THE ROLE OF INOSITOL 1,3,4,5-TETRAKISPHOSPHATE IN INTERNAL CA2+ MOBILIZATION FOLLOWING HISTAMINE H-1 RECEPTOR STIMULATION IN DDT1 MF-2 CELLS

Receptor-activated formation of inositol phosphates results in mobilization of intracellular stored Ca2+ in a variety of cells, including vas deferens derived DDT1 MF-2 cells. Stimulation of the histamine H-1 receptor on these cells caused a pronounced formation of inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P-4) with respect to that of inositol 1,4,5-trisphosphate (Ins(1,4,5)P-3). In this study, the role of inositol phosphates, in particular Ins(1,3,4,5)P-4 on the internal Ca2+-releasing process was investigated in permeabilized and histamine-stimulated intact DDT1 MF-2 cells. In permeabilized cells, Ins(1,4,5)P-3 induced a concentration-dependent release of intracellular stored Ca2+. Addition of Ins(1,3,4,5)P-4 did not cause Ca2+ mobilization, but its presence enhanced the amount of Ca2+ released by Ins(1,4,5)P-3, thereby increasing the total Ca2+-releasing capacity. The effect of both inositol phosphates was inhibited by heparin, known to block Ins(1,4,5)P-3-sensitive receptors. Thus, the additional amount of Ca2+ released by Ins(1,3,4,5)P-4 is mediated, either via Ins(1,4,5)P-3-sensitive Ca2+ channels, or via different heparin-sensitive Ca2+ channels activated by both Ins(1,4,5)P-3 and Ins(1,3,4,5)P-4. Histamine H-1 receptor stimulation in intact cells induced a Ca2+-dependent K+ current, representing Ca2+ release from internal stores if receptor-activated Ca2+ entry from the extracellular space was prevented under Ca2+-free conditions or in the presence of La3+. This transmembrane current was abolished in the presence of intracellularly applied heparin. Depletion of Ins(1,4,5)P-3-sensitive Ca2+ stores by internal application of Ins(1,4,5)P-3 reduced the histamine evoked K+ current to some extent if the contribution of external Ca2+ was excluded. However, depletion of both Ins(1,4,5)P-3 and Ins(1,3,4,5)P-4-sensitive Ca2+ compartments in advance caused abolition of the histamine-activated Ca2+ regulated K+ current. These results show that Ins(1,3,4,5)P-4 plays an important role in the Ca2+-releasing process in DDT1 MF-2 cells. It contributes to the development of the intracellular Ca2+ signal following histamine H-1 receptor stimulation by enhancing the total Ins(1,4,5)P-3-sensitive Ca2+-releasing capacity via a discrete Ca2+ compartment.