Biomedical subjects

G Elzinga

Publications and source records attributed to G Elzinga.

At least 19 recordsLinked to original sources

PubMed1997

Mitochondrial function is not decreased in stunned papillary muscle at 20 degrees C.

It is unclear to what extent mitochondrial function in vivo is changed after brief anoxia. Heat measurements allow evaluation of mitochondrial function within intact cardiac muscle. Heat production was determined using fast metal-film thermopiles, during contraction and post-contractile recovery in control and stunned superfused rabbit papillary muscles at 20 degrees C. Heat rate was measured for a train of ten twitches (0.2 Hz) before anoxia and after 40 min anoxia followed by 2 h of normoxic recovery. During anoxia muscles were stimulated at 0.2 Hz (group A) or at 1.0 Hz (group B). A normoxic control group C was stimulated at 0.2 Hz. After 2 h recovery, tension was 77 +/- 5% (S.E.M.), 72 +/- 7% and 94 +/- 3% of initial values, for group A, B and C respectively, indicating stunning by anoxia. The economy of contraction or the ratio of recovery heat to initial heat did not change significantly in groups A and B when compared with control, indicating that stunning with this protocol is not associated with mitochondrial uncoupling. Post-contractile recovery heat initially decayed exponentially with time constant 24.9 +/- 2.2 s for all groups and with 22.7 +/- 1.1, 22.0 +/- 0.8 and 41.7 +/- 4.4 s at the end for group A, B and C respectively. The cause of the remarkable slowing of the recovery rate over time in controls is unknown, but is mimicked by blocking fatty acid utilization. No slowing of metabolic recovery is observed in the stunned papillary muscles. We conclude that stunning is not associated with a decrease in mitochondrial function or oxidative capacity in cardiac muscle.

Analysis of Variance↗

PubMed1997

Force and myosin content variation in isolated intact single muscle fibres from Rana temporaria.

We studied the relation between force normalized by dry mass per unit length and the myosin fraction of muscle dry mass. The two tibialis anterior muscles were dissected from 12 frogs (Rana temporaria). Then, from one muscle, two single fast-twitch fibres were isolated. Each fibre was mounted isometrically in Ringer's solution, and electrically stimulated using a standardized protocol. Peak force production, normalized by the fibre's dry mass per unit length, varied by a factor of 1.4. Little variation in normalized force was measured between fibres from the same animal, whereas between animals a significant difference was found (P<0.05). The contralateral muscle was used to determine the myosin fraction of the dry mass. The relationship between the fraction myosin of the dry mass and force normalized by dry mass per unit length showed a high correlation (r = 0.81; n = 12). From this we conclude that variation in normalized tetanic force is determined greatly (65%) by variations in myosin content.

Animals↗

PubMed1997

Mechanical and energy characteristics during shortening in isolated type-1 muscle fibres from Xenopus laevis studied at maximal and submaximal activation.

The mechanical and energy characteristics of isolated fast-twitch muscle fibres (type 1) of Xenopus laevis in isometric- and isovelocity contractions were measured at 20 degrees C. The fibres were stimulated at either 60 Hz or 20 Hz to produce contractions at different levels of activation. The high stimulation frequency gave fused contractions, while at the low stimulation frequency tension fluctuated. When maximum isometric force had been reached, the fibres were shortened by 10% of the fibre length at different velocities. At 60 Hz stimulation during shortening the rate of heat production increased above the isometric rate of heat production. At 20 Hz stimulation during shortening, however, the rate of heat production was not different from the isometric rate of heat production. Mechanical efficiency was the same at the high and low level of activation. The actomyosin efficiency (i.e. the mechanical efficiency corrected for "activation heat") was highest at the low level of activation. We conclude that in fast-twitch muscle fibres from X. laevis, actomyosin efficiency is highest for partially activated muscle. From a comparison of the present results with those obtained from a study of slow-twitch muscle fibres presented earlier, it is concluded that fast-twitch muscle fibres are less efficient than slow-twitch muscle fibres.

