PubMed Health⌕ Search

Biomedical subjects

G Elzinga

Publications and source records attributed to G Elzinga.

At least 55 records · Page 3Linked to original sources
PubMed1989

Variation in the normalized tetanic force of single frog muscle fibres.

1. The forces produced in maximal fixed-end tetani of single fibres isolated from the anterior tibialis muscle of the frog Rana temporaria have been measured at sarcomere lengths of 2.2 microns and temperatures near 0 and 10 degrees C. 2. When normalized by either cross-sectional area or dry weight per unit length at a sarcomere length of 2.2 microns, the forces vary over a twofold range. 3. The normalized force is not significantly correlated with the velocity of unloaded shortening or the twitch characteristics of the fibres. Lack of variability of these two quantities (together with histochemical evidence) suggest that only one fibre type is present in the experimental sample. 4. The steady rate of energy liberation (stable, heart rate) of the fibres during isometric tetani is positively correlated with the normalized force, indicating that extra ATP splitting is required to produce higher forces. 5. Fibres with a higher ratio of dry weight per unit length to cross-sectional area ('dry density') show a higher force when normalized by area, but not when normalized by dry weight per unit length. 6. Fibres with a more circular cross-sectional profile produce more force when normalized by either cross-sectional area or dry weight per unit length. The significance of this correlation is unclear. 7. The contribution of various sources to the total overall variation in normalized force is assessed. It is suggested that a diffusible substance or substances may be involved in modulating fibre force.

Adenosine Triphosphatases↗
PubMed1989

Isometric force production before and after chemical skinning in isolated muscle fibres of the frog Rana temporaria.

1. The force produced in single fibres isolated from the anterior tibialis muscle of the frog Rana temporaria has been measured in tetani near 4 degrees C, and then in calcium-activated contractures of segments of the same fibres after chemical demembranation. All measurements were made at a sarcomere length of 2.3 microns. Force was normalized for fibre cross-section by the dry weight per unit length of the segments, which is proportional to cross-sectional area (Elzinga, Howarth, Rall, Wilson & Woledge, 1989). 2. The ratio of the force developed by the skinned segments to that produced by the intact fibres was inversely related to segment cross-section (dry weight per unit length), falling from approximately 1.0 for the thinnest segments to 0.6 for the thickest segments. 3. It is calculated that the accumulation of orthophosphate ion within contracting segments can account for a significant part of the decline in relative force in thicker segments. 4. The absolute forces in intact fibres and their derived segments were strongly correlated, but normalization by segment cross-section removed the correlation. 5. It is concluded that the sources of the approximately twofold variation in normalized force in both intact and skinned preparations are different. The existence of diffusible, force-modulating factors in intact fibres, which may be removed during skinning, is considered.

Animals↗
PubMed1989

Substrate dependence of energy metabolism in isolated guinea-pig cardiac muscle: a microcalorimetric study.

1. The effects of glucose, pyruvate and lactate on basal metabolism and on contraction-related energy expenditure of thin trabeculae isolated from guinea-pig heart were studied using a microcalorimetric technique. 2. Resting heat rates of cardiac ventricular muscle measured in the presence of substrate-free solution (56 +/- 20 mW (g dry weight)-1), 10 mM-lactate (54 +/- 12 mW (g dry weight)-1) and 10 mM-glucose (63 +/- 24 mW (g dry weight)-1) did not differ significantly. Increasing the external glucose concentration (up to 100 mM) and/or adding insulin (up to 80 units l-1) had virtually no effect on the measured resting heat rate. 3. With 10 mM-pyruvate as substrate resting heat rate was substantially larger (106 +/- 40 mW (g dry weight)-1) than with glucose, lactate or substrate-free solution. The concentrations of pyruvate producing a half-maximal increase in resting heat rate as compared to substrate-free solution ranged between 0.4 and 1.2 mM. 4. In order to test whether the development of an anoxic core contributed to the substrate dependence of resting heat production the critical PO2 (i.e. the PO2 that produced a just-noticeable decrease in heat rate) was determined in cylindrical preparations of various diameters. It was found that none of the preparations had an anoxic core at rest in a solution equilibrated with 100% oxygen. 5. From the dependence of the critical PO2 on the diameter of the preparation the diffusion coefficient of oxygen through cardiac muscle was calculated using a modification of Hill's equation (Hill, 1928). The O2 diffusion coefficient was found to be 1.09 X 10(-5) cm2 s-1. 6. Contraction-related heat production was also found to be dependent on the substrate used. In the presence of 10 mM-pyruvate it was about 60% larger than in the presence of 10 mM-glucose, 10 mM-lactate or with substrate-free solution. 7. Isometric force of contraction showed the same substrate dependence as contraction-related heat production and increased with a similar time course during repetitive stimulation. 8. The possible mechanisms underlying the substrate dependence of myocardial energy metabolism are discussed. It is suggested that the increased energy expenditure observed in the presence of pyruvate may be related to a decrease in intracellular phosphate and/or to an increase in intracellular pH.

