Biomedical subjects

G Elzinga

Publications and source records attributed to G Elzinga.

At least 19 recordsLinked to original sources

PubMed1992

Energetic determinants of stunning and cell damage following reoxygenation of rabbit myocardium.

The energetic conditions in anoxia which lead to stunning and cell damage upon reoxygenation of the myocardium are studied in isolated papillary muscles of rabbit contracting isometrically at 20 degrees C. Before anoxia and thereafter the muscles are stimulated at 0.2 Hz, while during anoxia stimulation frequency is varied. Total creatine (CrT) content is taken as a measure of cell damage. The degree of stunning is estimated from the difference in force before anoxia and at the end of the recovery period when no creatine is lost. In anoxic rabbit papillary muscles glycogen is the substrate for lactate formation. The 'ATP reserve' of the muscle can then be calculated from: 2 ATP + PCr + ADP + 3 (glycosyl units). In control muscles it equals 482.2 mumol.g-1 dw of ATP. By varying stimulus frequency (0, 0.2, 0.5, or 1 Hz) during and the length of the anoxic period (20 to 80 min), the fraction of the ATP reserve used in anoxia is varied. It is found that the degree of stunning depends on the degree of ATP reserve depletion. When the ATP reserve becomes virtually zero force recovers to only 67% of the preanoxic value. When anoxia continues after the ATP reserve is virtually zero, creatine in the muscle decreases during reoxygenation, indicating cell damage. Apparently a hierarchy of cellular ATPases exists in the myocardium when energy is short preserving cell integrity as long as possible.

Adenosine Triphosphate↗

PubMed1992

Metabolic changes with fatigue in different types of single muscle fibres of Xenopus laevis.

1. Peak isometric force of single fast (type 1) and slow (type 3) muscle fibres of Xenopus decreased when fibres were stimulated intermittently above their predicted sustainable duty cycle at 20 degrees C. Type 1 fibres could be fatigued to zero force. In most type 3 fibres force did not decrease below 50% of the original (P0) before activation failure, as indicated by irregular contractions. 2. Fibres were rapidly frozen at different force levels and analysed by high-performance liquid chromatography (HPLC) for ATP, IMP, phosphocreatine (PCr) and creatine (Cr). Lactate was determined enzymatically in type 1 fibres only. The relationships between force and PCr, and between force and ATP during fatigue were, apart from the range of values obtained, the same for both fibre types. When force had fallen to about 60-80% of original, PCr was fully reduced. At lower force levels, the ATP content-decreased, and a concomitant rise of IMP content was found. At zero force, ATP had fallen to about 25% of its value in rested type 1 fibres, and up to 200 mumol lactate (g dry weight)-1 had accumulated. 3. Recovery from fatigue was studied in fibres where force had fallen to 0.6 P0 (both fibre types) and 0.2 P0 (type 1 only). After 1 h of recovery ATP had in all cases returned to the level measured in rested fibres. In fibres fatigued to 0.6 P0, force almost returned to its original value. However, in type 1 fibres fatigued to 0.2 P0, it returned to only 0.3 P0. After 1 h of recovery the PCr/Cr ratio in type 1 fibres was lower (probability, P less than 0.05) than in control fibres, whereas in type 3 fibres it was not significantly different from controls. 4. The relationship between peak force and stimulus frequency, which had a sigmoid shape in fully rested fibres, was drastically changed by fatiguing stimulation. Immediately after fatiguing stimulation of type 1 fibres, force hardly increased with stimulus frequency, corresponding to the observation that calcium efflux from the sarcoplasmic reticulum was decreased at high stimulus frequencies. The force-frequency relationship of type 3 fibres was the same before and after intermittent stimulation.

Adenosine Triphosphate↗

PubMed1992

Mechanical properties of skinned rabbit psoas and soleus muscle fibres during lengthening: effects of phosphate and Ca2+.

