TRAAK is a mammalian neuronal mechano-gated K+ channel

The novel structural class of mammalian channels with four transmembrane segments and two pore regions comprise background K+ channels (TWIK-1, TREK-1, TRAAK, TASK, and TASK-2) with unique physiological functions (1-6). Unlike its counterparts, TRAAK is only expressed in neuronal tissues, including brain, spinal cord, and retina (1). This report shows that TRAAK, which was known to be activated by arachidonic acid (3), is also opened by membrane stretch. Mechanical activation of TRAAK is induced by a convex curvature of the plasma membrane and can be mimicked by the amphipathic membrane crenator trinitrophenol. Cytoskeletal elements are negative tonic regulators of TRAAK. Membrane depolarization and membrane crenation synergize with stretch-induced channel opening. Finally, TRAAK is reversibly blocked by micromolar concentrations of gadolinium, a well known blocker of stretch-activated channels. Mechanical activation of TRAAK in the central nervous system may play an important role during growth cone motility and neurite elongation.

Identification of the receptor subtype involved in the analgesic effect of neurotensin

The neuropeptide neurotensin (NT) elicits hypothermic and naloxone-insensitive analgesic responses after brain injection. Recent pharmacological evidence obtained with NT agonists and antagonists suggests that these effects are mediated by a receptor distinct from the initially cloned high-affinity NT receptor (NTR1). The recent cloning of a second NT receptor (NTR2) prompted us to evaluate its role in NT-induced analgesia. Intracerebroventricular injections in mice of two different antisense oligodeoxynucleotides from the NTR2 markedly decreased NTR2 mRNA and protein and reduced NT-induced analgesia. This effect was specific, because NTR1 levels were unaffected, and sense or scramble oligodeoxynucleotides had no effect. Structure-activity studies revealed a close correlation between the analgesic potency of NT analogs and their affinity for the NTR2 and disclosed potent and selective agonists of this receptor. These data confirm that NTR1 is involved in the NT-elicited turning behavior and demonstrate that the NTR2 mediates NT-induced analgesia.

A mammalian two pore domain mechano-gated S-like K+ channel

Aplysia S-type K+ channels of sensory neurons play a dominant role in presynaptic facilitation and behavioural sensitization. They are closed by serotonin via cAMP-dependent phosphorylation, whereas they are opened by arachidonic acid, volatile general anaesthetics and mechanical stimulation. We have identified a cloned mammalian two P domain K+ channel sharing the properties of the S channel. In addition, the recombinant channel is opened by lipid bilayer amphipathic crenators, while it is closed by cup-formers. The cytoplasmic C-terminus contains a charged region critical for chemical and mechanical activation, as well as a phosphorylation site required for cAMP inhibition.

Disruption of mitochondrial respiration inhibits volume-regulated anion channels and provokes neuronal cell swelling

Hypoxia and inhibitors of mitochondrial respiration impair the regulatory volume decrease (RVD) of cerebellar granule neurons after hypotonic swelling. RVD is linked to the opening of volume-regulated anion channels (VRACs). VRACs are outwardly rectifying, inactivate slowly during maintained depolarization, and are permeable to the cellular organic osmolyte taurine. Channel activation requires nonhydrolytic ATP binding and is not modulated by intracellular ADP. VRAC opening is reversibly depressed by hypoxia and by mitochondrial inhibitors such as oligomycin, rotenone, and antimycin A. These results demonstrate that neuronal VRAC activation and swelling are both tightly linked to cellular energy. Moreover, the findings reported in this work may have a particular significance for inherited mitochondrial human diseases, such as mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), which cause brain swelling and edema.

Kv2.1/Kv9.3, a novel ATP-dependent delayed-rectifier K+ channel in oxygen-sensitive pulmonary artery myocytes

The molecular structure of oxygen-sensitive delayed-rectifier K+ channels which are involved in hypoxic pulmonary artery (PA) vasoconstriction has yet to be elucidated. To address this problem, we identified the Shab K+ channel Kv2.1 and a novel Shab-like subunit Kv9.3, in rat PA myocytes. Kv9.3 encodes an electrically silent subunit which associates with Kv2.1 and modulates its biophysical properties. The Kv2.1/9.3 heteromultimer, unlike Kv2.1, opens in the voltage range of the resting membrane potential of PA myocytes. Moreover, we demonstrate that the activity of Kv2.1/Kv9.3 is tightly controlled by internal ATP and is reversibly inhibited by hypoxia. In conclusion, we propose that metabolic regulation of the Kv2.1/Kv9.3 heteromultimer may play an important role in hypoxic PA vasoconstriction and in the possible development of PA hypertension.

