Blockade by calmodulin inhibitors of Ca2+ channels in smooth muscle from rat vas deferens

Biological responses of Eisenia fetida towards the exposure and metabolism of tris (2-butoxyethyl) phosphate

The toxicity of various organophosphorus flame retardants (OPFRs) is of increasing concern. However, there is still a lack of research on the toxicity of OPFRs to terrestrial invertebrates and its metabolism in vivo. Herein, earthworms (Eisenia fetida) were exposed to soil spiked with 0, 0.05, 0.5, and 5 mg/kg tris(2-butoxyethyl) phosphate (TBOEP, a typical alkyl OPFRs) for 28 d to study the biological responses to the exposure and metabolism of TBOEP. TBOEP exposure inhibited the activity of acetyl-cholinesterase (64.4-68.6% of that in the control group), increased the energy consumption level, and affected calcium-dependent pathways of E. fetida, which caused a 3.6-12.4% reduction in the weight gain rate (developmental toxicity), a 10.6-69.4% reduction in the number of juveniles (reproduction toxicity), and neurotoxicity to E. fetida. The 5 mg/kg TBOEP exposure caused a significant accumulation of malondialdehyde (1.68 times higher than that in the control group) in E. fetida, which indicated that the balance of oxidation and anti-oxidation of E. fetida was broken. Meanwhile, E. fetida maintained the absorption and metabolic abilities to TBOEP under the environmental condition. The removal rate of soil TBOEP was increased by 25.1-35.5% by the presence of E. fetida. Importantly, TBOEP could accumulate in E. fetida (0.09-76.0 μg/kg) and the activation of cytochrome P450 and glutathione detoxification pathway promoted the metabolism of TBOEP in E. fetida. These findings link the biological responses and metabolic behavior of earthworms under pollution stress and provide fundamental data for the environmental risk assessment and pollution removal of OPFRs in soil.

Calmodulin Bidirectionally Regulates Evoked and Spontaneous Neurotransmitter Release at Retinal Ribbon Synapses

For decades, a role for the Ca²⁺-binding protein calmodulin (CaM) in Ca²⁺-dependent presynaptic modulation of synaptic transmission has been recognized. Here, we investigated the influence of CaM on evoked and spontaneous neurotransmission at rod bipolar (RB) cell→AII amacrine cell synapses in the mouse retina. Our work was motivated by the observations that expression of CaM in RB axon terminals is extremely high and that [Ca²⁺] in RB terminals normally rises sufficiently to saturate endogenous buffers, making tonic CaM activation likely. Taking advantage of a model in which RBs can be stimulated by expressed channelrhodopsin-2 (ChR2) to avoid dialysis of the presynaptic terminal, we found that inhibition of CaM dramatically decreased evoked release by inhibition of presynaptic Ca channels while at the same time potentiating both Ca²⁺-dependent and Ca²⁺-independent spontaneous release. Remarkably, inhibition of myosin light chain kinase (MLCK), but not other CaM-dependent targets, mimicked the effects of CaM inhibition on evoked and spontaneous release. Importantly, initial antagonism of CaM occluded the effect of subsequent inhibition of MLCK on spontaneous release. We conclude that CaM, by acting through MLCK, bidirectionally regulates evoked and spontaneous release at retinal ribbon synapses.

Trifluoperazine Attenuates Store-Dependent Ca2+ Entry in Macrophages

Using Fura-2AM microfluorimetry, we have shown for the first time that preincubation of macrophages with the calsequestrin inhibitor neuroleptic trifluoperazine leads to a significant inhibition of the store-dependent Ca²⁺ entry induced by endoplasmic Ca²⁺-ATPase inhibitors thapsigargin or cyclopiazonic acid in rat peritoneal macrophages. The results suggest calsequestrin involvement in the regulation of the store-dependent Ca²⁺ entry in macrophages.

Bilobal architecture is a requirement for calmodulin signaling to CaV1.3 channels

Calmodulin (CaM) regulation of voltage-gated calcium (Ca_V) channels is a powerful Ca²⁺ feedback mechanism that adjusts Ca²⁺ influx, affording rich mechanistic insights into Ca²⁺ decoding. CaM possesses a dual-lobed architecture, a salient feature of the myriad Ca²⁺-sensing proteins, where two homologous lobes that recognize similar targets hint at redundant signaling mechanisms. Here, by tethering CaM lobes, we demonstrate that bilobal architecture is obligatory for signaling to Ca_V channels. With one lobe bound, Ca_V carboxy tail rearranges itself, resulting in a preinhibited configuration precluded from Ca²⁺ feedback. Reconstitution of two lobes, even as separate molecules, relieves preinhibition and restores Ca²⁺ feedback. Ca_V channels thus detect the coincident binding of two Ca²⁺-free lobes to promote channel opening, a molecular implementation of a logical NOR operation that processes spatiotemporal Ca²⁺ signals bifurcated by CaM lobes. Overall, a unified scheme of Ca_V channel regulation by CaM now emerges, and our findings highlight the versatility of CaM to perform exquisite Ca²⁺ computations.