1. Mechanical properties of permeabilized single fibres from rabbit psoas and soleus muscle were determined by measuring the length responses due to abrupt changes in load and the force responses due to isovelocity length changes at different phosphate and Ca2+ concentrations. 2. The length responses due to abrupt increases in load from psoas fibres showed a rapid lengthening during the change in load followed by a phase of lengthening during which the velocity gradually decreased. In soleus fibres an abrupt lengthening during the change in load was followed by a phase of lengthening during which the velocity remained constant or decreased slightly for increases in load to less than 1.45 of the isometric force (F0). For larger increases in load the velocity during this later phase first increased and thereafter decreased. 3. The initial force-velocity curve, derived from the early part of the isotonic responses after the change in load, as well as the late force-velocity curve derived from the force level attained during isovelocity length changes, were sensitive to phosphate. Phosphate caused a shift of the absolute force-velocity curves of both psoas and soleus fibres towards lower values of force. In psoas fibres, the relative force-velocity curves derived by normalization of the force level to the force developed isometrically was shifted by phosphate to smaller velocities. In soleus fibres, the initial velocity at low and intermediate relative loads (less than 1.75 F0) was increased by phosphate but at higher loads it decreased, while the late force-velocity curve showed an overall decrease in velocity. 4. The force responses during isovelocity lengthening of psoas fibres showed an early rapid increase in force followed by a slow rise in force. The position of this break point in force was sensitive to the phosphate concentration. In soleus fibres, the force responses without phosphate showed an overshoot followed by a slow rise in force. The overshoot diminished with increasing phosphate concentration. 5. Phosphate and Ca2+ affected the force responses in psoas and soleus fibres in different ways. When the isometric starting levels were the same, force during and after the length change at submaximal activation was always less than at maximal activation in the presence of 15 mM-phosphate. 6. The changes in the mechanical performance during lengthening caused by phosphate in psoas as well as in soleus fibres, are in agreement with a decrease in the average force per attached crossbridge.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1992

ATP formation and energy demand in anoxic heart muscle of the rabbit.

In quiescent rabbit papillary muscle at 20 degrees C, the formation of ATP in nitrogen, estimated from the production of lactate, is 21% of that in oxygen. Stimulating the anoxic muscles at 0.2 Hz causes a threefold increase in ATP formation. In this study we want to determine 1) whether glycolytic ATP formation can be increased to a rate that would meet the aerobic ATP demand at rest and 2) what the maximum glycolytic rate attainable through stimulation is. Glycolytic rate is estimated from the amount of lactate produced at various times over 40 min of anoxia. Nucleotides and creatine compounds are also determined. Lactate formation at the onset of anoxia is proportional to stimulus frequency. The amount of lactate formed is correlated to the breakdown of glycogen; glucose is not used. Therefore the amount of glycogen present in the muscle at the onset of anoxia is the main determinant of the amount of ATP formed when oxidative phosphorylation is inhibited. The rate of lactate formation at the onset of anoxia increases from 1.22 mumol.g dry wt-1.min-1 in resting muscles to 18.5 mumol.g dry wt-1.min-1 in 1-Hz-stimulated muscles. This implies that in anoxic myocardium, glycolysis can provide ATP at more than three times the rate found in the muscle at rest in ample oxygen.

Adenosine Triphosphate↗

PubMed1991

Normalized input impedance and arterial decay time over heart period are independent of animal size.

