An heuristic hypothesis of chilling injury in plants: a role for calcium as the primary physiological transducer of injury

Abstract. It is suggested that increased levels of free cytosolic calcium ([Ca²⁺]_cyt) may serve as the primary physiological transducer of chilling injury in plants. Numerous similarities between the effects of [Ca²⁺]_cyt-raising treatments on plants and the effects of chilling temperatures on chilling-sensitive (CS) plants are noted. It is proposed that chilling temperatures may lead to increases in [Ca²⁺]_cyt in CS plant cells by reducing the rate at which they exclude Ca²⁺ from their cytosol and that rapid cooling (coldshock) may cause rapid increases in [Ca²⁺]_cyt due to the activation of voltage-dependent cation channels. Chill-induced increases in [Ca²⁺]_cyt in the cells of CS plants may reflect either an inherent inability of such plants to maintain homeostatic levels of Ca²⁺ at low temperatures or a stress-induced reaction which has evolved to enable such cells to cope more effectively with the short-term hardships imposed by cold. Previous proposals concerning the physiological transduction of chilling injury are also discussed. It is argued that there is little evidence to suggest that the immediate effects of low temperatures on CS cells include either decreases in ATP levels, general increases in the passive permeability of membranes, or increased rates of fermentation.     

Effects of Somatostatin on Intracellular Calcium Concentration in PC12 Cells

Abstract: Somatostatin (SS) is a neuropeptide that is distributed in various regions of the CNS, where it may act as a neurotransmitter or neuromodulator. SS produces multiple effects in the CNS through interactions with membrane receptors. In particular, SS inhibits various secretory responses in different cell types. In the present study, we have investigated the effects of exogenous application of SS on the intracellular free Ca²⁺ concentration ([Ca²⁺]_i) in PC12 cells, a rat pheochromocytoma cell line. SS did reduce the magnitude of the secondary, maintained Ca²⁺ influx brought about by K⁺ depolarization. Similar effects were obtained with the application of SS analogues, such as d -Trp⁸-SS, d -Trp⁸- d -Cys¹⁴-SS, CGP-23996, and SMS-201995. In addition, treatment with cyclo-SS, a SS antagonist, did not alter [Ca²⁺]_i. Experiments with selective blockers of different voltage-dependent Ca²⁺ channels, such as methoxyverapamil (D600) and Ω-conotoxin GVIA, demonstrated that the effects of SS on [Ca²⁺]_i were mediated by voltage-dependent Ca²⁺ channels of the L type. Control experiments with a membrane potential indicator, i.e., the fluorescent dye bisoxonol, excluded that SS influenced the level of the membrane potential. SS effects on PC12 cells suggest the possibility that this neuropeptide plays a role in the modulation of cell functional activity by altering Ca²⁺ influx.     

Actin Dynamics Regulates Voltage-Dependent Calcium-Permeable Channels of the Vicia faba Guard Cell Plasma Membrane

Free cytosolic Ca²⁺ ([Ca²⁺]_cyt) is an ubiquitous second messenger in plant cell signaling, and [Ca²⁺]_cyt elevation is associated with Ca²⁺-permeable channels in the plasma membrane and endomembranes regulated by a wide range of stimuli. However, knowledge regarding Ca²⁺ channels and their regulation remains limited in planta . A type of voltage-dependent Ca²⁺-permeable channel was identified and characterized for the Vicia faba L. guard cell plasma membrane by using patch-clamp techniques. These channels are permeable to both Ba²⁺ and Ca²⁺, and their activities can be inhibited by micromolar Gd³⁺. The unitary conductance and the reversal potential of the channels depend on the Ca²⁺ or Ba²⁺ gradients across the plasma membrane. The inward whole-cell Ca²⁺ (Ba²⁺) current, as well as the unitary current amplitude and NP_o of the single Ca²⁺ channel, increase along with the membrane hyperpolarization. Pharmacological experiments suggest that actin dynamics may serve as an upstream regulator of this type of calcium channel of the guard cell plasma membrane. Cytochalasin D, an actin polymerization blocker, activated the NPo of these channels at the single channel level and increased the current amplitude at the whole-cell level. But these channel activations and current increments could be restrained by pretreatment with an F-actin stabilizer, phalloidin. The potential physiological significance of this regulatory mechanism is also discussed.     

