Optimization of glucose level to determine the stimulation index of isolated rat islets

Although the stimulation index (SI) is widely used as a parameter that reflects the insulin releasing function of isolated islets, comparison between the outcomes of different institutions is problematic. This basic study was conducted for 2 objectives as we aimed to determine the optimal high glucose level for rat islets and clarify the available evidence in this field.

We isolated islets of 3 rat strains (Lewis, Sprague-Dawley, and Wistar--n = 5 per strain) and measured the viability of the islets. We measured the amount of insulin released after high and low glucose stimulation. The SI was calculated as a ratio of the insulin value after high glucose stimulation divided by insulin value after low glucose stimulation. We examined the correlation between SI and viability at the high glucose levels.

Isolated islets released insulin corresponding to the changes of high glucose level. The highest correlation coefficient between SI and viability was seen at a glucose level of 16.5 mmol/L (R2 = 0.546, correlation coefficient = 0.154) followed by 19.3 mmol/L (R2 = 0.541, correlation coefficient = 0.169).

We could optimize the high glucose level at 16.5 to 19.3 mmol/L (300-350 mg/dL) for isolated rat islets, with evidence acquired from correlation studies between SI and viability.