Cardiac contractile dysfunction during acute hyperglycemia due to impairment of SERCA by polyol pathway-mediated oxidative stress

Hyperglycemia is an indication of poor outcome for heart attack patients, even for nondiabetic patients with stress-induced hyperglycemia. Previous studies showed that inhibition of aldose reductase, the first and rate-limiting enzyme of the polyol pathway, attenuated contractile dysfunction in diabetic animals, but the mechanism is unclear. We therefore wanted to find out whether the polyol pathway also contributes to acute hyperglycemia-induced cardiac contractile dysfunction, and determine the mechanism involved. Rat hearts were isolated and retrogradely perfused with Krebs buffer containing either normal or high concentrations of glucose for 2 h. Short exposure to high-glucose medium led to contractile dysfunction as indicated by decreased -dP/dt(max), as well as elevation in left ventricular end-diastolic pressure. Cardiomyocytes incubated in high-glucose medium showed abnormal Ca2+ signaling, most likely because of decreased activity of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) inactivated by oxidative stress. Inhibition of aldose reductase or sorbitol dehydrogenase, the second enzyme in the polyol pathway, ameliorated contractile dysfunction, attenuated oxidative stress, and normalized Ca2+ signaling and SERCA activity caused by high glucose, indicating that the polyol pathway is the major contributor to acute hyperglycemia-induced oxidative stress leading to the inactivation of SERCA and contractile dysfunction.

Polyol pathway impairs the function of SERCA and RyR in ischemic-reperfused rat hearts by increasing oxidative modifications of these proteins

A number of studies have shown that the polyol pathway, consisting of aldose reductase (AR) and sorbitol dehydrogenase (SDH), contributes to ischemia-reperfusion (I/R)-induced myocardial infarction due to depletion of ATP. In this report we show that the polyol pathway in I/R heart also contributes to the impairment of sacro/endoplasmic reticulum Ca(2+)-ATPase (SERCA) and ryanodine receptor (RyR), two key players in Ca(2+) signaling that regulate cardiac contraction. Rat hearts were isolated and retrogradely perfused with either Krebs' buffer containing 1 microM AR inhibitor, zopolrestat, or 200 nM SDH inhibitor, CP-170,711, and challenged by 30 min of regional ischemia and 45 min of reperfusion. We found that post-ischemic contractile function of the isolated perfused hearts was improved by pharmacological inhibition of the polyol pathway. I/R-induced contractile dysfunction is most likely due to impairment in Ca(2+) signaling and the activities of SERCA and RyR. All these abnormalities were significantly ameliorated by treatment with ARI or SDI. We showed that the polyol pathway activities increase the level of peroxynitrite, which enhances the tyrosine nitration of SERCA and irreversibly modifies it to form SERCAC674-SO(3)H. This leads to reduced level of S-glutathiolated SERCA, contributing to its inactivation. The polyol pathway activities also deplete the level of GSH, leading to decreased active RyR, the S-glutathiolated RyR. Thus, in I/R heart, inhibition of polyol pathway improved the function of SERCA and RyR by protecting them from irreversible oxidation.

Testosterone-augmented contractile responses to α1- and β1-adrenoceptor stimulation are associated with increased activities of RyR, SERCA, and NCX in the heart

We hypothesized that testosterone at physiological levels enhances cardiac contractile responses to stimulation of both α1- and β1-adrenoceptors by increasing Ca2+ release from the sarcoplasmic reticulum (SR) and speedier removal of Ca2+ from cytosol via Ca2+-regulatory proteins. We first determined the left ventricular developed pressure, velocity of contraction and relaxation, and heart rate in perfused hearts isolated from control rats, orchiectomized rats, and orchiectomized rats without and with testosterone replacement (200 μg/100 g body wt) in the presence of norepinephrine (10−7 M), the α1-adrenoceptor agonist phenylephrine (10−6 M), or the nonselective β-adrenoceptor agonist isoprenaline (10−7 M) in the presence of 5 × 10−7 M ICI-118,551, a β2-adrenoceptor antagonist. Next, we determined the amplitudes of intracellular Ca2+ concentration transients induced by electrical stimulation or caffeine, which represent, respectively, Ca2+ release via the ryanodine receptor (RyR) or releasable Ca2+ in the SR, in ventricular myocytes isolated from the three groups of rats. We also measured 45Ca2+ release via the RyR. We then determined the time to 50% decay of both transients, which represents, respectively, Ca2+ reuptake by sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) and removal via the sarcolemmal Na+/Ca2+ exchanger (NCX). We correlated Ca2+ removal from the cytosol with activities of SERCA and its regulator phospholamban as well as NCX. The results showed that testosterone at physiological levels enhanced positive inotropic and lusitropic responses to stimulation of α1- and β1-adrenoceptors via the androgen receptor. The increased contractility and speedier relaxation were associated with increased Ca2+ release via the RyR and faster Ca2+ removal out of the cytosol via SERCA and NCX.

Short-term synaptic depression and recovery at the mature mammalian endbulb of Held synapse in mice

The endbulb of Held synapses between the auditory nerve fibers (ANF) and cochlear nucleus bushy neurons convey fine temporal information embedded in the incoming acoustic signal. The dynamics of synaptic depression and recovery is a key in regulating synaptic transmission at the endbulb synapse. We studied short-term synaptic depression and recovery in mature (P22-38) CBA mice with stimulation rates that were comparable to sound-driven activities recorded in vivo. Synaptic depression in mature mice is less severe ( approximately 40% at 100 Hz) than reported for immature animals and the depression is predominately due to depletion of releasable vesicles. Recovery from depression depends on the rate of activity and accumulation of intracellular Ca2+ at the presynaptic terminal. With a regular stimulus train at 100 Hz in 2 mM external [Ca2+], the recovery from depletion was slow (tauslow, approximately 2 s). In contrast, a fast (taufast, approximately 25 ms), Ca2+-dependent recovery followed by a slower recovery (tauslow, approximately 2 s) was seen when stimulus rates or external [Ca2+] increased. In normal [Ca2+], recovery from a 100-Hz Poisson-like train is rapid, suggesting that Poisson-like trains produce a higher internal [Ca2+] than regular trains. Moreover, the fast recovery was slowed by approximately twofold in the presence of calmidazolium, a Ca2+/calmodulin inhibitor. Our results suggest that endbulb synapses from high spontaneous firing rate auditory nerve fibers normally operate in a depressed state. The accelerated synaptic recovery during high rates of activity is likely to ensure that reliable synaptic transmission can be achieved at the endbulb synapse.

