Correlation of intracellular and extracellular calcium ion concentrations with synergy between 1,2-dioctanoyl-sn-glycerol and ionomycin in platelet arachidonic acid mobilization

Two distinct effects of 12S-hydroxyeicosatetraenoic acid on platelet aggregation by zooxanthellatoxin-A and ionomycin

The marine toxin zooxanthellatoxin-A (ZT-A) and the Ca2+ ionophore ionomycin caused aggregation in rabbit platelets. While ZT-A-induced platelet aggregation was inhibited by indomethacin, ionomycin-induced aggregation was potentiated by the drug. In contrast, both ZT-A- and ionomycin-induced accumulations of thromboxane B2 (TXB2), a stable metabolite of thromboxane A2 (TXA2), were completely inhibited by indomethacin. 12S-Hydroxyeicosatetraenoic acid (12-HETE), which could be accumulated in the presence of indomethacin, inhibited ZT-A-induced aggregation, but it potentiated the ionomycin-induced one. These results suggest that the different effects of indomethacin may be attributed to the distinct effects of 12-HETE on ZT-A- and ionomycin-induced platelet aggregations.

Role of protein kinase C in bradykinin-induced prostaglandin formation in osteoblasts

Bradykinin (1 microM, 5 min) induced translocation of protein kinase C (PKC) to the plasma membrane fraction in osteoblastic MC3T3-E1 cells. Bradykinin also enhanced the binding of phorbol 12,13-dibutyrate (PDBu) to intact cells, a measure of PKC activation. Addition of bradykinin (1 microM) to cells preincubated with [3H]PDBu (10 nM, 20 min) caused an increase in specific PDBu binding that was maximal after 5-10 min. The bradykinin-induced enhancement of PDBu binding was seen at 1 nM and was maximal at 10 nM. The bradykinin B1 receptor agonist des-Arg9-bradykinin (1 microM) did not enhance specific PDBu binding to intact MC3T3-E1 cells. PDBu at and above 3 nM stimulated the formation of prostaglandin E2 (PGE2) in MC3T3-EI cells. This stimulatory effect was seen after 15-20 min incubation. The Ca2+ ionophore A23187 at and above 1 microM induced a rapid (within seconds) burst of PGE2 formation in MC3T3-E1 cells. The effect of PDBu and A23187 on PGE2 formation was synergistic. The PKC inhibitor staurosporine (200 nM) inhibited basal as well as bradykinin-induced prostaglandin-formation in MC3T3-E1 cells.

IN CONCLUSION

bradykinin enhances PKC activation in osteoblastic MC3T3-E1 cells. This kinase activation may be involved in bradykinin-induced prostaglandin formation.

Effect of vitamin C and vitamin E on prostaglandin synthesis by fibroblasts and squamous carcinoma cells

Dietary levels of vitamins C and E have been associated with cancer prevention and to a lesser extent with therapeutic enhancement of cancer treatment. Inhibition of prostaglandins (PGs) by pharmacological agents has been demonstrated to enhance immunocompetence, and to suppress growth of tumors in animals and humans. We report here on the effect of vitamins C and E on PGE2 production by human gingival fibroblasts and SCC-25 oral squamous carcinoma cells. The results indicate: 1. vitamins C and E exert a dose-dependent effect on arachidonic acid (AA) release and PGE2 synthesis; 2. vitamin E has a biphasic effect which is stimulatory at 1 and 10 microM and inhibitory at 100 microM; 3. vitamin E is considerably more potent than vitamin C in its inhibitory effect on AA and PGE2 in both cell types; 4. a combination of the two vitamins has a consistent dose-dependent inhibitory effect on AA and PGE2; 5. vitamin C stimulates PGE2 synthesis from exogenous AA in fibroblasts, and inhibits it in SCC-25 cells. The in vivo significance of these findings requires further investigation.

Intercellular communication between follicular angiotensin receptors and Xenopus laevis oocytes: medication by an inositol 1,4,5-trisphosphate-dependent mechanism

In Xenopus laevis oocytes, activation of angiotensin II (AII) receptors on the surrounding follicular cells sends a signal through gap junctions to elevate cytoplasmic calcium concentration ([Ca2+]i) within the oocyte. The two major candidates for signal transfer through gap junctions into the oocyte during AII receptor stimulation are Ins(1,4,5)P3 and Ca2+. In [3H]inositol-injected follicular oocytes, AII stimulated two- to fourfold increases in phosphoinositide hydrolysis and production of inositol phosphates. Injection of the glycosaminoglycan, heparin, which selectively blocks Ins(1,4,5)P3 receptors, prevented both AII-stimulated and Ins(1,4,5)P3-induced Ca2+ mobilization in Xenopus follicular oocytes but did not affect mobilization of Ca2+ by ionomycin or GTP. These results indicate that the AII-regulated process of gap junction communication between follicular cells and the oocyte operates through an Ins(1,4,5)P3-dependent mechanism rather than through transfer of Ca2+ into the ooplasm and subsequent Ca(2+)-induced Ca2+ release.

