Food Deprivation Attenuates Seizures through CaMKII and EAG K+ Channels

Accumulated research has demonstrated the beneficial effects of dietary restriction on extending lifespan and increasing cellular stress resistance. However, reducing nutrient intake has also been shown to direct animal behaviors toward food acquisition. Under food-limiting conditions, behavioral changes suggest that neuronal and muscle activities in circuits that are not involved in nutrient acquisition are down-regulated. These dietary-regulated mechanisms, if understood better, might provide an approach to compensate for defects in molecules that regulate cell excitability. We previously reported that a neuromuscular circuit used in Caenorhabditis elegans male mating behavior is attenuated under food-limiting conditions. During periods between matings, sex-specific muscles that control movements of the male's copulatory spicules are kept inactive by UNC-103 ether-a-go-go–related gene (ERG)–like K⁺ channels. Deletion of unc-103 causes ~30%–40% of virgin males to display sex-muscle seizures; however, when food is deprived from males, the incidence of spontaneous muscle contractions drops to 9%–11%. In this work, we used genetics and pharmacology to address the mechanisms that act parallel with UNC-103 to suppress muscle seizures in males that lack ERG-like K⁺ channel function. We identify calcium/calmodulin-dependent protein kinase II as a regulator that uses different mechanisms in food and nonfood conditions to compensate for reduced ERG-like K⁺ channel activity. We found that in food-deprived conditions, calcium/calmodulin-dependent protein kinase II acts cell-autonomously with ether-a-go-go K⁺ channels to inhibit spontaneous muscle contractions. Our work suggests that upregulating mechanisms used by food deprivation can suppress muscle seizures.     

Cell excitability necessary for male mating behavior in Caenorhabditis elegans is coordinated by interactions between big current and ether-a-go-go family K(+) channels

Variations in K(+) channel composition allow for differences in cell excitability and, at an organismal level, provide flexibility to behavioral regulation. When the function of a K(+) channel is disrupted, the remaining K(+) channels might incompletely compensate, manifesting as abnormal organismal behavior. In this study, we explored how different K(+) channels interact to regulate the neuromuscular circuitry used by Caenorhabditis elegans males to protract their copulatory spicules from their tail and insert them into the hermaphrodite's vulva during mating. We determined that the big current K(+) channel (BK)/SLO-1 genetically interacts with ether-a-go-go (EAG)/EGL-2 and EAG-related gene/UNC-103 K(+) channels to control spicule protraction. Through rescue experiments, we show that specific slo-1 isoforms affect spicule protraction. Gene expression studies show that slo-1 and egl-2 expression can be upregulated in a calcium/calmodulin-dependent protein kinase II-dependent manner to compensate for the loss of unc-103 and conversely, unc-103 can partially compensate for the loss of SLO-1 function. In conclusion, an interaction between BK and EAG family K(+) channels produces the muscle excitability levels that regulate the timing of spicule protraction and the success of male mating behavior.     

K+ channels involved in contractility of rabbit small intestine

Most excitable cells, including gastrointestinal smooth muscle cells, express several types of K+ channels. The aim of this study was to examine the types of K' channels involved in the contractility of longitudinal smooth muscle of rabbit small intestine in vitro. Spontaneous contractions and KCl-stimulated contractions were reduced by atropine, phentolamine, propranolol, suramin, tetrodotoxin and indomethacin. The amplitude and tone of spontaneous contractions were increased by apamin, charybdotoxin, iberiotoxin, E4031, tetraetylammonium (TEA) and BaCl2. The frequency of contractions was reduced in the presence of apamin and TEA and increased by charybdotoxin. It was found that 4-aminopyridine increased the tone of spontaneous contractions and reduced the amplitude and frequency of contractions. Glibenclamide did not modify the amplitude, frequency or tone of contractions. KCl-stimulated contractions were increased by E4031, were not modified by apamin, glibenclamide, NS1619 or diazoxide, and were reduced by charybdotoxin, TEA, 4-aminopyridine or BaCl2. These results suggest that both Ca2+-activated K+ channels of small and high conductance, and HERG K+ channels and inward rectifier K+ channels participate in spontaneous contractions of small intestine. On the other hand, voltage-dependent K+ channels, HERG K+ channels, inward rectifier K+ channels and high conductance Ca2+-activated K+ channels are involved in KCl-stimulated contractions.     

