Substrate effects in the post-ischemic myocardium

Cardiodynamics and infarct size in regional and global ischemic isolated heart model: comparison of 1 hour and 2 hours reperfusion

Background and Objectives

We investigated whether 1 hour reperfusion is enough to assess cardiodynamics and infarct size in both regional ischemia (RI) and global ischemia (GI) in isolated rat heart models.

Materials and Methods

Hearts were randomly assigned to one of the following groups (each n=14): 1) Sham hearts for 1 hour; 2) Sham hearts for 2 hours; 3) 30 minutes RI followed by 1 hour reperfusion; 4) 30 minutes of RI followed by 2 hours reperfusion; 5) 30 minutes GI followed by 1 hour reperfusion; and 6) 30 minutes GI followed by 2 hours reperfusion.

Results

There were no significant differences in infarct size between 1 hour and 2 hours reperfusion in both RI and GI. Left ventricular developed pressure was significantly decreased at both 1 hour and 2 hours reperfusion in groups of RI and GI compared to baseline (p<0.01). Rate-pressure product and +dP/dt_max also significantly decreased compared to baseline level at both 1 hour and 2 hours reperfusion in groups of RI and GI (p<0.05).

Conclusion

There was no significant difference in infarct size between 1 hour and 2 hours reperfusion in groups of RI and GI. Cardiodynamic variables measured at 1 hour and 2 hours reperfusion significantly decreased compared to baseline level. Our data suggests that reperfusion of 1 hour is sufficient to assess cardiodynamics in both regional and global ischemic isolated hearts model.

Glucose and glycogen utilisation in myocardial ischemia--changes in metabolism and consequences for the myocyte

Experimentally, enhanced glycolytic flux has been shown to confer many benefits to the ischemic heart, including maintenance of membrane activity, inhibition of contracture, reduced arrhythmias, and improved functional recovery. While at moderate low coronary flows, the benefits of glycolysis appear extensive, the controversy arises at very low flow rates, in the absence of flow; or when glycolytic substrate may be present in excess, such that high glucose concentrations with or without insulin overload the cell with deleterious metabolites. Under conditions of total global ischemia, glycogen is the only substrate for glycolytic flux. Glycogenolysis may only be protective until the accumulation of metabolites (lactate, H+, NADH, sugar phosphates and Pi ) outweighs the benefit of the ATP produced. The possible deleterious effects associated with increased glycolysis cannot be ignored, and may explain some of the controversial findings reported in the literature. However, an optimal balance between the rate of ATP production and rate of accumulation of metabolites (determined by the glycolytic flux rate and the rate of coronary washout), may ensure optimal recovery. In addition, the effects of glucose utilisation must be distinguished from those of glycogen, differences which may be explained by functional compartmentation within the cell.

Substrate metabolism as a determinant for postischemic functional recovery of the heart

The mammalian myocardium meets its high energy needs through the oxidation of a variety of substrates, chiefly fatty acids. This review examines the hypothesis that efficient energy transfer in the heart occurs through a series of moiety-conserved cycles, which makes the heart an obligatory "omnivore." Ischemia results in a transformation of efficient metabolic cycles to less-efficient linear pathways. Substrate metabolism during reperfusion requires the replenishment of depleted cycles and is a major determinant for the return of contractile function. Although there is growing recognition of the concept that regulation of substrate flux through metabolic pathways is shared by several of the pathway enzymes it is apparent that glucose oxidation and glycogen resynthesis promote the return of normal contractile function in the postischemic heart. This concept is supported by clinical observations on the beneficial effects of a solution containing glucose, insulin, and potassium (GIK) for treatment of refractory left ventricular contractile failure after hypothermic ischemic arrest during cardiac surgery.

Hemodynamic effects of pyruvate infusion in dogs

PURPOSE

Although pyruvate supplementation enhances endurance in humans and increases cardiac output in dogs, its effects on cardiac and peripheral vascular function are not known. Thus, we assessed the cardiovascular effects of pyruvate infusion.

MATERIALS AND METHODS

Aortic, left ventricular (LV), and pulmonary (Ppa) pressures and LV stroke volume (Svlv; derived from aortic flow probe) were measured after thoracotomy in eight anesthetized dogs. LV area or volume changes were measured using either an epicardial echocardiography (n = 6) or a conductance catheter (n = 2). LV end-systolic elastance (Eeslv) and preload recruitable stroke force (PRSFlv) relations, as estimates of contractility, were generated by transient inferior vena cava occlusion. Simultaneous stroke volume to arterial pressure relations during the occlusions were used to measure arterial elastance (Ea), and steady-state systemic and pulmonary vascular resistances were used as measures of arterial tone. Graded doses of pyruvate (8, 16, and 32 mg/kg/min), dobutamine (positive control) and propranolol (negative control) and placebo (volume control) were sequentially given.

