the actin accessible to the myosin
head for binding. When Ca++
is removed from
the TN-C, the troponin-tropomyosin com-
plex resumes its inactivated position, thereby
inhibiting myosin-actin binding. As a clini-
cal aside, both TN-I and TN-T are used as
diagnostic markers for myocardial infarction
because of their release into the circulation
when myocytes die.
Excitation-Contraction Coupling
TRANSVERSE TUBULES AND THE
SARCOPLASMIC RETICULUM
The coupling between myocyte action poten-
tials and contraction is called excitation-
contraction
coupling.
To
understand
this
process, the internal structure of the myo-
cyte needs to be examined in more detail.
The sarcolemmal membrane of the myocyte
surrounds the bundle of myofibrils and has
deep
invaginations
called
transverse
(T)
tubules (Fig. 3.3), particularly in ventricular
myocytes. The T tubules, being a part of the
external sarcolemma, are open to the exter-
nal environment of the cell. This permits ions
to exchange between extracellular and intra-
cellular compartments to occur deep within
the myocyte during electrical depolarization
and repolarization of the myocyte. Within
the cell, and in close association with the
T tubules, is an extensive, branching tubular
network called the sarcoplasmic reticulum
that surrounds the myofilaments. The pri-
mary function of this structure is to regulate
intracellular calcium concentrations, which
is involved with contraction and relaxation.
Terminal cisternae are end pouches of the
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