Chronic intermittent hypoxia alters Ca2+ handling in rat cardiomyocytes by augmented Na+/Ca2+ exchange and ryanodine receptor activities in ischemia-reperfusion

This study examined Ca(2+) handling mechanisms involved in cardioprotection induced by chronic intermittent hypoxia (CIH) against ischemia-reperfusion (I/R) injury. Adult male Sprague-Dawley rats were exposed to 10% inspired O(2) continuously for 6 h daily from 3, 7, and 14 days. In isolated perfused hearts subjected to I/R, CIH-induced cardioprotection was most significant in the 7-day group with less infarct size and lactate dehydrogenase release, compared with the normoxic group. The I/R-induced alterations in diastolic Ca(2+) level, amplitude, time-to-peak, and the decay time of both electrically and caffeine-induced Ca(2+) transients measured by spectrofluorometry in isolated ventricular myocytes of the 7-day CIH group were less than that of the normoxic group, suggesting an involvement of altered Ca(2+) handling of the sarcoplasmic reticulum (SR) and sarcolemma. We further determined the protein expression and activity of (45)Ca(2+) flux of SR-Ca(2+)-ATPase, ryanodine receptor (RyR) and sarcolemmal Na(+)/Ca(2+) exchange (NCX) in ventricular myocytes from the CIH and normoxic groups before and during I/R. There were no changes in expression levels of the Ca(2+)-handling proteins but significant increases in the RyR and NCX activities were remarkable during I/R in the CIH but not the normoxic group. The augmented RyR and NCX activities were abolished, respectively, by PKA inhibitor (0.5 microM KT5720 or 0.5 microM PKI(14-22)) and PKC inhibitor (5 microM chelerythrine chloride or 0.2 microM calphostin C) but not by Ca(2+)/calmodulin-dependent protein kinase II inhibitor KN-93 (1 microM). Thus, CIH confers cardioprotection against I/R injury in rat cardiomyocytes by altered Ca(2+) handling with augmented RyR and NCX activities via protein kinase activation.

Calcium homeostasis in rat cardiomyocytes during chronic hypoxia: a time course study

The present study determined Ca2+ handling in the hearts of rats subjected to chronic hypoxia (CH). Spectrofluorometry was used to measure intracellular Ca2+ concentration ([Ca2+]i) and its responses to electrical stimulation, caffeine, and isoproterenol in myocytes from the right ventricle of rats breathing 10% oxygen for 1, 3, 7, 14, 21, 28, and 56 days and age-matched controls. The protein expression of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) and its ryanodine receptor (RyR) were measured. The uptake of 45Ca2+ by SERCA, release by RyR, and extrusion by Na+/Ca2+ exchange (NCX) were determined. It was found that Ca2+ homeostasis and Ca2+ responses to beta-adrenoceptor stimulation reached a new equilibrium after 4 wk of CH. Ca2+ content in the sarcoplasmic reticulum (SR) was reduced, but cytosolic Ca2+ remained unchanged after CH. Expression of SERCA and its Ca2+ uptake, Ca2+ release via RyR, and NCX activity were suppressed by CH. The results indicate impaired Ca2+ handling, which may be responsible for the attenuated Ca2+ responses to beta-adrenoceptor stimulation in CH.

Cyclic AMP-mediated regulation of striatal glutamate release: interactions of presynaptic ligand- and voltage-gated ion channels and G-protein-coupled receptors

The presynaptic regulation of striatal glutamate transmission was investigated using D-[3H]aspartate and mouse striatal slices. Functional changes in voltage-dependent and glutamate receptor-gated ion channels were elicited by pharmacologically modifying intracellular cyclic AMP formation via G-protein-coupled receptor stimulation. The kainate (KA)-evoked release was potentiated by the stimulatory G-protein (G(s))-coupled beta-adrenoceptor agonist isoproterenol (ISO) in a concentration-dependent manner. This effect was mimicked by the specific calmodulin (CaM) antagonists trifluoperazine and calmidazolium. Tetrodotoxin (TTX), a blocker of Na(+) channels, did not affect the basal release but inhibited to the same degree the releases evoked by kainate alone and by kainate and isoproterenol together. Vinpocetine, a blocker of voltage-dependent Na(+) channels, did not alter the basal or the evoked release. The Na(+) channel activator veratridine enhanced the basal release in a concentration-dependent manner and isoproterenol attenuated this effect. The opposite effects of isoproterenol on the kainate- and veratridine-evoked releases may reflect prevention of the cyclic AMP-protein kinase A (PKA) phosphorylation cascade in striatal glutamatergic signal transduction. In addition, the calmidazolium-induced potentiation of kainate-evoked release was thwarted by LY354740 and L-2-amino-4-phosphonobutanoate, agonists of the inhibitory G-protein (G(i))-coupled metabotropic group II and III glutamate receptors (mGluRs). Vinpocetine, which inhibits the CaM-dependent phosphodiesterase (PDE1), was likewise inhibitory. In turn, selective agonists and antagonists of the G(q)-protein-coupled group I mGluRs and (S)-3,5-dihydroxyphenylglycine (3,5-DHPG) and (RS)-1-aminoindan-1,5-dicarboxylate (AIDA), which modulate the intracellular Ca(2+) levels, did not alter the kainate-evoked release. The beta-adrenoceptor-mediated cyclic AMP accumulation seems to downregulate Na(+) channels but to enhance glutamate release by means of upregulation of kainate receptors. This regulation of presynaptic ligand- and voltage-gated ion channels is affected by the cAMP-protein kinase A-dependent phosphorylation cascade and controlled by G(i)-protein-coupled mGluRs.

