The temporal constraints of protection of neuronal damage by post-ischemic hypothermia was investigated in the gerbil model of global ischemia. Three experimental paradigms were used: 1) Hypothermia was initiated prior to ischemia followed by warming to normothermia immediately post ischemia; 2) Hypothermia of different durations was initiated immediately after reflow and 3) Six hours of hypothermia was initiated at various times following reperfusion. Hypothermia during 5 minutes of ischemia followed by warming to normal body temperature immediately post ischemia resulted in near complete protection of the hippocampus from CA1 cell loss. Hypothermic durations of 1/2, 1, 2, 4, and 6 hours beginning immediately following reperfusion resulted in progressively increased protection from ischemic damage (6 +/- 6%, 21 +/- 10%, 34 +/- 15%, 75 +/- 16% and 77 +/- 12%, respectively). Six hours of hypothermia delayed for 1 hour after reperfusion resulted in 49 +/- 9% protection. No reduction of ischemic damage was observed if 6 hours of hypothermia was delayed for 3 hour after reflow. These data suggest that: 1) Hypothermia during ischemia protects the brain from damage; 2) Hypothermia initiated immediately following reperfusion must have a duration of 2 hours or more to be effective and 3) Six hours of hypothermia is effective if initiated within 1 hour of reperfusion.

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Observations of the diurnal variations of OClO and BrO during austral spring, 1987, using long‐path visible and near‐ultraviolet absorption spectroscopy are presented and compared to simplified model calculations. It is shown that care must be taken to compare model calculations and measurements along the line of sight of the instrument. Evening twilight observations of OClO are found to be broadly consistent with current photochemical schemes, assuming ClO and BrO levels near 50 mbar of about 0.5 parts per billion by volume (ppbv) and 7 parts per trillion by volume (pptv), respectively, throughout the observing period from late August to mid‐October. Nighttime observations of OClO obtained using the Moon as a light source display evidence for growth after sunset in late August, but not in late September. Further, the observed morning twilight OClO abundances are in agreement with model calculations in late August, but they generally fall below calculations in late September and October. Observations of BrO in mid‐September show far greater evening than morning twilight abundances. It is shown that the diurnal variations of BrO and OClO in mid‐September and October can be explained by formation of the BrONO2 reservoir species at night, although other reservoir species with comparably long lifetimes could also explain the observations. If formation of BrONO2 is the correct explanation for these data, the observations suggest that NO2 levels in the Antarctic lower stratosphere are of the order of a few pptv or less in late August, a few tens of pptv in mid‐September, and a few hundred pptv in October.

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The interactions of methylene blue (MB, a cationic redox indicator and biological stain) and sodium dodecyl sulfate (SDS, a micelle-forming, anionic surfactant) in aqueous solution have been examined by using Rayleigh scattering, UV-visible absorption, and fluorescence spectroscopy. At SDS concentrations significantly below the critical micelle concentration (cmc), MB forms noncovalent dimers and aggregates with SDS that scatter light but do not fluoresce. For solutions containing 1 μM MB and < 3–5 mM SDS, shifts in the absorption spectrum characteristic of the formation of MB H-aggregates are noted. There appears to be little effect on the fluorescence emission spectrum, indicating that these MB aggregates do not fluoresce appreciably. At and above the known SDS cmc, MB is observed to interact with the micelles. The MB excited-state fluorescence lifetime (380 ps) remains constant until SDS micelles form, then increases to 615 ps. The MB rotational reorientation time similarly increases from 105 to 500 ps between 6 and 8 mM SDS. This finding suggests that the MB is encountering, on average, a microenvironment in the SDS micelles that is 5-fold more viscous than liquid water or the molar volume of the MB/SDS species that is reorienting is 5-fold larger than MB in water.

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