RGS3 and RGS3T suppress GnRH-stimulated IP3 concentrations in COS-1 cells. The cells were co-transfected with cDNAs representing the GnRH receptor and RGS3 or RGS3T; 72 h later the cells were treated with 10-7M GnRH for 0, 10, 20, or 30 sec before being collected for IP3 measurements using receptor radioassay in which IP3 and 3H-IP3 competed for binding to IP3 receptors prepared from calf cerebellum. Data presented are the mean ± SEM of three independent experiments. The SEM is not shown for some data points because it was smaller than the symbol. The RGS3 and RGS3T groups are significantly lower than the control group (p < 0.05), and the RGS3T group is significantly lower than the RGS3 group (p < 0.05).