Norepinephrine, vasopressin, glucagon, and A23187 induce efflux of calcium from an exchangeable pool in isolated rat hepatocytes

Isolated rat hepatocytes do not actively accumulate Ca(2+) during prolonged incubation in vitro; however, these cells do exhibit a limited exchange of intracellular with extracellular Ca(2+). The exchangeable pool represents about 2 nmol of Ca(2+) per mg of protein. In medium containing either a low (20 muM) or high (1 mM) concentration of Ca(2+), the divalent cation ionophore, A23187 (at concentrations of 0.03-0.1 nmol/mg of protein), causes release of (45)Ca(2+) from this exchangeable pool but does not allow net influx of extracellular Ca(2+) detectable by the use of a Ca(2+)-sensitive electrode. Like A23187, the hormones norepinephrine, vasopressin, and glucagon (at concentrations that stimulate gluconeogenesis) each induces a similar net efflux of Ca(2+). Treatment with one hormone decreases the subsequent reponse to the others, whereas treatment with A23187 abolishes the hormonal effects upon both Ca(2+) release and gluconeogenesis. The action of norepinephrine, but not of glucagon, upon Ca(2+) efflux is prevented by the alpha-adrenergic antagonist, phenoxybenzamine. The action of norepinephrine is not prevented by the beta-adrenergic antagonist, propranolol. Together these results indicate that the release of Ca(2+) from a common pool of exchangeable Ca(2+) is important to the action of a variety of hormones on hepatocytes. This Ca(2+) pool in the isolated hepatocyte is characterized as being similar in size and having exchange kinetics that are comparable to those reported for the major intracellular pool of Ca(2+) in the intact liver. The possibility that this pool is intramitochondrial calcium is discussed.

Posttraumatic GABA(A)-mediated [Ca2+]i increase is essential for the induction of brain-derived neurotrophic factor-dependent survival of mature central neurons

A shift of GABA(A)-mediated responses from hyperpolarizing to depolarizing after neuronal injury leads to GABA(A)-mediated increase in [Ca2+](i). In addition, central neurons become dependent on BDNF for survival. Whether these two mechanisms are causally interrelated is an open question. Here, we show in lesioned CA3 hippocampal neurons in vitro and in axotomized corticospinal neurons in vivo that posttraumatic downregulation of the neuron-specific K-Cl cotransporter KCC2 leads to intracellular chloride accumulation by the Na-K-2Cl cotransporter NKCC1, resulting in GABA-induced [Ca2+](i) transients. This mechanism is required by a population of neurons to survive in a BDNF-dependent manner after injury, because blocking GABA(A)-depolarization with the NKCC1 inhibitor bumetanide prevents the loss of neurons on BDNF withdrawal. The resurgence of KCC2 expression during recovery coincides with loss of BDNF dependency for survival. This is likely mediated through BDNF itself, because injured neurons reverse their response to this neurotrophin by switching the BDNF-induced downregulation of KCC2 to upregulation.

Pacific ciguatoxin-1b effect over Na+ and K+ currents, inositol 1,4,5-triphosphate content and intracellular Ca2+ signals in cultured rat myotubes

1. The action of the main ciguatoxin involved in ciguatera fish poisoning in the Pacific region (P-CTX-1b) was studied in myotubes originated from rat skeletal muscle cells kept in primary culture. 2. The effect of P-CTX-1b on sodium currents at short times of exposure (up to 1 min) showed a moderate increase in peak Na+ current. During prolonged exposures, P-CTX-1b decreased the peak Na+ current. This action was always accompanied by an increase of leakage currents, tail currents and outward Na+ currents, resulting in an intracellular Na+ accumulation. This effect is blocked by prior exposure to tetrodotoxin (TTX) and becomes evident only after washout of TTX. 3. Low to moderate concentrations of P-CTX-1b (2-5 nM) partially blocked potassium currents in a manner that was dependent on the membrane potential. 4. P-CTX-1b (2-12 nM) caused a small membrane depolarization (3-5 mV) and an increase in the frequency of spontaneous action potential discharges that reached in general low frequencies (0.1-0.5 Hz). 5. P-CTX-1b (10 nM) caused a transient increase of intracellular inositol 1,4,5-trisphosphate (IP(3)) mass levels, which was blocked by TTX. 6. In the presence of P-CTX-1b (10 nM) and in the absence of external Ca2+, the intracellular Ca2+ levels show a transient increase in the cytoplasm as well as in the nuclei. The time course of this effect may reflect the action of IP(3) over internal stores activated by P-CTX-1b-induced membrane depolarization.

Cardiac overexpression of insulin-like growth factor 1 attenuates chronic alcohol intake-induced myocardial contractile dysfunction but not hypertrophy: Roles of Akt, mTOR, GSK3beta, and PTEN

Chronic alcohol intake leads to the development of alcoholic cardiomyopathy manifested by cardiac hypertrophy and contractile dysfunction. This study was designed to examine the effects of transgenic overexpression of insulin-like growth factor 1 (IGF-1) on alcohol-induced cardiac contractile dysfunction. Wild-type FVB and cardiac-specific IGF-1 mice were placed on a 4% alcohol or control diet for 16weeks. Cardiac geometry and mechanical function were evaluated by echocardiography and cardiomyocyte and intracellular Ca(2+) properties. Histological analyses for cardiac fibrosis and apoptosis were evaluated by Masson trichrome staining and TUNEL assay, respectively. Expression and phosphorylation of Cu/Zn superoxide dismutase (SOD1), Ca(2+) handling proteins, and key signaling molecules for survival including Akt, mTOR, GSK3beta, Foxo3a, and the negative regulator of Akt, phosphatase and tensin homolog on chromosome 10 (PTEN), as well as mitochondrial proteins UCP-2 and PGC1alpha, were evaluated by Western blot analysis. Chronic alcohol intake led to cardiac hypertrophy, interstitial fibrosis, reduced mitochondrial number, compromised cardiac contractile function and intracellular Ca(2+) handling, decreased SOD1 expression, elevated superoxide production, and overt apoptosis, all of which, with the exception of cardiac hypertrophy, were abrogated by the IGF-1 transgene. Immunoblotting data showed reduced phosphorylation of Akt, mTOR, GSK3beta, and Foxo3a; upregulated Foxo3a and PTEN; and dampened SERCA2a, PGC1alpha, and UCP-2 after alcohol intake. All these alcohol-induced changes in survival and mitochondrial proteins were alleviated by IGF-1. Taken together, these data favor a beneficial role for IGF-1 in alcohol-induced myocardial contractile dysfunction independent of cardiac hypertrophy.

