Endogenous Calcium Channels Research Articles

Protein kinase A modulates an endogenous calcium channel, but not the calcium-activated chloride channel, in Xenopus oocytes

In Xenopus oocytes, Ca 2+ influx through an endogenous voltage-gated Ca 2+ channel activates a transient outward Cl − current ( I CI(Ca)), which is potentiated by cAMP increase. The site of cAMP effect appears to be the Ca 2+ channel instead of the Ca 2+-activated Cl − channel, because cAMP potentiates the Ba 2+ current through the Ca 2+ channel in a similar way to the I Cl(Ca), and cAMP does not potentiate the Ca 2+-dependent CI − current in cells treated with Ca 2+ ionophore. Using the catalytic subunit of protein kinase A (PKA) and PKA inhibitors, it was shown that PKA is both necessary and sufficient for the cAMP effect on I CI(Ca). Furthermore, the cAMP/PKA-mediated potentiation of I cI(Ca) was inhibited by both type 1 and type 2A protein phosphatases.

Evidence that the FSH receptor itself is not a calcium channel.

FSH has been shown to stimulate the uptake of calcium in cultured rat Sertoli cells, resulting in an increase in cytosolic calcium concentrations. One possibility which has been put forth is that the FSH receptor per se may act as a calcium channel. This is all the more tantalizing given the proposed structure of this receptor which, like all other G protein-coupled receptors, is thought to have the putative transmembrane helices forming a bundle-like structure in the plasma membrane. To test whether the FSH receptor could function as a calcium channel, we performed whole-cell voltage clamp experiments on 293 and 293F(wt1) cells, which are a clonal line of 293 cells expressing high levels of rat FSH receptors. The 293 cells, which do not express FSH receptors, were found to lack any detectable inward calcium currents, and therefore, serve as an excellent model for transfecting with potential calcium conducting FSH receptors. When the 293F(wt1) cells were then tested, no inward calcium currents could be detected in either control or FSH-stimulated cells. These results suggest that the FSH receptor itself is not a calcium channel and, therefore, FSH must be stimulating endogenous calcium channels in rat Sertoli cell plasma membranes.

Endogenous Xenopus-oocyte Ca-channels are regulated by protein kinases A and C

Calcium entry into Xenopus oocyte occurs mainly through voltage-dependent calcium channels. These channels were characterized as belonging to a particular type of calcium channel insensitive to dihydropyridines, ω-conotoxin, and Agelenopsis aperta venom, but blocked by divalent cations (Co, Cd, Ni). Intracellular injection of cAMP, or bath application of phorbol ester, induced a marked increase in calcium current amplitude and a slowing of the inactivation time-course. Despite their different pharmacology, endogenous calcium channels, like cardiac or neuronal calcium channels, could be thus regulated by protein kinases A and C.

Mg 2+ Administered up to twenty-four hours following reperfusion prevents ischemic damage of the CA1 neurons in the rat hippocampus

Inductively coupled plasma emission spectrometry analysis was applied to determine ischemiainduced changes of Mg 2+ and Ca 2+ in vulnerable regions of rat brain. This method can provide an accurate quantification and lower detection limits, as compared to atomic absorption spectrophotometry or several other methods. In the hippocampus, Mg 2+ content significantly increases 24 h following 20 min of ischemia, followed by a gradual decrease between 48 and 72 h. Ca 2+ accumulation was found at 48 and 72 h. At the cell membrane, Mg 2+ plays a role as an endogenous calcium channel blocker of both the receptor-operated and voltage-dependent gates and, in the mitochondria, Mg 2+ inhibits Ca 2+ uptake processes. We propose that the mobilization of Mg 2+ after 24 h reperfusion may counteract the process of ischemia-induced neuronal damage and that decreases of Mg 2+ may be correlated with the degree of brain injury. However, in the natural concentration of Mg 2+, the counteraction may not be sufficient for a neuroprotective effect. Therefore, after 24 h reperfusion, an artificial enhancement of Mg 2+ is necessary for neuroprotection. In order to test the above hypothesis, MgCl 2 (50 mM) was administered directly to the CA1 sector of the rat hippocampus before and at various intervals following 20 min of ischemia. Pyramidal cells were evaluated seven days later and neuronal density was determined. Consistent with the hypothesis, a neuroprotective effect was observed, even when MgCl 2 was administered 24 h, but not 48 h, after the ischemic episode.