SK but not IK channels regulate human detrusor smooth muscle spontaneous and nerve-evoked contractions

Animal studies suggest that the small (SK) and intermediate (IK) conductance Ca(2+)-activated K(+) channels may contribute to detrusor smooth muscle (DSM) excitability and contractility. However, the ability of SK and IK channels to control DSM spontaneous phasic and nerve-evoked contractions in human DSM remains unclear. We first investigated SK and IK channels molecular expression in native human DSM and further assessed their functional role using isometric DSM tension recordings and SK/IK channel-selective inhibitors. Quantitative PCR experiments revealed that SK3 channel mRNA expression in isolated DSM single cells was ～12- to 44-fold higher than SK1, SK2, and IK channels. RT-PCR studies at the single-cell level detected mRNA messages for SK3 channels but not SK1, SK2, and IK channels. Western blot and immunohistochemistry analysis further confirmed protein expression for the SK3 channel and lack of detectable protein expression for IK channel in whole DSM tissue. Apamin (1 μM), a selective SK channel inhibitor, significantly increased the spontaneous phasic contraction amplitude, muscle force integral, phasic contraction duration, and muscle tone of human DSM isolated strips. Apamin (1 μM) also increased the amplitude of human DSM electrical field stimulation (EFS)-induced contractions. However, TRAM-34 (1 μM), a selective IK channel inhibitor, had no effect on the spontaneous phasic and EFS-induced DSM contractions suggesting a lack of IK channel functional role in human DSM. In summary, our molecular and functional studies revealed that the SK, particularly the SK3 subtype, but not IK channels are expressed and regulate the spontaneous and nerve-evoked contractions in human DSM.     

Regulation of urinary bladder smooth muscle contractions by ryanodine receptors and BK and SK channels

This study examines the roles of voltage-dependent Ca(2+) channels (VDCC), ryanodine receptors (RyRs), large-conductance Ca(2+)-activated K(+) (BK) channels, and small-conductance Ca(2+)-activated K(+) (SK) channels in the regulation of phasic contractions of guinea pig urinary bladder smooth muscle (UBSM). Nisoldipine (100 nM), a dihydropyridine inhibitor of VDCC, abolished spontaneous UBSM contractions. Ryanodine (10 microM) increased contraction frequency and thereby integrated force and, in the presence of the SK blocker apamin, had a greater effect on integrated force than ryanodine alone. Blocking BK (iberiotoxin, 100 nM) or SK (apamin, 100 nM) channels increased contraction amplitude and duration but decreased frequency. The contractile response to iberiotoxin was more pronounced than to apamin. The increases in contraction amplitude and duration to apamin were substantially augmented with ryanodine pretreatment. These results indicate that BK and SK channels have prominent roles as negative feedback elements to limit UBSM contraction amplitude and duration. RyRs also appear to play a significant role as a negative feedback regulator of contraction frequency and duration, and this role is influenced by the activity of SK channels.     

Inhibition of ion channels and heart beat in Drosophila by selective COX-2 inhibitor SC-791

Recent findings suggest that modulation of ion channels might be implicated in some of the clinical effects of coxibs, selective inhibitors of cyclooxygenase-2 (COX-2). Celecoxib and its inactive analog 2,5-dimethyl-celecoxib, but not rofecoxib, can suppress or augment ionic currents and alter functioning of neurons and myocytes. To better understand these unexpected effects, we have recently investigated the mechanism of inhibition of human K(v)2.1 channels by a highly selective COX-2 inhibitor SC-791. In this study we have further explored the SC-791 action on ion channels and heartbeat in Drosophila, which lacks cyclooxygenases and thus can serve as a convenient model to study COX-2-independent mechanisms of coxibs. Using intracellular recordings in combination with a pharmacological approach and utilizing available Drosophila mutants, we found that SC-791 inhibited voltage-activated K(+) and L-type Ca(2+) channels in larval body-wall muscles and reduced heart rate in a concentration-dependent manner. Unlike celecoxib and several other K(+) channel blockers, SC-791 did not induce arrhythmia. Instead, application of SC-791 resulted in a dramatic slowing of contractions and, at higher concentrations, in progressively weaker contractions with gradual cessation of heartbeat. Isradipine, a selective blocker of L-type Ca(2+) channels, showed a similar pattern of heart arrest, though no prolongation of contractions was observed. Ryanodine was the only channel modulating compound of those tested additionally that was capable of slowing contractions. Like SC-791, ryanodine reduced heart rate without arrhythmia. However, it could not stop heartbeat completely even at 500 μM, the highest concentration used. The magnitude of heart rate reduction, when SC-791 and ryanodine were applied together, was smaller than expected for independent mechanisms, raising the possibility that SC-791 might be interfering with excitation-contraction coupling in Drosophila heart.     