Mitochondrial calcium regulates rat liver regeneration through the modulation of apoptosis

Subcellular Ca(2+) signals control a variety of responses in the liver. For example, mitochondrial Ca(2+) (Ca(mit)(2+)) regulates apoptosis, whereas Ca(2+) in the nucleus regulates cell proliferation. Because apoptosis and cell growth can be related, we investigated whether Ca(mit)(2+) also affects liver regeneration. The Ca(2+)-buffering protein parvalbumin, which was targeted to the mitochondrial matrix and fused to green fluorescent protein, was expressed in the SKHep1 liver cell line; the vector was called parvalbumin-mitochondrial targeting sequence-green fluorescent protein (PV-MITO-GFP). This construct properly localized to and effectively buffered Ca(2+) signals in the mitochondrial matrix. Additionally, the expression of PV-MITO-GFP reduced apoptosis induced by both intrinsic and extrinsic pathways. The reduction in cell death correlated with the increased expression of antiapoptotic genes [B cell lymphoma 2 (bcl-2), myeloid cell leukemia 1, and B cell lymphoma extra large] and with the decreased expression of proapoptotic genes [p53, B cell lymphoma 2-associated X protein (bax), apoptotic peptidase activating factor 1, and caspase-6]. PV-MITO-GFP was also expressed in hepatocytes in vivo with an adenoviral delivery system. Ca(mit)(2+) buffering in hepatocytes accelerated liver regeneration after partial hepatectomy, and this effect was associated with the increased expression of bcl-2 and the decreased expression of bax.

CONCLUSION

Together, these results reveal an essential role for Ca(mit)(2+) in hepatocyte proliferation and liver regeneration, which may be mediated by the regulation of apoptosis.

Stereoselective modulatory actions of oleamide on GABA(A) receptors and voltage-gated Na(+) channels in vitro: a putative endogenous ligand for depressant drug sites in CNS

1. cis-9,10-octadecenoamide ('oleamide') accumulates in CSF on sleep deprivation. It induces sleep in animals (the trans form is inactive) but its cellular actions are poorly characterized. We have used electrophysiology in cultures from embryonic rat cortex and biochemical studies in mouse nerve preparations to address these issues. 2. Twenty microM cis-oleamide (but not trans) reversibly enhanced GABA(A) currents and depressed the frequency of spontaneous excitatory and inhibitory synaptic activity in cultured networks. 3. cis-oleamide stereoselectively blocked veratridine-induced (but not K(+)-induced) depolarisation of mouse synaptoneurosomes (IC(50), 13. 9 microM). 4. The cis isomer stereoselectively blocked veratridine-induced (but not K(+)-induced) [(3)H]-GABA release from mouse synaptosomes (IC(50), 4.6 microM). 5. At 20 microM cis-oleamide, but not trans, produced a marked inhibition of Na(+) channel-dependent rises in intrasynaptosomal Ca(2+). 6. The physiological significance of these observations was examined by isolating Na(+) spikes in cultured pyramidal neurones. Sixty-four microM cis-oleamide did not significantly alter the amplitude, rate of rise or duration of unitary action potentials (1 Hz). 7. cis-Oleamide stereoselectively suppressed sustained repetitive firing (SRF) in these cells with an EC(50) of 4.1 microM suggesting a frequency- or state-dependent block of voltage-gated Na(+) channels. 8. Oleamide is a stereoselective modulator of both postsynaptic GABA(A) receptors and presynaptic or somatic voltage-gated Na(+) channels which are crucial for synaptic inhibition and conduction. The modulatory actions are strikingly similar to those displayed by sedative or anticonvulsant barbiturates and a variety of general anaesthetics. 9. Oleamide may represent an endogenous modulator for drug receptors and an important regulator of arousal.

Malondialdehyde inhibits cardiac contractile function in ventricular myocytes via a p38 mitogen-activated protein kinase-dependent mechanism

(1) Increased oxidative stress plays a significant role in the etiology of cardiovascular disease. Lipid peroxidation, initiated in the presence of hydroxy radicals resulting in the production of malondialdehyde, directly produces oxidative stress. This study was designed to examine the direct impact of malondialdehyde on ventricular contractile function at the single cardiac myocyte level. Ventricular myocytes from adult rat hearts were stimulated to contract at 0.5 Hz, and mechanical and intracellular Ca(2+) properties were evaluated using an IonOptix Myocam system. Contractile properties analyzed included peak shortening amplitude (PS), time-to-PS (TPS), time-to-90% relengthening (TR(90)), maximal velocity of shortening/relengthening (+/-dLdt), and Ca(2+)-induced intracellular Ca(2+) fluorescence release (CICR) and intracellular Ca(2+) decay (tau). p38 mitogen-activated protein (MAP) kinase phosphorylation was assessed with Western blot. (2) Our results indicated that malondialdehyde directly depressed PS, +/-dLdt and CICR in a concentration-dependent manner and shortened TPS without affecting TR(90) and tau. Interestingly, the malondialdehyde-induced cardiac mechanical effect was abolished by both the p38 MAP kinase inhibitor SB203580 (1 and 10 micro M) and the antioxidant vitamin C (100 micro M). Western blot analysis confirmed direct phosphorylation of p38 MAP kinase by malondialdehyde. (3) These findings revealed a novel role of malondialdehyde and p38 MAP kinase in lipid peroxidation and oxidative stress-associated cardiac dysfunction.