Regulation of glutamatergic neurotransmission in the striatum by presynaptic adenylyl cyclase-dependent processes

The aim here was to examine the possible roles of adenylyl cyclase- and protein kinase A (PKA)-dependent processes in ionotropic glutamate receptor (iGluR)-mediated neurotransmission using superfused mouse striatal slices and a non-metabolized L-glutamate analogue, D-[3H]aspartate. The direct and indirect presynaptic modulation of glutamate release and its susceptibility to changes in the intracellular levels of cyclic AMP (cAMP), Ca(2+) and calmodulin (CaM) and in protein phosphorylation was characterized by pharmacological manipulations. The agonists of iGluRs, 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and kainate, stimulated the basal release of D-[3H]aspartate, while N-methyl-D-aspartate (NMDA) was without effect. Both the AMPA- and kainate-mediated responses were accentuated by the beta-adrenoceptor agonist isoproterenol. These facilitatory effects were mimicked by the permeable cAMP analogue dibutyryl-cAMP. The beta-adrenoceptor antagonist propranolol, the adenylyl cyclase inhibitor MDL12,330A, the inhibitor of PKA and PKC, H-7, and the PKA inhibitor H-89 abolished the isoproterenol effect on the kainate-evoked release. The dibutyryl-cAMP-induced potentiation was also attenuated by H-7. Isoproterenol, propranolol and MDL12,330A failed to affect the basal release of D-[3H]aspartate, but dibutyryl-cAMP was inhibitory and MDL12,330A activatory. In Ca(2+)-free medium, the kainate-evoked release was enhanced, being further accentuated by the CaM antagonists calmidazolium and trifluoperazine, though these inhibited the basal release. The potentiating effect of calmidazolium on the kainate-stimulated release was counteracted by both MDL12,330A and H-7. We conclude that AMPA- and kainate-evoked glutamate release from striatal glutamatergic terminals is potentiated by beta-adrenergic receptor-mediated adenylyl cyclase activation and cAMP accumulation. Glutamate release is enhanced if the Ca(2+)- and CaM-dependent, kainate-evoked processes do not prevent the excessive accumulation of intracellular cAMP.

Modulation of a voltage-gated calcium channel linked to activation of glutamate receptors and calcium-induced calcium release in the catfish retina

1. Catfish (Ictalurus punctatus) retinal cone horizontal cells contain an L-type calcium current that has been proposed to be involved in visual processing. Here we report on the modulation of this current by activation of glutamate receptors and calcium-induced calcium release (CICR) from intracellular calcium stores. 2. Fluorescence data obtained from isolated horizontal cells loaded with indo-1 provided evidence of calcium release from an intracellular calcium store sensitive to caffeine, calcium and ryanodine. In the presence of caffeine, ryanodine-sensitive stores released calcium in a transient manner. Release of calcium was blocked when cells were preincubated in BAPTA, in the presence of ruthenium red, or in low concentrations of ryanodine. 3. The release of calcium from ryanodine-sensitive stores directly corresponded with a decrease of the voltage-gated L-type calcium current amplitude. Caffeine-induced modulation of the calcium current was reduced in the presence of ruthenium red. 4. Activation of ionotropic kainate receptors on catfish cone horizontal cells triggered CICR from ryanodine-sensitive stores and mimicked inhibition of the voltage-gated calcium current. Kainate-induced inhibition of the calcium current was diminished when intracellular calcium stores were inhibited with ruthenium red or depleted with ryanodine, or when calmodulin antagonists or CaM kinase II inhibitors were present. 5. These results provide evidence that activation of an ionotropic glutamate receptor on catfish cone horizontal cells is linked to calcium release from ryanodine-sensitive intracellular calcium stores and modulation of the L-type calcium current activity. Inhibition of this calcium current directly or indirectly involves calmodulin and CaM kinase II and represents a possible mechanism used by horizontal cells to affect response properties of these cells.

Calmodulin Signals Capacitation and Triggers the Agonist-Induced Acrosome Reaction in Mouse Spermatozoa

Capacitated acrosome-intact spermatozoa interact with specific sugar residues on neoglycoproteins (ngps) or solubilized zona pellucida (ZP), the egg's extracellular glycocalyx, prior to the initiation of a signal transduction cascade that results in the fenestration and fusion of the sperm plasma membrane and the outer acrosomal membrane at multiple sites and exocytosis of acrosomal contents (i.e., induction of the acrosome reaction (AR)). The AR releases acrosomal contents at the site of sperm-zona binding and is thought to be a prerequisite event that allows spermatozoa to penetrate the ZP and fertilize the egg. Since Ca(2+)/calmodulin (CaM) plays a significant role in several cell signaling pathways and membrane fusion events, we have used a pharmacological approach to examine the role of CaM, a calcium-binding protein, in sperm capacitation and agonist-induced AR. Inclusion of CaM antagonists (calmodulin binding domain, calmidazolium, compound 48/80, ophiobolin A, W5, W7, and W13), either in in vitro capacitation medium or after sperm capacitation blocked the npg-/ZP-induced AR. Purified CaM largely reversed the AR blocking effects of antagonists during capacitation. Our results demonstrate that CaM plays an important role in priming (i.e., capacitation) of mouse spermatozoa as well as in the agonist-induced AR. These data allow us to propose that CaM regulates these events by modulating sperm membrane component(s).

