High-performance liquid chromatography analysis of circulating insulins distinguishes between endogenous insulin production (a potential pitfall with streptozotocin diabetic rats) and islet xenograft function.

Porcine islets of Langerhans were microencapsulated according to the alginate-polylysine procedure, and implanted into the peritoneal cavity of 15 streptozotocin-induced (70 mg/kg) diabetic rats (6000 microencapsulated islets per rat). In four animals, a sustained decrease in plasma glucose level below 8.3 mmol/l was observed for up to nine months. However, it was possible to recover microcapsules from the peritoneal cavity of only one rat, and they were found to be damaged and containing no detectable tissue. When insulin in the plasma of three of these animals was analysed by reversed phase high-performance liquid chromatography, only rat insulins I and II, but not porcine insulin was detectable, indicating unambiguously that at the time of analysis, the correction of diabetes in these animals was due to the function of the recipient's own pancreas rather than the continued, long-term, function of the implanted porcine islets. These data confirm that in this model of diabetes, function of the host pancreas can resume following islet transplantation, leading in turn to the potential for a major bias in the interpretation of the data. In the case of an islet xenograft, when the donor's and recipient's insulins can be separated by high-performance liquid chromatography, this non-invasive analytical method should prove useful for identifying the source of insulin in the circulation, and thus the relative functional status of the endogenous and transplanted islets.