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PubMed1994

Basal forebrain control of cortical blood flow and tissue gases in conscious aged rats.

Cholinergic projections from the basal forebrain are capable of influencing local cortical blood flow (CoBF). The effect of age on this influence was investigated by measuring CoBF and tissue gas partial pressures (PtO2, PtCO2) by mass spectrometry in conscious young adult (2-4 months) and aged (22-28 months) Fischer 344 rats. Electrical stimulation (50 microA) of the substantia innominata (SI) increased frontal (+100.9%) and parietal (+28.4%) CoBF in young rats, but the effects were less in aged rats (frontal, +48.6%, P < 0.05; parietal, +18.9%, difference N.S.). Frontal PtO2 was increased in young but not aged rats (P < 0.01.). During standard hypercapnia, changes in CoBF, PtO2 and PtCO2 did not differ between young and aged rats. Under physostigmine infusion (0.15 mg/kg/h, i.v.), the CoBF increases to SI stimulation were approximately doubled in both cortices, in young and aged rats, and PtO2 increases were also significantly greater. However, frontal PtO2 increases were significantly smaller in aged (+7.6%) than in young (32.7%) rats, as were frontal PtCO2 reductions. We conclude: (i) the influence of the SI on frontal CoBF and PtO2 is substantially reduced with age; (ii) although physostigmine treatment potentiates this influence in both groups, the beneficial effects are relatively limited for aged rats.

Aging↗
PubMed1976

Tracer studies of cholesterol degradation induced by ionized gases.

Multiple molecular layers of cholesterol-4-C14 were bombarded in an ultra high vacuum system with various excited molecular, ionic and atomic species derived from oxygen, methane and carbon monoxide. The degradation products were analyzed by thin layer chromatography followed by autoradiography and the decomposition patterns were compared with those obtained from the autoxidation of cholesterol. Bombardment with excited and ionized species derived from both molecular oxygen and carbon monoxide leads to a number of known polar cholesterol oxidation products, similar to those formed during the natural oxidation of cholesterol. In contrast, species derived from methane lead only to the formation of products less polar than cholesterol with none of the established autoxidation products at detectable levels.

Carbon Monoxide↗
PubMed1990

Mass fractionation of noble gases in diffusion-limited hydrodynamic hydrogen escape.

Mass fractionation by hydrodynamic hydrogen escape is a promising mechanism for explaining the observed elemental and isotopic abundance patterns in terrestrial planet atmospheres. Previous work has considered only pure hydrogen winds. Here, the theory of mass fractionation by hydrogen escape is extended to atmospheres in which hydrogen is not the only major constituent. Analytical solutions are derived for cases in which all relevant atmospheric constituents escape; both analytical and numerical solutions are obtained for cases in which important heavy constituents are retained. In either case the fractionation patterns that result can differ significantly from those produced by pure hydrogen winds. Three applications of the theory are discussed: (1) The observed fractionation of terrestrial atmospheric neon with respect to mantle neon can be explained as a by-product of diffusion-limited hydrogen escape from a steam atmosphere toward the end of accretion. (2) The anomalously high Martian (SNC) 38Ar/36Ar ratio is attributed to hydrodynamic fractionation by a vigorously escaping, nearly pure hydrogen wind. (3) It is possible that the present high Martian D/H ratio was established during the same hydrodynamic escape phase that fractionated argon, but the predicted degree of D/H enrichment is sensitive to other, less well constrained parameters.

Argon↗
PubMedDate not supplied

Laboratory simulations of Titan's atmosphere: organic gases and aerosols.

Titan, the main satellite of Saturn, has been observed by remote sensing for many years, both from interplanetary probes (Pioneer and Voyager's flybys) and from the Earth. Its N2 atmosphere, containing a small fraction of CH4 (approximately 2%), with T approximately 90 K and P approximately 1.5 bar at the ground level, is irradiated by solar UV photons and deeply bombarded by energetic particles, i.e. Saturn mangetospheric electrons and protons, interplanetary electrons and cosmic rays. The resulting energy deposition, which takes place mainly below 1000 km, initiates chemical reactions which yield gaseous hydrocarbons and nitriles and, through polymerisation processes, solid aerosol particles which grow by coagulation and settle down to the ground. At the present time, photochemical models strongly require the results of specific laboratory studies. Chemical rate constants are not well known at low temperatures, charged-particle-induced reactions are difficult to model and laboratory simulations of atmospheric processes are therefore of great interest. Moreover, the synthesis of organic compounds which have not been detected to date provides valuable information for future observations. The origin and chemical composition of aerosols depend on the nature of chemical and energy sources. Their production from gaseous species may be monitored in laboratory chambers and their optical or microphysical properties compared to those deduced from the observations of Titan's atmosphere. The development of simulation chambers of Titan's extreme conditions is necessary for a better understanding of past and future observations. Space probes will sound Titan's atmosphere by remote sensing and in situ analysis in the near future (Cassini-Huygens mission). It appears necessary, as a preliminary step to test on-board experiments in such chambers, and as a final step, when new space data have been acquired, to use them for more general scientific purposes.

Aerosols↗