The Rat Pancreas Is Not an Appropriate Model for Testing the Preservation of the Human Pancreas with the Two-Layer Method (2006)

Expanding the available human donor pool (e.g., to nonheartbeating donors) is very important for islet transplantation and the well executed study recently published in Transplantation by Tanaka and co-workers (1) was aimed at contributing to this goal. In this study the authors investigated the ability of cold storage with the two-layer method (TLM) to improve islet isolation outcome after exposure of rat pancreata to warm ischemia (WI) for times similar to those expected with human nonheartbeating donors. The authors demonstrated that 3-hour cold storage with the TLM following WI resulted in significant improvements in islet yield and quality when WI duration was 30 min but not 45. These improvements correlated with increased pancreas ATP levels prior to islet isolation, suggesting improved oxygenation, which is widely believed to be associated with the TLM. Based on these findings the authors concluded that the TLM may improve islet isolation outcomes from pancreata derived from human nonheartbeating donors. However, as long as improved oxygenation remains a plausible explanation for the TLM's mechanism of action, the use of the rat pancreas as a model for clinical applications is problematic. The problem is that oxygen transfer inside a pancreas preserved with the TLM depends on diffusion, making the organ's level of oxygenation highly dependent on its thickness. Direct oxygen partial pressure (pO2) measurements carried out in 10 mm thick porcine pancreatic segments preserved with the TLM at 8°C have shown that pO2 in their core was virtually zero (2). Theoretical calculations show that even at 4°C and in the absence of pO2 gradients in preservation media equilibrated with pure oxygen (pO2=760 mm Hg at the medium/organ interface), oxygen penetration depth into the organ is only about 1 mm (2). Our predictions for oxygenation of preserved human and rat pancreata are shown in Figure 1. The diffuse rat pancreas is just 1–2 mm thick in the head region and about 4 mm thick in the splenic region (Bonner-Weir S, personal communication). For these small thicknesses, the predicted oxygenated volume fraction at 4°C is 100% and about 60%, respectively, in agreement with the high ATP levels observed by Tanaka and co-workers (1). In contrast, the predicted oxygenated volume fraction for a 25 mm thick human organ is less than 20% (2). The TLM should indeed enhance pancreas oxygenation by decreasing temporal and spatial pO2 gradients in the preservation media and possibly by improving oxygen diffusion inside the pancreas due to perfluorocarbon penetration into the organ. However, the fraction of the pancreas volume that can be oxygenated is much higher in a small organ. Therefore, any phenomenon that depends on oxygenation is likely to be affected differently in a rat versus a human pancreas.FIGURE 1.: Predicted fraction of rat and human pancreas volume that is oxygenated by diffusion during preservation with the TLM at 4°C. This fraction is 100% for the 1- to 2-mm thick head region of a rat pancreas, close to 60% for the 4-mm thick splenic region of a rat pancreas, and less than 20% for a cylindrical 25-mm diameter pancreas (simulating a human organ). See Reference 2 for assumptions used for these predictions.The purpose of this letter is to alert the community to the concept that, as far as oxygenation and possibly other diffusion phenomena are concerned, the rat (and to a lesser extent even the canine) pancreas is not an appropriate model for investigating preservation methods for clinical applications. Efstathios S. Avgoustiniatos Bernhard J. Hering Klearchos K. Papas Diabetes Institute for Immunology and Transplantation Department of Surgery University of Minnesota Minneapolis, MN