Nuclear magnetic resonance analysis of water in natural and deuterated mouse muscle above and below freezing.

Measurements of absolute proton signal intensities, free induction decays, spin-spin relaxation times, and local fields in the rotating frame in natural and fully deuterated mouse muscle at five temperatures in the range 293-170 K are reported. The analysis is carried out at three time windows on the free induction decay. The contribution to the magnetization from protons on large molecules and water are analyzed.     

Pulsed nuclear magnetic resonance study of 17-O, 2-D, and 1-H of water in frog striated muscle.

Whole gastrocnemius muscles were incubated in Ringer's solution enriched with H2-17O; the paired contralateral gastrocnemius muscles were incubated in a similar solution enriched with deuterons, as well. Subsequently, the longitudinal relaxation times (T1) were measured 17-O, 2-D, and 1-H, both at 8.1 MHz and at 4.3 MHz. The results indicate that: (a) the absolute values of T1 characterizing the three nuclides are different in muscle and pure water. (b) the longitudinal relaxation rates of all three have an identical frequency dependence over the range studied, (c) the ratio (T1)2D/(T1)17ois the same in muscle water and pure water, while the ratio (T1)1H/(T1)17o is 2.1 times greater in pure water than it is in muscle water, and (d) 30-49 percent substitution of 2-D for 1-H has very little effect on the spin-lattice relaxation of tissue water protons. These data suggest that muscle water is in rapid exchange between a small fraction of immobilized molecules and a large fraction of free water. The results render unlikely the possibility that hypothetical ordering of muscle water significantly contributes to its longitudinal relaxation.     

NMR spin grouping and correlation exchange analysis. Application to low hydration NaDNA paracrystals.

The NMR spin-grouping technique is applied to low hydration oriented fibers of NaDNA to study the role of exchange in determining the apparent (observed) spin relaxation of the system. The analysis proceeds in three steps: first, the apparent proton relaxation is measured at high fields, with both selective and nonselective inversion pulse sequences, and in the rotating frame. The spin-grouping technique is used in all spin-lattice relaxation measurements to provide the optimum apparent relaxation characterization of the sample. Next, all apparent results are analyzed for exchange. In this analysis the results from the high field and rotating frame experiments (which probe the exchange at two different time scales) are correlated to determine the inherent (or true) spin relaxation parameters of each of the proton groups in the system. The results of selective inversion T1 measurements are also incorporated into the exchange analysis. Finally, the dynamics of each spin group are inferred from the inherent relaxation characterization. The low hydration NaDNA structure is such that the exchange between the protons on the water and those on the NaDNA is limited, a priori, to dipolar mixing. The results of the exchange analysis indicate that the dipolar mixing between water and NaDNA protons is faster than the spin diffusion within the NaDNA proton group itself. The spin-diffusion on the macromolecule is the bottleneck for the exchange between the water protons and the NaDNA protons. The water protons serve as the relaxation sink both at high fields and in the rotating frame for the total NaDNA-water spin bath. The inherent relaxation of the water is characteristic of water undergoing anisotropic motion with a fast reorientational correlation time about one axis (5 X 10(-10) less than or equal to tau r less than or equal to 8 X 10(-9)S) which is about three orders of magnitude slower than that of water in the bulk; and a slow tumbling correlation time for this axis (1.5 x 10(-7) less than or equal to tau t less than or equal to 8 x 10(-7)S) which is two orders of magnitude slower yet.     

A study of molecular dynamics and freezing phase transition in tissues by proton spin relaxation

Muscle, spleen, and kidney tissues from 4-wk-old C57 black mice were studied by proton magnetic resonance. Spin-lattice relaxation times at high fields and in the rotating frame, as well as the spin-spin relaxation times, are reported as a function of temperature in the liquid and frozen phase. Motions of large molecules and of water molecules and their changes at the freezing phase transition are studied. The shortcomings of the two-state fast-exchange relaxation model are discussed.     

