Markedly Decreased Oxygen Tension in Transplanted Rat Pancreatic Islets Irrespective of the Implantation Site

Abstract

In this study, we syngeneically transplanted islets to three different implantation sites of diabetic and nondiabetic rats, then 9 -12 weeks later we measured the blood perfusion and compared the tissue partial pressure of oxygen (PO 2 ) levels of these transplanted islets to endogenous islets. Modified Clark microelectrodes (outer tip diameter 2-6 m) were used for the oxygen tension measurements, and islet transplant blood perfusion was recorded by laser-Doppler flowmetry (probe diameter 0.45 mm). The islet graft blood perfusion was similar in all islet grafts, irrespective of the implantation site. In comparison, the three implantation organs (the kidney cortex, liver, and spleen) differed markedly in their blood perfusion. There were no differences in islet graft blood perfusion between diabetic and nondiabetic recipients. Within native pancreatic islets, the mean PO 2 was ϳ40 mmHg; however, all transplanted islets had a mean PO 2 of ϳ5 mmHg. The oxygen tension of the grafts did not differ among the implantation sites. In diabetic recipients, an even lower PO 2 level was recorded in the islet transplants. We conclude that the choice of implantation site seems less important than intrinsic properties of the transplanted islets with regard to the degree of revascularization and concomitant blood perfusion. Furthermore, the mean PO 2 level in islets implanted to the kidney, liver, and spleen was markedly decreased at all three implantation sites when compared with native islets, especially in diabetic recipients. These results are suggestive of an insufficient oxygenization of revascularized transplanted islets, irrespective of the implantation site. Diabetes 50: -495, 2001A factor of potential importance in the failure of islet grafts is poor or inadequate engraftment of the islets in the implantation organ. Normally, pancreatic islets have a dense glomerular-like capillary network in which the capillaries course through the islet in a tortuous fashion that is ideal for the delivery of oxygen and nutrients to the islet cells and for the dispersal of the secreted hormones to the target organs (1,2). This pancreatic islet angioarchitecture entails a blood perfusion of the pancreatic islets that is 10 times higher than that in the exocrine pancreas and similar to that seen in the renal cortex (ϳ5-7 ml ⅐ min Ϫ1 ⅐ g -1 ) (3-6). However, during the process of isolation and in vitro culture of pancreatic islets preceding transplantation, the islet vasculature dedifferentiates or degenerates (7,8). Therefore, immediately after transplantation, the pancreatic islets are supplied with oxygen and nutrients solely by diffusion from the surrounding tissues. The revascularization process is initiated within a few days, and the islets are generally thought to be fully revascularized by 1 month posttransplantation (9,10). We had previously observed markedly decreased oxygen tension in islets transplanted beneath the renal capsule at 1 month posttransplantation, a decrease that was even more pronounced in diabetic animals (11)....

Better vascular engraftment and function in pancreatic islets transplanted without prior culture

“…At the time of transplantation, 250 (rat) or 200 (mouse) freshly isolated (<4 h after pancreas extirpation) or cultured (4-7 days of culture) islets were packed in a braking pipette and implanted beneath the renal capsule on the dorsal side of the left kidney of syngeneic normoglycaemic rats that had been anaesthetised with pentobarbital (60 mg/kg, i.p., Apoteket, Umeå, Sweden) or of syngeneic alloxan-diabetic mice that had been anaesthetised with Avertin (0.02 ml/g i.p, of a 2.5% [vol/vol] solution of 10 g 97% [vol/vol] 2,2,2,-tribromoethanol [Sigma-Aldrich] in 10 ml of 2-methyl-2-butanol [Kemila, Stockholm, Sweden]). For practical reasons, normoglycaemic rats were, used as recipients in the engraftment studies, since there is no difference in revascularisation and oxygenation of transplanted islets, compared to when the islets are implanted to cure diabetic recipients instead [19,24]. In all cases exocrine contamination of the transplanted islets was avoided as much as possible, since this has previously been observed to have a negative influence on the engraftment process of transplanted islets [25].…”

“…However, recent studies suggest that endothelial cells originating from the donor may also contribute significantly, and be important for the revascularisation process [16,17]. In our previous studies, we have observed that grafts composed of cultured rodent or human islets are not sufficiently revascularised, which results in a low graft oxygen tension and tissue acidosis [18][19][20][21][22]. The present study tested the hypothesis that syngeneic islet grafts composed of freshly isolated rodent islets become more efficiently revascularised than islets transplanted after culture, and that this might result in an improved islet graft function.…”

Function of a genetically modified human liver cell line that stores, processes and secretes insulin

“…This suggests that regulated insulin secretion in these grafts was small, with lowering of blood glucose levels in the animals being largely the result of constitutive release of insulin. This functional difference between the cells in vitro and in vivo may be caused by the effects of transplantation, such as lower levels of oxygen, 23 which are critical for insulin secretion. When the cells in the grafts were removed from the mice and cultured in vitro, the insulin content was similar to that of untransplanted cells, 5.1-5.6 pmol/10 6 cells.…”

Stimulation of vascularization of a subcutaneous scaffold applicable for pancreatic islet‐transplantation enhances immediate post‐transplant islet graft function but not long‐term normoglycemia

“…The liver as transplantation side is considered to be a major contributor to these low success rates. Inadequate revascularization,5 the large numbers of natural killer T cells in the liver,6 high concentrations of immunosuppressive drugs,7, 8 and the instant blood‐mediated inflammatory reaction9 play a role in islet graft failure after intraportal transplantation. Therefore, a more adequate transplantation site is required for this technology to gain widespread acceptance and practice.…”