Abstract

The Ca²⁺ receptor is a G protein-coupled receptor that enables parathyroid cells and certain other cells in the body to respond to changes in the concentration of extracellular Ca²⁺. In this study, two novel phenylalkylamine compounds, NPS 467 and NPS 568, were examined for effects on Xenopus laevis oocytes expressing the bovine or human parathyroid Ca²⁺ receptors. Increases in chloride current (I_Cl) were elicited in oocytes expressing the bovine Ca²⁺ receptor when the extracellular Ca²⁺concentration was raised above 1.5 mm, whereas Ca²⁺ concentrations > 3 mm were generally necessary to elicit responses in oocytes expressing the human Ca²⁺ receptor. NPS 467 and NPS 568 potentiated the activation of I_Cl by extracellular Ca²⁺ in oocytes expressing either Ca²⁺ receptor homolog, and this resulted in a leftward shift of the Ca²⁺concentration-response curve. Neither compound was active in the absence of extracellular Ca²⁺. Certain inorganic and organic cations known to activate the Ca²⁺ receptor were substituted for elevated levels of extracellular Ca²⁺ to increase I_Cl and the effects of these agonists were also potentiated by NPS 568 or NPS 467. The effects of NPS 568 were stereoselective and the R-enantiomer was about 10-fold more potent than the corresponding S-enantiomer. Neither NPS 467 nor 568 affected I_Cl in water-injected oocytes or in oocytes expressing the substance K receptor or the metabotropic glutamate receptor 1a. These results provide compelling evidence that NPS 467 and NPS 568 act directly upon the parathyroid Ca²⁺receptor to increase its sensitivity to activation by extracellular Ca²⁺. This activity suggests that these compounds are positive allosteric modulators of the Ca²⁺ receptor. As such, these compounds define a new class of pharmacological agents with potent and selective actions on the Ca²⁺ receptor.