Effect of length and caffeine on isometric tetanus relaxation of frog sartorius muscles

In an isometric tetanus of frog sartorius muscle the total relaxation time increased linearly with change in length from 0.7 to 1.4 times rest length. Maximal rate of relaxation, measured from the time derivative (dp/dt) of tension decay, decreased with both decrease and increase from rest length in correlation with the generated tetanus tension. Stretching the muscle did not significantly affect the times to maximal rate, positive and negative inflexion points but greatly increased the time to total relaxation from the negative inflexion point. Caffeine at 2 mM, acting on muscles at rest length, also slowed the relaxation and decreased the maximal rate of tension decay. However, caffeine increased the times to maximal rate, positive and negative inflexion points without significantly affecting time to total relaxation from the negative inflexion point. These results suggest that caffeine slows an earlier step in relaxation, while stretch slows a later step. It is proposed that muscle relaxation is a two step process: an initial step that is regulated by the rate of Ca2+ uptake by sarcoplasmic reticulum, and a later step that is mostly controlled by the speed of dissociation of remaining cross-bridges.     

The hydration response of poly (L-lysine) dynamics measured by 13C-NMR spectroscopy.

13C-nmr measurements are reported for samples of poly (L-lysine) both static and spinning at the magic angle in the beta-sheet form as a function of water content. The addition of water decreases the side-chain line widths considerably. Measurements of the cross-polarization time constants indicate that hydration by either H2O or D2O increases the time constant. Measurements of spin-lattice relaxation times in the laboratory frame and the rotating frame indicate that hydration does not change the dynamics of the backbone carbon atoms in the beta-sheet structure appreciably, but the side-chain atoms experience considerable increase in local mobility with increasing hydration. Deuteration of the exchangeable protons or the water has only small effects on the carbon relaxation times, indicating that relaxation is driven by intramolecular dipole-dipole interactions.     

A macroscopic description of stress relaxation in a muscle subject to stepwise deformation

Stress relaxation regimes arising in a muscle subject to stepwise deformation are described on the basis of a recent phenomenological model of fully activated muscle tissue which is presented in the form of a second-order constitutive equation. It is shown that this model reproduces the qualitative form of relaxation curves observed experimentally. Relations between rheological parameters which correspond to different types of stress relaxation are found for the case where the jump duration is much smaller than the relaxation times of the sample. As illustrated by the simplest model of a slow length jump (linear deformation), the qualitative form of the stress relaxation depends on the jump duration in this case. This effect can lead to rough errors in determination of rheological and molecular parameters of muscle tissue in mechanical experiments in which the relation between jump duration and relaxation times is not controlled.     

Pulsed NMR studies of water in striated muscle non-freezing factor of water II. Spin-lattice relaxation times and the dynamics of the non-freezing fraction of water

Spin-lattice relaxation times for the water protons in frog gastrocnemius muscle are reported over the temperature range 193 to 283 °K at Larmor frequencies of 30 and 60 MHz. Results of measurements under similar conditions of the transverse relaxation times are also reported. The relaxation times of the non-freezing 20% of the muscle water are interpreted in terms of water molecules, absorbed on or interacting with the proteins, and which are undergoing anisotropic motion, probably with a distribution of correlation times. Proton spin-lattice relaxation times are also reported for muscles under tension, the tension being produced by loading of the muscles with varying weights.     

Water and Ions in Muscles and Model Systems

The nuclear magnetic reasonance (NMR) relaxation times of protons in toad muscle water have been measured at three frequencies: 2.3, 8.9, and 30 MHz. The results are analyzed in terms of a distribution of correlation times, and it is found that only a few percent of the observed protons have mobilities more than two orders of magnitude smaller than normal. Sodium and chloride ion chemical potentials in some hydrated materials with similar proton NMR characteristics to toad muscle have been found to be heightened, but not sufficiently to account for the distribution of sodium ions in muscle.     

