Effect of isoproterenol on regional myocardial segment work, O2 consumption, and oxygen balance

Modulation of cardiac A1-adenosine receptors in rats following treatment with agents affecting heart rate

Effects of chronic treatment affecting heart rate on A1 adenosine receptor levels and their functions were studied. Treatment of rats with isoproterenol for 10 days accelerated heart rate and increased the level of adenosine receptors, in both the atria and ventricles. Negative dromotropic response of isolated heart to adenosine was enhanced in isoproterenol-treated rats. Similar results were obtained following treatment with atropine sulfate, or swimming training but not after treatment with thyroxine. On the other hand, treatment with amiodarone, which normally causes a decrease in heart rate, also increased the level of adenosine receptors in both atria and ventricles. The sensitivity of the isolated heart to the negative dromotropic and chronotropic effects of adenosine was not enhanced in the amiodarone treated rats. Similar results were obtained following treatment with propranolol, while treatment with PTU (6-n-propyl-2-thiouracil) increased adenosine sensitivity in the isolated heart. It was concluded that the levels of A1 adenosine receptors in the heart correspond to heart rate, and to cardiac efficiency. While an increase in heart rate was followed by up-regulation of A1 adenosine receptors, a decrease in heart rate caused a moderate elevation of these receptors.

Hypervolaemia improves global and local function and efficiency in postischaemic myocardium

1. In the present study, we investigated the effects of blood volume on postischaemic function and efficiency. In 14 anaesthetized dogs, following recovery from a period of 15 min occlusion of the left anterior descending coronary artery, the effects of hypervolaemia (HYPER; 15% increased volume produced by fast infusion of Hespan; B Braun Medical, Irvine, CA, USA), normovolaemia (NORMO) and hypovolaemia (HYPO) were studied. 2. Although myocardial O2 consumption was not significantly increased by volume (6.37+/-0.94 vs 6.89+/-1.1 mL/min per 100 g for HYPO and HYPER, respectively), local work of the stunned myocardium was markedly elevated (8.8+/-1.7 vs 22.5+/-3.5 g.mm/ beat, for HYPO and HYPER, respectively; P < 0.05). External work of the heart was also significantly improved (71.8+/-12.7 vs 139.5+/-16.2 mmHg.L/min for HYPO and HYPER, respectively). These data indicate markedly improved efficiency produced by volume, because work was increased with no change in myocardial O2 consumption. 3. Local dysfunction was characterized by several parameters, including systolic bulge, end-diastolic length, delay to onset of shortening, end shortening time delay (EST) and tail work ratio. Hypervolaemia reduced EST compared with hypovolaemia (98.6+/-18.3 vs 110.7+/-14.9 msec, respectively; P < 0.05) and improved tail work ratio (28.0+/-7.0 vs 36.0+/-7.0%, respectively; P < 0.05), with no effects on systolic bulge, end-diastolic length and delay to onset of shortening. 4. Thus, even in the postischaemic myocardium, increasing work by volume is energetically efficient and is accompanied by partial improvement of local dysfunction.

Reduction in the level of cardiac cyclic GMP worsens contractile delay in myocardial stunning

We tested the hypothesis that a reduction in the level of myocardial cyclic GMP would worsen the contractile delay associated with myocardial stunning. Two groups of 12 anesthetized open-chest New Zealand white rabbits were utilized. Myocardial stunning was produced by two 15-min occlusions of the left anterior descending coronary artery followed by 15 min of reperfusion. Either control vehicle (saline + 1% DMSO) or 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ 10(-4) M, a guanylate cyclase inhibitor) was topically applied to the left ventricular surface of the rabbit hearts. Left ventricular and aortic pressures along with wall thickness parameters were determined. Coronary blood flow (microspheres) and O(2) extraction (microspectrophotometry) were used to determine myocardial O(2) consumption. Myocardial stunning was observed in the control group through an increased delay in onset of wall thickening (46.2 +/- 7.3 vs 76.6 +/- 17.5 ms). There was no significant effect of stunning on the rate of wall thickening (21.8 +/- 9.5 vs 18.1 +/- 3.4 mm/s) or O(2) consumption (stun 4.6 +/- 0.6, control 4.8 +/- 0.4 ml O(2)/min/100 g). After treatment with ODQ 10(-4) M, both delay (43.9 +/- 9.6 vs 134.1 +/- 30.0 ms) and myocardial O(2) consumption (stun 5.9 +/- 0.6, control 5.9 +/- 0. 7) increased significantly compared to control. There was no significant change in the rate of wall thickening. We conclude that decreasing cyclic GMP worsens stunning by increasing delay in onset of wall thickening and increasing local O(2) costs in the stunned region.

