Abstract: A cDNA encoding a guinea pig histamine H1 receptor was stably expressed in Chinese hamster ovary (CHO) cells. In one resulting clone, named CHO(H1), the H1 receptor was found to be coupled to several major signal transduction pathways. In each case the involvement of a Gi/Go protein with pertussis toxin (PTX) was assessed, as well as the influence of extracellular Ca2+ and of protein kinase C activation by phorbol 12-myristate 13-acetate (PMA). Histamine induced, in a PTX- and PMA-insensitive manner, a biphasic increase in the intracellular Ca2+ level of which only the second sustained phase was dependent on the extracellular Ca2+ level. Histamine also caused a threefold elevation of inositol phosphate production, which was PTX-insensitive, but slightly inhibited by PMA and reduced by 75% in the absence of extracellular Ca2+. Histamine also caused a massive release of arachidonic acid, which occurred in a Ca2+- and PMA-sensitive manner, probably through the activation of a cytosolic phospholipase A2, which partly involves coupling to a PTX-sensitive G protein. In comparison, in HeLa cells endowed with a native H1 receptor, the histamine-induced arachidonic acid release was also Ca2+- and PMA-sensitive, but totally PTX-insensitive. Finally, in CHO(H1) cells, histamine in very low concentrations potentiated the cyclic AMP accumulation induced by forskolin. This response appeared to be insensitive to PTX, extracellular Ca2+, and PMA. These various observations show that stimulation of a single receptor subtype, the guinea pig H1 receptor, can trigger four major intracellular signals through coupling to several G proteins that are variously modulated by extracellular Ca2+ and protein kinase C activation.