共查询到20条相似文献,搜索用时 31 毫秒
1.
Phasic release of calcium from the sarcoplasmic reticulum occurs in all mammalian cardiac preparations when the intracellular calcium concentration is sufficiently high. The phasic calcium release is often sufficient to trigger electrophysiological responses and aftercontractions. These can be detrimental to normal cardiac function. We induced phasic calcium release in ferret papillary muscles loaded with the calcium indicator aequorin. Development of phasic calcium release was associated with an increase in resting and peak [Ca 2+] i. Inhibiting sodium channels with yohimbine reduced resting [Ca 2+] i and prevented phasic calcium release. We propose a mechasism where by reduced [Na +] i, and the subsequent increased efflux of calcium via sodium/calcium exchange reduced [Ca 2+] i. 相似文献
2.
Human leukocytes were exposed to N-(5-vinyl-1,3-thiazolidin-2-ylidene)phenylamine (5-VTPA), a postulated impurity in the case oils that caused the Spanish Toxic Oil Syndrome in 1981. Changes induced by 5-VTPA alone and together with a chemotactic peptide, formyl-methionyl-leucyl-phenylalanine (FMLP), a tumor promoter, phorbol myristate acetate (PMA), or a synthetic diacylglycerol, dioctanoyl- s, n-glycerol (DiC 8) in free intracellular calcium levels ([Ca 2+] i) and in the induction of oxidative burst were measured. 5-VTPA elevated dose-dependently [Ca 2+] i and induced the production of reactive oxygen metabolites in leukocytes. 5-VTPA also amplified FMLP-induced increase in [Ca 2+] i, but was without an effect on FMLP-induced oxidative burst. On the contrary, 5-VTPA amplified dose-dependently PMA- and DiC 8-induced respiratory burst. The present results indicate that 5-VTPA may interfere with transmembrane signalling in human leukocytes. 5-VTPA may elevate [Ca 2+] i by acting directly on the membrane, or by acting through Ca 2+-mobilizing receptors. Moreover, 5-VTPA also clearly amplified responses produced through protein kinase C stimulation. Thus, 5-VTPA may act on human leukocytes by affecting Ca 2+-metabolism and the activity of protein kinase C. 相似文献
3.
The effects of 7-chloro-3,5-dihydro-5-phenyl-1H-4,1-benzothiazepine-2-on (CGP37157), an inhibitor of mitochondrial Na +/Ca 2+ exchange, on depolarization-induced intracellular free Ca 2+ concentration ([Ca 2+] i) transients were studied in cultured rat dorsal root ganglion neurons with indo-1-based microfluorimetry. A characteristic plateau in the recovery phase of the [Ca 2+] i transient resulted from mitochondrion-mediated [Ca 2+] i buffering. It was blocked by metabolic poisons and was not dependent on extracellular Ca 2+. CGP37157 produced a concentration-dependent decrease in the amplitude of the mitochondrion-mediated plateau phase (IC 50=4±1 μM). This decrease in [Ca 2+] i was followed by an increase in [Ca 2+] i upon removal of the drug, suggesting that Ca 2+ trapped in the matrix was released when the CGP37157 was removed from the bath. CGP37157 also inhibited depolarization-induced Ca 2+ influx at the concentrations required to see effects on [Ca 2+] i buffering. Thus, CGP37157 inhibits mitochondrial Na +/Ca 2+ exchange and directly inhibits voltage-gated Ca 2+ channels, suggesting caution in its use to study [Ca 2+] i regulation in intact cells. 相似文献
4.