Effects of mibefradil and nifedipine on arteriolar myogenic responsiveness and intracellular Ca(2+)

1. Ca(2+) entry mechanisms underlying spontaneous arteriolar tone and acute myogenic reactivity remain uncertain. These studies aimed to compare the effects of nifedipine and the putative T-channel blocker, mibefradil, on arteriolar myogenic responsiveness and intracellular Ca(2+) (Ca(2+)(i)). 2. First order cremaster muscle arterioles (1A) were isolated from rats, cannulated, pressurized to 70 mmHg in the absence of intraluminal flow, and mechanical responses studied by video microscopy. The Ca(2+)(i) was measured using fluorescence imaging of Fura 2 loaded arterioles. 3. Both nifedipine and mibefradil showed dose-dependent inhibition of spontaneous myogenic tone (at 70 mmHg; pEC(50) 7.04+/-0.17 vs 6.65+/-0.20 respectively, n=6 for both, n.s.) and KCl-induced vasoconstriction (at 70 mmHg; pEC(50) 6.93+/-0. 38 vs 6.45+/-0.27 respectively, n=6 for both, n.s.). 4. In arterioles maintained at 50 mmHg, nifedipine (10(-7) and 10(-5) M) caused a concentration dependent reduction in Ca(2+)(i), however, mibefradil (10(-7) and 10(-5) M) had no effect. Furthermore nifedipine significantly attenuated the increase in Ca(2+)(i) associated with an acute pressure step (50 - 120 mmHg) whereas mibefradil was considerably less effective. 5. Mibefradil (10(-7) M) significantly attenuated contractile responses to 60 mM KCl without altering the KCl-induced increase in Ca(2+)(i), in contrast to nifedipine (10(-7) M) which reduced both Ca(2+)(i) and contraction. 6. Membrane potential of arterioles with spontaneous myogenic tone (70 mmHg) was -41.5+/-1. 0 mV. Nifedipine (10(-7) or 10(-5) M) had no effect on membrane potential, however mibefradil (10(-5) M) caused significant depolarization. 7. In summary, both mibefradil and nifedipine inhibit arteriolar spontaneous tone and acute myogenic reactivity. While there may be overlap in the mechanisms by which these agents inhibit tone, differences in effects on membrane potential and intracellular Ca(2+) levels suggest mibefradil exhibits actions other than blockade of Ca(2+) entry in skeletal muscle arterioles.

Mechanisms underlying the impairment of endothelium-dependent relaxation in the pulmonary artery of monocrotaline-induced pulmonary hypertensive rats

1. It has been reported that endothelium-dependent relaxation is impaired in pulmonary hypertensive vessels. The underlying mechanisms for this phenomenon, however, have not yet been identified. In this study, the mechanisms responsible for decreased endothelium-dependent relaxation in the pulmonary artery isolated from monocrotaline (MCT)-induced pulmonary hypertensive rat (MCT rat) were examined. MCT (60 mg kg-1), or its vehicle was administered by a single subcutaneous injection to 6-week-old male Sprague Dawley rats. 2. Endothelium-dependent relaxation induced by carbachol or ionomycin in the MCT rat artery was significantly smaller than that in vehicle-treated rat (control rat) artery. Cyclic GMP levels, measured by enzyme-immunoassay, under resting or stimulation with carbachol or ionomycin were also smaller in the MCT rat artery. However, sodium nitroprusside-induced cyclic GMP accumulation in the endothelium-denuded artery was similar in control and MCT rats. These results suggest that MCT treatment decreases endothelial nitric oxide (NO) production. 3. Resting endothelial Ca2+ levels ([Ca2+]i) in the fura-PE3-loaded MCT rat artery, were not different from those in the control rat. However, the increase in endothelial [Ca2+]i elicited by carbachol was attenuated in the MCT rat. 4. In quantitative RT - PCR analysis, the expression of mRNA encoding endothelial NO synthase was rather increased in the MCT rat artery, suggesting an up-regulation of eNOS expression. 5. These results provide evidence that impaired NO-mediated arterial relaxation in the MCT rat is due to dissociation between eNOS expression and NO production. This dissociation may be derived from an inhibition of receptor-mediated Ca2+ metabolism and also from the apparent decrease in Ca2+ sensitivity of eNOS.

Effects of chronic drug treatments on increases in intracellular calcium mediated by nicotinic acetylcholine receptors in SH-SY5Y cells

