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1.
The stiffness of the sarcomeres was studied during the diastolic interval of 18 stimulated (0.5 Hz) cardiac trabeculae of rat (pH 7.4; temperature = 25°C). Sarcomere length (SL) and force ( F) were measured using, respectively, laser diffraction techniques (resolution: 4 nm) and a silicon strain gauge (resolution: 0.63 μN). Sinusoidal perturbations (frequency = 500 Hz) were imposed to the length of the preparation. The stiffness was evaluated from the corresponding F and SL sinusoids by analysis of both signals together either in the time domain or in the frequency domain. A short burst (duration = 30 ms) of sinusoidal perturbations was repeated at 5 predetermined times during diastole providing 5 measurements of stiffness during the time interval separating two twitches. These measurements revealed that stiffness increases by 30% during diastole, while a simultaneous expansion of the sarcomeres (amplitude = 10-60 nm) was detected. Measurements of the fluorescence of fura-2 under the same conditions revealed a continuous exponential decline of [Ca 2+] i from 210 to 90 nM (constant of time 300 ms) during diastole. In order to test the possibility that the increase of sarcomere stiffness and the decline of [Ca 2+] i were coupled during diastole of intact trabeculae, we studied the effect of different free Ca 2+-concentrations ([Ca 2+]) between 1 and 430 nM on sarcomere stiffness in rat cardiac trabeculae skinned by saponin ( n = 17). Stiffness was studied using 500 Hz sinusoidal perturbations of muscle length (ML). We found that, below 70 nM, the stiffness was independent of [Ca 2+]; between 70 and 200 nM, the stiffness declined with increase of [Ca 2+]; above 200 nM, the stiffness increased steeply with [Ca 2+]. The data fitted accurately to the sum of two sigmoids (Hill functions): (1) at [Ca 2+] < 200 nM the stiffness decreased with [Ca 2+] (EC 50 = 160 ± 13 nM; n = −2.6±0.7) and (2) at [Ca 2+] > 200 nM, stiffness increased with [Ca 2+] (EC 50 = 3.4±0.3 μM; n = 2.1±0.2) due to attachment of cross-bridges. From these results, it was possible to reproduce accurately the time course of diastolic stiffness observed in intact trabeculae and to predict the effect on stiffness of a spontaneous elevation of the diastolic [Ca 2+]. Identical stiffness measurements were performed in 4 skinned preparations exposed to a cloned fragment of titin (Ti I-II) which has been shown to exhibit a strong interaction with F-actin in vitro. It was anticipated that Ti I-II would compete with endogenous titin for the same binding site on actin in the I-band. Below 200 nM, Ti I-II (2 μM) eliminated the Ca 2+-dependence of stiffness. These results are consistent with the hypothesis that the Ca 2+-sensitivity of the sarcomeres at [Ca 2+] < 200 nM, i.e. where the myocytes in intact muscle operate during diastole, involves an association between titin molecules and the thin filament. 相似文献
2.
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
3.
Mechanical compression of cartilage is associated with a rise in the interstitial osmotic pressure, which can alter cell volume and activate volume recovery pathways. One of the early events implicated in regulatory volume changes and mechanotransduction is an increase of intracellular calcium ion ([Ca 2+] i). In this study, we tested the hypothesis that osmotic stress initiates intracellular Ca 2+ signaling in chondrocytes. Using laser scanning microscopy and digital image processing, [Ca 2+] i and cell volume were monitored in chondrocytes exposed to hyper-osmotic solutions. Control experiments showed that exposure to hyper-osmotic solution caused significant decreases in cell volume as well as transient increases in [Ca 2+] i. The initial peak in [Ca 2+] i was generally followed by decaying oscillations. Pretreatment with gadolinium, a non-specific blocker of mechanosensitive ion channels, inhibited this [Ca 2+] i increase. Calcium-free media eliminated [Ca 2+] i increases in all cases. Pretreatment with U73122, thapsigargin, or heparin (blockers of the inositol phosphate pathway), or pertussis toxin (a blocker of G-proteins) significantly decreased the percentage of cells responding to osmotic stress and nearly abolished all oscillations. Cell volume decreased with hyper-osmotic stress and recovered towards baseline levels throughout the duration of the control experiments. The peak volume change with 550 mOsm osmotic stress, as well as the percent recovery of cell volume, was dependent on [Ca 2+] i. These findings indicate that osmotic stress causes significant volume change in chondrocytes and may activate an intracellular second messenger signal by inducing transient increases in [Ca 2+] i. 相似文献
4.
