共查询到20条相似文献,搜索用时 15 毫秒
1.
Phospholipase A 2 added directly to superfused [ 3H]norepinephrine-labeled synaptosomes could cause the release of neurotransmitter molecules. Chloroquine and quinacrine, which block the action of phospholipase A 2, inhibited either the phospholipase A 2-stimulated or the high potassium-stimulated release of [ 3H]norepinephrine from synaptosomes. Only quinacrine blocked the high potassium-stimulated influx of Ca 2+. It appears that during stimulation of synaptosomes, Ca 2+ influx leads to the activation of phospholipase A 2, which in turn, hydrolyzes membrane phospholipids in situ. The formation of lysophospholipids may alter the microenvironment and the physicochemical properties of membranes, resulting in the release of neurotransmitter through exocytosis. 相似文献
2.
In this study we investigated the release of Ca 2+ in brain microsomes after Ca 2+ loading by the Ca 2+-ATPase or by the Na +/Ca 2+ exchanger. The results show that in microsomes loaded with Ca 2+ by the Ca 2+-ATPase, Ins(1,4,5)P 3 (5 μM) release 21±2% of the total Ca 2+ accumulated, and that in the microsomes loaded with Ca 2+ by the Na 2+/Ca 2+ exchanger, Ins(1,4,5)P 3 released 28±3% of the total Ca 2+ accumulated. These results suggest that receptors of Ins(1,4,5)P 3 may be co-localized with the Na 2+/Ca 2+ exchanger in the endoplasmic reticulum membrane or that there are Ins(1,4,5)P 3 receptors in the plasma membrane where the Na 2+/Ca 2+ exchanger is normally present, or both. We also found that Ins(1,4,5)P 3 inhibited the Ca 2+-ATPase by 33.7%, but that it had no significant effect on the Na 2+/Ca 2+ exchanger. 相似文献
3.
Using a new fluorescence imaging technique, LAMP, we recently reported that Ca 2+ influx through store operated Ca 2+ channels (SOCs) strongly inhibits cell coupling in primary human fibroblasts (HF) expressing Cx43. To understand the mechanism of inhibition, we studied the involvement of cytosolic pH (pH i) and Ca 2+([Ca 2+] i) in the process by using fluorescence imaging and ion clamping techniques. During the capacitative Ca 2+ influx, there was a modest decline of pH i measured by BCECF. Decreasing pH i below neutral using thioacetate had little effect by itself on cell coupling, and concomitant pH i drop with thioacetate and bulk [Ca 2+i rise with ionomycin was much less effective in inhibiting cell coupling than Ca 2+ influx. Moreover, clamping pH i with a weak acid and a weak base during Ca 2+ influx largely suppressed bulk pH i drop, yet the inhibition of cell coupling was not affected. In contrast, buffering [Ca 2+i with BAPTA, but not EGTA, efficiently prevented cell uncoupling by Ca 2+ influx. We concluded that local Ca 2+ elevation subjacent to the plasma membrane is the primary cause for closing Cx43 channels during capacitative Ca 2+ influx. To assess how Ca 2+ influx affects junctional coupling mediated by other types of connexins, we applied the LAMP assay to Hela cells expressing Cx26. Capacitative Ca 2+ influx also caused a strong reduction of cell coupling, suggesting that the inhibitory effect by Ca 2+ influx may be a more general phenomenon. 相似文献
4.
Measurements of Ca 2+ influx and [Ca 2+] i changes in Fura-2/AM-loaded prothoracic glands (PGs) of the silkworm, Bombyx mori, were used to identify Ca 2+ as the actual second messenger of the prothoracicotropic hormone (PTTH) of this insect. Dose-dependent increases of [Ca 2+] i in PG cells were recorded in the presence of recombinant PTTH (rPTTH) within 5 minutes. The rPTTH-mediated increases of [Ca 2+] i levels were dependent on extracellular Ca 2+. They were not blocked by the dihydropyridine derivative, nitrendipine, an antagonist of high-voltage-activated (HVA) Ca 2+ channels, and by bepridil, an antagonist of low-voltage-activated (LVA) Ca 2+ channels. The trivalent cation La 3+, a non-specific blocker of plasma membrane Ca 2+ channels, eliminated the rPTTH-stimulated increase of [Ca 2+] i levels in PG cells and so did amiloride, an inhibitor of T-type Ca 2+ channels. Incubation of PG cells with thapsigargin resulted in an increase of [Ca 2+] i levels, which was also dependent on extracellular Ca 2+ and was quenched by amiloride, suggesting the existence of store-operated plasma membrane Ca 2+ channels, which can also be inhibited by amiloride. Thapsigargin and rPTTH did not operate independently in stimulating increases of [Ca 2+] i levels and one agent’s mediated increase of [Ca 2+] i was eliminated in the presence of the other. TMB-8, an inhibitor of intracellular Ca 2+ release from inositol 1,4,5 trisphosphate (IP 3)-sensitive Ca 2+ stores, blocked the rPTTH-stimulated increases of [Ca 2+] i levels, suggesting an involvement of IP 3 in the initiation of the rPTTH signaling cascade, whereas ryanodine did not influence the rPTTH-stimulated increases of [Ca 2+] i levels. The combined results indicate the presence of a cross-talk mechanism between the [Ca 2+] i levels, filling state of IP 3-sensitive intracellular Ca 2+ stores and the PTTH-receptor’s-mediated Ca 2+ influx. 相似文献
5.