1. SH-SY5Y cells express alpha7 and alpha3* subtypes of nicotinic acetylcholine receptors (AChR). Numbers of these receptors are upregulated by chronic treatment with nicotinic agonists or KCl. In this study we have examined the functional consequences of these drug treatments on nicotine- or KCl-evoked increases in [Ca(2+)](i), in SH-SY5Y cells. 2. In untreated cells, nicotine increased [Ca(2+)](i) (EC(50) 7.5 microM). Responses to 10 microM nicotine were abolished by the non-selective nicotinic antagonist mecamylamine and were partially blocked by alpha7-selective antagonists, the alpha3beta2*-selective antagonist alpha-conotoxin-MII, and by cadmium and verapamil. 3. After treatment for 4 days with nicotinic agonists, nicotine-evoked increases in [Ca(2+)](i) were significantly decreased by about 25%. Nicotine-evoked responses were paradoxically increased in the presence of acute methyllycaconitine (MLA; an alpha7-selective antagonist) although other alpha7-selective antagonists were without effect, while alpha-conotoxin-MII gave a partial inhibition. The increase observed with MLA was abolished by mecamylamine but not by alpha-conotoxin-MII and was still observed 24 h after chronic nicotine treatment. 4. After treatment for 4 days with KCl, nicotine-evoked increases in [Ca(2+)](i) were also decreased by 25%, but acute MLA was without effect. Responses to 20 mM KCl were unchanged by prior treatment with nicotine or KCl. Treatment for 4 days with 5 microM verapamil reduced responses to both nicotine and KCl by about 50%. 5. Multiple nicotinic AChR subtypes contribute to nicotine-evoked increases in [Ca(2+)](i) in SH-SY5Y cells. Responses to acute nicotine are reduced after chronic nicotine or KCl treatment, with loss of the component attributed to the alpha7 subtype. However, in nicotine-treated cells this effect is reversed when nicotine stimulation is applied in the presence of acute MLA. The antagonist may assist in converting a non-functional alpha7 nicotinic AChR to a conducting state.

NMDA receptor characterization and subunit expression in rat cultured mesencephalic neurones

1. NMDA-induced changes in free intracellular Ca2+ concentration ([Ca2+]i) were determined in individual cultured rat mesencephalic neurones by the fura-2 method. mRNA expression encoding NMDA receptor subunits (NR1, NR2A-D) was examined by RT-PCR. 2. NMDA (1-100 microM, plus 10 microM glycine) induced a concentration-dependent increase in [Ca2+]i (EC50 = 5.7 microM). The effect of NMDA was virtually insensitive to tetrodotoxin (0.3 microM) and nitrendipine (1 microM), but dependent on extracellular Ca2+. 5,7-Dichlorokynurenic acid (10 microM), a specific antagonist at the glycine binding site on the NMDA receptor, abolished the NMDA response. 3. Memantine, an open-channel blocker, and ifenprodil, a preferential non-competitive NR1/NR2B receptor antagonist diminished the NMDA effect with an IC50 value of 0.17 and 1 microM, respectively. Ethanol at 50 and 100 mM caused about 25 and 45%-inhibition, respectively. 4. Agarose gel analysis of the PCR products followed by ethidium bromide fluorescence or CSPD chemiluminescence detection revealed an almost exclusive expression of the NR1 splice variants lacking exon (E) 5 and E22. The 3' splice form without both E21 and E22 exceeded that containing E21 by approximately 4 fold. The relative amounts of NR2A, NR2B, NR2C corresponded to approximately 1:2:1. NR2D mRNA was also detectable. 5. In conclusion, mesencephalic neurones bear ethanol-sensitive NMDA receptors which might be involved in the development of ethanol dependence and withdrawal. The high affinity of NMDA to this receptor, its sensitivity to ifenprodil and memantine may suggest that the mesencephalic NMDA receptor comprises the NR1 splice variant lacking E5, NR2B, and NR2C, respectively.

Inositol trisphosphate 3-kinases: focus on immune and neuronal signaling

The localized control of second messenger levels sculpts dynamic and persistent changes in cell physiology and structure. Inositol trisphosphate [Ins(1,4,5)P(3)] 3-kinases (ITPKs) phosphorylate the intracellular second messenger Ins(1,4,5)P(3). These enzymes terminate the signal to release Ca(2+) from the endoplasmic reticulum and produce the messenger inositol tetrakisphosphate [Ins(1,3,4,5)P(4)]. Independent of their enzymatic activity, ITPKs regulate the microstructure of the actin cytoskeleton. The immune phenotypes of ITPK knockout mice raise new questions about how ITPKs control inositol phosphate lifetimes within spatial and temporal domains during lymphocyte maturation. The intense concentration of ITPK on actin inside the dendritic spines of pyramidal neurons suggests a role in signal integration and structural plasticity in the dendrite, and mice lacking neuronal ITPK exhibit memory deficits. Thus, the molecular and anatomical features of ITPKs allow them to regulate the spatiotemporal properties of intracellular signals, leading to the formation of persistent molecular memories.

Somatostatin-induced control of cytosolic free calcium in pituitary tumour cells

1. In rat pituitary tumour cells (GC cells), spontaneous oscillations of the intracellular concentration of Ca2+ ([Ca2+]i) induce growth hormone (GH) secretion that is inhibited by octreotide, a somatostatin (SRIF) agonist which binds to SRIF subtype (sst) receptor 2. The effects of its functional activation on the control of [Ca2+]i were investigated using fluorimetric measurements of [Ca2+]i. 2. SRIF decreases the basal [Ca2+]i and the [Ca2+]i rise in response to forskolin (FSK) through the inhibition of L-type voltage-dependent Ca2+ channels. 3. Pretreatment with octreotide or with L-Tyr8++ Cyanamid 154806, a sst2 receptor antagonist, abolishes the SRIF-induced inhibition of [Ca2+]i. Octreotide is known to operate through agonist-induced desensitization, while the antagonist operates through receptor blockade. 4. sst1 and sst2 receptor-immunoreactivities (-IRs) are localized to cell membranes. sst2, but not sst1 receptor-IR, internalizes after cell exposure to octreotide. 5. SRIF-induced inhibition of basal [Ca2+]i or FSK-induced Ca2+ entry is blocked by pertussis toxin (PTX). 6. FSK-induced cyclic AMP accumulation is only partially decreased by SRIF or octreotide, indicating that sst2 receptors are coupled to intracellular pathways other than adenylyl cyclase (AC) inhibition. 7. In the presence of H-89, an inhibitor of cyclic AMP-dependent protein kinase (PKA), SRIF-induced inhibition of basal [Ca2+]i is still present, although reduced in amplitude. 8. SRIF inhibits [Ca2+]i by activating sst2 receptors. Inhibition of AC activity is only partly responsible for this effect, and other transduction pathways may be involved.