The intracellular free Ca 2+ ion concentration ([Ca 2+]i) was measured using fura-2 microspec-trofluorimetry in individual rat pancreatic β-cells prepared by enzymatic digestion and fluorescence-activated cell sorting. The mean basal concentration of [Ca 2+]i in β-cells in the presence of 4.4 mM glucose and 1.8 mM Ca 2+ was 112±1.6 nM (n=207). The action of acetylcholine (ACh) was concentration-dependent, and raising the concentration resulted in [Ca 2+]i spikes of increasing amplitude and duration in some, but not all of the β-cells. In addition, the β-cells demonstrated variable sensitivity to ACh. The increases in [Ca 2+]i were rapid, transient and were blocked by atropine at 10 -6M. A brief exposure to 50 mM K + resulted in a transient increase in [Ca 2+]i similar to that induced by ACh, but resistant to atropine. A high concentration of ACh (100μL 10 -4M or 10 -3M) induced [Ca 2+]i oscillations in 11 out of 57 β-cells in the presence of 4.4 mM glucose. Using calcium channel blockers and Ca 2+ free medium, the source of the increase in [Ca 2+]i was deduced to be from extracellular spaces. Changing the temperature from 22 to 37°C did not affect the action of ACh on [Ca 2+]i. These data strongly suggest that ACh exerted a direct action on [Ca 2+]i in normal rat pancreatic β-cells and support a role for Ca 2+ as a second messenger in the action of ACh. 相似文献
5.
The effect of five lignans isolated from Hernandia nymphaeifolia on estrogenic compounds (17β-estradiol, tamoxifen and clomiphene)-induced Ca 2+ mobilization in human neutrophils was investigated. The five lignans were epi-yangambin, epi-magnolin, epi-aschantin, deoxypodophyllotoxin and yatein. In Ca 2+–containing medium, the lignans (50–100 μM) inhibited 10 μM 17β-estradiol- and 5 μM tamoxifen-induced increases in intracellular free Ca 2+ levels ([Ca 2+] i) without changing 25 μM clomiphene-induced [Ca 2+] i increase. 17β-estradiol and tamoxifen increased [Ca 2+] i by causing Ca 2+ influx and Ca 2+ release because their responses were partly reduced by removing extracellular Ca 2+. In contrast, clomiphene solely induced Ca 2+ release. The effect of the lignans on these two Ca 2+ movement pathways underlying 17β-estradiol- and tamoxifen-induced [Ca 2+] i increases was explored. All the lignans (50–100 μM) inhibited 10 μM 17β-estradiol-and 5 μM tamoxifen-induced Ca 2+ release, and 17β-estradiol-induced Ca 2+ influx. However, only 100 μM epi-aschantin was able to reduce tamoxifen-induced Ca 2+ influx while the other lignans had no effect. Collectively, this study shows that the lignans altered estrogenic compounds-induced Ca 2+ signaling in human neutrophils in a multiple manner. 相似文献
6.
Isocyanato and isothiocyanatopolypyridineruthenium complexes, [Ru(NCX)Y(bpy)(py) 2] n+ (bpy=2,2′-bipyridine, PY=pyridine; X=O, Y=NO 2 for n=0, and Y=py for n=1; X=S, Y=NO 2 for n=0, Y=NO for n=2, and Y=py for n=1), were synthesized by the reaction of polypyridineruthenium complexes with potassium cyanate or sodium thiocyanate salt. Isocyanatoruthenium(II) complexes, [Ru(NCO)(NO 2)(bpy)(py) 2] and [Ru(NCO)(bpy)(py) 3] +, react under acidic conditions to form the corresponding ammineruthenium complexes, [Ru(NO)(NH 3)(bpy)(py) 2] 3+. The molecular structures of [Ru(NCO)(bpy)(py) 3]ClO 4, [Ru(NCS)(NO)(bpy)(py) 2](PF 6) 2 and [Ru(NO)(NH 3)(bpy)(py) 2](PF 6) 3 were determined by X-ray crystallography. 相似文献
7.
