G. Elzinga

Pressure and Flow Generated by the Left Ventricle against Different Impedances

To study selective changes in capacitive and resistive load under constant left atrial filling pressure, isolated cat hearts were loaded with a hydraulic model simulating the input impedance of the systemic arteries. The model was constructed so that resistive (peripheral resistance) and capacitive (total arterial compliance) characteristics could be changed independently. Aortic and left ventricular pressure and aortic flow, as generated by the left ventricle against the different impedances, were measured. An increase in resistance resulted in an increase in systolic, diastolic, and mean aortic pressure. A decrease in capacitance caused a small increase in systolic pressure and a decrease in diastolic and mean aortic pressure. Mean left ventricular pressure increased when resistance increased or capacitance decreased. Both peak flow and mean flow decreased when resistance increased or capacitance decreased. We attempted to explain these observations by the concept of source impedance. This concept had been, until now, incorrectly approached, because the nonlinearity arising from the aortic valves had been neglected. The correct computations met with difficulties, but it was shown that the isolated heart, with constant atrial filling pressure, behaved as neither a flow source nor a pressure source.

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

SummaryThree different types of single living muscle fibre were dissected from the iliofibularis muscle ofXenopus 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‡ 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 (<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 mitochrondria 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 inXenopus laevis and that the maximum rate of oxygen consumption of single muscle fibres is determined by the volume density of mitochondria.

ATP utilization for calcium uptake and force production in skinned muscle fibres of Xenopus laevis.

1. A method has been developed to discriminate between the rate of ATP hydrolysis associated with calcium uptake into the sarcoplasmic reticulum (SR) and force development of the contractile apparatus in mechanically or saponin‐skinned skeletal muscle fibres. The rate of ATP hydrolysis was determined in fibres of different types from the iliofibularis muscle of Xenopus laevis by enzymatic coupling of ATP re‐synthesis to the oxidation of NADH. 2. The ATPase activity was determined before and after exposure of the preparations for 30 min to a solution containing 0.5% Triton X‐100, which effectively abolishes the SR ATPase activity. The fibres were activated in a solution containing 5 mM caffeine to ensure that calcium uptake into the SR was maximal. 3. At saturating Ca2+ concentrations the actomyosin (AM) and SR ATPase activities in fast‐twitch fibres, at 4.3 degrees C, amounted to 1.52 +/‐ 0.07 and 0.58 +/‐ 0.10 mumol s‐1 (g dry wt)‐1, respectively (means +/‐ S.E.M.; n = 25). The SR ATPase activity was 25% of the total ATPase activity. At submaximal calcium concentrations the AM ATPase activity varied in proportion to the isometric force. 4. The calcium sensitivity of the SR ATPase was larger than that of the AM ATPase and its dependence on [Ca2+] was less steep. The AM ATPase activity was half‐maximal at a pCa of 6.11 (pCa = ‐log [Ca2+]) whereas the SR ATPase activity was half‐maximal at a pCa of 6.62. 5. In Triton X‐100‐treated fibres, at different 2,3‐butanedione monoxime (BDM) concentrations, the AM ATPase activity and isometric force varied proportionally. The SR ATPase activity determined by extrapolation of the total ATPase activity in mechanically skinned or saponin‐treated fibres to zero force, was independent of the BDM concentration in the range studied (0‐20 mM). The values obtained for the SR ATPase activity in this way were similar to those obtained with Triton X‐100 treatment. 6. The AM ATPase activity in slow‐twitch fibres amounted to 0.74 +/‐ 0.13 mumol s‐1 (g dry wt)‐1, i.e. about a factor of two smaller than in fast‐twitch fibres. The SR ATPase activity amounted to 0.47 +/‐ 0.07 mumol s‐1 (g dry wt)‐1, i.e. rather similar to the value in fast‐twitch fibres. The proportion of the total ATPase activity that was due to SR ATPase (40%) was larger than in fast‐twitch fibres. 7. The temperature dependence of the AM and SR ATPase activities in fast‐twitch fibres differed. In the temperature range 5‐10 degrees C, the relative changes in AM and SR ATPase activities for a 10 degrees C temperature change (Q10) were 3.9 +/‐ 0.3 and 7.2 +/‐ 1.5, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparison of models used to calculate left ventricular wall force.