Ca(2+) mobilization induced by W-7 in MG63 human osteosarcoma cells

The effect of N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7), a widely used calmodulin inhibitor, on intracellular free Ca(2+)levels ([Ca(2+)](i)) in MG63 human osteosarcoma cells was explored using fura-2 as a Ca(2+)probe. W-7 (20-1000 micro m) induced an increase in [Ca(2+)](i)in a dose-dependent manner, with an EC(50)of 100 microm. The [Ca(2+)](i)signal comprised an initial rise and a sustained plateau without significant decay within 5 min. External Ca(2+)removal decreased the Ca(2+)signals by reducing the peak and sustained phase, indicating W-7-activated intracellular Ca(2+)release and extracellular Ca(2+)influx. W-7 (500 microm) failed to induce a [Ca(2+)](i)increase in a Ca(2+)-free medium after pre-treatment with thapsigargin (1 microm), an endoplasmic reticulum Ca(2+)pump inhibitor. Conversely, W-7 pre-treatment abolished the Ca(2+)release induced by thapsigargin. This suggests that W-7 (500 microm ) released internal Ca(2+)mainly from the endoplasmic reticulum. The addition of 3 mm Ca(2+)increased [Ca(2+)](i)dose-dependently after preincubation with 20-1000 microm W-7 in a Ca(2+)-free medium, implying that W-7 induced capacitative Ca(2+)entry. W-7-induced Ca(2+)release was not altered by inhibiting phospholipase C with 2 microm 1-(6-((17 beta - 3-methoxyestra-1,3, 5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione) (U73122). Tryphan blue assay demonstrated that W-7 (200 microm) caused gradual cell death within 30 min of the initial drug exposure. Together, it was found that W-7 induced [Ca(2+)](i)increases in human osteosarcoma cells by releasing internal Ca(2+)from the endoplasmic reticulum, and also by triggering Ca(2+)influx. W-7 may be cytotoxic to osteosarcoma cells.

Inhibition of a mammalian large conductance, calcium-sensitive K+ channel by calmodulin-binding peptides

The large conductance, calcium-sensitive K+ channel (BKCa channel) is a voltage-activated ion channel in which direct calcium binding shifts gating to more negative cellular membrane potentials. We hypothesized that the calcium-binding domain of BKCa channels may mimic the role played by calmodulin (CaM) in the activation of calcium-CaM-dependent enzymes, in which a tonic inhibitory constraint is removed on CaM binding. To examine such a hypothesis, we used peptides from the autoregulatory domains of CaM kinase II (CK291-317) and cNOS (the constitutive nitric oxide synthase; cNOS725-747) as probes for the calcium-dependent activation of murine BKCa channels transiently expressed in HEK 293 cells. We found that these CaM-binding peptides produced potent, time-dependent inhibition of mammalian BKCa channel current following voltage-dependent activation. Inhibition was observed in both the presence and the absence of cytosolic free calcium. Similar application of CK291-31 had no effect on either the amplitude or kinetics of voltage-dependent, macroscopic currents recorded from rabbit smooth muscle Kv1.5 potassium channels transiently expressed in HEK 293 cells. Cytosolic application of both CK291-317 and tetraethylammonium (TEA) produced an additive and non-competitive block of BKCa current. This finding suggests that the peptide-binding site is distinct (e.g. outside the pore region of the channel) from that of TEA. Our results are thus consistent with a model in which the BKCa channel's voltage-dependent gating process is under an intramolecular constraint that is relieved upon calcium binding. The intrinsic calcium sensor of the channel may thus interact with an inhibitory domain present in the BKCa channel, and by doing so, remove an inhibitory 'constraint' that permits voltage-dependent gating to occur at more negative potentials.

Calmidazolium-induced rises in cytosolic calcium concentrations in Madin Darby canine kidney cells

The effect of calmidazolium on Ca(2+) signaling in Madin Darby canine kidney (MDCK) cells was investigated using fura-2 as a Ca(2+) probe. Calmidazolium at 2-5 microM increased [Ca(2+)](i) concentration dependently. The [Ca(2+)](i) rise induced by 2-5 microM calmidazolium comprised an immediate rise and a slow decay. External Ca(2+) removal partly inhibited the Ca(2+) signals, suggesting that calmidazolium activated external Ca(2+) influx and internal Ca(2+) release. In Ca(2+)-free medium, pretreatment with 3 microM calmidazolium abolished the Ca(2+) release induced by 1 microM thapsigargin, an endoplasmic reticulum Ca(2+) pump inhibitor, and, vice versa, pretreatment with thapsigargin inhibited calmidazolium-induced Ca(2+) release. This indicates that thapsigargin-sensitive Ca(2+) store was the source of calmidazolium-induced Ca(2+) release. Calmidazolium (3 microM) induced Mn(2+) quench of fura-2 fluorescence at 360 nm excitation wavelength, which was suppressed by 0.1 mM La(3+). Addition of 3 mM Ca(2+) increased [Ca(2+)](i) after pretreatment with 3-5 microM calmidazolium in Ca(2+)-free medium. This implies that calmidazolium activated concentration-dependent capacitative Ca(2+) entry. Calmidazolium (3 microM) augmented the capacitative Ca(2+) entry induced by 1 microM thapsigargin or 0.1 mM ATP by 38%. Calmidazolium (3 microM)-induced Ca(2+) release was blocked by pretreatment with 40 microM aristolochic acid and 2 microM U73122 (2 microM) to inhibit phospholipase A(2) and phospholipase, respectively, but pretreatment with 0.1 mM propranolol to inhibit phospholipase D had no effect. This suggests that calmidazolium induced internal Ca(2+) release in a manner dependent on phospholipases C- and A(2)-coupled events.

