Assessment of intracellular insulin content during all steps of human islet isolation procedure

This study investigated the recovery of pancreatic insulin content during human islet isolation prior to and after digestion-filtration, continuous Hanks-Ficoll gradient purification (n = 20), and 3-4 day culture at 22°C (n = 6). The native insulin content varied in a wide range from 28.4 U to 360.8 U/pancreas. After digestion the initially measured average insulin content of 115.8 ± 20.8 U/pancreas (mean ± SEM) increased to 264.6 ± 22.8% (p < 0.001). This increase of insulin during pancreas digestion was attributed to the asymetrical distribution of insulin within the pancreas. Sampling of insulin within the pancreatic caput seemed not to be representative for the insulin content of the complete native organ, because the ratio of insulin per gram tissue within the pancreatic cauda compared to the caput (n = 5) was 2.4 ± 0.4 (p < 0.05). After purification total insulin recovery was 55.3 ± 4.8% (p < 0.001). Because recovery of islet equivalent number (IEQ) (83.7 ± 4.4%) exceeded insulin recovery, insulin/IEQ ratio decreased from 656.8 ± 70.6 μU/IEQ before purification to 436.4 ± 58.1 μU/IEQ (p < 0.001) after purification. After 22°C culture (n = 6) recovery of insulin and IEQ was 80.1 ± 8.1% (p < 0.05) and 92.8 ± 3.5% (p = Ns), respectively. Insulin content per IEQ decreased to 85.8 ± 6.5% (p < 0.05). This study clearly shows that most of islet insulin is lost during purification. This seems to be caused rather by an amplified insulin release than by the loss of islets itself. This release may facilitate the separation of endocrine and exocrine tissue by gradient centrifugation, but may also accelerate islet exhaustion detrimental for long-term insulin independence.