Slow kinetics of inositol 1,4,5-trisphosphate-induced Ca2+ release: is the release 'quantal' or 'non-quantal'?

Vesicularization of the endoplasmic reticulum is a fast response to plasma membrane injury

The endoplasmic reticulum of most cell types mainly consists of an extensive network of narrow sheets and tubules. It is well known that an excessive increase of the cytosolic Ca(2+) concentration induces a slow but extensive swelling of the endoplasmic reticulum into a vesicular morphology. We observed that a similar extensive transition to a vesicular morphology may also occur independently of a change of cytosolic Ca(2+) and that the change may occur at a time scale of seconds. Exposure of various types of cultured cells to saponin selectively permeabilized the plasma membrane and resulted in a rapid swelling of the endoplasmic reticulum even before a loss of permeability barrier was detectable with a low-molecular mass dye. The structural alteration was reversible provided the exposure to saponin was not too long. Mechanical damage of the plasma membrane resulted in a large-scale transition of the endoplasmic reticulum from a tubular to a vesicular morphology within seconds, also in Ca(2+)-depleted cells. The rapid onset of the phenomenon suggests that it could perform a physiological function. Various mechanisms are discussed whereby endoplasmic reticulum vesicularization could assist in protection against cytosolic Ca(2+) overload in cellular stress situations like plasma membrane injury.

A novel Ca2+-induced Ca2+ release mechanism in A7r5 cells regulated by calmodulin-like proteins

Intracellular Ca2+ release is involved in setting up Ca2+ signals in all eukaryotic cells. Here we report that an increase in free Ca2+ concentration triggered the release of up to 41 +/- 3% of the intracellular Ca2+ stores in permeabilized A7r5 (embryonic rat aorta) cells with an EC50 of 700 nm. This type of Ca2+-induced Ca2+ release (CICR) was neither mediated by inositol 1,4,5-trisphosphate receptors nor by ryanodine receptors, because it was not blocked by heparin, 2-aminoethoxydiphenyl borate, xestospongin C, ruthenium red, or ryanodine. ATP dose-dependently stimulated the CICR mechanism, whereas 10 mm MgCl2 abolished it. CICR was not affected by exogenously added calmodulin (CaM), but CaM1234, a Ca2+-insensitive CaM mutant, strongly inhibited the CICR mechanism. Other proteins of the CaM-like neuronal Ca2+-sensor protein family such as Ca2+-binding protein 1 and neuronal Ca2+ sensor-1 were equally potent for inhibiting the CICR. Removal of endogenous CaM, using a CaM-binding peptide derived from the ryanodine receptor type-1 (amino acids 3614-3643) prevented subsequent activation of the CICR mechanism. A similar CICR mechanism was also found in 16HBE14o-(human bronchial mucosa) cells. We conclude that A7r5 and 16HBE14o-cells express a novel type of CICR mechanism that is silent in normal resting conditions due to inhibition by CaM but becomes activated by a Ca2+-dependent dissociation of CaM. This CICR mechanism, which may be regulated by members of the family of neuronal Ca2+-sensor proteins, may provide an additional route for Ca2+ release that could allow amplification of small Ca2+ signals.

Influence of the in vivo calcium status on cellular calcium homeostasis and the level of the calcium-binding protein calreticulin in rat hepatocytes

Little attention has been given to the consequences of the in vivo calcium status on intracellular calcium homeostasis despite several pathological states induced by perturbations of the in vivo calcium balance. The aim of these studies was to probe the influence of an in vivo calcium deficiency on the resting cytoplasmic Ca2+ concentration and the inositol-1,4,5-trisphosphate-sensitive Ca2+ pools. Studies were conducted in hepatocytes (a cell type well characterized for its cellular Ca2+ response) isolated from normal and calcium-deficient rats secondary to vitamin D depletion. Both resting cytoplasmic Ca2+ concentration and Ca2+ mobilization from inositol-1,4,5-trisphosphate-sensitive cellular pools were significantly lowered by calcium depletion. In addition, Ca deficiency was shown to significantly reduce calreticulin messenger RNA and protein levels but calcium entry through store-operated calcium channels remained unaffected, indicating that the Ca2+ entry mechanisms are still fully operational in calcium deficiency. The effects of calcium deficiency on cellular calcium homeostasis were reversible by repletion with oral calcium feeding alone or by the administration of the calcium-regulating hormone 1,25-dihydroxyvitamin D3, further strengthening the tight link between extra- and intracellular calcium. These data, therefore, challenge the currently prevailing hypothesis that extracellular Ca2+ has no significant impact on cellular Ca2+ by demonstrating that despite the large Ca2+ gradient between extra- and intracellular Ca2+ concentrations, calcium deficiency in vivo significantly alters the hormone-sensitive cellular calcium homeostasis.