Epidermal induction and inhibition of neural fate by translation initiation factor 4AIII

Bone Morphogenetic Protein-4 (BMP-4) is a potent epidermal inducer and inhibitor of neural fate. We have used differential screening to identify genes involved in epidermal induction downstream of BMP-4 and report here evidence of a novel translational mechanism that regulates the division of the vertebrate ectoderm into regions of neural and epidermal fate. In dissociated Xenopus ectoderm, addition of ectopic BMP-4 leads to an increase in the expression of translation initiation factor 4AIII (eIF-4AIII), a divergent member of the eIF-4A gene family until now characterized only in plants. In the gastrula embryo, Xenopus eIF-4AIII (XeIF-4AIII) expression is elevated in the ventral ectoderm, a site of active BMP signal transduction. Moreover, overexpression of XeIF-4AIII induces epidermis in dissociated cells that would otherwise adopt a neural fate, mimicking the effects of BMP-4. Epidermal induction by XeIF-4AIII requires both an active BMP signaling pathway and an extracellular intermediate. Our results suggest that XeIF-4AIII can regulate changes in cell fate through selective mRNA translation. We propose that BMPs and XeIF-4AIII interact through a positive feedback loop in the ventral ectoderm of the vertebrate gastrula.

In vivo evidence for trigeminal nerve guidance by the cement gland in Xenopus

In Xenopus embryos growth cones of the mandibular trigeminal nerve contact cells located in the posterior end of the cement gland. We report three lines of evidence suggesting that cues from the posterior cells of the target influence the behavior of trigeminal neurites. First, target ablation in vivo results in failure of axons to stop and to arborize. Second, 180 degree rotation in the anteroposterior axis of the target results in anterior innervation of the cement gland. Third, ectopic cement gland implantation in the path of neurites migration leads to ectopic innervation. These results provide evidence for short-range cues (< 50 microns) in the posterior part of the cement gland controlling the stopping and the branching of the mandibular trigeminal nerve. Furthermore, we show that an ectodermic explant expressing follistatin mimics the natural target in homing trigeminal neurites. We finally propose to use this novel in vivo assay to isolate the trigeminal nerve target-recognition molecule(s).

Glibenclamide opens ATP-sensitive potassium channels in Xenopus oocyte follicular cells during metabolic stress

Follicular cells from Xenopus oocytes offer a particularly interesting system to study ATP-sensitive K+ channels (KATP channels). In these cells, as in many other cell types, glibenclamide is a classical blocker of KATP channels. Metabolic inhibition with dinitrophenol (DNP) converts this inhibitory effect into an activation. Follicular cells treated with DNP keep their sensitivity to the KATP channel opener P1060, but this opening effect becomes insensitive to glibenclamide inhibition. Glibenclamide activation of KATP channels in DNP-treated follicular cells occurs with an EC50 of 3 microM. Glibenclamide activation is antagonized by blockers of KATP channels that do not belong to the sulfonylurea family, such as U-37883A, tedisamil, and LH 35. Other sulfonylureas display the same activating behavior as does glibenclamide in DNP-treated cells. Two of the properties of KATP channels in follicular cells are activation by cAMP through protein kinase A and inhibition by muscarinic effectors through protein kinase C activation. The stimulating effects of cAMP and glibenclamide in DNP-treated cells seem to be synergistic as are the cAMP and P1060 effects in control follicular cells. Glibenclamide-activated KATP channels in DNP-treated cells (conductance of 15 pS) are also inhibited by acetylcholine and by phorbol esters. The internal acidosis produced by metabolic exhaustion with DNP appears to be the key element in the conversion of glibenclamide from a blocker to an activator of KATP channels.

Biophysical, pharmacological and developmental properties of ATP-sensitive K+ channels in cultured myotomal muscle cells from Xenopus embryos

Unlike mammalian muscle cells in culture, cultured myotomal muscle cells of Xenopus embryos express ATP-sensitive K+ (KATP) channels. The KATP channels are blocked by internal ATP (half-maximal inhibition K0.5 = 16 microM) and to a lesser extent by internal ADP, are voltage independent, have an inward rectification at positive potentials and are inhibited by glibenclamide (K0.5 = 2 microM). Surprisingly, these KATP channels are not sensitive to K+ channel openers such as cromakalim. Opening of these KATP channels does not occur under normal physiological conditions. It is elicited by metabolic exhaustion of the muscle cell and it precedes the development of an irreversible rigor state. Neither intracellular acidosis nor an increase of intracellular Ca2+ are involved in KATP channel opening. Different types of K+ channels are successively expressed after plating of myotomal muscle cells: (1) sustained delayed-rectifier K+ channels; (2) KATP channels; (3) inward-rectifier K+ channels; (4) transient delayed-rectifier K+ channels. The current density associated with KATP channels far exceeds that of voltage-dependent K+ channels. Innervation controls the expression of these KATP channels. Co-culture of muscle cells with neurons from the neural tube decreases the number of active KATP channels per patch. Similarly, in situ innervated submaxillaris muscle of tadpoles at stage 50-55 has a very low density of KATP channels.

