ARACHIDONIC-ACID IS FUNCTIONING AS A 2ND-MESSENGER IN ACTIVATING THE CA2-1-HISTAMINOCEPTOR STIMULATION IN DDT1 MF-2 CELLS( ENTRY PROCESS ONH)

This study was carried out to identify the cellular component activati ng the histamine-stimulated Ca2+ entry in vas-deferens-derived DDT1 MF -2 cells. H-1-histaminoceptor stimulation resulted in a rise in intrac ellular Ca2+ concentration, caused by Ca2+ release from inositol phosp hate-sensitive Ca2+ stores and Ca2+ entry from the extracellular space , accompanied by a transient Ca2+-activated outward K+ current. The hi stamine-evoked K+ current was still observed after preventing inositol phosphate-induced Ca2+ mobilization by intracellularly applied hepari n. This current was activated by Ca2+ entry from the extracellular spa ce, because it was abolished in the presence of the Ca2+-channel block er La3+ or under Ca2+-free conditions. H-1 histaminoceptor-activated C a2+ entry was also observed in the presence of intracellularly applied Ins(1,4,5)P-3 and Ins(1,3,4,5)P-4, depleting their respective Ca2+ st ores and pre-activating the inositol phosphate-regulated Ca2+ entry. T hus the ability of histamine to activate Ca2+ entry independently of C a2+ mobilization and the formation of inositol phosphates suggests tha t another component is involved to initiate the Ca2+-entry process. It was observed that H-1-histaminoceptor stimulation resulted in a prono unced release of arachidonic acid (AA) in DDT1 MF-2 cells. Exogenously applied AA induced a concentration-dependent increase in internal Ca2 + due to activation of Ca2+ entry from the extracellular space. Slow i nactivation of the AA-sensitive Ca2+ channels is suggested by the slow decline in Ca2+ entry. In accord, the histamine-induced Ca2+ entry wa s not observed with AA-pre-activated Ca2+ channels. Inhibition of the lipoxygenase and cyclo-oxygenase pathway did not affect the AA-induced Ca2+ entry. The histamine-induced change in internal Ca2+ and the con comitant Kf current were decreased in the presence of AA and caused by Ca2+ mobilization from internal stores. Blocking this internal Ca2+ r elease by heparin, in the presence of AA, resulted in abolition of the histamine-induced Ca2+-regulated K+ current. These observations show that AA, released on H-1-histaminoceptor stimulation in DDT1 MF-2 cell s, is functioning as a second messenger to activate plasmamembrane Ca2 + channels promoting Ca2+ entry from the extracellular space.