Effects of three different Ca(2+) pump ATPase inhibitors on evoked contractions in rabbit aorta and activities of Ca(2+) pump ATPases in porcine aorta

ATP Analogues for Structural Investigations: Case Studies of a DnaB Helicase and an ABC Transporter

Nucleoside triphosphates (NTPs) are used as chemical energy source in a variety of cell systems. Structural snapshots along the NTP hydrolysis reaction coordinate are typically obtained by adding stable, nonhydrolyzable adenosine triphosphate (ATP) -analogues to the proteins, with the goal to arrest a state that mimics as closely as possible a physiologically relevant state, e.g., the pre-hydrolytic, transition and post-hydrolytic states. We here present the lessons learned on two distinct ATPases on the best use and unexpected pitfalls observed for different analogues. The proteins investigated are the bacterial DnaB helicase from Helicobacter pylori and the multidrug ATP binding cassette (ABC) transporter BmrA from Bacillus subtilis, both belonging to the same division of P-loop fold NTPases. We review the magnetic-resonance strategies which can be of use to probe the binding of the ATP-mimics, and present carbon-13, phosphorus-31, and vanadium-51 solid-state nuclear magnetic resonance (NMR) spectra of the proteins or the bound molecules to unravel conformational and dynamic changes upon binding of the ATP-mimics. Electron paramagnetic resonance (EPR), and in particular W-band electron-electron double resonance (ELDOR)-detected NMR, is of complementary use to assess binding of vanadate. We discuss which analogues best mimic the different hydrolysis states for the DnaB helicase and the ABC transporter BmrA. These might be relevant also to structural and functional studies of other NTPases.

Inhibition of Sarco-Endoplasmic Reticulum Ca2+ ATPase Extends the Lifespan in C. elegans Worms

The sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA) refills the endoplasmic reticulum (ER) with Ca²⁺ up to the millimolar range and is therefore the main controller of the ER [Ca²⁺] level ([Ca²⁺]_ER), which has a key role in the modulation of cytosolic Ca²⁺ signaling and ER-mitochondria Ca²⁺ transfer. Given that both cytosolic and mitochondrial Ca²⁺ dynamics strongly interplay with energy metabolism and nutrient-sensitive pathways, both of them involved in the aging process, we have studied the effect of SERCA inhibitors on lifespan in C. elegans. We have used thapsigargin and 2,5-Di-tert-butylhydroquinone (2,5-BHQ) as SERCA inhibitors, and the inactive analog 2,6-Di-tert-butylhydroquinone (2,6-BHQ) as a control for 2,5-BHQ. Every drug was administered to the worms either directly in the agar or via an inclusion compound with γ-cyclodextrin. The results show that 2,6-BHQ produced a small but significant increase in survival, perhaps because of its antioxidant properties. However, 2,5-BHQ produced in all the conditions a much higher increase in lifespan, and the potent and specific SERCA inhibitor thapsigargin also extended the lifespan. The effects of 2,5-BHQ and thapsigargin had a bell-shaped concentration dependence, with a maximum effect at a certain dose and smaller or even toxic effects at higher concentrations. Our data show therefore that submaximal inhibition of SERCA pumps has a pro-longevity effect, suggesting that Ca²⁺ signaling plays an important role in the aging process and that it could be a promising novel target pathway to act on aging.

TRP channels: potential drug target for neuropathic pain

Neuropathic pain is a debilitating disease which affects central as well as peripheral nervous system. Transient receptor potential (TRP) channels are ligand-gated ion channels that detect physical and chemical stimuli and promote painful sensations via nociceptor activation. TRP channels have physiological role in the mechanisms controlling several physiological responses like temperature and mechanical sensations, response to painful stimuli, taste, and pheromones. TRP channel family involves six different TRPs (TRPV1, TRPV2, TRPV3, TRPV4, TRPM8, and TRPA1) which are expressed in pain sensing neurons and primary afferent nociceptors. They function as transducers for mechanical, chemical, and thermal stimuli into inward currents, an essential first step for provoking pain sensations. TRP ion channels activated by temperature (thermo TRPs) are important molecular players in acute, inflammatory, and chronic pain states. Different degree of heat activates four TRP channels (TRPV1-4), while cold temperature ranging from affable to painful activate two indistinctly related thermo TRP channels (TRPM8 and TRPA1). Targeting primary afferent nociceptive neurons containing TRP channels that play pivotal role in revealing physical stimuli may be an effective target for the development of successful pharmacotherapeutics for clinical pain syndromes. In this review, we highlighted the potential role of various TRP channels in different types of neuropathic pain. We also discussed the pharmacological activity of naturally and synthetically originated TRP channel modulators for pharmacotherapeutics of nociception and neuropathic pain.

