Effect of temperature on the fluidity of boar sperm membranes

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Abstract
Fluidity was used to assess changes in molecular organization of boar spermatozoa plasma membranes from (1) the head (2) rest of the sperm body and acrosome as a consequence of temperature. The initial fluidity of the head membranes at 25.degree.C was less than of the sperm body membranes (P < 0.05). When held at 25.degree.C, the fludity of the head membranes decreased for 105 .+-. 8 min and then stabilized for the remainder of the 160-min incubation. Calcium (10 mM) caused a significantly greater decrease in fludity. The fludity of the sperm body membranes increased slightly over time in the absence of Ca2+, but decreased significantly with Ca2+. Cooling from 25 to 5.degree.C and subsequent heating to 40.degree.C (0.4.degree.C/min) caused marked alterations in the fluidity of each membrane. Cooling the head membranes prevented the fluidity increase seen at 25.degree.C, while reheating caused a dramatic decrease in fludity. Fluidity of the head membranes was now unaffected by Ca2+. Lipid phase transitions, indicated by sharp break points in data curves, were detected at the onset of reheating (7 .+-. 3.degree.C) and at 23 .+-. 4.degree.C during reheating. Fluidity of the sperm body membranes decreased slightly and in a linear fashion with Ca2+. Without Ca2+, the sperm body membranes showed an additional lipid phase shift at 31 .+-. 5.degree.C, which led to a rapid fall in fluidity. These results suggest that the fludity, and therefore the molecular structure, of sperm head and body membranes differ. The head plasma membranes, and to a lesser extent the sperm body membranes, undergo a fluidity change over time, which may reflect the structural reogranization of capacitation. This fluidity pattern is significantly disrupted by cooling and reheating.

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