CGRP-induced activation of KATP channels in follicular Xenopus oocytes

The two-microelectrode voltage-clamp technique was used to monitor K+ channel activity in Xenopus oocyte follicular cells, which are electrically coupled to the oocyte itself by gap junctions. Endogenous vasodilators such as calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP), prostaglandin E2 (PGE2) and adenosine activate glibenclamide-ATP-sensitive K+ (KATP) channels in Xenopus oocyte follicular cells. The mechanism of action of CGRP was studied in detail. CGRP effects undergo a rapid desensitization. CGRP acts via CGRPI receptors. Its effects are antagonised by the amino-truncated CGRP analog hCGRP(8-37). The second messenger for CGRP activation of KATP channels is cAMP. Phosphodiesterase inhibition by 3-isobutyl-1-methylxanthine enhances the CGRP response while adenyl cyclase inhibition by either 2',5'-dideoxyadenosine or progesterone nearly completely depresses the CGRP response. Vasoconstrictors such as ACh and angiotensin II also have receptors in follicular cells. ACh strongly inhibits the CGRP activation of K+ channels as it inhibits the activation of KATP channels by P1060, but angiotensin II does not. It is concluded that as in vascular smooth muscle cells, CGRP and probably other hyperpolarizing vasodilators open KATP channels in follicular cells by protein kinase A activation.

Injection of a K+ channel (Kv1.3) cRNA in fertilized eggs leads to functional expression in cultured myotomal muscle cells from Xenopus embryos

The synthetic cRNA encoding for the major T lymphocyte K+ channel (Kv1.3) was injected into Xenopus fertilized eggs. Somites from embryos of stage 20-22 (about 40 h post-fertilization at 19 degrees C) were dissociated and myotomal muscle cells were cultured in vitro for 2 days. The whole cell configuration of the tight seal patch-clamp technique was used to record K+ channel activity in cultured myocytes. These myocytes have two endogenous delayed-rectifiers (sustained and transient) and an inward-rectifier K+ currents, all of which are insensitive to the scorpion toxin charybdotoxin. Cultured myocytes dissociated from embryos injected with the Kv1.3 cRNA expressed the exogenous Kv1.3 channel. The Kv1.3 channel was identified by its physiological (a very low recovery from inactivation) and its pharmacological properties (a high sensitivity to charybdotoxin). This work demonstrates that Xenopus cultured myotomal muscle cells represent a very efficient and practical assay system for the functional expression of cloned ion channels.

Molecular biology of voltage-gated K+ channels in heart

The recent cloning of numerous voltage-activated K+ channels provides new information concerning the architecture of K+ channel proteins. The combination of molecular genetic and biophysical methods gives us a new insight into the molecular mechanisms of K+ channel pharmacology.

External blockade of the major cardiac delayed-rectifier K+ channel (Kv1.5) by polyunsaturated fatty acids

The present work shows that arachidonic acid and some other long chain polyunsaturated fatty acids such as docosahexaenoic acid, which is abundant in fish oil, produce a direct open channel block of the major voltage-dependent K+ channel (Kv1.5) cloned in cardiac cells. The inhibitory action of these selected fatty acids is seen when they are applied extracellularly but not when they are included in the patch pipette. Fatty acids then appear to bind to an external site on the Kv1.5 channel structure. Inhibition of Kv1.5 channel activity by polyunsaturated fatty acids (acceleration of the apparent inactivation and decrease of the peak current) is similar to that produced by the class III antiarrhythmic tedisamil. Docosahexaenoic acid and arachidonic acid also inhibit the delayed-rectifier K+ channel currents in cultured mouse and rat cardiomyocytes. These results are discussed in the light of the reported fatty acids effects on cardiac function in diseased states. Since Kv1.5 is also present in the brain, the results reported here could also have a significance in terms of processes such as long-term potentiation or depression.

