ALLOSTERIC ACTIVATION OF THE CA2-LAEVIS OOCYTES BY NPS-467 OR NPS-568( RECEPTOR EXPRESSED IN XENOPUS)

The Ca2+ receptor is a G protein-coupled receptor that enables parathy roid cells and certain other cells in the body to respond to changes i n the concentration of extracellular Ca2+. In this study, two novel ph enylalkylamine compounds, NPS 467 and NPS 568, were examined for effec ts on Xenopus laevis oocytes expressing the bovine or human parathyroi d Ca2+ receptors. Increases in chloride current (I-Cl) were elicited i n oocytes expressing the bovine Ca2+ receptor when the extracellular C a2+ concentration was raised above 1.5 mM, whereas Ca2+ concentrations >3 mM were generally necessary to elicit responses in oocytes express ing the human Ca2+ receptor. NPS 467 and NPS 568 potentiated the activ ation of I-Cl by extracellular Ca2+ in oocytes expressing either Ca2receptor homolog, and this resulted in a leftward shift of the Ca2+ co ncentration-response curve. Neither compound was active in the absence of extracellular Ca2+. Certain inorganic and organic: cations known t o activate the Ca2+ receptor were substituted for elevated levels of e xtracellular Ca2+ to increase I-Cl and the effects of these agonists w ere also potentiated by NPS 568 or NPS 467. The effects of NPS 568 wer e stereoselective and the R-enantiomer was about 10-fold more potent t han 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 pro vide compelling evidence that NPS 467 and NPS 568 act directly upon th e 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 c ompounds define a new class of pharmacological agents with potent and selective actions on the Ca2+ receptor.