Biomedical subjects

R E Godt

Publications and source records attributed to R E Godt.

At least 37 records · Page 2Linked to original sources

PubMed1990

Inositol trisphosphate has no direct effect on the contractile apparatus of skinned cardiac muscles.

Alpha-adrenoceptor stimulation produces a positive inotropic effect in heart muscle via mechanisms that are not well understood. The purpose of our study was to test the hypothesis that the increase in inositol 1,4,5 trisphosphate [Ins(1,4,5)P3] concentration that accompanies alpha stimulation contributes to the inotropic effect by increasing the calcium sensitivity of the contractile proteins, an effect which Ins(1,4,5)P3 has been shown to have in skeletal muscle. We determined the calcium sensitivity of the contractile apparatus of small, chemically skinned bundles from papillary muscles of rabbit, rat and dog hearts. These preparations were chosen because they exhibit a range of sensitivity to alpha agonists. In addition, we measured the calcium sensitivity of chemically skinned, single fibers from rabbit psoas muscle. All preparations were skinned with Triton X-100, a non-ionic detergent that disrupts the sarcolemmal, sarcoplasmic reticular, and mitochondrial membranes. In all cardiac preparations, we found that 38 microM Ins(1,4,5)P3 had no effect on either the calcium sensitivity or maximum calcium-activated force. Thus, there was no correlation between inotropic response to alpha stimulation and myocardial response to Ins(1,4,5)P3. On the other hand, the maximum calcium-activated force of skinned skeletal muscle was slightly increased by Ins(1,4,5)P3. Moreover, Ins(1,4,5)P3 significantly increased the sensitivity of these fibers to calcium.

Animals↗

PubMed1990

Inhibitory influence of phosphate and arsenate on contraction of skinned skeletal and cardiac muscle.

It has been widely observed that Pi decreases maximum calcium-activated force (Fmax) and calcium sensitivity of skinned skeletal and cardiac muscle. However, whether a particular ionic species of Pi (i.e., H2PO4-) is responsible for these effects is controversial. To clarify this issue, we examined the influence of Pi and its structural analogue arsenate (Asi) on contraction of skinned rabbit psoas (fast twitch), soleus (slow twitch), and cardiac papillary muscle. Asi decreased Fmax of all three muscles types to a greater extent than Pi. Both Pi and Asi decreased calcium sensitivity of psoas and cardiac muscles, with Asi having the greater effect. The effect of the protonated form of Pi and Asi on Fmax was evaluated by measuring the response to 30 mM total Pi or Asi at pH 7.4, 7.0, 6.6, and 6.2. In psoas fibers we found that both Pi and Asi were more effective in decreasing Fmax as the pH was lowered (i.e., as the concentration of the diprotonated forms increased). On the contrary, soleus and cardiac fibers did not exhibit this behavior. These differences in the effects of Pi and Asi on Fmax in psoas vs. cardiac and soleus muscles may be related to differences in their myosin heavy chains other than the binding site for the gamma-phosphate of ATP which appears to be conserved for all myosins.

Animals↗

PubMed1990

Effects of temperature and concomitant change in pH on muscle.

Contractile performance decreases with a decrease in temperature and increases with an increase in pH. In general, a decrease in ambient temperature is associated with an increase of the pH of the intracellular and extracellular fluids of ectotherms. Thus the concomitant increase in pH will to some extent counteract the effect of the decrease in temperature. We review the magnitude of this effect and show that it is modest for force (24%) but is small or negligible for speed or for variables involving time. Experiments with skinned fibers yield similar results to those with intact fibers. We argue that one important effect of the concomitant increase in pH is that it causes an increase in calcium sensitivity and that there may be a considerable metabolic saving associated with releasing less calcium at lower temperatures.

Animals↗

PubMed1989

Equilibrium distribution of ions in a muscle fiber.

We have developed a mathematical description of the equilibrium (Donnan) distribution of mobile ions between two phases containing fixed charges. This differs from the classical Donnan derivation by including mobile polyvalent ions such as those present in intact muscle fibers and in solutions used with skinned muscle fibers. Given the average concentrations of ionic species present in intact frog muscle, we calculate that the myofibrillar fixed charge density (-42 meq/liter cytoplasmic fluid) is in close agreement with estimates from amino acid analysis of myofibrillar proteins. As expected, with negative fixed charges in the myofibril, anions are excluded from the myofibrillar space while cations are concentrated in this space; the ratio between the average intra- and extramyofibrillar concentrations for an ion of valence n is (1.11)n. This model allowed us to design a bathing solution for skinned muscle fibers in which the intramyofibrillar ion concentrations closely approximate those found in intact frog muscle cells. Our model, applied to the A- and I-bands of the sarcomere, suggests that likely differences in fixed charge densities in these regions accounts for only a small fraction of the extreme concentration of phosphocreatine observed in the I-bands of intact frog muscle.