Actomyosin↗

PubMed1997

Variation in normalized isometric tetanic force of isolated fast-twitch muscle fibres of Rana temporaria.

The origin of the threefold variation found previously in isometric force normalized to cross-sectional area of single fast-twitch tibialis anterior muscle fibres of the frog Rana temporaria was studied by using (1) a strictly defined stimulus protocol, and (2) influencing the condition of the frog using artificial hibernation. Variation in normalized force was found to be influenced by the length of the rest period between tetani. After a long rest (> 6h), tetanic force production was less than for a tetanus produced after 1 h. The length of the rest period accounted for a factor of 1.24 of the total variation in normalized force. The condition of the frog also influenced normalized force production. Little variation in normalized force was observed between different fibres from the same animal, whereas a significant difference was found between animals. After artificial hibernation, force normalized to cross-sectional area remained unchanged, but force normalized to dry mass per unit length increased; the total variation increased from a factor of 1.37 to a factor of 1.64. Force normalized to muscle protein mass per unit length, however, was not affected by artificial hibernation. We conclude that variation in normalized tetanic force can be partly reduced by standardization of the stimulation protocol and normalization to protein content per unit length.

Animals↗

PubMed1996

Force-dependent and force-independent heat production in single slow- and fast-twitch muscle fibres from Xenopus laevis.

1. The origin of labile heat production, i.e. a heat component which rapidly decays after the onset of stimulation, and of stable (maintenance) heat production was investigated in intact single fast-twitch (type 1) and slow-twitch (type 3) iliofibularis muscle fibres from Xenopus laevis, at 20 degrees C, by varying stimulation frequency and by varying sarcomere length and the concentration of 2,3-butanedione 2-monoxime (BDM) added. 2. The labile heat produced consisted of a force-independent and a force-dependent part. The average parvalbumin (PA) content found in type 1 fibre bundles (0.84 +/- 0.08 mM; mean +/- S.E.M.; n = 5) and in type 3 fibre bundles (0.12 +/- 0.02 mM; n = 5) indicates that the force-independent labile heat is explained by Ca(2+)-Mg2+ exchange on PA, and amounts to a molar enthalpy change of -78 kJ (molPA)-1. 3. Force-dependent labile heat during fused contractions was similar to the calculated heat production resulting from the formation of force-generating cross-bridges, assuming an enthalpy change associated with cross-bridge formation of -30 kJ mol-1. 4. Activation heat, i.e. the part of the total stable heat that is not related to the contractile apparatus, and of which the calcium sequestration by the sarcoplasmic reticulum is the most important contributor, determined by varying sarcomere length or BDM concentration, was identical. For fused contractions the fraction activation heat of the stable maintenance rate of heat production was 34 +/- 4% (mean +/- S.E.M.; n = 13) in type 1 fibres, and 52 +/- 4% (n = 15) in type 3 fibres. In unfused contractions this was 48 +/- 5% (n = 13) in type 1 fibres, and 35 +/- 2% (n = 11) in type 3 fibres. 5. From the force-dependent stable rate of heat production the economy of cross-bridge cycling, expressed as the force-time integral for a single myosin head per ATP molecule hydrolysed, was calculated. It followed that cross-bridge interaction in type 3 fibres is more economical than in type 1 fibres, and that fused contractions are more economical than unfused contractions.

Animals↗

PubMed1996

The effect of hyperosmolality on the rate of heat production of quiescent trabeculae isolated from the rat heart.

