Thermotropic Phase Transitions in the Plasma Membrane of Ram Spermatozoa

A steady-state fluorescence polarization technique, using the membrane probe 1,6-diphenyl-1,3,5-hexatriene (DPH), showed that separately detectable transitions occurred in the regions of 17, 26 and 36 degrees C in isolated preparations of ram sperm plasma membrane. An independent technique based on the temperature-related behaviour of calcium- and magnesium-activated ATPase detected a single phase transition in the region of 24 degrees C. Modulation of ATPase by neighbouring lipid composition was inferred from findings that phospholipase A2 caused significant stimulation of the enzyme. Cholesterol-rich liposomes caused an upward shift of the phase-transition temperature from 24 degrees C to 30 degrees C, but the reasons for this are unclear. It is considered that these phase transitions may have profound effects on sperm survival and physiology, both during normal fertilization processes and in response to cryostorage.