Comparative models of type 2 diabetes mellitus: The role of islet amyloid polypeptide in pathogenesis

Feline pancreatic islets of adequate quantity and viability for in vitro studies cannot be isolated using many of the digestion and gradient separation techniques effective in other species. However, occasional smooth-bordered oval islets released early in digestion, presumably interlobular islets, maintained >90% viability with intense cytoplasmic immunofluorescence for insulin after nine days in culture. Isolated islets from human islet amyloid polypeptide (hIAPP) transgenic mice had flocculent IAPP-immunoreactive deposits in the halo region of insulin secretory vesicles after isolation as well as flocculent and fibrillar extracellular deposits within deep invaginations of the beta-cell border after culture in high (28 mM) glucose media for eight days. Expression of the endoplasmic reticulum chaperone protein BiP was increased in islets cultured in high glucose relative to low (5.5 mM) glucose, but no difference was detectable BiP expression or apoptosis between hIAPP transgenic islets and controls. Plasma IAPP and insulin concentrations were measured in normal, overweight non-diabetic, non-ketotic diabetic and ketoacidotic cats after a 12-hour fast and after intravenous glucagon. The fasting IAPP concentration in normal cats was 8.3 +/- 3.5 pmol/L. Ketoacidotic cats had significantly lower IAPP concentrations than other groups. Similar to that seen in humans, fasting insulin concentrations were similar between normal and diabetic cats and higher in overweight cats. In contrast to normal cats, the diabetic animals in this study had no increase in insulin or IAPP in response to glucagon. Only 5 of 21 (24%) of diabetic cats had improved glycemic control after treatment with glipizide, but both responders and non-responders had higher fasting IAPP and insulin concentrations during treatment with glipizide relative to treatment with insulin. An enzyme-linked immunosorbent assay for IAPP was validated for use with feline plasma. Acceptable accuracy and precision were noted throughout the assay range of 2.5--100 pmol/L. However, matrix effects were suspected based on recovery and dilution studies and discordant results suggest the possibility of additional interferences such as heterophilic antibodies. Although there was poor correlation with radioimmunoassay results, biological specificity was demonstrated with higher IAPP concentrations in overweight cats relative to normal cats as well as after glucagon administration.