Intervillous blood was collected from term placentae at delivery, and sera were tested for phospholipase A 2 under various experimental conditions. Enzyme activity was found to develop upon extended storage in the cold or at 37°C. The enzyme is reversibly inhibited by dithiothreitol, requires Ca ++ ions for activity, and tolerates various detergents. The apparent molecular weight is 42 kDa. In all these parameters the serum enzyme behaves similar to the 42 kDa phospholipase A 2 which we recently purified to homogeneity from thoroughly washed placental tissue. Serum phospholipase A 2 appears to be generated by proteolytic processing from a slightly larger inactive precursor which was detected immunochemically. Most likely this protein originates from fetal cells and may be released by membrane damage. We conclude that both placental serum and tissue harbour a novel type of phospholipase A 2 which is distinct from cytosolic and secretory phospholipases A 2. Preference for arachidonate containing substrate suggests a role in eicosanoid production within gestational tissues. 相似文献
6.
固废基改良剂是盐碱地改良的有效手段之一.活性焦作为一种多孔废弃物,应用于盐碱地预期可以起到改善土壤性质、减缓植物盐碱胁迫的作用.为了阐明活性焦对晋北盐碱地的改良效应,本研究设置不同用量活性焦(CK,0g·kg-1;A10,10 g·kg-1;A20,20 g·kg-1;A50,50 g·kg-1),分析其对盐碱地土壤性... 相似文献
7.
Airway myocytes are the primary effectors of airway reactivity which modulates airway resistance and hence ventilation. Stimulation of airway myocytes results in an increase in the cytosolic Ca 2+ concentration ([Ca 2+] i) and the subsequent activation of the contractile apparatus. Many contractile agonists, including acetylcholine, induce [Ca 2+] i increase via Ca 2+ release from the sarcoplasmic reticulum through InsP 3 receptors. Several models have been developed to explain the characteristics of InsP 3-induced [Ca 2+] i responses, in particular Ca 2+ oscillations. The article reviews the modelling of the major structures implicated in intracellular Ca 2+ handling, i.e., InsP 3 receptors, SERCAs, mitochondria and Ca 2+-binding cytosolic proteins. We developed theoretical models specifically dedicated to the airway myocyte which include the major mechanisms responsible for intracellular Ca 2+ handling identified in these cells. These biocomputations pointed out the importance of the relative proportion of InsP 3 receptor isoforms and the respective role of the different mechanisms responsible for cytosolic Ca 2+ clearance in the pattern of [Ca 2+] i variations. We have developed a theoretical model of membrane conductances that predicts the variations in membrane potential and extracellular Ca 2+ influx. Stimulation of this model by simulated increase in [Ca 2+] i predicts membrane depolarisation, but not great enough to trigger a significant opening of voltage-dependant Ca 2+ channels. This may explain why airway contraction induced by cholinergic stimulation does not greatly depend on extracellular calcium. The development of such models of airway myocytes is important for the understanding of the cellular mechanisms of airway reactivity and their possible modulation by pharmacological agents. 相似文献
8.