The arterial system of mammals in the weight range from 0.6 to 70 kg is characterized by the three-element windkessel, a succinct representation of the arterial tree consisting of the parameters peripheral resistance (Rp), total arterial compliance (C), and aortic characteristic impedance (Zc). The values of these parameters in resting conditions are related to body mass (M). The time constant, or decay time (tau), of the arterial system (defining rate of decay of aortic pressure in diastole), the product of Rp and C, is also evaluated. The dependencies of the heart period (T, inverse of heart rate), and durations of ejection (Ts) and of diastole (Td) in resting conditions are also determined as a function of M. It is found that Rp = Rp0M-0.93; Zc = Zc0M-0.97; and C = C0M+1.23, where Rp0, Zc0, and C0 are proportionality constants. Zc is thus a constant fraction of Rp in all mammals. tau is related to M as tau = tau 0M+0.29; T and Td are related to M as T = T0M+0.27 and Td = Td0M+0.30, where tau 0, T0, and Td0 are proportionality constants. The duration of diastole is thus a constant fraction of T, and the ratios T/tau and Td/tau are independent of M. The findings indicate that arterial input impedance, normalized to aortic Zc and plotted as a function of frequency normalized to heart rate, is similar for all mammals. The finding that the ratio Td/tau is the same in mammals (and Ts/T and stroke volume/M are constant) explains the constancy of pulse pressure (systolic minus diastolic pressure).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1991

Matching between ventricle and arterial load. An evolutionary process.

The hemodynamic properties of the ventricle are related to those of the arterial load. However, the precise nature of this relation is not known. At least three different matching criteria have been described in the literature: optimization of heart rate, of power output, and of external efficiency. Although these suggestions are based on experimental findings, there is little understanding of the underlying principles. We now suggest that the balance between the ventricle and its load is a result of the evolutionary process. To support our view, three simple assumptions are proposed regarding the evolutionary determinants underlying the relation between ventricle and arterial load: 1) Arterial pressure and flow to be generated by the ventricular pump under normal (control) conditions are set by the demands of the body. 2) Mechanical properties of contractile machinery and arterial wall material are given. 3) The heart and arterial system should have minimum size. On the basis thereof, we argue that heart rate is related to maintenance of diastolic pressure and show that the ventricle operates close to optimum power and efficiency to attain minimum size.

Animals↗

PubMed1991

Heart rate and cardiac energetics.

The heart and arterial system are matched in the sense that the heart pumps at maximal external power. Why external power is optimized and what regulatory mechanisms are responsible for this optimization are not known. In the present report we will suggest a mechanism of matching on the basis of the following arguments. 1) Mean aortic pressure, the perfusion pressure for most organs, most notably the cerebral circulation, is similar in mammals. 2) Heart period (T), a cardiac parameter, is related to body mass in a similar way as the decay time (tau = RpC, where R is peripheral resistance and C is total arterial compliance) of aortic pressure in diastole, a vascular parameter. In other words, the ratio of T/tau is similar in all mammals and assures a similar aortic pressure in diastole so that coronary perfusion, which mainly takes place in diastole, is guaranteed. 3) Assuming given mechanical properties of mammalian cardiac muscle, optimal power delivery relates to a minimal cardiac size; in other words, during evolution total cardiac volume was minimized, resulting in a heart that pumps at maximal power.

Animals↗

PubMed1990

Myocardial force production and energy turnover in anoxia.

ATP turnover of isolated rabbit papillary muscles, contracting isometrically at 20 degrees C, was determined in oxygen and during 40 min of exposure to nitrogen (anoxia). Stimulus frequency was 0.2 hertz (Hz) in oxygen and 0.2 or 1.0 Hz in nitrogen. In oxygen, ATP turnover was determined from oxygen consumption using a P/O2 ratio of 6.3. The time-dependent rate of ATP turnover in nitrogen was found from the production of lactate, and the changes in adenine nucleotides and phosphocreatine, measured in rapidly frozen preparations at different time-points during the anoxic period. A P/lactate ratio of 1.5 was used. In muscles stimulated at 0.2 Hz, twitch force dropped during the anoxic period to 33% while force production of muscles stimulated at 1.0 Hz stopped completely. However, in the latter muscles, resting force rose to 19% of the twitch force in oxygen. The rate of ATP hydrolysis in anoxia depended strongly on stimulus frequency, indicating that it is not solely determined by the glycolytic capacity. In the 0.2 Hz-stimulated muscles the decrease in energy turnover occurred in parallel with the drop in force. However, the rise in resting force in muscles stimulated at 1.0 Hz occurred when ATP turnover was close to zero. It was concluded that anoxia hardly affects the energy required for twitch force production, but that the rise of resting force measured when twitch force had disappeared occurred when the rates of cross-bridge cycling and calcium turnover were very low.

