Cells expressing either wild type or the indicated mutants were stimulated with a concentration range of glucagon in the presence of increasing concentrations of MK-0893. After determination of levels of cAMP generated, the data were analysed by global fitting to the half-maximum effective concentration (EC50) allosteric shift equation. pKB is the negative log of the antagonist affinity that mediates the allosteric inhibition of the glucagon response. Data are an average of three independent experiments and error bars represent s.e.m. P values are derived from an unpaired two-tailed t-test.