Direct block of Ca2+ channels by calmidazolium in cultured vascular smooth muscle cells

We investigated the action of calmidazolium (CMZ), an inhibitor of calmodulin (CaM), on the L-type Ca2+ currents (ICa(L)) of cultured vascular smooth muscle (VSM) cells (A7r5 cell line), by using the whole-cell voltage-clamp method. All experiments were conducted at room temperature (24-25 degrees C). The peak IBa (Ca2+ channel current with 5 mM Ba2+ as charge carrier) was evoked every 15 s by a test potential to +10 mV from a holding potential of -60 mV. To elevate intracellular free Ca2+ concentration ([Ca]i) to pCa 6.5, the pipette solution contained a Ca2+-EGTA buffer (pCa 6.5) to allow equilibration with the cells. Bath application of 1 microM CMZ reduced the peak amplitude of IBa to 36.7+/-4.9% (n = 8); maximal effect occurred within 7-8 min. Peak IBa continued to decrease even after washing out the CMZ. Recovery of IBa was not observed even after 10 min of washout. Even in presence of an peptide inhibitor of CaM-dependent protein kinase-II (5.2 microM) in the pipette solution, CMZ inhibited IBa to 27.8 +/-5.3% (n = 7). To exclude the possibility that other Ca2+/ CaM-dependent kinases and phosphatases may regulate Ca2+ channel activity, we examined the effect of CMZ on IBa when [Ca]i was reduced by use of Ca2+/EGTA-buffered pipette solutions. At pCa approximately equal to 10 (10 mM EGTA and only contaminant Ca2+), CMZ inhibited IBa to 33.4+/-5.9% (n = 14) with a median inhibitory concentration (IC50) value of 0.29 microM. The activation curve (pCa approximately equal to 10) was shifted in the positive direction by 6.3 mV; the inactivation curve was shifted in the negative direction by 5.0 mV. CMZ decreased IBa progressively during repetitive step depolarizations. CMZ did not slow the rate of recovery from inactivation. In conclusion, CMZ inhibits Ca2+ channel current in a use-dependent manner. This inhibition is independent of CaMK-II and other Ca2+/CaM-dependent pathways. Therefore it is likely due to direct blockade of Ca2+ channels by CMZ. CMZ may reduce the outer surface charge and block the open state of the Ca2+ channels.

Stimulation of expression for the adenosine A2A receptor gene by hypoxia in PC12 cells. A potential role in cell protection

The purpose of this study was to examine the regulation of adenosine A2A receptor (A2AR) gene expression during hypoxia in pheochromocytoma (PC12) cells. Northern blot analysis revealed that the A2AR mRNA level was substantially increased after a 3-h exposure to hypoxia (5% O2), which reached a peak at 12 h. Immunoblot analysis showed that the A2AR protein level was also increased during hypoxia. Inhibition of de novo protein synthesis blocked A2AR induction by hypoxia. In addition, removal of extracellular free Ca2+, chelation of intracellular free Ca2+, and pretreatment with protein kinase C inhibitors prevented A2AR induction by hypoxia. Moreover, depletion of protein kinase C activity by prolonged treatment with phorbol 12-myristate 13-acetate significantly inhibited the hypoxic induction of A2AR. A2AR antagonists led to a significant enhancement of A2AR mRNA levels during hypoxia, whereas A2AR agonists caused down-regulation of A2AR expression during hypoxia. This suggests that A2AR regulates its own expression during hypoxia by feedback mechanisms. We further found that activation of A2AR enhances cell viability during hypoxia and also inhibits vascular endothelial growth factor expression in PC12 cells. Thus, increased expression of A2AR during hypoxia might protect cells against hypoxia and may act to inhibit hypoxia-induced angiogenic activity mediated by vascular endothelial growth factor.

Pharmacological analysis of intracellular Ca2+ signalling: problems and pitfalls

The complex changes in intracellular Ca2+ concentration that follow cell stimulation reflect the concerted activities of Ca2+ channels in the plasma membrane and in the membranes of intracellular stores, and the opposing actions of the mechanisms that extrude Ca2+ from the cytosol. Disentangling the roles of each of these processes is hampered by the lack of adequately selective pharmacological tools. In this review, Colin Taylor and Lisa Broad summarize the more serious problems associated with some of the commonly used drugs, and describe specific situations in which the multiple effects of drugs on Ca2(+)-signalling pathways have confused analysis of these pathways.

Calmodulin regulation of Ca2+ entry in Jurkat T cells

We have previously proposed a role for calmodulin (CaM) in the regulation of initiation of Ca2+ entry in Jurkat T cells, as well as in the regulation of the current that mediates Ca2+ entry, IT. In this report, we provide evidence for the mechanism of CaM action. We have previously shown that activation-induced Ca2+ entry into Jurkat T cells is mediated by a current we have called IT. In the whole cell variation, but not the perforated patch variation, of the patch clamp technique, this current is short-lived (under 6 min) suggesting that the current is under the control of a diffusible component of the cytosol. Addition of CaM to the whole cell recording pipette solution maintained IT for up to 20 min, suggesting that CaM may be this diffusible component. Pharmacological inhibitors of CaM blocked the augmentation of IT normally induced by an activating stimulus. Cells electroporated in the presence of anti-CaM antibodies had reduced influx of extracellular Ca2+, with no change in release of Ca2+ from the internal stores. These observations suggest that T cell receptor engagement initiates Ca2+ influx by a pathway that likely includes CaM, which may in turn regulate IT. Influx of extracellular Ca2+ is required for cellular proliferation, and inhibition of CaM by pharmacological inhibitors reduced cellular proliferation. This same inhibition of proliferation was seen in cells electroporated with anti-CaM antibodies. This suggests that inhibition of CaM and/or IT may be a target for therapeutic inhibition of inappropriate T cell proliferation.

