Luminescence data (peak integration) were calculated as described previously2 (link) using MikroWin 2000 software (Berthold). Statistical and curve-fitting analyses were performed using Prism 4.0 (GraphPad) software. Data were expressed in percentage (% RLU) of the maximal luminescence that was detected with 10−4 M SP (without antagonist). The competitive nature of antagonism was evaluated using the Schild plot method.22 (link) All antagonists analyzed in this study provided linear regression plots and were considered competitive. The pA2 values were calculated using the Schild’s equation.22 (link)