Adenine Nucleotides↗

PubMed1990

Geometry and pump function in cardiac ventricular hypertrophy.

Ventricular pump function can be quantified by the inverse relation between pressure and output, i.e., the pump function graph, which is obtained by varying arterial load without changing end-diastolic volume, inotropic state and heart rate. The ratio of pressure and output, i.e., the peripheral resistance, can be represented in the same graph by a line through the origin. The 2 pressure-output relations intersect in the working point, i.e., the pressure and flow at the prevailing steady state. In normal, anesthetized cats the ventricle appears to be matched to the arterial load in the sense that the working point is found at the optimal power, i.e., the optimal value of the product of pressure and output along the pump function graph. To maintain this matching criterion during pressure overload, the ventricular volume has to remain the same while thickening of the wall takes place: concentric hypertrophy. With volume overload, matching would be preserved with eccentric hypertrophy. Because volume and pressure overloads typically lead to eccentric and concentric hypertrophy, respectively, the matching criterion may be a valuable predictor of the geometric changes found with changes in load. This idea was further investigated experimentally by determining the position of the working point in the perinephritic cat that had 1 kidney removed and the other wrapped in cellophane for 15 to 26 weeks. The working point was no longer found at the optimal power, indicating that either matching was permanently comprised or that the ventricle was still trying to restore matching.

Animals↗

PubMed1990

Metabolic recovery of mouse extensor digitorum longus and soleus muscle.

Heat produced by a 1-s isometric tetanus of mouse extensor digitorum longus muscle (EDL; n = 6) and a 1.5-s isometric tetanus of soleus muscle (n = 7) was measured with thermopiles at 20 degrees C, and separated into initial heat (I) and recovery heat (R). In EDL the initial heat was 190 +/- 40 (SD) mJ g-1 and in soleus 52 +/- 9 (SD) mJ g-1. The recovery heat production rate immediately following the tetanus was almost zero in both muscles. It rose in 12 +/- 6 s (EDL) and in 30 +/- 3 s (soleus) to a maximum, to decrease thereafter monoexponentially with a time constant of 30.7 +/- 5.7 s (EDL) and 41.7 +/- 7.2 s (soleus). The measured recovery ratio (R/I) differed between EDL (0.95 +/- 0.14) and soleus (1.54 +/- 0.22). The value for soleus muscles was significantly different from the theoretical value of 1.13. EDL muscles were freeze-clamped at rest (n = 10) and during the recovery phase, 1 min after the onset of the tetanus (n = 10), to determine lactate and creatine phosphate. It was found that no significant amount of net lactate was produced. The amount of creatine phosphate reformed corresponded to the recovery heat produced. The results suggest that metabolic recovery after short tetani of EDL and soleus muscles occurs predominantly through oxidative phosphorylation, but knowledge of respiratory control in the living cell is insufficient to explain its slow onset immediately following contraction and the finding that EDL recovers faster than soleus.

Adenosine Triphosphate↗

PubMed1990

Depression of force by phosphate in skinned skeletal muscle fibers of the frog.