Exogenously administered cannabinoids are neuroprotective in several different cellular and animal models. In the current study, two cannabinoid CB1 receptor ligands (WIN 55,212-2, CP 55,940) markedly reduced hippocampal cell death, in a time-dependent manner, in cultured neurons subjected to high levels of NMDA (15 μM). WIN 55,212-2 was also shown to inhibit the NMDA-induced increase in intracellular calcium concentration ([Ca 2+] i) indicated by FURA-2 fluorescence imaging in the same cultured neurons. Changes in [Ca 2+] i occurred with similar concentrations (25–100 nM) and in the same time-dependent manner (pre-exposure 1–15 min) as CB1 receptor mediated neuroprotective actions. Both effects were blocked by the CB1 receptor antagonist SR141716A. An underlying mechanism was indicated by the fact that (1) the NMDA-induced increase in [Ca 2+] i was inhibited by ryanodine, implicating a ryanodine receptor (RyR) coupled intracellular calcium channel, and (2) the cannabinoid influence involved a reduction in cAMP cAMP-dependent protein kinase (PKA) dependent phosphorylation of the same RyR levels that regulate channel. Moreover the time course of CB1 receptor mediated inhibition of PKA phosphorylation was directly related to effective pre-exposure intervals for cannabinoid neuroprotection. Control studies ruled out the involvement of inositol-trisphosphate (IP3) pathways, enhanced calcium reuptake and voltage sensitive calcium channels in the neuroprotective process. The results suggest that cannabinoids prevent cell death by initiating a time and dose dependent inhibition of adenylyl cyclase, that outlasts direct action at the CB1 receptor and is capable of reducing [Ca 2+] i via a cAMP/PKA-dependent process during the neurotoxic event. 相似文献
5.
The effects of the L-type (nifedipine and verapamil) and the T-type (mibefradil) Ca 2+ channel blockers on the increase in intracellular Ca 2+ concentration ([Ca 2+] i) induced by NaCN metabolic inhibition and hyperkalemia were examined in chicken cardiomyocytes using fluorescence imaging with Fura-2. NaCN induced a slow and sustained rise in [Ca 2+] i, which was not affected by pretreating the cells for 5 min with nifedipine, verapamil, or mibefradil at 100 nM or 10 μM. Pretreatment of the cells with 10 μM nifedipine, verapamil, or mibefradil for 5 min remarkably inhibited the K +-induced increase in [Ca 2+] i. These inhibitory effects diminished after 48-h pretreatment with nifedipine or verapamil but not with mibefradil. Ryanodine also induces an increase in [Ca 2+] i, and this effect was enhanced by 48-h pretreatment of the cells with 10 μM verapamil but not with 10 μM mibefradil. We conclude that the NaCN-induced increase in [Ca 2+] i is independent of the Ca 2+ influx though the L-type or T-type Ca 2+ channels. Chronic inhibition of the L-type Ca 2+ channels but not T-type channels may enhance the ryanodine receptor-mediated Ca 2+ release, which may be responsible for the development of tolerance to their inhibitory effects on K +-induced increase in [Ca 2+] i. 相似文献
6.
The mechanisms responsible for somatostatin (SRIF)-induced increases in intracellular Ca 2+ concentration ([Ca 2+] i) and subsequent desensitisation were studied in CHO-K1 cells expressing human sst 5 receptors (CHOsst 5 cells). To study the nature of the desensitisation, interactions with uridine triphosphate (UTP) were examined. SRIF (pEC 50 7.10) and UTP (pEC 50 5.14) caused concentration-dependent increases in [Ca 2+] i but the SRIF maximum was about 40% of that to UTP. SRIF-, but not UTP-, induced increases in [Ca 2+] i were transient and abolished by pertussis toxin. SRIF and UTP caused sustained increases in Ins(1,4,5)P 3 but the SRIF maximum was about 30% of that to UTP. Removal of [Ca 2+ ] e attenuated the SRIF-induced peak rise in [Ca 2+] i but had no effect on the peak increases in Ins(1,4,5)P 3. UTP-induced increases in [Ca 2+] i and Ins(1,4,5)P 3 were attenuated in the absence of [Ca 2+] e. Following pre-exposure to SRIF (1 μM) or UTP (100 μM) for 5 min, subsequent SRIF responses were desensitised. Similar results were obtained in the absence of [Ca 2+] e. Pre-exposure to SRIF had no effect on subsequent responses to UTP but in the absence of [Ca 2+] e, responses to UTP were attenuated. The results suggest that SRIF but not UTP-induced increases in [Ca 2+] i in CHOsst 5 cells are mediated by pertussis toxin sensitive G proteins and are caused by an entry of extracellular Ca 2+ and release from an Ins(1,4,5)P 3 sensitive Ca 2+ store. Homologous or heterologous desensitisation of agonist-induced increases in [Ca 2+] i could be demonstrated in the presence or absence of extracellular Ca 2+ respectively, and the latter appeared to involve depletion of a common intracellular Ca 2+ store. 相似文献
7.
