Role of acetylstrophanthidin in augmenting myocardial oxygen consumption. Relation of increased O-2 consumption to changes in velocity of contraction

Surrogates for myocardial power and power efficiency in patients with aortic valve disease

We aimed to assess surrogate markers for left ventricular (LV) myocardial power and efficiency in patients with isolated aortic stenosis (AS) and combined stenosis/regurgitation (AS/AR). In AS (n = 59), AS/AR (n = 21) and controls (n = 14), surrogates for LV myocardial power and circulatory/external myocardial efficiency were obtained from cardiac MRI. Median surrogate LV myocardial power was increased in AS, 7.7 W/m² (interquartile range 6.0–10.2; p = 0.010) and AS/AR, 10.8 W/m² (8.9–13.4; p < 0.001) when compared to controls, 5.4 W/m² (4.2–6.5), and was lower in AS than AS/AR (p < 0.001). Surrogate circulatory efficiency was decreased in AS, 8.6% (6.8–11.1; p < 0.001) and AS/AR, 5.4% (4.1–6.2; p < 0.001) when compared to controls, 11.8% (9.8–16.9). Surrogate external myocardial efficiency was higher in AS, 15.2% (11.9–18.6) than in AS/AR, 12.2% (10.1–14.2; p = 0.031) and was significantly lower compared to controls, 12.2% (10.7–18.1) in patients with reduced ejection fraction (EF), 9.8% (8.1–11.7; p = 0.025). In 16% of all cases, left ventricular mass/volume indices and EF were within normal ranges, wheras surrogate LV myocardial power was elevated and patients were symptomatic. Although influenced by pressure/volume load, the myocardium is additionally affected by remodelling processes. Surrogates for circulatory efficiency and LV myocardial power gradually reflect alterations in patients with AS and AS/AR, even when surrogate external myocardial efficiency, EF, mass and volume indices still remain compensated.

Energetics of the Frank-Starling effect in rabbit myocardium: economy and efficiency depend on muscle length

We tested the hypothesis that economy and efficiency are independent of length in intact cardiac muscle over its normal working range. We measured force, force-time integral, force-length area, and myocardial oxygen consumption in eight isometrically contracting rabbit right ventricular papillary muscles. 2,3-Butanedione monoxime was used to partition nonbasal oxygen consumption into tension-independent and tension-dependent components. Developed force, force-time integral, and force-length area increased by factors of 2.4, 2.7, and 4.8, respectively, as muscle length was increased from 90% to 100% maximal length, whereas tension-dependent oxygen consumption increased only 1.6-fold. Economy (the ratio of force-time integral to tension-dependent oxygen consumption) increased significantly with muscle length, as did contractile efficiency, the ratio of force-length area to tension-dependent oxygen consumption. The average force-length area-nonbasal oxygen consumption intercept was more than the twice tension-independent oxygen consumption. We conclude that economy and efficiency increase with length in rabbit myocardium. This conclusion is consistent with published data in isolated rabbit and dog hearts but at odds with studies in skinned myocardium.

Constitutive properties of hypertrophied myocardium: cellular contribution to changes in myocardial stiffness

Recent studies have suggested that pressure overload hypertrophy (POH) alters the viscoelastic properties of individual cardiocytes when studied in isolation. However, whether these changes in cardiocyte properties contribute causally to changes in the material properties of the cardiac muscle as a whole is unknown. Accordingly, a selective, isolated, acute change in cardiocyte constitutive properties was imposed in an in vitro system capable of measuring the resultant effect on the material properties of the composite cardiac muscle. POH caused an increase in both myocardial elastic stiffness, from 20.5 +/- 1.3 to 28.4 +/- 1.8, and viscous damping, from 15.2 +/- 1.1 to 19.8 +/- 1.5 s (normal vs. POH, P < 0.05), respectively. Recent studies have shown that cardiocyte constitutive properties could be acutely altered by depolymerizing the microtubules with colchicine. Colchicine caused a significant decrease in the viscous damping in POH muscles (19.8 +/- 1.5 s at baseline vs. 14.7 +/- 1.3 s after colchicine, P < 0.05). Therefore, myocardial material properties can be altered by selectively changing the constitutive properties of one element within this muscle tissue, the cardiocyte. Changes in the constitutive properties of the cardiocytes themselves contribute to the abnormalities in myocardial stiffness and viscosity that develop during POH.