Effect of neuromuscular electrical stimulation on motor cortex excitability upon release of tonic muscle contraction

The aim of the present study was to investigate the neurophysiological triggers underlying muscle relaxation from the contracted state, and to examine the mechanisms involved in this process and their subsequent modification by neuromuscular electrical stimulation (NMES). Single-pulse transcranial magnetic stimulation (TMS) was used to produce motor-evoked potentials (MEPs) and short-interval intracortical inhibition (SICI) in 23 healthy participants, wherein motor cortex excitability was examined at the onset of voluntary muscle relaxation following a period of voluntary tonic muscle contraction. In addition, the effects of afferent input on motor cortex excitability, as produced by NMES during muscle contraction, were examined. In particular, two NMES intensities were used for analysis: 1.2 times the sensory threshold and 1.2 times the motor threshold (MT). Participants were directed to execute constant wrist extensions and to release muscle contraction in response to an auditory “GO” signal. MEPs were recorded from the flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles, and TMS was applied at three different time intervals (30, 60, and 90?ms) after the “GO” signal. Motor cortex excitability was greater during voluntary ECR and FCR relaxation using high-intensity NMES, and relaxation time was decreased. Each parameter differed significantly between 30 and 60?ms. Moreover, in both muscles, SICI was larger in the presence than in the absence of NMES. Therefore, the present findings suggest that terminating a muscle contraction triggers transient neurophysiological mechanisms that facilitate the NMES-induced modulation of cortical motor excitability in the period prior to muscle relaxation. High-intensity NMES might facilitate motor cortical excitability as a function of increased inhibitory intracortical activity, and therefore serve as a transient trigger for the relaxation of prime mover muscles in a therapeutic context.     

Pulsed NMR studies on water in striated muscle III. The effects of water content

Pulsed NMR is used to study the kinetics of dehydration of frog gastrocnemius muscle. In addition, measurements are reported of the variation of the spin-lattice (T₁) and transverse (T₂) nuclear magnetic relaxation times of the water protons as a function of water content. The proton transverse relaxation and freezing properties of the water in muscles which had been dehydrated and then rehydrated are also investigated. Correlation of the double-exponential dehydration kinetics with the transverse relaxation at various water contents provides strong evidence for the evidence of a fraction of muscle water (10–20%) which is sufficiently strongly held to the solid substance of the muscle to make it relatively slowly removed under conditions of zero relative humidity but which is still dynamically very mobile on average. This is supported by the dependence of T₁ on water content. The relaxation times are interpreted qualitatively in terms of a number of possible effects which are at present not distinguishable. The properties of the dehydrated-rehydrated muscles indicated changes in the muscle proteins which affect the transverse relaxation of the water protons and the freezing properties of the muscle water.     

A new nuclear magnetic resonance property of lung

Inflated lung has a nuclear magnetic resonance (NMR) free-induction decay (FID) which is short compared with that of collapsed lung and those of other body tissues. An almost identically short FID is obtained from a slurry of 5-micron alumina particles in water. Interfaces between air and water in lung and between alumina and water in the slurry appear to be the source of spatial internal magnetic inhomogeneities which produce NMR line broadening and the short FID. Paired images that included lung, taken with paired symmetric and asymmetric NMR spin-echo sequences, permit the generation of an image, by subtraction, of the lung isolated from surrounding tissue. These new lung images are neither proton density, T1 (spin-lattice relaxation time), nor T2 (spin-spin relaxation time) images. They complement current NMR images and provide information about regional lung inflation. This previously unrecognized NMR property of lung tissue has potential application in NMR imaging, in quantitative determination of lung water and its distribution, and in the quantitation of regional lung inflation.     

Isometric force and endurance in soleus muscle of thyroid hormone receptor-alpha(1)- or -beta-deficient mice

The specific role of each subtype of thyroid hormone receptor (TR) on skeletal muscle function is unclear. We have therefore studied kinetics of isometric twitches and tetani as well as fatigue resistance in isolated soleus muscles of R-alpha(1)- or -beta-deficient mice. The results show 20-40% longer contraction and relaxation times of twitches and tetani in soleus muscles from TR-alpha(1)-deficient mice compared with their wild-type controls. TR-beta-deficient mice, which have high thyroid hormone levels, were less fatigue resistant than their wild-type controls, but contraction and relaxation times were not different. Western blot analyses showed a reduced concentration of the fast-type sarcoplasmic reticulum Ca(2+)-ATPase (SERCa1) in TR-alpha(1)-deficient mice, but no changes were observed in TR-beta-deficient mice compared with their respective controls. We conclude that in skeletal muscle, both TR-alpha(1) and TR-beta are required to get a normal thyroid hormone response.     

