Table 2

Histamine-induced cAMP production in HEK-293 cells transiently expressing the D115A or D115N mutant H₂ receptor or different levels of the wild-type H₂ receptor

H₂Receptor	B_max	EC₅₀	E_max
	fmol/mg	nM	pmol/well
WT341	341 ± 70	180 ± 32	12 ± 2
WT632	632 ± 100	151 ± 63	20 ± 1^*
WT1536	1536 ± 157	151 ± 51	42 ± 6^**
D115N	231 ± 34	121 ± 46	88 ± 12^***
D115A	305 ± 60	222 ± 95	66 ± 13^**

Cells were incubated with increasing concentrations of histamine for 10 min at 37°C in DMEM in the presence of 300 μM IBMX and 25 mM HEPES, pH 7.4. Data represent the mean ± S.E. of three to six independent experiments. The asterisks indicate a significant difference from HEK-293 cells transiently expressing the wild-type receptor at a density of 341 fmol/mg protein (see Materials and Methods).