Regulation of ryanodine receptor opening by lumenal Ca(2+) underlies quantal Ca(2+) release in PC12 cells

Graded or "quantal" Ca(2+) release from intracellular stores has been observed in various cell types following activation of either ryanodine receptors (RyR) or inositol 1,4,5-trisphosphate receptors (InsP(3)R). The mechanism causing the release of Ca(2+) stores in direct proportion to the strength of stimulation is unresolved. We investigated the properties of quantal Ca(2+) release evoked by activation of RyR in PC12 cells, and in particular whether the sensitivity of RyR to the agonist caffeine was altered by lumenal Ca(2+). Quantal Ca(2+) release was observed in cells stimulated with 1 to 40 mM caffeine, a range of caffeine concentrations giving a >10-fold change in lumenal Ca(2+) content. The Ca(2+) load of the caffeine-sensitive stores was modulated by allowing them to refill for varying times after complete discharge with maximal caffeine, or by depolarizing the cells with K(+) to enhance their normal steady-state loading. The threshold for RyR activation was sensitized approximately 10-fold as the Ca(2+) load increased from a minimal to a maximal loading. In addition, the fraction of Ca(2+) released by low caffeine concentrations increased. Our data suggest that RyR are sensitive to lumenal Ca(2+) over the full range of Ca(2+) loads that can be achieved in an intact PC12 cell, and that changes in RyR sensitivity may be responsible for the termination of Ca(2+) release underlying the quantal effect.

Determination of time-dependent inositol-1,4,5-trisphosphate concentrations during calcium release in a smooth muscle cell

The level of [InsP3]cyt required for calcium release in A7r5 cells, a smooth muscle cell line, was determined by a new set of procedures using quantitative confocal microscopy to measure release of InsP3 from cells microinjected with caged InsP3. From these experiments, the [InsP3]cyt required to evoke a half-maximal calcium response is 100 nM. Experiments with caged glycerophosphoryl-myo-inositol 4, 5-bisphosphate (GPIP2), a slowly metabolized analogue of InsP3, gave a much slower recovery and a half-maximal response of an order of magnitude greater than InsP3. Experimental data and highly constrained variables were used to construct a mathematical model of the InsP3-dependent [Ca2+]cyt changes; the resulting simulations show high fidelity to experiment. Among the elements considered in constructing this model were the mechanism of the InsP3-receptor, InsP3 degradation, calcium buffering in the cytosol, and refilling of the ER stores via sarcoplasmic endoplasmic reticulum ATPase (SERCA) pumps. The model predicts a time constant of 0.8 s for InsP3 degradation and 13 s for GPIP2. InsP3 degradation was found to be a prerequisite for [Ca2+]cyt recovery to baseline levels and is therefore critical to the pattern of the overall [Ca2+]cyt signal. Analysis of the features of this model provides insights into the individual factors controlling the amplitude and shape of the InsP3-mediated calcium signal.

Magnolol induces cytosolic-free Ca2+ elevation in rat neutrophils primarily via inositol trisphosphate signalling pathway

In the present study, we describe the role of inositol trisphosphate in the signalling pathway that leads to the elevation of cytosolic-free Ca2+ in rat neutrophils stimulated with magnolol, a compound isolated from the cortex of Magnolia officinalis. Magnolol increased [Ca2+]i, by stimulating Ca2+ release from internal stores and Ca2+ influx across the plasma membrane, in a concentration-dependent manner. Ni2+ and [6-[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H -pyrrole-2,5-dione (U73122), but not pertussis toxin, inhibited the magnolol-induced Ca2+ influx. Measurement of cellular levels of inositol trisphosphate showed a clear increase upon exposure to magnolol. U73122 but not ryanodine suppressed the Ca2+ release from internal stores caused by magnolol. Pretreatment of cells with formyl-Met-Leu-Phe (fMLP) or cyclopiazonic acid greatly reduced the [Ca2+]i changes caused by the subsequent addition of magnolol. Collectively, these findings suggest that a pertussis toxin-insensitive inositol trisphosphate signalling pathway is involved in the magnolol-induced [Ca2+]i elevation in rat neutrophils.