Modeling of proton spin relaxation in muscle tissue using nuclear magnetic resonance spin grouping and exchange analysis.

NMR spin relaxation experiments performed on healthy mouse muscle tissue at 40 MHz and 293 K are reported. The spin-lattice relaxation experiments were performed using different combinations of selective and nonselective radio frequency pulses. Relaxation experiments in the rotating frame at H1 = 10, 5 and 1 G are also reported. The experimental results were analyzed using the spin-grouping method, which yields the sizes of the resolved magnetization components as well as their T2's and T1's (or T1p's) for the nonexponential relaxation functions. These results were analyzed further for the exchange between different spin groups. It has been found that to explain all of these experimental data it was necessary to use a four-compartment model of the muscle tissue that consists of a lipid spin group, a "solid-like" spin group (mainly proteins), a "bulk water" spin group and a "bound water" spin group. The chemical exchange rate between "bulk" and "bound" water was found to be 29 +/- 9s-1 at room temperature. The exchange rate between the bound water and the solid moderator was estimated to be approximately 500 s-1.     

Time-resolved rotational dynamics of phosphorescent-labeled myosin heads in contracting muscle fibers.

We have measured the microsecond rotational motions of myosin heads in contracting rabbit psoas muscle fibers by detecting the transient phosphorescence anisotropy of eosin-5-maleimide attached specifically to the myosin head. Experiments were performed on small bundles (10-20 fibers) of glycerinated rabbit psoas muscle fibers at 4 degrees C. The isometric tension and physiological ATPase activity of activated fibers were unaffected by labeling 60-80% of the heads. Following excitation of the probes by a 10-ns laser pulse polarized parallel to the fiber axis, the time-resolved emission anisotropy of muscle fibers in rigor (no ATP) showed no decay from 1 microsecond to 1 ms (r infinity = 0.095), indicating that all heads are rigidly attached to actin on this time scale. In relaxation (5 mM MgATP but no Ca2+), the anisotropy decayed substantially over the microsecond time range, from an initial anisotropy (r0) of 0.066 to a final anisotropy (r infinity) of 0.034, indicating large-amplitude rotational motions with correlation times of about 10 and 150 microseconds and an overall angular range of 40-50 degrees. In isometric contraction (MgATP plus saturating Ca2+), the amplitude of the anisotropy decay (and thus the amplitude of the microsecond motion) is slightly less than in relaxation, and the rotational correlation times are about twice as long, indicating slower motions than those observed in relaxation. While the residual anisotropy (at 1 ms) in contraction is much closer to that in relaxation than in rigor, the initial anisotropy (at 1 microsecond) is approximately equidistant between those of rigor and relaxation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of temperature and Na0+ on the relaxation of phasic and tonic tension of guinea-pig ureter muscle

Effects of temperature and Na0+ on the relaxation of guinea-pig ureter smooth muscle were studied. Relaxation of phasic contraction was found to be highly temperature-dependent, practically independent of Na0+ and Ca02+, and resistant to vanadate. The relaxation of the tonic tension of both high-K and low-Na contracture was less temperature-dependent and affected by Na0+. The relaxation of tonic tension produced by introduction of Na0+ was about 3-5 times faster than that produced by Ca-free solution. La3+ ions were found to block the relaxation of the tonic component of the Na+-free contracture initiated by removal of Ca02+. Three systems of regulation of cell calcium are suggested to be operative in the ureter muscle: a fast one which is highly temperature-dependent and responsible for the relaxation of the phasic contraction (probably the sarcoplasmic reticulum), and two slow membrane-linked carriers, one of which is dependent on Na0+ (probably Na-Ca exchange) and another one which is independent of Na0+ and inhibited by La3+ (probably Ca-pump).     