The relation between isometric force and phosphate concentration in skinned skeletal muscle fibers of the frog is found to depend on fiber size. Force decreased with increasing phosphate concentration, but depression of force in thick fibers was smaller than in thin segments. When the external phosphate concentration was abruptly altered during a sustained contracture, force changed. The half-time of the force change was proportional to the cross-sectional area of the preparation. From this relation, a value for the diffusion constant of phosphate in skinned fibers of 0.9 x 10(-10) m2/s was derived. The rate of phosphate production was determined photometrically via the enzymatic coupling of the resynthesis of ATP to the oxidation of nicotinamide adenine dinucleotide. The average value (+/- SE) of the rate of ATP hydrolysis (at 4 degrees C) was 2.7 +/- 0.3 mumol.s-1.g dry wt-1, which corresponds to 0.34 mmol.l-1.s-1. From a calculation based on the diffusion constant and the rate of phosphate production determined, it follows that the dependency of the force-phosphate relation on fiber diameter is due to phosphate accumulation inside the fiber.

Adenosine Triphosphatases↗

PubMed1990

Oxidative and glycolytic ATP formation of rabbit papillary muscle in oxygen and nitrogen.

Contraction-related O2 consumption of rabbit papillary muscles was determined at 20 degrees C by measuring change in saline PO2 during and after trains of 120 twitches at 0.125-1 Hz in a microrespirometer. Although anoxic cores occurred at twitch frequencies greater than 0.2 Hz, no lactate was found in saline after twitch train. To measure lactate accumulation in muscle, fully oxygenated muscles were frozen at rest and during steady-state twitches at 0.2 Hz. We also measured nucleotides and creatine (Cr) compounds. There were no differences in lactate, ATP, and phosphocreatine (PCr) content between the resting and active muscles. When a P-to-O2 ratio of 6.3 is assumed, aerobic ATP formation was compared with glycolytic ATP formation during anoxia at a stimulus frequency of 0.2 Hz. The latter value was obtained by freezing muscles between 6 and 25 min after changing from O2- to N2-saturated saline. Withdrawal of O2 caused the ratio of PCr to total Cr to fall in less than 6 min from 0.77 to 0.23, while ATP remained at approximately 15 mumol/g dry wt. Force fell initially within 4 min to approximately 70% of control value, decreasing thereafter more slowly to approximately 40% at 20 min. From the relationship between amount of lactate formed and duration of anoxia, rate of anaerobic ATP formation was calculated assuming a P-to-lactate ratio of 1. We found that despite continuing contractile activity, anaerobic ATP formation was less than that required by a fully oxygenated resting muscle and was about the same magnitude as the estimated ATP hydrolysis for the contractions in N2. We conclude that in fully oxygenated rabbit papillary muscles no net lactate is produced during stimulation and that in anoxia anaerobic glycolytic capacity may not provide sufficient ATP for processes other than the uptake of Ca by the sarcoplasmic reticulum and cross-bridge cycling.

Adenosine Triphosphate↗

PubMed1990

Analysis of thermopile records from contracting isolated cardiac muscle.

Recovery heat production after contraction in rabbit papillary muscle at 20 degrees C occurs at an exponentially declining rate. The time constant describing this decline is 25 s; it is not different when 10 twitches or when a steady-state twitch train is studied, and it is unaltered by changing stimulus frequency from 0.125 to 0.2 Hz. The same value has previously been found after single twitches. If it is assumed that phosphocreatine (PCr) resynthesis is the cause of recovery heat production and that it occurs also during contractions at a rate proportional to the amount of PCr depletion, it is possible to divide the total heat production for any period of stimulation into that caused by this recovery process (R) and that caused by initial (I) processes (presumed to be PCr splitting). The value of R/I obtained by using this method is 1.10 +/- 0.04 (means +/- SE, n = 27 muscles), close to the theoretical value of 1.13. The correspondence between the measured and the predicted ratio supports the assumptions underlying the measurement. Thus in heart muscle the heat produced during and after contraction can be explained by PCr splitting and reformation. The older Bugnard method of analysis applied to the same data gives an R/I value of 1.5; the reasons for the discrepancy are described.

Animals↗

PubMed1990

Heat released during relaxation equals force-length area in isometric contractions of rabbit papillary muscle.