Mechanism Underlying the ATP-Induced Increase in the Cytosolic Ca2+ Concentration in Chick Ciliary Ganglion Neurons

Abstract: We examined the mechanism underlying the ATP-induced increase in the cytosolic Ca²⁺ concentration ([Ca]_in) in acutely isolated chick ciliary ganglion neurons, using fura-2 microfluorometry. The ATP-induced increase in [Ca]_in was dependent on external Ca²⁺, was blocked in a dose-dependent manner by reactive blue 2, and was substantially inhibited by both L- and N-type Ca²⁺ channel blockers. ATP was effective in increasing [Ca]_in in the presence of a desensitizing concentration of nicotine (100 µ M ), and simultaneous addition of maximal doses of ATP and nicotine caused an additive increase in [Ca]_in, suggesting that ATP acts on a site distinct from nicotinic acetylcholine receptors. ATP also increased the cytosolic Na⁺ concentration as determined by sodium-binding benzofuran isophthalate microfluorometry. These results suggest that ATP increases Na⁺ influx through P₂ purinoceptor-associated channels resulting in membrane depolarization, which in turn increases Ca²⁺ influx through voltage-dependent Ca²⁺ channels. However, ATP still caused a small increase in [Ca]_in under Na⁺-free conditions, and this [Ca]_in increase was little affected by Ca²⁺ channel blockers. ATP also increased Mn²⁺ influx under Na⁺-free conditions, as indicated by quenching of fura-2 fluorescence. These results suggest that nonselective cationic channels activated by ATP are permeable not only to Ca²⁺ but also to Mn²⁺, in addition to monovalent cations.     

Plasma Ca2+ concentration limits melatonin night production in two fish species

In freshwater (FW) rainbow trout Oncorhynchus mykiss of spontaneously low plasma calcium concentrations ([Ca²⁺]_pl), plasma melatonin at night was significantly lower than that measured in FW fish with the highest [Ca²⁺]_pl. In brackish water adapted rainbow trout with originally high [Ca²⁺]_pl, plasma melatonin concentration at night was elevated. In cannulated flounder Platichthys flesus , night plasma melatonin increases (ΔMel) corresponded to [Ca²⁺]_pl. It is postulated that in physiological steady-state conditions, melatonin synthesis capacity is coupled to free calcium concentration in plasma of O. mykiss and P. flesus .     

Early Events in Free Radical-Mediated Damage of Isolated Nerve Terminals: Effects of Peroxides on Membrane Potential and Intracellular Na+ and Ca2+ Concentrations

Abstract: The effects of peroxides were investigated on the membrane potential, intracellular Na⁺ ([Na⁺]_i) and intracellular Ca²⁺ ([Ca²⁺]_i) concentrations, and basal glutamate release of synaptosomes. Both H₂O₂ and the organic cumene hydroperoxide produced a slow and continuous depolarization, parallel to an increase of [Na⁺]_i over an incubation period of 15 min. A steady rise of the [Ca²⁺]_i due to peroxides was also observed that was external Ca²⁺ dependent and detected only at an inwardly directed Ca²⁺ gradient of the plasma membrane. These changes did not correlate with lipid peroxidation, which was elicited by cumene hydroperoxide but not by H₂O₂. Resting release of glutamate remained unchanged during the first 15 min of incubation in the presence of peroxides. These alterations may indicate early dysfunctions in the sequence of events occurring in the nerve terminals in response to oxidative stress.     

