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E Homsher

Publications and source records attributed to E Homsher.

At least 55 records · Page 3Linked to original sources
PubMed1984

The effect of shortening on energy liberation and high energy phosphate hydrolysis in frog skeletal muscle.

It is generally assumed that the increased rate of energy liberation (as heat and work, h+w) accompanying shortening stems from an increased rate of crossbridge cycling and ATP hydrolysis. Experiments were performed to test two premises of this assumption: first, is the increased rate of heat production accompanying shortening derived from crossbridge activity? This question was answered by measuring the amount of shortening heat produced by a fixed displacement of 0.3 micron/sarcomere in the sarcomere length range of 2.25-3.75 microns. Shortening heat declines linearly with decreasing amounts of thick and thin filament overlap and becomes zero at a sarcomere spacing of ca. 3.70 microns. Secondly, the extent to which the measured consumption of high energy phosphate accounts for the measured tetanic (h+w) production during and after shortening for 300 ms at a velocity of Vmax or 1/ 2Vmax was examined. The results of these experiments showed that within 700 ms of the end of shortening at both velocities, all the (h+w) could be explained by the hydrolysis of ATP. At Vmax all the (h+w) produced by the end of shortening could be explained by the measured ATP hydrolysis. However, at Vmax less than half of the (h+w) produced by the end of shortening could be explained by the measured ATP splitting and there was a high rate of ATP splitting after the end of shortening. These results suggest that while shortening at velocities less than or equal to Vmax the energy liberation is indeed derived from an increased rate of ATP hydrolysis by crossbridges, at Vmax the crossbridge ATPase cycle differs somewhat from that at lower shortening velocities.

Adenosine Diphosphate↗
PubMed1984

The dependence on the distance of shortening of the energy output from frog skeletal muscle shortening at velocities of Vmax, 1/2Vmax and 1/4Vmax.

Shortening heat and work were measured, during and after the shortening, in contractions in which muscles shortened various distances (all ending at 2.05 micron of sarcomere length) at various velocities. Shortening heat and work were produced as a non-linear function of the distance shortened at velocities greater than 1/ 2Vmax , which was probably caused by a redistribution of the crossbridge population to different state(s) during the approach to the steady state.

Animals↗
PubMed1984

Energy balance studies in frog skeletal muscles shortening at one-half maximal velocity.

High-energy phosphate metabolism and energy liberated as heat and work were measured in 3-s tetani of frog sartorius muscle at 0 degree C. Two contraction periods were studied: (a) a 0.35-s period of shortening near half-maximum velocity beginning after 2 s of isometric stimulation, and (b) a 0.65-s isometric period immediately following the shortening. There were no significant changes in levels of ATP, ADP, or AMP in the two contraction periods. The observed changes in inorganic phosphate and creatine levels indicated that the only significant reaction occurring was phosphocreatine splitting. The mean rate of high-energy phosphate splitting during the shortening, 1.60 +/- 0.23 mumol X g-1 X s-1 (n = 24), was about fivefold higher than that in the 1-s period in the isometric tetanus, 0.32 +/- 0.11 mumol X g-1 X s-1 (n = 17), observed in our previous study. The mean rate in the post-shortening period, 0.46 +/- 0.13 mumol X g-1 X s-1 (n = 17), was not significantly different from that in the 1-s period in the isometric tetanus. A large amount of heat plus work was produced during the shortening period, and this could be accounted for by simultaneous chemical changes. In the post-shortening period, the observed enthalpy was also accounted for by simultaneous chemical reactions. Thus, the present result is in sharp contrast to that obtained from a similar study performed at a shortening at Vmax, where an enthalpy excess was produced during shortening and an enthalpy deficit was produced during the period following the shortening.

Animals↗
PubMed1984

The kinetics of magnesium adenosine triphosphate cleavage in skinned muscle fibres of the rabbit.

