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Biochemical, morphological and functional alterations in acute myocardial ischaemia

University of British Columbia

Changes in biochemical, functional and morphological indices of myocardial cell injury have been observed after short periods of ischaemia produced by coronary occlusion. These alterations are generally reversible if blood flow is re-instated within approximately 30 min post-1igation; however, reperfusion after this time cannot reverse the damage and actually exacerbates the existing injury. These studies have investigated ischaemia/reperfusion injury, assessing cell damage using several different techniques in order to avoid the inherent bias of any single method. The experiments concerned the molecular changes that occur during the transition from reversible to irreversible damage and their modification by a variety of drugs, especially the calcium channel-blocker, D-600 and the β-adrenergic antagonist, propranolol. Ischaemic or ischaemic/reperfused tissue was obtained by ligation (with or without release) of the left circumflex coronary artery of the open-chest, anaesthetized rabbit. This area was isolated and samples removed for the various biochemical, chemical and ultrastructural analyses. The functional integrity of mitochondrial (MITO) and sarcolemmal (SL) membranes was assessed using membrane fractions purified by differential and density gradient centrifugation. Myocardial ischaemia was shown to produce progressive decreases in the activities of the membrane-bound MITO ATPase and SL Na⁺,K⁺-ATPase enzyme as well as a decline in tissue ATP. Interestingly, reperfusion fully restored only the MITO enzyme activity. The enzyme inhibition was not a generalized phenomenon as shown by the lack of effect of ischaemia on another MITO enzyme, namely cytochrome c oxidase. The irreversible decrease in Na⁺,K⁺-ATPase activity was not accompanied by changes in K⁺ and Mg²⁺ affinity or alterations to the SL protein or phospholipid profiles although more subtle changes in the lipid or protein environment may have occurred. The specificity of the decrease to the SL membrane was shown by the concomitant increase in the tissue Na⁺ /K⁺ ratio in the absence of Ca²⁺ /Mg²⁺ changes. Results from the 20 min ligation study in which D-600 pretreatment provided significant protection against ischaemia-induced ATP depletion and MITO ATPase inhibition prompted the examination of the efficacy of D-600 pretreatment prior to a 40 min ligation and 60 min reperfusion. Propranolol was also included in this study with the aim of determining the relative importance of decreased energy consumption vs. calcium channel-blockade in attenuating ischaemia/reperfusion injury. Both agents were shown to significantly reduce the biochemical and morphological damage, including Ca²⁺ accumulation, that resulted from reperfusion of irreversibly-damaged tissue, although D-600 was consistently superior. Despite the beneficial effects on MITO and SL integrity, neither drug preserved ATP levels or contractile function. These data indicate that the predominant beneficial effects are probably the result of decreased energy requirements during ischaemia. This hypothesis was further supported by observations from the study in which D-600 was administered just prior to reperfusion. This method of drug administration reduced calcium influx to the same degree as when the drug was given prior to ischaemia. However, this intervention significantly preserved only MITO ATPase activity with no protection of SL function. MITO protection in the absence of preservation of SL integrity would not be expected to provide sustained benefit to the ischaemic/reperfused myocardium. Even the observed protection of SL integrity in the drug-pretreated hearts was not accompanied by an improvement in contractility. Thus, the maintenance of intact permeability barriers does not necessarily result in an improvement of contractile function. Long-term studies would be necessary to determine the ultimate benefit of drug treatment in preventing or reducing the deleterious sequelae of myocardial ischaemia.

Text

2010-06-13

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http://hdl.handle.net/2429/25651

Pharmacology

University of British Columbia

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