The effect of temperature on the basal metabolism of cardiac muscle

Protecting the aged heart during cardiac surgery: single-dose del Nido cardioplegia is superior to multi-dose del Nido cardioplegia in isolated rat hearts

Background:

Aged hearts are particularly vulnerable to reperfusion injury. We recently showed that single-dose del Nido cardioplegia was superior to ‘standard’ multi-dose 4:1 blood cardioplegia in aged rat hearts. This study seeks to determine if multi-dose del Nido cardioplegia offers additional benefits over single-dose del Nido cardioplegia.

Methods:

Functional recovery after 60 min of cardioplegic arrest was assessed in isolated, working, senescent rat hearts. Single-dose del Nido cardioplegia (n=14) was compared to multi-dose del Nido cardioplegia (n=12) delivered every 20 min.

Results:

Troponin release during reperfusion was similar in the single (0.263±0.056 ng/ml) and multi-dose groups (0.261±0.055 ng/ml). Although functional recovery was similar early after reperfusion (stroke work 91±6 ml*mmHg*g−1 vs. 91±8 ml*mmHg*g−1 for single- vs. multi-dose), it declined over time in the multi-dose group (71±9 vs. 43±9 ml*mmHg*g−1 at 60 min, p=0.0175)

Conclusions:

In aged rat hearts, a single-dose del Nido cardioplegia strategy results in superior functional recovery compared to a multi-dose del Nido cardioplegia strategy.

Stress development, heat production and dynamic modulus of rat isolated cardiac trabeculae revealed in a flow-through micro-mechano-calorimeter

Progress toward understanding the thermo-mechanical behavior of isolated cardiac muscle, excised from either healthy or diseased heart, is contingent on being able to measure simultaneously the stress (force per cross-sectional area) and heat production. Determination of dynamic modulus (dynamic stiffness times muscle length per cross-sectional area) sheds further light on the behavior of the force-and heat-generating actin-myosin cross-bridges. We are in a unique position to perform such measurements, given the recent completion of a micro-mechano-calorimeter. In this paper, we characterize the micro-mechano-calorimeter and present experimental results of twitch stress, heat per twitch and dynamic modulus measured in rat right-ventricular trabeculae at varied stimulus frequencies and muscle lengths. The minute radial dimensions of cardiac trabeculae (which approximate those of a human hair) ensure adequate provision of oxygen and nutrients via diffusion from the continuously replenished superfusate flowing through the measurement chamber. This enables investigation of the thermo-mechanical performance of cardiac trabeculae for many hours.

Modelling diffusive O(2) supply to isolated preparations of mammalian skeletal and cardiac muscle

The purpose of this study was to use A. V. Hill's equation describing diffusion of O(2) into cylindrical muscles to assess the adequacy of O(2) supply for commonly used isolated preparations of mammalian cardiac and skeletal muscles. The diffusion equation was solved numerically to give the maximum, steady state O(2) diffusion distances (i.e. the distance from the surface of the muscle to the radial location where P(O(2)) is 0) for both resting and contracting muscles and for a range of temperatures. Non-steady state solutions for the rest-to-work transition were also determined to estimate how long contractile activity could be continued before anoxia develops at the muscle centre. The influence on muscle oxygenation of myoglobin-facilitated O(2) diffusion was also assessed. The analysis was performed for typical sized, whole muscles from adult rats and mice, for frog sartorius muscle and for a range of temperatures. Muscle O(2) consumption rates were taken from the literature. The results indicated that (1) diffusive O(2) supply would be adequate to support resting metabolism of soleus and EDL muscles of rat and mouse but may not be adequate to support the transient high resting metabolic rate of papillary muscles shortly after dissection, (2) during steady contractile activity of soleus and EDL muscles, particularly those from the rat, over a reasonable range of duty cycles, adequate O(2) supply could only be ensured if the radii of preparations was substantially smaller than those of whole muscles and (3) for cardiac muscles, diffusive O(2) supply could only support steady-state metabolism at twitch frequencies <1 Hz for whole papillary muscles from rat and <3 Hz for those from mouse. Reducing experimental temperature markedly enhances O(2) supply to skeletal, but not cardiac, muscle. O(2) supply from myoglobin had only minimal effects on oxygenation under typical isolated muscle conditions.

Resting metabolism of mouse papillary muscle

The aims of this study were to measure the resting metabolic rate of isolated mouse papillary muscles and to determine whether diffusive O2 supply is adequate to support the resting metabolism. Resting metabolism of left ventricular papillary muscles was measured in vitro (27 degrees C) using the myothermic technique. The rate of resting metabolism declined exponentially with time towards a steady value, with a time constant of 18+/-2 min (n=13). There was no alteration in isometric force output during this time. The magnitude of the resting metabolism, which depended inversely on muscle mass, more than doubled following a change in substrate from glucose to pyruvate and was increased 2.5-fold when the osmolarity of the bathing solution was increased by addition of 300 mM sucrose. Addition of 30 mM 2, 3-butanedione monoxime affected neither the time course of the decline in metabolic rate nor the eventual steady value. Analysis of the diffusive oxygen supply to the isolated preparation indicated that small papillary muscles (mass <1 mg), which have a very high resting metabolic rate early in an experiment, are unlikely to be adequately oxygenated.