The time course of magnesium adenosine triphosphate (Mg ATP) cleavage in chemically skinned muscle fibres of the rabbit was measured by a method in which Mg ATP cleavage was initiated by photolytic release of ATP from P3-1-(2-nitro)phenylethyladenosine 5'-triphosphate (caged ATP) and terminated by rapid freezing 50 ms to 8 s later. Up to 5 mM-ATP was released following a single 50 ns laser pulse at 347 nm. Mg ATP cleavage was measured at 19 degrees C in the presence and absence of calcium ions, for fibres near rest length and stretched beyond overlap of the myofilaments. At full overlap and in the absence of calcium (less than 10(-8) M) and nucleotide, the fibres developed rigor tension. Following the laser pulse the tension decreased to that of a relaxed fibre in two distinct phases. The first phase lasted about 40 ms and was followed by a second phase during which tension decreased to zero with an approximately exponential time course with a rate constant of 11 s-1. In the presence of 2 X 10(-5) M-free calcium ions, the initial phase following the laser flash lasted approximately 13 ms, and was followed by an exponential rise of tension with a rate constant of 28 s-1. The active tension reached by the muscle fibres was 54 kN/m2. For fibres stretched beyond overlap, no change in tension was observed following the release of Mg ATP. Under all conditions the time course of Mg ATP cleavage was biphasic, and consisted of a rapid initial burst of ADP formation, complete within 50 ms, followed by a slower steady-state rate of Mg ATP cleavage. The number of molecules of Mg ATP cleaved during the burst was approximately equal to the number of myosin subfragment 1 heads for fibres at full myofilament overlap, and equal to 0.7 molecules per myosin subfragment 1 head for fibres stretched beyond overlap. At full overlap in the presence of calcium ions, the steady-state rate equalled 1.8 mol Mg ATP cleaved per mole myosin subfragment 1 head per second. In all other cases the steady-state rate of Mg ATP cleavage was at least 10-fold less. When fibres at full overlap were pre-incubated with 2 mM-ADP, the initial phase of the tension response was somewhat prolonged, but the burst of ADP formation was also complete within 50 ms.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine Diphosphate↗
PubMed1983

The variation in shortening heat with sarcomere length in frog muscle.

Pairs of frog semitendinosus muscles were stimulated tetanically for 2 s at 0 degree C and, after 0.75 s of isometric contraction, were released at constant velocity for 0.16 s. The distance shortened was 0.3 micron per sarcomere. The extra heat (shortening heat) associated with the release was determined by comparison with isometric control tetani. The mean sarcomere length at the start of stimulation was varied between 2.25 and 3.75 microns. At the greater initial lengths there was considerable resting tension and heat was absorbed during releases of unstimulated muscles. This thermoelastic effect was also present during stimulation, so shortening heat values were calculated from the difference between the heat production of the stimulated and unstimulated muscle for releases over the same length range and at the same velocity. Laser diffraction was used to measure mean sarcomere length in the part of the muscle from which heat was recorded. At the greater initial lengths these central sarcomeres slowly elongated by a small amount during nominally isometric tetani. When a release was given during a tetanus the central sarcomere shortening was slightly less than that in a release of the unstimulated muscle with the same change of muscle length. In the period of a tetanus after the release had ended the rate of central sarcomere elongation was greater than that in an isometric tetanus. Shortening heat production had a large early component, during and just after the release, then continued to increase slowly up to the end of the tetanus. It is likely that sarcomere length redistribution along the muscle is responsible for the apparent slow phase, but has little effect on the early component. Shortening heat (determined as the early component) decreased linearly with increasing muscle length in the range studied. The intercept on the length axis was 3.78 +/- 0.08 micron per sarcomere (mean +/- S.E. of mean, n = 7, based on sarcomere length before stimulation). The corresponding intercept for the isometric tension before the release was 3.78 +/- 0.03 micron per sarcomere. The isometric heat rate at this time also decreased with increasing muscle length, but at 3.75 microns per sarcomere was still 50.0 +/- 2.0% of its maximum value. The results suggest that shortening heat, like isometric tension, is produced by the interaction of thick and thin filaments in contracting muscle.

Animals↗
PubMed1982

Unexplained enthalpy production in contracting skeletal muscles.