Viscoelastic properties of pressure overload hypertrophied myocardium: effect of serine protease treatment

To determine whether and to what extent one component of the extracellular matrix, fibrillar collagen, contributes causally to abnormalities in viscoelasticity, collagen was acutely degraded by activation of endogenous matrix metalloproteinases (MMPs) with the serine protease plasmin. Papillary muscles were isolated from normal cats and cats with right ventricular pressure overload hypertrophy (POH) induced by pulmonary artery banding. Plasmin treatment caused MMP activation, collagen degradation, decreased the elastic stiffness constant, and decreased the viscosity constant in both normal and POH muscles. Thus, whereas many mechanisms may contribute to the abnormalities in myocardial viscoelasticity in the POH myocardium, changes in fibrillar collagen appear to play a predominant role.

Cardiac basal metabolism

We endeavor to show that the metabolism of the nonbeating heart can vary over an extreme range: from values approximating those measured in the beating heart to values of only a small fraction of normal--perhaps mimicking the situation of nonflow arrest during cardiac bypass surgery. We discuss some of the technical issues that make it difficult to establish the magnitude of basal metabolism in vivo. We consider some of the likely contributors to its magnitude and point out that the biochemical reasons for a sizable fraction of the heart's basal ATP usage remain unresolved. We consider many of the physiological factors that can alter the basal metabolic rate, stressing the importance of substrate supply. We point out that the protective effect of hypothermia may be less than is commonly assumed in the literature and suggest that hypoxia and ischemia may be able to regulate basal metabolic rate, thus making an important contribution to the phenomenon of cardiac hibernation.

Role of microtubules in the contractile dysfunction of hypertrophied myocardium

OBJECTIVES

We sought to determine whether the ameliorative effects of microtubule depolymerization on cellular contractile dysfunction in pressure overload cardiac hypertrophy apply at the tissue level.

BACKGROUND

A selective and persistent increase in microtubule density causes decreased contractile function of cardiocytes from cats with hypertrophy produced by chronic right ventricular (RV) pressure overloading. Microtubule depolymerization by colchicine normalizes contractility in these isolated cardiocytes. However, whether these changes in cellular function might contribute to changes in function at the more highly integrated and complex cardiac tissue level was unknown.

METHODS

Accordingly, RV papillary muscles were isolated from 25 cats with RV pressure overload hypertrophy induced by pulmonary artery banding (PAB) for 4 weeks and 25 control cats. Contractile state was measured using physiologically sequenced contractions before and 90 min after treatment with 10(-5) mol/liter colchicine.

RESULTS

The PAB significantly increased RV systolic pressure and the RV weight/body weight ratio in PAB; it significantly decreased developed tension from 59+/-3 mN/mm2 in control to 25+/-4 mN/mm2 in PAB, shortening extent from 0.21+/-0.01 muscle lengths (ML) in control to 0.12+/-0.01 ML in PAB, and shortening rate from 1.12+/-0.07 ML/s in control to 0.55+/-0.03 ML/s in PAB. Indirect immunofluorescence confocal microscopy showed that PAB muscles had a selective increase in microtubule density and that colchicine caused complete microtubule depolymerization in both control and PAB papillary muscles. Microtubule depolymerization normalized myocardial contractility in papillary muscles of PAB cats but did not alter contractility in control muscles.

CONCLUSIONS

Excess microtubule density, therefore, is equally important to both cellular and to myocardial contractile dysfunction caused by chronic, severe pressure-overload cardiac hypertrophy.

