Hypotonic stress activates BK channels in clonal kidney cells via purinergic receptors, presumably of the P2Y subtype

Inhibitory effects of patchouli alcohol on stress-induced diarrhea-predominant irritable bowel syndrome

AIM

To elucidate the mechanism of patchouli alcohol (PA) in treatment of rat models of diarrhea-predominant irritable bowel syndrome (IBS-D).

METHODS

We studied the effects of PA on colonic spontaneous motility using its cumulative log concentration (3 × 10-7 mol/L to 1 × 10-4 mol/L). We then determined the responses of the proximal and distal colon segments of rats to the following stimuli: (1) carbachol (1 × 10-9 mol/L to 1 × 10-5 mol/L); (2) neurotransmitter antagonists including Nω-nitro-L-arginine methyl ester hydrochloride (10 μmol/L) and (1R*, 2S*)-4-[2-Iodo-6-(methylamino)-9H-purin-9-yl]-2-(phosphonooxy)bicyclo[3.1.0]hexane-1-methanol dihydrogen phosphate ester tetraammonium salt (1 μmol/L); (3) agonist α,β-methyleneadenosine 5'-triphosphate trisodium salt (100 μmol/L); and (4) single KCl doses (120 mmol/L). The effects of blockers against antagonist responses were also assessed by pretreatment with PA (100 μmol/L) for 1 min. Electrical-field stimulation (40 V, 2-30 Hz, 0.5 ms pulse duration, and 10 s) was performed to observe nonadrenergic, noncholinergic neurotransmitter release in IBS-D rat colon. The ATP level of Kreb's solution was also determined.

RESULTS

PA exerted a concentration-dependent inhibitory effect on the spontaneous contraction of the colonic longitudinal smooth muscle, and the half maximal effective concentration (EC50) was 41.9 μmol/L. In comparison with the KCl-treated IBS-D group, the contractile response (mg contractions) in the PA + KCl-treated IBS-D group (11.87 ± 3.34) was significantly decreased in the peak tension (P < 0.01). Compared with CCh-treated IBS-D rat colon, the cholinergic contractile response of IBS-D rat colonic smooth muscle (EC50 = 0.94 μmol/L) was significantly decreased by PA (EC50 = 37.43 μmol/L) (P < 0.05). Lack of nitrergic neurotransmitter release in stress-induced IBS-D rats showed contraction effects on colonic smooth muscle. Pretreatment with PA resulted in inhibitory effect on L-NAME-induced (10 μmol/L) contraction (P < 0.05). ATP might not be the main neurotransmitter involved in inhibitory effects of PA in the colonic relaxation of stress-induced IBS-D rats.

CONCLUSION

PA application may serve as a new therapeutic approach for IBS-D.

Stimulatory actions of a novel thiourea derivative on large-conductance, calcium-activated potassium channels

In this study, we examine whether an anti-inflammatory thiourea derivative, compound #326, actions on ion channels. The effects of compound #326 on Ca²⁺ -activated K⁺ channels were evaluated by patch-clamp recordings obtained in cell-attached, inside-out or whole-cell configuration. In pituitary GH₃ cells, compound #326 increased the amplitude of Ca²⁺ -activated K⁺ currents (I_K(Ca) ) with an EC₅₀ value of 11.6 μM, which was reversed by verruculogen, but not tolbutamide or TRAM-34. Under inside-out configuration, a bath application of compound #326 raised the probability of large-conductance Ca²⁺ -activated K⁺ (BK_Ca ) channels. The activation curve of BK_Ca channels was shifted to less depolarised potential with no modification of the gating charge of the curve; consequently, the difference of free energy was reduced in the presence of this compound. Compound #326-stimulated activity of BK_Ca channels is explained by a shortening of mean closed time, despite its inability to alter single-channel conductance. Neither delayed-rectifier nor erg-mediated K⁺ currents was modified. Compound #326 decreased the peak amplitude of voltage-gated Na⁺ current with no clear change in the overall current-voltage relationship of this current. In HEK293T cells expressing α-hSlo, compound #326 enhanced BK_Ca channels effectively. Intriguingly, the inhibitory actions of compound #326 on interleukin 1β in lipopolysaccharide-activated microglia were significantly reversed by verruculogen, whereas BK_Ca channel inhibitors suppressed the expressions of inducible nitric oxide synthase. The BK_Ca channels could be an important target for compound #326 if similar in vivo results occur, and the multi-functionality of BK_Ca channels in modulating microglial immunity merit further investigation.