Inhibition of phosphodiesterases relaxes detrusor smooth muscle via activation of the large-conductance voltage- and Ca²⁺-activated K⁺ channel

Detrusor smooth muscle (DSM) exhibits increased spontaneous phasic contractions under pathophysiological conditions such as detrusor overactivity (DO). Our previous studies showed that activation of cAMP signaling pathways reduces DSM contractility by increasing the large-conductance voltage- and Ca(2+)-activated K(+) (BK) channel activity. Here, we tested the hypothesis whether inhibition of phosphodiesterases (PDEs) can reduce guinea pig DSM excitability and contractility by increasing BK channel activity. Utilizing isometric tension recordings of DSM isolated strips and the perforated patch-clamp technique on freshly isolated DSM cells, we examined the mechanism of DSM relaxation induced by PDE inhibition. Inhibition of PDEs by 3-isobutyl-1-methylxanthine (IBMX), a nonselective PDE inhibitor, significantly reduced DSM spontaneous and carbachol-induced contraction amplitude, frequency, duration, muscle force integral, and tone in a concentration-dependent manner. IBMX significantly reduced electrical field stimulation-induced contractions of DSM strips. Blocking BK channels with paxilline diminished the inhibitory effects of IBMX on DSM contractility, indicating a role for BK channels in DSM relaxation mediated by PDE inhibition. IBMX increased the transient BK currents (TBKCs) frequency by ～3-fold without affecting the TBKCs amplitude. IBMX increased the frequency of the spontaneous transient hyperpolarizations by ～2-fold and hyperpolarized the DSM cell resting membrane potential by ～6 mV. Blocking the BK channels with paxilline abolished the IBMX hyperpolarizing effects. Under conditions of blocked Ca(2+) sources for BK channel activation, IBMX did not affect the depolarization-induced steady-state whole cell BK currents. Our data reveal that PDE inhibition with IBMX relaxes guinea pig DSM via TBKCs activation and subsequent DSM cell membrane hyperpolarization.     

Role of vascular potassium channels in the regulation of renal hemodynamics

K(+) conductance is a major determinant of membrane potential (V(m)) in vascular smooth muscle (VSMC) and endothelial cells (EC). The vascular tone is controlled by V(m) through the action of voltage-operated Ca(2+) channels (VOCC) in VSMC. Increased K(+) conductance leads to hyperpolarization and vasodilation, while inactivation of K(+) channels causes depolarization and vasoconstriction. K(+) channels in EC indirectly participate in the control of vascular tone by several mechanisms, e.g., release of nitric oxide and endothelium-derived hyperpolarizing factor. In the kidney, a change in the activity of one or more classes of K(+) channels will lead to a change in hemodynamic resistance and therefore of renal blood flow and glomerular filtration pressure. Through these effects, the activity of renal vascular K(+) channels influences renal salt and water excretion, fluid homeostasis, and ultimately blood pressure. Four main classes of K(+) channels [calcium activated (K(Ca)), inward rectifier (K(ir)), voltage activated (K(V)), and ATP sensitive (K(ATP))] are found in the renal vasculature. Several in vitro experiments have suggested a role for individual classes of K(+) channels in the regulation of renal vascular function. Results from in vivo experiments are sparse. We discuss the role of the different classes of renal vascular K(+) channels and their possible role in the integrated function of the renal microvasculature. Since several pathological conditions, among them hypertension, are associated with alterations in K(+) channel function, the role of renal vascular K(+) channels in the control of salt and water excretion deserves attention.     

Ionic mechanisms underlying TRH-induced prolactin secretion in rat lactotrophs.