To study the cellular basis of the neurotoxicity of methylmercury, the effects of methylmercury on dissociated rat cerebellar neurons were examined using a flow cytometer, a confocal laser microscope and three fluorescent dyes, fluo-3 for monitoring the changes in intracellular Ca 2+ concentration ([Ca 2+] i) and for detecting live neurons, ethidium for assessing the neurons that are dead or have compromised membranes, and 5-chloromethylfluorescein (CMF) for estimating the cellular content of nonprotein thiols. Methylmercury at concentrations of 1 μM or greater increased the [Ca 2+] i of almost all neurons. Prolonged exposure to methylmercury (3 and 10 μM) produced a further increase in [Ca 2+] i, in association with compromising membranes in some neurons. Thereafter, methylmercury induced blebs on membranes of some neurons with increased [Ca 2+] i. Methylmercury at concentrations of 0.3 μM or greater dose-dependently decreased the cellular content of nonprotein thiols. Results suggest that methylmercury may induce the loss of membrane integrity through destabilized Ca 2+ homeostasis and oxidative stress in mammalian brain neurons. 相似文献
8.
To determine the mechanism of endothelium-dependent relaxation by bradykinin, we simultaneously measured changes in cytosolic calcium concentrations ([Ca 2+] i) and force of fura-2-loaded strips of porcine coronary artery. We also examined effects of captopril, an angiotensin converting enzyme inhibitor, on bradykinin-induced relaxation. Bradykinin, in a concentration-dependent manner (10 −10 to 10 −7 M), decreased both [Ca 2+] i and force to resting levels, during 10 −5 M prostaglandin F 2-induced contractions, only when endothelium was intact. Treatment with 10 −5 M captopril enhanced the bradykinin-induced decreases in [Ca 2+] i and force and shifted the concentration-response bradykinin induced a greater relaxation than that expected from the reduction in [Ca 2+] i. Captopril had no effects on the relationship between reduction in [Ca 2+] i and relaxation induced by bradykinin. Bradykinin relaxes porcine coronary artery in an endothelium-dependent manner, by decreasing [Ca 2+] i and also by controlling the Ca 2+ sensitivity of the contractile apparatus of smooth muscle. Captopril enhanced the bradykinin-induced relaxation, with no apparent direct effect on Ca 2+ sensitivity of the contractile apparatus. 相似文献
9.
Because recent reports point to Na + channel blockers as protective agents directed against anoxia-induced neuronal damage including protection of anaerobic glycolysis, the influences of tetrodotoxin (TTX) and (±)-kavain on anoxic rat brain vesicles were investigated with respect to lactate synthesis, vesicular ATP content and cytosolic free Na + and Ca 2+ ([Na +] i, [Ca 2+] i), both of the latter determined fluorometrically employing SBFI and FURA-2, respectively. After anoxia, basal lactate production was increased from 2.9 to 9.8 nmol lactate/min/mg protein. Although lactate synthesis seemed to be stable for at least 45 min of anoxia, as deduced from the linearity of lactate production, the ATP content declined continuously with a half life (τ
) af 14.5 min, indicating that anaerobic glycolysis was insufficient to cover the energy demand of anoxic vesicles. Correspondingly, [Na +] i and [Ca 2+] i increased persistently after anoxia by 22.1 mmol/l Na + and 274.9 nmol/l Ca 2+, determined 6.3 min after onset. An additional stimulation of vesicles with veratridine accelerated the drop of ATP (τ
= 5.1 min) and provoked a massive Na + overload, which levelled off to 119 mmol/l Na + within a few minutes. Concomitantly, [Ca 2+] i increased linearly with a rate of 355 nmol Ca 2+/l/min. Despite the massive perturbation of ion homeostasis, lactate production was unaffected during the first 8 min of veratridine stimulation. However, complete inhibition of lactate synthesis took place 30 min after veratridine was added. The Na + channel blockers TTX and (±)-kavain, if applied before anoxia, preserved vesicular ATP content, diminished anoxia-induced increases in [Na +] i and [Ca 2+] i and prevented both the veratridine-induced increases of [Na +] i and [Ca 2+] i and the inhibition of lactate production. The data indicate a considerable Na + influx via voltage-dependent Na + channels during anoxia, which speeds up the decline in ATP and provokes an increase in [Ca 2+] i. A massive Na + and Ca 2+ overload induced by veratridine failed to influence lactate synthesis directly, but initiated its inhibition. © 1997 Elsevier Science Ltd. All rights reserved. 相似文献
10.
