Activation Of Calcium-dependent Potassium Channels Research Articles

Paradoxical Excitatory Impact of SK Channels on Dendritic Excitability.

Dendritic excitability regulates how neurons integrate synaptic inputs and thereby influences neuronal output. As active dendritic events are associated with significant calcium influx they are likely to be modulated by calcium-dependent processes, such as calcium-activated potassium channels. Here we investigate the impact of small conductance calcium-activated potassium channels (SK channels) on dendritic excitability in male and female rat cortical pyramidal neurons in vitro and in vivo Using local applications of the SK channel antagonist apamin in vitro, we show that blocking somatic SK channels enhances action potential output, whereas blocking dendritic SK channels paradoxically reduces the generation of dendritic calcium spikes and associated somatic burst firing. Opposite effects were observed using the SK channel enhancer NS309. The effect of apamin on dendritic SK channels was occluded when R-type calcium channels were blocked, indicating that the inhibitory impact of apamin on dendritic calcium spikes involved R-type calcium channels. Comparable effects were observed in vivo Intracellular application of apamin via the somatic whole-cell recording pipette reduced the medium afterhyperpolarization and increased action potential output during UP states. In contrast, extracellular application of apamin to the cortical surface to block dendritic SK channels shifted the distribution of action potentials within UP states from an initial burst to a more distributed firing pattern, while having no impact on overall action potential firing frequency or UP and DOWN states. These data indicate that somatic and dendritic SK channels have opposite effects on neuronal excitability, with dendritic SK channels counter-intuitively promoting rather than suppressing neuronal output.SIGNIFICANCE STATEMENT Neurons typically receive input from other neurons onto processes called dendrites, and use electrical events such as action potentials for signaling. As electrical events in neurons are usually associated with calcium influx they can be regulated by calcium-dependent processes. One such process is through the activation of calcium-dependent potassium channels, which usually act to reduce action potential signaling. Although this is the case for calcium-dependent potassium channels found at the cell body, we show here that calcium-dependent potassium channels in dendrites of cortical pyramidal neurons counter-intuitively promote rather than suppress action potential output.

Efferent Inhibition of the Cochlea.

Cholinergic efferent neurons originating in the brainstem innervate the acoustico-lateralis organs (inner ear, lateral line) of vertebrates. These release acetylcholine (ACh) to inhibit hair cells through activation of calcium-dependent potassium channels. In the mammalian cochlea, ACh shunts and suppresses outer hair cell (OHC) electromotility, reducing the essential amplification of basilar membrane motion. Consequently, medial olivocochlear neurons that inhibit OHCs reduce the sensitivity and frequency selectivity of afferent neurons driven by cochlear vibration of inner hair cells (IHCs). The cholinergic synapse on hair cells involves an unusual ionotropic ACh receptor, and a near-membrane postsynaptic cistern. Lateral olivocochlear (LOC) neurons modulate type I afferents by still-to-be-defined synaptic mechanisms. Olivocochlear neurons can be activated by a reflex arc that includes the auditory nerve and projections from the cochlear nucleus. They are also subject to modulation by higher-order central auditory interneurons. Through its actions on cochlear hair cells, afferent neurons, and higher centers, the olivocochlear system protects against age-related and noise-induced hearing loss, improves signal coding in noise under certain conditions, modulates selective attention to sensory stimuli, and influences sound localization.

Tonic Activation of Extrasynaptic NMDA Receptors Decreases Neuronal Excitability in Alzheimer's Disease