Myocardial wall force per area (=stress) is a major determinant of muscle function and oxygen consumption. It cannot be measured accurately but has to be derived from a mathematical model. Many models have been presented in the literature but a comparison between models has not been available. In this study angiographic data from the literature are used to calculate left ventricular wall force for normal and diseased hearts using a thin-walled spherical model, a thick-walled spherical model and six ellipsoidal models, and the results are compared. There appeared to be large differences between the stresses yielded by the models for the same cardiac geometry. The thick-walled sphere yields circumferential stresses that are approximately 25% lower than the stresses yielded by most of the ellipsoidal models. Of the ellipsoidal models the one suggested by Streeter el al. gives circumferential stresses that are 25% higher than those of the other ellipsoids. Similar differences are found for left ventricular wall stress in the longitudinal direction. However, all models correspond closely in the prediction of the deviation from normal stress in the various pathological states studied.Some of the models give information about the stress distribution over the thickness of the wall as well. We found substantial differences in this predicted stress distribution for models that employ similar assumptions. These differences plus the uncertainties with regard to the properties of the myocardial wall material, that change during the cardiac cycle, call for some scepticism concerning the calculated stress distribution over the wall. The ellipsoidal model suggested by Falsetti et al. is very simple and yields approximately the same mean wall stress values as the more complicated models that we studied. This model therefore appears to be the best choice.

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)

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.

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.

ATP formation and ATP hydrolysis during fatiguing, intermittent stimulation of different types of single muscle fibres from Xenopus laevis.

SummaryThis report describes changes of the rate of ATP hydrolysis in single, intact muscle fibres during the development of fatigue induced by intermittent tetanic stimulation. High (type 3) and low (type 1) oxidative muscle fibres dissected from the iliofibularis muscle of Xenopus laevis were studied at 20°C. The rate of ATP hydrolysis was calculated during different time intervals from changes in the content of nucleotides, creatine compounds and lactate, as well as lactate efflux and oxygen uptake. During the first phase of intermittent stimulation, phosphocreatine is fully reduced while the rate of oxygen consumption increases to its maximum, the lactate content increases to a maximum level, and a small amount of IMP is formed; the rate of ATP hydrolysis in type 3 fibres is constant while force decreases, whereas the rate decreases approximately in proportion to force in type 1 fibres. After the first phase, the rate of ATP hydrolysis in type 3 fibres decreases slightly and the fibres reach a steady metabolic state in which the rates of ATP formation and hydrolysis are equal; in type 1 fibres a drastic change of the rate of ATP hydrolysis occurs and a steady metabolic state is not reached. On the basis of the time courses of the metabolic changes, it is concluded that the rate of ATP hydrolysis in type 3 fibres is reduced by acidification and/or a reduced calcium efflux from the sarcoplasmic reticulum, whereas in type 1 fibres inorganic phosphate and/or acidification inhibit the rate initially and ADP is a likely candidate to explain the drastic fall of the rate of ATP hydrolysis during late phases of fatiguing stimulation.

Myofibrillar ATPase activity and mechanical performance of skinned fibres from rabbit psoas muscle.

1. The relationship between energy turnover and mechanical performance was investigated in chemically skinned single fibres from rabbit psoas muscle at 15 degrees C, pH = 7.1, with MgATP, 5 mM; free Mg2+, 1 mM; ionic strength, 200 mM and sarcomere length, 2.4 microns by measuring force production and myofibrillar ATP turnover during isometric contractions as well as during repetitive changes in length. ATP hydrolysis was stoichiometrically coupled to the breakdown of NADH, which was measured photometrically via the absorption of near UV light at 340 nm. 2. Force and ATPase activity were measured during square‐wave length changes of different amplitudes (1‐10% of the fibre length, Lo) and different frequencies (2.5‐167 Hz). The average force during the length changes was less than the isometric value and decreased with increasing amplitude and frequency. At full activation (pCa 4.5), the isometric ATP turnover rate (+/‐ S.E.M.) was 2.30 +/‐ 0.05 s‐1 per myosin head. ATP turnover increased monotonically with increasing amplitude as well as with increasing frequency until saturation was reached. The greatest increase observed was 2.4 times the isometric value. 3. Force and ATPase activity were also determined for ramp shortenings followed by fast restretches. The average force decreased with increasing shortening velocity in a hyperbolic fashion. The ATP turnover increased with ramp velocity up to 0.5 L0 s‐1 and stayed almost constant (at 2.2 times the isometric value) for larger velocities. 4. Isometric force and ATPase activity both decreased as the calcium concentration was decreased. They did not vary in proportion at low Ca2+ concentrations, but this could largely be accounted for by the presence of a residual, Ca(2+)‐dependent, membrane‐bound ATPase. At high calcium concentrations ATPase activity during square‐wave length changes was higher than the isometric value, but at low calcium concentrations (pCa > 6.1), the ATPase activity during the length changes decreased below the isometric value and reached a minimum of 40% of the isometric level. 5. ATPase activity and average force obtained during changes in length show a high, movement protocol‐independent correlation. During the length changes the rate of ATP turnover divided by the average force level (tension cost) was larger than the isometric tension cost. The largest value found, for 10% length changes at 23 Hz, was 17 times the tension cost under isometric conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

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.