Effects of kappa-opioid receptor stimulation in the heart and the involvement of protein kinase C

1. The role of protein kinase C (PKC) in mediating the action of kappa-receptor stimulation on intracellular Ca2+ and cyclic AMP production was determined by studying the effects of trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl] cyclohexyl) benzeneacetamide methanesulphonate (U50,488H), a selective kappa-receptor agonist, and phorbol 12-myristate 13-acetate (PMA), a PKC agonist, on the electrically-induced [Ca2+]i transient and forskolin-stimulated cyclic AMP accumulation in the presence and absence of a PKC antagonist, staurosporine or chelerythrine, in the single rat ventricular myocyte. 2. U50,488H at 2.5-40 microM decreased both the electrically-induced [Ca2+]i transient and forskolin-stimulated cyclic AMP accumulation dose-dependently, effects which PMA mimicked. The effects of the kappa-agonist, that were blocked by a selective kappa-antagonist, nor-binaltorphimine, were significantly antagonized by the PKC antagonists, staurosporine and/or chelerythrine. The results indicate that PKC mediates the actions of kappa-receptor stimulation. 3. To determine whether the action of PKC was at the sarcoplasmic reticulum (SR) or not, the [Ca2+]i transient induced by caffeine, that depletes the SR of Ca2+, was used as an indicator of Ca2+ content in the SR. The caffeine-induced [Ca2+]i transient was significantly reduced by U50,488H at 20 microM. This effect of U50,488H on caffeine-induced [Ca2+]i transient was significantly attenuated by 1 microM chelerythrine, indicating that the action of PKC involves mobilization of Ca2+ from the SR. When the increase in IP3 production in response to K-receptor stimulation with U50,488H in the ventricular myocyte was determined, the effect of U50,488H was the same in the presence and absence of staurosporine, suggesting that the effect of PKC activation subsequent to kappa-receptor stimulation does not involve IP3. The observations suggest that PKC may act directly at the SR. 4. In conclusion, the present study has provided evidence for the first time that PKC may be involved in the action of kappa-receptor stimulation on Ca2+ in the SR and cyclic AMP production, both of which play an essential role in Ca2+ homeostasis in the heart.

Intracellular calcium changes in rat aortic smooth muscle cells in response to fluid flow

Vascular smooth muscle cells (VSM) are normally exposed to transmural fluid flow shear stresses, and after vascular injury, blood flow shear stresses are imposed upon them. Since Ca2+ is a ubiquitous intracellular signaling molecule, we examined the effects of fluid flow on intracellular Ca2+ concentration in rat aortic smooth muscle cells to assess VSM responsiveness to shear stress. Cells loaded with fura 2 were exposed to steady flow shear stress levels of 0.5-10.0 dyn/cm2 in a parallel-plate flow chamber. The percentage of cells displaying a rise in cytosolic Ca2+ ion concentration ([Ca2+]i) increased in response to increasing flow, but there was no effect of flow on the ([Ca2+]i) amplitude of responding cells. Addition of Gd3+ (10 microM) or thapsigargin (50 nM) significantly reduced the percentage of cells responding and the response amplitude, suggesting that influx of Ca2+ through ion channels and release from intracellular stores contribute to the rise in ([Ca2+]i) in response to flow. The addition of nifedipine (1 or 10 microM) or ryanodine (10 microM) also significantly reduced the response amplitude, further defining the role of ion channels and intracellular stores in the Ca2+ response.

Phenanthrolines--a new class of CFTR chloride channel openers

1. A number of phenanthrolines and benzoquinolines were examined for their ability to activate epithelial chloride secretion by measuring short circuit current (SCC) using the mouse colon epithelium. 1,10 phenanthroline stimulated electrogenic chloride secretion with an EC(50) of 612+/-10 microM and a Hill slope of 4.9+/-0.3. A similar pharmacology was demonstrated by both 1,7 and 4,7 phenanthrolines, 7,8 benzoquinoline and phenanthridine. 2. Evidence that the increase in SCC caused by 1,10 phenanthroline was due to chloride secretion is based upon (a) inhibition of the current by furosemide, (b) failure of cystic fibrosis (CF) colons to respond and (c) an associated net flux of (36)Cl(-). 3. 1,10 Phenanthroline affected neither the generation of cyclic AMP or the concentration of intracellular Ca(2+) in colonic epithelial cells. 4. 1,10 phenanthroline affected the chloride conductance of the apical membrane, as shown by an increase in chloride current in 'apical membrane only' preparations in the presence of an apical to basolateral chloride gradient. The increase in chloride current was inhibited by 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and was not present in CF colons. 5. Additionally, 1,10 phenanthroline activated basolateral K(+) channels, both Ca(2+)- and cyclic AMP-sensitive channels, as shown by inhibitor studies with charybdotoxin (ChTX) and XE991, and after the apical membrane was permeabilized with nystatin. 6. The phenanthrolines and benzoquinolines described here, with dual actions affecting CFTR and basolateral K(+) channels, may constitute useful lead compounds for adjunct therapy in CF.

Anti-arrhythmic effect of acupuncture pretreatment in the rats subjected to simulative global ischemia and reperfusion--involvement of intracellular Ca2+ and connexin 43

BACKGROUND

The previous study showed that the cardiac arrhythmias induced by myocardial ischemia and reperfusion were attenuated by the pretreatment of acupuncture; however, the related mechanism is not understood. The present study was therefore designed to determine whether intracellular Ca(2+) ([Ca(2+)]i) and connexin 43 (Cx43) are involved in the mediation of the anti-arrhythmic effect of electro-acupuncture (EA) pretreatment in the rats subjected to simulative global ischemia and reperfusion (SGIR).