Inhibition of Bradykinin-Induced Calcium Increase by Phosphatase Inhibitors in Neuroblastoma × Glioma Hybrid NG108-15 Cells

Abstract: Prior treatment of NG108-15 cells with phosphatase inhibitors including okadaic acid and calyculin A inhibited the elevation of cytosolic Ca²⁺ concentration ([Ca²⁺]_i) induced by bradykinin by ∼63%. This inhibition was dependent on the concentration of okadaic acid with an IC₅₀ of 0.15 n M . Okadaic acid treatment only lowered the maximal response of [Ca²⁺]_i increase and had no effect on the EC₅₀ value for bradykinin regardless of the presence of extracellular Ca²⁺. Neither the capacity of ⁴⁵Ca²⁺ accumulation within intracellular nonmitochondrial Ca²⁺ stores nor the magnitude of [Ca²⁺]_i increase induced by thapsigargin was reduced by the treatment of okadaic acid. In contrast, the same phosphatase inhibitor treatment inhibited the bradykinin-evoked inositol 1,4,5-trisphosphate (IP₃) generation, the Mn²⁺ influx, and the capacity of mitochondrial Ca²⁺ accumulation. Furthermore, the sensitivity of IP₃ in the Ca²⁺ release was suppressed by okadaic acid pretreatment. Our results suggest that the reduction of bradykinin-induced [Ca²⁺]_i rise by the promotion of protein phosphorylation was attributed to the reduced activity of phospholipase C, the decreased sensitivity to IP₃, and the slowed rate of Ca²⁺ influx. Thus, phosphorylation plays a role in bradykinin-sensitive Ca²⁺ signaling cascade in NG108-15 cells.     

Catecholamine Secretion from Bovine Adrenal Chromaffin Cells: The Role of the Na+/Ca2+ Exchanger and the Intracellular Ca2+ Pool

Abstract: The role of the Na⁺/Ca²⁺ exchanger and intracellular nonmitochondrial Ca²⁺ pool in the regulation of cytosolic free calcium concentration ([Ca²⁺]_i) during catecholamine secretion was investigated. Catecholamine secretion and [Ca²⁺]_i were simultaneously monitored in a single chromaffin cell. After high-K⁺ stimulation, control cells and cells in which the Na⁺/Ca²⁺ exchange activity was inhibited showed similar rates of [Ca²⁺]_i elevation. However, the recovery of [Ca²⁺]_i to resting levels was slower in the inhibited cells. Inhibition of the exchanger increased the total catecholamine secretion by prolonging the secretion. Inhibition of the Ca²⁺ pump of the intracellular Ca²⁺ pool with thapsigargin caused a significant delay in the recovery of [Ca²⁺]_i and greatly enhanced the secretory events. These data suggest that both the Na⁺/Ca²⁺ exchanger and the thapsigargin-sensitive Ca²⁺ pool are important in the regulation of [Ca²⁺]_i and, by modulating the time course of secretion, are important in determining the extent of secretion.     

Hypoxia-Induced Catecholamine Release and Intracellular Ca2+ Increase via Suppression of K+ Channels in Cultured Rat Adrenal Chromaffin Cells

Abstract: Hypoxia (5% O₂) enhanced catecholamine release in cultured rat adrenal chromaffin cells. Also, the intracellular free Ca²⁺ concentration ([Ca²⁺]_i) increased within 3 min in ∼50% of the chromaffin cells under hypoxic stimulation. The increase depended on the presence of extracellular Ca²⁺. Nifedipine and ω-conotoxin decreased the population of the cells that showed the hypoxia-induced [Ca²⁺]_i increase, showing that the Ca²⁺ influx was attributable to L- and N-type voltage-dependent Ca²⁺ channels. The membrane potential was depolarized during the perfusion with the hypoxic solution and returned to the basal level following the change to the normoxic solution (20% O₂). Membrane resistance increased twofold under the hypoxic condition. The current-voltage relationship showed a hypoxia-induced decrease in the outward K⁺ current. Among the K⁺ channel openers tested, cromakalim and levcromakalim, both of which interact with ATP-sensitive K⁺ channels, inhibited the hypoxia-induced [Ca²⁺]_i increase and catecholamine release. The inhibitory effects of cromakalim and levcromakalim were reversed by glibenclamide and tolbutamide, potent blockers of ATP-sensitive K⁺ channels. These results suggest that some fractions of adrenal chromaffin cells are reactive to hypoxia and that K⁺ channels sensitive to cromakalim and glibenclamide might have a crucial role in hypoxia-induced responses. Adrenal chromaffin cells could thus be a useful model for the study of oxygen-sensing mechanisms.     