Amyloids β (Aβ) are a hallmark of Alzheimer's disease. They affect the communication between neurons. They can also bind to neuronal targets and thereby affect both intracellular signalling and neuronal electrical activity. During the onset of Alzheimer's disease, a positive feedback loop between Aβ40/42 and cytosolic calcium is thoughtto accelerate the progression of the disease. If intracellular calcium and Aβ reinforce themselves through this mechanism, one would expect that the neurons targets of Aβ may display an altered electrical activity caused by the increase in cytoplasmic calcium as it is known that there is a tight coupling between calcium dynamics and the electrical excitabilty. The aim of this work is to test this assumption when considering one of the privileged target of Aβ, the activity of extrasynaptic NMDA receptors. Our theoretical model is a simple description of neuronal electrical activity based on the Hodgkin-Huxley like formalism, including a term that corresponds to the activity of the NMDA receptor and a cytosolic calcium compartment. When the tonic activity of extrasynaptic NMDA receptors is increased, neurons are less excitable. This is a counterintuive result as NMDA receptors exert an excitatory effect. Further analysis show that this inhibitory effect is due to the activation of calcium-dependent potassium channels, which hyperpolarize the neurons. Activation of extrasynaptic NMDA receptors also provokes a marked increase in intracellular calcium concentration, thus reinforcing the feed-forward relation between Aβ production and calcium.

Assessment of the expression and role of the α1-nAChR subunit in efferent cholinergic function during the development of the mammalian cochlea.

Hair cell (HC) activity in the mammalian cochlea is modulated by cholinergic efferent inputs from the brainstem. These inhibitory inputs are mediated by calcium-permeable nicotinic acetylcholine receptors (nAChRs) containing α9- and α10-subunits and by subsequent activation of calcium-dependent potassium channels. Intriguingly, mRNAs of α1- and γ-nAChRs, subunits of the "muscle-type" nAChR have also been found in developing HCs (Cai T, Jen HI, Kang H, Klisch TJ, Zoghbi HY, Groves AK. J Neurosci 35: 5870-5883, 2015; Scheffer D, Sage C, Plazas PV, Huang M, Wedemeyer C, Zhang DS, Chen ZY, Elgoyhen AB, Corey DP, Pingault V. J Neurochem 103: 2651-2664, 2007; Sinkkonen ST, Chai R, Jan TA, Hartman BH, Laske RD, Gahlen F, Sinkkonen W, Cheng AG, Oshima K, Heller S. Sci Rep 1: 26, 2011) prompting proposals that another type of nAChR is present and may be critical during early synaptic development. Mouse genetics, histochemistry, pharmacology, and whole cell recording approaches were combined to test the role of α1-nAChR subunit in HC efferent synapse formation and cholinergic function. The onset of α1-mRNA expression in mouse HCs was found to coincide with the onset of the ACh response and efferent synaptic function. However, in mouse inner hair cells (IHCs) no response to the muscle-type nAChR agonists (±)-anatoxin A, (±)-epibatidine, (-)-nicotine, or 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) was detected, arguing against the presence of an independent functional α1-containing muscle-type nAChR in IHCs. In α1-deficient mice, no obvious change of IHC efferent innervation was detected at embryonic day 18, contrary to the hyperinnervation observed at the neuromuscular junction. Additionally, ACh response and efferent synaptic activity were detectable in α1-deficient IHCs, suggesting that α1 is not necessary for assembly and membrane targeting of nAChRs or for efferent synapse formation in IHCs.

Synaptic calcium regulation in hair cells of the chicken basilar papilla.

Cholinergic inhibition of hair cells occurs by activation of calcium-dependent potassium channels. A near-membrane postsynaptic cistern has been proposed to serve as a store from which calcium is released to supplement influx through the ionotropic ACh receptor. However, the time and voltage dependence of acetylcholine (ACh)-evoked potassium currents reveal a more complex relationship between calcium entry and release from stores. The present work uses voltage steps to regulate calcium influx during the application of ACh to hair cells in the chicken basilar papilla. When calcium influx was terminated at positive membrane potential, the ACh-evoked potassium current decayed exponentially over ∼100 ms. However, at negative membrane potentials, this current exhibited a secondary rise in amplitude that could be eliminated by dihydropyridine block of the voltage-gated calcium channels of the hair cell. Calcium entering through voltage-gated channels may transit through the postsynaptic cistern, since ryanodine and sarcoendoplasmic reticulum calcium-ATPase blockers altered the time course and magnitude of this secondary, voltage-dependent contribution to ACh-evoked potassium current. Serial section electron microscopy showed that efferent and afferent synaptic structures are juxtaposed, supporting the possibility that voltage-gated influx at afferent ribbon synapses influences calcium homeostasis during long-lasting cholinergic inhibition. In contrast, spontaneous postsynaptic currents ("minis") resulting from stochastic efferent release of ACh were made briefer by ryanodine, supporting the hypothesis that the synaptic cistern serves primarily as a calcium barrier and sink during low-level synaptic activity. Hypolemmal cisterns such as that at the efferent synapse of the hair cell can play a dynamic role in segregating near-membrane calcium for short-term and long-term signaling.

