Mechanical and Metabolic Characterization of Ischemic Contracture in the Neonatal Pig Heart

Abstract

Isolated, paced, isovolumetrically beating piglet hearts ( n = 37) underwent retrograde aortic perfusion with a crystalloid solution during three periods: 1) baseline (coronary perfusion pressure 60 mm Hg), 2) ischemia (coronary flow 10% of baseline for -80 min), and 3) reperfusion (perfusion pressure returned to baseline). In one group of hearts, glycolysis (using "H,o formation from [3H]glucose) was assessed. During baseline, peak systolic pressure (PSP) was 101.1 i 5.0 mm Hg, end diastolic pressure (EDP) 4.4 z 0.5 mm Hg, glycolysis 970.5 -C 65.3 nmol/min/g,,,, and myocardial glycogen 234.8 2 12.0 pmoll g,,,. During ischemia, PSI' decreased to 23.3 t 2.7 mm Hg, BDP increased to 12.3 2 0.7 mm Hg, myocardial glycogen decreased to 181.5 2 30.3 pmol/g,,,, and lactate (-154 pmoll g,,,) and glycerol (-930 nmol/g,,,) were released. Myocardial contracture correlated with a decrease in lactate release. Glycolysis decreased to -400 nmol/min/g,,, and remained stable, accounting for =SO% of the lactate producetl. During reperfusion, PSP recovered to 79.8 2 3.5 mm llg, EDP 6.6 2 1.7 mm Hg, and glycolysis 1103.9 + 81 nmol/rnin/g,,,. In a second group of hearts, with similar mechanical responses, giucose oxidation (using "CO, formation from ['4C]glucose) was evaluated. During baseline, glucose oxidation was 165.4 -t 15.9 nmol/min/g,,, and correlated closely (r = 0.957) with rnechanical activity. With ischemia, glucose oxidation decreased to =17Myocardial contracture represents a state of decreased diastolic compliance of the ventricle secondary to a depletion of high energy stores in the region of the contractile apparatus (1).With ischemia, contracture occurs as a consequence of myocardial underperfusion, which results in inadequate substrate delivery and impaired ATP production. Insufficient ATP leads to a progressive increase in the number of myocin-actin complexes that are unable to dissociate and to derangements in resting cytosolic calcium levels (2). (3,4). Little is known about myocardial contracture in the setting of low-flow ischemia, where hearts are beating, and there is washout of metabolites. Additionally, the modalities of energy production and utilization associated with contracture are poorly understood, yet these processes ultimately determine myocardial viability. An understanding of these processes in neonatal hearts has significant clinical ramifications, because contracture is thought to represent a precursor to an irreversible myocardial injury (5).The objectives of the present investigation were to study myocardial contracture in isolated, isovolumetrically beating, neonatal pig hearts subjected to normothermic, low-flow isch-

Performance of the Neonatal Pig Heart Subjected to Oxygen Insufficiency

“…Therefore, baseline oxygen delivery in these experiments was adequate, as evidenced by normal and sustained LV pressure development, without the hearts requiring complete extraction of coronary arterial oxygen. We have previously shown that piglet hearts perfused with this preparation are functionally stable for at least 90 min, the time period required for the proposed experiments [11,14].…”

“…Any fluid within the ventricular chamber, e.g., from Thebesian circulation, was vented through an "1-mm apical stab wound. This maneuver reduces the possibility that a fluid collection around the balloon could after EDP [11]. The coronary flow was measured by timed collections of the pulmonary artery (coronary venous) effluent.…”

“…Using the techniques originally described by Neely et al [8], and modified for our preparation [11], the 3 H 2 O was separated from [ 3 H]glucose using columns containing Dowex 1 ! 4 anion-exchange resin (200-400 mesh) suspended in a 0.2 M potassium tetraborate.…”

“…The glucose oxidation from exogenously supplied glucose was estimated by measuring the 14 CO 2 production (liberated at the pyruvate dehydrogenase step and from the Krebs cycle) from [U-14 C]glucose (100 ÌCi/l), using the method described previously [11]. Briefly, a 3-ml sample of perfusate was obtained anaerobically and injected into a stoppered Erlenmeyer flask containing 1 N HCl (to release CO 2 ) and a center well with Hyamine hydroxide (to collect CO 2 ).…”

“…Hearts from a total of 32 piglets were excised and perfused as described previously [1,6,11,13,14]. A crystalloid perfusate was used to more easily create the desired states of oxygen deprivation.…”