Biomedical Engineering Reference
In-Depth Information
Table 5.10.
Calcium efflux in ventricular myocytes during relaxation. Relative participation of
the involved carriers. The Ca
2
+
amount entering the mitochondria is insignificant with respect
to excitation-contraction coupling, but cumulative changes in intra-mitochondrial
Ca
2
+
]
i
can
stimulate dehydrogenases that produce nicotinamide adenine dinucleotide (NADH) and ATP to
match increased energetic demands [
464
]. The relative activity (%) of the carriers depends on their
concentrations as well as animal species, here given for rabbit and rat. Calcium efflux in ferret,
dog, cat, guinea pig, and human ventricle is quantitatively similar to the rabbit. (Rat ventricles do
not mimic the huma
n heart with respect to Ca
2
+
efflux.)
[
Carrier
Rabbit
Rat
Sarcoplamic reticulum Ca
2
+
AT P a s e
∼
70
∼
92
Sarcolemmal Na
+
-Ca
2
+
exchanger
∼
28
∼
7
Sarcolemmal Ca
2
+
AT P a s e a n d
∼
1
∼
1
Mitochondrial Ca
2
+
uniporter
IP
3
-dependent Ca
2
+
release contributes to atrial Ca
2
+
signaling. Receptor IP
3
R
interacts with membrane-associated adaptor ankyrin-2. It localizes to caveolae with
hyaluronan
90
and cholesterol.
5.10.3.4
Sarco(endo)plasmic Reticulum Calcium ATPases
The cytoplasmic Ca
2
+
concentration is controlled by a pump in the membrane of
the calcium-storing sarcoplasmic reticulum that temporarily sequesters Ca
2
+
ions.
Sarco(endo)plasmic reticulum
Ca
2
+
ATPases
(SERCA) transport calcium ions from
the sarcomere and cytosol into the sarcoplasmic reticulum.
91
Plasma membrane
Ca
2
+
ATPases
,Na
+
-Ca
2
+
exchangers
92
in their forward
mode, and mitochondrial Ca
2
+
uniporters also participate in Ca
2
+
removal from
the cytosol of cardiomyocytes (Table
5.10
).
5.10.3.5
Regulators of Calcium Flux and Calcium Handling
Upon stimulation of G-protein-coupled receptors by their agonists, such as
endothelin-1 or angiotensin-2, members of the heterotrimeric G-protein class
provoke acute and chronic cardiac responses.
β
-Adrenergic receptors control the
90
A.k.a. hyaluronic acid and hyaluronate. It is an anionic, non-sulfated glycosaminoglycan. It
is ubiquitous. It is synthesized by integral membrane hyaluronan synthases (HAS1-HAS3) and
degraded by hyaluronidases. It also resides in the extracellular matrix.
91
Diverse SERCA types exist: SERCA1 in fast-twitch skeletal muscles; SERCA2a in the myo-
cardium and in slow-twitch skeletal muscles; SERCA2b in smooth myocytes; and SERCA3 in
endothelial cells.
92
NCX carrier competes with PMCA for Ca
2
+
efflux, but it may induce less than one-third of Ca
2
+
efflux.
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