Cardiac basal and activation metabolism

Cardiac basal metabolism is the rate of energy expenditure of the quiescent myocardium. It is species dependent and increases with pre-load. It has small contributions from membrane-bound cation pumps. The contribution of protein metabolism remains open to question. Calculations show that mitochondrial proton pumping may account for a large fraction of the cardiac basal metabolism. Nevertheless this component remains essentially ill-understood. Cardiac activation metabolism is the supra-basal rate of energy expenditure associated with those processes that activate contraction. In isolated muscle preparations it is typically measured as the rate of heat production or oxygen consumption of a muscle, pre-shortened to a length where active force production is negligible, although it is also estimated by pharmacological intervention. In whole-heart studies it is indexed by the supra-basal rate of oxygen consumption of the empty, beating but non-working heart. Activation metabolism underwrites electrical excitation (the ECG) and excitation-contraction coupling (the cycling of calcium ions). It is increased by agents that increase contractility; it probably increases with pre-load, via the phenomenon of length-dependent activation. The basal and activation components each account for one-quarter to one-third of the total energy expenditure of the heart under normal conditions.

Beta adrenergic receptor blockade of feline myocardium. Cardiac mechanics, energetics, and beta adrenoceptor regulation

Myocardial oxygen consumption is regulated by interrelated mechanical and inotropic conditions; there is a parallel increase in the aerobic metabolism and inotropic state during beta-adrenergic stimulation under fixed mechanical conditions. In contrast, there is some evidence that beta-blockade may reduce oxygen consumption through effects independent of its influence on mechanical conditions and contractile state, and that prolonged beta-blockade may sensitize the myocardium to beta-adrenergic stimulation. To clarify these two points, the present study examined the relationship of myocardial energetics to mechanics and inotropism during acute beta-blockade and after the withdrawal of long-term beta-blockade, whereupon the basis for any effect observed was sought by characterizing the number, affinity, and affinity states of the beta-receptors as well as the coupling of activated beta-receptors to cyclic AMP generation. Studies of right ventricular papillary muscles from control and chronically beta-blocked cats demonstrated contractile and energetic properties as well as dose-response behavior and inotropic specificity suggestive of an increase in myocardial sensitivity to beta-adrenoceptor stimulation in the latter group. Assays of cardiac beta-adrenoceptors from further groups of control and pretreated cats, both in cardiac tissue and in isolated cardiac muscle cells, failed to define a difference between the two groups either in terms of receptor number and affinity or in terms of the proportion of receptors in the high-affinity state. However, coupling of the activated beta-adrenoceptors to cyclic AMP generation was enhanced in cardiac muscle cells from chronically beta-blocked cats. These data demonstrate that beta-adrenoceptor blockade (a) produces parallel effects on inotropic state and oxygen consumption without an independent effect on either and (b) increases myocardial sensitivity to beta-adrenergic stimulation after beta-blockade withdrawal, not by "up-regulation" of the cardiac beta-adrenoceptors, but instead by more effective coupling of these receptors when activated to cyclic AMP generation.

Myocardial oxygen consumption with isoproterenol versus calcium chloride in hypocalcemic ventricular failure in dogs