Animals↗

PubMed1989

Changes of intracellular milieu with fatigue or hypoxia depress contraction of skinned rabbit skeletal and cardiac muscle.

1. Maximal calcium-activated force (Fmax) and calcium sensitivity were markedly decreased in detergent-skinned fibres from skeletal and cardiac muscle by solutions that mimicked the total milieu changes associated with fatigue and hypoxia. Further experiments determined the relative contribution of each of the individual changes in milieu. 2. Both Ca2+ sensitivity and Fmax of skeletal and cardiac fibres were decreased with increased [H+] or inorganic phosphate (Pi). These effects were greater in cardiac muscle. 3. Decreasing MgATP over the range observed with fatigue and hypoxia (6.8-4.7 mM) had no effect on Fmax or Ca2+ sensitivity of either muscle type. 4. Decreasing phosphocreatine (PCr: 15-1 mM) increased Fmax but had little effect on Ca2+ sensitivity in both muscle types. In cardiac fibres, the effect on Fmax could be mimicked by inhibition of endogenous creatine kinase. 5. ADP (0.7 mM) increased Fmax and Ca2+ sensitivity, while AMP (0.06 mM) slightly increased Fmax but had no effect on Ca2+ sensitivity of either skeletal or cardiac fibres. 6. Creatine (25 mM) had no significant effect on either Ca2+ sensitivity or Fmax of skeletal and cardiac muscle fibres. At higher levels (50 mM), however, creatine depressed Fmax and slightly altered Ca2+ sensitivity. 7. Thiophosphorylation of myosin P light chains (phosphorylatable light chains of myosin) in rabbit psoas fibres had no effect on Ca2+ sensitivity, yet slightly but significantly increased Fmax under fatigue conditions. 8. Reducing the affinity for ATP hydrolysis (by adding ADP, AMP and creatine) over the range calculated for fatigue/hypoxia (60-45 kJ/mol) produced the enhancement in Fmax expected from added ADP and AMP in cardiac but not skeletal muscle, indicating that changes in affinity influence Fmax of skeletal muscle. Reducing affinity produced little change in Ca2+ sensitivity of skeletal muscle. In contrast, the change produced in cardiac muscle was greater than that expected from addition of ADP and AMP; i.e. decreasing affinity increases calcium sensitivity of the heart. 9. Simple summation of all significant changes expected from each constituent altered by fatigue/hypoxia adequately predicted the observed changes in Fmax and Ca2+ sensitivity in both cardiac and skeletal muscle fibres with but one exception (the change in Ca2+ sensitivity of skeletal muscle at pH 7 was slightly overestimated).

Animals↗

PubMed1988

On the composition of the cytosol of relaxed skeletal muscle of the frog.

This review summarizes a variety of estimates for the concentrations of the principal cytosolic constituents in frog skeletal muscle. From these estimates (listed in the APPENDIX), we chose representative values and used electroneutrality and osmotic considerations to ensure that all major constituents have been considered. Given total cytosolic concentrations of these constituents from the literature, we employed a computer program to calculate the concentrations of all the major ionic species in the cytosol. In relaxed muscle, electroneutrality and osmotic constraints are fulfilled if, in addition to diffusible species, the charge contribution of the myofilaments is considered. Mean buffer power of the diffusible cytosolic species is calculated to be less than one-third of that experimentally determined for whole muscle. Computations indicate that recent estimates of intracellular free magnesium concentration approximately 1 mM are likely to be correct.

Animals↗

PubMed1987

It is diprotonated inorganic phosphate that depresses force in skinned skeletal muscle fibers.

The increases in the intracellular concentrations of inorganic phosphate and hydrogen ion accompanying fatigue of skeletal muscle appear to be the most important metabolic changes associated with the decrease in contractile force. Experiments on chemically skinned single fibers from rabbit psoas muscle with pH ranging between 6 and 7.25 demonstrate that the depression of maximal calcium-activated force by inorganic phosphate correlates nicely with the concentration of the acidic (diprotonated) species. Therefore, in addition to the well-known depressant effect on the contractile machinery of lowering pH per se, any decrease of intracellular pH associated with fatigue further depresses force production by converting more of the total inorganic phosphate within the cell to the inhibitory diprotonated form.

