Creatine and Creatinine Metabolism - Physiological Reviews
Creatine and Creatinine Metabolism - Physiological Reviews
Creatine and Creatinine Metabolism - Physiological Reviews
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
July 2000 CREATINE AND CREATININE METABOLISM 1127<br />
D. CK<br />
<strong>Physiological</strong> <strong>and</strong> biochemical aspects of the CK<br />
isoenzyme system have been briefly introduced at the<br />
beginning of this article <strong>and</strong> were the subject of recent<br />
reviews (837, 838, 1084, 1124). Instead of repeating the<br />
relevant arguments, some major achievements made in<br />
the last few years are summarized here.<br />
Transgenic animals lacking single isoenzymes of CK<br />
or combinations thereof have recently been analyzed<br />
carefully (for reviews, see Refs. 502, 838; see also Refs. 73,<br />
527, 818, 848, 942, 1026, 1047, 1091). Mutant mice lacking<br />
the muscle form of cytosolic CK, M-CK, are viable <strong>and</strong><br />
fertile <strong>and</strong> display neither overt abnormalities nor alterations<br />
in absolute muscle force (1043). However, the gastrocnemius-plantaris-soleus<br />
muscle complex of these M-<br />
CK knockout mice lacks the ability to perform burst<br />
activity, i.e., at 1- <strong>and</strong> 5-Hz stimulation, the isometric<br />
twitch force is considerably lower for the first 70 <strong>and</strong> 25 s,<br />
respectively, than in controls. After these periods, the<br />
ratios invert <strong>and</strong> mutants develop even greater force than<br />
controls.<br />
Graded mutants displaying a two-, three-, or sixfold<br />
depression of MM-CK activity relative to controls showed<br />
intermediary properties in as far as force generation is<br />
concerned (1045). Interestingly, no flux through the CK<br />
reaction could be detected by the 31 P-NMR inversion<br />
transfer technique in muscles having 0, 16, <strong>and</strong> 34% of<br />
wild-type MM-CK activity, whereas significant fluxes of<br />
similar size were measured in muscles having 50 <strong>and</strong> 100%<br />
of wild-type activity. These findings clearly indicate that a<br />
considerable portion of the CK flux in muscle must be<br />
NMR invisible, <strong>and</strong> therefore question the usefulness of<br />
NMR experiments for measuring total CK flux.<br />
Single mutants lacking the sarcomeric mitochondrial<br />
CK (Mi b-CK) gene showed no abnormalities in skeletal<br />
muscle morphology <strong>and</strong> short-term performance (939).<br />
Oxidative phosphorylation capacity also was normal in<br />
both skeletal muscle <strong>and</strong> heart, except for an impaired<br />
stimulation of mitochondrial respiration by Cr (73, 939).<br />
These findings might raise questions about the physiological<br />
functions of Mi-CK. It has to be kept in mind, however,<br />
that the proportion of Mi-CK to total CK activity in<br />
skeletal muscle is generally rather low. Because this proportion<br />
is considerably higher in rodent heart, it will be<br />
more interesting to study the effects of the Mi b-CK knockout<br />
mutation on heart function.<br />
Double mutants deficient in both M-CK <strong>and</strong> Mi b-CK,<br />
despite being apparently normal, displayed even more<br />
pronounced disturbances in gastrocnemius <strong>and</strong> diaphragm<br />
muscle performance than M-CK-deficient single<br />
mutants (527, 938, 1091). Tetanic force, power, work, <strong>and</strong><br />
the rates of tension development <strong>and</strong> relaxation were<br />
considerably decreased. The enhanced endurance performance<br />
seen in M-CK-deficient gastrocnemius muscle was<br />
not observed in double mutants. Cultured myotubes of<br />
CK-deficient double mutants showed abnormal Ca 2� h<strong>and</strong>ling<br />
in response to depolarization by ACh or KCl, as<br />
evidenced by a 30% decrease in the [Ca 2� ] amplitude, a<br />
60% increase in the plateau level of Ca 2� after stimulation,<br />
<strong>and</strong> decreased rates of Ca 2� release <strong>and</strong> sequestration.<br />
Furthermore, double mutants displayed ultrastructural<br />
abnormalities, most notably conspicuous tubular aggregates<br />
of SR membranes in fast skeletal muscle fibers.<br />
Such tubular aggregates are also seen in various myopathies<br />
with electrolyte disturbances <strong>and</strong> may represent a<br />
metabolic adaptation to abnormal Ca 2� h<strong>and</strong>ling.<br />
Isolated perfused hearts from wild-type, M-CK-deficient,<br />
<strong>and</strong> both M- <strong>and</strong> sarcomeric Mi-CK-deficient mice<br />
displayed comparable isovolumic left ventricular contractile<br />
performance at baseline <strong>and</strong> responded to increased<br />
heart rate <strong>and</strong> perfusate [Ca 2� ] with similar increases in<br />
rate-pressure product (848). During baseline perfusion,<br />
[ADP] was significantly higher in M- plus Mi-CK-deficient<br />
hearts (214 �M) than in either wild-type or M-CK-deficient<br />
hearts (150 �M). In contrast, the �G for ATP hydrolysis<br />
was not significantly different between the three groups.<br />
Increasing heart rate <strong>and</strong> perfusate [Ca 2� ] increased<br />
[ADP] in M- plus Mi-CK-deficient hearts to 407 �M <strong>and</strong><br />
decreased |�G ATP| by 3.6 kJ/mol. On the other h<strong>and</strong>, in<br />
either wild-type or M-CK-deficient hearts, it had no significant<br />
effect on [ADP] <strong>and</strong> decreased |�G ATP| by only 1.7<br />
kJ/mol. Consequently, increases in cardiac work in Mplus<br />
sarcomeric Mi-CK-deficient mice become more “energetically<br />
costly” in terms of high-energy phosphate use,<br />
accumulation of ADP, <strong>and</strong> decreases in the free energy of<br />
ATP hydrolysis (848).<br />
Because the importance of the CK/PCr/Cr system for<br />
high-energy phosphate transport had been demonstrated<br />
most elegantly in sea urchin spermatozoa (1011), it was<br />
straightforward to test the effects of a disruption of the<br />
proposed PCr shuttle in mouse spermatozoa which contain<br />
ubiquitous mitochondrial CK (Mi a-CK) <strong>and</strong> the brain<br />
isoenzyme of cytosolic CK (B-CK) (942). Surprisingly,<br />
transgenic mice deficient in ubiquitous Mi-CK displayed<br />
normal viability <strong>and</strong> fertility as well as normal motility<br />
patterns of isolated spermatozoa. In summary, the mild<br />
phenotype of some of the CK knockout mutants was<br />
rather unexpected. Upon closer examination, however,<br />
the CK-knockout mice stress the importance of the CK<br />
system for high-energy phosphate metabolism <strong>and</strong> transport,<br />
proper Ca 2� h<strong>and</strong>ling <strong>and</strong>, thus, muscle performance.<br />
In contrast to these CK-knockout mice, gain of function<br />
effects were studied in transgenic mice expressing<br />
B-CK or ubiquitous Mi-CK in liver, a tissue that is normally<br />
devoid of CK (32, 96, 347, 454, 502, 642, 643, 717, 846).<br />
These livers, in the presence of Cr <strong>and</strong> PCr, revealed a<br />
protection of [ATP] <strong>and</strong> decreased adenine nucleotide<br />
degradation during a fructose load (96), delayed ATP