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

Receptor-activated formation of inositol phosphates results in mobiliz ation of intracellular stored Ca2+ in a variety of cells, including va s deferens derived DDT1 MF-2 cells. Stimulation of the histamine H-1 r eceptor 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 inosito l 1,4,5-trisphosphate (Ins(1,4,5)P-3). In this study, the role of inos itol phosphates, in particular Ins(1,3,4,5)P-4 on the internal Ca2+-re leasing process was investigated in permeabilized and histamine-stimul ated intact DDT1 MF-2 cells. In permeabilized cells, Ins(1,4,5)P-3 ind uced a concentration-dependent release of intracellular stored Ca2+. A ddition of Ins(1,3,4,5)P-4 did not cause Ca2+ mobilization, but its pr esence enhanced the amount of Ca2+ released by Ins(1,4,5)P-3, thereby increasing the total Ca2+-releasing capacity. The effect of both inosi tol phosphates was inhibited by heparin, known to block Ins(1,4,5)P-3- sensitive receptors. Thus, the additional amount of Ca2+ released by I ns(1,3,4,5)P-4 is mediated, either via Ins(1,4,5)P-3-sensitive Ca2+ ch annels, 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 stimula tion in intact cells induced a Ca2+-dependent K+ current, representing Ca2+ release from internal stores if receptor-activated Ca2+ entry fr om 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 r educed the histamine evoked K+ current to some extent if the contribut ion 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 cause d abolition of the histamine-activated Ca2+ regulated K+ current. Thes e 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 developm ent of the intracellular Ca2+ signal following histamine H-1 receptor stimulation by enhancing the total Ins(1,4,5)P-3-sensitive Ca2+-releas ing capacity via a discrete Ca2+ compartment.