TABLE 4

Analysis of the effects of histamine and mepyramine and determination of the constitutive activity of hH₁R, bH₁R, rH₁R, and gpH₁R in the steady-state GTPase assay

Sf9 cell membranes expressing hH₁R, bH₁R, rH₁R, or gpH₁R in combination with RGS4 were used to study the constitutive activity. GTPase assays were performed as described under Materials and Methods. The concentration-response curves of the inverse agonist mepyramine (MEP) 18 were determined in a concentration range from 0.1 nM to 10 μM. Data were analyzed by nonlinear regression and were best fit to sigmoidal concentration-response curves. The efficacy of histamine was set 1.00. The ΔHA value refers to the difference of maximal stimulation with 100 μM histamine relative to basal activity. The coefficient ΔHA/B_max was calculated based on the data given for ΔHA in this table and B_max values in Table 2. The ΔMEP value refers to the difference of basal activity relative to GTP hydrolysis in presence of 10 μ M MEP. The relative effect of MEP in the last column is the percentage of effect of MEP relative to HA. Data shown are means ± S.E.M. of three experiments performed in duplicates each.