Animals↗
PubMed1989

Metabolic recovery of acidotic rabbit cardiac muscle: effects of low pH and oxygen shortage.

Heat production during and after contraction of isolated rabbit papillary muscles was measured at 20 degrees C with metal-film thermopiles. Under control conditions (0.2 Hz, pH 7.4) total heat and steady-state force production due to 120 twitches were 1.0 +/- 0.4 J/g and 29.5 +/- 5.1 mN/mm2 (mean +/- SD; n = 5), respectively. Increasing the CO2 of the bicarbonate-buffered superfusate from 5% (pH 7.4) to 24% (pH 6.6) led to a decrease of force and heat production, 54% and 72%, respectively, of the control value. The recovery heat-time constant, reflecting the time course of oxidative phosphorylation, increased from 23.0 +/- 5.1 seconds at pH 7.4 to 69.5 +/- 34.7 seconds at pH 6.6. The ratio of recovery and initial heat equaled 1.06 under both conditions. Enhancing the metabolic rate by increasing the stimulation frequency to 1.0 Hz led, after an initial maximum, to a decline of force and heat presumably as the consequence of shortage of oxygen in the muscle core. The recovery phase in this case was characterized by a double exponential function having time constants of 7.6 and 64.4 seconds. When pH was lowered to 6.6 together with the enhancement of the stimulation frequency to 1 Hz, an additional exothermal process, unrelated to force production, was observed during contraction and for some time thereafter. It was concluded that severe acidosis slows down the rate of oxidative phosphorylation and may reduce the economy of contraction. However, it does not change the nature of recovery and initial heat processes.(ABSTRACT TRUNCATED AT 250 WORDS)

Acidosis↗
PubMed1989

Energy turnover in hypoxic heart muscle.

In isolated rabbit papillary muscles at 20 degrees C, we studied the relationship between demand, supply, and utilisation of energy in hypoxia. Since shortage of energy is an important factor in loss of contractile performance following an hypoxic period, we tried to find a relationship between the loss of force production upon reoxygenation and the demand, supply, and utilisation of energy. Energy demand in hypoxia was defined as the -P turnover found in oxygen under further identical conditions. Energy supply in hypoxia was determined from lactate formation, using a P/lactate ratio of 1, while energy utilisation in hypoxia was obtained from the sum of: -P supply, the decrease of PCr and ATP, and the increase of AMP. Energy demand in hypoxia was varied by stimulating the hypoxic muscles at 0.2 Hz or not at all. For contracting hypoxic muscles energy supply by glycolysis was only 23% of energy demand. After 40 min of hypoxia force did only partially recover (78%) upon reoxygenation. For muscles at rest in hypoxia, force recovery was significantly (p less than 0.05) better (94% of control). However, energy supply was no more than 15% of the demand because supply was almost proportionally lower. It is concluded that the loss of force production upon reoxygenation is not related to the supply: demand ratio. By taking the integrated difference after 40 min of hypoxia between energy demand and utilisation, the energy deficit built up in hypoxia was estimated. In contracting muscle this was 2 times higher than in resting ones. It is suggested that a relationship could exist between the energy deficit and the loss of contractile performance upon reoxygenation. Experiments to test this idea are discussed.

Animals↗
PubMed1988

Recovery heat production of isolated rabbit papillary muscle at 20 degrees C.

Using metal-film thermopiles, heat production of isolated rabbit papillary muscles was measured under aerobic conditions at 20 degrees C. The time course of total heat production resulting from a single contraction (average of 10) and a twitch train of 10 contractions (0.2 Hz) was separated into initial (I) and recovery heat (R). The time course of recovery heat production of single twitches was characterized by a time constant of 25.4 +/- 1.7 s (mean +/- SE; n = 10). The recovery ratio, R/I, was 1.18 +/- 0.08 (mean +/- SE; n = 7). Total heat produced 25.2 +/- 2.9 mJ.gdw-1 (mean +/- SE; n = 11). After trains of 10 contractions a time constant of 25.2 +/- 1.6 s (mean +/- SE; n = 9) was found. The recovery ratio was 1.14 +/- 0.09 (mean +/-SE; n = 9). Total heat produced was 489 +/- 41 mJ.gdw-1 (mean +/- SE; n = 9). Time constants and recovery ratios for 1 and 10 twitches were not significantly different. This suggests that only the extent but not the nature of the chemical processes after contraction changes when the preparation produces about 20 times more heat. Since the recovery ratio values did not differ largely from the value derived theoretically the conclusion is justified that, under normal aerobic conditions, PCr splitting and its oxidative resynthesis are the major metabolic processes responsible for the energy supply of isolated cardiac muscle.