In 30 dogs on right heart bypass we compared the effects of isoproterenol with those of calcium chloride on myocardial oxygen consumption and on left ventricular function in the setting of ventricular depression produced by ionized hypocalcemia. In 22 dogs (Groups A and B) either isoproterenol or calcium chloride was infused, left ventricular function curves were generated, and end-diastolic pressure vs segment length plots were obtained. In 8 dogs (Group C), with initial hypocalcemia, both isoproterenol and calcium chloride were infused separately in random order to produce an equal decrease in left ventricular end-diastolic pressure at constant mean aortic pressure, heart rate, and cardiac output. Myocardial oxygen consumption and indices of left ventricular function were obtained. In Groups A and B, both drugs, when administered to the ventricle depressed by hypocalcemia, displaced left ventricular function curves upward and to the left. Left ventricular stroke work at constant left ventricular end-diastolic pressure increased (from 13.0 +/- 1.3 to 31.2 +/- 2.3 g X m for isoproterenol; from 13.9 +/- 2.5 to 32.5 +/- 2.5 g X m for calcium chloride). In Group C, there were no significant differences between left ventricular end-diastolic pressure, end-diastolic internal diameter, myocardial oxygen consumption, or peak left ventricular dP/dt in the hypocalcemic periods preceding isoproterenol and calcium chloride infusion. When the two drugs caused matched decreases in left ventricular end-diastolic pressure (-7.4 +/- 0.5 cm H2O for isoproterenol; -7.3 +/- 0.8 cm H2O for calcium chloride) there were similar decreases in end-diastolic internal diameter. However, isoproterenol was associated with a significantly greater (P less than 0.001) myocardial oxygen consumption (13.7 +/- 0.4 ml X 100 g-1 X min-1) than calcium chloride infusion (11.9 +/- 0.4 ml X 100 g-1 X min-1), as well as a greater peak left ventricular dP/dt (P less than 0.005).

Factors affecting the metabolism of resting rabbit papillary muscle

The rate of resting heat production of 12 right ventricular rabbit papillary muscles was measured myothermically. Resting heat rate was measured at 4 temperatures (15, 20, 25 and 30 degrees C) in either 45% or 95% O2 while the muscle was passively stretched with various pre-loads. The metabolic substrate was pyruvate (10 mmol X 1(-1)). The mean resting heat rate, averaged across all treatment conditions, was 2.88 mW/g with no significant difference between the two oxygen concentrations. The calculated Q10 of the resting heat rate was surprisingly low--only about 1.4--but is shown to be in general agreement with literature values from whole heart oxygen consumption studies when the time-dependent decline is taken into account. Stretching the muscle beyond its rest length increased the rate of resting heat production. This response appeared unrelated to muscle diameter. The results are discussed in terms of the possible diffusion limitation of isolated papillary muscle preparations.

Stretch-induced increase in resting metabolism of isolated papillary muscle

A mathematical model of oxygen diffusion into quiescent papillary muscles in vitro is developed. The model incorporates a continuous sigmoidal function relating the rate of oxygen consumption and the partial pressure of oxygen within the tissue. The behavior of the model is explored over a wide range of external oxygen partial pressures, oxygen consumption/partial pressure relations, oxygen diffusivities, muscle dimensions, and resting metabolic rates, while the muscle is subjected to stimulated stretches of various extents in order to test the assertion that the stretch-induced increase in basal metabolic rate observed experimentally implies the existence of an anoxic core region of papillary muscles in vitro. The model predicts the existence of an oxygen diffusion-mediated stretch response of resting papillary muscle metabolism, but one which is quantitatively insignificant compared with experimentally observed values. The classic Hill diffusion model which explicitly predicts an anoxic core, likewise predicts stretch effects of magnitudes smaller than those frequently observed. It is concluded that the increment in basal metabolism of papillary muscles subjected to stretch in vivo cannot be taken as evidence of oxygen diffusion limitation in unstretched preparations.

Comparative influence of ouabain, norepinephrine and heart rate on myocardial oxygen consumption and inotropic state in dogs

The purpose of this study was to compare the myocardial oxygen cost of augmented inotropic state produced by ouabain, norepinephrine, or increased heart rate. This problem was examined in dogs using an isovolumically contracting left ventricular preparation. Inotropic state was measured as the maximum observed contractile element velocity at the lowest common level of wall stress (MAX V). Peak left ventricular wall stress was maintained constant in each dog so that it would not influence changes in myocardial oxygen consumption (MVO2). Ouabain (4 X 10(-2) mumoles/Kg.) and norepinephrine (2 X 10(-3) mumoles/Kg./minute) always augmented inotropic state (MAX V) and increased MVO2. The positive slopes of the regression of MVO2 on MAX V for ouabain (45.4 +/- 12.5 microliter/beat/100 Gm./muscle length/sec; mean +/- SEM) and norepinephrine (34.5 +/- 5.6 microliter/beat/100 Gm./muscle length/sec; mean +/- SEM) were not significantly different, indicating that for an equal augmentation of inotropic state, ouabain increases myocardial oxygen demands to the same extent as does norepinephrine. When the results with ouabain or norepinephrine were compared to results obtained by altering heart rate, it was found that increasing inotropic state by these pharmacologic agents is more costly in terms of myocardial energy demands than when inotropic state is enhanced by increasing heart rate.

