Augmentation of myocardial oxygen consumption in hyperthyroid cats

Triiodothyronine optimizes sheep ventriculoarterial coupling for work efficiency

BACKGROUND

Triiodothyronine (T3) administration after cardiopulmonary bypass has been shown to significantly improve cardiac performance. The present study was undertaken to elucidate the effects of T3, when administered as an intravenous bolus, on both cardiac energetics and stroke work-oxygen utilization (EW/LVVO2) efficiency.

METHODS

In both unstressed and stressed hearts, energetics were evaluated at baseline and 2 hours after intervention in an in vivo sheep preparation. In the first group (n = 5) sheep received saline vehicle. In the second group (n = 9) sheep received an intravenous bolus of 1.2 micrograms/kg of T3. In the third group (n = 7) sheep received a 2-hour intravenous infusion of dobutamine at a rate of 5 micrograms/kg/min.

RESULTS

In the unstressed heart, T3 improved cardiac function at no cost in oxygen consumption by decreasing afterload and hence improved EW/LVVO2 efficiency. In contrast, dobutamine improved unstressed cardiac function by increasing contractility at the cost of increased oxygen consumption and thus decreased EW/LVVO2 efficiency. Triiodothyronine optimized ventriculoarterial coupling for efficiency, but dobutamine optimized coupling for maximal work. In the stressed heart, T3 again improved EW/LVVO2 efficiency, but dobutamine had the opposite effect.

CONCLUSIONS

The bolus administration of T3 improves unstressed cardiac performance through optimization of ventriculoarterial coupling for EW/LVVO2 efficiency, primarily through vasodilation. Triiodothyronine also increases efficiency in the stressed heart. This study supports the use of T3 in cardiac operations to improve cardiac performance with no cost in oxygen consumption characteristic of inotropic agents.

The direct effects of 3,5,3'-triiodo-L-thyronine (T3) on myocyte contractile processes. Insights into mechanisms of action

Administration of 3,5,3'-triiodo-L-thyronine (T3) has recently been suggested to acutely improve left ventricular performance. However, the cellular and molecular mechanisms responsible for this improvement in left ventricular function with T3 remained unknown. Accordingly, the present study examined the direct effects of T3 administration on myocyte contractile function and the sarcolemmal systems that might potentially contribute to these effects. In isolated porcine left ventricular myocytes (n = 81), velocity of shortening increased in the presence of 80 pmol/L T3 compared with that in untreated myocytes (117.0 +/- 5.0 versus 77.3 +/- 3.3 microns/sec, p < 0.05). In a separate series of experiments (n = 29), myocyte velocity of shortening increased in the presence of both T3 and beta-adrenergic receptor stimulation (25 nmol/L isoproterenol) to greater than that with beta-adrenergic receptor stimulation alone (274.3 +/- 16.9 versus 203.7 +/- 16.2 microns/sec, p < 0.05). Cyclic adenosine monophosphate generation was next examined in isolated myocyte preparations (n = 9). In the presence of T3, no significant increase in cyclic-adenosine monophosphate generation was observed compared with that in untreated myocytes (39.1 +/- 8.3 versus 24.7 +/- 5.8 fmols/myocyte, p = 0.17). However, in the presence of both T3 and beta-adrenergic receptor stimulation, cyclic-adenosine monophosphate generation increased significantly to greater than that with beta-adrenergic receptor stimulation alone (224.4 +/- 61.1 versus 120.1 +/- 35.5 fmoles/myocyte, p < 0.05). Because cyclic-adenosine monophosphate modulates intracellular Ca2+ processes, L-type Ca+2 channel current (patch clamp methods; -picoamp/picofarad, n = 15) and peak intracellular Ca+2 levels (fura 2 ionic measurement, n = 47) were next measured. In the presence of T3, a shift in the activation voltage at peak L-type Ca+2 channel current was observed from baseline (5.5 +/- 1.4 versus 9.0 +/- 1.0 mV, p < 0.05). Furthermore, in the presence of both T3 and beta-adrenergic receptor stimulation, peak L-type Ca+2 channel current (8.9 +/- 0.7 versus 6.3 +/- 1.0 mV, p < 0.05) and peak intracellular Ca+2 levels (189.9 +/- 8.4 versus 171.7 +/- 8.3 nmol/L, p < 0.05) increased compared with values obtained with beta-adrenergic receptor stimulation alone. Important findings from the present study were twofold: (1) T3 improved myocyte contractile processes through a cyclic-adenosine monophosphate-independent mechanism and (2) T3 potentiated the effects of beta-adrenergic receptor stimulation transduction by increasing cyclic-adenosine monophosphate production, L-type Ca+2 channel current, and Ca+2 availability to the myocyte contractile apparatus.(ABSTRACT TRUNCATED AT 400 WORDS)