Animals↗

PubMed1986

Inositol trisphosphate enhances calcium release in skinned cardiac and skeletal muscle.

Experiments from other laboratories suggest that inositol trisphosphate (InsP3) may be involved in the excitation-contraction coupling (ECC) process of cardiac and skeletal muscle. Our results support this hypothesis. Studying fiber bundles (less than 200-microns diameter) from guinea pig papillary muscles skinned with saponin and mechanically skinned single fibers from frog semitendinosus muscle, we find that calcium-induced force oscillations (observed in solutions containing low ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid and pCa 7.0) are enhanced in magnitude and frequency by InsP3 at concentrations as low as 1 microM. InsP3 at 10 microM can often induce such oscillations in mechanically skinned frog skeletal muscle. In skinned cardiac fibers, InsP3 increases the magnitude of caffeine contractures at submaximal caffeine concentrations to a greater extent than at near-maximal caffeine concentrations. InsP3 (30 microM) has no effect on either the calcium sensitivity or maximal force generated by the contractile apparatus of skinned cardiac muscle. We conclude that InsP3 has no direct effect on the contractile machinery but that it can modulate ECC by enhancing the calcium-induced release of calcium from the sarcoplasmic reticulum, possibly from the same pool and through the same mechanism as caffeine.

Animals↗

PubMed1984

Contractile responses to MgATP and pH in a thick filament regulated muscle: studies with skinned scallop fibers.

The striated adductor of the Atlantic deep sea scallop ( Placopecten magellanicus ), a thick filament regulated muscle, contains little or no troponin. We examined the effect on activation of two agents (MgATP and pH) that alter the contractile threshold of thin filament regulated muscle, presumably through effects on troponin, to see if they also alter that of thick filament regulated muscle. We find that decreasing MgATP from 2 to 0.1 mM shifts the force-pCa curve of chemically skinned scallop muscle to the left by about 0.8 log units (i.e. Ca2+ sensitivity increases some six-fold). Under similar conditions the force-pCa relation of frog skinned fibers shifts leftward by almost the same amount, 0.7 log units ( Godt , 1974). The force-pCa curve of scallop was unaffected by a decrease in pH from 7 to 6.5. It is especially interesting because: A) the force-pCa relation of skinned fibers from frog (Robertson and Kerrick , 1979) and striated adductor of the Pacific scallop ( Chlamys hastata hericia ) ( Donaldson , unpublished observations) is shifted to the right by about 0.5 log units over this pH range. Furthermore, B) decreasing pH is reported to decrease the calcium affinity of Placopecten myofibrils ( Chantler et al., 1981). Thus the molecular details of thick filament regulation appear to be more complex and varied than hitherto supposed.

Adenosine Triphosphate↗

PubMed1984

Potential and K+ activity in skinned muscle fibers. Evidence against a simple Donnan equilibrium.

It has been suggested that potentials measured with conventional microelectrodes in chemically or mechanically skinned muscle fibers arise from a Donnan equilibrium due to myofilament fixed charges. This hypothesis was tested in mechanically skinned frog (Rana pipiens) semitendinosus fibers by measuring the distribution potential (Ed) between fiber and bath with 3 M KCl-filled microelectrodes and the K+ activity gradient (aik/aok) with K+ ion-selective microelectrodes (KISE). If skinned fibers are a Donnan system, Ed should become more positive as pH is decreased, altering the fixed charge on the myofilaments. Consistent with this expectation, Ed was -4.4, -0.6, and +4.8 mV in ATP-containing solutions and -6.5, -2.2, and +8.4 mV in ATP-free solutions at pH 7, 6, and 5, respectively. Donnan equilibrium also requires that all mobile ionic species be in electrochemical equilibrium. In ATP-containing solutions, this was true for K+ at pH 7. At pH 5, however, KISE indicated that K+ was not in equilibrium; average Ed was 5.9 mV positive to the K+ equilibrium potential, and aik/aok was 1.04, while the Donnan prediction was 0.83. In contrast, KISE measurements in ATP-free solutions indicated that K+ was in equilibrium at all pH studied. Skinned fibers in ATP-containing media are not equilibrium systems because ATPase reactions occur. Under our conditions, frog myofibrils hydrolyze 0.4 and 0.08 mumol ATP/min X mg myofibrillar protein at pH 7 and 5, respectively. It is suggested that in the presence of ATP, Ed is a superposition of Donnan and diffusion potentials, the latter arising from differences in the mobilities of anionic substrate and products that diffuse through the charged myofilament lattice. A coupling to diffusion of K+, the predominant counter ion, is required for macroscopic electroneutrality. This coupling may be the origin of the nonequilibrium K+ distribution.

