Summary. Fluidity was used to assess changes in molecular organization of boar spermatozoa plasma membranes from (1) the head and (2) the rest of the sperm body and acrosome as a consequence of temperature. The initial fluidity of the head membranes at 25°C was less than that of the sperm body membranes (P < 0·05). When held at 25°C, the fluidity 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 fluidity. The fluidity of the sperm body membranes increased slightly over time in the absence of Ca2+, but decreased significantly with Ca2+. Cooling from 25 to 5°C and subsequent heating to 40°C (0·4°C/min) caused marked alterations in the fluidity of each membrane. Cooling the head membranes prevented the fluidity increase seen at 25°C, while reheating caused a dramatic decrease in fluidity. 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°C) and at 23 ± 4°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°C, which led to a rapid fall in fluidity. These results suggest that the fluidity, 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 reorganization of capacitation. This fluidity pattern is significantly disrupted by cooling and reheating.
Keywords: boar; spermatozoa; membranes; fluidity; temperature
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