Animals↗
PubMed1988

Dependency of the force-velocity relationships on Mg ATP in different types of muscle fibers from Xenopus laevis.

MgATP binding to the actomyosin complex is followed by the dissociation of actin and myosin. The rate of this dissociation process was determined from the relationship between the maximum velocity of shortening and the MgATP concentration. It is shown here that the overall dissociation rate is rather similar in different types of muscle fibers. The relation between MgATP concentration and the maximum shortening velocity was investigated in fast and slow fibers and bundles of myofibrils of the iliofibularis muscle of Xenopus laevis at 4 degrees C from which the sarcolemma was either removed mechanically or made permeable by means of a detergent. A small segment of each fiber was used for a histochemical determination of fiber type. At 5 mM MgATP, the fast fibers had a maximum shortening velocity (Vmax) of 1.74 +/- 0.12 Lo/s (mean +/- SEM) (Lo: segment length at a sarcomere length of 2.2 microns). For the slow fibers Vmax was 0.41 +/- 0.15 Lo/s. In both cases, the relationship between Vmax and the ATP concentration followed the hyperbolic Michaelis-Menten relation. A Km of 0.56 +/- 0.06 mM (mean +/- SD) was found for the fast fibers and of 0.16 +/- 0.03 mM for the slow fibers. Assuming that Vmax is mainly determined by the crossbridge detachment rate, the apparent second order dissociation rate for the actomyosin complex in vivo would be 3.8.10(5) M-1s-1 for the fast fibers and 2.9.10(5) M-1 s-1 for the slow fibers. Maximum power output as a function of the MgATP concentration was derived from the force-velocity relationships. At 5 mM MgATP, the maximum power output in fast fibers was (73 +/- 8) mW.g-1 dry weight and (15 +/- 5) mW.g-1 in slow fibers. The Km for MgATP for the maximum power output for the fast fibers was (0.15 +/- 0.03) mM, which is about a factor of 4 lower than the Km for Vmax. The implications of these results are discussed in terms of a kinetic scheme for crossbridge action.

Adenosine Triphosphate↗
PubMed1988

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.

2,4-Dinitrophenol↗
PubMed1988

Oxygen consumption of single muscle fibres of Rana temporaria and Xenopus laevis at 20 degrees C.

1. Oxygen consumption of contracting single muscle fibres of Rana temporaria and Xenopus laevis was investigated at 20 degrees C. 2. Single fibres of the tibialis anterior muscle of Rana and the iliofibularis muscle of Xenopus were mounted in a chamber containing Ringer solution. The solution was stirred and its partial pressure of oxygen (PO2) was continuously measured polarographically. 3. Steady-state rates of oxygen consumption (VO2) of single fibres were determined as a function of twitch frequency (0.2-12 stimuli s-1, depending on the type of fibre). VO2 increased with twitch frequency until a plateau value (VO2,max) was reached. VO2,max of different fibres ranged from 0.042 to 0.169 nmol O2 s-1 mg-1 dry weight in Rana and from 0.045 to 0.412 nmol O2 s-1 mg-1 dry weight in Xenopus. Under VO2,max conditions oxygen availability was not the limiting factor. 4. VO2 after injection of the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) into the chamber correlated with VO2,max, suggesting that VO2,max is determined by mitochondrial density. This suggestion was confirmed by the observation that a close relationship exists between VO2,max and succinate dehydrogenase activity in three different fibre types of Xenopus. 5. At VO2,max a considerable amount of oxygen was taken up after the twitch train by most fibres, indicating that the oxidative ATP synthesis cannot match ATP hydrolysis. Xenopus muscle fibres with high oxidative capacity did not show this phenomenon. 6. The results are discussed in relation to the occurrence of anoxic cores in muscle fibres and the maximum steady-state contractile activity attainable by the fibres.

Animals↗
PubMed1988

Matching between feline left ventricle and arterial load: optimal external power or efficiency.

