Adenine nucleotide content and regional function during ischemia and reperfusion in canine ventricular myocardium

High-energy phosphates, myocardial contractile function and material properties after short periods of oxygen deficiency

To investigate myocardial performance and diastolic properties after repeated periods of oxygen deficiency auxotonic and isovolumic measurements were performed after three periods (4 min) of asphyxia in Wistar rats (n = 19). Additionally, the response of the peak isovolumic left ventricular pressure to postextrasystolic potentiation was measured. The hemodynamic results were compared to the levels of high-energy phosphates. Already after 15 min of recovery from asphyxia auxotonic measures of systolic function were completely normal compared to the control group (n = 19). Isovolumic measurements after 20 min of postasphyctic recovery, however, demonstrated a considerable reduction of the peak left ventricular pressure (226.5 +/- 7.5 mm Hg vs. 262.6 +/- 3.4 mm Hg in controls, mean +/- SEM (p less than 0.01) indicating persistence of decreased postischemic contractile performance. The relative effect of postextrasystolic potentiation was similar in both groups, but could not compensate for the reduced performance of the postasphyctic hearts: the absolute postextrasystolic peak isovolumic pressure of the postasphyctic hearts was lower than the value of the regular isovolumic peak pressure in the controls. Diastolic properties (pressure/volume and stress/strain relationships) of the postasphyctic myocardium remained unchanged. The total sum of the adenine-nucleotides decreased from 7.2 +/- 0.2 to 5.6 +/- 0.3 mumol/gww (p less than 0.01). ATP was reduced from 4.8 +/- 0.2 to 3.9 +/- 0.3 mumol/gww (p less than 0.01). Phosphocreatine was elevated to 7.0 +/- 0.6 mumol/gww, x +/- SEM (p less than 0.01). Our results demonstrated normal postasphyctic basal hemodynamics and material properties. Thus, the energy supply was sufficient to maintain steady state conditions - in spite of decreased overall adenine-nucleotide levels. Isovolumic measurements and postextrasystolic potentation tests, however, indicated that the contractile performance of the postischemic myocardium was still reduced. This functional limitation cannot be explained by altered material properties and is probably not causally related to the decreased overall ATP content.

Disruption of myofibrillar energy use: dual mechanisms that may contribute to postischemic dysfunction in stunned myocardium

The abnormalities in regional function produced by myocardial ischemia persist after the ischemic episode resolves. Since a close functional coupling exists between myofibrillar creatine kinase and myosin ATPase, a disruption of this coupling could adversely influence myocardial function and might provide a mechanism for the myocardial dysfunction observed. The purpose of the present study was to determine if an alteration in the activity of creatine kinase associated with the myofibril occurs in the postischemic period. Anesthetized open-chest dogs (n = 6) underwent coronary occlusion for 15 minutes, followed by reperfusion for 15 minutes. In reperfused myocardium, adenine nucleotide content was decreased (72 +/- 10% of nonischemic myocardium, p less than 0.05), documenting the presence of previous ischemia. The creatine phosphate content of reperfused myocardium returned to normal, indicating resumption of myocardial energy production. The creatine kinase activity of purified myofibrils isolated from reperfused myocardium was decreased by 17 +/- 7% compared to that of nonischemic myofibrils (p less than 0.03). In addition, the free adenosine diphosphate concentration in reperfused myocardium was calculated to be 96 microM and was less than the Km of adenosine diphosphate determined for myofibrillar creatine kinase (105 microM). The results suggest two putative mechanisms for disruption of energy use in postischemic myocardium: decreased creatine kinase activity associated with the myofibril, and limitation of substrate necessary for maximal creatine kinase activity.

Repeated short periods of regional myocardial ischemia: effect on local function and high energy phosphate levels

The effect of recurrent periods of ischemia on the myocardium was investigated in 15 open-chest dogs. Ischemia was produced by 3 minutes of proximal occlusion of the left anterior descending coronary artery. Each occlusion was followed by reperfusion of 3 minutes duration. Forty occlusions with a total of 120 minutes of ischemia were performed, and regional function (sonomicrometry) as well as high energy phosphates (needle biopsies) were determined at the end of the 5th, 20th, and 40th period of ischemia and reperfusion. The first periods of ischemia had a cumulative effect both on regional postischemic function (44% and 59% respectively of preischemic control after 20 occlusions) and on the ATP content, but with increasing number of occlusions the additive effects became smaller (ATP reduction/mumol/g w w/per occlusion). The ATP breakdown per occlusion was diminished with increasing number of periods of ischemia, and no significant adenosine was measured in the ischemic myocardium. Higher than normal postischemic creatine phosphate levels (9.1 mumol/g w w at the 40th reperfusion vs. 6.7 mumol/g w w control) indicated a functioning oxidative phosphorylation in the presence of an ATP utilization problem at the sarcomere level, because indicators of the cellular energy level (energy charge, free energy change of ATP hydrolysis) quickly normalized during reperfusion. Stunned myocardium is therefore not a problem of energy supply but rather of energy utilization. Reduced ATP utilization and regional dysfunction are the expressions of the same cellular defect which resides either in the ATP-splitting contractile apparatus or in the electromechanical coupling. Contractile dysfunction during reperfusion protects the heart against subsequent periods of ischemia because ATP turnover is reduced.