Joseph Kedem

Effect of cyclic GMP reduction on regional myocardial mechanics and metabolism in experimental left ventricular hypertrophy.

We tested the hypotheses that decreased myocardial cyclic GMP levels produced by intracoronary injection of methylene blue would increase local myocardial work and O2 consumption while decreasing intracellular cyclic GMP and that the relation between work, O2 consumption, and cyclic GMP may be altered in left ventricular hypertrophy (LVH) produced by aortic valve plication. In 8 control and 8 LVH open-chest anesthetized dogs, 1 mg/kg/min methylene blue was infused into the left anterior descending coronary artery (LAD); the circumflex region (CFX) served as control area. Regional work was calculated as the integrated product of force (miniature transducer) and segment shortening (sonomicrometry). Regional myocardial O2 consumption was calculated from flow measurements (radioactive microspheres), and regional O2 saturations (microspectrophotometry). A radioimmunoassay was used to determine intracellular level of cyclic GMP in the myocardium. Global hemodynamics and blood gases were unchanged by methylene blue in both control and LVH animals. Intracoronary methylene blue increased regional work from 762 +/- 129 to 1,451 +/- 307 g center dot mm/min in controls and from 912 +/- 173 to 1581 +/- 253 g center dot mm/min in the LVH groups. No significant changes in CFX regional work were observed. Regional blood flow, O2 extraction, and O2 consumption remained unchanged after injection of methylene blue in both control and LVH animals. The basal levels of cyclic GMP in the LVH group were fivefold higher than that in controls. In both groups, cyclic GMP levels were significantly decreased by methylene blue and to a greater extent in the LVH animals (from 6.16 +/- 1.2 to 3.34 +/- 0.44 pmol/g) than in the control animals (from 1.32 +/- 0.20 to 1.09 +/- 0.19 pmol/g). Therefore, intracoronary methylene blue increased regional myocardial work equally in control and LVH hearts without affecting regional metabolism (i.e., increased efficiency). For the same increased mechanical function, the hypertrophic myocardium exhibited a greater reduction in cyclic GMP pool size.

β-adrenergic stimulation of reperfused myocardium after 2-hour ischemia

Postischemic myocardium possesses considerable contractile and metabolic reserves, but their mobilization could result in increased cell death. We tested the hypothesis that beta-adrenergic stimulation of reperfused myocardium would increase segment work more than O2 consumption, thereby improving efficiency without increased cell death. In 16 open-chest anesthetized dogs, the left anterior descending coronary artery (LAD) was ligated for 2 h; during the reperfusion period, isoproterenol (ISO; 0.1 microg/kg/min, i.v.) was administered to nine of the animals. Regional myocardial segment length and force were measured in the anterior (LAD) and posterior circumflex coronary artery (CFX) regions of the left ventricular myocardium. Work was calculated as the integrated products of force and shortening for each region. Regional myocardial O2 consumption was obtained from LAD flow and arterial and local venous O2 saturations. Infarct size (tetrazolium) was measured in the treated and untreated hearts at the end of the experiment. In untreated hearts, the first derivative of left ventricular pressure, cardiac output, and external work were significantly depressed during reperfusion; ISO restored all values to preocclusion levels. Regional myocardial work in both LAD and CFX regions was significantly increased by ISO (from 564 +/- 207 to 1,635 +/- 543 g/mm/min in LAD, and from 753 +/- 90 to 1,426 +/- 245 g/mm/min in CFX). Efficiency (work/oxygen consumption) of the reperfused region was similarly increased. LAD flow was significantly increased by ISO, and O2 extraction was unchanged. Infarct size was 28.2 +/- 4.7% in untreated hearts and 29.0 +/- 3.5% in ISO hearts. Thus isoproterenol stimulation significantly improved both regional and global function without subsequent evidence of increased cell death.

Differential effects of various inotropic agents on the intracellular NADH redox level in the in vivo dog heart.

A similar inotropic response was elicited by either increasing heart rate or infusing noradrenaline or ouabain in the open-chest dog preparation. Changes in local coronary blood supply and intracellular NADH redox level produced by these inotropic interactions were examined. Contractile tension was measured using a strain gauge arch; coronary flow, using a thermistor probe; and NADH redox level, by surface fluorometry. For each inotropic agent, isometric tension increased by about 40%. However, the mean increase in coronary flow was 80 ± 9.7% for adrenaline, 67 ± 18% for tachycardia, and 1 ± 10.8% for ouabain. The mean changes in intracellular NADH redox level were −17 ± 4.4%, 49 ± 8.4%, and −6 ± 6.4% for noradrenaline, tachycardia, and ouabain, respectively. The time course of changes in the various parameters was different following the onset of each inotropic stimulus. Furthermore, inducing tachycardia while the heart was under the influence of the various inotropic agents caused a reduction in contractility at different rates. These results indicate a large variation in the oxygen cost of contraction produced by these inotropic interventions, and also demonstrate notable variations in the intracellular oxygen balance. The possible relation between the intracellular NADH level and the “mechanical reserve” of cardiac muscle is discussed.