Normal contractile state of hypertrophied myocardium after pulmonary artery constriction in the cat

The contractile function of right ventricular papillary muscles from normal cats and cats in which the pulmonary artery had been constricted for 6 or 24 wk was examined. Acute pulmonary artery constriction reduced cross-sectional area by an average of 70%, resulting in a 30% mortality from congestive heart failure, all such deaths occurring within the first 3 wk after banding. The increase in right ventricular mass in animals surviving for 6 or 24 wk was similar, averaging 70%. No banded animals had evidence of congestive heart failure at the time of sacrifice, and cardiac output and right atrial pressures were similar to those in control animals.6 wk after banding, the active length-tension curve, maximal rate of rise of isometric force, force-velocity relations, and isometric force with paired stimulation and norepinephrine were all significantly depressed when compared to their respective values in control animals. In contrast, none of these variables was significantly different from control values in animals banded for 24 wk. These observations indicate that depressed contractile state is not a fundamental characteristic of pressure-induced hypertrophied myocardium and reemphasize the important temporal relationship between contractile state and the imposition of sudden sustained loads.

The effects of intra-aortic counterpulsation on cardiac performance and metabolism in shock associated with acute myocardial infarction

The effect of intra-aortic counterpulsation (IACP, 22-94 hr) on hemodynamics and cardiac energetics was evaluated in 10 patients in shock after acute myocardial infarction. The data clearly indicate that IACP improves myocardial oxygenation, enhances peripheral perfusion, and probably improves myocardial contractility in the severely diseased heart. Before treatment, decreases in cardiac index (mean value, 1.22 liter/min per m(2)), systolic ejection rate (67 ml/sec), and time-tension index per minute (1280 mm Hg.sec/min) were observed. Systemic vascular resistance varied widely. Low coronary blood flow (68 ml/min per 100 g) was associated with increased myocardial oxygen extraction (79%), low coronary sinus oxygen tension (20 mm Hg), and abnormal myocardial lactate-pyruvate metabolism. During 4-6 hr of IACP, systolic pressure and left ventricular outflow resistance decreased by 18% and 24%, respectively, while cardiac index improved by 38%. Diastolic arterial pressure rose 98%. Increase in coronary blood flow from an average of 68 to 91 ml/100 g per min (P < 0.001) was significantly correlated with rise in mean arterial pressure (r = 0.685). This correlation was best expressed in a third-order curve, which intercepts the point of no flow at a mean aortic pressure of 30 mm Hg. The flow-pressure curve is relatively flat above 65-70 mm Hg, but becomes steeper as mean aortic pressure falls below this point. Myocardial oxygen consumption remained essentially unchanged during early IACP and tended to rise during the later stages. However, the relationship of cardiac work performed to oxygen availability was markedly improved. Myocardial lactate production of 6% shifted to 15% extraction (P < 0.001). After termination of IACP, hemodynamics and myocardial perfusion and metabolism remained improved in the four patients who could be reevaluated. Although the acute shock state was reversed in all patients, only one left the hospital. Extensive myocardial damage limits the long-term survival of such patients. Therefore early IACP seems desirable, when subtle evidence of pump failure after acute myocardial infarction occurs. Early use of IACP may prevent the development of severe coronary shock or may stabilize cardiac energetics in severe shock facilitating subsequent surgical intervention.