We previously demonstrated that oxysterols added to the culture medium of NRK 49F cells labelled with [ 14C] arachidonic acid potentiated arachidonic acid (AA) release and prostaglandin (PG) E 2 biosynthesis induced by the activation of these cells with fetal calf serum (FCS). In the absence of FCS, oxysterols had no effect on AA release. As phospholipase (Plase) A 2 activity is Ca 2+-dependent, we investigated whether oxysterol potentiating effect on AA release was related to an effect of these compounds on cell Ca 2+ concentration. In this paper, we show that the intensity of potentiation by oxysterol varies with the external cell Ca 2+ concentration; when external Ca 2+ is chelated by EGTA, the oxysterol effect persists, though it is decreased. The Ca 2+ channel inhibitor nifedipine does not decrease the potentiating effect of 25-OH cholesterol, indicating that, if oxysterol favours Ca 2+ entry into the cell, the nifedipine inhibited channel is not involved. At the usual concentration (5 μm/ml), oxysterols are not able to increase, mimmediately or after a short time of contact (90 min) the concentration of intracellular free Ca 2+ ([Ca 2+]) i measured by fluorescence of Quinn-2; at very high concentration of oxysterol (25 μm/ml), [Ca 2+] i only slightly increases (+30%). The liberation of AA induced by cell activation with the Ca 2+ ionophore ionomycin is also potentiated by 25-OH cholesterol. All these observations are not in favour of a proper effect o oxysterols on cell Ca 2+ level. 相似文献
9.
Ca 2+ mobilization elicited by simulation with brief pulses of high K + were monitored with confocal laser scanned microscopy in intact, guinea pig cardiac myocytes loaded with the calcium indicator fluo-3. Single wavelength ratioing of fluorescence images obtained after prolonged integration times revealed non-uniformities of intracellular Ca 2+ changes across the cell, suggesting the presence of significant spatial Ca 2+ gradients. Treatment with 20 μM ryanodine, an inhibitor of Ca 2+ release from the SR, and 10 μM verapamil, a calcium channel blocker, reduced by 42% and 76% respectively the changes in [Ca 2+] i elicited by membrane depolarization. The overall spatial distribution of [Ca 2+] i changes appeared unchanged. Ca 2+ transients recorded in the presence of verapamil and ryanodine (about 20% of the size of control responses), diminished in the presence of 50 μM 2-4 Dichlorbenzamil (DCB) or 5 mM nickel, two relatively specific inhibitors of the
exchange mechanism. Conversely, when the reversal potential of the
exchange was shifted to negative potentials by lowering [Na +] 0 or by increasing [Na +] i by treatment with 20 μM monensin, the amplitude of these Ca 2+ transients increased. Ca 2+ transients elicited by membrane depolarization and largely mediated by reverse operation of Na +-Ca 2+ exchange could be recorded in the presence of ryanodine, verapamil and monensin. These findings suggest that in intact guinea pig cardiac cells, Ca 2+ influx through the
exchange mechanism activated by a membrane depolarization in the physiological range can be sufficient to play a significant role in excitation-contraction coupling. 相似文献
10.
Intravenous administration of ovokinin(2–7), a cleavage peptide derived from ovalbumin, dose-dependently (0.1–5 mg/kg) lowered the mean arterial pressure (MAP) that was not accompanied by a significant change in the heart rate (HR) of urethane-anesthetized rats. The hypotensive effects of ovokinin(2–7) were five orders of magnitude lower compared to that of bradykinin and were largely prevented by pretreatment with the bradykinin B 2 receptor antagonist HOE140 (81.6±18.4%) and moderately affected by the B 1 receptor antagonist [des-Arg 10]-HOE140 (26.3±15.5%). Intracellular Ca 2+ levels, as measured by Fur 2-AM, were significantly elevated in cultured aorta smooth muscle cells by ovokinin(2–7). The increases were abolished by HOE140 and unaffected by [des-Arg 10]-HOE140. The elevation of intracellular Ca 2+ by ovokinin(2–7) was dependent on Ca 2+ entry from extracellular space as it was reduced in a Ca 2+-free solution. Pretreatment of the cells with the phospholipase C inhibitor U73122 (2 μM) eliminated the Ca 2+ increase by the peptide. PA phosphohydrolase and phospholipase A 2 inhibitors significantly reduced the responses as well. Our results show that ovokinin(2–7) modulates cardiovascular activity by interacting with B 2 bradykinin receptors. 相似文献
11.
