Differential freezing tolerance of rat pancreatic islets depending on their size variation

We have investigated the freezing tolerance of rat pancreatic islets. Freshly isolated rat pancreatic islets were divided into three groups based on their longest diameter (small; 100 - 200 microns, medium; 201 - 300 microns, large; > 300 microns). They were then cryopreserved at a slow cooling rate (-0.3 degrees C/min) in the presence of dimethyl sulfoxide (Me2SO) or ethylene glycol (EG). After storage at -196 degrees C for 1 - 4 weeks, they were thawed and their ability to secrete insulin in response to fluctuations in glucose concentration was examined during three consecutive static incubations in vitro (1st; 2.8 mM, 2nd; 16.7 mM, 3rd; 2.8 mM). Morphological examination of the beta-granule population was determined by image analysis, and correlation with islets size was analyzed. The amount of insulin released from large-sized islets was significantly suppressed in EG (p < 0.05) and Me2SO (p < 0.01) groups compared to unfrozen islets. However, the mean volume of the large-sized islets isolated from one rat accounted for 43.0% of the total volume. On the other hand, the amount of insulin released from small- and medium-sized islets did not differ from those of unfrozen islets, and their mean volumes were 13.2 and 43.8% respectively. The percentage of cells with beta-granules was significantly correlated with size in both EG (r = -0.52) and Me2SO (r = -0.35) groups, but no significant correlation was observed in the unfrozen islets groups. These findings suggest that large-sized islets are more susceptible to freezing injury than small- or medium-sized islets. Moreover, the volume distribution of isolated islets indicated that it may be important to retain the ability of insulin secretion from the large-sized islets.