Energy balance experiments, testing the hypothesis that the observed enthalpy production can be accounted for by high-energy phosphate splitting, have shown that two additional sources of heat production not directly associated with high-energy phosphate splitting per se can be observed during contraction. The first is an energy production occurring during the first 5 s of a maintained isometric tetanus (0 C), which amounts to about 25 mJ/g of muscle. Because the amount of this unexplained enthalpy is not altered by varying the amount of thick and thin filament overlap, it is suggested that it is associated with Ca2+ redistribution (and related binding) within the muscle cell during contraction. The second type of unexplained enthalpy production, amounting to 5-8 mJ/g, occurs when thick and thin filaments slide rapidly past one another. It is suggested that its production is associated with a redistribution of cross-bridge states upon transition from isometric to rapidly shortening contraction. Both types of unexplained enthalpy production are thought to be reversed by an ATP splitting subsequent to their production.

Adenosine Triphosphate↗
PubMed1981

High-energy phosphate metabolism and energy liberation associated with rapid shortening in frog skeletal muscle.

1. High-energy phosphate metabolism and energy liberated as heat and work were measured in 3 sec tetani of frog sartorius muscles at 0 degrees C.2. Three contraction periods were studied: (a) shortening at near-maximum velocity for 0.3 sec from sarcomere length 2.6 to 1.8 mum, beginning after 2 sec of isometric stimulation, (b) the 0.7 sec isometric period immediately following such rapid shortening, (c) the period from 2 to 3 sec in an isometric tetanus at sarcomere length 1.8 mum.3. There were no significant changes in levels of ATP, ADP or AMP in any contraction period. The observed changes in inorganic phosphate and creatine levels indicated that the only significant reaction occurring was phosphocreatine splitting.4. The mean rate of high-energy phosphate splitting during rapid shortening, 0.48 +/- 0.24 mumole/g.sec (mean +/- s.e. of mean, n = 29; ;g' refers to blotted muscle weight), was not significantly different from that in the 1 sec period in the isometric tetanus, 0.32 +/- 0.11 mumole/g.sec (n = 17). The mean rate in the post-shortening period, 0.71 +/- 0.10 mumole/g.sec (n = 22), was greater than that in the 1 sec period in the isometric tetanus, and this difference is significant (P < 0.02, t test).5. A large quantity of heat plus work was produced during the rapid shortening period, but less than half of this could be accounted for by simultaneous chemical reactions. The unexplained enthalpy production was 6.5 +/- 2.6 mJ/g (mean +/- s.e. of mean). No significant unexplained enthalpy was produced in the 1 sec period in the isometric tetanus.6. In the post-shortening period the observed enthalpy was less, by 6.2 +/- 2.6 mJ/g, than that expected from the simultaneous chemical reactions.7. The results are interpreted in terms of an exothermic shift in the population of cross-bridge states during rapid shortening. It is suggested that a relatively slow subsequent step prevents many of these cross-bridges from completing the cycle and splitting ATP until after the end of shortening.

Adenosine Triphosphate↗
PubMed1979

The time-course of energy balance in an isometric tetanus.

Unpoisoned sartorius muscles of Rana temporaria were stimulated tetanically in isometric contractions lasting up to 20 s at 0 degrees C. The observed enthalpy (heat + work) production and the chemical changes in these contractions were measured, and a comparison was made between the observed enthalpy and the enthalpy that could be explained by the chemical changes. Like earlier workers, we found that the only net known reaction of energetic significance that occurred was dephosphorylation of n-phosphoryl creatine (PC), and we found a significant evolution of unexplained enthalpy (UE), a portion of the observed enthalpy which could not be explained by the extent of PC dephosphorylation. We measured the total quantity and the rate of production of the UE, and we found that its rate of evolution, which was most rapid during the first 750 ms of contraction, fell progressively to zero by the 8th s of contraction: i.e., after 8 s of contraction, all the observed enthalpy is adequately explained by PC dephosphorylation. The time-course of evolution of the UE was slower than that of the labile enthalpy (a component of the enthalpy evolved in isometric contraction whose rate of production declines exponentially at approximately 1 s-1). We conclude that, although the magnitudes of these enthalpy quantities may be similar, they are not derived from the same chemical reaction in muscle.

Animals↗
PubMed1978

Preparation and characterization of frog muscle myosin subfragment 1 and actin.

