Effects of cell concentration on viability and metabolic activity during cryopreservation

Increasing the cell concentration during the cryopreservation of red blood cells (RBC) increased hemolysis. Similarly, increasing the cell concentration during the cryopreservation of hepatocytes reduced both the viability of the cells as assessed by trypan blue exclusion and the metabolic activity of the trypan blue-excluding cells. In both cell types, significant damage appeared at cell concentration levels exceeding 60%. With tightly packed RBC, hemolysis reached 63%, while for hepatocytes after thawing and dilution, trypan blue viability was reduced to 41% of that of the cells initially isolated. The viability of cryopreserved hepatocytes, estimated by trypan blue exclusion, was considerably reduced following incubation for 1 h at 37 degrees C and the extent of this reduction was also a function of cell concentration during freezing and thawing. The trypan blue viability of hepatocytes that were tightly packed during cryopreservation and then incubated at a concentration of 5 x 10(6) viable cells/ml fell dramatically to only 12.5%. The protein-synthesizing activity and the membrane transport activity of these cells, expressed in terms of cells that excluded trypan blue immediately following thawing and removal of the cryoprotectant, were reduced to 38.7 and 33.5%, respectively, of the activity in cells that were packed at 10% cell concentration. The fall in metabolic activity may have been due to complete loss of activity in some of the cells or reduced activity in most or all of the cells or any combination of these factors. It is concluded that there may be many mechanisms involved, but the most important factor is probably stresses produced by unphysiological cell-cell contacts occurring during the freeze-thaw cycle.