Cytoplasmic free Ca2+ dynamics in single tomato (Lycopersicon esculentum) protoplasts subjected to chilling temperatures

Protoplasts were isolated from leaves of tomato seedlings ( Lycopersicon esculentum Mill., cv. Marmande) at the 2nd to 4th true leaf stage and were loaded with the calcium binding tetra[acetoxymethyl⁺] ester of the fluorescent stilbene chromophore, Fura 2. Although the loading efficiency of the dye in these protoplasts was low, many protoplasts loaded only in the cytosol were always obtained. Changes in the cytosolic calcium concentration ([Ca²⁺]_cyt) were determined in single protoplasts in a temperature-controlled perfusion chamber by use of fluorescence photometry microscopy after excitation at 340 and 380 nm. When the protoplasts were subjected to chilling temperatures (10–15°C) by a circulating solution, the [Ca²⁺]_cyt increased in 64% of the analysed protoplasts. Depending on the initial resting level of [Ca²⁺]_cyt, three main types of kinetics were obtained in these protoplasts: (1) In 21% of the protoplasts, [Ca²⁺]_cyt increased to a maximum within 10–20 s from the start of temperature decrease, followed by a fast decrease; (2) in 11% of the protoplasts, the [Ca²⁺]_cyt both increased and decreased somewhat slower; and (3) in 32% a constant increase of [Ca²⁺]_cyt was obtained 1 min after the start of temperature decrease.     

Cyclothiazide Modulates AMPA Receptor-Mediated Increases in Intracellular Free Ca2+ and Mg2+ in Cultured Neurons from Rat Brain

Abstract: We investigated the modulation of (±)-α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-induced increases in intracellular free Ca²⁺ ([Ca²⁺]_i) and intracellular free Mg²⁺ ([Mg²⁺]_i) by cyclothiazide and GYKI 52466 using microspectrofluorimetry in single cultured rat brain neurons. AMPA-induced changes in [Ca²⁺]_i were increased by 0.3–100 µ M cyclothiazide, with an EC₅₀ value of 2.40 µ M and a maximum potentiation of 428% of control values. [Ca²⁺]_i responses to glutamate in the presence of N -methyl- d -aspartate (NMDA) receptor antagonists were also potentiated by 10 µ M cyclothiazide. The response to NMDA was not affected, demonstrating specificity of cyclothiazide for non-NMDA receptors. Almost all neurons responded with an increase in [Ca²⁺]_i to both kainate and AMPA in the absence of extracellular Na⁺, and these Na⁺-free responses were also potentiated by cyclothiazide. GYKI 52466 inhibited responses to AMPA with an IC₅₀ value of 12.0 µ M . Ten micromolar cyclothiazide significantly decreased the potency of GYKI 52466. However, the magnitude of this decrease in potency was not consistent with a competitive interaction between the two ligands. Cyclothiazide also potentiated AMPA- and glutamate-induced increases in [Mg²⁺]_i. These results are consistent with the ability of cyclothiazide to decrease desensitization of non-NMDA glutamate receptors and may provide the basis for the increase in non-NMDA receptor-mediated excitotoxicity produced by cyclothiazide.     