The effects of isoquinoline derivatives, HA1077 (1-[5-isoquinolinesulfonyl]-homopiperazine) and H-7 (1-[5-isoquinoline-sulfonyl]-2-methylpiperazine), on cytosolic Ca 2+ levels ([Ca 2+] i) and muscle tension were examined in vascular smooth muscle of rat aorta. High K + (72.7 mM) and norepinephrine (1 μM) induced a sustained contraction with a sustained increase in [Ca 2+] i. HA1077 and H-7 (3–10 μM) inhibited the increse in muscle tension more strongly than the increase in [Ca 2+] i. Verapamil (10 μM) completely inhibited the increase in [Ca 2+] i and the contraction induced by K + whereas it inhibited the increase in [Ca 2+] i more strongly than the contraction due to norepinephrine. The verapamil-insensitive portion of the norepinephrine-induced contraction was inhibited by HA1077 or H-7. In Ca 2+-free solution, 0.1 μM norepinephrine induced a transient increase in [Ca 2+] i and muscle tension. The transient contraction was inhibited by 10 μM HA1077 or 10 μM H-7 without inhibiting the increase in [Ca 2+] i. 12-Deoxyphorbol 13-isobutyrate (DPB) (1 μM) caused a sustained contraction, and this contraction was inhibited by HA1077 and H-7 at similar concentrations needed to inhibit the contractions induced by high K + or norepinephrine. In rabbit mesenteric artery permeabilized with Staphylococcus aureus -toxin, 100 μM HA1077 and 100 μM H-7 inhibited the contraction induced by 0.3 μM Ca 2+. These results suggest that the inhibitory effects of isoquinoline derivatives, HA1077 and H-7, are due to a decrease in [Ca 2+] i and in the Ca 2+ sensitivity of contractile elemenst in vascular smooth muscle. 相似文献
11.
The effects of puerarin on behaviour and brain neuronal activity in animal studies have been described previously. However, molecule mechanisms underlying these effects were poorly understood. Here, we examined the regulation of puerarin on the Ca 2+ signals in primary rat hippocampal neurons using Fura-2 based calcium imaging techniques. Application of puerarin had no effect on the basal intracellular calcium concentration ([Ca 2+] i), but potentiated the KCl-evoked [Ca 2+] i transient in 87% of recorded neurons. Dantrolene or ruthenium red, the inhibitors of ryanodine receptors, completely blocked this potentiation induced by puerarin. Moreover, in Ca 2+-free solution, pre-application of puerarin significantly augmented the elevation of [Ca 2+] i evoked by caffeine (3 mM), which is a specific agent to activate the ryanodine receptors. In contrast, nifedipine failed to prevent the potentiation induced by puerarin. Similarly, in the experiments of whole-cell patch-clamp recording, puerarin did not show any effect on calcium currents generated by depolarization pulses. These data demonstrated that the potentiation induced by puerarin was attributed to the facilitation of Ca 2+-induced Ca 2+ release (CICR) via ryanodine receptors, rather than extracellular Ca 2+ influx. Using estrogen receptor antagonist ICI 182780 and tamoxifen, we further demonstrated that the potentiation induced by puerarin was mediated by the estrogen receptor. Furthermore, the membrane-permeant inhibitor of protein kinase A (PKA) H89 completely inhibited this potentiation. However, U-73122, the inhibitor of phospholipase C (PLC) had no effect, indicating that the cyclic AMP/PKA signaling pathway was involved in the activation of CICR by puerarin. 相似文献
12.