Nitroglycerin improves the distribution of regional oxygenation in partially ischemic canine myocardium.

The effect of nitroglycerin administration on regional oxygen supply and consumption of partially ischemic areas of myocardium was investigated in 14 open-chest anesthetized dogs. Flow in the left anterior descending coronary artery (LAD) was reduced by 50% with the aid of a specially designed screw clamp placed below the first bifurcation. Seven animals received nitroglycerin (20 micrograms/kg i.v.) 5 min following partial occlusion. The remaining animals served as controls. Regional blood flow as measured with radioactive microspheres (15 +/- 3 micron in diameter) was determined before and 10 min following partial occlusion. Regional O2 saturations in small arteries and veins were determined using a microspectrophotometric technique. Oxygen supply, A-V O2 difference, and consumption were calculated in both subepicardial and subendocardial regions of ischemic and non-ischemic myocardium after occlusion. Tissue blood flow to the partially occluded regions was reduced by about 40% in both treated and untreated animals. Nitroglycerin prevented a significant reduction in arterial and venous O2 saturation of the partially occluded region. In the occluded subendocardium, nitroglycerin reduced the skewness of the venous O2 saturation distribution. The O2 supply/consumption ratio was significantly improved by nitroglycerin in the ischemic subendocardium, compared with the same region in the control group. It is suggested that nitroglycerin improves the microregional relationship between blood flow and metabolism even in the absence of a change in total regional blood supply.

Effect of coronary vasodilation produced by hypopnea upon regional myocardial oxygen balance.

An attempt was made to differentiate between autoregulatory coronary vasodilation and changes in vasomotor tone produced by factors extrinsic to the heart. this was done by investigating the relation between local cardiac force and local coronary blood supply. Intracellular NADH redox levels were also measured in order to further elucidate the oxygen balance under various experimental conditions. In anaesthetized open-chest dogs, local blood supply was estimated with the aid of a thermistor probe, and the oxidation-reduction state of mitochondrial pyridine nucleotide was measured by a surface fluorometric technique. Local myocardial contractile force, as well as blood pressure and ECG were recorded simultaneously with the above parameters. The heart was paced at frequencies from 60/min to 300/min with an electronic stimulator, under both normoxic and hypopneic conditions. It was found that elevation of heart rate caused a progressive increase in local blood flow during both normal and hypopneic ventilation. The absolute flow values during hypopnea were approximately double those during normoxia. Heart rates above 120/min or 150/min resulted in a progressive increase in NADH fluorescence. This response to elevated heart rate was less prominent or absent during hypopnea. Contractile force during hypopnea was greater at elevated heart rates than during normal breathing. Data are brought which suggest that whereas vasodilation following increased heart rate is probably due to an autoregulatory mechanism, the marked vasodilatatory effect of hypopnea is related to elevated arterial CO2 levels. It is suggested that hypercapnia markedly stimulates extrinsic coronary vasodilation thereby supplying enough oxygen to maintain contractility even at very high heart rates. Moreover, intracellular O2 concentration (mitochondrial NADH level) is maintained at a normal level despite the greatly increased demand.

Specific effects of nitroprusside on myocardial O2 balance following coronary ligation in the dog heart

Summary: The effect of gradual infusion of nitroprusside was studied in healthy and in ischemic hearts. In two areas of the left ventricular surface (ischemic and nonischemic) local coronary blood flow was measured by a thermistor technique. Isometric contractile tension was recorded with strain gauge arches, and nicotinamideadenine-dinucleotide (NADH) redox state was measured simultaneously in both regions using a two-channel fluorometer. Aortic blood pressure was also recorded. It was found that at an infusion rate of 1.0 µg/kg/min, nitroprusside increased regional coronary blood supply in the healthy heart as well as in the ischemic and nonischemic areas of left anterior descending artery (LAD)-ligated hearts. Flow elevation was similar in all regions (37.0 ± 6.1, 42.5 ± 13.5 and 45.36 ± 14.8%, respectively). At higher doses, a decrease of 6–10% in blood pressure had a detrimental effect on the coronary flow to the ischemic region without reducing flow to the nonischemic region. The NADH redox level was not significantly improved by nitroprusside in spite of elevated coronary blood supply to all regions examined. Moreover, higher doses of nitroprusside resulted in a significant elevation in NADH levels that could be correlated to the decrease in blood pressure. It is concluded that the effect of nitroprusside on coronary blood supply and myocardial O2 balance may be strongly dependent on the magnitude of its effect on blood pressure.

