Plasma membrane fluidity gradients of human peripheral blood leukocytes

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Abstract
The shape of the fluidity gradient of the outer hemileaflet of the plasma membrane of normal, living, human white blood cells was determined using a series of n‐(9‐anthroyloxy) fatty acid probes where n = 2, 3, 6, 7, 9, 11, 12, and 16, to establish a baseline for future studies on the consequences of various pathological states. Fluorescence uptake and steady‐state anisotropy values were obtained with a flow cytometer capable of continuous recording over time of vertical and horizontal emission intensities, with the output of these intensities as calculated anisotropy values. The fluorescence uptake of all of the membrane probes was rapid up to about 15 min. The magnitudes of the uptake of fluorescence was, for the n‐(9‐anthroyioxy) series, in the order 2 > 3 > 6 > 7 > 9 > 11 =12 = 16 for neutrophils, lymphocytes, and monocytes. Anisotropy values were constant from 5 to 30 min after addition of the various probes. The orders of the anisotropy magnitudes, indicative of the shapes of the fluidity gradient, were, for neutrophils, 6 > 7 > 9 > 2 = 3 = 11 = 12 > 16, and for lymphocytes, 7 > 6 > 9 > 11 > 2 = 3 > 11 = 12 > 16, and for monocytes, 9 > 7 > 6 > 11 > 2 = 3 > 12 > 16. The kinetics of anisotropy from 1 to 5 min after addition of the probes differed for each of the three cell types. Probes with an n‐value less than or equal to the maxima (n = 6, neutrophils; n = 7, lymphocytes; n = 9, monocytes) rapidly (1.2 min) reached equilibrium, whereas those probes with n‐values greater than the maxima took progressively longer times to equilibrate as n increased. This behavior is consistent with the existence of an energy barrier just below the approximate region sensed by the probes, which would correspond to just below 6AS for neutrophils, 7AS for lymphocytes, and 9AS for monocytes.

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