The in vitro effects of the insecticide lindane have been investigated in rat testis peritubular myoid cells (PMCs). Upon PMC exposure to lindane, polarity increase and decrease of dipole dynamics were seen at the membrane level (EC50 20 μM), leading to a partial dissipation of the membrane intrinsic dipole potential. The initial membrane depolarization was increased by Cl − efflux and limited by Ca 2+-activated repolarizing currents. Concomitantly, lindane produced an increase in [Ca 2+] i (EC50 125 μM) resulting from both Ca 2+ release from an inositol 1,4,5-trisphosphate-sensitive intracellular store and a voltage-dependent Ca 2+ influx from the extracellular medium. Of particular interest from a toxicologic point of view, insecticide concentrations well below those effective in altering ion homeostasis potently inhibited both [Ca 2+] i increase and contraction induced by the natural agonists vasopressin and endothelin-1 (IC50s < 10 μM). These data demonstrate that PMCs are highly susceptible to lindane and suggest that the insecticide may exert testicular toxicity by interfering with hormone-regulated PMC function. 相似文献
13.
The present study established a model of RyR 2 knockdown cardiomyocytes and elucidated the role of RyR 2 in aconitine-induced arrhythmia. Cardiomyocytes were obtained from hearts of neonatal Sprague–Dawlay rats. siRNAs were used to down-regulate RyR 2 expression. Reduction of RyR 2 expression was documented by RT-PCR, western blot, and immunofluorescence. Ca 2+ signals were investigated by measuring the relative intracellular Ca 2+ concentration, spontaneous Ca 2+ oscillations, caffeine-induced Ca 2+ release, and L-type Ca 2+ currents. In normal cardiomyocytes, steady and periodic spontaneous Ca 2+ oscillations were observed, and the baseline [Ca 2+] i remained at the low level. Exposure to 3 μM aconitine increased the frequency and decreased the amplitude of Ca 2+ oscillations; the baseline [Ca 2+] i and the level of caffeine-induced Ca 2+ release were increased but the L-type Ca 2+ currents were inhibited after application of 3 μM aconitine for 5 min. In RyR 2 knockdown cardiomyocytes, the steady and periodic spontaneous Ca 2+ oscillations almost disappeared, but were re-induced by aconitine without affecting the baseline [Ca 2+] i level; the level of caffeine-induced Ca 2+ release was increased but L-type Ca 2+ currents were inhibited. Alterations of RyR 2 are important consequences of aconitine-stimulation and activation of RyR 2 appear to have a direct relationship with aconitine-induced arrhythmias. The present study demonstrates a potential method for preventing aconitine-induced arrhythmias by inhibiting Ca 2+ leakage through the sarcoplasmic reticulum RyR 2 channel. 相似文献
14.
The effects of the organochlorine (OC) liver tumor promoter heptachlor epoxide (HE) and a related non-tumor promoting OC, delta-hexachlorocyclohexane (δ-HCH), on the dynamics of intracellular calcium (Ca 2+) were investigated in mouse 1c1c7 hepatoma cells. HE induced a non-capacitative, Ca 2+ entry-like phenomenon, which was transient and concentration-dependent with 10 and 50 μM HE. The plasma membrane Ca 2+ channel blocker SKF-96365 antagonized this HE-induced Ca 2+ entry. δ-HCH failed to induce Ca 2+ entry, rather it antagonized the HE-induced Ca 2+ entry. Both HE and δ-HCH induced Ca 2+ release from endoplasmic reticulum (ER) at treatment concentrations as low as 10 μM; at 50 μM, the former induced 5× as much Ca 2+ release as the latter. The HE-induced Ca 2+ release from the ER was antagonized using the IP 3 receptor/channel blocker xestospongin C, suggesting that HE induces ER Ca 2+ release through the IP 3 receptor/channel pore. These results show that the effect of HE on cellular Ca 2+ mimics that of mitogens such as epidermal and hepatocyte growth factors. They also provide insight into the similarities and differences between tumorigenic and non-tumorigenic OCs, in terms of the mechanisms and the extent of the [Ca 2+] i increased by these agents. 相似文献
15.