Adenosine Triphosphatases↗

PubMed1984

Influence of deuterium oxide on calcium transients and myofibrillar responses of frog skeletal muscle.

The influence of substituting D2O for H2O on calcium transients and on contraction was studied in intact single skeletal muscle fibres injected with aequorin and in mechanically skinned fibres from frogs. Most experiments were carried out at 10 degrees C. Experiments performed in vitro established that the calcium concentration-effect curve for aequorin is depressed and the rate of change of aequorin light emission after sudden changes of [Ca2+] is decreased when D2O is substituted for H2O. D2O substitution greatly reduces the amplitude of the aequorin signals of intact fibres both in twitches and in tetani. This is partly the result of the influence of D2O on aequorin, but the amplitude of the calcium transients is reduced as well. In both H2O and D2O Ringer solutions, the amplitude and time course of the sarcoplasmic calcium transient during a tetanus vary greatly with the stimulus frequency. In H2O, frequencies high enough to produce mechanical fusion normally produce cytoplasmic calcium concentrations high enough to saturate the myofibrils, with the result that stimulus frequency has very little influence on the mechanical response. Saturation does not occur in D2O, and the amplitude and form of fully fused tetani are greatly influenced by the frequency of stimulation. Aequorin was used as a calcium indicator to estimate the influence of D2O substitution on the apparent equilibrium association constant for the calcium-EGTA complex. The constant was reduced by more than one order of magnitude in D2O at pD = pH = 7.0. Experiments on mechanically skinned muscle fibres showed that D2O substitution decreased the apparent sensitivity of the myofibrils to calcium, but increased the force of contraction at saturating calcium concentrations. The latter effect is probably responsible for the potentiation of tetanic tension that is observed in certain D2O/H2O mixtures or in H2O solutions immediately after a switch from D2O. In intact muscle fibres the potentiation of twitch or tetanic tension after a switch from D2O to H2O declines with a half-time of about 25 s; this probably reflects the time course of exchange of intracellular D2O for H2O. The effects of D2O on the intracellular calcium transient appeared and disappeared with a half-time of less than 1 s; this time is of the same order of magnitude as that calculated for the exchange of H2O and D2O in the T-tubules. We conclude that D2O affects calcium release at a site in the T-tubule which is more accessible to the extracellular space than to the intracellular space.

Action Potentials↗

PubMed1984

Effects of hypertonic solutions on contraction of frog tonic muscle fibers.

The influence of solution hypertonicity on contraction was studied in small bundles of tonic muscle fibers from the iliofibularis muscle of the frog Rana pipiens. Muscles were activated with high-K+ solutions that had osmolalities which were increased with tris(hydroxymethyl)aminomethanepropionate. Peak potassium contracture force decreased monotonically with tonicity and was zero in solutions with 2.5 or 3 times the osmolality of control Ringer. Contracture force at all tonicities studied (less than or equal to 3 X Ringer) was increased by increasing Ca2+ in the media 10-fold (to 20 mM) and/or by adding caffeine (10-20 mM). Nevertheless, this potentiated force also was diminished as tonicity increased. Force of single, mechanically skinned tonic fibers taken from these bundles was activated by Ca2+ over the same concentration range as that of twitch fibers. Moreover, maximal Ca2+-activated force, normalized per cross-sectional area, was similar in skinned tonic and twitch fibers. As was shown previously in twitch fibers, maximal Ca2+-activated force was decreased when ionic strength was increased. These data suggest that, as with twitch fibers, increased tonicity depresses contraction of tonic fibers by increasing the intracellular ionic strength, which in turn inhibits the ability of the contractile apparatus to generate force. Unlike twitch fibers, however, disruption of the excitation-contraction coupling process probably plays a more significant role in the action of hypertonicity on tonic fibers.

Animals↗

PubMed1982

Influence of temperature upon contractile activation and isometric force production in mechanically skinned muscle fibers of the frog.