Whole cell patch-clamp experiments were performed in clonal rat pituitary cells (GH3/B6 cells) to further analyze an inward-rectifying K current (IK, IR) which is suggested to be involved in the thyrotropin-releasing hormone (TRH)-induced increase in prolactin secretion from these cells. Using the class III antiarrhythmic agent E-4031 which is known as specific blocker of ether-á-go-go-related gene (ERG) K channels, the inward-rectifying K current could be isolated as the drug-sensitive current. To elucidate in molecular basis of this current, comparative experiments were performed in CHO cells which served as heterologous expression system for RERG, the rat homologue of the human ERG (HERG). It is shown that the inward-rectifying K current has properties identical to those mediated by channels encoded by RERG. In external 5 mM K+ solution, the ERG-like current IK, IR was an outward current in the physiological potential range, and this outward current could be strongly reduced by TRH. A specific block of IK, IR was able to mimick the second phase of the TRH-response which is characterized by a depolarization and/or by an increase in the frequency of Ca action potentials. These data show, that the ERG-like current in GH3/B6 cells contributes to the maintainance of the resting membrane potential and that it plays an important part in the mechanisms of the effects of TRH leading to an increase in prolactin secretion.     

Effects of extracellular HCO3(-) on fatigue, pHi, and K+ efflux in rat skeletal muscles.

Elevated plasma HCO(3)(-) can improve exercise endurance in humans. This effect has been related to attenuation of the work-induced reduction in muscle pH, which is suggested to improve performance via at least two mechanisms: 1) less inhibition of muscle enzymes and 2) reduced opening of muscle K(ATP) channels with less ensuing reduction in excitability. Aiming at determining whether the ergogenic effect of HCO(3)(-) is related to effects on muscles, we examined the effect of elevating extracellular HCO(3)(-) from 25 to 40 mM (pH from 7.4 to 7.6) on fatigue, intracellular pH (pH(i)), and K(+) efflux in isolated rat skeletal muscles contracting isometrically. Fatigue induced by 30-Hz stimulation at 30 and 37 degrees C was similar between soleus muscles incubated in high and normal HCO(3)(-) concentrations. In extensor digitorum longus muscles stimulated at 60 Hz, elevated HCO(3)(-) did not affect fatigue at 30 degrees C. In soleus muscles, 30-Hz stimulation induced a approximately 0.2 unit reduction in pH(i), as determined by using the pH-sensitive probe 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. This reduction in pH(i) was not affected by elevated HCO(3)(-). Estimation of K(+) efflux using (86)Rb(+) showed that elevated HCO(3)(-) did not affect K(+) efflux at rest or during contractions. Similarly, other modifications of the intra- and extracellular pH had little effect on K(+) efflux during contraction. In conclusion, elevated extracellular HCO(3)(-) had no significant effect on muscle fatigue, pH(i), and K(+) efflux. These findings indicate that alternative mechanisms must be considered for the ergogenic effect of HCO(3)(-) observed in integral exercise studies.     

The potential-dependent action of the opioid peptide dermorphin]

Dermorphin action was studied on cross-section strip of frog stomach muscle by a mechanographic recorder. The results show that dermorphin (10(-5)-10(-8) M) blocks acetylcholine effects, spontaneous activity and muscle contractions induced by direct electrical stimulation. All the above effects are hardly reversible. Dermorphin fails to block spontaneous and evoked activities if it is injected into the incubated medium during K(+)-depolarization (KCI--100 mM) of the muscle. Thus, dermorphin has voltage-dependent action. The discussion deals with dermorphin action on voltage-dependent Ca(2+)-channels of muscle cell membrane.     

Nitric oxide induces muscular relaxation via cyclic GMP-dependent and -independent mechanisms in the longitudinal muscle of the mouse duodenum.

