Improved islet isolation by 10% albumin does not influence graft angiogenesis and vascularization

Addition of 10% albumin to the digestion medium has been suggested to enhance yield and integrity of harvested islets by inhibition of proteolytic activities and to improve endocrine function early after transplantation. The aim of this study was to evaluate in vivo by means of intravital fluorescence microscopy whether this rapid reversal of hyperglycemia after transplantation is due to improved graft vascularization. Pancreatic islets were isolated from Syrian golden hamsters by collagenase digestion using either solely Hank's balanced salt solution (HBSS) or HBSS supplemented with 10% human serum albumin. Islets were then transplanted into the dorsal skinfold chamber of syngeneic animals (control: N = 8 animals, n = 50 islets; albumin: N = 7, n = 41). The grafts' microvasculature was analysed on days 6, 10, and 14 after transplantation. Immunohistochemical staining for insulin was performed at the end of the microscopic observation period. Islet isolation with albumin supplementation did not increase islet yield. However, photomicroscopic analysis suggested a beneficial effect on the isolation process with improved islet integrity and prevention of outer margin irregularities, in particular in large islets. Analysis of revascularization 6 days after transplantation revealed in the control group a functional capillary density (FCD) of 477 +/- 47 cm-1. On day 10 FCD increased to 680 +/- 42 cm-1 with no further changes on day 14, indicating complete revascularization. Islets in the albumin group demonstrated a comparable FCD of 598 +/- 49 cm-1 on day 10 and complete revascularization on day 14 (655 +/- 45 cm-1). The angio-architecture of the islets was found similar in both groups, presenting with a glomerulum-like capillary network, comparable to that of pancreatic islets in situ. We conclude that the addition of 10% serum albumin to the collagenase digestion medium improves the preservation of the structural integrity of isolated pancreatic islets, however, does not influence the process of graft vascularization. Thus, improved early graft function may rather be due to superior preservation of islet cell integrity and function.