Glucose-induced insuline release, glucose-induced rises in intracellular free Ca 2+ concentration ([Ca 2+] i), and voltage-dependent Ca 2+ channel activity were assessed in monolayer cultures of β-vells 3–5 day-old rats. The glucose-stimulated insulin secretory responses and [Ca 2+] i rises were like those in adult rat β-cells rather than fetal rat β-cells. Voltage-dependent Ca 2+ channel antagonists decreased glucose-induced insulin secretion, aborted the [Ca 2+] 2 rise and, like deprivation of extracellular Ca 2+, prevented the glucose-induced rise in [Ca 2+] i when added before the glucose challenge. The presence of nifedipine-sensitive, voltage-dependent Ca 2+ channels was demonstrated directly by measuring Ca 2+ currents using the whole-cell configuration of the patch-clamp technique and indirectly by measuring [Ca 2+] 1 after membrane depolarization by 45 mMm K + or 200 μM tolbutamide. Thus, in cultured β-cells of 3–5 day-old rats the coupling of glucose stimulation to Ca 2+ influx is essentially mature, in contrast to what has been reported for fetal or very early neonatal cells. 相似文献
8.
Tumour-promoting phorbol esters (phorbol-12-myristate-13-acetate, PMA; phorbol-12,13-dibutyrate, PDBu) but not 4β-phorbol, activate protein kinase C. Using human platelets pre-labelled with quin2 or 32PO 4 we examined the effects of these compounds on human platelet cytosolic free Ca 2+ ([Ca 2+] j) and on [ 32]phosphatidic acid ([ 32P]PtdOH). PMA and PDBu, but not 4β-phorbol inhibited thrombin-, PAF- and vasopressin-induced elevation of [Ca 2+], and [ 2+P]PtdOH formation. It is suggested that protein kinase C may act to terminate the transduction processes that link receptor occupancy to cellular activation. 相似文献
9.
Regulation of the increase in inositol phosphate (IP) production and intracellular Ca 2+ concentration ([Ca 2+] i by protein kinase C (PKC) was investigated in cultured rat vascular smooth muscle cells (VSMCs). Pretreatment of VSMCs with phorbol 12-myristate 14-acetate (PMA, 1 μM) for 30 min almost abolished the BK-induced IP formation and Ca 2+ mobilisation. This inhibition was reduced after incubating the cells with PMA for 4 h, and within 24 h the BK-induced responses were greater than those of control cells. The concentrations of PMA giving a half-maximal (pEC 50) and maximal inhibition of BK induced an increase in [Ca 2+] i, were 7.8 ± 0.3 M and 1 μM, n = 8, respectively. Prior treatment of VSMCs with staurosporine (1 μM), a PKC inhibitor, inhibited the ability of PMA to attenuate BK-induced responses, suggesting that the inhibitory effect of PMA is mediated through the activation of PKC. Paralleling the effect of PMA on the BK-induced IP formation and Ca 2+ mobilisation, the translocation and downregulation of PKC isozymes were determined by Western blotting with antibodies against different PKC isozymes. The results revealed that treatment of the cells with PMA for various times, translocation of PKC-, βI, βII, δ, ε, and ζ isozymes from the cytosol to the membrane were seen after 5 min, 30 min, 2 h, and 4 h of treatment. However, 24-h treatment caused a partial downregulation of these PKC isozymes in both fractions. Treatment of VSMCs with 1 μM PMA for either 1 or 24 h did not significantly change the KD and Bmax of the BK receptor for binding (control: KD = 1.7 ± 0.2 nM; Bmax = 47.3 ± 4.4 fmol/ mg protein), indicating that BK receptors are not a site for the inhibitory effect of PMA on BK-induced responses. In conclusion, these resuts demonstrate that translocation of PKC-, βI, βII, δ, ε, and ζ induced by PMA caused an attenuation of BK-induced IPs accumulation and Ca 2+ mobilisation in VSMCs. 相似文献
10.
A wasp venom, mastoparan, rapidly increased the cytosolic free Ca 2+ concentration ([Ca 2+] i) and activated phosphorylase in rat hepatocytes in a concentration-dependent manner. Mastoparan could increase [Ca 2+] i even in the absence of extracellular Ca 2+, but a larger increase was observed in the presence of extracellular Ca 2+. Thus, mastoparan mobilized Ca 2+ from intracellular and extracellular Ca 2+ stores. It also activated inositol triphosphate (IP 3) accumulation, but did not stimulate cAMP production. From these results, we conclude that mastoparan activates rat hepatic glycogenolysis mediated by the accumulation of IP 3, which causes an increase of [Ca 2+] i but not that mediated by cAMP. 相似文献
11.
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
12.
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
13.