Increasing temperature (4-22 degrees C) increases the Ca2+ concentration required for activation of mechanically skinned frog muscle fibers. The pCa required for 50% maximal force (pCa50) was inversely proportional to absolute temperature. Assuming that relative force is directly related to fractional occupancy of the Ca2+-binding sites on troponin that regulate force, the shift was consistent with a Gibbs free energy change of binding (delta G) of about -7.8 kcal/mol. This is close to the delta G for Ca2+ binding to the calcium-specific sites on troponin C reported by others. Decreasing Mg2+ from 1 mM to 60 microM shifts the force-pCa curves at either 4 or 22 degrees C to higher pCa, but the shift of pCa50 with temperature over this range (0.4 log units) was the same at low and high Mg2+. Maximal force increased with temperature for the entire range 4-22 degrees C with a Q10 of 1.41, and over the restricted range 4-15 degrees C with a Q10 of 1.20. From the dual effects of temperature on Ca2+ activation and maximal force, one would expect that force would respond differently to temperature change at high or low Ca2+. At high Ca2+, a temperature increase will lead to an increased force. However, at low to intermediate Ca2+ levels (below the intersection of the force-pCa curves for the initial and final temperatures), steady state force should decrease with increasing temperature. The inverse responses should occur with a decrease in temperature. These responses are observed when temperature is changed by rapid solution exchange.

Animals↗

PubMed1981

Influence of osmotic compression on calcium activation and tension in skinned muscle fibers of the rabbit.

Single skinned muscle fibers were osmotically compressed back to and below their in situ size by addition of a large, random-coil polymer (Deytran T500; MN = 180,000; MW = 461,000) to the bathing medium. Maximal Ca2+-activated tension in fibers swollen (zero Dextran, fiber width 21% above in situ) or near in situ size (5% Dextran, in g/100 ml final solution) was similar, but compression to 86% of in situ width with 10% Dextran decreased maximal force by 15% relative to polymer-free control. While the relative tension-pCa relation in 0 and 10% Dextran was similar, with a pCa of 6.37 required for 50% activation, that in 5% Dextran was more sensitive to Ca2+, with a pCa50 of 6.66. We feel these effects are most likely due to changes in interfilament spacing with compression and that alterations in Ca2+-sensitivity might be explained by changes in cross-bridge angle or in the concomitant attachment-detachment rate constants which would be expected to influence the troponin-Ca2+ binding equilibrium, as has been proposed by others.

Animals↗

PubMed1981

Inhibition of force production in compressed skinned muscle fibers of the frog.

Calcium-activated force development in skinned frog muscle fibers is inhibited by osmotically compressing the fiber, probably owing to a decrease in spacing between the myofilaments. This inhibition depends upon sarcomere length in that fibers at long lengths must be compressed further than those at short lengths to achieve the same degree of inhibition. As a result, this length dependency of inhibition tends to compensate for the reduction of force due solely to the decrease in interfilament spacing which occurs with stretch in intact fibers.

Animals↗

PubMed1981

A simple electrostatic model can explain the effect of pH upon the force-pCa relation of skinned frog skeletal muscle fibers.

The relative force-pCa relation of skinned frog skeletal muscle fibers is shifted along the pCa axis by changes in pH. This shift has been interpreted as arising from competition between H+ and Ca2+ for a binding site on troponin. Unfortunately, binding studies have been unable to confirm such competition. Alternatively, however, the data fit a model where H+ influences the degree of dissociation of ionizable groups on the surface of the thin filaments, thus altering the electrostatic potential surrounding the filaments. Alterations in the potential will, in turn, change the concentration of Ca2+ near the troponin binding sites in accordance with the Boltzmann relation. A simple model, based upon the Gouy-Chapman relation between surface potential and charge density, provides a quantitative explanation for the shift of the relative force-pCa curve with pH, given a reasonable estimate of the surface charge density on the thin filament. A best fit is obtained when the ionizable groups giving rise to the potential have a log proton ionization constant (pKa) of 6.1, similar to that for the imidazole group on histidine, and when the density of these groups is near that estimated from amino acid analysis of thin filament proteins and from filament geometry. In preliminary experiments, reaction of skinned frog fibers with diethylpyrocarbonate (DEP) at pH 6 shifted the force-pCa curve toward lower Ca2+. This would be expected in the model since DEP at pH 6 is reported to specifically react with histidine imidazole groups and to irreversibly decrease their pKa, which would increase the net negative charge of the filaments.

Animals↗

PubMed1981

Radial forces within muscle fibers in rigor.

