31P-NMR studies of respiratory regulation in the intact myocardium.

The mechanism by which mitochondrial respiration is coupled to ATP consumption in intact tissues is unclear. We determined the relationship between high-energy phosphate levels and oxygen consumption rate in rat hearts operating over a range of workloads and perfused with different substrates. With pyruvate +glucose perfusion, ADP levels were in general very low, and varied with MVO2 yielding an apparent Km of 25 +/- 5 microM, suggesting regulation of oxidative phosphorylation through availability of ADP. In contrast, with glucose perfusion in the presence or absence of insulin, ADP levels, ADP/ATP ratio or the phosphate potential were relatively constant over the workload range examined and generally not correlated with alterations in MVO2; it is suggested that under these conditions, carbon substrate delivery to the mitochondria may control mitochondrial respiration. The common feature of both of the suggested regulatory mechanisms is substrate limitation which, however, is exercised at different metabolic points depending on the carbon substrate available to the myocardium.

Bacterial endocarditis presenting as acute myocardial infarction: a cautionary note for the era of reperfusion.

Coronary embolism is a known complication of bacterial endocarditis that sometimes causes acute myocardial infarction. The necessity for rapidly restoring coronary artery perfusion and the time constraints governing clinical decisions may prevent endocarditis from being diagnosed before pharmacologic or mechanical thrombolysis. This report describes the first documented cases of coronary angioplasty in two patients with acute myocardial infarction caused by bacterial endocarditis, and reviews the literature on coronary artery complications of bacterial endocarditis. The first patient developed a coronary artery mycotic aneurysm at the dilatation site; the second experienced a small intracerebral hemorrhage following reperfusion. It is, of course, unwise to generalize from two cases, but we believe that in patients who are most likely to have endocarditis as the cause of acute myocardial infarction, the impulse to follow conventional strategies for coronary reperfusion should be tempered by thoughts of possible consequences.

Ischemic contracture begins when anaerobic glycolysis stops: a 31P-NMR study of isolated rat hearts.

The relationships among myocardial ATP, intracellular pH, and ischemic contracture in Langendorff-perfused rat hearts were investigated by 31P nuclear magnetic resonance spectroscopy during total global normothermic ischemia while the left ventricular pressure was recorded continuously via an intraventricular balloon. Glucose-perfused hearts (n = 63) were divided into five groups based on the time of onset of contracture (TOC), and three other groups of hearts were treated to vary the ischemic glycogen availability. ATP levels, which showed no evidence of accelerated ATP depletion during contracture, were significant and variable at TOC. Intracellular pH initially declined and then leveled off at TOC, with lower final pH in hearts with later TOC. We conclude that contracture began when anaerobic glycolysis (and thus glycolytic ATP synthesis) stopped. These results, though consistent with the concept that ischemic contracture in normal hearts results from rigor bond formation due to low ATP levels at the myofibrils, suggest that TOC is more closely related to glycolytic ATP production than to total cellular ATP content, thus providing evidence of some degree of subcellular compartmentation or metabolite channeling. In glycolytically inhibited hearts, the quite early contracture may have a Ca2+ component.

Alterations in oxidative function and respiratory regulation in the post-ischemic myocardium.

In the normal and post-ischemic, isovolumic Langendorff perfused rat hearts, 31P NMR spectra and mechanical performance were evaluated over a wide range of myocardial oxygen consumption rates (MVO2). Hearts were perfused with either glucose and insulin, palmitate and glucose, or pyruvate and glucose as exogenous carbon sources. After ischemia at 38 degrees C until the onset of ischemic contracture and subsequent reperfusion, the "free" ADP levels were significantly reduced as compared to controls. In the control palmitate + glucose and glucose + insulin groups, the ADP levels were virtually independent of approximately 2.5-fold variation in MVO2; in contrast, they changed 4-fold with a approximately 30% variation in MVO2 in the post-ischemic myocardium following ischemia to contracture. In the pyruvate + glucose group, ADP levels varied with MVO2 in controls and post-ischemia; however, MVO2-ADP relationship was significantly altered following ischemia. Analysis of these observations within the concept of kinetic regulation of oxidative phosphorylation yielded the following significant conclusions: 1) the mode of respiratory regulation changed from a non-ADP to an "ADP:Pi limited" domain with non-pyruvate carbon sources; 2) respiratory regulation was in the ADP:Pi limited domain before and after ischemia in the pyruvate + glucose group; however, the Km for the relationship between MVO2 and ADP was reduced following the ischemia/reperfusion insult; 3) the post-ischemic oxidative capacity (Vmax for MVO2) was significantly reduced in all groups and this reduction would limit maximal post-ischemic mechanical performance.

Regulation of the oxidative phosphorylation rate in the intact cell.

The mechanisms that underlie the balance between the consumption and oxidative generation of ATP in the intact cell are not well-defined. Cytosolic inorganic phosphate (Pi) and ADP levels, the cytosolic ATP/ADP ratio, and the cytosolic phosphorylation potential (PP) have all been proposed as major regulatory variables, the latter as a component of a "near-equilibrium" thermodynamic regulatory scheme. Therefore, the potential regulatory roles of these variables in the intact cell were evaluated with 31P NMR and Langendorff perfused rat hearts; in this preparation, the tissue oxygen consumption rate (MVO2) can be varied over a wide range. When the exogenous carbon source was varied, none of the proposed regulatory parameters, i.e., the ATP/ADP ratio, PP, or cytosolic ADP level, were found to be uniquely related to MVO2. Rather, ADP levels at a given MVO2 decreased progressively for the exogenous carbon sources in the following order: glucose, glucose + insulin, palmitate + glucose, lactate, pyruvate + glucose, and octanoate + glucose. In the octanoate and pyruvate groups, MVO2(-1) was linearly dependent upon [ADP]-1 with apparent Km values being in the range previously observed in isolated mitochondria. A similar trend was observed in the MVO2-[Pi] relationship. The present findings suggest that exogenous carbon sources which effectuate deregulation of intramitochondrial NADH generation lower cytosolic ADP and Pi to levels which are limiting to the rate of oxidative phosphorylation. For other carbon sources, the processes controlling the rate of NADH generation also participate in determining the rate of oxidative ATP synthesis. However, this control must be exerted kinetically rather than through a near-equilibrium thermodynamic mechanism as indicated by the present data and prior kinetic studies of the ATP synthetic process in both isolated mitochondria and intact myocardium [La Noue, K. F., et al. (1986) Biochemistry 25, 7667-7675; Kingsley-Hickman, P., et al. (1987) Biochemistry 26, 7501-7510].