The physical basis of renal fibrosis: effects of altered hydrodynamic forces on kidney homeostasis

Healthy kidneys are continuously exposed to an array of physical forces as they filter the blood: shear stress along the inner lumen of the tubules, distension of the tubular walls in response to changing fluid pressures, and bending moments along both the cilia and microvilli of individual epithelial cells that comprise the tubules. Dysregulation of kidney homeostasis via underlying medical conditions such as hypertension, diabetes, or glomerulonephritis fundamentally elevates the magnitudes of each principle force in the kidney and leads to fibrotic scarring and eventual loss of organ function. The purpose of this review is to summarize the progress made characterizing the response of kidney cells to pathological levels of mechanical stimuli. In particular, we examine important, mechanically responsive signaling cascades and explore fundamental changes in renal cell homeostasis after cyclic strain or fluid shear stress exposure. Elucidating the effects of these disease-related mechanical imbalances on endogenous signaling events in kidney cells presents a unique opportunity to better understand the fibrotic process.

Purinergic signalling - a possible mechanism for KCNQ1 channel response to cell volume challenges

AIM

A number of K(+) channels are regulated by small, fast changes in cell volume. The mechanisms underlying cell volume sensitivity are not known, but one possible mechanism could be purinergic signalling. Volume activated ATP release could trigger signalling pathways that subsequently lead to ion channel stimulation and cell volume back-regulation. Our aim was to investigate whether volume sensitivity of the voltage-gated K(+) channel, KCNQ1, is dependent on ATP release and regulation by purinergic signalling.

METHODS

We used Xenopus oocytes heterologously expressing human KCNQ1, KCNE1, water channels (AQP1) and P2Y2 receptors. ATP release was monitored by a luciferin-luciferase assay and ion channel conductance was recorded by two-electrode voltage clamp.

RESULTS

The luminescence assay showed that oocytes released ATP in response to mechanical, hypoosmotic stimuli and hyperosmotic stimuli. Basal ATP release was approx. three times higher in the KCNQ1 + AQP1 and KCNQ1 injected oocytes compared to the non-injected ones. Exogenously added ATP (0.1 mm) did not have any substantial effect on volume-induced KCNQ1 currents. Nevertheless, apyrase decreased all currents by about 50%. Suramin inhibited about 23% of the KCNQ1 volume sensitivity. Expression of P2Y2 receptors stimulated endogenous Cl(-) channels, but it also led to 68% inhibition of the KCNQ1 currents. Adenosine (0.1 mm) also inhibited the KCNQ1 currents by about 56%.

CONCLUSION

Xenopus oocytes release ATP in response to mechanical stimuli and cell volume changes. Purinergic P2 and P1 receptors confer some of the KCNQ1 channel volume sensitivity, although endogenous adenosine receptors and expressed P2Y2 receptors do so in the negative direction.

Coupled ATP and potassium efflux from intercalated cells

Increased flow in the distal nephron induces K secretion through the large-conductance, calcium-activated K channel (BK), which is primarily expressed in intercalated cells (IC). Since flow also increases ATP release from IC, we hypothesized that purinergic signaling has a role in shear stress (τ; 10 dynes/cm(2)) -induced, BK-dependent, K efflux. We found that 10 μM ATP led to increased IC Ca concentration, which was significantly reduced in the presence of the P(2) receptor blocker suramin or calcium-free buffer. ATP also produced BK-dependent K efflux, and IC volume decrease. Suramin inhibited τ-induced K efflux, suggesting that K efflux is at least partially dependent on purinergic signaling. BK-β4 small interfering (si) RNA, but not nontarget siRNA, decreased ATP secretion and both ATP-dependent and τ-induced K efflux. Similarly, carbenoxolone (25 μM), which blocks connexins, putative ATP pathways, blocked τ-induced K efflux and ATP secretion. Compared with BK-β4(-/-) mice, wild-type mice with high distal flows exhibited significantly more urinary ATP excretion. These data demonstrate coupled electrochemical efflux between K and ATP as part of the mechanism for τ-induced ATP release in IC.

P2Y1 receptors mediate an activation of neuronal calcium-dependent K+ channels

Molecularly defined P2Y receptor subtypes are known to regulate the functions of neurons through an inhibition of K(V)7 K(+) and Ca(V)2 Ca(2+) channels and via an activation or inhibition of Kir3 channels. Here, we searched for additional neuronal ion channels as targets for P2Y receptors. Rat P2Y(1) receptors were expressed in PC12 cells via an inducible expression system, and the effects of nucleotides on membrane currents and intracellular Ca(2+) were investigated. At a membrane potential of 30 mV, ADP induced transient outward currents in a concentration-dependent manner with half-maximal effects at 4 μm. These currents had reversal potentials close to the K(+) equilibrium potential and changed direction when extracellular Na(+) was largely replaced by K(+), but remained unaltered when extracellular Cl() was changed. Currents were abolished by P2Y(1) antagonists and by blockade of phospholipase C. ADP also caused rises in intracellular Ca(2+), and ADP-evoked currents were abolished when inositol trisphosphate-sensitive Ca(2+) stores were depleted. Blockers of K(Ca)2, but not those of K(Ca)1.1 or K(Ca)3.1, channels largely reduced ADP-evoked currents. In hippocampal neurons, ADP also triggered outward currents at 30 mV which were attenuated by P2Y(1) antagonists, depletion of Ca(2+) stores, or a blocker of K(Ca)2 channels. These results demonstrate that activation of neuronal P2Y(1) receptors may gate Ca(2+)-dependent K(+) (K(Ca)2) channels via phospholipase C-dependent increases in intracellular Ca(2+) and thereby define an additional class of neuronal ion channels as novel effectors for P2Y receptors. This mechanism may form the basis for the control of synaptic plasticity via P2Y(1) receptors.