We have measured the rate of heat production of isolated, quiescent, right ventricular trabeculae of the rat under isosmotic and hyperosmotic conditions, using a microcalorimetric technique. In parallel experiments, we measured force production and intracellular calcium concentration ([Ca2+]i). The rate of resting heat production under isosmotic conditions (mean +/- SEM, n = 32) was 100 +/- 7 mW (g dry wt)-1; it increased sigmoidally with osmolality, reaching a peak that was about four times the isosmotic value at about twice normal osmotic pressure. The hyperosmotic thermal response was: (a) abolished by anoxia, (b) attenuated by procaine, (c) insensitive to verapamil, ouabain, and external calcium concentration, and (d) absent in chemically skinned trabeculae bathed in low-Ca2+ "relaxing solution." Active force production was inhibited at all osmolalities above isosmotic. Passive (tonic) force increased to, at most, 15% of the peak active force developed under isosmotic conditions while [Ca2+]i increased, at most, 30% above its isosmotic value. We infer that hyperosmotic stimulation of resting cardiac heat production reflects, in large part, greatly increased activity of the sarcoplasmic reticular Ca2+ ATPase in the face of increased efflux via a procaine-inhibitable Ca(2+)-release channel.

Animals↗

PubMed1995

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)

Actomyosin↗

PubMed1995

Energetics of shortening depend on stimulation frequency in single muscle fibres from Xenopus laevis at 20 degrees C.

Single intact slow-twitch (type 3) muscle fibres from the iliofibularis muscle of Xenopus laevis were shortened at a constant velocity (0.4 L0/S, where L0 is the initial length at different levels of activation (40, 15, 12.5, and 10 Hz). A stimulation frequency of 40 Hz gave fused tetanic records. At this frequency the mean heat production rate during shortening (0.38 +/- 0.05 W/g dry weight) was slightly higher than the isometric heat production rate (0.33 +/- 0.03 W/g dry weight). The lower stimulation frequencies gave unfused tetanic contractions, the average isometric force of which was 40 +/- 3% of the isometric force at 40 Hz. In these unfused tetani during shortening the heat production rate (0.18 +/- 0.02 W/g dry weight) significantly decreased below the isometric heat production rate (0.25 +/- 0.02 W/g dry weight). At full activation the rate of total energy production (mechanical power plus heat production rate) during shortening was 1.88 +/- 0.32 times the isometric total energy production rate. This effect, i.e. an increase in energy turnover with shortening, is known as the Fenn effect. At sub-maximal stimulation the energy output during shortening was only 1.07 +/- 0.08 times the isometric value. These results show that the Fenn effect is dependent on the level of activation. The efficiency (ratio of mechanical power to total energy output) was independent of the stimulation frequency (0.37 +/- 0.06).

Animals↗

PubMed1995

Exposure of energy-depleted rat trabeculae to low pH improves contractile recovery: role of calcium.

The beneficial effect of low pH during cardiac ischemia on reperfusion injury has often been attributed to its energy-saving effect due to inhibition of contraction. The role of low pH on Ca2+ accumulation and muscle tension was assessed in energy-depleted tissue by changing the pH of the medium from 7.4 to 6.2 at onset of rigor development during metabolic inhibition (MI), i.e., in the energy-depleted phase. Cytosolic free Ca2+ ([Ca2+]i) and intracellular H+ (pHi) were measured in rat trabeculae at 20 degrees C with fura 2 and 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein, respectively, and tension was recorded. The preparations were energy depleted by stimulation at 1 Hz in glucose-free Tyrode solution with 2 mM NaCN. Rigor developed within 20 min, indicating energy depletion. Resting [Ca2+]i was followed during 50 min (group I) or 100 min (group II) of rigor, and recovery was followed for 60 min in glucose-containing Tyrode solution at 0.2-Hz stimulation. Resting [Ca2+]i rose within 50 min (group I) but stabilized in the 50- to 100-min period (group II). All preparations from group I (n = 5) resumed contraction in the recovery period but in group II (n = 10) 70% failed to recover, and [Ca2+]i remained elevated compared with those that recovered. An extracellular pH of 6.2, resulting in similar pHi, from onset of rigor development (group III) led to only a modest rise in [Ca2+]i during the 100-min rigor period, and all preparations resumed contraction after approximately 3 min in normal medium. ATP was very low in all groups at the end of MI but was still significantly lower in group II than in groups I and III. A beneficial energy-sparing effect of low pH during the rigor phase can therefore not be excluded. We conclude that 1) the capacity of trabeculae to recover from MI depends on the time period and magnitude of the [Ca2+]i rise in the energy-depleted phase and 2) low pH in energy-depleted trabeculae protects against Ca overload, improving recovery after normalization of perfusion conditions.

