ischaemic preconditioning of the human heart. - Leicester Research ...

the rat skeletal muscle and liver. Certainly, mitochondrial
and sarcolernmal K,,, channels appear to exhibit minor
differences in structure but the function of the mito-
chondrial KATP channels appear to be intimately involved
in matrix volume control as opposed to electrical activity
for sarcolemmal KATP channels. In this instance, opening
of the mitoKATP channel leads to membrane depolariza-
tion, matrix swelling, slowing of ATP synthesis, and
accelerated respiration [29]. There is good evidence that
diazoxide and 5-HD show good selectivity for mitochon-
drial over cardiac sarcolemmal KATP channels [13,20] and
our present study confirms that preconditioning can be
mimicked with diazoxide and abolished with both 5-HD
and glibenclamide. These results are in close agreement
with those of Garlid et al. [30] and further suggest that the
mitoKATP channels are the end-effectors of cardioprotec-
tion produced by ischernic preconditioning. To more
clearly address this issue, we used a specific sarcolemmal
KATP channel blocker, HMR 1883. The novel blocker
HMR 1883 shows good selectivity for the cardiac sar-
colemmal KATP channel over those of pancreatic beta cells
and the vasculature [201. Further, a brief report suggests
that HMR 1883 did not block the protection induced by
ischemic preconditioning in the rabbit [211. Our results are
in agreement with this, since we found that the protective
effect of PC persisted in the presence of HMR 1883. Taken
together with our findings with 5-HD and diazoxide, this
suggests that HMR 1883 does not block mito KATP
channels of human myocardium at the concentration used
(10 ýLM), and that blockade of the sarcolemmal channel
does not abolish protection.
The mechanisms by which the opening of mitoKATP
channels exert the cardioprotective effect of precondition-
ing are not fully understood. Consequences of opening of
the mito KATP channel include depolarization of the
intramitochondrial membrane as K+ enters leading to
decreased calcium in-port and matrix swelling. The mecha-
nism of how this ultimately stimulates mitochondrial
respiration and consequently the cytoprotective effect is
much less clear and warrants further research.
A potential limitation of our study was the use of atrial
tissue as opposed to ventricular myocardium and therefore
any extrapolation must be conducted with caution; how-
ever, Yellon and colleagues have suggested that precondi-
tioning exerts identical protection in both tissues [16,311.
Undoubtedly, KATP channels are present in both atrium and
ventricle [321, although their density in both tissues is
unknown. Another possible limitation might be that right
atrial appendages were obtained from patients subjected to
various medical treatments (e. g. nitrates, P-blockers, cal-
cium antagonists) and that in principle may have in-
fluenced ischemia/reperfusion injury and the protection
induced by preconditioning. However; it should be empha-
sized, that all medication was stopped the day before
surgery when specimens were taken for the study and that
a significant effect of the medication was unlikely since all
S. Ghosh et al. / Cardiovascular Research 45 (2000) 934-940
939
preparations responded to ischemia/reperfusion with a
similar degree of injury and preconditioning was protective
in all instances when applied. It should be mentioned that
the preparation used in this study was not electrically
stimulated (i. e. non-beating) and therefore one should be
cautious when extrapolating to the in vivo situation.
It should also be emphasized, that in common with
previous studies in non-human hearts [13,14,20], our
evidence for the involvement of mitoKATp rather than
sarcolemmal KATP channels depends strongly on the
selectivity of the KATP channel openers and blockers used,
in particular diazoxide, 5-HD, and HMR1883. In this
context, the concentration of diazoxide that we used (100
l. LM) has been reported to activate mitoKATp but not
sarcolemmal KATP in rabbit ventricular myocytes [14].
HMR 1883 should be an effective blocker of sarcolemmal
KATP channels at 10 ýLM, since its reported Ki for these
channels is 0.8 l. LM [21], though its effects on mitoKATP
have not been tested directly. 5-HD is an effective blocker
of mitoKATP provided that the channel is opened by a
physiological or pharmacological opener [14,18], but its
selectivity for mitoKATP over sarcolemmal KATP merits
further study. Notsu et al. [331 reported block of sarcolem-
mal KATP channels in guinea pig myocytes by 5-HD,
though more recently Hu et al. [34] have argued that 5-HD
at a concentration of 0.5 mM selectively blocks mitoKATP
without affecting sarcolemmal KATP channels of rabbit
ventricular cells. Certainly, 5-HD at a concentration of I
mM was an effective blocker of cardioprotection in our
human model.
In conclusion, the present study provides strong evi-
dence that mitochondrial rather than sarcolemmal KATP
channels are effectors of ischemic preconditioning in the
human myocardium. The finding has obvious important
clinical implications, however, the mechanism by which
the opening of mito KATP channels is protective is not fully
understood and merits further investigation.
Acknowledgments
This study was supported in part by grants from The
Wellcome Trust, British Heart Foundation, Rhone-Poulenc
Rorer (UK) and the University of Leicester.
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