Skeletal muscle relaxation rate after fasting or hypocaloric feeding

The effect of malnutrition on skeletal muscle relaxation is not entirely clear; some studies indicate no change and others a slowing of the relaxation rate. We investigated whether these different results were due to type of malnutrition, muscle fiber type composition, or the index used to express relaxation rate. The effect of a 2-day fast (16% body wt loss) or 1 wk of hypocaloric feeding (22.6% wt loss) on relaxation rates of soleus and extensor digitorum longus (EDL) muscles was studied in situ with the use of anesthetized adult Wistar rats. Relaxation rates were assessed for twitch contractions using half-relaxation times and exponential phase half-times and for tetanic contractions using exponential phase half-times. The rate of relaxation was unaffected by fasting, whereas hypocaloric feeding reduced relaxation rates after twitch and tetanic contractions in both soleus and EDL muscles. We conclude that slowing of skeletal muscle relaxation rate occurs after 1 wk of hypocaloric feeding but not after 2 days of fasting. The slowing is independent of muscle fiber composition, type of contraction, or the index used to express relaxation rate.     

23Na and flame photometric studies of the NMR visibility of sodium in rat muscle.

23Na nuclear magnetic resonance spectroscopy (NMR) is increasingly being used to study Na+ gradients and fluxes in biological tissues. However, the quantitative aspects of 23Na NMR applied to living systems remain controversial. This paper compares sodium concentrations determined by 23Na NMR in intact rat hindlimb (n = 8) and excised rat gastrocnemius muscle (n = 4) with those obtained by flame photometric methods. In both types of samples, 90% of the sodium measured by flame photometry was found to be NMR-visible. This is much higher than previously reported values. The NMR measurements for intact hindlimb correlated linearly with the flame photometric measurements, implying that one pool of sodium, predominantly extracellular, is 100% visible. From measurements on excised muscle, in which extracellular space is more clearly defined, the NMR visibility of intracellular Na+ was calculated to be 70%, assuming an extracellular space of 12% of the total tissue water volume and an extracellular NMR visibility of 100%. 23Na transverse relaxation measurements were carried out using a Hahn spin echo on both intact hindlimb (n = 1) and excised muscle (n = 2) samples. These showed relaxation curves that could each be described adequately using two relaxation times. The rapidly relaxing component showed a T2 value of 3-4 ms and the slowly relaxing component a T2 of 21-37 ms. A spin lattice relaxation (T1) measurement on intact hindlimb yielded a value of 51 ms. These relatively long relaxation times show that the quadrupolar relaxation effect of Na+ complexing to large macromolecules or being otherwise motionally restricted is relatively weak. This is consistent with the high NMR visibilities reported here.     

Carbon-13 and proton magnetic resonance of mouse muscle.

It is shown that roughly 4 mmol carbon atoms/g mouse muscle can give rise to a "high resolution" 13C NMR spectrum. From the 13C spectrum, it is estimated that the protons from mobile organic molecules or molecular segments amount to 6-8%of total nonrigid protons (organic plus water) in muscle. Their spin-spin relaxation times (T2) are of the order of 0.4-2 ms. At 37 degrees C, the proton spin-echo decay of mouse muscle changes rapidly with time after death, while that of mouse brain does not.     

Mechanics of caudal artery relaxation in control and hypertensive rats

Alterations of smooth muscle function can just as easily stem from mechanical alterations in its ability to relax as from alteration in contraction. Since a failure of arterial smooth muscle to relax may contribute to the development of hypertension, we felt it necessary to study the relaxation process in greater depth. The effect of load on the time course of relaxation of rat caudal artery smooth muscle was analyzed either by comparing afterloaded contractions against various loads or by imposing abrupt alterations in load. Unlike mammalian striated muscles in which relaxation was reported sensitive to loading conditions, relaxation in the smooth muscle of the rat caudal artery (n = 17) was found to be largely independent of loading conditions. This type of relaxation has been termed "inactivation-dependent" relaxation; it is typical of muscle tissue in which the calcium sequestering apparatus is poorly developed. Our results suggest that calcium resequestration, or some biochemical process downstream to it, is the rate-limiting step during relaxation in arterial smooth muscle and that this is not qualitatively different for hypertensive arterial smooth muscle. These analytic techniques were used in the study of relaxation of hypertensive vessels. Quantitative analysis of the relaxation curves showed that both isometric and isotonic relaxation time was prolonged in hypertensive arterial smooth muscle. Prolonged isotonic relaxation indicates that hypertensive arteries remain narrowed for prolonged periods compared with normotensive vessels. Such narrowed vessels may be a factor in the increased total peripheral resistance seen in genetic hypertension.