Venous gas embolism as a predictive tool for improving CNS decompression safety

A key process in the pathophysiological steps leading to decompression sickness (DCS) is the formation of inert gas bubbles. The adverse effects of decompression are still not fully understood, but it seems reasonable to suggest that the formation of venous gas emboli (VGE) and their effects on the endothelium may be the central mechanism leading to central nervous system (CNS) damage. Hence, VGE might also have impact on the long-term health effects of diving. In the present review, we highlight the findings from our laboratory related to the hypothesis that VGE formation is the main mechanism behind serious decompression injuries. In recent studies, we have determined the impact of VGE on endothelial function in both laboratory animals and in humans. We observed that the damage to the endothelium due to VGE was dose dependent, and that the amount of VGE can be affected both by aerobic exercise and exogenous nitric oxide (NO) intervention prior to a dive. We observed that NO reduced VGE during decompression, and pharmacological blocking of NO production increased VGE formation following a dive. The importance of micro-nuclei for the formation of VGE and how it can be possible to manipulate the formation of VGE are discussed together with the effects of VGE on the organism. In the last part of the review we introduce our thoughts for the future, and how the enigma of DCS should be approached

OXYGEN DEPENDENCE OF PHOTOINHIBITION AT LOW-TEMPERATURE IN INTACT PROTOPLASTS OF VALERIANELLA-LOCUSTA L

Photoinhibition of photosynthesis in vivo is shown to be considerably promoted by O2 under circumstances where energy turnover by photorespiration and photosynthetic carbon metabolism are low. Intact protoplasts of Valerianella locusta L. were photoinhibited by 30 min irradiation with 3000-mu-mol photons.m-2.s-1 at 4-degrees-C in saturating [CO2] at different oxygen concentrations, corresponding to 2-40% O2 in air. The photoinhibition of light-limited CO2-dependent photosynthetic O2 evolution increased with increasing oxygen concentration. The uncoupled photochemical activity of photosystem II, measured in the presence of the electron acceptor 1,4-benzoquinone, and maximum variable fluorescence, F(V), were strongly affected and this inhibition was closely correlated to the O2 concentration. The effect of O2 did not saturate at the highest concentrations applied. An increase in photoinhibitory fluorescence quenching with [O2], although less pronounced than in protoplasts, was also observed with intact leaves irradiated at 4-degrees-C in air. Initial fluorescence, F(O), was slightly (about 10%) increased by the inhibitory treatments but not influenced by [O2]. A long-term cold acclimation of the plants did not substantially alter the O2-sensitivity of the protoplasts under the high-light treatment. From these experiments we conclude that oxygen is involved in the photoinactivation of photosystem II by excess light in vivo

PHOTOINHIBITION OF PHOTOSYSTEM-II INVIVO IS PRECEDED BY DOWN-REGULATION THROUGH LIGHT-INDUCED ACIDIFICATION OF THE LUMEN - CONSEQUENCES FOR THE MECHANISM OF PHOTOINHIBITION INVIVO

The mechanism of photoinhibition of photosystem II (PSII) was studied in intact leaf discs of Spinacia oleracea L. and detached leaves of Vigna unguiculata L. The leaf material was exposed to different photon flux densities (PFDs) for 100 min, while non-photochemical (q(N)) and photochemical quenching (q(P)) of chlorophyll fluorescence were monitored. The 'energy' and redox state of PSII were manipulated quite independently of the PFD by application of different temperatures (5-20-degrees-C), [CO2] and [O2] at different PFDs. A linear or curvilinear relationship between qp and photoinhibition of PSII was observed. When [CO2] and [O2] were both low (30 mul.l-1 and 2%, respectively), PSII was less susceptible at a given q(P) than at ambient or higher [CO2] and photoinhibition became only substantial when q(P) decreased below 0.3. When high levels of energy-dependent quenching (q(E)) (between 0.6 and 0.8) were reached, a further increase of the PFD or a further decrease of the metabolic demand for ATP and NADPH led to a shift from q(E) to photoinhibitory quenching (q(I)). This shift indicated that photoinhibition was preceded by down-regulation through light-induced acidification of the lumen. We propose that photoinhibition took place in the centers down-regulated by q(E). The shift from q(E) to q(I) occurred concomitant with q(P) decreasing to zero. The results clearly show that photoinhibition does not primarily depend on the photon density in the antenna, but that photoinhibition depends on the energy state of the membrane in combination with the redox balance of PSII. The results are discussed with regard to the mechanism of photoinhibition of PSII, considering, in particular, effects of light-induced acidification on the donor side of PSII. Interestingly, cold-acclimation of spinach leaves did not significantly affect the relationship between q(P), q(E) and photoinhibition of PSII at low temperature

KINETIC RESOLUTION OF DIFFERENT RECOVERY PHASES OF PHOTOINHIBITED PHOTOSYSTEM-II IN COLD-ACCLIMATED AND NON-ACCLIMATED SPINACH LEAVES

Leaf discs from spinach were exposed to a photon flux density of 1 250 mumol m-2 s-1 at 5-degrees-C for 2 or 3 h in ambient air. Photoinhibition of photosystem II (PS II) was measured by means of chlorophyll fluorescence. Recovery of photosystem II was followed at 6-degrees-C and 20-degrees-C in low light or darkness for periods up to 12 h. The experimental setup allowed kinetic resolution of different phases of recovery. The experiments revealed a temperature dependent dark recovery phase and two distinct light- and temperature dependent phases: (1) A relatively fast, light dependent recovery phase occurred in parallel with partial recovery of basic fluorescence at 6-degrees-C and 20-degrees-C. A population of PS II centers with very slow fluorescence induction kinetics, which had accumulated during photoinhibition treatment, disappeared during this phase. This fast recovery phase is proposed to represent reactivation of photoinhibited PS II, without dissassembly or incorporation of new D1-protein. (2) A relatively slow light-dependent recovery phase took place at 20-degrees-C, but not at 6-degrees-C. In the presence of the chloroplast translation inhibitor streptomycin, part of the 2nd phase was inhibited. This phase is proposed to involve assembly of new Photosystem II centers, which is partly dependent on de novo synthesis of D1-reaction center protein, but presumably is also using a preexisting pool of D1-protein. Cold acclimation of the leaves resulted in a decreased sensitivity for photoinhibition of photosystem II. Recovery of photoinhibited photosystem II at 6-degrees-C of the cold-acclimated leaves was faster than in non-acclimated leaves, but this effect can be ascribed to diminished photoinhibitory damage