Role of Nicotinic Acetylcholine Receptor on Efferent Inhibition in Cochlear Hair Cell

The α9α10 nicotinic acetylcholine receptors (nAChRs) mediates efferent inhibition of hair cell function within the auditory sensory organ. Gating of the nAChRs leads to activation of calcium-dependent potassium channels to hyperpolarize the hair cell. In efferent system, main calcium providers to SK channel are nAChR and synaptic cistern, which contribution to efferent inhibition is different between avian and mammalian species. Calcium permeation is more effective in nAChRs of mammalian cochlea than avian cochlea, and mammalian calcium permeability of nAChRs is about 3 times more than avian hair cell. Thus, mammalian nAChRs is a main component of efferent inhibition in cochlear hair cell system.

A chloroform extract of the herb feverfew blocks voltage-dependent potassium currents recorded from single smooth muscle cells.

We have studied the effects of a chloroform extract of fresh leaves from the herb feverfew (Tanacetum parthenium) on potassium currents in smooth muscle. The currents were recorded from single cells dissociated from the rat anococcygeus and the rabbit ear artery using the whole-cell patch-clamp technique. When applied to cells isolated from the rat anococcygeus, the extract reduced the inactivating voltage-dependent potassium current in a concentration-related manner, with an IC50 value (the concentration that reduced the current by 50%) of 56 micrograms mL-1. Complete block of the current occurred at 1 mg mL-1. In addition to reducing the peak current, feverfew decreased the time to peak of the current and increased the rate of decay of the current. These effects can be explained by the feverfew extract blocking open potassium channels. In single cells isolated from rabbit ear artery the feverfew extract again reduced the voltage-dependent potassium current, whilst at the same time having no effect on the spontaneous transient outward currents which arise as a consequence of activation of calcium-dependent potassium channels. These results suggest that chloroform extracts of feverfew leaf contain an as yet unidentified substance capable of producing a selective, open-channel block of voltage-dependent potassium channels.

KCaChannels as Therapeutic Targets in Episodic Ataxia Type-2

Episodic ataxia type-2 (EA2) is an inherited movement disorder caused by mutations in the gene encoding the Ca(v)2.1alpha1 subunit of the P/Q-type voltage-gated calcium channel that result in an overall reduction in the P/Q-type calcium current. A consequence of these mutations is loss of precision of pacemaking in cerebellar Purkinje cells. This diminished precision reduces the information encoded by Purkinje cells and is thought to contribute to symptoms associated with this disorder. The loss of the precision of pacemaking in EA2 is the consequence of reduced activation of calcium-dependent potassium channels (K(Ca)) by the smaller calcium current and in vitro can be pharmacologically restored by K(Ca) activators. We used a well established mouse model of EA2, the tottering (tg/tg) mouse, to examine the potential therapeutic utility of one such Food and Drug Administration (FDA)-approved compound, chlorzoxazone (CHZ). Compared with wild-type Purkinje cells, we found the firing rate of tg/tg Purkinje cells in acutely prepared cerebellar slices to be very irregular. Bath application of CHZ successfully restored the precision of pacemaking in a dose-dependent manner. Oral administration of CHZ to tg/tg mice improved their baseline motor performance and reduced the severity, frequency, and duration of episodes of dyskinesia without producing any adverse effects. We propose the use of CHZ, which is currently FDA approved as a muscle relaxant, as a safe and novel treatment of EA2.

