PHARMACOLOGICAL CHARACTERIZATION OF THE HUMAN HISTAMINE H-2-RECEPTOR STABLY EXPRESSED IN CHINESE-HAMSTER OVARY CELLS

1 The gene for the human histamine H-2 receptor was stably expressed in Chinese hamster ovary (CHO) cells and characterized by [I-125]-iodoaminopotentidine binding studies. In addition, the coupling of the expressed receptor protein to a variety of signal transduction pathways was investigated. 2 After cotransfection of CHO cells with pCMVhumH(2) and pUT626, a phleomycine-resistant clonal cell line (CHOhumH(2)) was isolated that expressed 565 +/- 35 fmol kg(-1) protein binding sites with high affinity (0.21 +/- 0.02 nM) for the H-2 antagonist, [I-125]-iodoaminopotentidine. 3 Displacement studies with a variety of H-2 antagonists indicated that the encoded protein was indistinguishable from the H-2 receptor identified in human brain membranes and guinea-pig right atrium. The K-i-values observed in the various preparations correlated very well (r(2) = 0.996-0.920). 4 Displacement studies with histamine showed that a limited fraction (32 +/- 6%) of the binding sites showed a high affinity for histamine (2 +/- 1.2 mu M); the shallow displacement curves were reflected by a Hill-coefficient significantly different from unity (n(H) = 0.58 +/- 0.09). The addition of 100 mu M Gpp(NH)p resulted in a steepening of the displacement curve (n(H) = 0.79 +/- 0.02) and a loss of high affinity sites for histamine. 5 Displacement studies with other agonists indicated that the recently developed specific H-2 agonists, amthamine and amselamine, showed an approximately 4-5 fold higher affinity for the human H-2 receptor than histamine. 6 Stimulation of CHOhumH(2) cells with histamine resulted in a rapid rise of the intracellular cyclic AMP levels. After 10 min an approximately 10 fold increase in cyclic AMP could be measured. The EC(50) value for this response was 7 +/- 1 nM for histamine. This response was effectively blocked by tiotidine and cimetidine, resulting in K-i values of 8 +/- 1 nM and 0.56 +/- 0.24 mu M respectively. 7 Stimulation of CHOhumH(2) cells with histamine neither inhibited the A23187-induced release of [H-3]-arachidonic acid nor changed the intracellular IP3 levels. 8 These results show that the cloned human gene encodes a histamine H-2 receptor that is indistinguishable from the H-2 receptor identified in human brain tissue. This receptor is functionally coupled to the adenylate cyclase in CHO cells, but does not influence the inositolphosphate turnover or arachidonic acid release.