It has been claimed that the mechanical performance and the related energy turnover of the left ventricle can be reliably predicted on the basis of its time-varying elastance behavior. In its most elementary form, this behavior can be mathematically described by E(t) = P(t)/[V(t)-Vd], where E is ventricular elastance, t is time, P is ventricular pressure, V is ventricular volume, and Vd is the intercept of the end-systolic pressure-volume line on the volume axis. To find out how this behavior of the ventricle as a whole is related to the properties of the myocardium, we tested the energetic prediction for the ventricle that the pressure-volume area of an isovolumic contraction equals the energy released in relaxation in experiments on isolated rabbit papillary muscle at 20 degrees C. To that end, the energy (joules) contained by the force-length area of the muscles, contracting isometrically, was compared with the heat (joules) liberated in relaxation as measured with thermopiles. Mechanical performance of the muscles was varied by altering initial muscle length and external calcium. The slope of the resulting relation between force-length area and heat liberated in relaxation (n = 26) was not significantly different from unity. Thus, the energetic prediction of the time-varying elastance model developed for the whole left ventricle was confirmed by experiments on rabbit papillary muscle at 20 degrees C.

Animals↗

PubMed1990

Energy demand, supply, and utilization in hypoxia, and force recovery after reoxygenation in rabbit heart muscle.

In rabbit papillary muscle contracting at 20 degrees C in nitrogen at 0.2 Hz, glycolytic ATP formation is just enough to support the diminished contractile activity. Basal metabolism, important to maintain cellular function and integrity, is strongly inhibited. In the present study, we address the question of whether the inhibition of basal processes in hypoxia determines redevelopment of force in reoxygenation. By not stimulating the muscle during hypoxia, we try to make more ATP available for basal processes. Isometric force of papillary muscles (0.2-Hz stimulation) is measured before, during, and after 40 minutes of hypoxia. ATP formation and utilization in hypoxia are estimated from lactate production and changes in nucleotides and creatine compounds. After reoxygenation, muscles stimulated during hypoxia produce a steady-state force of 78% of the aerobic control; resting muscles recover to 94%. In contrast to expectation, lactate production in hypoxic resting muscles is only 30% of that in contracting ones. The findings indicate that basal metabolic rate of hypoxic muscles at rest is 14% of that of quiescent, well-oxygenated myocardium. We conclude that in hypoxic myocardium little ATP is available for basal metabolism, irrespective of the energy demand of the contractile system. It is therefore unlikely that the lower force found after reoxygenation in muscles stimulated during hypoxia is related to the degree of inhibition of basal processes.

Adenosine Triphosphate↗

PubMed1989

Maximum rate of oxygen consumption and quantitative histochemistry of succinate dehydrogenase in single muscle fibres of Xenopus laevis.

Three different types of single living muscle fibre were dissected from the iliofibularis muscle of Xenopus laevis. The fibres were mounted in a glass chamber and their rate of oxygen consumption was determined as a function of twitch frequency at 20 degrees C. The rate of oxygen consumption increased with twitch frequency until it levelled off and reached a maximum. The maximum rate of oxygen consumption varied between fibres (0.019 to 0.161 nmol O2 s-1 mm-3) and was reached at different twitch frequencies (less than 0.2 to 5.7 stimuli s-1). After the determination of the maximum rate of oxygen consumption, the succinate dehydrogenase activity in cross sections of the fibre was determined by means of a quantitative histochemical method. A proportional relationship between the maximum rate of oxygen consumption and the succinate dehydrogenase activity was found. The maximum rate of oxygen consumption and the succinate dehydrogenase activity are also proportional to the volume density of mitochondria in the three fibre types reported by Smith and Ovalle (1973; J. Anat., Lond. 116, 1-24). It is concluded that quantitative histochemistry of succinate dehydrogenase reliably predicts the maximum rate of oxygen consumption of muscle fibres in Xenopus laevis and that the maximum rate of oxygen consumption of single muscle fibres is determined by the volume density of mitochondria.

Animals↗

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↗