We tested the hypothesis that the feline left ventricle normally works at optimal external power as opposed to optimal efficiency by (re)analyzing data from five isolated, blood-perfused cat hearts and 39 open-thorax cats. In the isolated hearts, we measured pump function, external steady power, myocardial oxygen consumption, and efficiency. Optimal external power and optimal efficiency were found at different left ventricular outputs (6.94 +/- 0.33 and 8.35 +/- 0.37 ml/s, respectively; P less than 0.001). In the in situ cat hearts the working point was found at an output of 4.72 +/- 0.32 ml/s, whereas optimal external power was found at 4.84 +/- 0.26 ml/s. These values were not significantly different. Assuming that the point of optimal efficiency was located at the same fraction of the maximal unloaded left ventricular output (Fmax) as in the isolated hearts, i.e., 0.7, we found the point of optimal efficiency for the in situ heart at a flow of 5.83 +/- 0.32 ml/s, which was significantly different (P less than 0.001) from the flow in the working point. Our data therefore indicate that the left ventricle in the open-thorax cat is matched to the arterial load such that its external power output rather than efficiency is optimized.

Algorithms↗
PubMed1988

The energetics of work and heat production by single muscle fibres from the frog.

During active shortening the heat rate in isolated muscle fibres is greater than during isometric contraction, and increases with velocity of shortening (V), but at a decreasing rate as the maximum velocity (V0) is approached. For shortening at V less than 0.25 V0 the amount of extra heat produced during a period of shortening is proportional to the distance shortened, but for rapid shortening (V greater than 0.5 V0) the extra heat increases less than proportionally with the distance shortened. After a period of shortening the higher heat rate returns to the isometric level over a period of several hundred milliseconds. A similar period of increased heat rate is seen after a quick release. After shortening 10% of muscles slack length at 0.1 V0 the amount of heat produced during tension redevelopment is similar to that after a quick release. But after more rapid shortening there is less heat production than after a quick release.

Adenosine Triphosphate↗
PubMed1987

ATPase activity of intact single muscle fibres of Xenopus laevis is related to the rate of force redevelopment after rapid shortening.

Five different fibre types have been recognized in the iliofibularis muscle of Xenopus laevis. The force-velocity and histochemical characteristics of these fibres vary considerably and differences are also found in their myosin composition. In this study a comparison was made between the rate of ATP hydrolysis estimated from the stable maintenance heat rate and the mechanical performance of fibres of type 1, 2 and 3. In the experiments, firstly the force-velocity relation of a fibre was determined, and subsequently, heat production during isometric tetanic contractions at 20 degrees C was measured. Force redevelopment following the fastest shortening used for the measurement of the force-velocity relationship was fitted to a single exponential. The rate of ATP hydrolysis, estimated from the heat production, was found to be roughly proportional to the rate of force redevelopment. Crossbridge attachment rate was determined by using a simulation of a four state model of the crossbridge cycle. It appears that crossbridge attachment rate is proportional to the in vivo actomyosin ATPase activity during an isometric tetanic contraction.

Adenosine Triphosphatases↗
PubMed1987

Mechanical determinants of myocardial energy turnover.

Energy turnover of the left ventricle does not differ in isovolumic contractions and contractions where pressure is released from peak to zero. This experimental result corresponds to predictions from a time varying elastance model of the mechanical and energetic properties of the left ventricle. To assess the validity of this model for cardiac muscle in general, experiments were designed to investigate whether mechanical and energetic behaviour of isolated cardiac muscle preparations could also be predicted from the time varying elastance model. The results obtained so far indicate, however, that not all experimental results can be accommodated by the model. This suggests that the value of the model may be limited.

Animals↗
PubMed1987

Heat production and oxygen consumption following contraction of isolated rabbit papillary muscle at 20 degrees C.

The time course of oxygen uptake following isometric twitch contractions of isolated rabbit papillary muscles was measured using a polarographic oxygen electrode. Using a diffusion model we eliminated the effect of oxygen storage on the measured time course of oxygen uptake to determine the time constant of mitochondrial "off" kinetics. Two different approaches were followed. In Method 1, two steady-state levels were compared, whereas in Method 2, the time course of mitochondrial "off" kinetics was studied. Using Method 1 we found tau = 20 +/- 8 seconds (n = 7), whereas Method 2 yielded tau = 26 +/- 9 seconds (n = 11). These findings were compared with preliminary measurements of recovery heat production of the same preparation and at the same temperature. Heat produced after a train of 10 twitch contractions appeared to follow a monoexponential time course with a time constant of 24.9 +/- 9.5 seconds (n = 9). These results suggest that aerobic metabolism in isolated rabbit papillary muscle constitutes the only recovery process.

Animals↗