Response of human insulinoma cells to extracellular calcium is different from normal B cells

The preoperative determination of the localization of a small insulinoma is sometimes difficult using routine imaging techniques. We have used the selective arterial calcium injection (SACI) test to determine the location of the tumor preoperatively. The pathophysiologic basis of the SACI test is based on the responsiveness of insulinomas to calcium injected into the feeding artery. In this study, we demonstrated the in vitro response of the insulinoma cells to the extracellular calcium challenge by using primary-cultured insulinoma cells. Human insulinoma cells were obtained from three patients. MIN6 cells (normal pancreatic B cells) were used as a control; their insulin response to various stimuli resembles that of normal B cells. The insulin secretory dynamics in response to extracellular calcium were observed using a perfusion system. Second, the change of the concentration of cytosolic free calcium ([Ca2+]i) was monitored by fluorometry using fura-2/AM. When the concentration of extracellular calcium ([Ca2+]o) was changed from 2.54 mM to 10 mM, insulin secretion from the insulinoma cells was markedly increased within 6 min (10- to 18-fold at maximum), and rapidly returned to the basal level; at the same time, [Ca2+]i was immediately elevated and reached a peak within 1 min. In contrast, in the MIN6 cells, the insulin secretion and [Ca2+]i were not significantly changed when [Ca2+]o was switched to 10 mM. The results of these in vitro experiments agreed with the clinical results of the SACI test. The positive response of the insulinoma to the SACI test is probably due to the different response of insulinoma cells to the extracellular calcium challenge compared with normal B cells. The role of [Ca2+]i may be important in the mechanism underlying the SACI test.