Introduction: Subcutaneous islet transplantation offers various advantages, however poor engraftment and low efficiency hinder its prevalence. One of contributing factors to this low efficiency is the hypoxic condition of the subcutaneous space (SC), thus improving this subcutaneous hypoxic condition was expected to enhance the outcomes of subcutaneous islet transplantation. We have developed a novel method for measuring tissue oxygen pressure (pO₂) in vivo using optical sensor system and reported that pO₂ in the SC was lower than previously reported. In this study, we investigated the influence of hypoxic conditions at the SC on islet graft viability in vivo.
Method: The oxygen delivery device was implanted in the SC of rats, and the oxygenation condition was adjusted to achieve subcutaneous pO₂ levels similar to those of renal subcapsular space using the device. After embedding the islet grafts in collagen gel and transplanting them subcutaneously with the device for improving hypoxia in the SC, the gel was retrieved three days after transplantation. Insulin extraction from the transplanted islet grafts in the gel was evaluated to compare the residual amount of the grafts. Next, the grafts were collected by dissolving the gels using the enzymes (collagenase and thermolysin), and the impact of subcutaneous pO₂ levels on graft energy status was evaluated using ATP/DNA assay. In addition, rat islets were cultured in culture medium mixed with or without enzymes, and the influence of the enzyme treatment on islet viability was also evaluated using ATP/DNA assay.
Results: No significant difference was observed in ATP/DNA ratio of islets after culture with or without the enzymes (p=0.64), suggesting that the enzyme treatment used in the present study did not affect islet viability. In terms of the amount of insulin in the islet grafts, there was no significant difference by the pO2 elevation in the SC (p=0.81). However, the ATP/DNA of islet grafts transplanted in the SC tended to be higher when pO₂ levels were elevated to those of renal subcapsular space (p=0.07).
Conclusion: The use of collagen gel for subcutaneous islet transplantation may allow for the recovery of samples post-transplantation in vivo and the assessment of graft viability. This study suggests that even slight improvements in the subcutaneous hypoxic environment could potentially enhance islet engraftment.