The preparation, structural and steady-state kinetic characteristics of contractile proteins from the leg muscle of frogs Rana temporaria and Rana pipiens are described. Actin and myosin from the two frog species are indistinguishable. The proteins have structural and steady-state kinetic properties similar to those from rabbit fast-twitch skeletal muscle. Chymotrypsin digestion of frog myosin or myofibrils in the presence of EDTA yields subfragment 1, which is separated by chromatography into two components that are distinguished by their alkali light-chain content.

Actins↗
PubMed1978

Reaction mechanism of the magnesium ion-dependent adenosine triphosphatase of frog muscle myosin and subfragment 1.

The Mg2+-dependent ATPase (adenosine 5'-triphosphatase) mechanism of myosin and subfragment 1 prepared from frog leg muscle was investigated by transient kinetic technique. The results show that in general terms the mechanism is similar to that of the rabbit skeletal-muscle myosin ATPase. During subfragment-1 ATPase activity at 0-5 degrees C pH 7.0 and I0.15, the predominant component of the steady-state intermediate is a subfragment-1-products complex (E.ADP.Pi). Binary subfragment-1-ATP (E.ATP) and subfragment-1-ADP (E.ADP) complexes are the other main components of the steady-state intermediate, the relative concentrations of the three components E.ATP, E.ADP.Pi and E.ADP being 5.5:92.5:2.0 respectively. The frog myosin ATPase mechanism is distinguished from that of the rabbit at 0-5 degrees C by the low steady-state concentrations of E.ATP and E.ADP relative to that of E.ADP.Pi and can be described by: E + ATP k' + 1 in equilibrium k' - 1 E.ATP k' + 2 in equilibrium k' - 2 E.ADP.Pi k' + 3 in equilibrium k' - 3 E.ADP + Pi k' + 4 in equilibrium k' - 4 E + ADP. In the above conditions successive forward rate constants have values: k' + 1, 1.1 X 10(5)M-1.S-1; k' + 2 greater than 5s-1; k' + 3, 0.011 s-1; k' + 4, 0.5 s-1; k'-1 is probably less than 0.006s-1. The observed second-order rate constants of the association of actin to subfragment 1 and of ATP-induced dissociation of the actin-subfragment-1 complex are 5.5 X 10(4) M-1.S-1 and 7.4 X 10(5) M-1.S-1 respectively at 2-5 degrees C and pH 7.0. The physiological implications of these results are discussed.

Actins↗
PubMed1976

A temporal dissociation of energy liberation and high energy phosphate splitting during shortening in frog skeletal muscles.

Measurements of the time course of high energy phosphate splitting and energy liberation were performed on rapidly shortening Rana pipiens skeletal muscles. In muscles contracting 30 times against small loads (less the 0.02P), the ratio of explained heat + work (H + W) (calculated from the measured high energy phosphate splitting) to observed H + W (from myothermal and mechanical measurements) was 0.68 +/- 0.08 and is in agreement with results obtained in isometric tetani of R. pipiens skeletal muscle. In lightly afterloaded muscles which were tetanized for 0.6a and whose metabolism was arrested at 3.0 s after the beginning of stimulation, a similar ratio of explained H + W to observed H + W was obtained. However, in identical contractions in which metabolism was arrested at 0.5-0.75 s after the beginning of stimulation, the ratio of explained H + W to observed H + W declined significantly to values ranging from 0.15 to 0.40. These results suggest that rapid shortening at the beginning of contraction induces a delay between energy production and measurable high energy phosphate splitting. This interpretation was tested and confirmed in experiments in which one muscle of a pair contracted isometrically while the other contracted against a small afterload. The afterload and stimulus pattern were arranged so that at the time metabolism was arrested, 0.5 s after the beginning of stimulation, the total energy production by both muscles was the same. Chemical analysis revealed that the isotonically contracting muscle spilt only 25% as much high energy phosphate as did the isometrically contracting muscle.

Animals↗
PubMed1975

Energy liberation and chemical change in frog skeletal muscle during single isometric tetanic contractions.