Real-time imaging of calcium dynamics in rhythmically active neurons of thespinal network for locomotion

Event Abstract Back to Event Real-time imaging of calcium dynamics in rhythmically active neurons of thespinal network for locomotion Di Wang1*, Sten Grillner1 and Peter Wallén1 1 Karolinska Institutet, Sweden The neuronal network underlying locomotion in vertebrates has been investigated in considerable detail in the lamprey spinal cord in vitro model. The isolated spinal cord preparation can be made to produce well-coordinated rhythmic activity, with left-right alternation and intersegmental phase-coupling, corresponding to locomotion (swimming). The rhythm-generating capacity of the network is the result of an interplay between network connectivity properties and the dynamic membrane properties of the individual network neurons. These different kinds of membrane properties typically rely on localized influx of calcium, leading to, for instance, the activation of calcium-dependent potassium channels (KCa-channels). Among the key calcium-dependent membrane properties are the NMDA-receptor-dependent membrane potential pacemaker oscillations, the slow afterhyperpolarization (sAHP) following the action potential, and the postinhibitory rebound depolarization following a period of hyperpolarization. Although much is known about the detailed operation of the network, specific information on the calcium dynamics – both in the temporal and the spatial domain – is still to a large degree lacking. We are therefore investigating different membrane properties of lamprey spinal neurons by utilizing confocal calcium imaging in combination with intracellular membrane potential recordings. During rhythmic network operation (fictive locomotion), calcium fluctuations can be detected in dendrites of individual neurons, even during subthreshold membrane potential oscillations (cf. Bacskai et al., Neuron 14:19-28, 1995). Furthermore, during NMDA-dependent pacemaker oscillations in the presence of TTX, dendritic calcium fluctuations are also seen, timed to the membrane potential oscillations (Fig. 1). During burst firing, the sAHP´s following each spike will add and produce a pronounced summed sAHP, which contributes to the termination of the burst. Calcium imaging during spike trains has revealed a distinct influx of calcium in conjunction with the sAHP. This calcium enters via N (CaV 2.2)- and P/Q (CaV 2.1) types of HVA calcium channels, while L-type channels (CaV 1) are not involved. Correspondingly, blockade of N- and P/Q-type channels reduced the calcium influx in the soma and proximal dendrites, while blockade of L-type channels did not. This indicates that the calcium channels responsible for activating the KCa channels involved in the sAHP are primarily located in the soma and proximal dendrites of the cell. In contrast, L-type calcium channels appear to be mainly localized to distal dendrites in lamprey spinal neurons. Confocal fluorescence imaging of calcium influx, and other dynamic events, with high spatial and temporal resolution, in combination with electrophysiological recordings from the same neuron in the functional network, holds promise to reveal important insights into the cellular and subcellular mechanisms governing network operation. tn_image2 conference image2 conference Conference: Neuroinformatics 2008, Stockholm, Sweden, 7 Sep - 9 Sep, 2008. Presentation Type: Poster and Short Oral Presentation Topic: Neuroimaging Citation: Wang D, Grillner S and Wallén P (2008). Real-time imaging of calcium dynamics in rhythmically active neurons of thespinal network for locomotion. Front. Neuroinform. Conference Abstract: Neuroinformatics 2008. doi: 10.3389/conf.neuro.11.2008.01.060 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 25 Jul 2008; Published Online: 25 Jul 2008. * Correspondence: Di Wang, Karolinska Institutet, Stockholm, Sweden, di.wang@ki.se Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract Supplemental Data The Authors in Frontiers Di Wang Sten Grillner Peter Wallén Google Di Wang Sten Grillner Peter Wallén Google Scholar Di Wang Sten Grillner Peter Wallén PubMed Di Wang Sten Grillner Peter Wallén Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Elevated Blood Pressure in Transgenic Lipoatrophic Mice and Altered Vascular Function