Control of calcium channels in neuroblastoma cells (N1E-115)

Neuroblastoma cells (N1E-115) were used as models of transient (T) and long-lasting (L) Ca ++ channels. The whole cell version of the patch clamp technique was used to measure inward Ca ++ currents, and the fluorescent indicator, Fura-2, was used to measure changes in intracellular Ca ++. Cells were cultured and selected during recording so that predominately T or L channel currents were measured. T channel currents did not respond to dihydropyridine or parathyroid hormone, whereas L channel currents did. BAY-K-8644 increased and nifedipine decreased L channel currents. After a 15 mM KCl challenge, cells with predominately T channels responded with a transient change in intracellular Ca ++, while cells with predominately L channels showed a sustained response. PTH inhibited the increase in intracellular Ca ++ in cells with L channels, but not in those with T channels. PTH may be an example of an endogenous calcium channel blocker, at least in neuroblastoma cells.

Endogenous photoproteins, calcium channels and calcium transients during metamorphosis in hydrozoans.

There are species of hydrozoans, Eutonina victoria, Mitrocomella polydiademata, and Phialidium gregarium whose eggs contain calcium-specific photoproteins. These cytoplasmic photoproteins are synthesized during oogenesis. During the cleavage stages of embryogenesis they are distributed to all of the cells of the developing planula larva. The amount of photoprotein slowly declines during the development of the planula larva, and markedly declines when the planula undergoes metamorphosis to become a polyp.Oocytes, unfertilized eggs, and fertilized eggs prior to the first cleavage do not produce light when treated with KCl. The ability to respond to KCl appears about the time of first cleavage, and is correlated with the appearance of active membrane responses. Both the KCl response and the action potentials will occur in sodium-free sea water, and both are inhibited by calcium channel blockers. These and other experiments suggest that voltage sensitive calcium channels first become active at about the time of first cleavage. These channels also appear on the same schedule in both unfertilized eggs and in enucleated egg fragments, which have been artificially activated with A23187.Developing planulae produce few or no spontaneous light responses before gastrulation. Later the frequency and magnitude of spontaneous light production increases presumably due to an increasing frequency and magnitude of calcium transients. Both the natural trigger of metamorphosis (bacteria) and an artificial trigger (CsCl) cause a conspicuous series of calcium transients. When these transients are inhibited by calcium channel blockers, metamorphosis is also inhibited.

The effects of maitotoxin on 45Ca2+ flux and hormone release in GH3 rat pituitary cells.

Maitotoxin has been reported to activate calcium channels and stimulate calcium-dependent functions in several tissues, but a thorough investigation of 45Ca2+ fluxes is lacking. To characterize the influence of maitotoxin on 45Ca2+ flux in greater detail, we incubated dispersed GH3 pituitary tumor cells in 45Ca2+ with maitotoxin and other agents affecting calcium channels. Within 10 sec of exposure, maitotoxin induced a net calcium influx in cells at isotopic equilibrium. Calcium uptake was concentration dependent between 0.4 and 40 ng/ml maitotoxin and was inhibited by antagonists of voltage-dependent calcium channels but not by inhibitors of sodium channels. PRL and GH release from perifused GH3 cells was stimulated within 1 min by maitotoxin. We conclude that maitotoxin causes a rapid, concentration-dependent influx of calcium through presumed voltage-dependent endogenous calcium channels, culminating in enhanced hormone release. This potent toxin may provide a more precise understanding of the role of calcium in the stimulus-secretion coupling process.