Propranolol and thyroxine-induced hypertrophic rabbit hearts: effect on heart size and regional O2 supply/consumption variables

The purpose of this study was to determine the effect of acute and chronic propranolol on heart size and regional O2 supply/consumption variables in thyroxine (T4)-treated rabbit hearts. New Zealand white rabbits were given 0.5 mg/kg T4 for 3 or 16 days with and without concomitant 2 mg/kg propranolol. Another group was given 16 days of propranolol alone and another 3-day T4 group was given 2 mg/kg propranolol 1 h before the experiment began. Another group served as control. Myocardial blood flows were determined using radioactive microspheres and small arteriolar and venous O2 saturations were determined using microspectrophotometry. Treatment with T4 for 3 or 16 days increased the heart weight/body weight ratio, myocardial blood flow, and regional O2 consumption. 16-day T4 treatment resulted in myocardial flow 195% and O2 consumption 300% above control group values. When propranolol was given chronically along with T4, heart weight/body weight ratios did not increase to the degree seen with 3 or 16 days of T4, alone. Propranolol given acutely in 3-day T4-treated animals, resulted in a reduced O2 consumption and O2 extraction, though not to the extent seen with chronic propranolol treatment of T4-treated animals. Acute propranolol treatment slightly reduced myocardial blood flow in 3-day T4-treated animals, while chronic treatment significantly reduced it. Chronic propranolol treatment in 16-day T4-treated animals resulted in a significant reduction in flow and O2 consumption. Thus, T4 treatment increased O2 consumption, flow, and heart size and these effects could be attenuated using acute and chronic propranolol.

Spatial energy balance within a structural model of the left ventricle

A model describing the local instantaneous energetic needs within the left ventricle (LV) myocardium is presented. The model, which combines the myocardial oxygen consumption (MVO2) with the mechanical activity of the cardiac muscle, is based on the theory of cross bridge kinetics between the actin and myosin fibers within the sarcomere. The microscale relationship between the stress, stress development, strain rate and basal metabolism demand is incorporated into the LV model which describes the mechanical activities of different layers within the myocardium. The model shows a significant increase in the oxygen consumption in the endocardial layers as compared with the epicardial layers. Integrating the spatial and temporal oxygen consumption distribution within the myocardium yields the total myocardial oxygen consumption. The quantitative relationships between the heat rate, stress, contractility and external work and the MVO2 are in agreement with known data. The model thus offers a tool to assess the local instantaneous as well as the time averaged overall energy consumption, over a wide range of loading conditions of the LV.

Role of thyroid hormone in postnatal circulatory and metabolic adjustments

To assess the role of the early postnatal surge in plasma thyroid hormone concentrations on cardiovascular and metabolic adaptations, we measured cardiac output, total oxygen consumption, and plasma triiodothyronine (T3) concentrations in three groups of lambs in the first 6 h after delivery. 15 fetal lambs were prepared at gestational ages of 128-129 d by placing catheters in the brachiocephalic artery, descending aorta, distal inferior vena cava, left atrium, and pulmonary artery so that measurements could be made soon after delivery. They were divided into three groups: Group I comprised five control animals; Group II consisted of five fetuses in which thyroidectomy was performed at surgery at 129 d gestation; and Group III consisted of five animals in which thyroidectomy was performed at term gestation during delivery by caesarian section, prior to severing the umbilical cord. The lambs in Group I exhibited a rapid postnatal rise in T3 concentrations, similar to that described previously, reaching a peak value of about 5 ng/ml. Although the postnatal surge in T3 concentration was arrested in Group II and III animals, Group II had no detectable plasma T3, while the Group III animals had T3 concentrations of about 0.8 ng/ml, which were within the range previously reported for term lamb fetuses. The lambs in group II showed 40-50% lower left ventricular outputs (190 vs. 297 ml/kg per min), systemic blood flows (155 vs. 286 ml/kg per min), and oxygen consumptions (9.8 vs. 20.2 ml/kg per min) as compared with Group I animals over the entire 6-h period. The lambs in Group II also had significantly lower heart rates (131 vs. 192 beats/min) and mean systemic arterial pressures (56 vs. 72 torr). However, there were no significant differences for any of these measurements between the Group III and Group I lambs. The reduction in cardiac output in the Group II animals were reflected in a significantly lower blood flow to the peripheral circulation, but there were no significant differences in blood flow to other organs in the three groups. These studies indicate that plasma thyroid concentrations in the 2-3 wk prior to delivery and not the increase in thyroid hormone concentrations which occur after birth are important for postnatal cardiovascular and metabolic adjustments. We speculate that lack of circulating triiodothyronine in late gestation may affect postnatal cardiovascular adaptation by modifying normal beta adrenergic receptor development.