METHODS

SGIR was made in the isolated heart by a low flow perfusion followed by a flow restoration. Four groups of animals are involved in the present study, including normal control group, SGIR group, EA group and EA plus 18 beta-glycyrrhetinic acid (EAG) group. For EA pretreatment, bilateral Neiguan acupoints (PC6) of the rats were stimulated for 30 min once a day in 3 consecutive days. Cx43 antagonist was given to the rats in EAG group 30 minutes before the EA pretreatment. The resting [Ca(2+)]i concentration, calcium oscillation, the contents of total Cx43 and non-phosphrylated Cx43 and arrhythmia score were compared among different groups.

RESULTS

In EA group, the arrhythmic score, the resting [Ca(2+)]i concentration and the number of [Ca(2+)]i oscillations were all significantly less than those in SGIR group (all P < 0.05), and interestingly, after EA pretreatment, the contents of nonphosphated Cx43 in the EA group were significantly lower than that in SGIR group respectively (P < 0.05). However, when the rats were treated with Cx43 antagonist prior to the EA pretreatment, the protection effects induced by EA pretreatment were reversed.

CONCLUSIONS

The results showed that EA pretreatment could produce anti-arrhythmic effect in the rats subjected to SGIR. The anti-arrhythmic effect of EA pretreatment may be due at least partially to the inhibition of SGIR-induced calcium overload and [Ca(2+)]i oscillations, reduction of non-phosphorylated Cx43 and the enhancement of the corresponding phosphorylated Cx43 in the cardiac cells.

Mechanism of action of angiotensin II in human isolated subcutaneous resistance arteries

1. Human isolated subcutaneous arteries were mounted in a myograph and isometric tension measured. In some experiments, intracellular calcium [Ca(2+)]i was also measured using fura-2. 2. Angiotensin II (100 pM - 1 microM) increased [Ca(2+)]i and tone in a concentration-dependent manner. The effects of angiotensin II (100 nM) were inhibited by an AT1-receptor antagonist, candesartan (100 pM). 3. Ryanodine (10 microM), had no effect on angiotensin II-induced responses, but removal of extracellular Ca(2+) abolished angiotensin II-induced rise in [Ca(2+)]i and tone. Inhibition of Ca(2+) entry by Ni(2+) (2 mM), also inhibited angiotensin II responses. The dihydropyridine, L-type calcium channel antagonist, amlodipine (10 microM), only partially attenuated angiotensin II responses. 4. Inhibition of protein kinase C (PKC) by chelerythrine (1 microM), or by overnight exposure to a phorbol ester (PDBu; 500 nM) had no effect on angiotensin II-induced contraction. 5. Genistein (10 microM), a tyrosine kinase inhibitor, inhibited angiotensin II-induced contraction, but did not inhibit the rise in [Ca(2+)]i, suggesting that at this concentration it affected the calcium sensitivity of the contractile apparatus. Genistein did not affect responses to norepinephrine (NE) or high potassium (KPSS). 6. A selective MEK inhibitor, PD98059 (30 microM), inhibited both the angiotensin II-induced contraction and rise in [Ca(2+)]i, but had no effect on responses to NE or KPSS. 7. AT1 activation causes Ca(2+) influx via L-type calcium channels and a dihydropyridine-insensitive route, but does not release Ca(2+) from intracellular sites. Activation of tyrosine kinase(s) and the ERK 1/2 pathway, but not classical or novel PKC, also play a role in angiotensin II-induced contraction in human subcutaneous resistance arteries.

Regulation of Ca²⁺ release through inositol 1,4,5-trisphosphate receptors by adenine nucleotides in parotid acinar cells

Secretagogue-stimulated intracellular Ca(2+) signals are fundamentally important for initiating the secretion of the fluid and ion component of saliva from parotid acinar cells. The Ca(2+) signals have characteristic spatial and temporal characteristics, which are defined by the specific properties of Ca(2+) release mediated by inositol 1,4,5-trisphosphate receptors (InsP(3)R). In this study we have investigated the role of adenine nucleotides in modulating Ca(2+) release in mouse parotid acinar cells. In permeabilized cells, the Ca(2+) release rate induced by submaximal [InsP(3)] was increased by 5 mM ATP. Enhanced Ca(2+) release was not observed at saturating [InsP(3)]. The EC(50) for the augmented Ca(2+) release was ∼8 μM ATP. The effect was mimicked by nonhydrolysable ATP analogs. ADP and AMP also potentiated Ca(2+) release but were less potent than ATP. In acini isolated from InsP(3)R-2-null transgenic animals, the rate of Ca(2+) release was decreased under all conditions but now enhanced by ATP at all [InsP(3)]. In addition the EC(50) for ATP potentiation increased to ∼500 μM. These characteristics are consistent with the properties of the InsP(3)R-2 dominating the overall features of InsP(3)R-induced Ca(2+) release despite the expression of all isoforms. Finally, Ca(2+) signals were measured in intact parotid lobules by multiphoton microscopy. Consistent with the release data, carbachol-stimulated Ca(2+) signals were reduced in lobules exposed to experimental hypoxia compared with control lobules only at submaximal concentrations. Adenine nucleotide modulation of InsP(3)R in parotid acinar cells likely contributes to the properties of Ca(2+) signals in physiological and pathological conditions.

