| Abstrakt: |
Receptor-activated formation of inositol phosphates results in mobilization of intracellular stored Ca in a variety of cells, including vas deferens derived DDT 1 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 Ca-releasing process was investigated in permeabilized and histamine-stimulated intact DDT 1 MF-2 cells. In permeabilized cells, Ins(1,4,5)P 3 induced a concetration-dependent release of intracellular stored Ca. Addition of Ins(1,3,4,5)P 4 did not cause Ca mobilization, but its presence enhanced the amount of Ca released by Ins(1,4,5)P 3 , thereby increasing the total Ca-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 Ca released by Ins(1,3,4,5)P 4 is mediated, either via Ins(1,4,5)P 3 -sensitive Ca channels, or via different heparin-sensitive Ca 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 Ca-dependent K current, representing Ca release from internal stores if receptor-activated Ca entry from the extracellular space was prevented under Ca-free conditions or in the presence of La. This transmembrane current was abolished in the presence of intracellularly applied heparin. Depletion of Ins(1,4,5)P 3 -sensitive Ca 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 Ca was excluded. However, depletion of both Ins(1,4,5)P 3 and Ins(1,3,4,5)P 4 -sensitive Ca compartments in advance caused abolition of the histamine-activated Ca regulated K current. These results show that Ins(1,3,4,5)P 4 plays an important role in the Ca-releasing process in DDT 1 MF-2 cells. It contributes to the development of the intracellular Ca signal following histamine H 1 receptor stimulation by enhancing the total Ins(1,4,5)P 3 -sensitive Ca-releasing capacity via a discrete Ca compartment. |