The role of perivascular fat in the control of vascular function was studied using lipoatrophic A-ZIP/F1 transgenic mice. Only a small amount of brown fat was found around the aorta but not around mesenteric arteries. Blood pressure of A-ZIP/F1 mice became higher than wild-type (WT) mice from 10 weeks of age. The presence of perivascular fat reduced the contraction of WT aorta to phenylephrine and serotonin, whereas this effect was either absent or less prominent in A-ZIP/F1 aorta. In WT mice, transfer of solution incubated with aorta with fat to aorta with fat removed caused a relaxation response, but not in A-ZIP/F1 mice, indicating the release of a relaxation factor from perivascular fat in WT aorta. This factor was acting through the activation of calcium-dependent potassium channels. Perfusion of phenylephrine to the isolated mesenteric bed caused a higher increase in perfusion pressure in A-ZIP/F1 than in WT mice. Contractile response of aorta to angiotensin II (Ang II) was mediated by Ang II type 1 receptors and was higher in A-ZIP/F1 than in WT mice. Expression of Ang II type 1 receptors but not Ang II type 2 receptors was higher in aorta of A-ZIP/F1 than WT mice. Treatment with an Ang II type 1 receptor antagonist (TCV 116, 10 mg/kg per day) for 2 weeks normalized the blood pressure of A-ZIP/F1 mice. These results suggest that the absence of perivascular fat tissue, which enhances the contractile response of the blood vessels to agonists, and an upregulation of vascular Ang II type 1 receptors in A-ZIP/F1 mice, are some of the mechanisms underlying the blood pressure elevation in these lipoatrophic mice.

CGMP-Dependent Protein Kinase in Regulation of the Pulmonary Circulation

Cyclic GMP-dependent protein kinase (PKG) plays a central role in the responses of the pulmonary vasculature to nitric oxide, nitrovasodilators, and natriuretic peptides. In mammalian cells, PKG is present in two forms, PKG I and PKG II, and the type I isoform is the primary enzyme involved in cGMP-dependent regulation of cardiovascular function. PKG can mediate vasodilation by lowering cytosolic Ca2+ levels by stimulating calcium-activated potassium channels and by phosphorylating inositol 1,4,5-triphosphate receptor associated PKG I substrate (IRAG). PKG can also cause vasodilation by reducing the Ca2+ sensitivity of myofilments by directly stimulating myosin light chain phosphatases (MLCP) and by interfering with the inhibition of MLCP exerted by Rho kinase. Recent studies show that PKG activity is regulated by oxygen and may be an important mechanism involved in postnatal adaptation of the pulmonary circulation. In chronically hypoxic lungs, PKG-dependent activation of calcium-dependent potassium channels may be impaired and thus contribute to an abnormally high vasomotor tone. A decrease in PKG activity may also contribute to the tolerance that the pulmonary vasculature develops to nitric oxide after prolonged therapy with nitric oxide inhalation for persistent pulmonary hypertension of the newborn (PPHN). Thus, the PKG appears to have multiple roles in the pulmonary circulation that can be affected by pathological conditions. With an increasing understanding of the mechanisms of PKG actions, the development of more selective tools to target the PKG pathway may lead to potential therapy for pulmonary vascular disease.

Perivascular adipose tissue modulates vascular function in the human internal thoracic artery

Recent studies have shown that perivascular adipose tissue from the rat aorta secretes a substance that can dilate the aorta. The purpose of the present study was to examine whether this vasodilator is also present in human internal thoracic arteries. Vascular function of human internal thoracic arteries with and without perivascular adipose tissue was assessed with wire myography, and morphology was examined with light microscopy. The presence of perivascular adipose tissue attenuated the maximal contraction to U 46619 and the contraction to phenylephrine (1 micromol/L) by 37% and 24%, respectively. Transfer of the solution incubated with a perivascular adipose tissue-intact vessel (donor) to a vessel without perivascular adipose tissue (recipient) induced a significant relaxation (36%) in the recipient artery precontracted with phenylephrine. Transfer of incubation solution with perivascular adipose tissue alone also induced a relaxation response in the recipient vessel (37%). The relaxation of the recipient artery induced by the transfer of incubation solution from the donor (artery with intact perivascular adipose tissue or perivascular adipose tissue alone) was absent in vessels precontracted by KCl (60 mmol/L) and was prevented by calcium-dependent potassium channel blockers (tetraethylammonium chloride, 1 mmol/L; iberiotoxin, 100 nmol/L), but not by the voltage-dependent potassium channel blocker 4-aminopyridine (1 mmol/L) and the adenosine triphosphate-dependent potassium channel blocker glibenclamide (10 micromol/L). Perivascular adipose tissue in human internal thoracic arteries releases a transferable relaxation factor that acts through the activation of calcium-dependent potassium channels. Because perivascular adipose tissue is often removed in coronary artery bypass grafting, retaining perivascular adipose tissue might be helpful in reducing the occurrence of vasospasm of the graft vessels.