Sodium-gated cation channel implicated in the activation of lobster olfactory receptor neurons

The role of Na+-activated channels in cellular function, if any, is still elusive. We have attempted to implicate a Na+-activated nonselective cation channel in the activation of lobster olfactory receptor neurons. We show that a Na+-activated channel occurs in the odor-detecting outer dendrites. With the use of pharmacological blockers of the channel together with ion substitution, we show that a substantial part of the odor-evoked depolarization in these cells can be ascribed to a Na+-activated conductance. We hypothesize, therefore, that the Na+-activated channel amplifies the receptor current as a result of being secondarily activated by the primary odor transduction pathway.

Effects of protein kinase and phosphatase inhibitors on slow shortening of guinea pig cochlear outer hair cells

The intracellular mechanisms of slow shortening in isolated guinea pig cochlear outer hair cells were investigated using inhibitors and/or an activator of protein kinases and protein phosphatases. The slow shortening was induced by tetanic electrical field stimulation, and changes in the cell length, volume and intracellular Cl- concentration were microscopically monitored using a chloride-sensitive fluorescent dye. The slow shortening was inhibited by a calmodulin inhibitor, W-7, and a calcium calmodulin-dependent protein kinase II (CaMKII) inhibitor, KN-62. The inhibition by W-7 or KN-62, was abolished by the supplemented conductance of K+ with valinomycin. Among the protein phosphatase inhibitors tested, a type 1 and 2A protein phosphatase inhibitor, calyculin A, inhibited the slow shortening. The inhibition by calyculin A was abolished by the increased Cl- permeability, but neither by the increased K+ conductance with valinomycin nor by the increased Ca2+ conductance with A23187. A protein serine/threonine phosphatase activator, N-acetylsphingosine, inhibited the shortening, which was abolished by either valinomycin or a type 2A protein phosphatase inhibitor, okadaic acid, but not by calyculin A. These findings suggest the following signaling mechanisms in the slow shortening of outer hair cells; the K+ channel opening is facilitated through protein phosphorylation by CaMKII and suppressed via okadaic acid-sensitive dephosphorylation, and the Cl- channel opening depends on calyculin A-sensitive protein phosphatase activity.

Mouse sperm membrane potential: changes induced by Ca2+

Mouse sperm resting membrane potential (Er) (-42 +/- 8.8 mV), determined with a potential sensitive dye, depended on extracellular K+ and, in the absence of extracellular Ca2+ ([Ca2+]e), on external Na+ ([Na+]e). Ca2+ addition (> 5 microM) to sperm in Ca-free media induced a transient hyperpolarization (Ca-ith) which strongly depended on [Na+]e and less on external Cl- ([Cl-]e). Cd2+ and Mn2+ (microM) mimicked the Ca2+ effect, but not Ba2+. The Ca-ith was partially inhibited by ouabain (74%, IC50 = 5.8 microM) and niflumic acid (38%, IC50 = 240 microM), indicating the participation of the Na-K ATPase and Cl- channels. In Ca-free low-Na+ media, Ca2+ addition caused a depolarization sensitive to: nimodipine (25 microM), trifluoperazine (12.5 microM) and Mg2+ (1.2 mM), suggesting the participation of Ca2+ channels. Since some inhibitors of the sperm Ca-ith block the acrosome reaction (AR), both processes may share transport systems.

Role of calmodulin in the activation of carbachol-activated cationic current in guinea-pig gastric antral myocytes

In mammalian gastrointestinal myocytes, it is known that muscarinic stimulation activates nonselective cation channels through a G-protein and a Ca2+-dependent pathway. We recorded inward cationic currents following application of carbachol (ICCh) to guinea-pig gastric myocytes, which were held at -20 mV using the whole-cell patch-clamp method. ICCh was suppressed by nicardipine or removal of Ca2+ from the bath solution. The peak value of inward current induced by repetitive applications of carbachol (CCh) decreased progressively (run-down phenomenon). This run-down was significantly alleviated by the addition of calmodulin to the pipette solution (0.15 mg/ml) or by using the perforated-patch whole-cell voltage-clamp technique. Moreover, W-7[N-6(aminohexyl)-5-chloro-1-naphthalenesulphonamide], a calmodulin antagonist, was a reversible inhibitor of ICCh. However, @-7 had only a weak inhibitory effect on the same cationic current which was induced by guanosine 5'-O-(3-thiotriphosphate) (GTP¿gammaS] 0.2 mM) in the pipette solution. This GTP[gammaS]-induced cationic current was still markedly suppressed by the Ca2+-free bath solution. W-7 itself had a weak inhibitory effect on voltage-operated Ca2+ channels as well as the effects on ICCh. These data suggest that multiple Ca2+-dependent pathways are involved in the activation of CCh-gated cation channels in guinea-pig antral myocytes and a Ca2+/calmodulin-dependent pathway would be one of them.