The aim of this study was to investigate, in mouse duodenum, the role of nitric oxide (NO) in the relaxation of longitudinal muscle evoked by nerve activation and the coupled action mechanism. Electrical field stimulation (EFS; 0.5 ms, 10-s train duration, supramaximal voltage, at various frequencies) under nonadrenergic noncholinergic conditions evoked muscular relaxation occasionally followed, at the higher stimulus frequencies, by rebound contractions. Inhibition of the synthesis of NO by N(omega)-nitro-L-arginine methyl ester (L-NAME; 100 microM) virtually abolished the evoked relaxation. The relaxation was reduced also by apamin (0.1 microM) and by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 1 microM), a guanylyl cyclase inhibitor. The coadministration of apamin and ODQ produced additive effects on the responses to EFS. Sodium nitroprusside (0.1-100 microM) produced a concentration-dependent reduction of the phasic spontaneous activity and at the highest dose used suppressed phasic activity and induced muscular relaxation. These effects were tetrodotoxin and L-NAME resistant and were antagonized both by apamin and by ODQ. 8-Bromoguanosine 3',5'-cyclic monophosphate (0.1-100 microM) reduced in a concentration-dependent manner the spontaneous mechanical activity and at 100 microM suppressed the phasic activity and induced muscular relaxation, not antagonized by apamin. This study indicates that NO is the primary transmitter released by inhibitory nerves supplying the longitudinal muscle of mouse duodenum and that guanylate cyclase stimulation and opening of Ca(2+)-dependent K(+) channels are independent mechanisms working in parallel to mediate NO action.     

CyPPA, a positive modulator of small-conductance Ca(2+)-activated K(+) channels, inhibits phasic uterine contractions and delays preterm birth in mice

Organized uterine contractions, including those necessary for parturition, are dependent on calcium entry through voltage-gated calcium channels in myometrial smooth muscle cells. Recent evidence suggests that small-conductance Ca(2+)-activated potassium channels (K(Ca)2), specifically isoforms K(Ca)2.2 and 2.3, may control these contractions through negative feedback regulation of Ca(2+) entry. We tested whether selective pharmacologic activation of K(Ca)2.2/2.3 channels might depress uterine contractions, providing a new strategy for preterm labor intervention. Western blot analysis and immunofluorescence microscopy revealed expression of both K(Ca)2.2 and K(Ca)2.3 in the myometrium of nonpregnant (NP) and pregnant (gestation day 10 and 16; D10 and D16, respectively) mice. Spontaneous phasic contractions of isolated NP, D10, and D16 uterine strips were all suppressed by the K(Ca)2.2/2.3-selective activator CyPPA in a concentration-dependent manner. This effect was antagonized by the selective K(Ca)2 inhibitor apamin. Whereas CyPPA sensitivity was reduced in D10 and D16 versus NP strips (pIC(50) 5.33 ± 0.09, 4.64 ± 0.03, 4.72 ± 0.10, respectively), all contractions were abolished between 30 and 60 μM. Blunted contractions were associated with CyPPA depression of spontaneous Ca(2+) events in myometrial smooth muscle bundles. Augmentation of uterine contractions with oxytocin or prostaglandin F(2α) did not reduce CyPPA sensitivity or efficacy. Finally, in an RU486-induced preterm labor model, CyPPA significantly delayed time to delivery by 3.4 h and caused a 2.5-fold increase in pup retention. These data indicate that pharmacologic stimulation of myometrial K(Ca)2.2/2.3 channels effectively suppresses Ca(2+)-mediated uterine contractions and delays preterm birth in mice, supporting the potential utility of this approach in tocolytic therapies.     

KV2.1 and electrically silent KV channel subunits control excitability and contractility of guinea pig detrusor smooth muscle

Voltage-gated K(+) (K(V)) channels are implicated in detrusor smooth muscle (DSM) function. However, little is known about the functional role of the heterotetrameric K(V) channels in DSM. In this report, we provide molecular, electrophysiological, and functional evidence for the presence of K(V)2.1 and electrically silent K(V) channel subunits in guinea pig DSM. Stromatoxin-1 (ScTx1), a selective inhibitor of the homotetrameric K(V)2.1, K(V)2.2, and K(V)4.2 as well as the heterotetrameric K(V)2.1/6.3 and K(V)2.1/9.3 channels, was used to examine the role of these K(V) channels in DSM function. RT-PCR indicated mRNA expression of K(V)2.1, K(V)6.2-6.3, K(V)8.2, and K(V)9.1-9.3 subunits in isolated DSM cells. K(V)2.1 protein expression was confirmed by Western blot and immunocytochemistry. Perforated whole cell patch-clamp experiments revealed that ScTx1 (100 nM) inhibited the amplitude of the K(V) current in freshly isolated DSM cells. ScTx1 (100 nM) did not significantly change the steady-state activation and inactivation curves for K(V) current. However, ScTx1 (100 nM) decreased the activation time-constant of the K(V) current at positive voltages. Although our patch-clamp data could not exclude the presence of the homotetrameric K(V)2.1 channels, the biophysical characteristics of the ScTx1-sensitive current were consistent with the presence of heterotetrameric K(V)2.1/silent K(V) channels. Current-clamp recordings showed that ScTx1 (100 nM) did not change the DSM cell resting membrane potential. ScTx1 (100 nM) increased the spontaneous phasic contraction amplitude, muscle force, and muscle tone as well as the amplitude of the electrical field stimulation-induced contractions of isolated DSM strips. Collectively, our data revealed that K(V)2.1-containing channels are important physiological regulators of guinea pig DSM excitability and contractility.     