Phospholipase A 2 and acyltransferase were assayed and characterized in pure axoplasm and neural tissues of squid. Intracellular phospholipase A 2 activity was highest in giant fiber lobe and axoplasm, followed by homogenates from retinal fibers, optic lobe and fin nerve. In most preparations, exogenous calcium (5 mM) caused a slight stimulation of activity. EGTA (2 mM) was somewhat inhibitory, indicating that low levels of endogenous calcium may be required for optimum activity. Phospholipase A 2 was inhibited by 0.1 mM p-bromophenacylbromide, and was completely inactivated following heating. The level of acylCoA: lysophosphatidylcholine acyltransferase activity was higher in axoplasm and giant fiber lobe than in other neural tissues of the squid. Km (apparent) and Vmax (apparent) for oleoyl-CoA and lysophosphatidylcholine were quite similar for axoplasm and giant fiber lobe enzyme preparations. Acyltransferase activity was inactivated by heat treatment, and greatly inhibited by 0.2 mM p-chloromercuribenzoate, and to a lesser extent by 20 mM N-ethylmaleimide. Phospholipase A2 activity was present in fractions enriched in axolemmal membranes (separated from squid retinal fibers and garfish olfactory nerve) from both tissues, and it was also highly concentrated in vesicles derived from squid axoplasm. In all three preparations, phospholipase A2 activity was stimulated by Ca++ (5 mM) and inhibited by EGTA (2 mM). In addition, axoplasmic cytosol (114,000 g supernatant) retained a substantial portion of a Ca++-independent phospholipase A2, active in the presence of 2 mM EGTA. Acyltransferase activity was present at high content in both axolemma membrane rich fractions, and among subaxoplasmic fractions and axoplasmic vesicles. 相似文献
12.
Infusion of inositol-3,4,5,6-tetrakisphosphate (Ins(3,4,5,6)P 4) from the patch pipette into the cytoplasm, produced a biphasic intracellular free Ca 2+ concentration ([Ca 2+] i) increase in ras-transformed NIH/3T3 (DT) cells. The Ins(3,4,5,6)P 4-induced increase in DT cells depended upon extracellular Ca 2+ and was enhanced by membrane hyperpolarization. Identical [Ca 2+] i increases were observed with intracellular application of inositol-1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P 4) and inositol-1,3,4,6-tetrakisphosphate but not with inositol-1,2,4,5-tetrakisphosphate, inositol-1,4,5-trisphosphate or inositol-1,3,4,5,6-pentakisphosphate. Stimulation of DT cells with bradykinin increased the levels of Ins(3,4,5,6)P 4 and Ins(1,3,4,5)P 4. These results suggest that Ins(3,4,5,6)P 4 may serve as a second messenger for continuous Ca 2+ influx along with other tetrakisphosphates downstream from bradykinin receptors in DT cells. 相似文献
13.
To elucidate the relationship between intracellular free Ca 2+ concentration ([Ca 2+] i) and Ca 2+-signalling by the sarcoplasmic reticulum (SR) in Ca 2+-overloaded heart muscle cells, the direct effects of “basal” [Ca 2+] i on calcium waves were investigated by altering the membrane potential. When basal inter-calcium wave (BCW) [Ca 2+] i was maintained at a high level, (i) calcium waves showed more gradual and more rapidly suppressed increase in [Ca 2+]-profile ( P < 0.005), and (ii) calcium waves occurred at a significantly higher frequency and velocity (259% and 137%), than when low BCW [Ca 2+] i was maintained. Similar investigations on inhibition of the Na +-Ca 2+ exchanger, however, showed that membrane potential did not elicit direct effects on calcium waves. These results showed that the elevation of BCW [Ca 2+] i per se directly influences Ca 2+-signalling in heart muscle cells through non-equilibrated release-restoration Ca 2+-handling by the SR. 相似文献
14.
We have used a continuous spectrofluorimetric method to analyse the role of cytosolic free Ca 2+ ([Ca 2+] i) in the lysosomal enzyme release from the azurophilic granules in human neutrophils stimulated with f-Met-Leu-Phe (fMLP) in the presence of cytochalasin B. Measurements were performed with the β-glucuronidase substrate 4-methylumbelliferyl-β-
-glucuronide. We found that the transient rise in [Ca 2+] i induced by fMLP is a necessary signal to obtain to obtain maximal degranulation. When this Ca 2+ transient is prevented by the Ca 2+ chelator BAPTA, degranulation can still be induced by a stimulated Ca 2+ influx, albeit to a lower extent. We also studied the degranulation process in the neutrophils of a patient with a generalized chemotactic defect. Release of β-glucuronidase from the patient's neutrophils could not be induced despite the occurrence of a normal Ca 2+ response and normal degranulation of specific granules. We conclude that, besides an increase in [Ca 2+] i], an additional signal is required for the fusion of azurophilic granules with the plasma membrane in human neutrophils. 相似文献
15.