Prostaglandins.

Prostaglandins.

Mechanisms for lateral turns in lamprey in response to descending unilateral commands: a modeling study.

Straight locomotion in the lamprey is, at the segmental level, characterized by alternating bursts of motor activity with equal duration and spike frequency on the left and the right sides of the body. Lateral turns are characterized by three main changes in this pattern: (1) in the turn cycle, the spike frequency, burst duration, and burst proportion (burst duration/cycle duration) increase on the turning side; (2) the cycle duration increases in both the turn cycle and the succeeding cycle; and (3) in the cycle succeeding the turn cycle, the burst duration increases on the non-turning side (rebound). We investigated mechanisms for the generation of turns in single-segment models of the lamprey locomotor spinal network. Activation of crossing inhibitory neurons proved a sufficient mechanism to explain all three changes in the locomotor rhythm during a fictive turn. Increased activation of these cells inhibits the activity of the opposite side during the prolonged burst of the turn cycle, and slows down the locomotor rhythm. Secondly, this activation of the crossing inhibitory neurons is accompanied by an increased calcium influx into the cells. This gives a suppressed activity on the turning side and a contralateral rebound after the turn, through activation of calcium-dependent potassium channels.

Arachidonate dilates basilar artery by lipoxygenase-dependent mechanism and activation of K(+) channels.

Dilatation of cerebral arterioles in response to arachidonic acid is dependent on activity of cyclooxygenase. In this study, we examined mechanisms that mediate dilatation of the basilar artery in response to arachidonate. Diameter of the basilar artery (baseline diameter = 216 +/- 7 micrometer) (means +/- SE) was measured using a cranial window in anesthetized rats. Arachidonic acid (10 and 100 microM) produced concentration-dependent vasodilatation that was not inhibited by indomethacin (10 mg/kg iv) or N(G)-nitro-L-arginine (100 microM) but was inhibited markedly by baicalein (10 micrometerM) or nordihydroguaiaretic acid (NDGA; 10 microM), inhibitors of the lipoxygenase pathway. Dilatation of the basilar artery was also inhibited markedly by tetraethylammonium ion (TEA; 1 mM) or iberiotoxin (50 nM), inhibitors of calcium-dependent potassium channels. For example, 10 microM arachidonate dilated the basilar artery by 19 +/- 7 and 1 +/- 1% in the absence and presence of iberiotoxin, respectively. Measurements of membrane potential indicated that arachidonate produced hyperpolarization of the basilar artery that was blocked completely by TEA. Incubation with [(3)H]arachidonic acid followed by reverse-phase and chiral HPLC indicated that the basilar artery produces relatively small quantities of prostanoids but large quantities of 12(S)-hydroxyeicosatetraenoic acid (12-S-HETE), a lipoxygenase product. Moreover, the production of 12-HETE was inhibited by baicalein or NDGA. These findings suggest that dilatation of the basilar artery in response to arachidonate is mediated by a product(s) of the lipoxygenase pathway, with activation of calcium-dependent potassium channels and hyperpolarization of vascular muscle.

