Introduction: The islets of Langerhans of the pancreas (multicellular spheroidal organoids constituting 1-3% of the pancreas volume) are the primary endocrine cells regulating glucose homeostasis via two primary hormones, insulin, and glucagon. Insulin is secreted from beta cells and glucagon from alpha cells. The two hormones have opposing effects on glucose homeostasis: insulin promotes glucose uptake into several cell types and thus lowers blood glucose levels; glucagon drives glucose production in the liver, thus raising glucose levels.  Secretion of both hormones is a highly dynamic process, involving complex mechanisms including crosstalk within islets, which are not fully understood. Most of these dynamic processes have been studied primarily with rodent islets, whereas human islets studies are limited.
Methods: For our study, we used a Biorep perifusion system to collect samples for the concurrent measurements of insulin and glucagon secreted from human islets exposed to two glucose concentrations, as well as KCl or arginine stimulation.  Human islets were isolated at the Institute for Cellular Transplantation at the University of Arizona. Perifusions were performed on 25 individual islet isolations in which insulin and glucagon were simultaneously measured using a multiplex assay.
Results: Glucose stimulation (16.7mM) caused a biphasic insulin response with the 1st phase peaking ~ 2 minutes and lasting 6-8 minutes, followed by a sustained 2nd phase. As predicted, glucagon levels decreased during glucose stimulation and remained repressed until islets were again exposed to low glucose (2.8mM).  KCL (30mM) was used to mimic depolarization, which induced a monophasic peak for both glucagon and insulin secretion. Arginine stimulation (10mM) under low glucose conditions initiated a biphasic glucagon response and a modest monophasic increase in insulin.
Conclusions: Our data provides a physiological template to assess islet function for clinical and research use, as well as a method to test novel drugs to potentiate/inhibit secretion of these hormones.