REGULATION OF HISTAMINE-INDUCED AND UTP-INDUCED INCREASES IN INS(1,4,5)P-3, INS(1,3,4,5)P-4 AND CA2+ BY CYCLIC-AMP IN DDT1 MF-2 CELLS

1 Stimulation of P-2U-purinoceptors with UTP or histamine H-1-receptors with histamine gave rise to the formation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P-3) and inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P-4) in DDT1 MF-2 smooth muscle cells. 2 Stimulation of P-2U-purinoceptors or histamine H-1-receptors caused an increase in cytoplasmic Ca2+ consisting of an initial peak, representing the release of Ca2+ from internal stores and a sustained phase representing Ca2+ influx. 3 The P-2U-purinoceptor-mediated Ca2+-entry mechanism was more sensitive to UTP than Ca2+-mobilization (EC(50): 3.3 mu M +/- 0.4 mu M VS 55.1 mu M +/- 9.2 mu M), in contrast to these processes activated by histamine H-1-receptors (EC(50): 5.8 mu M +/- 0.6 mu M VS 3.1 mu M +/- 0.5 mu M). 4 Pre-stimulation of cells with several adenosine 3':5'-cyclic monophosphate (cyclic AMP) elevating agents, reduced the histamine H-1-receptor-mediated formation of Ins(1,4,5)P-3 and Ins(1,3,4,5)P-4, Forskolin completely inhibited Ins(1,4,5)P-3 formation (IC50: 158 +/- 24 nM) whereas Ins(1,3,4,5)P-4 formation was inhibited by only 45% (IC50: 173 +/- 16 nM). The P-2U-purinoceptor-mediated production of these inositol phosphates was not affected by cyclic AMP. 5 Forskolin and isoprenaline reduced the histamine-induced increase in cytoplasmic Ca2+, as measured in Ca2+ containing medium and in nominally Ca2+-free medium but did not change the UTP-induced increase in cytoplasmic Ca2+. 6 These results clearly demonstrate that cyclic AMP differentially regulates components of the histamine induced phospholipase C signal transduction pathway. Furthermore, cyclic AMP does not affect the phospholipase C pathway activated by stimulation of P-2U-purinoceptors in DDT1 MF-2 cells.