Cardiovascular Physiology
381
coupling are phosphorylated by the kinase, which alters their
activity. These include:
1.
DHP receptors (L-type calcium channels) in the plasma
membrane;
2.
the ryanodine receptor and associated proteins in the
sarcoplasmic reticulum membrane;
3.
thin fi lament proteins; in particular, troponin;
4.
thick fi lament proteins associated with the cross-
bridges; and
5.
proteins involved in pumping calcium back into the
sarcoplasmic reticulum.
Due to these alterations, cytosolic calcium concentration rises
more quickly and reaches a greater value during excitation, cal-
cium returns to its pre-excitation value more quickly following
excitation, and the rate of cross-bridge activation and cycling
are accelerated. The net result is the stronger, faster contrac-
tion observed during sympathetic activation of the heart.
There is little parasympathetic innervation of the ventri-
cles, so the parasympathetic system normally has only a negli-
gible direct effect on ventricular contractility.
Table 12–3
summarizes the effects of the autonomic
nerves on cardiac function.
+
1
2
3
4
5
Norepinephrine
Epinephrine
DHP receptor
cAMP
ATP
Ca
2+
Ca
2+
αα
β
γ
β
γ
Inactive
cAMP-dependent
protein kinase
Active
cAMP-dependent
protein kinase
Thin filament
activation
(Ca
2+
-troponin)
Cross-bridge cycling,
thick and thin filament sliding,
force generation
β
-adrenergic
receptor
Extracellular fluid
Plasma membrane
Intracellular fluid
Ryanodine receptor
Sarcoplasmic
reticulum
Force and Velocity of Contraction
Adenylyl
cyclase
Figure 12–27
Mechanisms of sympathetic effects on cardiac muscle cell contractility. In some of the pathways, the kinase phosphorylates proteins that are
not shown.
Table 12–3
Effects of Autonomic Nerves on the Heart
Area Affected
Sympathetic Nerves
(norepinephrine on
β
-adrenergic receptors)
Parasympathetic Nerves
(ACh on muscarinic receptors)
SA node
Increased heart rate
Decreased heart rate
AV node
Increased conduction rate
Decreased conduction rate
Atrial muscle
Increased contractility
Decreased contractility
Ventricular muscle
Increased contractility
No signifi cant effect
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