Animals↗

PubMed1994

Lactate efflux from fatigued fast-twitch muscle fibres of Xenopus laevis under various extracellular conditions.

1. Isolated, fast-twitch, low-oxidative muscle fibres from the iliofibularis muscle of Xenopus laevis were fatigued by intermittent tetanic stimulation at 20 degrees C in different Ringer solutions and the amount of lactate released was determined. 2. The rate of lactate efflux was constant during 10 min of intermittent stimulation while lactate in the fibres accumulated, and lactate efflux was not hampered by an unstirred layer surrounding the isolated muscle fibre. 3. The rate of lactate efflux at extracellular pH 7.2 was the same as that at pH 7.8, but depended on the type of buffer used; the highest efflux rate (mean +/- S.E.M., 7.4 +/- 2.2 mumol min-1 (g dry weight)-1, n = 8) was observed in bicarbonate-buffered Ringer solution. This rate was about 2.5 times higher than the rate in phosphate-buffered Ringer solution (2.9 +/- 1.3 mumol min-1 (g dry weight)-1, n = 8), indicating that lactate-bicarbonate exchange is the most important route for lactate extrusion in vivo. 4. The highest rate of lactate efflux corresponds to a rate of glycolytic ATP production which is only about 30% of the oxidative rate of ATP production (calculated from the maximum rate of oxygen consumption determined previously). 5. In the presence of 5 mM alpha-cyano-4-hydroxycinnamate (CHC) the lowest lactate efflux rate (1.5 +/- 0.6 mumol min-1 (g dry weight)-1, n = 16) was found. This rate was independent of the composition of the Ringer solution. Assuming that 5 mM CHC completely inhibits lactate transporters in the sarcolemma, the rate of lactate efflux in the presence of 5 mM CHC can be explained by passive diffusion, but only if most lactate is extruded via the T-tubules.

Animals↗

PubMed1994

Influence of phosphate and pH on myofibrillar ATPase activity and force in skinned cardiac trabeculae from rat.

1. The effects of inorganic phosphate (Pi) and pH on maximal calcium-activated isometric force and MgATPase activity were studied in chemically skinned cardiac trabeculae from rat. ATP hydrolysis was coupled enzymatically to the breakdown of NADH, and its concentration was determined photometrically. Measurements were performed at 2.1 microns sarcomere length and 20 degrees C. ATPase activity and force were also determined when square-wave-shaped length changes were applied, with a frequency of 23 Hz and an amplitude of 2.5%. 2. At pH 7.0 without added Pi, the average isometric force (+/- S.E.M.) was 51 +/- 3 kN m-2 (n = 23). The average isometric ATPase activity was 0.43 +/- 0.02 mM s-1 (n = 23). During the changes in length ATPase activity increased to 152 +/- 3% of the isometric value, while the average force level decreased to 48 +/- 2%. 3. Isometric force gradually decreased to 31 +/- 2% of the control value when the Pi concentration was increased to 30 mM. Isometric ATPase activity, however, remained constant for Pi concentrations up to 5 mM and decreased to 87 +/- 3% at 30 mM Pi. When Pi accumulation inside the preparation due to ATP hydrolysis was taken into account, a linear relationship was found between isometric force and log [Pi]. The decrease in relative force was found to be 44 +/- 4% per decade. 4. During the length changes, ATPase activity and average force showed, apart from the increase in ATPase activity and decrease in average force, the same dependence on Pi as the isometric values. Stiffness, estimated from the amplitude of the force responses during the length changes, decreased in proportion to isometric force when the Pi concentration was increased. The changes in the shape of the force responses due to the repetitive changes in length as a function of the Pi concentration were relatively small. These results suggest that the effect of Pi on the transitions which influence ATP turnover is rather insensitive to changes in cross-bridge strain. 5. Isometric force, normalized to the control value at pH 7.0, increased gradually from 54 +/- 1% at pH 6.2 to 143 +/- 10% at pH 7.5. ATPase activity remained practically constant for pH values from 6.8 to 7.2 but decreased to 80 +/- 1% at pH 6.2 and to 83 +/- 5% at pH 7.5. ATPase activity during the length changes was reduced more than the isometric ATPase activity when pH was lowered.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine Triphosphatases↗

PubMed1994

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)

Adenosine Triphosphatases↗

PubMed1993

Stunning does not change the relation between calcium and force in skinned rat trabeculae.

