Four experiments were conducted to determine the permeability coefficient of human sperm to water (Lp) and its activation energy (Ea). Critical tonicity (tonicity at which 50% of the cells swell and lyse) was determined by equilibrating sperm to 22 degrees C (experiments 1a and 1b), 30, 22, 8, or 0 degrees C (experiment 2a), and 0, -1, -3, -5, or -7 degrees C (experiment 2b) and then exposing them to various hypotonic media (215-3 mOsm). For Lp determination, sperm were equilibrated to 30, 22, 8, or 0 degrees C (experiment 3a), 8, 0, or -3 degrees C (experiment 3b), and -1, -3, -5, or -7 degrees C (experiment 3c), and then were exposed for increasing times to hypotonic (40 mOsm) media. Activation energies were calculated from the results of the latter experiments (experiment 4). Results indicate a temperature-dependent (p < 0.05) critical tonicity, with sperm exhibiting an increased membrane fragility at 8, 0, and -7 degrees C, relative to 30, 22, -1, -3, or -5 degrees C (67.5 +/- 2.4, [mean +/- SEM], 62.7 +/- 2.3, and 61.9 +/- 3.7 mOsm vs. 57.4 +/- 3.4, 57 +/- 1.2, 54.8 +/- 3.4, 60.1 +/- 5.3, and 59.8 +/- 5.2 mOsm, respectively). Human sperm have an Lp of 2.40 +/- 0.20 microns/min/atm at 22 degrees C and an Ea of 3.92 +/- 0.59 kcal/mol between 30 and -7 degrees C. The Ea for cells incubated at temperatures above 0 degrees C (3.92 kcal/mol) show an apparent discontinuity (p < 0.004) in water permeability in supercooled conditions (7.48 kcal/mol). These data suggest that 1) human sperm have a high Lp and low Ea, relative to other cell types, above 0 degrees C; and 2) this high Lp and its low Ea change significantly below 0 degrees C.
