The calcium-sensing receptor elicits differential dephosphorylation of its threonine-888 residue thus determining whether agonist challenges induce oscillatory or sustained intracellular calcium mobilisation.

The calcium-sensing receptor (CaR) is the key controller of Ca2+o homeostasis via its regulation of parathyroid hormone secretion and renal Ca2+ reabsorption. CaRT888 is a crucial intracellular signalling determinant whose phosphorylation by protein kinase C inhibits CaR-elicited Ca2+i mobilisation (Davies et al., 2007; Jiang et al., 2002). Modest CaR stimulation elicits oscillatory Ca2+i mobilisation whereas greater CaR stimulation induces sustained responses. However, if modest CaR activation induces dynamic CaRT888 phosphorylation to produce oscillatory Ca2+i mobilisation, then how do greater levels of CaR activity overcome such PKC-mediated inhibition to elicit sustained responses?Thus, to address this paradox we examined the concentration-dependence of Ca2+o on CaRT888 phosphorylation by semi-quantitative immunoblotting (Davies et al., 2007). HEK-293 cells stably expressing human CaR (CaR-HEK) were incubated for 10mins at 37oC in Hepes-containing buffer. Raising [Ca2+]o from 0.5 to 2.5mM caused a 5.5-fold increase in CaRT888 phosphorylation (160kDa mature, membrane-localised receptor; P