Thrombin-induced calcium oscillation in human platelets and MEG-01, a megakaryoblastic leukemia cell line

Digital imaging microscopy revealed that human platelets show periodic intracellular Ca++ elevation in response to 0.01 U/ml thrombin. MEG-01, a megakaryoblastic leukemia cell line, also responded with oscillatory intracellular Ca++ elevation (0.7-1 times/min) to thrombin (0.001-0.003U/ml). Ca++ transients appears to be fused with higher thrombin doses. With extracellular Ca++ concentrations of 0.1 mM or less, Ca++ oscillation could not be elicited, or even when present, it disappeared after a few spikings of [Ca++]i. Extracellular Ca++ concentrations of 0.3 mM or more were required to facilitate ongoing Ca++ oscillation, suggesting an important role of Ca++ influx for Ca++ oscillation.

Inhibition of PGE2 production in macrophages from vitamin E-treated rats

Rat peritoneal macrophages from vitamin E-treated rats (5 mg per rat for 6 successive days) contained 403.3 +/- 90.7 ng alpha-tocopherol/10(6) cells, whereas control macrophages contained 1.2 +/- 0.4 ng. PGE2 production in the macrophages from vitamin E-treated rats was significantly suppressed when stimulated with PMA and calcium ionophore A23187. The mechanism of vitamin E inhibition of PGE2 production in macrophages was investigated. The release of (14C)-arachidonic acid from pre-labeled macrophages and the conversion of (14C)-arachidonic acid to PGE2 by the homogenate of the cells were remarkably reduced. These results strongly suggested that the inhibition of PGE2 production by vitamin E results from the inhibition of the activities of both phospholipase A2 and cyclooxygenase.

Inhibition of phosphatidic acid production and lysophospholipid formation in collagen-stimulated human platelets by staurosporine, a protein kinase inhibitor

To investigate a possible regulatory role of protein kinase C (PKC) on collagen-induced phospholipase activity, human platelets were prelabelled with either [3H] arachidonic acid or [14C]stearic acid and stimulated with collagen (2 micrograms/ml) in the presence or absence of the protein kinase inhibitor, staurosporine (1 microM). The collagen-induced release of [3H]arachidonic acid and formation of [14C]stearoyl-labelled lysophospholipids was inhibited by prior incubation with staurosporine, as was the formation of 3H-labelled thromboxane B2, thereby suggesting inhibition of the collagen-induced phospholipase A2 activity. The degradation of phosphatidylinositol (PI) and elevation of phosphatidic acid (PA) in platelets prelabelled with either radiotracer were also completely blocked by staurosporine pretreatment, indicating a suppression of collagen-stimulated phospholipase C activity. Suppressed phospholipase C activity may have been due to diminished thromboxane A2 formation since treatment with the dual cyclo-oxygenase/lipoxygenase inhibitor, BW755C, also resulted in an inhibition of the collagen-stimulated loss of 14C-labelled PI and rise in PA by 75-80%. Our results suggest that protein kinase, possible PKC, may be involved in the regulation of these phospholipases in collagen-stimulated human platelets.

Ionomycin, a Ca++ ionophore, increases platelet volume independently of the Na+/H+ exchanger

A Ca++-ionophore, ionomycin, increased the volume of human platelets suspended in a Ca++-containing buffer. This change in cell volume was dependent upon ionomycin and extracellular Ca++ concentrations, suggesting that the volume change occurs when the intracellular Ca++ reaches a certain level (greater than uM as determined by aequorin method). The ionomycin-induced volume increase was suppressed by replacement of extracellular Na+ with membrane-impermeable N-methyl-D-glucamine or Cs+, but not with Li+, K+, or Rb+. Ethylisopropylamiloride, a potent inhibitor of the Na+/H+ exchanger, had only weak inhibitory effect, and the apparent Km for Na+ was approximately 350 mM, which is much larger than that of the Na+/H+ exchanger. It is suggested that certain mechanisms other than the Na+/H+ exchanger are responsible for ionomycin-induced volume increase.