Thermotropic lipid phase separations in human platelet and rat liver plasma membranes

Electron spin resonance (ESR) studies were conducted on human platelet plasma membranes using 5-nitroxide stearate, I(12,3). The polarity-corrected order parameter S and polarity-uncorrected order parameters S(T parallel) and S(T perpendicular) were independent of probe concentration at low I(12.3)/membrane protein ratios. At higher ratios, S and S(T perpendicular) decreased with increasing probe concentration while S(T parallel) remained unchanged. This is the result of enhanced radical interactions due to probe clustering. A lipid phase separation occurs in platelet membranes that segregates I(12,3) for temperatures less than 37 degrees C. As Arrhenius plots of platelet acid phosphatase activity exhibit a break at 35 to 36 degrees C, this enzyme activity may be influenced by the above phase separation. Similar experiments were performed on native [cholesterol/phospholipid ratio (C/P) = 0.71] and cholesterol-enriched [C/P = 0.85] rat liver plasma membranes. At 36 degrees C, cholesterol loading reduces I(12,3) flexibility and decreases the probe ratio at which radical interactions are apparent. The latter effects are attributed to the formation of cholesterol-rich lipid domains, and to the inability of I(12,3) to partition into these domains because of steric hinderance. Cholesterol enrichment increases both the high temperature onset of the phase separation occurring in liver membranes from 28 degrees to 37 degrees C and the percentage of probe-excluding, cholesterol-rich lipid domains at elevated temperatures. A model is discussed attributing the lipid phase separation in native liver plasma membranes to cholesterol-rich and -poor domains. As I(12,3) behaves similarly in cholesterol-enriched liver and human platelet plasma membranes, cholesterol-rich and -poor domains probably exist in both systems at physiologic temperatures.     

SPRING MIGRATION OVER PUERTO RICO AND THE WESTERN ATLANTIC, A RADAR STUDY

Migration over Puerto Rico was recorded by time-lapse filming of the display of a long-range surveillance radar on 40 days and 37 nights in the period 2 March-29 May 1971. Moderate density movements occurred every night; low density movements occurred on most days. Many birds, primarily passerines, took off from Puerto Rico each evening at 20–45 minutes after sunset.
Almost all birds flew to the west, NW or north. Birds were seen approaching from the direction of the Windward Islands and Venezuela, over Puerto Rico, and departing towards the Bahamas and eastern coast of the U.S. Uni- and multivariate analyses showed that the number of birds departing W-N each evening was positively correlated with following winds.
There is less night-to-night variation in the amount of migration at Puerto Rico than in eastern North America. However, this is apparently the result of less variable weather in the tropics, not the result of any lesser degree of meteorological selectivity by the migrants.
The tracks of the birds were correlated with wind direction. Birds moved WNW-NW with NE side winds but NW-NNW with SE following winds. The tracks were rarely exactly downwind. The variance amongst the directions of individual birds at any given time was usually small and not correlated with cloud cover or magnetic disturbances. The estimated headings of the birds varied from day to day in a pattern suggesting adjustment of headings to compensate at least partially for lateral wind drift.
In autumn many birds approach Puerto Rico from the north or even east of north; in spring few birds moved in the opposite directions. This difference in routes takes advantage of prevailing wind patterns.