Page 85 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Energy Supply for Muscle Contraction constant (! p. 75 B). The several minutes that
pass before this steady state is achieved are
Adenosine triphosphate (ATP) is a direct bridged by anaerobic energy production, in-
source of chemical energy for muscle contrac- creased O 2 extraction from the blood and
tion (! A, pp. 40 and 64). However, a muscle depletion of short-term O 2 reserves in the
cell contains only a limited amount of ATP– muscle (myoglobin). The interim between the
only enough to take a sprinter some 10 to 20 m two phases is often perceived as the “low
or so. Hence, spent ATP is continuously re- point” of physical performance.
The O 2 affinity of myoglobin is higher than
generated to keep the intracellular ATP con- that of hemoglobin, but lower than that of res-
Nerve and Muscle, Physical Work 1. Dephosphorylation of creatine phosphate oxygen to the mitochondria during brief arte-
centration constant, even when large quanti-
piratory chain enzymes. Thus, myoglobin is
ties of it are needed. The three routes of ATP re-
normally saturated with O 2 and can pass on its
generation are (! B ):
rial oxygen supply deficits.
2. Anaerobic glycolysis
The endurance limit, which is some 370 W
3. Aerobic oxidation of glucose and fatty acids.
(! 0.5 HP) in top athletes, is mainly dependent
Routes 2 and 3 are relatively slow, so creatine
on the speed at which O 2 is supplied and on
phosphate (CrP) must provide the chemical
how fast aerobic oxidation takes place. When
energy needed for rapid ATP regeneration. ADP
derived from metabolized ATP is immediately
the endurance limit is exceeded, steady state
transformed to ATP and creatine (Cr) by mito-
ously (! p. 75 B). The muscles can temporarily
CrP reserve of the muscle is sufficient for
compensate for the energy deficit (see above),
2 chondrial creatine kinase (! B1 and p. 40). The cannot occur, the heart rate then rises continu-
short-term
high-performance
+
but the H -consuming lactate metabolism can-
bursts
of
10–20 s (e.g., for a 100-m sprint). not keep pace with the persistently high level
Anaerobic glycolysis occurs later than CrP of anaerobic ATP regeneration. An excess of
dephosphorylation (after a maximum of ca. lactate and H ions, i.e. lactacidosis, therefore
+
30 s). In anaerobic glycolysis, muscle glycogen develops. If an individual exceeds his or her
is converted via glucose-6-phosphate to lactic endurance limit by around 60%, which is about
+
acid (! lactate + H ), yielding 3 ATP molecules equivalent to maximum O 2 consumption
for each glucose residue (! B2). During light (! p. 74), the plasma lactate concentration
exercise, lactate is broken down in the heart will increase sharply, reaching the so-called
!
+
and liver whereby H ions are used up. Aerobic anaerobic threshold at 4 mmol/L. No significant
oxidation of glucose and fatty acids takes place increase in performance can be expected after
approx. 1 min after this less productive an- that point. The systemic drop in pH results in
aerobic form of ATP regeneration. If aerobic ox- increasing inhibition of the chemical reactions
idation does not produce a sufficient supply of needed for muscle contraction. This ultimately
ATP during strenuous exercise, anaerobic gly- leads to an ATP deficit, rapid muscle fatigue
colysis must also be continued. and, finally, a stoppage of muscle work.
CrP metabolism and anaerobic glycolysis
In this case, however, glucose must be imported enable the body to achieve three times the per-
from the liver where it is formed by glycogenolysis
and gluconeogenesis (see also p. 282f.). Imported formance possible with aerobic ATP regenera-
glucose yields only two ATP for each molecule of glu- tion, albeit for only about 40 s. However, these
cose, because one ATP is required for 6-phosphoryla- processes result in an O 2 deficit that must be
tion of glucose. compensated for in the post-exercise recovery
phase (O 2 debt). The body “pays off” this debt
Aerobic regeneration of ATP from glucose
(2 + 34 ATP per glucose residue) or fatty acids is by regenerating its energy reserves and break-
required for sustained exercise (! B3). The car- ing down the excess lactate in the liver and
diac output (= heart rate"stroke volume) and heart. The O 2 debt after strenuous exercise is
total ventilation must therefore be increased much larger (up to 20 L) than the O 2 deficit for
72 to meet the increased metabolic requirements several reasons.
of the muscle; the heart rate then becomes
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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