Release Of Free Ca2 Research Articles

Na+ controls hypoxic signalling by the mitochondrial respiratory chain.

All metazoans depend on the consumption of O2 by the mitochondrial oxidative phosphorylation system (OXPHOS) to produce energy. In addition, the OXPHOS uses O2 to produce reactive oxygen species that can drive cell adaptations1-4, a phenomenon that occurs in hypoxia4-8 and whose precise mechanism remains unknown. Ca2+ is the best known ion that acts as a second messenger9, yet the role ascribed to Na+ is to serve as a mere mediator of membrane potential10. Here we show that Na+ acts as a second messenger that regulates OXPHOS function and the production of reactive oxygen species by modulating the fluidity of theinner mitochondrial membrane. A conformational shift in mitochondrial complex I during acute hypoxia11 drives acidification of the matrix and the release of free Ca2+ from calcium phosphate (CaP) precipitates. The concomitant activation of the mitochondrial Na+/Ca2+ exchanger promotes the import of Na+ into the matrix. Na+ interacts with phospholipids, reducing inner mitochondrial membrane fluidity and the mobility of free ubiquinone between complex II and complex III, but not inside supercomplexes. As a consequence, superoxide is produced at complex III. The inhibition of Na+ import through the Na+/Ca2+ exchanger is sufficient to block this pathway, preventing adaptation to hypoxia. These results reveal that Na+ controls OXPHOS function and redox signalling through an unexpected interaction with phospholipids, with profound consequences for cellular metabolism.

Role of inositol 1,4,5-trisphosphate receptor in ventral signaling in Xenopus embryos.

The inositol 1,4,5-trisphosphate (IP3) receptor is a calcium ion channel involved in the release of free Ca2+ from intracellular stores. For analysis of the role of IP3-induced Ca2+ release (IICR) on patterning of the embryonic body, monoclonal antibodies that inhibit IICR were produced. Injection of these blocking antibodies into the ventral part of early Xenopus embryos induced modest dorsal differentiation. A close correlation between IICR blocking potencies and ectopic dorsal axis induction frequency suggests that an active IP3-Ca2+ signal may participate in the modulation of ventral differentiation.

A localized elevation of cytosolic free calcium is associated with cytokinesis in the zebrafish embryo.

Cytokinesis, a key step in cell division, is known to be precisely regulated both in its timing and location. At present, the regulatory mechanism of cytokinesis is not well understood, although it has been suggested that calcium signaling may play an important role in this process. To test this notion, we introduced a sensitive fluorescent Ca2+ indicator into the zebrafish embryo and used confocal microscopy to measure the spatiotemporal variation of intracellular free Ca2+ concentration ([Ca2+]i) during cell cleavage. It was evident that a localized elevation of [Ca2+]i is closely associated with cytokinesis. First, we found that during cytokinesis, the level of free Ca2+ was elevated locally precisely at the cleavage site. Second, the rise of free Ca2+ was very rapid and occurred just preceding the initiation of furrow contraction. These observations strongly suggest that cytokinesis may be triggered by a calcium signal. In addition, we found that this cytokinesis-associated calcium signal arose mainly from internal stores of Ca2+ rather than from external free Ca2+; it could be blocked by the antagonist of inositol trisphosphate (InsP3) receptors. These findings suggest that the localized elevation of [Ca2+]i is caused by the release of free Ca2+ from the endoplasmic reticulum through the InsP3-regulated calcium channels.

Ca2+Mobilization is Related to the Lethal Effect of X-irradiation in L5178Y Cells

Radiation-resistant L5178Y-R (LY-R) and radiation-sensitive L5178Y-S (LY-S) murine lymphoma cells were X-irradiated and their free Ca2+ concentration was examined with Fura-2 in Ca2(+)-free salt solution. The release of free Ca2+ from intracellular stores was linear between 10 and 60 min after irradiation (1-5 Gy X-rays) and was higher in LY-S than in LY-R cells. Pre-treatment with 2 mM benzamide (Bz) further increased the concentration of free Ca2+ in LY-S cells but not in LY-R cells. In contrast with LY-R cells, LY-S cells had previously been found to be radiosensitized by continuous 2 mM Bz treatment (Szumiel et al. 1984b). Thus, there was a parallel effect of Bz on survival and on the increase in free Ca2+ concentration in LY cells. Moreover, the rates of Ca2+ release after irradiation with 1-5 Gy of X-rays with or without Bz treatment were inversely related to the respective surviving fractions of LY cells.

Release of Free Ca²⁺ from the Nematocysts of Aiptasia mutabilis during the Discharge

Although the mechanism of discharge of nematocysts is still controversial, most authors agree that bound calcium plays an definite role in stabilizing the resting state of capsules. Therefore, the kinetics of the release of Ca²⁺ during the discharge is expected to provide useful information about the mechanism of discharge itself. The investigation was performed on microbasic mastigophores extruded in 1 M citrate from the acontia of Aiptasia mutabilis. The release of free Ca²⁺ during the discharge of a population of nematocysts induced by treatment with hypoosmotic medium was revealed by the light emission of aequorin. The time course of light emission was related to the discharge curve. The recorded light signal had two different components: a short-latency one preceding the discharge and a longer one paralleling the discharge. The effectiveness of the hypoosmotic medium in inducing the discharge was not constant in various samples. Unresponsive nematocysts gave rise to a different light signal. These data suggest that in microbasic mastigophores of Aiptasia some change in the outer layer of the capsule wall is associated with the discharge. In conclusion, in the discharge of the microbasic mastigophores of Aiptasia mutabilis, the unbinding of calcium from protein molecules of the capsule fluid could be involved. However, also in these nematocysts, the osmotically generated energy for discharge is not effective unless the "stopper" is opened.