Variations in left and right ventricular oxygen balance produced by paired electrical stimulations

The possible differential effect of positive inotropic stimulation upon regional myocardial oxygen balance in the two ventricles was investigated during tachycardia and paired electrical stimulation. Isometric contractile force was measured by strain gauge arches; local coronary blood supply was measured by thermistor probes and intracellular NADH redox level was recorded using surface fluorometry. It was found that when contractility was increased by paired stimulation at a basic rate of 140 bpm, the inotropic response was more pronounced in the right ventricle (97.2 +/- 11.5%) than in the left (63.1 +/- 12.6%). Coronary blood supply to the left ventricle increased by 117.8 +/- 30.4% and the corresponding NADH redox level increased by 54.3 +/- 19.9%. When the contractile force was increased to the same extent (64.1 +/- 8.9%) by single stimulation at a rate of 210 bpm, the coronary flow to the left ventricle was increased by only 36.4 +/- 11.0% and the NADH state rose by 67.1 +/- 12.1%. It is concluded that paired stimulation reduced the mechanical limitation to flow during tachycardia, thus allowing coronary blood supply to increase in response to positive inotropic stimulation, thereby preserving a relatively improved oxygen state. It was also observed that the ratio contractile force/blood supply (contraction efficiency) was usually proportional to the NADH redox level (oxygen balance). Nevertheless, variations observed in the force/supply ratio for the left ventricle indicate that the NADH redox level cannot be predicted quantitatively by the force/supply ratio.

Relation between myocardial substrate utilization, oxygen consumption and regional oxygen balance in the dog heart in vivo

The interaction between myocardial function, oxygen consumption and energy production was examined in the left ventricular myocardium during various physiological conditions. Myocardial function was measured by both LV dP/dTmax and by local contractile tension. Coronary blood flow was measured from the coronary sinus; regional coronary blood supply was recorded using a thermistor placed on the epicardial surface. Intracellular oxygen balance was estimated using NADH fluorescence. Myocardial oxygen consumption and utilization of glucose, pyruvate, lactate and free fatty acids were calculated from their concentrations in the arterial and coronary sinus blood. The effects of tachycardia at 180 and 240 bpm, noradrenaline infusion (25 micrograms kg-1 min-1), and increased coronary blood flow caused by hypopneic respiration were examined. During pacing, contractile force, coronary flow and NADH fluorescence increased. At 240 bpm, the lactate/pyruvate ratio increased from 5.98 +/- 0.92 to 8.76 +/- 1.41 and NADH fluorescence increased from 50 to 71.7 +/- 3.73 (as compared to control), indicating impairment of myocardial oxygenation. Hypopneic respiration produced a marked elevation of coronary blood flow. Both noradrenaline infusion and hypopnea produced a decrease in both NADH fluorescence and the lactate/pyruvate ratio. No significant difference was found between the FORCE/ATP, FORCE/MVO2 and ATP/MVO2 ratios during pacing and noradrenaline. However, during hypopnea, the amount of ATP apparently formed (as calculated by substrate utilization assuming the formation of 3 ATP molecules per oxygen) was disproportionately greater than contractile force and oxygen consumption. It is suggested that this discrepancy may be due to the uncoupling of oxidative phosphorylation.

Preservation of myocardial oxygen balance and functional reserve by coronary vasodilators

Reduced myocardial function at very high heart rates may be due to limited coronary blood supply. The effects of the vasodilators nitroglycerin (10 micrograms kg-1 min-1) and elevated CO2 upon regional function during tachycardia were studied. In open-chest anaesthetized dogs, regional contractile force, epicardial tissue blood flow and local NADH redox level were recorded during graded ventricular pacing. It was found that the vasodilating action of nitroglycerin in the unpaced heart was much lower than produced by CO2 (23.6 +/- 5.8% vs. 137.6 +/- 33.5%). Maximal pacing at 275 bpm caused only a moderate flow elevation in control (20 +/- 6.8%) and CO2 conditions (20.3 +/- 4.03%), but marked vasodilation during nitroglycerin infusion (85.2 +/- 14.6%). Regional function during tachycardia was improved similarly by both vasodilators. NADH levels increased with heart rates under all experimental conditions, but the absolute NADH levels were consistently lower following vasodilator treatments. The lowest NADH levels were observed during nitroglycerin treatment at all heart rates. It is suggested that nitroglycerin augments myocardial functional reserve by preserving oxygen balance more than predicted by its vasodilatory effect alone.

Effect of coronary vasodilators and pacing upon regional oxygen balance of the ischaemic myocardium.

In anaesthetized open-chest dogs, regional contractile force, epicardial tissue blood flow, and local NADH redox levels were recorded during graded ventricular pacing in the range 150-285 bpm. These parameters were measured before, and 30 min following LAD coronary artery occlusion. It was found that during pacing, blood supply to the untreated ischaemic region was reduced by 65.4 +/- 11% of control values at a rate of 150 bpm, and fell to -105 +/- 40.2% at a rate of 225 bpm. Hypopneic respiration prevented this pacing induced flow reduction. Pacing in the presence of nitroglycerin resulted in a marked increase in regional flow. Similarly, the vasodilator treatments prevented the marked elevation in NADH levels (77.5 +/- 15.6%) produced by pacing in the untreated ischaemic myocardium. The reduction in regional contractile force in the ischaemic region produced following pacing (-30.5%) was reversed during both vasodilator treatments (+47.2% during nitroglycerin and +23.4% during hypopnea). It was concluded that vasodilation improves regional ischaemic myocardial oxygen balance, thus expanding the functional reserve of the ischaemic muscle. Nitroglycerin is more active.