RESULTS

Dobutamine increased Eeslv, PRSFlv, whereas propranolol had the opposite effect on Eeslv and PRSFlv. Pyruvate at 32 mg/kg/min increased heart rate, Ppa, and SVlv and decreased LV end-diastolic area, and systemic vascular resistance without changing arterial pressure, Eeslv, PRSFlv, or Ea.

CONCLUSIONS

We conclude that pyruvate infusion in normal dogs induces venodilation but does not alter either cardiac contractility or arterial tone.

Preconditioning against myocardial dysfunction after ischemia and reperfusion by an alpha 1-adrenergic mechanism

Preconditioning may find ready applicability in humans facing scheduled global cardiac ischemia-reperfusion (IR) during bypass or transplantation, where such a maneuver is feasible before arrest. Our objective was to delineate and exploit the endogenous preconditioning mechanism triggered by transient ischemia (TI) and thereby attenuate myocardial postischemic mechanical dysfunction by clinically acceptable means. Preconditioning by 2 minutes of TI followed by 10 minutes of normal perfusion protected isolated rat left ventricle function assessed after 20 minutes of global, 37 degrees C ischemia and 40 minutes of reperfusion. Final recovery of developed pressure (DP) was improved (91.5 +/- 1.9% of equilibration DP versus unconditioned IR control, 57.4 +/- 2.4%, P < .01) and was accompanied by increased contractility (+/- dP/dt). Norepinephrine release increased after TI, and reserpine pretreatment abolished TI preconditioning. This suggests that endogenous norepinephrine mediates functional preconditioning in rat. Brief pretreatment (2 minutes) with exogenous norepinephrine reproduced the protection (89.1 +/- 1.4%) of postischemic function. Functional protection persisted after the hemodynamic effects had resolved. Norepinephrine-induced preconditioning was simulated by phenylephrine and blocked by alpha 1-adrenergic receptor antagonist. TI preconditioning was similarly lost after selective alpha 1-adrenergic receptor blockade. We conclude that transient ischemic preconditioning is mediated by the sympathetic neurotransmitter release and alpha 1-adrenergic receptor stimulation. Although the postreceptor mechanism remains unclear, functional protection after IR does not seem related to the magnitude of ATP depletion and elevation of resting pressure during ischemia. Rather, the endogenous mechanisms facilitate both recovery of mechanical function and ATP repletion during reperfusion.

Relation between glycolysis and calcium homeostasis in postischemic myocardium

This study examined the hypothesis that glycolysis is required for functional recovery of the myocardium during reperfusion by facilitating restoration of calcium homeostasis. [Ca2+]i was measured in isolated perfused rabbit hearts by using the Ca2+ indicator 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (5F-BAPTA) and 19F nuclear magnetic resonance spectroscopy. In nonischemic control hearts, inhibition of glycolysis with iodoacetate did not alter [Ca2+]i. In hearts subjected to 20 minutes of global zero-flow ischemia, [Ca2+]i increased from 260 +/- 80 nM before ischemia to 556 +/- 44 nM after 15 minutes of ischemia (p less than 0.05). After reperfusion with 5 mM pyruvate as a carbon substrate, [Ca2+]i increased further in hearts with intact glycolysis to 851 +/- 134 nM (p less than 0.05 versus ischemia) during the first 10 minutes of reperfusion, before returning to preischemic levels. In contrast, inhibition of glycolysis during the reperfusion period resulted in persistent severe calcium overload ([Ca2+]i, 1,380 +/- 260 nM after 15 minutes of reperfusion, p less than 0.02 versus intact glycolysis group). Furthermore, despite the presence of pyruvate and oxygen, inhibition of glycolysis during early reperfusion resulted in greater impairment of functional recovery (rate/pressure product, 3,722 +/- 738 mm Hg/min) than did reperfusion with pyruvate and intact glycolysis (rate/pressure product, 9,851 +/- 590 mm Hg/min, p less than 0.01). Inhibition of glycolysis during early reperfusion was also associated with a marked increase in left ventricular end-diastolic pressure during reperfusion (41 +/- 5 mm Hg) compared with hearts with intact glycolysis (16 +/- 2 mm Hg, p less than 0.01). The detrimental effects of glycolytic inhibition during early reperfusion were, however, prevented by initial reperfusion with a low calcium solution ([Ca]o, 0.63 mM for 30 minutes, then 2.50 mM for 30 minutes). In these hearts, the rate/pressure product after 60 minutes of reperfusion was 12,492 +/- 1,561 mm Hg/min (p less than 0.01 versus initial reflow with [Ca]o of 2.50 mM). These findings indicate that the functional impairment observed in postischemic myocardium is related to cellular Ca2+ overload. Glycolysis appears to play an important role in restoration of Ca2+ homeostasis and recovery of function of postischemic myocardium.