The mechanism of the vasodilator effect of pinacidil was examined. Pinacidil (0.1–100 μM) inhibited the increases in cytosolic Ca 2+ ([Ca 2+] i) and muscle tension due to norepinephrine in rat aorta. In contrast, a Ca 2+ channel blocker, verapamil, inhibited the norepinephrine-stimulated [Ca 2+] i more strongly than the contraction. Higher concentrations of pinacidil (3–100 μM) inhibited the verapamil-insensitive portion of the contraction and [Ca 2+] i. An inhibitor of ATP-sensitive K + channels, glibenclamide, antagonized the inhibitory effect of low concentrations ( 10 pM) of pinacidol. Pinacidil did not change the contraction induced by Ca 2+ in vascular smooth muscle permeabilized with Staphylococcus aureus -toxin. Norepinephrine (in the presence of GTP), 12-deoxyphorbol 13-isobutyrate (in the absence of GTP), and treatment with GTP γS potentiated the contraction of permeabilized smooth muscle induced by the addition of Ca 2+. Pinacidil (100 μM) inhibited the potentiation due to GTP γS or noepinephrine but not to phorbol ester. These results suggest that pinacidil has dual effects on vascular smooth muscle contraction. At lower concentrations (>0.1 μM), it decreases [Ca 2+] i, possibly by activating ATP-sensitive K+ channels. At higher concentrations (> 3 μM), it may additionally inhibit the receptor-mediated, GTP-binding protein-coupled phosphatidyl inositol turnover. 相似文献
16.
Prolonged exposure to cannabinoids results in desensitization of cannabinoid receptors. Here, we compared the desensitization produced by the partial agonist, Δ 9-tetrahydrocannabinol (THC) to that produced by the full agonist Win55,212-2 on cannabinoid-mediated inhibition of glutamatergic synaptic transmission. Synaptic activity between rat hippocampal neurons was determined from network-driven increases in the intracellular Ca 2+ concentration ([Ca 2+] i spikes). To assess the effects of prolonged treatment, cultures were incubated with cannabinoids, washed in 0.5% fatty-acid-free bovine serum albumin to ensure the removal of the lipophilic drug and then tested for inhibition of [Ca 2+] i spiking by Win55,212-2. In control experiments, 0.1 μM Win55,212-2 inhibited [Ca 2+] i spiking by 93 ± 5%. Win55,212-2 produced significantly less inhibition of [Ca 2+] i spiking following 18–24 h treatment with 1 μM THC (48 ± 5%) or treatment with 1 μM Win55,212-2 (29 ± 6%). Thus, THC produced significantly less functional desensitization than Win55,212-2. The desensitization produced by THC was maximal at 0.3 μM, remained stable between 1 and 7 days of preincubation and shifted the EC 50 of acute inhibition by Win55,212-2 from 27 to 251 nM. Differences in the long-term effects of cannabinoid receptor agonists on synaptic transmission may prove important for evaluating their therapeutic and abuse potential. 相似文献
18.
The release of glutamate from striatal synaptosomes induced by depolarisation with 4-aminopyridine (4-AP) was studied by a method based on the fluorescent properties of the NAPDH formed by the metabolism of the neurotransmitter by glutamate dehydrogenase. Ca2+-dependent, depolarisation-induced glutamate release was inhibited in a concentration-dependent manner by the selective histamine H3 agonist immepip. Best-fit estimates were: maximum inhibition 60±10% and IC50 68±10 nM. The effect of 300 nM immepip on depolarisation-evoked glutamate release was reversed by the selective H3 antagonist thioperamide in a concentration-dependent manner (EC50 23 nM, Ki 4 nM). In fura-2-loaded synaptosomes, the increase in the intracellular concentration of Ca2+ ([Ca2+]i) evoked by 4-AP-induced depolarisation (resting level 167±14 nM; Δ[Ca2+]i 88±15 nM) was modestly, but significantly reduced (29±5% inhibition) by 300 nM immepip. The action of the H3 agonist on depolarisation-induced changes in [Ca2+]i was reversed by 100 nM thioperamide. Taken together, our results indicate that histamine modulates the release of glutamate from corticostriatal nerve terminals. Inhibition of depolarisation-induced Ca2+ entry through voltage-dependent Ca2+ channels appears to account for the effect of H3 receptor activation on neurotransmitter release. Modulation of glutamatergic transmission in rat striatum may have important consequences for the function of basal ganglia and therefore for the control of motor behaviour. 相似文献
19.