Modulation of Ion Gradients and Glutamate Release in Cultured Cerebellar Granule Cells by Ouabain

Abstract: Upon addition of the cardiac glycoside ouabain to cultured cerebellar granule cells, an immediate increase in intracellular free sodium is evoked mediated by two pathways, a voltage-sensitive channel blocked by tetrodotoxin and a channel sensitive to flunarizine. Ouabain induces a steady plasma membrane depolarization in low Ca²⁺ medium; whereas in the presence of Ca²⁺, a distinct discontinuity is observed always preceded by a large increase in intracellular free Ca²⁺ ([Ca²⁺]_c). The plateau component of the increase can be inhibited additively by the L-type Ca²⁺ channel antagonist nifedipine, the spider toxin Aga-Gl, and the NMDA receptor antagonist MK-801. Single-cell imaging reveals that the [Ca²⁺]_c increase occurs asynchronously in the cell population and is not dependent on a critical level of extracellular glutamate or synaptic transmission between the cells. A prolonged release of glutamate is also observed that is predominantly Ca²⁺ dependent for the first 6–10 min after the evoked increase in [Ca²⁺]_c. This release is four times as large as that observed with 50 m M KCl and is predominantly exocytotic because release was inhibited by tetanus toxin, the V-type ATPase inhibitor bafilomycin, and Aga-Gl. It is proposed, therefore, that ouabain induces a period of membrane excitability culminating in a sustained exocytosis above that observed upon permanent depolarization with KCl.     

Involvement of Calcium Influx in Muscarinic Cholinergic Regulation of Phospholipase C in Cerebellar Granule Cells

Abstract: Inositol phosphate accumulation on carbachol stimulation of rat cerebellar granule cells shows a marked dependence on factors affecting cytosolic Ca²⁺ concentration ([Ca²⁺]_c). After 5 min, potassium depolarisation caused a modest accumulation of inositol phosphates but augmented the response to carbachol by a factor of 2–3. These effects of potassium were dependent on an extracellular source of calcium and could be partially blocked by specific (nifedipine) and nonspecific (verapamil) calcium channel blockers. Measurements of [Ca²⁺]_c under a range of stimulatory conditions demonstrated a close correlation between the elevation of [Ca²⁺]_c and agonist-stimulated phospholipase C (PLC) activity. The maximal potentiation of carbachol-stimulated inositol phosphate accumulation was achieved using 20 m M KCl, which increased [Ca²⁺]_c from ∼20 to ∼75 n M , indicating the involvement of relatively low threshold Ca²⁺ channels and the high sensitivity of the relevant PLC to small changes in [Ca²⁺]_c. By contrast, increases in [Ca²⁺]_c induced by the Ca²⁺ ionophore ionomycin were associated with more modest and less potent effects on agonist-stimulated PLC. These results demonstrate a cooperative interaction between a receptor/G protein-regulated PLC and voltage-stimulated elevations of [Ca²⁺]_c, which may function to integrate ionotropic and metabotropic signalling mechanisms in cerebellar granule cells.     

Endothelin-1 Increases Arachidonic Acid Release in C6 Glioma Cells Through a Potassium-Modulated Influx of Calcium

Abstract: Endothelin-1 (Et-1) but not a range of other receptor agonists stimulated the release of arachidonic acid (AA) in C6 glioma. Et-1 activation was concentration dependent and was inhibited by chelation of extracellular calcium. The calcium ionophores A23187 and ionomycin could also stimulate release of AA. Et-1 caused an early increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) followed by a sustained but lower plateau level. The sensitivity of the response to quinacrine, its dependence on Ca²⁺, and the demonstration of an increase in phospholipase A₂ (PLA₂) activity that was insensitive to dithiothreitol suggested that the release of AA was due to activation of cytosolic PLA₂ in the cells. Staurosporine, a protein kinase C (PKC) inhibitor, had no effect on Et-1-induced AA release but abolished that by phorbol 12-myristate 13-acetate, demonstrating that the Et-1 response was PKC independent. Raised levels of extracellular KCI inhibited both AA release and the increase in [Ca²⁺]_i triggered by Et-1, whereas valinomycin, which causes K⁺ efflux, not only caused a rapid rise in [Ca²⁺]_i but also caused AA mobilisation. The results therefore suggest that Et-1 activation of PLA₂ in this cell type requires calcium influx dependent on K⁺ efflux.     

