Clinical measures of islet function: usefulness to characterize defects in diabetes

In healthy individuals, the ability of the pancreatic islets to sense and respond appropriately to changes in plasma glucose levels maintains plasma glucose levels within a narrow range despite broad fluctuations in nutrient intake and variable "demand" for insulin imposed by changes in insulin sensitivity. This ability of the pancreatic islets is lost in type 2 diabetes (T2DM). For studies on the pathophysiology of T2DM, methods for analyzing islet function are therefore required. Many methods of varying degrees of complexity have been developed and used to measure pancreatic beta-cell function in humans and to characterize the defects existing in patients with T2DM or precursors thereof (impaired fasting glucose [IFG] and impaired glucose tolerance [IGT]). Significant, although perhaps less progress has been made toward development of methods to characterize alpha-cell function. This work presents an overview of clinical measures of islet function, from simple static measures such as HOMA-beta to the more complex dynamic measures such as those utilizing stepped hyperglycemic clamps and acute administration of arginine to obtain more detailed information regarding the interaction of glucose and non-glucose secretagogues. We emphazise the need for accurate measures of alpha-cell function, and we discuss the strengths and limitations of the various methods, highlighting the many aspects of both alpha- and beta-cell function that become impaired during development of T2DM.