Endocrine Abstracts

Objective: The extracellular calcium-sensing receptor (CaR) is expressed on many different cell types other than those involved in the systemic regulation of plasma calcium. We have demonstrated previously that insulin-producing beta cells express the CaR. In this study we have used a calcimimetic CaR agonist (A568) to investigate the effects of CaR activation on beta cell function.

Methods: Experiments used the insulin secreting MIN6 cell line or isolated human islets of Langerhans. Single cell calcium microfluorimetry measurements were performed using Fura-2 loaded cells. Insulin secretion was measured in a multichannel perifusion system. Cell proliferation was measured by bromodeoxyuridine incorporation.

Results: In the absence of extracellular Ca²⁺ A568 (0.01-1 micromolar) produced elevations in cytosolic Ca²⁺ in 21/36 cells. Addition of A568 in the presence of extracellular Ca²⁺ (0.25-2.0mM) evoked marked increases in cytosolic Ca²⁺ (p<0.05). A568 alone caused a small but significant stimulation of insulin secretion from human islets (194 plus/minus 35% control, p<0.01, n=12), that was further enhanced by the addition of extracellular Ca²⁺ (0.2mM Ca²⁺, 907 plus/minus 205% basal; 1.2mM Ca²⁺, 3347 plus/minus 572%, p<0.05). Similar effects were observed in MIN6 cells configured as pseudoislets, in which A568 alone caused a small stimulation of secretion above basal (264 plus/minus 54% control, p<0.01, n=9), that was further enhanced by the presence of extracellular Ca²⁺ (0.25-2.5mM). CaR activation stimulated cell proliferation in monolayers of serum starved (24h) MIN6 cells to a similar extent as serum replacement (A568, 290 plus/minus 19% controls, p<0.01, serum 257 plus/minus 23% p<0.01).

Conclusion: These results demonstrate that CaR activation initiates both secretory and proliferative responses in pancreatic beta cells, and suggest that signalling through the CaR may play an important regulatory role in normal beta cell function.