Involvement of Intracellular Stores in the Ca2+ Responses to N-Methyl-D-Aspartate and Depolarization in Cerebellar Granule Cells

Abstract: The [Ca²⁺]₁ of cerebellar granule cells can be increased in a biphasic manner by addition of NMDA or by depolarization (induced by elevating the extracellular K⁺ level), which both activate Ca²⁺ influx. The possibility that these stimuli activate Ca²⁺-induced Ca²⁺ release was investigated using granule cells loaded with fura 2-AM. Dantrolene, perfused onto groups of cells during the sustained plateau phase of the [Ca²⁺]₁ response to K⁺ or NMDA, was found to reduce the response to both agents in a concentration-dependent manner. Preincubation with thapsigargm (10 μ M ) substantially reduced the plateau phase of the [Ca²⁺], response to K⁺ and both the peak and plateau phases of the NMDA response. Preincubation with ryanodine (10 μ M ) also reduced both the K⁺-evoked plateau response and both phases of the NMDA response. Neither had a consistent effect on the peak response to K⁺. The effects of thapsigargin and ryanodine on the NMDA response were partially additive. These results demonstrate that in cerebellar granule cells a major component of both K⁺- and NMDA-induced elevation of [Ca²⁺]₁ appears to be due to release from intracellular stores. The partial additivity of the effects of thapsigargin and ryanodine suggests that these agents affect two overlapping but nonidentical Ca²⁺ pools.     

Fertilization envelope formation induced by melittin in sea urchin eggs does not depend on Ca2+ signalling

In sea urchin eggs, 10 μg/mL melittin was found to induce fertilization envelope formation without any increase in [Ca²⁺]_i (the intracellular free Ca²⁺ level). On the other hand, 10 μmol/L Br-A23187 and 100 μg/mL SDS induced fertilization envelope formation associated with [Ca²⁺]_i increase. If EGTA was injected into eggs to make an intracellular concentration of 2 mmol/L, [Ca²⁺]_i became quite low and was not altered by melittin, or by Br-A23187 and SDS. In eggs containing EGTA, fertilization envelope formation was induced by melittin even in Ca²⁺-free artificial sea water, but not by Br-A23187 or SDS. Thus [Ca²⁺]_i is essential for induction of a fertilization envelope in sea urchin eggs by Br-A23187 or SDS but not by melittin. Melittin probably activates some Ca²⁺-independent reaction downstream of Ca²⁺-dependent reactions in a sequential reaction system that finally results in fertilization envelope formation.     

Inhibition of Bradykinin-Induced Cytosolic Ca2+ Elevation by Muscarinic Stimulation Without Attenuation of Inositol 1,4,5-Trisphosphate Production in Human Neuroblastoma SK-N-BE(2)C Cells

Abstract: Cross talk between two phospholipase C (PLC)-linked receptor signalings was investigated in SK-N-BE(2)C human neuroblastoma cells. Sequential stimulation with two agonists at 5-min intervals was performed to examine the interaction between muscarinic and bradykinin (BK) receptors. Pretreatment of cells with a maximal effective concentration (5 µ M ) of BK did not affect the subsequent carbachol (CCh)-induced [Ca²⁺]_i rise, but CCh (1 m M ) pretreatment completely abolished the BK-induced [Ca²⁺]_i rise without inhibition of BK-induced inositol 1,4,5-trisphosphate (IP₃) generation. Thapsigargin (1 µ M ) pretreatment abolished the subsequent BK- and CCh-induced [Ca²⁺]_i rise, though it did not affect agonist-induced IP₃ generation. However, the addition of atropine at plateau phases of CCh-induced [Ca²⁺]_i rise and IP₃ production caused a rapid decline to the basal levels and then restored the [Ca²⁺]_i rise by BK. Treatment of cells with both CCh and BK at the same time showed additive effects in IP₃ production. However, the [Ca²⁺]_i rise induced by both agonists in the presence or absence of extracellular Ca²⁺ was the same as the responses triggered by CCh alone. The results suggest that each receptor or receptor-linked PLC activity is not influenced by pretreatment with the other agonist but IP₃-sensitive Ca²⁺ stores are shared by signal pathways from both receptors.     