To test the hypothesis that stunning is due to a decreased sensitivity of the myofibrils for calcium, we compared the isometric force-Ca2+ relation in skinned trabeculae from stunned and control hearts. Hearts were made ischemic for 40 min followed by 30 min reperfusion. In one group (Group 1) changes in left ventricular systolic and diastolic pressure were monitored. From another group (Group 2), trabeculae were isolated to determine the relation between force and Ca2+ concentration. Trabeculae isolated from hearts (Group 3) perfused aerobically for 90 min served as controls. Left ventricular developed pressure and end diastolic pressure were 5.9 +/- 0.7 kPa and 5.8 +/- 0.7 kPa, respectively in stunned hearts as compared to 9.9 +/- 1.2 kPa and 1.2 +/- 0.1 kPa prior to ischemia. The nucleotide content decreased from 24.9 +/- 3.4 mumol.g-1(dw) in control hearts to 9.3 +/- 0.8 mumol.g-1(dw) after ischemia and reperfusion while the creatine kinase levels were about the same. Force-Ca2+ relations obtained from trabeculae from control and stunned hearts were fitted to the Hill equation. Maximal isometric force, the midpoint and the steepness of the curves estimated for the two groups were not significantly different. We conclude that the maximum isometric force and the sensitivity of the contractile apparatus of skinned myocardium of stunned hearts do not differ from that of control hearts. This suggests that structural changes of the contractile proteins leading to a decreased sensitivity of the myofibrils to calcium are not involved in the mechanism responsible for stunning.

Animals↗

PubMed1993

Why smaller animals have higher heart rates.

Diastolic blood pressure is the main driving pressure for coronary perfusion. Diastolic pressure depends on mean pressure and the ratio of the decay time of aortic pressure in diastole (tau) and the duration of diastole (Td). The ratio of tau, a morphological, arterial parameter, and Td, a functional, cardiac parameter, is the same in all mammals. This could mean that smaller animals have higher heart rates i.e. shorter duration of diastole to match the shorter time constant of the diastolic pressure decay and to guarantee adequate coronary perfusion.

Animals↗

PubMed1993

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

This 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 degrees 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.

Adenosine Diphosphate↗

PubMed1993

Calcium modulates the influence of length changes on the myofibrillar adenosine triphosphatase activity in rat skinned cardiac trabeculae.

The relationship between adenosine triphosphate (ATP) turnover and muscle performance was investigated in skinned cardiac trabeculae of the rat at different [Ca2+] and two different sarcomere lengths (1.8 microns and 2.2 microns) at 20 degrees C. ATP turnover was measured photometrically by enzymatic coupling of the regeneration of ATP to the oxidation of reduced nicotinamide adenine dinucleotide. The trabeculae were studied under isometric conditions and when the length was altered repetitively at a frequency of 23 Hz, with a square wave, by 5% of the initial length. The isometric ATPase activity amounted to 0.48 mM/s. Isometric ATP turnover and force were proportional at different [Ca2+]. During length changes at maximal activation (pCa 4.27) and 2.2 microns sarcomere length, ATPase activity increased to up to 162% whereas at low [Ca2+], ATPase activity decreased with respect to the isometric value at that pCa. At pCa 5.5, ATPase activity was reduced to 33%. These results indicate that during the length changes the apparent cross-bridge detachment rate is increased and the apparent attachment rate is decreased. The findings suggest that the Fenn effect, i.e. the increase in energy turnover above the isometric value during shortening, is present in cardiac trabeculae at high levels of activation, but is absent or reversed at lower levels of activity.