Different mechanisms may be required for maintenance of NMDA receptor-dependent and independent forms of long-term potentiation

In hippocampal area CA1, long-term potentiation (LTP) is induced by tetanic stimulation protocols that activate N-methyl-D-aspartate (NMDA) receptors. In addition, some stimulation protocols can induce LTP during NMDA receptor blockade. An initial signal in both NMDA receptor-dependent and independent LTPs is increased intracellular Ca2+ concentration in postsynaptic neurons. It therefore seems possible that subsequent steps leading to expression and maintenance of potentiation are shared whether or not LTP is induced through NMDA receptor activation. We tested this hypothesis by applying a broad spectrum protein kinase inhibitor, previously shown to inhibit NMDA receptor-dependent LTP. In agreement with earlier reports, we found that H-7 inhibited NMDA receptor-dependent LTP when applied either during tetanic stimulation, or beginning 30 min following tetanic stimulation. In contrast, NMDA receptor-independent LTP was not inhibited by H-7 applied during or following tetanic stimulation. We also tested for mutual occlusion between NMDA receptor-dependent and independent LTPs. Although induction of NMDA receptor-independent LTP did not occlude later induction of NMDA receptor-dependent LTP, induction of NMDA receptor-dependent LTP did occlude NMDA receptor-independent LTP. While the kinase inhibitor experiment showed a clear difference between NMDA receptor-dependent and independent LTPs, the occlusion experiments suggest an interaction between the signalling pathways for the two LTPs.

Multifunctional Ca2+/calmodulin-dependent protein kinase mediates Ca(2+)-induced enhancement of the L-type Ca2+ current in rabbit ventricular myocytes

The intracellular mechanism underlying the Ca(2+)-induced enhancement of the L-type Ca2+ current (ICa) was examined in adult rabbit cardiac ventricular myocytes by using patch-clamp methodology. Internal Ca2+ was elevated by flash photolysis of the Ca2+ chelator Nitr 5, and intracellular Ca2+ levels were simultaneously monitored by Fluo 3 fluorescence. Flash photolysis of Nitr 5 produced a rapid (< 1-second) elevation of internal Ca2+, which led to enhancement (39% to 51% above control) of the peak inward Ca2+ current after a delay of 20 to 120 seconds. Internal dialysis of myocytes with synthetic inhibitory peptides derived from the pseudosubstrate (peptide 273-302) and calmodulin binding (peptide 291-317) regions within the regulatory domain of multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) blocked enhancement of ICa produced by elevation of internal Ca2+ but not that produced by beta-adrenergic stimulation. These inhibitory peptides also had no effect on the elevation of internal Ca2+ produced by flash photolysis of Nitr 5. A pseudosubstrate inhibitory peptide derived from protein kinase C had no significant effect on Ca(2+)-dependent enhancement of ICa. We conclude that CaM kinase mediates the Ca(2+)-induced enhancement of ICa in mammalian cardiac myocytes by a mechanism likely involving direct phosphorylation of the L-type Ca2+ channel complex or an associated regulatory protein.

Effects of Ca2+ channel antagonists and benzodiazepine receptor ligands in normal and skinned rat urinary bladder

The effects of Ca2+ channel antagonists and benzodiazepine receptor ligands against concentration-dependent contractions of rat urinary bladder induced by CaCl2 (0.1-50 mM, in K(+)-depolarized tissues), KCl (1-100 mM) and acetylcholine (0.1 microM to 1 mM) were studied. Nifedipine (0.001-0.1 microM), verapamil (0.01-1 microM), diltiazem (0.01-1 microM), cinnarizine (1-100 microM), and trifluoperazine (1-100 microM) each produced a concentration-related inhibition of the log concentration-effect curve for CaCl2. The rank order of potencies of these antagonists, measured as the IC50 against Ca2+ (25 mM)-induced contraction of depolarized bladder, was nifedipine (0.01 microM) > diltiazem (0.36 microM) approximately verapamil (0.41 microM) > or = cinnarizine (2.57 microM) > trifluoperazine (17.4 microM). These antagonists depressed KCl-induced contractions with an effectiveness and potency similar to that displayed against CaCl2-induced contractions. Nifedipine, verapamil, and diltiazem but not cinnarizine and trifluoperazine had a preferential inhibitory effect on the contractions elicited by KCl when compared to those elicited by acetylcholine. Ro 5-4864, diazepam, midazolam and the non-benzodiazepine PK 11195, each at 1-100 microM, depressed CaCl2- and KCl-induced contractions (IC50 values in the micromolar range). Benzodiazepines and PK 11195, all at 100 microM, markedly depressed acetylcholine-induced contractions. Flumazenil was scarcely effective. Cinnarizine (100 microM) and trifluoperazine (100 microM), but not the other Ca2+ channel antagonists and benzodiazepine receptor ligands tested, depressed Ca2+ (20 microM)-evoked contractions of skinned bladder. It is concluded that the action of nifedipine, verapamil, and diltiazem is restricted to the plasmalemma whereas cinnarizine and trifluoperazine also act on the intracellular contractile apparatus.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of olfactory cyclic nucleotide-activated current by calmodulin antagonists

1. In amphibian olfactory receptor neurones, much of the depolarizing current in response to odours is carried by cationic channels that are directly gated by cyclic AMP. The effects of four calmodulin antagonists on the cyclic AMP-activated receptor current were studied in single olfactory cilia of the frog. 2. Two antagonists, W-7 and trifluoperazine, were potent and reversible inhibitors of the cyclic AMP-activated current. IC50 values were 5 microM for W-7 and 13 microM for trifluoperazine. A third antagonist, calmidazolium, irreversibly blocked the current. The fourth, mastoparan, had little effect. 3. Calmodulin was unable to reverse the effects of W-7 and trifluoperazine, suggesting that these inhibitors act directly on the cyclic AMP-gated channels. 4. Neither W-7 nor trifluoperazine inhibited a Ca(2+)-activated Cl- current which also contributes to the odorant response. These compounds thus allow the two components of the olfactory receptor current to be discriminated.