Role of nucleotide cyclases in the inhibition of pregnant rat uterine contractions by the openers of potassium channels

Objective: Our objective was to study the involvement of adenylate and guanylate cylases in spontaneous uterine contractions and inhibition induced by the opening of potassium channels. Study Design: Uterine rings from rats at mid and term gestation and from rats at term gestation in labor were suspended in organ chambers for isometric tension recording. Concentration-response relationships to an opener of adenosine triphosphate–dependent potassium channels, levcromakalim, or to an opener of calcium-dependent potassium channels, NS 1619, were studied in the absence and presence of inhibitors of adenylate cyclase (MDL 12330 A, 2 × 10 –5 mol/L; SQ 22536, 10 –4 mol/L) or guanylate cyclase (LY 83583,3 × 10 –6 mol/L). Results: MDL 12330 A and SQ 22536 accentuated contractions in rings from rats at mid gestation but not at term gestation or at term gestation in labor. LY 83583 inhibited contractions in the rings from all 3 groups. Levcromakalim was equally effective in inhibiting contractions of rings from all 3 groups. MDL 12330 A, but not SQ 22536, decreased sensitivity and maximal inhibition induced by levcromakalim (term gestation greater than mid gestation greater than term gestation in labor). LY 83583 decreased the sensitivity to and maximal inhibition induced by levcromakalim in rings from pregnant rats at mid gestation. NS 1619 attenuated contraction of rings from rats at mid gestation and, to a lesser extent, at term gestation but accentuated contractions in rings from animals at term gestation in labor. MDL 12330 A, but not SQ 22536 or LY 83583, attenuated the changes induced by NS 1619 in rings from all 3 groups. Conclusions: (1) The influence of nucleotide cyclases on basal uterine contractility depends on gestational age. (2) The inhibition of uterine contractions that results from the opening of calcium-dependent potassium channels depends on adenylate cyclase, whereas that of adenosine triphosphate–dependent potassium channels depends on both adenylate and guanylate cyclases. 3. Activation of adenosine triphosphate–dependent potassium channels is more efficient than activation of calcium-dependent potassium channels. 4. The inhibition induced by calcium-dependent potassium channel openers, but not adenosine triphosphate–dependent potassium channel openers, decreases as pregnancy progresses, and at delivery the activation of spontaneous contractile activity is evident. (Am J Obstet Gynecol 2000;182:913-8.)

Apamin-sensitive, small-conductance, calcium-activated potassium channels mediate cholinergic inhibition of chick auditory hair cells.

Acetylcholine released from efferent neurons in the cochlea causes inhibition of mechanosensory hair cells due to the activation of calcium-dependent potassium channels. Hair cells are known to have large-conductance, "BK"-type potassium channels associated with the afferent synapse, but these channels have different properties than those activated by acetylcholine. Whole-cell (tight-seal) and cell-attached patch-clamp recordings were made from short (outer) hair cells isolated from the chicken basilar papilla (cochlea equivalent). The peptides apamin and charybdotoxin were used to distinguish the calcium-activated potassium channels involved in the acetylcholine response from the BK-type channels associated with the afferent synapse. Differential toxin blockade of these potassium currents provides definitive evidence that ACh activates apamin-sensitive, "SK"-type potassium channels, but does not activate carybdotoxin-sensitive BK channels. This conclusion is supported by tentative identification of small-conductance, calcium-sensitive but voltage-insensitive potassium channels in cell-attached patches. The distinction between these channel types is important for understanding the segregation of opposing afferent and efferent synaptic activity in the hair cell, both of which depend on calcium influx. These different calcium-activated potassium channels serve as sensitive indicators for functionally significant calcium influx in the hair cell.

Components of after-hyperpolarization in magnocellular neurones of the rat supraoptic nucleus in vitro.