Beta-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.

Local inotropic stimulation by methylene blue does not improve mechanical dysfunction due to myocardial stunning

We tested the hypothesis that reduction of intramyocardial cyclic guanosine monophosphate (GMP) by methylene blue (MB) would improve mechanical dysfunction in stunned myocardium. Regional stunning was produced in nine open-chest anesthetized dogs by a 12-min left anterior descending coronary artery (LAD) occlusion. MB was infused into the LAD during reperfusion (1 mg/kg per min). Stunning reduced LAD force development, introduced a significant time delay between the onset of force and shortening (delay) and caused significant systolic bulging to occur. Stunning reduced systolic regional work (the integrated product of force and segment shortening during systole), but did not significantly alter regional oxygen consumption or cyclic GMP levels. MB decreased cyclic GMP (1.8 +/- 0.2 to 0.9 +/- 0.1 pmol/g) and increased peak force (36 +/- 5 to 55 +/- 10 g). However, MB increased delay (93.9 +/- 18.4 to 233 +/- 19 ms) and systolic bulging (5.9 +/- 2.1% to 9.3 +/- 2.8%) and further reduced systolic regional work (control; 4204 +/- 933 g x mm/min; stunned: 2191 +/- 542 g x mm/min; MB: 1153 +/- 516 g x mm/min). MB increased regional myocardial oxygen consumption (7.4 +/- 1.0 to 15.6 +/- 2.7 ml O2/min per 100 g). These results suggest that depressed contractility, while present in myocardial stunning, is not the primary cause of mechanical dysfunction.

The oxygen wasting effect of isoproterenol is altered by chemical denervation and cardiac hypertrophy

We tested the hypothesis that isoproterenol would increase myocardial work and O2 consumption at reduced efficiency and that both left ventricular hypertrophy and chemical sympathectomy would lead to changes in this myocardial efficiency response. Left ventricular hypertrophy was produced by aortic valve plication in 23 puppies. Six months later, sympathetic denervation (6-hydroxydopamine) was produced in 12 hypertrophied and 10 non-hypertrophied dogs, 5 days prior to acute experiments. Ten non-hypertrophied and 11 hypertrophied animals were not denervated. Measurements were made before and during an isoproterenol infusion (0.5 microgram/kg/ min). Regional myocardial work was calculated as the integrated product of force (miniature transducer) and segment shortening (ultrasonic crystals). Regional O2 consumption was calculated from regional blood flow (microspheres) and regional O2 saturations (microspectrophotometry). In all groups, regional O2 consumption increased with isoproterenol (non-hypertrophied, non-sympathectomized 6.5 +/- 0.8 to 20.3 +/- 5 ml O2/min/100 g, non-hypertrophied, sympathectomized 5.0 +/- 0.7 to 10.0 +/- 1.5, hypertrophied, non-sympathectomized 9.8 +/- 1.3 to 16.2 +/- 2.2, hypertrophied, sympathectomized 6.1 +/- 0.5 to 13.3 +/- 1.6). Regional segment work also increased in all groups with isoproterenol stimulation (non-hypertrophied, non-sympathectomized 781 +/- 73 to 1197 +/- 61 g.mm/min, non-hypertrophied, sympathectomized 996 +/- 221 to 2118 +/- 412, hypertrophied, non-sympathectomized 1031 +/- 145 to 3262 +/- 753, hypertrophied, sympathectomized 721 +/- 116 to 1745 +/- 402). In the non-hypertrophied, non-sympathectomized group, efficiency (work/O2 consumption) was significantly decreased from 122 +/- 17 to 76 +/- 9 g.mm/ml O2/100 g demonstrating an "oxygen wasting" effect. In the hypertrophied, non-sympathectomized group, segment efficiency significantly increased from 94 +/- 19 to 250 +/- 63. In both sympathectomized groups, efficiency was not altered by isoproterenol. Thus the oxygen wasting effect of beta-adrenergic stimulation was reversed by left ventricular hypertrophy and blocked by sympathectomy.