Basal release of ATP: an autocrine-paracrine mechanism for cell regulation

Cells release adenosine triphosphate (ATP), which activates plasma membrane-localized P2X and P2Y receptors and thereby modulates cellular function in an autocrine or paracrine manner. Release of ATP and the subsequent activation of P2 receptors help establish the basal level of activation (sometimes termed "the set point") for signal transduction pathways and regulate a wide array of responses that include tissue blood flow, ion transport, cell volume regulation, neuronal signaling, and host-pathogen interactions. Basal release and autocrine or paracrine responses to ATP are multifunctional, evolutionarily conserved, and provide an economical means for the modulation of cell, tissue, and organismal biology.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

On the role of G-protein coupled receptors in cell volume regulation

Cell volume is determined genetically for each cell lineage, but it is not a static feature of the cell. Intracellular volume is continuously challenged by metabolic reactions, uptake of nutrients, intracellular displacement of molecules and organelles and generation of ionic gradients. Moreover, recent evidence raises the intriguing possibility that changes in cell volume act as signals for basic cell functions such as proliferation, migration, secretion and apoptosis. Cells adapt to volume increase by a complex, dynamic process resulting from the concerted action of volume sensing mechanisms and intricate signaling chains, directed to initiate the multiple adaptations demanded by a change in cell volume, among others adhesion reactions, membrane and cytoskeleton remodeling, and activation of the osmolyte pathways leading to reestablish the water balance between extracellular/intracellular or intracellular/intracellular compartments. In multicellular organisms, a continuous interaction with the external milieu is fundamental for the dynamics of the cell. It is in this sense that the recent surge of interest about the influence on cell volume control by the most extended family of signaling elements, the G proteins, acquires particular importance. As here reviewed, a large variety of G-protein coupled receptors (GPCRs) are involved in this interplay with cell volume regulatory mechanisms, which amplifies and diversifies the volume-elicited signaling chains, providing a variety of routes towards the multiple effectors related to cell volume changes.

Thrombin increases hyposmotic taurine efflux and accelerates % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbbjxAHX % garmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy0Hgip5wz % aebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbbf9v8qqaq % Fr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qq % Q8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaeaaakeaaca % qGjbGaae4qaiaabMeadaqhaaWcbaGaae4CaiaabEhacaqGLbGaaeiB % aiaabYgaaeaacqGHsislaaaaaa!3FBE! $$ {\text{ICI}}^{ - }_{{{\text{swell}}}} $$ and RVD in 3T3 fibroblasts by a src-dependent EGFR transactivation

The present study in Swiss3T3 fibroblasts examines the effect of thrombin on hyposmolarity-induced osmolyte fluxes and RVD, and the contribution of the src/EGFR pathway. Thrombin (5 U/ml) added to a 30% hyposmotic medium markedly increased hyposmotic 3H-taurine efflux (285%), accelerated the volume-sensitive Cl- current (ICI-swell) and increased RVD rate. These effects were reduced (50-65%) by preventing the thrombin-induced intracellular Ca2+ [Ca2+]i rise with EGTA-AM, or with the phospholipase C (PLC) blocker U73122. Ca2+calmodulin (CaM) and calmodulin kinase II (CaMKII) also participate in this Ca2+-dependent pathway. Thrombin plus hyposmolarity increased src and EGFR phosphorylation, whose blockade by PP2 and AG1478, decreased by 30-50%, respectively, the thrombin effects on hyposmotic taurine efflux, ICI-swell and RVD. Ca2+- and src/EGFR-mediated pathways operate independently as shown by (1) the persistence of src and EGFR activation when [Ca2+]i rise is prevented and (2) the additive effect on taurine efflux, ICI-swell or RVD by simultaneous inhibition of the two pathways, which essentially suppressed these events. PLC-Ca2+- and src/EGFR-signaling pathways operate in the hyposmotic condition and because thrombin per se failed to increase taurine efflux and ICI-swell under isosmotic condition it seems that it is merely amplifying these previously activated mechanisms. The study shows that thrombin potentiates hyposmolarity-induced osmolyte fluxes and RVD by increasing src/EGFR-dependent signaling, in addition to the Ca2+-dependent pathway.