Sprague-Dawley rats (Envigo Harlan, Houston, TX, USA) were used as the islet donor. Islet isolation was done using standard collagenase digestion and gradient purification5 (link). After isolation, the islets were cultured in CMRL-1066 (Mediatech, Manassas, VA, USA) supplemented with 10% newborn calf serum (Equitech Bio, Kerrville, TX, USA) and 1% penicillin/streptomycin (Fisher Scientific, Hampton, NH, USA) in a 37°C 5% CO2 incubator.