John S. Leigh

Myoglobin O2 desaturation during exercise. Evidence of limited O2 transport.

The assumption that cellular oxygen pressure (PO2) is close to zero in maximally exercising muscle is essential for the hypothesis that O2 transport between blood and mitochondria has a finite conductance that determines maximum O2 consumption. The unique combination of isolated human quadriceps exercise, direct measures of arterial, femoral venous PO2, and 1H nuclear magnetic resonance spectroscopy to detect myoglobin desaturation enabled this assumption to be tested in six trained men while breathing room air (normoxic, N) and 12% O2 (hypoxic, H). Within 20 s of exercise onset partial myoglobin desaturation was evident even at 50% of maximum O2 consumption, was significantly greater in H than N, and was then constant at an average of 51 +/- 3% (N) and 60 +/- 3% (H) throughout the incremental exercise protocol to maximum work rate. Assuming a myoglobin PO2 where 50% of myoglobin binding sites are bound with O2 of 3.2 mmHg, myoglobin-associated PO2 averaged 3.1 +/- .3 (N) and 2.1 +/- .2 mmHg (H). At maximal exercise, measurements of arterial PO2 (115 +/- 4 [N] and 46 +/- 1 mmHg [H]) and femoral venous PO2 (22 +/- 1.6 [N] and 17 +/- 1.3 mmHg [H]) resulted in calculated mean capillary PO2 values of 38 +/- 2 (N) and 30 +/- 2 mmHg(H). Thus, for the first time, large differences in PO2 between blood and intracellular tissue have been demonstrated in intact normal human muscle and are found over a wide range of exercise intensities. These data are consistent with an O2 diffusion limitation across the 1-5-microns path-length from red cell to the sarcolemma that plays a role in determining maximal muscle O2 uptake in normal humans.

Quantitation of time- and frequency-resolved optical spectra for the determination of tissue oxygenation

The recent development of near-infrared time- and frequency-resolved tissue spectroscopy techniques to probe tissue oxygenation and tissue oxygenation kinetics has led to the need for further quantitation of spectroscopic signals. In this paper, we briefly review the theory of light transport in strongly scattering media as monitored in the time and frequency domains, and use this theory to develop algorithms for quantitation of hemoglobin saturation from the photon decay rate (delta log R/delta t) obtained using time-resolved spectroscopy, and from the phase-shift (theta) obtained from frequency-resolved, phase-modulated spectroscopy. To test the relationship of these optical parameters, we studied the behavior of delta log R/delta t and theta as a function of oxygenation in model systems which mimicked the optical properties of tissue. Our results show that deoxygenation at varying hemoglobin concentrations can be monitored with the change in the photon decay kinetics, delta delta log R/delta t in the time-resolved measurements, and with the change in phase-shift, delta theta, in the frequency-resolved technique. Optical spectra of the adult human brain obtained with these two techniques show similar characteristics identified from the model systems.

Proteoglycan‐induced changes in T1ρ‐relaxation of articular cartilage at 4T

Proteoglycan (PG) depletion‐induced changes in T1ρ (spin‐lattice relaxation in rotating frame) relaxation and dispersion in articular cartilage were studied at 4T. Using a spin‐lock cluster pre‐encoded fast spin echo sequence, T1ρ maps of healthy bovine specimens and specimens that were subjected to PG depletion were computed at varying spin‐lock frequencies. Sequential PG depletion was induced by trypsinization of cartilage for varying amounts of time. Results demonstrated that over 50% depletion of PG from bovine articular cartilage resulted in average T1ρ increases from 110–170 ms. Regression analysis of the data showed a strong correlation (R2 = 0.987) between changes in PG and T1ρ. T1ρ values were highest at the superficial zone and decreased gradually in the middle zone and again showed an increasing trend in the region near the subchondral bone. The potentials of this method in detecting early degenerative changes of cartilage are discussed. Also, T1ρ‐dispersion changes as a function of PG depletion are described. Magn Reson Med 46:419–423, 2001.