T Cell Receptor Mediated Calcium Entry Requires Alternatively Spliced Cav1.1 Channels

The process of calcium entry in T cells is a multichannel and multi-step process. We have studied the requirement for L-type calcium channels (Ca_v1.1) α1S subunits during calcium entry after TCR stimulation. High expression levels of Ca_v1.1 channels were detected in activated T cells. Sequencing and cloning of Ca_v1.1 channel cDNA from T cells revealed that a single splice variant is expressed. This variant lacks exon 29, which encodes the linker region adjacent to the voltage sensor, but contains five new N-terminal exons that substitute for exons 1 and 2, which are found in the Ca_v1.1 muscle counterpart. Overexpression studies using cloned T cell Ca_v1.1 in 293HEK cells (that lack TCR) suggest that the gating of these channels was altered. Knockdown of Ca_v1.1 channels in T cells abrogated calcium entry after TCR stimulation, suggesting that Ca_v1.1 channels are controlled by TCR signaling.

Sildenafil does not enhance but rather attenuates vasorelaxant effects of antidiabetic agents

Type 2 diabetic men commonly experience erectile dysfunction for which phosphodiesterase-5 (PDE5) inhibitors like sildenafil (Viagra) are often recommended. By preventing degradation of cyclic guanosine monophosphate (cGMP) in vascular smooth muscle, these inhibitors also enhance arterial vasorelaxant effects of nitric oxide donors (which stimulate cGMP synthesis). In the present work, we confirmed this enhancing effect after co-administration of sildenafil with nitroprusside to freshly-isolated rat tail arterial tissues. However, in the same tissues we also observed that sildenafil does not enhance but rather attenuates vasorelaxant effects of three commonly-used antidiabetic drugs, i.e. the biguanide metformin and the thiazolidinediones pioglitazone and rosiglitazone. Indeed, sildenafil completely blocked vasorelaxant effects of low concentrations of these drugs. In addition, we found that this same novel anti-vasorelaxant interaction of sildenafil with these agents was abolished by either 1) omitting extracellular glucose or 2) inhibiting specific smooth muscle glycolytic pathways; pathways known to preferentially utilize extracellular glucose to fuel certain adenosine triphosphate (ATP)-dependent ion transporters: e.g. ATP-sensitive K channels, sarcoplasmic reticulum Ca-ATPase, plasma membrane Ca-ATPase and Na/K-ATPase. Accordingly, we suspect that altered activity of one or more of these ion transporters mediates the observed attenuating (anti-vasorelaxant) interaction of sildenafil with the antidiabetic drugs. The present results are relevant because hypertension is so common and difficult to control in Type 2 diabetes. The present data suggest that sildenafil might interfere with the known antihypertensive potential of metformin and the thiazolidinediones. However, they do not suggest that it will interact with them to cause life-threatening episodes of severe hypotension, as can occur when it is co-administered with nitrates.

Airway smooth muscle CXCR3 ligand production: regulation by JAK-STAT1 and intracellular Ca²⁺

In asthma, airway smooth muscle (ASM) chemokine (C-X-C motif) receptor 3 (CXCR3) ligand production may attract mast cells or T lymphocytes to the ASM, where they can modulate ASM functions. In ASM cells (ASMCs) from people with or without asthma, we aimed to investigate JAK-STAT1, JNK, and Ca²⁺ involvement in chemokine (C-X-C motif) ligand (CXCL)10 and CXCL11 production stimulated by interferon-γ, IL-1β, and TNF-α combined (cytomix). Confluent, growth-arrested ASMC were treated with inhibitors for pan-JAK (pyridone-6), JAK2 (AG-490), JNK (SP-600125), or the sarco(endo)plasmic reticulum Ca²⁺ATPase (SERCA) pump (thapsigargin), Ca²⁺ chelator (BAPTA-AM), or vehicle before and during cytomix stimulation for up to 24 h. Signaling protein activation as well as CXCL10/CXCL11 mRNA and protein production were examined using immunoblot analysis, real-time PCR, and ELISA, respectively. Cytomix-induced STAT1 activation was lower and CXCR3 ligand mRNA production was more sensitive to pyridone-6 and AG-490 in asthmatic than nonasthmatic ASMCs, but CXCL10/CXCL11 release was inhibited by the same proportion. Neither agent caused additional inhibition of release when used in combination with the JNK inhibitor SP-600125. Conversely, p65 NF-κB activation was higher in asthmatic than nonasthmatic ASMCs. BAPTA-AM abolished early CXCL10/CXCL11 mRNA production, whereas thapsigargin reduced it in asthmatic cells and inhibited CXCL10/CXCL11 release by both ASMC types. Despite these inhibitory effects, neither Ca²⁺ agent affected early activation of STAT1, JNK, or p65 NF-κB. In conclusion, intracellular Ca²⁺ regulated CXCL10/CXCL11 production but not early activation of the signaling molecules involved. In asthma, reduced ASM STAT1-JNK activation, increased NF-κB activation, and altered Ca²⁺ handling may contribute to rapid CXCR3 ligand production and enhanced inflammatory cell recruitment.