We investigated the restoration of [Ca 2+] i in fura-2-loaded human platelets following discharge of internal Ca 2+ stores in the absence of external Ca 2+. After stimulation by thrombin [Ca 2+] i returned from a peak level of 0.6 μM to resting levels within 4 min. When ionomycin discharged the internal stores the recovery was slower with [Ca 2+] i still elevated at around 0.5 μM after 5 min. Thrombin added shortly after ionomycin could accelerate the recovery of [Ca 2+] i and restore resting levels within 5 min, an effect that was mimicked by phorbol-12-myristate-13-acetate (PMA). Since the continued presence of ionomycin precluded reuptake into the internal stores we conclude that thrombin and PMA stimulate Ca 2+ efflux, perhaps via protein kinase C actions on a plasma membrane Ca 2+ pump. 相似文献
16.
The activation and deactivation of Ca 2+- and calmodulindependent neuronal nitric oxide synthase (nNOS) in the central nervous system must be tightly controlled to prevent excessive nitric oxide (NO) generation. Considering plasma membrane calcium ATPase (PMCA) is a key deactivator of nNOS, the present investigation aims to determine the key events involved in nNOS deactivation of by PMCA in living cells to maintain its cellular context. Using time-resolved F?rster resonance energy transfer (FRET), we determined the occurrence of Ca 2+-induced protein-protein interactions between plasma membrane calcium ATPase 4b (PMCA4b) and nNOS in living cells. PMCA activation significantly decreased the intracellular Ca 2+ concentrations ([Ca 2+] i), which deactivates nNOS and slowdowns NO synthesis. Under the basal [Ca 2+] i caused by PMCA activation, no protein-protein interactions were observed between PMCA4b and nNOS. Furthermore, both the PDZ domain of nNOS and the PDZ-binding motif of PMCA4b were essential for the protein-protein interaction. The involvement of lipid raft microdomains on the activity of PMCA4b and nNOS was also investigated. Unlike other PMCA isoforms, PMCA4 was relatively more concentrated in the raft fractions. Disruption of lipid rafts altered the intracellular localization of PMCA4b and affected the interaction between PMCA4b and nNOS, which suggest that the unique lipid raft distribution of PMCA4 may be responsible for its regulation of nNOS activity. In summary, lipid rafts may act as platforms for the PMCA4b regulation of nNOS activity and the transient tethering of nNOS to PMCA4b is responsible for rapid nNOS deactivation. 相似文献
17.
Treatment of U937 cells with a sublethal concentration of tert-butylhydroperoxide generates DNA single strand breakage in U937 cells and this response is increased by caffeine, ATP, pyruvate or antimycin A. As we previously reported (Guidarelli, Clementi, Brambilla and Cantoni, (1997) Biochem. J. 328, 801-806), the enhancing effects of antimycin A are mediated by inhibition of complex III and the ensuing formation of superoxides and hydrogen peroxide in a reaction in which ubisemiquinone serves as an electron donor. Active electron transport was required in pyruvate-supplemented cells since the increased genotoxic response occurred as a consequence of enforced mitochondrial Ca 2+ accumulation, a process driven by the increased electrochemical gradient. The enhancing effects of caffeine or ATP were also the consequence of mitochondrial Ca 2+ accumulation but these responses were independent on electron transport. The increased formation of DNA lesions resulting from exposure to tert-butylhydroperoxide associated with the Ca 2+-mobilizing agents or the respiratory substrate was mediated by arachidonic acid generated by Ca 2+-dependent activation of phospholipase A 2. Melittin, a potent phospholipase A 2 activator, and reagent arachidonic acid mimicked the effects of caffeine, ATP or pyruvate on the tert-butylhydroperoxide-induced DNA single strand breakage. 相似文献
18.