Pharmacological evaluation of the role of cytochrome P450 in intracellular calcium signalling in rat pancreatic acinar cells

We have investigated whether the cytochrome P450 system is involved in Ca(2+) signalling in rat pancreatic acinar cells. Intracellular free [Ca(2+)] ([Ca(2+)](i)) was measured in collagenase-isolated cells using fura-2 microspectrofluorimetry and imaging. The imidazole P450 inhibitor ketoconazole (5 - 50 microM) inhibited [Ca(2+)](i) oscillations induced by cholecystokinin octapeptide (CCK). However, ketoconazole also raised baseline [Ca(2+)](i) when applied in the absence of CCK. These effects were mimicked by 5 - 50 microM SKF96365, an imidazole widely used as an inhibitor of Ca(2+) entry. The non-imidazole P450 inhibitor proadifen (SKF525A) inhibited CCK-induced [Ca(2+)](i) oscillations at a concentration of 10 - 50 microM. Proadifen alone caused intracellular Ca(2+) release at 25 or 50 microM, but not at 10 microM. Octadecynoic acid and 1-aminobenzotriazole, structurally-unrelated non-imidazole P450 inhibitors, did not alter baseline [Ca(2+)](i) or CCK-evoked oscillations. We compared cumulative CCK dose-response relationship in control cells and in cells where P450 had been induced by prior injection of animals with beta-naphthoflavone. Only minor differences were apparent, with induced cells showing some decrease in responsiveness at moderate and higher concentration of CCK (30 pM - 3 nM). Direct assessment of depletion-activated Ca(2+) entry showed no clear differences between control and induced cells. In conclusion, we could find no compelling evidence for a role of P450 in controlling Ca(2+) signalling generally, or Ca(2+) entry in particular, in pancreatic acinar cells. Induction of P450 is therefore probably toxic to acinar cells via a Ca(2+)-independent mechanism.

Mechanisms of AVP-induced glucagon release in clonal alpha-cells in-R1-G9: involvement of Ca(2+)-dependent and -independent pathways

1. The mechanisms underlying AVP-induced increase in [Ca(2+)](i) and glucagon release in clonal alpha-cells In-R1-G9 were investigated. 2. AVP increased [Ca(2+)](i) and glucagon release in a concentration-dependent manner. After the administration of AVP, glucagon was released within 30 s, quickly reached the maximum within 2 min, and maintained a steady-state concentration for at least 15 min. 3. In Ca(2+)-containing medium, AVP increased [Ca(2+)](i) in a biphasic pattern; a peak followed by a sustained plateau. In Ca(2+)-free medium, the Ca(2+) response to AVP became monophasic with lower amplitude and no plateau. Both the basal and AVP-induced glucagon releases were lower in the absence than in the presence of extracellular Ca(2+). When [Ca(2+)](i) was stringently deprived by BAPTA, a Ca(2+) chelator, AVP still significantly increased glucagon release. 4. Pretreatment with thapsigargin, a microsomal Ca(2+) ATPase inhibitor, abolished both the Ca(2+) peak and sustained plateau. 5.AVP increased intracellular concentration of IP(3). 6. U-73122 (8 microM), a phospholipase C inhibitor, abolished AVP-induced increases in [Ca(2+)](i), but only reduced AVP-induced glucagon release by 39%. 7. Pretreatment with nimodipine, an L-type Ca(2+) channel blocker failed to alter AVP-induced glucagon release or increase in [Ca(2+)](i). 8. The results suggest that AVP causes glucagon release through both Ca(2+)-dependent and -independent pathways. For the Ca(2+)-dependent pathway, the G(q) protein activates phospholipase C, which catalyzes the formation of IP(3). IP(3) induces Ca(2+) release from the endoplasmic reticulum, which, in turn, triggers Ca(2+) influx. Both Ca(2+) release and Ca(2+) influx may contribute to AVP-induced glucagon release.

Acetylcholine and tachykinins involvement in the caffeine-induced biphasic change in intracellular Ca2+ in bovine airway smooth muscle

1. Caffeine has been widely used as a pharmacological tool to evaluate Ca(2+) release from the sarcoplasmic reticulum in isolated smooth muscle cells. However, in nervous tissue this drug also causes neurotransmitters release, which might cause additional effects when smooth muscle strips are evaluated. To assess this last possibility, simultaneous measurements of contraction and cytosolic Ca(2+) concentration (using Fura-2/AM) were carried out in bovine airway smooth muscle strips during caffeine stimulation. 2. A first stimulation (S1, n=11) with caffeine (10 mM) induced a biphasic change in cytosolic Ca(2+), which consisted of a transient Ca(2+) peak (254+/-40 nM, X+/-SEM) followed by a plateau (92+/-13 nM), and a transient contraction (204.72+/-31.56 mg tension mg tissue(-1)). A second caffeine stimulation (S2) produced a similar response but these parameters had a different magnitude. The S2/S1 ratios for these parameters were 0.69+/-0.02, 0.83+/-0.06 and 1.01+/-0.03, respectively. Addition of omega-conotoxin GVIA (1 micro M) and tetrodotoxin (3.1 micro M) before S2 significantly diminished these S2/S1 ratios (0.26+/-0.05, 0.26+/-0.09 and 0.64+/-0.11, respectively, n=5, P<0.05), implicating the neurotransmitters release involvement in the response to caffeine. A similar effect (P<0.01) was observed with atropine (1 micro M, n=4), the fragment 4-11 of substance P (SP) (an SP receptor antagonist, 10 micro M, n=5), and with both substances (n=4). 3. We discarded a direct effect of omega-conotoxin GVIA (1 micro M) plus tetrodotoxin (3.1 micro M) or of atropine (1 micro M) plus SP fragment 4-11 on smooth muscle cells because they did not modify caffeine responses in isolated tracheal myocytes. 4. We confirmed by HPLC that caffeine increased the release of acetylcholine (from 0.43+/-0.19 to 2.07+/-0.56 nM mg tissue(-1), P<0.02) in bovine airway smooth muscle strips. Detection of substance P by ELISA was not statistically different after caffeine stimulation (geometric means before and after caffeine, 0.69 vs. 1.97 pg ml(-1) mg tissue(-1), respectively, P=0.053). 5. We concluded that acetylcholine and tachykinins release are involved in the caffeine-induced biphasic changes in cytosolic Ca(2+) concentration.