Two distinct calcium pools in the endoplasmic reticulum of HEK-293T cells

Agonist-sensitive intracellular Ca2+ stores may be heterogeneous and exhibit distinct functional features. We have studied the properties of intracellular Ca2+ stores using targeted aequorins for selective measurements in different subcellular compartments. Both, HEK-293T [HEK (human embryonic kidney)-293 cells expressing the large T-antigen of SV40 (simian virus 40)] and HeLa cells accumulated Ca2+ into the ER (endoplasmic reticulum) to near millimolar concentrations and the IP3-generating agonists, carbachol and ATP, mobilized this Ca2+ pool. We find in HEK-293T, but not in HeLa cells, a distinct agonist-releasable Ca2+ pool insensitive to the SERCA (sarco/endoplasmic reticulum Ca2+ ATPase) inhibitor TBH [2,5-di-(t-butyl)-benzohydroquinone]. TG (thapsigargin) and CPA (cyclopiazonic acid) completely emptied this pool, whereas lysosomal disruption or manoeuvres collapsing endomembrane pH gradients did not. Our results indicate that SERCA3d is important for filling the TBH-resistant store as: (i) SERCA3d is more abundant in HEK-293T than in HeLa cells; (ii) the SERCA 3 ATPase activity of HEK-293T cells is not fully blocked by TBH; and (iii) the expression of SERCA3d in HeLa cells generated a TBH-resistant agonist-mobilizable compartment in the ER. Therefore the distribution of SERCA isoforms may originate the heterogeneity of the ER Ca2+ stores and this may be the basis for store specialization in diverse functions. This adds to recent evidence indicating that SERCA3 isoforms may subserve important physiological and pathophysiological mechanisms.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

The epoxy group of pramanicin is required for the optimal endothelium-dependent relaxation of rat aorta

The vascular effects of a newly discovered anti-fungal agent, pramanicin (PMC), and its two analogues, PMC-A, in which the epoxy group is replaced by a - HC = CH - bond, and PMC-B, on which the diene is converted to the saturated (CH(2))(4)-derivative, respectively, were investigated in rat aorta. All three compounds caused an initial endothelial-dependent relaxation, which is prevented either by removal of endothelium or inclusion of the nitric oxide synthase inhibitor L-NAME. Upon prolonged incubation with the aortic rings, they also caused endothelial cell dysfunction characterized as reduced relaxation to carbachol (CCh). These effects were the strongest for PMC, being completely inhibitory at 20 microM after 30 min incubation, whereas those of PMC-A and PMC-B were smaller and comparable with each other, causing 30 - 40% inhibition at 20 micro M. PMC and its analogues had no effect on KCl-induced contraction and also had no effect on relaxation induced by sodium nitroprusside, suggesting that these compounds had no effect on the basic mechanisms of the contractile elements. Phenylephrine (PE)-induced contraction, however, was significantly reduced in the presence of these compounds, the inhibitory effect being strongest with PMC, but this inhibitory action was rapidly reversible and not of the competitive mode with respect to PE. We conclude that the epoxy group in PMC is required for the optimal vascular effects. We have discussed and speculated upon the possible mechanisms of action of PMC. The potent, selective, and irreversible inhibitory effect of PMC on the endothelial function points to its potential development into an anti-angiogenic drug.

