Initial mechanical efficiency of isolated cardiac muscle.

The aim of this study was to determine whether the initial mechanical efficiency (ratio of work output to initial metabolic cost) of isolated cardiac muscle is over 60%, as has been reported previously, or whether it is approximately 30%, as suggested by an estimate based on the well-established net mechanical efficiency (ratio of work output to total, suprabasal energy cost) of 15%. Determination of initial efficiency required separation of the enthalpy output (i.e. heat + work) into initial and recovery components. The former corresponds to energy produced by reactions that use high-energy phosphates and the latter to energy produced in the regeneration of high-energy phosphates. The two components were separated mathematically. Experiments were performed in vitro (30 degrees C) using preparations dissected from rat left ventricular papillary muscles (N=13). Muscle work output and heat production were measured during a series of 40 contractions using a contraction protocol designed to mimic in vivo papillary muscle activity. Net mechanical efficiency was 13.3+/-0.7%. The total enthalpy output was 2.16 times greater than the initial enthalpy output, so that initial mechanical efficiency was 28.1+/-1.2%.

The energetics of rat papillary muscles undergoing realistic strain patterns.

Studies of cardiac muscle energetics have traditionally used contraction protocols with strain patterns that bear little resemblance to those observed in vivo. This study aimed to develop a realistic strain protocol, based on published in situ measurements of contracting papillary muscles, for use with isolated preparations. The protocol included the three phases observed in intact papillary muscles: an initial isometric phase followed by isovelocity shortening and re-lengthening phases. Realistic papillary muscle dynamics were simulated in vitro (27 degrees C) using preparations isolated from the left ventricle of adult male rats. The standard contraction protocol consisted of 40 twitches at a contraction rate of 2 Hz. Force, changes in muscle length and changes in muscle temperature were measured simultaneously. To quantify the energetic costs of contraction, work output and enthalpy output were determined, from which the maximum net mechanical efficiency could be calculated. The most notable result from these experiments was the constancy of enthalpy output per twitch, or energy cost, despite the various alterations made to the protocol. Changes in mechanical efficiency, therefore, generally reflected changes in work output per twitch. The variable that affected work output per twitch to the greatest extent was the amplitude of shortening, while changes in the duration of the initial isometric phase had little effect. Decreasing the duration of the shortening phase increased work output per twitch without altering enthalpy output per twitch. Increasing the contraction frequency from 2 to 3 Hz resulted in slight decreases in the work output per twitch and in efficiency. Using this realistic strain protocol, the maximum net mechanical efficiency of rat papillary muscles was approximately 15 %. The protocol was modified to incorporate an isometric relaxation period, thus allowing the model to simulate the main mechanical features of ventricular function.

Comparison of the efficiency of rat papillary muscles during afterloaded isotonic contractions and contractions with sinusoidal length changes.

The results of previous studies suggest that the maximum mechanical efficiency of rat papillary muscles is lower during a contraction protocol involving sinusoidal length changes than during one involving afterloaded isotonic contractions. The aim of this study was to compare directly the efficiency of isolated rat papillary muscle preparations in isotonic and sinusoidal contraction protocols. Experiments were performed in vitro (27 degrees C) using left ventricular papillary muscles from adult rats. Each preparation performed three contraction protocols: (i) low-frequency afterloaded isotonic contractions (10 twitches at 0.2 Hz), (ii) sinusoidal length change contractions with phasic stimulation (40 twitches at 2 Hz) and (iii) high-frequency afterloaded isotonic contractions (40 twitches at 2 Hz). The first two protocols resembled those used in previous studies and the third combined the characteristics of the first two. The parameters for each protocol were adjusted to those that gave maximum efficiency. For the afterloaded isotonic protocols, the afterload was set to 0.3 of the maximum developed force. The sinusoidal length change protocol incorporated a cycle amplitude of +/-5% resting length and a stimulus phase of -10 degrees. Measurements of force output, muscle length change and muscle temperature change were used to calculate the work and heat produced during and after each protocol. Net mechanical efficiency was defined as the proportion of the energy (enthalpy) liberated by the muscle that appeared as work. The efficiency in the low-frequency, isotonic contraction protocol was 21.1+/-1.4% (mean +/- s.e.m., N=6) and that in the sinusoidal protocol was 13.2+/-0.7%, consistent with previous results. This difference was not due to the higher frequency or greater number of twitches because efficiency in the high-frequency, isotonic protocol was 21.5+/-1.0%. Although these results apparently confirm that efficiency is protocol-dependent, additional experiments designed to measure work output unambiguously indicated that the method used to calculate work output in isotonic contractions overestimated actual work output. When net work output, which excludes work done by parallel elastic elements, rather than total work output was used to determine efficiency in afterloaded isotonic contractions, efficiency was similar to that for sinusoidal contractions. The maximum net mechanical efficiency of rat papillary muscles performing afterloaded isotonic or sinusoidal length change contractions was between 10 and 15%.

Effects of pyruvate on intracellular Ca2+ regulation in cardiac myocytes from normal and diabetic rats.

1. Pyruvate has been shown to enhance the contractile performance of cardiac muscle when provided as an alternative substrate to glucose. The aims of the present study were to determine whether the inotropic effects of pyruvate are due to increased mobilization of intracellular Ca2+ ([Ca2+]i) and to compare the effects of pyruvate on [Ca2+]i levels in myocytes from normal and diabetic animals. 2. Fura-2 was used to monitor [Ca2+]i in ventricular myocytes isolated from control and streptozotocin-treated male Wistar rats. The experiments were performed at 25 degrees C, with an extracellular [Ca2+] of 1.5 mmol/L and either 10 mmol/L glucose or 10 mmol/L pyruvate as the substrate. 3. In myocytes from both control and diabetic rats, increasing the stimulus frequency from 0.33 to 2.0 Hz resulted in significant increases in resting and peak [Ca2+]i as well as in the amplitude of the [Ca2+]i transient, irrespective of substrate. Compared with glucose, pyruvate significantly increased resting and peak [Ca2+]i and the amplitude of the [Ca2+]i transient at each stimulus frequency in myocytes from both control and diabetic animals. However, the extent of potentiation of the [Ca2+]i transient amplitude produced by pyruvate was significantly less in myocytes from the diabetic rats. 4. The rate of restitution of the [Ca2+]i transient was used as an index of the rate of Ca2+ cycling by the sarcoplasmic reticulum (SR). Pyruvate enhanced the rate of restitution in control but not diabetic rat cells. 5. The time course of decay of the [Ca2+]i transient was analysed as a measure of the rate of removal of Ca2+ from the cytosol. Pyruvate tended to increase the rate of decay in cells from control but not diabetic animals. The rate of decay was slower in cells from diabetic animals compared with controls. 6. The data reveal that pyruvate increases SR Ca2+ cycling, leading to greater Ca2+ release and an increase in the amplitude of the [Ca2+]i transient. Therefore, it seems highly likely that increased [Ca2+]i mobilization is responsible for the previously reported positive inotropic actions of pyruvate. These effects of pyruvate are attenuated in diabetic rat cells, which may reflect an impaired capacity of mitochondria in diabetic hearts to oxidize pyruvate, thus limiting potential energetic benefits.