Invited Symposium: Medicinal Plants and Drug Actions
Discussion and Conclusion
TET increases [Ca2+]i by itself in rat alveolar type II cells (18) and mobilizes Ca2+ from TG-sensitive intracellular stores in human leukaemic HL-60 cells (10). However, TET alone did not mobilize Ca2+ from extracellular medium and intracellular stores in glioma C6 cells (Figs. 2 and 5). We have recently reported that Ca2+-antagonistic actions of TET are dependent on cell type (12). The Ca2+-mobilizing action of TET might also be dependent on the cell type.
The blocking actions of TET on VOC (4-8) and ROC (10;12) are maximal at around 100 M. The doses needed to induce anti-inflammative and immunosuppressive effects of TET are higher than those to block VOC and ROC. In the present study using glioma C6 cells, 30 microM TET blocked Ca2+ entry evoked by bombesin and TG but they did not affect leakage entry of Ca2+ (Figs. 3 and 6). Ca2+ entry evoked by bombesin and TG is caused through ROC because C6 cells lack VOC (22). Therefore, TET are likely to act as ROC blockers in glioma C6 cells. The ROC blocking action of TET is confirmed in HL-60 cells (10), vascular smooth muscle cells (4;9) and endothelial cells (11). The blocking action of TET on VOC seems to be due to binding of the subunit of L-type VOC because TET completely blocks the benzothiazepine binding site (3). The mechanism of the activation, like the structure of ROC, is not well understood compared to that of VOC. A popular hypothesis for the activation of ROC is that it occurs via the capacitative Ca2+ entry pathway by which depletion of Ca2+ in the intracellular store activates Ca2+ entry (26). Capacitative Ca2+ entry is now confirmed in various types of cells, including excitable cells (21) and exists in C6 cells (27). Recently, it was reported that Ca2+ channels expressed by the Drosophila transient receptor potential gene, the product of which shows some structural similarity to VOC, is homologous to capacitative Ca2+ entry channels (28;29). Therefore, inhibitory actions of TET on Ca2+ entry could result from their binding to ROC .
TET also suppressed the peak responses of [Ca2+]i to bombesin and TG (Figs. 1 and 3), suggesting that they inhibited Ca2+ release from intracellular stores. This was confirmed by using permeabilized C6 cells (Fig. 5). There are three possible mechanisms by which TET affect Ca2+ release. First, TET could block Ca2+ uptake into the stores like TG, which inhibits microsomal Ca2+-ATPase (30). However, this is unlikely because TET alone caused neither an increase in [Ca2+]i (Fig. 1) nor release of Ca2+ from permeabilized cells (Fig. 5). Second, TET could directly block the IP3 receptor channel on Ca2+ stores. This is likely because Ca2+ release induced by IP3 was abolished by TET in permeabilized cells (Fig. 5). Third, TET could inhibit the leakage release of Ca2+ at the membrane of Ca2+ stores. This is also likely because Ca2+ release caused by TG is due to leakage through both the IP3 receptor channel and a non-specific pathway (30;31).
Ioannoni et al. (1989) reported that TET partially inhibited inositol monophosphate (IP1) accumulation evoked by concanavalin A in human lymphocytes. However, they were unable to examine the effect of TET on IP3 because concanavalin A did not increase IP3 accumulation. The present study showed that TET markedly suppressed IP3 generation induced by bombesin. The generation of IP3, but not IP1, has an important role in the initial Ca2+ release of Ca2+ mobilization induced by the activation of receptors on the plasma membrane (20;21). Therefore, the inhibitory effects of TET on Ca2+ release evoked by bombesin partially resulted from the suppression of IP3 generation. However, the mechanism of the inhibition of IP3 generation by TET is unknown. Further studies on the effects of TET on receptor-binding, GTP-binding protein and phospholipase C are necessary.
Glioma C6 cells have several receptors to increase [Ca2+]i and are widely used as a neuro-glial model of information processing in the central nervous system (32). Various inhibitory effects of TET on Ca2+ mobilization in glioma C6 cells seem to result in modification of changes in glial structure and function, including gene expression, development, metabolism and regulation of the extracellular milieu (32). The mechanisms of various pharmacological actions of TET such as hypotensive effects (2), suppression of fibroblast proliferation (33) and collagen synthesis (34), anti-silicotic effect (35), immunosuppressive effect (14) and anti-inflammatory effects (13;16;17;36) are not well known. It is, thus, tempting to speculate that these pharmacological actions of TET are caused by multiple inhibitions including those of VOC, ROC, Ca2+ release and IP3 production in various kinds of cells.
In conclusion, the mechanisms by which TET inhibited Ca2+ mobilization activated by agonists include at least inhibition of IP3 formation, Ca2+ entry and Ca2+ release in rat glioma C6 cells. The use of TET in the treatment of hypertension and angina pectoris would be clinically more effective than other VOC blockers because the clinically used dose of many VOC blockers mainly inhibits VOC but not ROC, Ca2+ release or IP3 formation.
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|Takemura, H.; (1998). Effects of Tetrandrine on Calcium Mobilization in Non-excitable Cells. Presented at INABIS '98 - 5th Internet World Congress on Biomedical Sciences at McMaster University, Canada, Dec 7-16th. Invited Symposium. Available at URL http://www.mcmaster.ca/inabis98/kwan/takemura0224/index.html|
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