The interaction of the novel CCK analogs JMV-180, JMV-320, and JMV-332 with CCK-B/gastrin receptors on small cell lung cancer (SCLC) cells was investigated. JMV-180, JMV-320, and JMV-332 potently inhibited specific binding of 125I-CCK-8 to CCK-B/gastrin receptors expressed on the SCLC cell line NCI-H345 (H345) with IC 50 values of 4.9, 1.8, and 7.0 n M, respectively. JMV-320 and JMV-332 stimulated intracellular calcium ([Ca 2+] i) release in a dose-dependent manner in cells preloaded with indo-1. JMV-180 did not stimulate [Ca 2+] i but inhibited the [Ca 2+] i release elicited by 10 n M CCK-8 in a dose-dependent manner. These data indicate that JMV-320 and JMV-332 function as CCK-B/gastrin receptor agonists while JMV-180 functions as a CCK-B/gastrin receptor antagonist in H345 cells. 相似文献
14.
The effect of clomiphene, an ovulation-inducing agent, on cytosolic free Ca 2+ levels ([Ca 2+] i) in populations of PC3 human prostate cancer cells was explored by using fura-2 as a Ca 2+ indicator. Clomiphene at concentrations between 10-50 μM increased [Ca 2+] i in a concentration-dependent manner. The [Ca 2+] i signal was biphasic with an initial rise and a slow decay. Ca 2+ removal inhibited the Ca 2+ signal by 41%. Adding 3 mM Ca 2+ increased [Ca 2+] i in cells pretreated with clomiphene in Ca 2+-free medium, confirming that clomiphene induced Ca 2+ entry. In Ca 2+-free medium, pretreatment with 50 μM brefeldin A (to permeabilize the Golgi complex), 1 μM thapsigargin (to inhibit the endoplasmic reticulum Ca 2+ pump), and 2 μM carbonylcyanide m-chlorophenylhydrazone (to uncouple mitochondria) inhibited 25% of 50 μM clomiphene-induced store Ca 2+ release. Conversely, pretreatment with 50 μM clomiphene in Ca 2+-free medium abolished the [Ca 2+] i increase induced by brefeldin A, thapsigargin or carbonylcyanide m-chlorophenylhydrazone. The 50 μM clomiphene-induced Ca 2+release was unaltered by inhibiting phospholipase C with 2 μM 1-(6-((17β-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione (U73122). Trypan blue exclusion assay suggested that incubation with clomiphene (50 μM) for 2-15 min induced time-dependent decrease in cell viability by 10-50%. Collectively, the results suggest that clomiphene induced [Ca 2+] i increases in PC3 cells by releasing store Ca 2+ from multiple stores in an phospholipase C-independent manner, and by activating Ca 2+ influx; and clomiphene was of mild cytotoxicity. 相似文献
15.
The cold inhibited functions of skin thermoreceptors, of the thermoregulation centre, and the respiration centre during deep hypothermia can be restored without rewarming the body. The methods used were developed to test the hypothesis that during deep hypothermia calcium ion concentration [Ca 2+] i in the cytoplasm increases. This causes a perturbation of cell metabolism, the impairment of cell membrane function that cause the inhibition of cell functioning, resulting in cell death. Such an increase in [Ca 2+] i most likely would result from an energy deficit in a deeply cooled cell, which would compromise the processes that maintain the [Ca 2+] i at about 10 −7 M. These processes require large amounts of energy since they occur against a large concentration gradient. With the use of EDTA the extracellular concentration of Ca 2+ has been lowered by 15–27%, so reducing the concentration gradient for Ca 2+ between the cell and the medium and in consequence facilitated the process the extrusion of cell Ca 2+. During a period of cooling, sufficient to impair normal functioning, the experimental lowering of blood Ca2+ allowed the restoration of normal function without the need to rewarm. In such cases the animals survived after cooling the body to temperatures at which they would normally have succumbed. The data presented support the stated hypothesis that the impairment of cellular function in mammals by low temperatures is the result of an uncorrected rise in [Ca2+]i. 相似文献
16.
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
17.