α-Synuclein modulation of Ca2+ signaling in human neuroblastoma (SH-SY5Y) cells

Parkinson's disease (PD) is characterized in part by the presence of α-synuclein (α-syn) rich intracellular inclusions (Lewy bodies). Mutations and multiplication of the α-synuclein gene ( SNCA ) are associated with familial PD. Since Ca²⁺ dyshomeostasis may play an important role in the pathogenesis of PD, we used fluorimetry in fura-2 loaded SH-SY5Y cells to monitor Ca²⁺ homeostasis in cells stably transfected with either wild-type α-syn, the A53T mutant form, the S129D phosphomimetic mutant or with empty vector (which served as control). Voltage-gated Ca²⁺ influx evoked by exposure of cells to 50 mM K⁺ was enhanced in cells expressing all three forms of α-syn, an effect which was due specifically to increased Ca²⁺ entry via L-type Ca²⁺ channels. Mobilization of Ca²⁺ by muscarine was not strikingly modified by any of the α-syn forms, but they all reduced capacitative Ca²⁺ entry following store depletion caused either by muscarine or thapsigargin. Emptying of stores with cyclopiazonic acid caused similar rises of [Ca²⁺]_i in all cells tested (with the exception of the S129D mutant), and mitochondrial Ca²⁺ content was unaffected by any form of α-synuclein. However, only WT α-syn transfected cells displayed significantly impaired viability. Our findings suggest that α-syn regulates Ca²⁺ entry pathways and, consequently, that abnormal α-syn levels may promote neuronal damage through dysregulation of Ca²⁺ homeostasis.     

Vitamin D3 affects growth and Ca2+ uptake by Phaseolus vulgaris roots cultured in vitro

Vitamin D₃ at low concentration (10⁻⁹ M) inhibited the growth of Phaseolus vulgaris L. (cv. Contrancha) roots in vitro as measured by elongation (14 h) and [³H]-leucine incorporation into protein (2 h), and increased their labelling with ⁴⁵Ca²⁺ (2 h). Cycloheximide and puromycin (50 u.M) blocked vitamin D₃ stimulation of root ⁴⁵Ca²⁺ labelling, indicating that it is mediated by de novo protein synthesis. The calcium ionophore X-537A (10⁻⁵JW) induced similar changes both in root elongation and ⁴⁵Ca²⁺ uptake (14 h). This may indicate that the inhibitory effects of the sterol on root growth are mediated by changes in Ca²⁺ fluxes. However, this interpretation should be further strengthened by additional studies as the ionophore may have acted on root growth, affecting physiological processes other than Ca²⁺ transport.     

Signal Flows from Two Phospholipase C-Linked Receptors Are Independent in PC12 Cells

Abstract: Bradykinin (BK) receptor and P₂-purinergic receptor are known to be coupled to phospholipase C (PLC) in PC12 cells. To study the interaction between these two PLC-linked receptors, the presence of both receptors on individual cells was demonstrated by sequential Ca²⁺ spikes caused by BK and ATP in a single fura-2-loaded cell. BK- and ATP-induced catecholamine (CA) secretions were desensitized within 5 min. However, in the sequential experiment, the BK-induced homologous desensitization of CA secretion did not block the ATP-induced secretion, and vice versa. Each agonist-induced an increase in inositol 1,4,5-trisphosphate (IP₃) production and intracellular free Ca²⁺ concentration also led to homologous desensitization. However, there was no heterologous desensitization between the two agonists. When the cells were treated with both BK and ATP simultaneously, the amounts of CA secretion, IP₃ production, internal Ca²⁺ mobilization, and Ca²⁺ influx were all additive. We also found that both IP₃-induced Ca²⁺ release from intracellular Ca²⁺ stores and Ca²⁺ influx from extracellular space were able to release [³H]norepinephrine, and the secretion induced by both agonists was exactly additive in the absence or presence of extracellular Ca²⁺. The data suggest that the CA secretions caused by BK or ATP may have separate secretory pathways even though they activate identical second messenger pathways.