Negative feedback regulation of nerve-mediated contractions by KCa channels in mouse urinary bladder smooth muscle

When the urinary bladder is full, activation of parasympathetic nerves causes release of neurotransmitters that induce forceful contraction of the detrusor muscle, leading to urine voiding. The roles of ion channels that regulate contractility of urinary bladder smooth muscle (UBSM) in response to activation of parasympathetic nerves are not well known. The present study was designed to characterize the role of large (BK)- and small-conductance (SK) Ca(2+)-activated K(+) (K(Ca)) channels in regulating UBSM contractility in response to physiological levels of nerve stimulation in UBSM strips from mice. Nerve-evoked contractions were induced by electric field stimulation (0.5-50 Hz) in isolated strips of UBSM. BK and SK channel inhibition substantially increased the amplitude of nerve-evoked contractions up to 2.45 +/- 0.12- and 2.99 +/- 0.25-fold, respectively. When both SK and BK channels were inhibited, the combined response was additive. Inhibition of L-type voltage-dependent Ca(2+) channels (VDCCs) in UBSM inhibited nerve-evoked contractions by 92.3 +/- 2.0%. These results suggest that SK and BK channels are part of two distinct negative feedback pathways that limit UBSM contractility in response to nerve stimulation by modulating the activity of VDCCs. Dysfunctional regulation of UBSM contractility by alterations in BK/SK channel expression or function may underlie pathologies such as overactive bladder.     

Pharmacological basis for the use of turmeric in gastrointestinal and respiratory disorders

This study was carried out to provide scientific basis for the medicinal use of turmeric (Curcuma longa) in gastrointestinal and respiratory disorders. The crude extract of turmeric (Cl.Cr), relaxed the spontaneous and K+ (80 mM)-induced contractions in isolated rabbit jejunum as well as shifted the CaCl2 concentration-response curves. In rabbit tracheal preparation, Cl.Cr inhibited carbachol and K(+)-induced contractions. In anesthetized rats, Cl.Cr produced variable responses on blood pressure with a mixture of weak hypertensive and hypotensive actions. In rabbit aorta, Cl.Cr caused a weak vasoconstrictor and a vasodilator effect on K+ and phenylephrine-induced contractions. In guinea-pig atria, Cl.Cr inhibited spontaneous rate and force of contractions at 14-24 times higher concentrations. Activity directed fractionation revealed that the vasodilator and vasoconstrictor activities are widely distributed in the plant with no clear separation into the polar or non-polar fractions. When used for comparison, both curcumin and verapamil caused similar inhibitory effects in all smooth muscle preparations with relatively more effect against K(+)-induced contractions and that both were devoid of any vasoconstrictor effect and curcumin had no effect on atria. These data suggest that the inhibitory effects of Cl.Cr are mediated primarily through calcium channel blockade, though additional mechanism cannot be ruled out and this study forms the basis for the traditional use of turmeric in hyperactive states of the gut and airways. Furthermore, curcumin, the main active principle, does not share all effects of turmeric.     

Influence of the redox-state potential of biophase on electrically stimulated skeletal muscles (myographic and voltage-clamp analysis)

Redox modulation of cholinergic and adrenergic mechanisms of excitatory tissues have been analyzed rather satisfactorily until recently. The aim of the present work is to give some initial guiding information about the redox modulation of electrogenic excitatory processes in skeletal muscles. It was observed, that on increasing the tissue redox-state potential (E0'), the amplitudes of muscle contractions are higher by 18.5 per cent than controls, but decreasing E0', the amplitudes of muscle contractions are lower by 10.5 per cents than controls. It means, that muscle contractions elicited by electric stimulation are also under redox control. One of the mechanisms responsible for this phenomenon is that following increased E0' values both peak current (Ip) and steady-state current (ISS) increases, but after decreased E0' values ISS decreases. The role of other site of actions and mechanisms, i.e. ion channels, active transport, excitation-contraction coupling, and Na(+)-Ca2+ exchange diffusion processes, are also discussed.     