ESR Rigid‐Lattice Line Shape in a System of Two Interacting Spins

A theory is developed to describe the rigid‐lattice line shape of an electron spin resonance signal which is influenced by dipolar coupling to a second spin. The polarization lifetime T1 of the dipolar field is assumed short. Computer generated plots are presented. Broadening, peak shifts, and amplitude changes are noted. Under certain conditions no observable broadening is predicted, merely an apparent decrease in spectrum amplitude. Experimental results for a nitroxide radical interacting with a paramagnetic ion are compared with theoretical curves. The results allow calculation of the distance between the two spins. Values for the directional angles of the second spin in the g tensor coordinate system of the observed spin are also calculated.

Primary processes in photosynthesis: Insitu ESR studies on the light induced oxidized and triplet state of reaction center bacteriochlorophyll

Summary ESR studies at liquid helium temperatures have been conducted on chromatophore and subchromatophore preparations from Chromatium D. If the primary electron acceptor of reaction center bacteriochlorophyll is chemically in a reduced state before illumination, the light activated exited state bacteriochlorophyll is prevented from undergoing oxidation. This is evidenced under these conditions by the absence of the familiar g ≅ 2 signal. Instead, a new ESR spectrum is generated in the light. This is comprised of both absorption and emission bands. The oxidation-reduction potential dependence and kinetics of the ESR changes, activated by laser pulses, suggest the signals represent bacteriochlorophyll in the triplet state. This state could be a primary intermediate in the early light activated transitions of photosynthesis.

A new method for measuring longitudinal relaxation times

The longitudinal relaxation times of two-tryptophan protons were determined by using a new pulse sequence: apply a 90° pulse, spoil field homogeneity, wait for a delay time, τ, apply a 90° sampling pulse. Relaxation times of 1.6 and 3.0 seconds were found for the low and high field aromatic doublets of tryptophan using this new pulse sequence. On the basis of the relaxation times an assignment of the low and high field aromatic doublets to the C4 and C7 ring protons is made.

Water magnetic relaxation dispersion in biological systems: The contribution of proton exchange and implications for the noninvasive detection of cartilage degradation

Magnetic relaxation has been used extensively to study and characterize biological tissues. In particular, spin-lattice relaxation in the rotating frame (T1ρ) of water in protein solutions has been demonstrated to be sensitive to macromolecular weight and composition. However, the nature of the contribution from low frequency processes to water relaxation remains unclear. We have examined this problem by studying the water T1ρ dispersion in peptide solutions (14N- and 15N-labeled), glycosaminoglycan solutions, and samples of bovine articular cartilage before and after proteoglycan degradation. We find in model systems and tissue that hydrogen exchange from NH and OH groups to water dominates the low frequency water T1ρ dispersion, in the context of the model used to interpret the relaxation data. Further, low frequency dispersion changes are correlated with loss of proteoglycan from the extra-cellular matrix of articular cartilage. This finding has significance for the noninvasive detection of matrix degradation.

Primary events in the photosynthetic reaction centre from Rhodopseudomonas spheroides strain R26: triplet and oxidized states of bacteriochlorophyll and the identification of the primary electron acceptor.

Abstract ESR studies at approximately 10 °K on the reaction centre complex of the photosynthetic bacterium Rhodopseudomonas spheroides (strain R26), have revealed bacteriochlorophyll triplet states and a component which has an ESR absorption centred at g = 1.82. The triplet-state bacteriochlorophyll is induced only in the light and is only detectable when the reaction-centre bacteriochlorophyll and its primary electron acceptor are reduced; the ESR triplet state signals are composed of both ESR absorption and ESR emission bands. The oxidation-reduction properties of the g = 1.82 component and its flash-induced kinetic behavior in relation to that of P870 are those expected for the primary electron acceptor in bacterial photosynthesis.

Relaxation times in systems with chemical exchange: Approximate solutions for the nondilute case

Abstract Approximate solutions for the relaxation times T 1 and T 2 in chemically exchanging systems are derived from the exact solutions without the usual assumptions that the system is dilute and that the relaxation time of the dilute species is much faster than that of the bulk species. The approximations are derived from less restrictive, more rigorous assumptions than have been used previously.

Relaxation times in systems with chemical exchange: Some exact solutions☆

Abstract Exact solutions for relaxation times T 1 and T 2 in chemically exchanging systems are obtained. From the solutions, approximations can be easily obtained for special conditions. These approximations are derived from less restrictive, more rigorous assumptions than have been used previously.