1. The pharmacological sensitivity of hyperpolarizing components of spike train after-potentials was examined in sixty-one magnocellular neurones of the rat supraoptic nucleus using intracellular recording techniques in a brain slice preparation. 2. In 26 % of all neurones a slow after-hyperpolarization (AHP) was observed in addition to a fast AHP. In 31 % of all neurones a depolarizing after-potential (DAP) was observed. 3. The fast AHP was blocked by apamin whereas the slow AHP was blocked by charybdotoxin (ChTX). The DAP was enhanced by ChTX or a DAP was unmasked if not present during the control period. 4. Low concentrations of TEA (0.15-1.5 mM) induced effects on the slow AHP and the DAP essentially resembling those of ChTX. The same was true for the effects of CoCl2 (1 mM). 5. Spike train after-potentials were not affected by either iberiotoxin (IbTX), a selective high-conductance potassium (BK) channel antagonist, or margatoxin (MgTX), a Kv1.3 alpha-subunit antagonist. 6. Kv1.3 alpha-subunit immunohistochemistry revealed that these units are not expressed in the somato-dendritic region of supraoptic neurones. 7. The effects of ChTX, IbTX, MgTX, TEA, CoCl2 and CdCl2 on spike train after-potentials are interpreted in terms of an induction of the slow AHP by the activation of calcium-dependent potassium channels of intermediate single channel conductance (IK channels). 8. The results suggest that at least the majority of supraoptic magnocellular neurones share the capability of generating both a slow AHP and a DAP. The slow AHP may act to control the expression of the DAP, thus regulating the excitability of magnocellular neurones. The interaction of the slow AHP and the DAP may be important for the control of phasic discharge.

Mechanisms of bradykinin-induced cerebral vasodilatation in rats. Evidence that reactive oxygen species activate K+ channels.

Relatively little is know regarding mechanisms by which reactive oxygen species produce dilatation of cerebral arterioles. The goal of this study was to test the hypothesis that vasodilator responses of cerebral arterioles to bradykinin, which produces endogenous generation of reactive oxygen species, involve activation of calcium-dependent potassium channels. We used a cranial window in anesthetized rats to examine effects of catalase (which degrades hydrogen peroxide) on responses to bradykinin. In addition, we examined effects of tetraethylammonium (TEA) and iberiotoxin, inhibitors of calcium-dependent potassium channels, on responses of cerebral arterioles to hydrogen peroxide, bradykinin, and papaverine. In cerebral arterioles (baseline diameter = 40 +/- 1 microns) (mean +/- SE), hydrogen peroxide (10 and 100 mumol/L) produced concentration-dependent dilatation. TEA (1 mmol/L), an inhibitor of calcium-dependent potassium channels, produced marked inhibition of vasodilatation in response to hydrogen peroxide. For example, 100 mumol/L hydrogen peroxide dilated arterioles by 13 +/- 2% in the absence and 4 +/- 1% (P < .05 versus control) in the presence of TEA. Bradykinin (10 nmol/L to 1 mumol/L) also produced concentration-dependent dilatation of cerebral arterioles that was inhibited completely by catalase (100 U/mL). TEA or iberiotoxin markedly inhibited vasodilatation in response to bradykinin. For example, 100 nmol/L bradykinin dilated arterioles by 21 +/- 3% in the absence and 2 +/- 2% (P < .05 vs control) in the presence of iberiotoxin (50 nmol/L). These findings suggest that dilatation of cerebral arterioles in the rat in response to hydrogen peroxide, or hydrogen peroxide produced endogenously in response to bradykinin, is mediated by activation of calcium-dependent potassium channels. Thus, activation of potassium channels may be a major mechanism of dilatation in response to reactive oxygen species in the cerebral microcirculation.

Caffeine-induced electrical responses of intact endothelial cells

The mechanism of caffeine-induced endothelial-dependent relaxation of vascular smooth muscle cells has been studied by recording caffeine application-induced electrical responses from intact guinea pig aortic endothelial cells. Depending on the values of the membrane potential, caffeine evoked either hyperpolarizing responses (Vm −45 mV). The mean amplitude of caffeine-induced hyperpolarization of endothelial cells was 11.2±5.5 mV, which is comparable with the amplitude of ATP-induced hyperpolarization. The amplitude of caffeine-induced depolarization was 8.9±3.4 mV, on average. It was shown that caffeine-induced hyperpolarization of endothelial cells is a result of calcium release from the intracellular stores with subsequent activation of calcium-dependent potassium channels. Intracellular calcium stores involved in caffeine-induced responses are different from those involved in ATP responses. It is concluded that calcium mobilization from the intracellular stores of endothelial cells and, possibly, activation of calcium entry contributes to the caffeine-induced endothelial-dependent relaxation of vascular smooth muscle cells.