Evidence for mitochondrial Ca(2+)-induced Ca2+ release in permeabilised endothelial cells

Generally most intracellular Ca2+ is stored in the endoplasmic reticulum (ER) and mitochondria. Recently a mitochondrial Ca(2+)-induced Ca2+ release (mCICR) mechanism, unconnected with ryanodine receptors (RyR's), has been shown in tumour cells. The existence of a mitochondrial Ca2+ release mechanism in BAE cells was investigated using saponin-permeabilised BAE cells. When buffered intracellular solution were 'stepped' from 10 nM to 10 microM free Ca2+, the mitochondrial inhibitors CN (2 mM), FCCP (1 microM), and RR (20 microM) significantly reduced total CICR by approximately 25%. The ER Ca(2+)-ATPase inhibitor thapsigargin (100 nM) had no effect. Furthermore, cyclosporin A (200 nM), an inhibitor of the mitochondrial permeability transition pore (PTP), abolished total CICR. Therefore, the novel ryanodine-caffeine insensitive CICR mechanism previously reported in BAE cells involves mitochondrial Ca2 release. It is proposed that in BAE cells, mCICR occurs via the mitochondrial PTP and may be physiologically important in endothelial cell Ca2+ signalling.

Electrophysiological properties of human astrocytic tumor cells In situ: enigma of spiking glial cells

To better understand physiological changes that accompany the neoplastic transition of astrocytes to become astrocytoma cells, we studied biopsies of low-grade, pilocytic astrocytomas. This group of tumors is most prevalent in children and the tumor cells maintain most antigenic features typical of astrocytes. Astrocytoma cells were studied with the use of whole cell patch-clamp recordings in acute biopsy slices from 4-mo- to 14-yr-old pediatric patients. Recordings from 53 cells in six cases of low-grade astrocytomas were compared to either noncancerous peritumoral astrocytes or astrocytes obtained from other surgeries. Astrocytoma cells almost exclusively displayed slowly activating, sustained, tetraethylammonium (TEA)-sensitive outward potassium currents (delayed rectifying potassium currents; IDR) and transient, tetrodotoxin (TTX)-sensitive sodium currents (INa). By contrast, comparison glial cells from peritumoral regions or other surgeries showed IDR and INa, but in addition these cells also expressed transient "A"-type K+ currents and inwardly rectifying K+ currents (IIR), both of which were absent in astrocytoma cells. IIR constituted the predominant conductance in comparison astrocytes and was responsible for a high-resting K+ conductance in these cells. Voltage-activated Na+ currents were observed in 37 of 53 astrocytoma cells. Na+ current densities in astrocytoma cells, on average, were three- to fivefold larger than in comparison astrocytes. Astrocytoma cells expressing INa could be induced to generate slow action potential-like responses (spikes) by current injections. The threshold for generating such spikes was -34 mV (from a holding potential of -70 mV). The spike amplitude and time width were 52.5 mV and 12 ms, respectively. No spikes could be elicited in comparison astrocytes, although some of them expressed Na+ currents of similar size. Comparison of astrocytes to astrocytoma cells suggests that the apparent lack of IIR, which leads to high-input resistance (>500 MOmega), allows glioma cells to be sufficiently depolarized to generate Na+ spikes, whereas the high resting K+ conductance in astrocytes prevents their depolarization and thus generation of spikes. Consistent with this notion, Na+ spikes could be induced in spinal cord astrocytes in culture when IIR was experimentally blocked by 10 microM Ba2+, suggesting that the absence of IIR in astrocytoma cells is primarily responsible for the unusual spiking behavior seen in these glial tumor cells. It is unlikely that such glial spikes ever occur in vivo.

Modelling of some potential effects of lumenal Ca2+ binding on the kinetics of Ca2+ release from the endoplasmic reticulum

The time course of Ca2+ release from intracellular stores during a prolonged exposure to a constant concentration of inositol 1,4,5-trisphosphate does not follow a single exponential but presents a progressive slowing down of the relative rate of efflux. Among the many factors that may contribute to this phenomenon, the possible contribution of lumenal Ca2+ buffering has been largely neglected. However, since the efflux rate depends on the free Ca2+ concentration whereas the total Ca2+ content is mainly determined by bound Ca2+, simple Ca2+ efflux kinetics can be expected only if there is a linear relation between free Ca2+ and bound Ca2+. Although little is known on lumenal Ca2+ binding, reasonable assumptions predict that the ratio of (Ca2+ bound)/(Ca2+ free) may decrease with increasing free Ca2+ concentrations, implying a continuously decreasing rate coefficient, as is experimentally observed. Simulated Ca2+ release curves show significant deviations from a single exponential if high-affinity binding sites are present in addition to low-affinity sites. Although this simple model does not predict several of the experimentally observed properties, it is concluded that a full understanding of Ca2+ release kinetics requires more detailed information on lumenal Ca2+ buffering within Ca2+ stores.