Multiple mRNA isoforms encoding the mouse cardiac Kv1-5 delayed rectifier K+ channel

The mouse Kv1-5 K+ channel cDNA has been cloned from heart. This channel was highly expressed in heart and, to a lesser extent, in other tissues, including brain and thymus. Two alternatively spliced isoforms were found. The longer form encoded a 602-amino acid protein, while in the short form (Kv1-5 delta 5'), the first 200 amino acids lying upstream the transmembrane segment S1 were deleted. RNase protection experiments showed that both Kv1-5 mRNA isoforms are present in the mouse tissues examined, the longer form being predominant. The short mRNA (Kv1-5 delta 5') arose by an unusual splicing event within the exonic sequence. An additional short cDNA clone (Kv1-5 delta 3') that codes for a carboxyl-terminal truncated protein has been isolated. The gene coding sequence contained a single exon and has been mapped on human chromosome 12 (p13) and on mouse chromosome 6 (band F). Expression in Xenopus oocytes revealed that the long (Kv1-5) and the amino-terminal deleted (Kv1-5 delta 5') isoforms elicited similar K+ currents with a drastically decreased efficacy for Kv1-5 delta 5'. The carboxyl-terminal truncated Kv1-5 delta 3' clone was not functional but inhibited the expression of the long isoform.

The protein IsK is a dual activator of K+ and Cl- channels

The protein IsK (M(r) 14,500) is present in epithelial cells, heart, uterus and lymphocytes and induces slowly activating K+ currents when expressed in Xenopus oocytes. The finding that mutations of its single transmembrane segment altered channel gating or selectivity has suggested that IsK is a channel-forming protein. But IsK does not exhibit the K+ channel hallmarks (a conserved K+ selective pore (H5) flanked by either six or two membrane-spanning regions). Here we report that IsK expression in Xenopus oocytes also induces a Cl- selective current very similar to the Cl- current produced by phospholemman expression and with biophysical, pharmacological and regulation characteristics very different from those of the IsK-induced K+ channel activity. IsK mutagenesis identifies amino- and carboxy-terminal domains as critical for the induction of Cl- and K+ channel activities, respectively. Our data lead to a model in which the IsK protein (now called IsK, Cl) acts as a potent activator of endogenous and otherwise silent K+ or Cl- channels.

Single-channel properties and regulation of pinacidil/glibenclamide-sensitive K+ channels in follicular cells from Xenopus oocyte

Follicular oocytes from Xenopus laevis contain K+ channels that are activated by members of the recently recognized class of vasorelaxants that includes the pinacidil derivative P1060. These channels are blocked by antidiabetic sulphonylureas such as glibenclamide. Opening of the glibenclamide-sensitive K+ channels with P1060 promotes follicular oocyte maturation. Whole-cell and single-channel patch-clamp configurations were used to monitor K+ channel activity in isolated follicular cells. In the presence of micromolar concentrations of intracellular Mg2+ATP, P1060 activated a whole-cell K+ current that was blocked by glibenclamide. The P1060 response was depressed by millimolar concentrations of intracellular ATP and ATP[gamma S]. Single-channel recordings identified two different types of K+ channel. These channels differed in their unitary conductances (19 pS and 150 pS), in their sensitivities to internal Ca2+, to charybdotoxin and to pinacidil and glibenclamide. Only the Ca(2+)-independent K+ channel (19 pS) was activated by the pinacidil derivative and blocked by glibenclamide. Opening of the 19-pS glibenclamide-sensitive K+ channel by P1060 critically required the presence of a low concentration of Mg2+ATP in the intracellular medium. The 19-pS K+ channel was opened by increasing intracellular cAMP. Similar effects were obtained by intracellular application of the catalytic subunit of protein kinase A in the presence of micromolar concentrations of Mg2+ATP. Both acetylcholine and the phorbol ester phorbol 12-myristate 13-acetate blocked the 19-pS K+ channel after it was activated by P1060.

Effects of the level of mRNA expression on biophysical properties, sensitivity to neurotoxins, and regulation of the brain delayed-rectifier K+ channels Kv1.2

Injection of 0.2 ng of cRNA encoding the brain Kv1.2 channel into Xenopus oocytes leads to the expression of a very slowly inactivating K+ current. Inactivation is absent in oocytes injected with 20 ng of cRNA although activation remains unchanged. Low cRNA concentrations generate a channel which is sensitive to dendrotoxin I (IC50 = 2 nM at 0.2 ng of cRNA/oocyte) and to less potent analogs of this toxin from Dendroaspis polylepis venom. A good correlation is found between blockade of the K+ current and binding of the different toxins to rat brain membranes. High cRNA concentrations generate another form of the K+ channel which is largely insensitive to dendrotoxin I (IC50 = 200 nM at 20 ng of cRNA per oocyte). At low cRNA concentrations, the expressed Kv1.2 channel is also blocked by other polypeptide toxins such as MCD peptide (IC50 = 20 nM), charybdotoxin (IC50 = 50 nM), and beta-bungarotoxin (IC50 = 50 nM), which bind to distinct and allosterically related sites on the channel protein. The pharmacologically distinct type of K+ channel expressed at high cRNA concentrations (20 ng of cRNA/oocyte) is nearly totally resistant to 100 nM MCD peptide and hardly altered by charybdotoxin and beta-bungarotoxin at concentrations as high as 1 microM. Both at low and at high cRNA concentrations, the expressed Kv1.2 channel is blocked by an increase in intracellular Ca2+ from the inositol trisphosphate sensitive pools and by the phorbol ester PMA that activates protein kinase C.