The effects of nifedipine and thapsigargin on the responses of pressurized rat mesenteric artery to 5-hydroxytryptamine and norepinephrine

The responses of isolated pressurized second order mesenteric resistance arteries of Wistar rats, superfused with physiological salt solution (PSS) were determined to 5-hydroxytryptamine (5-HT) and norepinephrine (NE). The contractility of the vessel was enhanced in response to 5-HT compared to NE (P<.001, ANOVA). The L-type calcium ion channel blocker, nifedipine (10(-6) M) abolished the response to either 5-HT or NE. In vessels with intact endothelium, thapsigargin (TG, 10(-6) M), which inhibits uptake of calcium ions into intracellular stores, significantly reduced the contractile response to 5-HT (P<.02) but had little or no effect on the response to NE (P=.2). However, in vessels denuded of the endothelium, there was no significant difference in the response of the mesenteric artery, after TG, to either 5-HT or NE. The results indicate that, in the rat mesenteric resistance vessel, both 5-HT and NE use calcium ions from extracellular sources for contraction, while NE relies mainly on extracellular ion influx with little or no contribution from intracellular sources. The reduced response of the de-endothelized vessel to 5-HT after TG suggests that the utilization of intracellular stores by this agonist is endothelium-dependent. These observations may explain the enhanced responsiveness of the mesenteric artery to 5-HT when compared with NE.

Discrete store-operated calcium influx into an intracellular compartment in rabbit arteriolar smooth muscle

This study tested the hypothesis that store-operated channels (SOCs) exist as a discrete population of Ca2+ channels activated by depletion of intracellular Ca(2+) stores in cerebral arteriolar smooth muscle cells and explored their direct contractile function. Using the Ca2+ indicator fura-PE3 it was observed that depletion of sarcoplasmic reticulum (SR) Ca2+ by inhibition of SR Ca2+-ATPase (SERCA) led to sustained elevation of [Ca2+]i that depended on extracellular Ca2+ and slightly enhanced Mn2+ entry. Enhanced background Ca2+ influx did not explain the raised [Ca2+]i in response to SERCA inhibitors because it had marked gadolinium (Gd3+) sensitivity, which background pathways did not. Effects were not secondary to changes in membrane potential. Thus SR Ca2+ depletion activated SOCs. Strikingly, SOC-mediated Ca2+ influx did not evoke constriction of the arterioles, which were in a resting state. This was despite the fura-PE3-indicated [Ca2+]i rise being greater than that evoked by 20 mM [K+]o (which did cause constriction). Release of endothelial vasodilators did not explain the absence of SOC-mediated constriction, nor did a change in Ca2+ sensitivity of the contractile proteins. We suggest SOCs are a discrete subset of Ca2+ channels allowing Ca2+ influx into a 'non-contractile' compartment in cerebral arteriolar smooth muscle cells.

Prevention of a hypoxic Ca(2+)(i) response by SERCA inhibitors in cerebral arterioles

1. The aim of the study was to investigate the mechanism of a novel effect of hypoxia on intracellular Ca(2+) signalling in rabbit cerebral arteriolar smooth muscle cells, an effect that was resistant to the L-type Ca(2+) channel antagonist methoxyverapamil (D600). 2.[Ca(2+)](i) of smooth muscle cells in intact arteriolar fragments was measured using the Ca(2+)-indicator dye fura-PE3. Hypoxia (PO(2) 10 - 20 mmHg) lowered basal [Ca(2+)](i) but did not inhibit Ca(2+) entry pathways measured by Mn(2+)-quenching of fura-PE3. 3. The effect of hypoxia was completely prevented by thapsigargin or cyclopiazonic acid, selective inhibitors of sarcoplasmic reticulum Ca(2+) ATPase (SERCA). Since these inhibitors do not block Ca(2+) extrusion or uptake via the plasma membrane, the data indicate that the effect of hypoxia depends on a functional sarcoplasmic reticulum. 4. Because actions of nitric oxide (NO) on vascular smooth muscle are also prevented by SERCA inhibitors it was explored whether the effect of hypoxia occurred via modulation of endogenous NO release. Residual NOS-I and NOS-III were detected by immunostaining, and there were NO-dependent effects of NOS inhibitors on Ca(2+)(i)-signalling. Nevertheless, inhibition of endogenous NO production did not prevent the effect of hypoxia on [Ca(2+)](i). 5. The experiments reveal a novel nitric oxide-independent effect of hypoxia that is prevented by SERCA inhibitors.