CALCULATED OPTIMAL COOLING RATES FOR RAM AND HUMAN SPERM CRYOPRESERVATION FAIL TO CONFORM WITH EMPIRICAL OBSERVATIONS

The permeability coefficient to water (I-p) and its associated activat ion energy (E(2)) were measured for ram (8.47 mu m/min/ atm at 25 degr ees C, 1.06 kcal/mol) and human (2.89 mu m/min/atm at 30 degrees C, 1. 93 kcal/mol) spermatozoa. By use of these figures, predictive water lo ss curves were calculated, from published equations, for different coo ling rates from 100 degrees C/min to 100 000 degrees C/min The calcula ted curves show that ram spermatozoa cooled at even the fastest rate w ould be in osmotic equilibrium by -20 degrees C, and human spermatozoa cooled at rates up to 10 000 degrees C/min would be in equilibrium by -15 degrees C. If the nucleation temperature for spermatozoa is taken to be between -20 degrees C and -30 degrees C, then ram and human spe rmatozoa cooled at these rates would apparently not exhibit any intrac ellular freezing. There is a significant discrepancy between these cal culated optimal cooling rates and the published empirically derived op timal rates of 50 degrees C/min for ram and 10 degrees C/min for human . The failure of ram and human spermatozoa to conform with the establi shed and previously successful model for prediction of optimal cooling rates suggests that damage sustained at high cooling rates may be unr elated to intracellular ice formation.