The effect of increasing extracellular potassium concentration on the resting heart rate of the isolated rat papillary muscle

The effect of elevating extracellular K+ concentration on the basal metabolism of the isolated rat left ventricular papillary muscle has been investigated. The preparation was mounted on a thermopile and connected to a force transducer, to allow simultaneous measurement of muscle heat production and force. The resting heat rate (RHR) of the quiescent preparation was measured as an index of basal metabolism. Throughout all of the experiments, the muscles were maintained under a resting force of 10 mN and all measurements of RHR were made at times when there was no active force present above this passive level. Elevating the extracellular K+ concentration from 5.9 to 20, 40, then 80 mM produced graded increases in the RHR. The increase in RHR produced by 40 mM K+ was observed to be time-dependent, its effect being significantly greater at 5-7 h than at 2-4 h after cardiectomy. Averaged over all times, the percentage increases in RHR produced by 20, 40, and 80 mM K+ in the presence of 2 mM Ca2+ were 6.4 +/- 2.0%, 28.7 +/- 2.3%, and 51.3 +/- 8.9% (mean +/- SEM) respectively. The high K(+)-induced increase in basal metabolism was also shown to be Ca2(+)-dependent, the increase in RHR produced by 40 mM K+ being greater the higher the extracellular Ca2+ concentration (0.5-8.0 mM). The addition of verapamil was found to partially inhibit the K(+)-induced increase in resting metabolism. These results show that elevation of the extracellular K+ concentration produces a graded increase in the RHR that is Ca2(+)-dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial blood flow and oxygen consumption after aortic cross-clamping

The effect of four repetitive periods of 10 min global ischemia, followed by 15 min of reperfusion on myocardial blood flow and oxygen consumption at 34 degrees C (group N) and 25 degrees C (group H) were studied in comparison with that of 60 min continuous ischemia combined with multidose St. Thomas Cardioplegia (group C) in dogs on cardiopulmonary bypass (CPB). Two groups, in which a protocol comparable to the groups N and H, respectively, but without repetitive periods of ischemia was followed, served as control. In all groups a hyperemic response was observed after release of aortic cross-clamping (AC). Myocardial blood flow was not diminished at the end of CPB as compared to the values at the start of CPB. We conclude that the no-reflow phenomenon did not occur after these procedures of intermittent or continuous AC. Immediately following release of AC the arterial-coronary sinus difference of oxygen content reached a peak value in groups N and H indicating rapid replenishment of the low tissue oxygen content. These peak values appeared to be much smaller after cardioplegia. After 10 min of reperfusion a significant lower oxygen consumption was observed at 25 degrees C (0.2 mumole.g-1.min-1 O2) as compared to 34 degrees C (1.5 mumole.g-1.min-1 O2). This difference cannot be explained by temperature alone because oxygen consumption did not decrease below 0.5 mumole.g-1.min-1 in the control group of dogs put on CPB at 25 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Heat production of quiescent ventricular trabeculae isolated from guinea-pig heart

1. A new calorimetric technique has been developed which allows continuous measurement of the rate of energy expenditure in superfused preparations of cardiac muscle. Thin trabeculae of guinea-pig ventricular muscle were mounted in a Perspex tube of 0.8 mm inner diameter and the temperature difference of the perfusate upstream and downstream of the preparation was measured. 2. The resting heat rate of trabeculae of 240-575 microns diameter from guinea-pig heart was determined repeatedly for up to 6 h after cardiectomy. It did not vary with time during the course of the experiment. 3. The average resting heat rate measured in HEPES-buffered Tyrode solution containing 20 mM-glucose and 2 mM-pyruvate as substrates was 130 +/- 29 mW/g dry weight or 36 +/- 8 mW/cm3 of tissue (n = 15). This is an order of magnitude larger than the resting heat rate reported in the literature for isolated cardiac preparations. 4. After omitting the pyruvate from the superfusate the resting heat rate decreased to 60-70% of its steady value within 4 min. After readmission of pyruvate this effect was reversed. The average resting heat rate with glucose as sole substrate was 23 +/- 4 mW/cm3. 5. Uncoupling of the mitochondria by 50 microM-2,4-dinitrophenol (DNP) increased the heat rate up to 170 mW/cm3. This effect could be maintained for several minutes and was fully reversible. Raising the external K+ concentration to 150 mM (NaCl replaced by KCl) induced a transient rise in the rate of heat production up to 115 mW/cm3. 6. The heat production during uncoupling of the mitochondria and during potassium contractures was inversely related to the diameter of the preparation. Calculation based on Hill's equation (Hill, 1928) indicated that this was caused by the development of anoxia at the core of the preparation. 7. In contrast, the rate of heat production of quiescent preparations was not correlated with diameter and calculation indicated that at rest there was no anoxic core. The high value of resting heat rate found in the present study is discussed within the context of the large variation of 1.7-25 mW/g reported in the literature for resting metabolic rate of cardiac muscle.

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.

The rate of resting heat production of rat papillary muscle

The rate of resting heat production of quiescent rat left ventricular papillary muscles was measured myothermically. The effects of contractile activity, stretch, oxygen partial pressure, temperature, amino acids and time were examined. The rate of basal heat production was the same throughout the day whether or not muscles contracted isotonically under a small pre-load. Passive stretch increased the rate of resting heat production; the stretch-induced increment was highly variable from muscle to muscle. The resting heat rate per se was only moderately sensitive to oxygen partial pressure and temperature, and was insensitive to the presence of amino acids in the bathing medium. The stretch-induced increase in resting heat rate was independent of these three factors. The rate of resting heat production declined exponentially with time to reach a plateau about 4 h following cardiectomy.