Reduction of infarct size with ischemic preconditioning and
monophosphoryl lipid a: role of adenosine regulating enzymes ?.
Przyklenk, Karin, Lin Zhao Ms, Robert A. Kloner & Gary T. Elliott.
Heart Institute, Good Samaritan Hospital and Department of
Medicine, Section of Cardiology, University of Southern California,
Los Angeles, CA and RIBI Immunochem Research, Inc., Hamilton MT
APStracts 3:0050H, 1996.
Both ischemic preconditioning and pretreatment with the endotoxin
derivative monophosphoryl lipid A (MLA) have been shown to reduce
infarct size caused by a subsequent sustained ischemic insult, yet
the cellular mechanisms responsible for the cardioprotection achieved
with either intervention is unknown. Using pentobarbital-anesthetized
dogs, we tested the hypothesis that increased activity of 5'
nucleotidase (NT), the enzyme that catalyzes the formation of
adenosine from AMP, may play a role. Twenty-two dogs underwent 1 h of
sustained coronary artery occlusion and 4 h of reperfusion: 8
controls received no intervention, 7 were preconditioned with 4 5-min
episodes of brief ischemia immediately before the sustained
occlusion, and 7 received MLA (35 [mu]g/kg IV) 24 h previously.
Collateral blood flow was measured during sustained occlusion by
injection of radiolabeled microspheres, infarct size was delineated
at the end of the protocol by tetrazolium staining, and myocardial
activities of cytosolic and ectosolic (membrane-associated) 5'NT were
assayed at 4 h after relief of ischemia by quantifying the release of
adenosine from AMP. Despite comparable values of collateral blood
flow in all groups, infarct size was reduced in preconditioned and
MLA-treated dogs vs controls (6+3%**, 11+3%* and 23+4% of the
myocardium at risk; *p<0.05; **p<0.01 vs control). In
addition, 5'NT activities were increased throughout the heart with
preconditioning and MLA treatment: ie. in the ischemic/reperfused
subendocardium, cytosolic 5'NT was 64.8+14.5*, 47.6+14.4* and
22.9+6.5 nmol/mg protein/min in the 3 groups, respectively. However,
single and multivariate regression analyses revealed no correlation
between infarct size and 5'NT activities for either treatment group:
in fact, in the preconditioned cohort, animals with the highest
enzyme activities paradoxically developed the largest infarcts. This
dissociation between infarct size and 5'NT suggests that increased
activity of 5'NT is not the mechanism by which preconditioning or MLA
treatment protect the canine heart against sustained coronary
occlusion.
Received 1 December 1995; accepted in final form 18 January 1996.
APS Manuscript Number H1117-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 8 February 96