Recent data obtained from Rana temporaria sartorius muscles during an isometric tetanus indicate that the time-course of phosphocreatine (PC) splitting cannot account for the total energy (heat + work) liberation (Gilbert et al. 1971. J. Physiol. (Lond.) 218:)63). As this conclusion is important to an understanding of the chemical energetics of contraction, similar experments were performed on unpoisoned, oxygenated Rana pipiens sartorius muscles. The muscles were tetanized (isometrically) at 0 degrees C for 0.6, 1, or 5 s; metabolism was rapidly arrested by freezing the muscles with a specially designed hammer apparatus, and the frozen muscles were chemically analyzed. Comparable myothermal measurments were made on frogs from the same batch. Results of these experiments indicate: (a) The energy liberation parallels the PC and ATP breakdown with a proportionality constant of 10.7 kcal/mol; (b) comparably designed experiments with sartorius muscles of R. temporaria revealed that the ratio of energy liberation to PC splitting was significantly greater than that observed in R. pipiens sartorius muscles; (c) there is no systematic difference between experiments in which metabolism was arrested by the hammer apparatus and others using a conventional immersion technique.

Adenosine Triphosphate↗
PubMed1975

Activation of phosphorylase in frog muscle as determined by contractile activity.

The state of activation of phosphorylation in muscle has been reinvestigated by combining the extraction procedures of Danforth, Helmreich, and Cori with the low-temperature techniques of this laboratory. In resting frog muscle, the phosphorylase-alpha content is usually below detectability. Upon contractile activity in series of twitches, activation of phosphorylase beta to alpha took place, without activation of phosphorylase beta kinase as defined by the assay procedure. Two different experimental designs were used to examine the relation between phosphorylase activation and the myothermally determined energy turnover per twitch, and these showed, identically, that the enzyme activation is proportional to the energy per twitch.

Animals↗
PubMed1973

Energetics of shortening muscles in twitches and tetanic contractions. I. A reinvestigation of Hill's concept of the shortening heat.

Shortening heat was defined by Hill as the "difference between heat produced when shortening occurs and that produced in a similar contraction without shortening." For the tetanus the "similar contraction" was an isometric one at or near l(o). By contrast, in a twitch the "similar contraction" was one in which only activation heat was produced. The applicability of Hill's concept of the shortening heat has been reexamined in both the twitch and tetanus of Rana pipiens semitendinosus muscles. Results of this investigation confirm the existence of an extra heat production accompanying shortening in the twitch and tetanus. In both cases, this shortening heat was proportional to distance shortened and relative afterload. However, at a given afterload the amount of shortening heat produced per distance shortened was greater in the twitch than the tetanus. This difference suggests that the base lines or "similar contractions" employed for the twitch and tetanus are not equivalent. The discrepancy is not remedied by utilizing in the tetanus the activation heat as the myothermic baseline and suggests that some heat producing factor(s) has been omitted in Hill's formulation of the shortening heat. Finally, the existence of Hill's feedback heat, an energy liberation associated with the presence of tension during mechanical relaxation, was not confirmed. This result strongly indicates that relaxation is energetically passive.

Animals↗
PubMed1973

Energetics of shortening muscles in twitches and tetanic contractions. II. Force-determined shortening heat.

The extra heat liberation accompanying muscular shortening, the force-determined shortening heat, is defined as the difference between the heat produced when shortening occurs and that produced in an isometric contraction developing the same amount of force and performing the same amount of internal work. Based on this definition, the initial energy production in twitches and tetanic contractions (E) is given by E = A + f (P, t) + alpha(F)x + W, where A is the activation heat, f(P, t), the tension-related heat (a heat production associated with the development and maintenance of tension), alpha(F)x, the force-determined shortening heat, and W, the external work. It is demonstrated that this equation accurately accounts for the time-course of heat evolution and the total initial energy production in both twitches and tetani at 0 degrees C. The force-determined shortening heat is liberated, during shortening, in direct proportion to (a) the distance shortened, and (b) the force against which shortening occurs. The normalized value of the force-determined shortening heat coefficient, alpha(F)/P(o), is the same in both the twitch and the tetanus. Finally, this formulation of the muscle's energy production also accounts for the total energy production in afterload isotonic twitches at 20 degrees C, where a Fenn effect is not demonstrable.

Animals↗