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.

Effect of hypoxic and carbon monoxide-induced hypoxia on regional myocardial segment work and O2 consumption

The purpose of this study was to investigate the potentially greater responses of regional myocardial work and O2 consumption to hypoxic hypoxia than to CO-induced hypoxia. Twenty open-chest anesthetized dogs were studied under control and four hypoxic conditions, hypoxic hypoxia induced with either 8% O2 (SaO2 = 56%) or 6% O2 (SaO2 = 40%) gas mixtures, or CO-induced hypoxia produced by a 1% CO gas mixture for either 7 min (SaO2 = 67%; SaCO = 30%) or 20 min (SaO2 = 40%; SaCO = 56%). Ultrasonic crystals and a force gauge were utilized to measure myocardial shortening and force. Regional myocardial segment work was calculated by integrating myocardial segment shortening multiplied by its corresponding force. Radioactive microspheres were used to measure regional coronary blood flow during each condition. Transmural biopsies were utilized to measure arterial and venous O2 saturation with a four-wavelength microspectrophotometric method. Regional O2 extraction and consumption were calculated. Regional coronary blood flow (77 +/- 38 ml/min per 100 g, control) increased with severe hypoxic hypoxia (293 +/- 206) and CO-induced hypoxia (150 +/- 128). Regional myocardial O2 extraction was decreased with both hypoxic and CO hypoxia. Regional myocardial O2 consumption was maintained even with severe hypoxic and CO hypoxia. Regional myocardial segment work/min increased from 343 +/- 205 g* mm/min with increasing levels of hypoxic hypoxia (564 +/- 677) and decreased with increasing levels of CO hypoxia (169 +/- 111). Regional segment work increased with increasing levels of hypoxic hypoxia and decreased with increasing CO hypoxia. The differential effects on segment work caused by the two types of hypoxia may be due to direct metabolic or autonomic differences.

Regional asynchrony of segmental contraction may explain the "oxygen consumption paradox" in stunned myocardium

Despite apparently depressed function, stunned myocardium maintains oxygen consumption and has the capacity to increase contractility with inotropic stimulation. We hypothesized that during stunning, O2 demand is maintained because regional segment work is performed, but is asynchronous with global left ventricular contraction, and that inotropic stimulation would restore regional work and synchrony. Thirteen open-chest anesthetized dogs were subjected to three left anterior descending (LAD) coronary artery occlusions (10 min) and reperfusions (15 min) to produce regional myocardial stunning. Segment shortening and force development were measured independently and simultaneously in the LAD (experimental) region and circumflex (control) regions. Regional myocardial work was calculated as the integrated product of instantaneous force and shortening, during two periods: 1) over the entire cardiac cycle (Positive Work), and 2) limited to the systolic portion of the cardiac cycle (Systolic Work). Regional myocardial O2 consumption (MVO2) was calculated from regional blood flow (radiolabeled microspheres) and O2 saturation data (microspectrophotometry). Occlusion of the LAD produced a delay in onset of segment shortening in the ischemic region, but not in regional force development. A time delay of 67-81 ms persisted through the three stages of occlusions and reperfusions. Systolic regional work was depressed to a greater extent (924 +/- 182 to 149 +/- 118 g*mm*min-1) than total positive regional work (1437 +/- 337 to 857 +/- 174 g*mm*min-1). Regional subepicardial MVO2 in the stunned region was not different than in the control region (7.3 +/- 1.5 vs. 6.9 +/- 1.4 ml O2*min-1*100 g-1). Local infusion of isoproterenol reversed the delay in regional shortening from 73 +/- 7 to 21 +/- 8 ms, thereby augmenting systolic work (298%) more than positive work (60%), without a significant increase in MVO2 (7.3 +/- 1.5 to 10.5 +/- 3.2 ml O2*min-1*100 g-1). It is concluded that myocardial stunning decreases regional systolic work due to regional mechanical asynchrony, while MVO2 is used supported total positive work which was not significantly reduced. Isoproterenol restores regional work by restoring synchrony, without greatly affecting regional MVO2.