Adenosine Triphosphatases↗

PubMed1993

Force relaxation, labile heat and parvalbumin content of skeletal muscle fibres of Xenopus laevis.

1. Measurements were made of stable (hb) and labile (ha) maintenance heat rate, slowing of relaxation as a function of tetanus duration, and parvalbumin (PA) content in intact single muscle fibres of types 1 and 2 from Xenopus laevis. The majority of experiments were performed at 20 degrees C. In addition, total and myofibrillar ATPase activity was measured in skinned Xenopus fibres, also of types 1 and 2; these studies were performed at 4 degrees C. 2. In agreement with a previous study hb was significantly higher in type 1 (175 +/- 13 mW (g wet wt)-1; n = 8) than in type 2 fibres (88 +/- 9 mW (g wet wt)-1; n = 7). The value of ha was 236 +/- 22 and 117 +/- 16 mW (g wet wt)-1, respectively (mean +/- S.E.M.). ha decayed with a time constant of 0.27 +/- 0.02 (n = 8) and 0.33 +/- 0.02 s (n = 7). 3. The early relaxation rate of tetanic force, extrapolated to the onset of stimulation (yo + yb; where yo is 'extra' rate of relaxation and yb steady rate) was 85.6 +/- 4.2 s-1 for type 1 fibres (n = 8) and 62.7 +/- 7.3 s-1 for type 2 fibres (n = 7). Relaxation rate at the end of a 1.8 s tetanus (yb) was 29.4 +/- 1.6 and 33.3 +/- 1.5 s-1, respectively; thus, there was more slowing with tetanus duration in type 1 fibres. The time constant for slowing of relaxation with tetanus duration was similar to that for decay of ha. 4. Parvalbumin concentration, [PA], was 0.45 +/- 0.04 mM in type 1 (n = 7) and 0.22 +/- 0.04 mM (n = 7) in type 2 fibres. 5. For individual fibres positive correlations were found between the 'extra' rate of relaxation (yo), labile heat (ha) and [PA]. Significantly more labile heat was liberated than can be accounted for by the enthalpy change of Ca2+ binding to PA. 6. For five fibres (type 1) studied both at 20 and 10 degrees C, the magnitude of slowing of relaxation, expressed as yo/(yo + yb), was 0.58 +/- 0.03 at 20 degrees C and 0.65 +/- 0.03 at 10 degrees C. 7. Both slowing of relaxation and labile heat were depressed in the second of two closely spaced tetani in type 1 fibres. Repriming of both effects followed similar, biphasic time courses and required more than 10 min for completion at 20 degrees C.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine Triphosphatases↗

PubMed1993

Uptake and caffeine-induced release of calcium in fast muscle fibers of Xenopus laevis: effects of MgATP and P(i).

To elucidate the origin of the reduction in force during prolonged muscle fatigue, the dependency of Ca2+ uptake and release on MgATP and P(i) concentration was studied in saponin-skinned fast skeletal muscle fibers of the iliofibularis muscle of Xenopus laevis at 3 degrees C. The sarcoplasmic reticulum was loaded with Ca2+ for 5 min at pCa 7.0. The amount of Ca2+ released was derived from the area of the caffeine-induced force response. Ca2+ uptake increased with the MgATP concentration present during loading. It was half maximal at 20 microM and saturated at higher concentrations. The kinetics of Ca2+ release were affected for MgATP concentrations between 0.1 and 0.5 mM or less, but the amount of Ca2+ released by caffeine in ATP-free solutions was substantial. Phosphate (15 mM) only slightly reduced Ca2+ uptake when the loading period was short (1 min). It is unlikely, therefore, that the reduction in MgATP concentration contributes to the depression of Ca2+ released from the sarcoplasmic reticulum during fatigue. The increase in P(i) concentration could play a small role by reducing Ca2+ uptake.

Adenosine Triphosphate↗