Effects of thyroid hormone on left ventricular function in patients treated for thyrotoxicosis

Systolic time intervals, echocardiographic indexes of left ventricular contractile function and serum triiodothyronine and thyroxine levels were measured before treatment in nine patients with hyperthyroidism, and again every 2 weeks for the first 2 months after therapy and then every 4 weeks until the subjects were clinically and chemically euthyroid. Six of the nine became transiently hypothyroid. Although the preejection period corrected for heart rate (preejection period index) increased as the patients became euthyroid, the change was not significant. Preejection period index increased dramatically in the patients becoming hypothyroid (p less than 0.005). Corrected left ventricular ejection time (left ventricular ejection time index) also increased as the patients became euthyroid (p less than 0.001), and increased again with the appearance of hypothyroidism (p less than 0.05). There was a linear correlation between velocity of circumferential fiber shortening and serum triiodothyronine level (r - 0.77) and between velocity of circumferential fiber shortening and serum thyroxine level (r = 0.70) at all stages of thyroid function. Thus thyroid hormone definitely enhances left ventricular function in human beings, and both excess and deficiency cause predictable reversible changes in myocardial contractile function. Thus thyroid hormone definitely enhances left ventricular function in human beings, and both excess and deficiency cause predictable reversible changes in myocardial contractile function. Furthermore echocardiographic measurements of velocity of circumferential fiber shortening provide rapid estimates of the chemical status of thyrotoxic patients before and after treatment.

Force-velocity relations of the portal vein of hyperthyroid rats

The effect of thyroxine on the elementary process of contraction of vascular smooth muscle was tested on 38 hyperthyroid and in 38 control rats. Hyperthyroidism was induced by i.p. injections of triiodthyronine or 1-htyroxine for 2 weeks. By means of afterloaded isotonic contractions of the tetanized portal vein force velocity relations were calculated. There was a slight increase in both the extrapolated velocity of shortening at zero load [from 0.93 +/- 0.04 ML/s (control rats) to 1.03 +/- 0.04 ML/s (hyperthyroid rats; P less than 0.05)], and the peak force generation (from 14.8 +/- 0.4 mN to 16.1 +/- 0.4 mN; P less than 0.005). The maximum of mechanical power development at a distinct extent of afterload was augmented from 1.35 +/- 0.06 muW/ML to 1.68 +/- 0.06 muW/ML (P less than 0.005). The influence of thyroxine on the elementary process of contraction in vascular smooth muscle is discussed in connection with the much stronger effect of hyperthyroidism on cardiac muscle contraction.

Thyroid hormone control of Na+-K+-ATPase and K+-dependent phosphatase in rat heart

To assess the possible role of the Na+ pump in mediating physiological responses to thyroid hormone in the rat myocardium, we examined the effects of L-3,5,3'-triiodothyronine (T3) on the activities of the closely associated enzymes, Na+-K+-dependent adenosine triphosphatase (Na-K-ATPase) and K+-dependent p-nitrophenyl phosphatase (K-dep-pNPPase). In hypothyroid rats, administration of T3 (50 microng/100 g body wt) resulted in significant increases (greater than 50%) in Na-K-ATPase and K-dep-pNPPase activities in both crude homogenates and microsomal fractions of the rat ventricle. Significant effects on Na-K-ATPase activity were also attained with low doses (1 microng/100 g body wt) of T3. A method was developed for assaying K-dep-pNPPase activity in cardiac slices. With this technique, enhancement in K-dep-pNPPase activity of 89.2% was found in ventricle slices after treatment of hypothyroid rats with T3 (50 microng/100 g body wt), implying that augmentation of the capacity of the Na+ pump is achieved in vivo. The potent analogue, L-3,5-diiodo-3' isopropyl thyronine (isopropyl T2) had the same effects on cardiac growth and Na-K-ATPase as T3, in hypothyroid rats. In contrast, the relatively inactive isomer, L-3,3',5'-triiodothyronine (reverse T3) had no significant effect on the heart weight-to-body weight ratio or on ventricular Na-K-ATPase activity.

