Kinetic basis of quantal calcium release from intracellular calcium stores

Glucosylceramide and glucosylsphingosine modulate calcium mobilization from brain microsomes via different mechanisms

We recently demonstrated that elevation of intracellular glucosylceramide (GlcCer) levels results in increased functional Ca2+ stores in cultured neurons, and suggested that this may be due to modulation of ryanodine receptors (RyaRs) by GlcCer (Korkotian, E., Schwarz, A., Pelled, D., Schwarzmann, G., Segal, M. and Futerman, A. H. (1999) J. Biol. Chem. 274, 21673-21678). We now systematically examine the effects of exogenously added GlcCer, other glycosphingolipids (GSLs) and their lyso-derivatives on Ca2+ release from rat brain microsomes. GlcCer had no direct effect on Ca2+ release, but rather augmented agonist-stimulated Ca2+ release via RyaRs, through a mechanism that may involve the redox sensor of the RyaR, but had no effect on Ca2+ release via inositol 1,4,5-trisphosphate receptors. Other GSLs and sphingolipids, including galactosylceramide, lactosylceramide, ceramide, sphingomyelin, sphingosine 1-phosphate, sphinganine 1-phosphate, and sphingosylphosphorylcholine had no effect on Ca2+ mobilization from rat brain microsomes, but both galactosylsphingosine (psychosine) and glucosylsphingosine stimulated Ca2+ release, although only galactosylsphingosine mediated Ca2+ release via the RyaR. Finally, we demonstrated that GlcCer levels were approximately 10-fold higher in microsomes prepared from the temporal lobe of a type 2 Gaucher disease patient compared with a control, and Ca2+ release via the RyaR was significantly elevated, which may be of relevance for explaining the pathophysiology of neuronopathic forms of Gaucher disease.

Kinetic studies of calcium-induced calcium release in cardiac sarcoplasmic reticulum vesicles

Fast Ca(2+) release kinetics were measured in cardiac sarcoplasmic reticulum vesicles actively loaded with Ca(2+). Release was induced in solutions containing 1.2 mM free ATP and variable free [Ca(2+)] and [Mg(2+)]. Release rate constants (k) were 10-fold higher at pCa 6 than at pCa 5 whereas Ryanodine binding was highest at pCa < or =5. These results suggest that channels respond differently when exposed to sudden [Ca(2+)] changes than when exposed to Ca(2+) for longer periods. Vesicles with severalfold different luminal calcium contents exhibited double exponential release kinetics at pCa 6, suggesting that channels undergo time-dependent activity changes. Addition of Mg(2+) produced a marked inhibition of release kinetics at pCa 6 (K(0.5) = 63 microM) but not at pCa 5. Coexistence of calcium activation and inhibition sites with equally fast binding kinetics is proposed to explain this behavior. Thimerosal activated release kinetics at pCa 5 at all [Mg(2+)] tested and increased at pCa 6 the K(0.5) for Mg(2+) inhibition, from 63 microM to 136 microM. We discuss the possible relevance of these results, which suggest release through RyR2 channels is subject to fast regulation by Ca(2+) and Mg(2+) followed by time-dependent regulation, to the physiological mechanisms of cardiac channel opening and closing.

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.

Age-related alterations in caffeine-sensitive calcium stores and mitochondrial buffering in rat basal forebrain

The properties of caffeine- and thapsigargin-sensitive endoplasmic reticulum calcium stores were compared in acutely dissociated basal forebrain neurons from young and aged F344 rats by ratiometric microfluorimetry. The ability of these stores to sequester and release calcium resembles that observed in other central neurons, with an important role of mitochondrial calcium buffering in regulating the response to caffeine. An age-related reduction in the filling state of the stores in resting cells appears to be mediated by increased rapid calcium buffering, which reduces the availability of calcium for uptake into the stores. An age-related decrease in the amplitude of maximal caffeine-induced calcium release was attributed to increased mitochondrial buffering. There were no age-related differences in the sensitivity to caffeine or in the calcium sequestration/release process at the level of the endoplasmic reticulum per se. These findings demonstrate the importance of interactions between cellular calcium buffering mechanisms and provide details regarding age-related changes in calcium homeostasis which have been thought to occur in these and other neurons associated with age-related neuronal dysfunctions.