Ca(2+) regulation of endocochlear potential in marginal cells

We examined the effect of the cytosolic Ca(2+) concentration ([Ca(2+)](c)) in marginal cells on the asphyxia- or furosemide-induced decrease in the endocochlear potential (EP) by perfusing the endolymph with or without a Ca(2+) chelator or inhibitors of Ca(2+)-permeable channels or Ca(2+)-pump during transient asphyxia or intravenous administration of furosemide. We obtained the following results. (1) Endolymphatic administration of SKF96365 (an inhibitor of TRPC and L-type Ca(2+) channels) or EGTA-acetoxymethyl ester (EGTA-AM) significantly inhibited both the transient asphyxia-induced decrease in EP (TAID) and the furosemide-induced decrease in EP (FUID). (2) Endolymphatic perfusion with nifedipine significantly inhibited the TAID but not the FUID. (3) The recovery from the FUID was significantly suppressed by perfusing the endolymph with EGTA-AM, nifedipine, or SKF96365. (4) Endolymphatic administration of thapsigargin inhibited both the FUID and TAID. (5) The recovery rate from the FUID was much slower than that from the TAID, indicating that furosemide may inhibit the Ca(2+)-pump. (6) A strong reaction in immunohistochemical staining for TRPC channels was observed in the luminal and basolateral membranes of marginal cells. (7) A positive staining reaction for the gamma subunit of epithelial Na(+) channels was observed in the luminal and basolateral membranes of marginal cells. (8) Positive EP was diminished toward 0 mV by the endolymphatic perfusion with 10 muM amiloride or 10 muM phenamil. Taken together, these findings suggest that [Ca(2+)](c) regulated by endoplasmic Ca(2+)-pump and Ca(2+)-permeable channels in marginal cells may regulate the positive EP, which is partly produced by the diffusion potential of Na(+) across the basolateral membrane in marginal cells.

Voltage- and calcium-dependent inactivation of calcium channels in Lymnaea neurons

Ca(2+) channel inactivation in the neurons of the freshwater snail, Lymnaea stagnalis, was studied using patch-clamp techniques. In the presence of a high concentration of intracellular Ca(2+) buffer (5 mM EGTA), the inactivation of these Ca(2+) channels is entirely voltage dependent; it is not influenced by the identity of the permeant divalent ions or the amount of extracellular Ca(2+) influx, or reduced by higher levels of intracellular Ca(2+) buffering. Inactivation measured under these conditions, despite being independent of Ca(2+) influx, has a bell-shaped voltage dependence, which has often been considered a hallmark of Ca(2+)-dependent inactivation. Ca(2+)-dependent inactivation does occur in Lymnaea neurons, when the concentration of the intracellular Ca(2+) buffer is lowered to 0.1 mM EGTA. However, the magnitude of Ca(2+)-dependent inactivation does not increase linearly with Ca(2+) influx, but saturates for relatively small amounts of Ca(2+) influx. Recovery from inactivation at negative potentials is biexponential and has the same time constants in the presence of different intracellular concentrations of EGTA. However, the amplitude of the slow component is selectively enhanced by a decrease in intracellular EGTA, thus slowing the overall rate of recovery. The ability of 5 mM EGTA to completely suppress Ca(2+)-dependent inactivation suggests that the Ca(2+) binding site is at some distance from the channel protein itself. No evidence was found of a role for serine/threonine phosphorylation in Ca(2+) channel inactivation. Cytochalasin B, a microfilament disrupter, was found to greatly enhance the amount of Ca(2+) channel inactivation, but the involvement of actin filaments in this effect of cytochalasin B on Ca(2+) channel inactivation could not be verified using other pharmacological compounds. Thus, the mechanism of Ca(2+)-dependent inactivation in these neurons remains unknown, but appears to differ from those proposed for mammalian L-type Ca(2+) channels.

Insulin-like growth factor I (IGF-1) deficiency ameliorates sex difference in cardiac contractile function and intracellular Ca(2+) homeostasis

Sex difference in cardiac contractile function exists which may contribute to the different prevalence in cardiovascular diseases between genders. However, the precise mechanisms of action behind sex difference in cardiac function are still elusive. Given that sex difference exists in insulin-like growth factor I (IGF-1) cascade, this study is designed to evaluate the impact of severe liver IGF-1 deficiency (LID) on sex difference in cardiac function. Echocardiographic, cardiomyocyte contractile and intracellular Ca(2+) properties were evaluated including ventricular geometry, fractional shortening, peak shortening, maximal velocity of shortening/relengthening (±dL/dt), time-to-peak shortening (TPS), time-to-90% relengthening (TR(90)), fura-fluorescence intensity (FFI) and intracellular Ca(2+) clearance. Female C57 mice exhibited significantly higher plasma IGF-1 levels than their male counterpart. LID mice possessed comparably low IGF-1 levels in both sexes. Female C57 and LID mice displayed lower body, heart and liver weights compared to male counterparts. Echocardiographic analysis revealed larger LV mass in female C57 but not LID mice without sex difference in other cardiac geometric indices. Myocytes from female C57 mice exhibited reduced peak shortening, ±dL/dt, longer TPS, TR(90) and intracellular Ca(2+) clearance compared with males. Interestingly, this sex difference was greatly attenuated or abolished by IGF-1 deficiency. Female C57 mice displayed significantly decreased mRNA and protein levels of Na(+)-Ca(2+) exchanger, SERCA2a and phosphorylated phospholamban as well as SERCA activity compared with male C57 mice. These sex differences in Ca(2+) regulatory proteins were abolished or overtly attenuated by IGF-1 deficiency. In summary, our data suggested that IGF-1 deficiency may significantly attenuated or mitigate the sex difference in cardiomyocyte contractile function associated with intracellular Ca(2+) regulation.