Experimental hyperthyroidism IV. Myocardial muscle mechanics and oxygen consumption in euand hyperthyroidism

Myocardial mechanics and oxygen consumption were studied in right ventricular papillary muscles taken from cats pretreated with cristalline L-thyroxine (1 mg/kg/day, i.p.) 8-18 days prior to the examination. Isotonic afterloaded and isometric contractions were employed. Oxygen consumption was determined polarographically. Data obtained were compared with control studies on papillary muscles taken from euthyroid cats. In isotonic afterloaded contractions the extent of shortening was nearly identical in both groups. However, maximum rate of isometric tension development and velocity of isotonic shortening were considerably increased in hyperthyroid myocardium. Myocardial oxygen consumption was significantly increased in hyperthyroidism, primarily due to an increased maximum rate of isometric tension development and-to a lesser extent-to increased isotonic contraction velocity. In isometric contractions maximum tension development (preload near Lmax) was similar in both groups. However, maximum rate of isometric tension development was markedly increased in hyperthyroidism. A close and linear relationship was found between maximum rate of isometric tension development (isometric contraction) and myocardial oxygen consumption. The results demonstrate increases of velocity factors of myocardial performance in experimental hyperthyroidism. Myocardial oxygen consumption is significantly increased. This increase in oxygen consumption quantitatively has its mechanical equivalent in increased isometric contraction velocity and, to a small amount, in increased isotonic contraction velocity.

Mechanisms for the abnormal energetics of pressure-induced hypertrophy of cat myocardium

Depressed myocardial contractility with paradoxically increased oxygen consumption has been demonstrated in previous studies of pressure overload hypertrophy. To determine whether altered mitochondrial respiration participates in the abnormal energetics of this muscle, right ventricular hypertrophy (RVH) was produced in 12 cats by pulmonary artery banding. A polarographic muscle bath was used to study eight control and eight RVH papillary muscles, and the respiration of mitochondria isolated from these right ventricles was characterized. RVH muscles demonstrated depressed force-velocity and length-tension curves. The myocardial oxygen consumption per gram of peak active tension was increased from 0.65 ± 0.05 nliters/mg beat -1 (control) to 1.10±0.07 nliters/mg beat -1 (RVH) ( P <0.001). Abnormal mitochondrial respiration was shown by an increase in the rate of state 4 oxygen consumption from 12.5±0.8 natoms/mg min -1 (control) to 19.9±0.8 natoms/mg min -1 (RVH) ( P < 0.001). The altered oxygen cost of active isometric tension in the RVH muscles was linearly correlated with the altered rate of mitochondrial state 4 oxygen consumption ( r = 0.91). Ruthenium red, a compound that blocks mitochondrial calcium uptake, reduced the rate of RVH state 4 oxygen consumption to the control level. The present study suggests a mechanism for the abnormal myocardial oxygen consumption in pressure overload hypertrophy and relates it to nonphosphorylating mitochondrial respiration linked to calcium transport.

Mechanochemistry of cardiac muscle. V. Influence of thyroid state on energy utilization

The possibility that alterations in the rate or efficiency of energy utilization could be involved in the control of cellular oxygen consumption by thyroid hormone was examined in right ventricular papillary muscles isolated from normal euthyroid cats and cats with experimentally induced hyperthyroidism and hypothyroidism. Energy production in the muscles was inhibited and isolated from the process of energy utilization by exposure to iodoacetic acid and nitrogen. After resting or performing variable amounts of contractile element work under isometric conditions, muscles were frozen, and the total amount of chemical energy ( approximately P = creatine phosphate + ATP) used was determined. The resting rate of energy utilization in muscles from euthyroid animals was 0.78+/-0.07 mumoles/g per min of approximately P. This rate was elevated in muscles from hyperthyroid cats to 1.00+/-0.09 mumoles/g per min and decreased in muscles from hypothyroid cats to 0.23+/-0.14 mumoles/g per min. Isometrically contracting muscles from cats with hypothyroidism utilized only 64% as much energy as muscles from euthyroid cats while performing 81% as much contractile element work at a moderately decreased level of contractile state. Muscles from hyperthyroid cats utilized an average of 41% more energy than did muscles from euthyroid cats while contracting an identical number of times and performing an equal amount of contractile element work at a slightly increased level of contractile state. These results suggest that thyroid hormone directly influences the rate of cellular energy utilization. Furthermore, the increase in energy utilization in muscles from hyperthyroid cats could not be attributed entirely to observed alterations in contractile behavior, which indicates that excess thyroid hormone may decrease the efficiency of the conversion of cellular energy to work. However, the opposite effect, an increased efficiency of energy utilization, was not observed in muscles from hypothyroid cats. Thus, it is concluded that the calorigenic effects of thyroid hormone may be explained, at least in part, by alterations in the process of energy utilization.