The influence of cooling rate and warming rate on the response of renal cortical slices frozen to -40 degrees C in the presence of 2.1 M cryoprotectant (ethylene glycol, glycerol, or dimethyl sulfoxide)

Thermal properties of ethylene glycol aqueous solutions

Preventing ice crystallization by transforming liquids into an amorphous state, vitrification can be considered as the most suitable technique allowing complex tissues, and organs cryopreservation. This process requires the use of rapid cooling rates in the presence of cryoprotective solutions highly concentrated in antifreeze compounds, such as polyalcohols. Many of them have already been intensively studied. Their glass forming tendency and the stability of their amorphous state would make vitrification a reality if their biological toxicity did not reduce their usable concentrations often below the concentrations necessary to vitrify organs under achievable thermal conditions. Fortunately, it has been shown that mixtures of cryoprotectants tend to reduce the global toxicity of cryoprotective solutions and various efficient combinations have been proposed containing ethanediol. This work reports on the thermal properties of aqueous solutions with 40, 43, 45, 48, and 50% (w/w) of this compound measured by differential scanning calorimetry. The glass forming tendency and the stability of the amorphous state are evaluated as a function of concentration. They are given by the critical cooling rates v(ccr)above which ice crystallization is avoided, and the critical warming rates v(cwr) necessary to prevent ice crystallization in the supercooled liquid state during rewarming. Those critical rates are calculated using the same semi-empirical model as previously. This work shows a strong decrease of averaged critical cooling and warming rates when ethanediol concentration increases, V(ccr) and V(cwr) = 1.08 x 10 (10) K/min for 40% (w/w) whereas V(ccr) = 11 and V(cwr) = 853 K/min for 50% (w/w). Those results are compared with the corresponding properties of other dialcohols obtained by the same method. Ethylene glycol efficiency is between those of 1,2-propanediol and 1,3-propanediol.

Biotransformation activity in vitrified human liver slices

In vitro testing of human liver for biotransformation or xenobiotic metabolism studies has been limited by unpredictable acquisition of samples. Consequently, it has become necessary to consider methods to cryopreserve and store these samples whenever they do become available for culture of the revived tissue at a more convenient time. Human liver slices were cryopreserved by vitrification, which allows for the transfer of aqueous media to low temperatures (-196 degrees C) without the formation of ice crystals. Human liver slices were exposed to increasing concentrations of 1,2-propanediol up to a final concentration of 4.76 M in fetal calf serum. Slices were then vitrified by direct immersion into liquid nitrogen and warmed by submersion in 37 degrees C fetal calf serum. Warming was done either immediately or after 4 and 8 weeks of storage under liquid nitrogen. The effects of vitrification, storage time, and warming on biotransformation were determined by assessing the integrated metabolism of 7-ethoxycoumarin (7-EC). Vitrified or fresh human liver slices were exposed to 50 microM 7-EC and its primary metabolite 7-hydroxycoumarin (7-HC) in organ culture for up to 6 hr. Metabolite production of both fresh and vitrified liver slices was compared. Retention of the inherent biotransformation rate was usually high and seemed independent of storage time. Integration of both cytochrome P450-mediated and secondary conjugation processes was retained in cryopreserved tissue. Vitrification offers a way to cryopreserve human liver slices for the study of xenobiotic metabolism in humans.

Cryopreservation of pig and human liver slices

The ability to cryopreserve human liver slices would greatly enhance the opportunities to test potentially hepatotoxic drugs and environmental contaminants as well as the metabolism of these compounds. This study focused on trying to cryopreserve pig and human liver slices. Since the acquisition of human liver tissue is unpredictable and scarce, an animal model was sought to predict problems associated with cryopreservation of human tissue. The pig liver was chosen because of its anatomical and physiological resemblance to human liver. The human liver tissues that did become available were obtained through the Arizona Organ Bank and the National Disease Research Interchange and from surgical liver resections. An in vitro culture system that employed precision-cut liver slices was used in this study. Different types and concentrations of cryoprotectants, cooling rates, and culture media were all tried in an attempt to cryopreserve pig and human liver slices. The viabilities of fresh and cryopreserved liver slices were evaluated using slice K+ retention and protein synthesis. Pig liver slices following cryopreservation retained between 80 and 85% of intracellular K+ content and protein synthesis as compared to controls using 1.4 M Me2SO, a 12 degrees C/min cooling rate, and a rapid rewarming rate of direct submersion of the slice into 37 degrees C fetal calf serum. Human liver slices following cryopreservation retained between 54 and 89% of intracellular K+ content and protein synthesis as compared to controls using the same protocol as for pigs, except that lower cooling rates were giving better results. The large variation seen in cryopreserved human liver slices was due to the length of warm and cold ischemia to which the tissue was exposed before arriving at the laboratory. This study indicated that pig and human liver slices can be cryopreserved and used for future toxicological and metabolic studies.