Creatine and Creatinine Metabolism - Physiological Reviews

July 2000 CREATINE AND CREATININE METABOLISM 1153
idative damage (i.e., lipid peroxidation) caused by perfusion
of the isolated heart with H 2O 2. Even though intravenous
administration of PCr increased the intracellular
concentrations of ATP and PCr in the heart of living rats
(194), PCr is generally accepted not to be membrane
permeable. Because in contrast to PCr, Cr plus P i did not
protect the ischemic myocardium, cardioprotection is believed
to be brought about by extracellular effects of PCr.
It may interact electrostatically with membrane phospholipids,
thus decreasing the fluidity and possibly increasing
the stability of the plasma membrane. The half-life of PCr
in blood or plasma was estimated to be between 4 and 5
min (505) and �1.3 h (194).
Close structural analogs like PCrn (821), creatinol
O-phosphate (288), and O-benzyl-phosphocreatine (1024)
were also shown to afford cardioprotection, possibly in
the same manner as PCr. In contrast, P i plus Cr, PArg (see
Refs. 505, 836), O-benzyl-phosphoglycocyamine (1024),
and O-benzyl-phosphocreatine ethyl ester (1025) were ineffective.
A substance displaying most likely still another mode
of action is cCr. As already discussed in section VIIIB, cCr
competes with Cr for uptake into heart (and other tissues)
where it is accumulated, together with its phosphorylation
product, PcCr, in large concentrations at the expense
of Cr and PCr. PcCr serves as a slowly hydrolyzable
reservoir of high-energy phosphates for ATP regeneration.
During cardiac ischemia upon long-term feeding of
rats and chickens with cCr, the reduction in tissue [ATP],
exhaustion of high-energy phosphates, and onset of rigor
tension were all significantly delayed (419, 738, 808, 1028,
1076). Upon reperfusion, the number of hearts recovering
mechanical function was significantly higher and the ratepressure
product comparable in cCr-treated rats relative
to controls, despite a considerably longer period of ischemia
(56 vs. 34 min) (738). Spontaneous defibrillation
upon reperfusion occurred sooner in hearts of cCr-fed
rats than in controls (178 vs. 346 s) (419).
Surprisingly, favorable effects were observed also
after short-term cCr or PcCr treatment. Intravenous injection
in dogs, rats, and rabbits of cCr or PcCr 30–120 min
before 1) killing of the animals and heart removal, 2)
aortic cross-clamping, or 3) coronary artery occlusion, all
followed by a period of ischemia, resulted in a significant
protection of the PCr and ATP pools (13, 218) and in a
remarkable improvement of cardiac function upon reperfusion
(13, 219, 383). In addition, cCr treatment reduced
the cardiac production of neutrophil chemotactic factors
(217, 218) as well as the accumulation of neutrophils in
the heart after ischemia and reperfusion (217). Neutrophil
accumulation in ischemic myocardium has been implicated
to be involved in postischemic damage of the heart
(see Refs. 217, 218). cCr has no inotropic or chronotropic
effect on the dog heart (383).
Unexpectedly, cCr showed no beneficial effects in
Syrian hamsters with congestive heart failure. If anything,
its effect was slightly negative when given in high
amounts to animals already experiencing heart failure.
Even though the reason for this discrepancy remains to be
established, the results on normal hearts are encouraging
and hint at the potential for cCr in organ transplantation.
To close the gap between animal experiments and organ
transplantation in humans, and because endothelial cells
may be the primary target of reperfusion injury (548), we
tested the effects of cCr on human umbilical vein endothelial
cells (HUVEC) in cell culture. Despite the rather
low levels of Cr, PCr, and CK activity in endothelial cells,
preincubation with cCr for 24 h delayed ATP depletion
during cold hypoxia, diminished release of lactate dehydrogenase
during subsequent reperfusion, and better preserved
the viability of the HUVEC in preliminary experiments
(M. Wyss, T. Eberl, Y. Ishida, R. Margreiter, and R.
Kaddurah-Daouk, unpublished results). Even though use
of cCr seems particularly attractive at present in heart
transplantation, beneficial effects on the preservation of
other organs (e.g., skin flaps in reconstructive surgery,
Ref. 147) should not be ruled out.
5. Conclusions
The findings discussed in this section provide a long
list of arguments, although admittedly still not conclusive,
for a correlation between the functional capacity of the
CK/PCr/Cr system and ischemic tolerance of a given tissue.
This has recently been corroborated by experiments
on transgenic mice expressing high levels of BB-CK in
liver, a tissue normally displaying only very low levels of
CK activity (642). The concentrations of Cr and PCr in
transgenic liver increased with the level of Cr in the diet.
In transgenic liver with an initial [PCr]/[ATP] of 4.5, a
delayed and less pronounced depletion of ATP as well as
a smaller drop in pH were observed during 40 min of
ischemia. Within 30 min of reperfusion, pH as well as P i
and ATP levels returned to preischemic values in transgenic
liver, whereas only incomplete recovery of [P i] and
[ATP] was seen in normal liver. In addition, during 90 min
of hypoxia, release of lactate dehydrogenase was prevented
in transgenic liver containing high levels of PCr.
Protection against low-oxygen stress was not seen in
normal liver containing elevated concentrations of Cr, or
in transgenic liver low in PCr. Therefore, protection
against low-oxygen stress and reperfusion injury depends
on the presence of both CK and PCr. In conclusion,
strategies toward improving the functional capacity of the
CK/PCr/Cr system may represent effective means for improving
the ischemic tolerance of a tissue. As shown
above, cCr is a promising candidate for such a purpose
and may improve cold preservation of organs, e.g., in
heart transplantation.