The relationship between the agonist-sensitive Ca 2+ pool and those discharged by the Ca 2+-ATPase inhibitor thapsigargin (TG) were investigated in canine tracheal smooth muscle cells (TSMCs). In fura-2-loaded TSMCs, 5-hydroxytryptamine (5-HT) stimulated a rapid increase in intracellular Ca 2+ ([Ca 2+] i), followed by a sustained plateau phase that was dependent on extracellular Ca 2+. In such cells, TG produced a concentration-dependent increase in [Ca 2+] i, which remained elevated over basal level for several minutes and was substantially attenuated in the absence of extracellular Ca 2+. Application of 5-HT after TG demonstrated that the TG-sensitive compartment partly overlapped the 5-HT-sensitive stores. Pre-treatment of TSMCs with TG significantly inhibited the increase in [Ca 2+] i induced by 5-HT in a time-dependent manner. Similar results were obtained with two other Ca 2+-ATPase inhibitors, cyclopiazonic acid and 2,5-di- t-butylhydroquinone. Although these inhibitors had no effect on phosphoinositide hydrolysis, Ca 2+-influx was stimulated by these agents. These results suggest that depletion of the agonist-sensitive Ca 2+ stores is sufficient for activation of Ca 2+ influx. Some characteristics of the Ca 2+-influx activated by depletion of internal Ca 2+ stores were compared with those of the agonist-activated pathway. 5-HT-stimulated Ca 2+ influx was inhibited by La 3+, membrane depolarisation, and the novel Ca 2+-influx blocker 1-{β-[3-(4-methoxyphenyl) propoxy]-4-methoxyphenethyl}-1H-imidazole hydrochloride (SKF96365). Likewise, activation of Ca 2+ influx by TG also was blocked by La 3+, membrane depolarisation, and SKF96365. These results suggest that (1) in the absence of PI hydrolysis, depletion of the agonist-sensitive internal Ca 2+ stores in TSMCs is sufficient for activation of Ca 2+ influx, and (2) the agonist-activated Ca 2+ influx pathway and the influx pathway activated by depletion of the inositol 1,4,5-trisphosphate-sensitive Ca 2+ pool are indistinguishable. 相似文献
19.
Upon glucose elevation, pancreatic beta-cells secrete insulin in a Ca 2+-dependent manner. In diabetic animal models, different aspects of the calcium signaling pathway in beta-cells are altered, but there is no consensus regarding their relative contributions to the development of beta-cell dysfunction. In this study, we compared the increase in cytosolic Ca 2+ ([Ca 2+] i) via Ca 2+ influx, Ca 2+ mobilization from endoplasmic reticulum (ER) calcium stores, and the removal of Ca 2+ via multiple mechanisms in beta-cells from both diabetic db/db mice and nondiabetic C57BL/6J mice. We refined our previous quantitative model to describe the slow [Ca 2+] i recovery after depolarization in beta-cells from db/db mice. According to the model, the activity levels of the two subtypes of the sarco-endoplasmic reticulum Ca 2+-ATPase (SERCA) pump, SERCA2 and SERCA3, were severely down-regulated in diabetic cells to 65% and 0% of the levels in normal cells. This down-regulation may lead to a reduction in the Ca 2+ concentration in the ER, a compensatory up-regulation of the plasma membrane Na +/Ca 2+ exchanger (NCX) and a reduction in depolarizationevoked Ca 2+ influx. As a result, the patterns of glucosestimulated calcium oscillations were significantly different in db/db diabetic beta-cells compared with normal cells. Overall, quantifying the changes in the calcium signaling pathway in db/db diabetic beta-cells will aid in the development of a disease model that could provide insight into the adaptive transformations of beta-cell function during diabetes development. 相似文献
20.
Influence of hypotonic swelling on Ca 2+ ( 45Ca 2+) uptake in rat brain synaptosomes was studied. A decrease in medium osmolality from 310 to 260-180 mOsm led to a progressive stimulation of 45Ca 2+ accumulation. The effect was blocked by verapamil (IC 50 = 5 μM), CoCl 50 = 58 μM) and retained at a fixed concentration of external sodium indicating the involvement of Ca 2+ channels rather than Na +/Ca 2+ exchange in swelling-induced Ca 2+ influx. The populations of calcium channels observed in hypoosmotic and depolarizing conditions are different in three aspects: (i) kinetics of 45Ca 2+ entry; (ii) insensitivity to dihydropyridines and ω-conotoxin GVIA; (iii) insensitivity to preliminary depolarization by high potassium. The effects of swelling and depolarization on Ca 2+ uptake were additive. No change in membrane potential monitored with diS-C 3-(5) was recorded during synaptosome hypotonic swelling. The results suggest the existence in synaptosomal plasma membrane of volume-dependent calcium-permeable channels with properties distinct from those of the voltage-dependent calcium channels. Activation of these channels may constitute an early event in volume regulation of nerve terminals in anisoosmotic conditions. 相似文献
|