Increased spontaneous tone in renal arteries of spontaneously hypertensive rats

The spontaneous tone of vascular smooth muscle is augmented in hypertension. The present study examined the role of nitric oxide (NO), cyclooxygenase (COX), thromboxane A(2)/prostanoid (TP) and PGE(2)/prostanoid (EP-1) receptors, reactive oxygen species, and large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels in the regulation of spontaneous tone in renal arteries of young and mature Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Rings of arteries, with and without endothelium, were suspended in a myograph for isometric force recording. Spontaneous tone (increase above initial tension) was observed only in arteries of mature SHR and was greater in arteries without endothelium. N(omega)-nitro-L-arginine methyl ester (L-NAME, an inhibitor of NO synthases) induced larger contractions in arteries of SHR than WKY. Indomethacin (a COX inhibitor), SC-19220 (an EP-1 receptor antagonist), and terutroban (a TP receptor antagonist) reduced the L-NAME-evoked contractions. Tiron (a superoxide anion scavenger), catalase (an enzyme that degrades H(2)O(2)), and deferoxamine (a hydroxyl radical scavenger) augmented the L-NAME-induced contractions in arteries of mature SHR. Charybdotoxin (a BK(Ca) channel blocker) caused contractions in arteries of mature SHR without endothelium and in arteries with endothelium incubated with L-NAME. A decreased protein level of endothelial NO synthase, an increased release of prostacyclin, and an increased expression of EP-1 receptors were observed in arteries of mature SHR. The present study suggests that spontaneous tone is precipitated by age and hypertension. The reduced production of NO, leading to decreased activation of BK(Ca) channels, may leave the actions of endogenous vasoconstrictors unopposed. COX products that activate EP-1 and TP receptors are involved in the development of spontaneous tone.     

Potassium transporters in plants--involvement in K+ acquisition, redistribution and homeostasis

Potassium is a major plant nutrient which has to be accumulated in great quantity by roots and distributed throughout the plant and within plant cells. Membrane transport of potassium can be mediated by potassium channels and secondary potassium transporters. Plant potassium transporters are present in three families of membrane proteins: the K(+) uptake permeases (KT/HAK/KUP), the K(+) transporter (Trk/HKT) family and the cation proton antiporters (CPA). This review will discuss the contribution of members of each family to potassium acquisition, redistribution and homeostasis.     

Eph and NMDA receptors control Ca2+/calmodulin-dependent protein kinase II activation during C. elegans oocyte meiotic maturation

Fertilization in the female reproductive tract depends on intercellular signaling mechanisms that coordinate sperm presence with oocyte meiotic progression. To achieve this coordination in Caenorhabditis elegans, sperm release an extracellular signal, the major sperm protein (MSP), to induce oocyte meiotic maturation and ovulation. MSP binds to multiple receptors, including the VAB-1 Eph receptor protein-tyrosine kinase on oocyte and ovarian sheath cell surfaces. Canonical VAB-1 ligands called ephrins negatively regulate oocyte maturation and MPK-1 mitogen-activated protein kinase (MAPK) activation. Here, we show that MSP and VAB-1 regulate the signaling properties of two Ca2+ channels that are encoded by the NMR-1 N-methyl D-aspartate type glutamate receptor subunit and ITR-1 inositol 1,4,5-triphosphate receptor. Ephrin/VAB-1 signaling acts upstream of ITR-1 to inhibit meiotic resumption, while NMR-1 prevents signaling by the UNC-43 Ca2+/calmodulin-dependent protein kinase II (CaMKII). MSP binding to VAB-1 stimulates NMR-1-dependent UNC-43 activation, and UNC-43 acts redundantly in oocytes to promote oocyte maturation and MAPK activation. Our results support a model in which VAB-1 switches from a negative regulator into a redundant positive regulator of oocyte maturation upon binding to MSP. NMR-1 mediates this switch by controlling UNC-43 CaMKII activation at the oocyte cortex.