We investigated the effect of lysophosphatidic acid (LPA), a bioactive phospholipid, on the response in cytosolic free Ca 2+ concentration ([Ca 2+] i) to mechanical stress in cultured bovine lens epithelial cells. Spritzing of bath solution onto cells as mechanical stress caused marked increase in [Ca 2+] i in the presence of LPA and this increase was concentration-dependent (1–10 μM), whereas neither addition of LPA alone nor the mechanical stress in the absence of LPA affected [Ca 2+] i. The mechanical stress-induced increase in [Ca 2+] i in the presence of LPA was inhibited by removing extracellular Ca 2+ or by addition of Gd 3+, a blocker of mechanosensitive cation channels, but not by nicardipine, thapsigargin, an inhibitor of endoplasmic reticulum-ATPase pump, or U73122, a phospholipase C inhibitor. These results show that LPA sensitises Ca 2+ influx through cation-selective mechanosensitive channels, but does not sensitise Ca 2+ release from intracellular stores, triggered by changes in mechanical stress. On the other hand, phosphatidic acid had less of a sensitising effect than LPA, and neither lysophosphatidylcholine nor chlorpromazine had any effect. Also Ca 2+ mobilising agonists, ATP, histamine and carbachol, did not sensitise Ca 2+ response to the mechanical stress. These results show that LPA sensitises mechanoreceptor-linked response in lens epithelial cells, suggesting that it plays a role in the development of cataracts due to increases in [Ca 2+] i induced by mechanical stress. 相似文献
18.
Stretch of the myocardium influences the shape and amplitude of the intracellular Ca 2+([Ca 2+] i) transient. Under isometric conditions stretch immediately increases myofilament Ca 2+ sensitivity, increasing force production and abbreviating the time course of the [Ca 2+] i transient (the rapid response). Conversely, muscle shortening can prolong the Ca 2+ transient by decreasing myofilament Ca 2+ sensitivity. During the cardiac cycle, increased ventricular dilation may increase myofilament Ca 2+ sensitivity during diastolic filling and the isovolumic phase of systole, but enhance the decrease in myofilament Ca 2+ sensitivity during the systolic shortening of the ejection phase. If stretch is maintained there is a gradual increase in the amplitude of the Ca 2+ transient and force production, which takes several minutes to develop fully (the slow response). The rapid and slow responses have been reported in whole hearts and single myocytes. Here we review stretch-induced changes in [Ca 2+] i and the underlying mechanisms. Myocardial stretch also modifies electrical activity and the opening of stretch-activated channels (SACs) is often used to explain this effect. However, the myocardium has many ionic currents that are regulated by [Ca2+]i and in this review we discuss how stretch-induced changes in [Ca2+]i can influence electrical activity via the modulation of these Ca2+-dependent currents. Our recent work in single ventricular myocytes has shown that axial stretch prolongs the action potential. This effect is sensitive to either SAC blockade by streptomycin or the buffering of [Ca2+]i with BAPTA, suggesting that both SACs and [Ca2+]i are important for the full effects of axial stretch on electrical activity to develop. 相似文献
19.
In cultured pituatary gonadotrophs, gonadotropin-releasing hormone (GnRH) caused dose-dependent and biphasic increases in cytoplasmic calcium concentration ([Ca 2+] i) and LH release. Both extra- and intracellular calcium pools participate in GnRH-induced elevation of [Ca 2+] i and LH secretion. The spike phase of the [Ca 2+] i response represents the primary signal derived predominantly from the rapid mobilization of intracellular Ca 2+. In contrast, the prolonged phase of the Ca 2+ signal depends exclusively on Ca 2+ entry from the extracellular pool. The influx of Ca 2+ occurs partially through dihydropyridine-sensitive calcium channels. Both [Ca 2+] i and LH responses to increasing concentrations of GnRH occur over very similar time scales, suggesting that increasing degrees of receptor occupancy are transduced into amplitude-modulated Ca 2+ responses, which in turn activate exocytosis in a linear manner. However, several lines of evidence indicated the complexity over the relationship between Ca 2+ signaling and LH exocytosis. In contrast to [Ca 2+] i measurements in cell suspension, single cell Ca 2+ measurements revealed the existence of a more complicated pattern of Ca 2+ response to GnRH, with a biphasic response to high agonist doses and prominent oscillatory responses to lower GnRH concentrations, with a log-linear correlation between GnRH dose and the frequency of Ca 2+ spiking. In addition, analysis of the magnitudes of the magnitudes of the [Ca 2+] i and LH responses of gonadotrophs to a wide range of GnRH concentrations in the presence and absence of extracellular Ca 2+, and to K + and phorbol ester stimulation, showed non-linearity between these parameters with amplification of [Ca 2+] i-mediated exocytosis. Studies on cell depleted of protein kinase C under conditions that did not change the LH pool suggested the participation of protein kinase C in this amplication, especially during the plateau phase of the secretory response to GnRH. 相似文献
20.
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
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