Non-frozen cold storage is favorable for islet function and morphology

Freezing has been shown to damage pancreatic islets and to disrupt their insulin release, probably because of intracellular ice formation. We compared frozen islets with fresh ones and with others stored at temperatures above freezing from a standpoint of insulin release response to glucose and transplantation. Group A islets, isolated from rats and immersed in 10% dimethyl sulfoxide in RPMI 1640, were stored at −2°C, and group B islets at −196°C, for 7 days. As for group B, the islets were cooled at 1°C/min from room temperature to −40°C, subsequently at 3°C/min to −80°C and then put into liquid nitrogen to be rapidly frozen to −196°C. The control islets were fresh. In vitro, basal release at 3.3 mM glucose was similar in group A to that in the controls, but was higher in group B than group A. Stimulated release against 16.7 mM glucose was lower in group A than group B. However, insulin responsiveness, i.e., the ratio of insulin release at 16.7 mM glucose to that at 3.3 mM glucose, was lost in group B. Freezing also caused damage to the group B cells visible under the light and electron microscopes, while group A islets were largely intact. In vivo, after 600 islets were transplanted into streptozotocin-induced diabetic rats, group A was better able to lower fasting blood glucose than was group B, and remained so for 4 weeks. Above sub-zero preservation in the non-frozen state thus seems adequate for the short-term storage for 7 days.