Positive Allosteric Modulation of the Calcium-sensing Receptor by Physiological Concentrations of Glucose

The calcium-sensing receptor (CaSR) is activated by various cations, cationic compounds, and amino acids. In the present study we investigated the effect of glucose on CaSR in HEK293 cells stably expressing human CaSR (HEK-CaSR cells). When glucose concentration in the buffer was raised from 3 to 25 mM, a rapid elevation of cytoplasmic Ca2+ concentration ([Ca2+](c)) was observed. This elevation was immediate and transient and was followed by a sustained decrease in [Ca2+](c). The effect of glucose was detected at a concentration of 4 mM and reached its maximum at 5 mM. 3-O-Methylglucose, a non-metabolizable analogue of glucose, reproduced the effect of glucose. Sucrose also induced an elevation of [Ca2+](c) in HEK-CaSR cells. Similarly, sucralose was nearly as effective as glucose in inducing elevation of [Ca2+](c). Glucose was not able to increase [Ca2+](c) in the absence of extracellular Ca2+. The effect of glucose on [Ca2+](c) was inhibited by NPS-2143, an allosteric inhibitor of CaSR. In addition, NPS-2143 also inhibited the [Ca2+](c) responses to sucralose and sucrose. Glucose as well as sucralose decreased cytoplasmic cAMP concentration in HEK-CaSR cells. The reduction of cAMP induced by glucose was blocked by pertussis toxin. Likewise, sucralose reduced [cAMP](c). Finally, glucose increased [Ca2+](c) in PT-r parathyroid cells and in Madin-Darby canine kidney cells, both of which express endogenous CaSR. These results indicate that glucose acts as a positive allosteric modulator of CaSR.