The force–frequency relationship (FFR) is an important intrinsic regulatory mechanism of cardiac contractility. The FFR in most mammalian ventricular myocardium is positive; that is, an increase in contractile force in association with an increase in the amplitude of Ca 2+ transients is induced by elevation of the stimulation frequency, which reflects the cardiac contractile reserve. The relationship is different depending on the range of frequency and species of animal. In some species, including rat and mouse, a ‘primary-phase’ negative FFR is induced over the low-frequency range up to approximately 0.5–1 Hz (rat) and 1–2 Hz (mouse). Even in these species, the FFR over the frequency range close to the physiological heart rate is positive and qualitatively similar to that in larger mammalian species, although the positive FFR is less prominent. The integrated dynamic balance of the intracellular Ca 2+ concentration ([Ca 2+] i) is the primary cellular mechanism responsible for the FFR and is determined by sarcoplasmic reticulum (SR) Ca 2+ load and Ca 2+ flux through the sarcolemma via L-type Ca 2+ channels and the Na +-Ca 2+ exchanger. Intracellular Na + concentration is also an important factor in [Ca 2+] i regulation. In isolated rabbit papillary muscle, over a lower frequency range (<0.5 Hz), an increase in duration rather than amplitude of Ca 2+ transients appears to be responsible for the increase in contractile force, while over an intermediate frequency range (0.5–2.0 Hz), the amplitude of Ca 2+ transients correlates well with the increase in contractile force. Over a higher frequency range (>2.5 Hz), the contractile force is dissociated from the amplitude of Ca 2+ transients probably due to complex cellular mechanisms, including oxygen limitation in the central fibers of isolated muscle preparations, while the amplitude of Ca 2+ transients increases further with increasing frequency (‘secondary-phase’ negative FFR). Calmodulin (CaM) may contribute to a positive FFR and the frequency-dependent acceleration of relaxation, although the role of calmodulin has not yet been established unequivocally. In failing ventricular myocardium, the positive FFR disappears or is inverted and becomes negative. The activation and overexpression of cardiac sarcoplasmic reticulum Ca 2+ ATPase (SERCA2a) is able to reverse these abnormalities. Frequency-dependent alterations of systolic and diastolic force in association with those of Ca 2+ transients and diastolic [Ca 2+] i levels are excellent indicators for analysis of cardiac excitation-contraction coupling, and for evaluating the severity of cardiac contractile dysfunction, cardiac reserve capacity and the effectiveness of therapeutic agents in congestive heart failure. 相似文献
20.
The effect of three different receptor-specific adenosine agonists on the rate of ureagenesis by isolated rat hepatocytes and the dependence on the external free Ca 2+ concentration ([Ca 2+] e) were investigated. In the presence of high [Ca 2+] e all adenosine receptor agonists increased ureagenesis to similar levels. However, with low [Ca 2+] e the effects of each agonist varied as follows: (i) the adenosine A 1 receptor agonist, 2-chloro- N6-cyclopentyl-adenosine, increased ureagenesis depending partially on [Ca 2+] e, (ii) the adenosine receptor A 2 agonist, 2- p-(-2-carboxy-ethyl) phenethylamino-5′- N-ethylcarboxyamido adenosine hydrochloride, increased ureagenesis independently of [Ca 2+] e and (iii) in contrast, the adenosine receptor A 3 agonist N6-2-(-4-aminophenyl) ethyladenosine, increased ureagenesis only in the presence of high [Ca 2+] e. The adenosine receptor A 1 antagonist, 1-allyl-3,7-dimethyl-8-phenyl xanthine, inhibited the effect of the adenosine receptor A 1 agonist on ureagenesis, but not the effect of the adenosine A 2 or A 3 receptor agonists. The adenosine A 2 receptor antagonist, 3,7-dimethyl-1-propargylxanthine, inhibited only the effect of the adenosine A 2 receptor agonist. Thus, in addition to A 1 and A 2 type adenosine receptors, rat hepatocytes possess an A 3-like adenosine receptor which responds to the addition of an adenosine A 3 agonist by accelerating ureagenesis a [Ca 2+] e dependent manner. Moreover, it was observed that in the presence of extracellular Ca 2+ each agonist increased [Ca 2+] i and this effect was inhibited by the appropriate specific antagonist. 相似文献
|