Considering the widely accepted cross-bridge model of muscle contraction (Huxley. 1969. Science [Wash. D. C.]. 164:1356-1366), one would expect that attachment of angled cross-bridges would give rise to radial as well as longitudinal forces in the muscle fiber. These forces would tend, in most instances, to draw the myofilaments together and to cause the fiber to decrease in width. Using optical techniques, we have observed significant changes in the width of mechanically skinned frog muscle fibers when the fibers are put into rigor by deleting ATP from the bathing medium. Using a high molecular weight polymer polyvinylpyrrolidone (PVP-40; number average mol. wt. (Mn) = 40,000) in the bathing solution, we were able to estimate the magnitude of the radial forces by shrinking the relaxed fiber to the width observed with rigor induction. With rigor, fiber widths decreased up to approximately 10%, with shrinking being greater at shorter sarcomere spacing and at lower PVP concentrations. At higher PVP concentrations, some fibers actually swelled slightly. Radial pressures seen with rigor in 2 and 4% PVP ranged up to 8.9 x 10(3) N/m2. Upon rigor induction, fibers exerted a longitudinal force of approximately 1 x 10(5) N/m2 that was inhibited by high PVP concentrations (greater than or equal to 13%). In very high PVP concentrations (greater than or equal to 20%), fibers exerted an anomalous force, independent of ATP, which ranged up to 6 x 10(4) N/m2 at 60% PVP. Assuming that all the radial force is the result of cross-bridge attachment, we calculated that rigor cross-bridges exert a radial force of 0.2 x 1.2 x 10(-9) N per thick filament in sarcomeres near rest length. This force is of roughly the same order of magnitude as the longitudinal force per thick filament in rigor contraction or in maximal (calcium-activated) contraction of skinned fibers in ATP-containing solutions. Inasmuch as widths of fibers stretched well beyond overlap of thick and thin filaments decreased with rigor, other radially directed forces may be operating in parallel with cross-bridge forces.

Adenosine Triphosphate↗

PubMed1980

A quantitative analysis of elastic, entropic, electrostatic, and osmotic forces within relaxed skinned muscle fibers.

The elastic behavior of mechanically skinned skeletal muscle fibers in relaxing solution is modelled using equations developed by Flory (1953) for the elasticity of non-biological polymers. Mechanically, the relaxed skinned fiber is considered to be a semi-crystalline network of inextensible polymer chains, which are periodically cross-linked and which are bathed in an aqueous medium. We consider (1) configurational elastic forces in the network, (2) entropic forces due to mixing of polymer and water, (3) electrostatic forces due to fixed charges on the muscle proteins and mobile charges in the bathing solution, and (4) compressive forces due to large colloids in the bathing solution. Van der Waals forces are not considered since calculations show that they are probably negligible under our conditions. We derive an expression which relates known quantities (ionic strength, osmotic compressive pressure, and fiber width), experimentally estimated quantities (fixed charge density and volume fraction of muscle proteins), and derived quantities (concentration of cross-links and a parameter reflecting the interaction energy between protein and water). The model was tested by comparison with observed changes in skinned fiber width under a variety of experimental conditions which included changes in osmotic compressive pressure, pH, sarcomere length, and ionic strength. Over a wide range of compressive pressure (0-36 atm) the theory predicted the nonlinear relation between fiber width and logarithm of pressure. The direction and magnitude of the decrease in width when pH was decreased to 4 could be modelled assuming the fixed charge density on the protein network was 0.34 moles of electrons per liter protein, a value in accordance with the estimates of others. The relation between width and sarcomere length over the complete range of compressive pressures could be modelled with the assumption that the number of cross-links increases somewhat with sarcomere length. Changes of width with ionic strength were modelled assuming that increasing salt concentration both increased the electrostatic shielding of fixed charges and decreased the number of cross-links. The decrease of fiber width in 1% glutaraldehyde was modelled by assuming that the concentration of crosslinks increased by some 10%. The theory predicted the order of magnitude but not the detailed shape of the passive tension-length relation which may indicate that, as with non-biological polymers, the theory does not adequately describe the behavior of semi-crystalline networks at high degrees of deformation. In summary, the theory provides a semiquantitative approach to an understanding of the nature and relative magnitudes of the forces underlying the mechanical behavior of relaxed skinned fibers. It indicates, for instance, that when fibers are returned to near their in vivo size with 3% PVP, the forces in order of their importance are: (elastic forces) approximately (entropic forces) greater than (electrostatic forces) approximately (osmotic compressive forces).

Animals↗