Mobilization of Ca2+ from intracellular stores of pancreatic beta-cells by the calcium store blocker TMB-8.

TMB-8 has been used experimentally in many cell types, including endocrine cells, because of its ability to block the efflux of Ca2+ from intracellular stores without affecting influx. Unexpectedly, TMB-8 potentiates stimulated insulin release from pancreatic islets, a process believed to be dependent on the level of cytosolic Ca2+. In the present study, while having no effect on basal insulin release (in the presence of 2.8 mM glucose), TMB-8 (10, 30, and 100 microM) caused a concentration-dependent increase in 45Ca2+ efflux from 45Ca2+-preloaded islets. TMB-8 (100 microM) stimulated 45Ca2+ efflux even in the absence of extracellular Ca2+. In the presence of 5.6 mM glucose, TMB-8 (30 and 100 microM) potentiated insulin release and again increased 45Ca2+ efflux in a concentration-dependent manner. Similarly, insulin release stimulated by isobutylmethylxanthine (IBMX) was potentiated significantly, and IBMX-stimulated 45Ca2+ efflux was increased by the simultaneous introduction of 30 microM TMB-8. Thus, in pancreatic islets, TMB-8 appears to mobilize Ca2+ from intracellular stores, rather than inhibit the efflux as has been commonly accepted. In further studies, using insulin-secreting beta-cells of the RINm5F cell line, TMB-8 was shown to increase the cytosolic Ca2+ concentration in the presence and absence of extracellular Ca2+. This confirmed that mobilization of intracellular Ca2+ was occurring in the pancreatic beta-cell in response to TMB-8. Furthermore, a rise in cytosolic Ca2+ of not more than 10 nM (as induced with KCl) was found to mimick the effect of TMB-8 in conjunction with IBMX. No additional effect of TMB-8 to alter Ca2+ handling at the plasma membrane was found when 45Ca2+ uptake experiments were performed. Therefore, the paradoxical mobilization of beta-cell Ca2+ by TMB-8 appears to be a sufficient explanation for its potentiating effect on the rate of insulin secretion.