Effect of temperature on the fluidity of boar sperm membranes

Abstract

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.