Different types of K+ channel current are generated by different levels of a single mRNA

A cloned human voltage-sensitive K+ channel HLK3 which is present in T-lymphocytes and in the brain was expressed in Xenopus oocytes and after permanent transfection of a human B-lymphocyte cell line (IM9). Injections of low cRNA concentrations into Xenopus oocytes led to the expression of a transient K+ current, with saturating current-voltage (I-V) relationship, which was abolished by repetitive stimulations due to a slow recovery from inactivation. This transient K+ channel current was fully inhibited by 10 nM charybdotoxin. Injection of high concentrations of the same RNA led to a non-inactivating K+ current, with linear I-V curve, which did not undergo use-dependent inactivation and was hardly sensitive to 10 nM charybdotoxin. Intermediate behaviour due to changing proportions of these two types of K+ channel expression were observed at intermediate RNA concentrations. Transient and non-inactivating K+ currents were also observed by both whole-cell and single channel patch-clamp recording from HLK3 transfected IM9 cells. The main conductance of the channel in the two different modes (inactivating and charybdotoxin-sensitive or non-inactivating and charybdotoxin-resistant) is the same (12-14 pS). Destruction of the cytoskeletal elements with cytochalasin D, colchicine or botulinum C2 toxin in oocyte experiments prevented expression of the sustained mode of the K+ channel. The results suggest that the sustained mode obtained at high RNA concentrations corresponds to channel clustering involving cytoskeletal elements. This differential functional expression of K+ channels associated with different levels of mRNA appears as a new important factor to explain the biophysical and pharmacological diversity of voltage-sensitive K+ channels.

Regulation of a major cloned voltage-gated K+ channel from human T lymphocytes

When expressed into Xenopus oocytes, HLK3 K+ channel (Kv1-3) induced a slowly inactivating voltage-dependent K+ current. We have studied the modulation of this K+ current by co-expressing a cloned 5-HT2 receptor together with HLK3 K+ channel protein. Application of 5-HT caused a long-lasting inhibition of the voltage-gated K+ current. This inhibitory modulation was mimicked by intracellular injection of inositol triphosphate or Ca2+, as well as by incubation with phorbol esters or diacylglycerol analogs. Oocytes pretreatment with staurosporine and EGTA fully prevented 5-HT inhibitory action. Elevation of cAMP and cGMP levels into oocytes did not produce any detectable effect on the current recorded in the absence or the presence of 5-HT. These data suggest that the second messengers generated by phospholipase C activation may be important modulators of HLK3 K+ channels in the immune and the central nervous systems.

Opening of glibenclamide-sensitive K+ channels in follicular cells promotes Xenopus oocyte maturation

The vasorelaxing K+ channel opener P1060 (a pinacidil analog), gonadotropins, and cAMP were shown to activate a glibenclamide-sensitive 86Rb+ efflux from fully grown follicle-enclosed Xenopus oocytes. Glibenclamide-sensitive K+ channels are located in follicular cells. Glibenclamide (i) depressed the gonadotropin- but not the progesterone-induced maturation and (ii) did not significantly modify progesterone production in oocytes exposed to Xenopus gonadotropin. In follicle-enclosed oocytes, the opener P1060 very significantly enhanced the oocyte sensitivity to progesterone. This increased sensitivity to the hormone induced by the K+ channel opener was reversed by glibenclamide. Thus these results suggest that the opening of glibenclamide-sensitive K+ channels in follicular cells by gonadotropins (and other activators of this channel) induces a hyperpolarization in the oocyte that greatly facilitates maturation by increasing the oocyte sensitivity to progesterone.

Cloning, functional expression, and regulation of two K+ channels in human T lymphocytes

Low stringency screening of a Jurkat cDNA library with a rat brain K+ channel (RCK1) probe has resulted in the isolation of HLK3, a voltage-gated K+ channel. In Xenopus oocytes, the HLK3 clone directs the expression of a rapidly activating transient outward K+ current similar to the type n K+ current recorded in Jurkat T cells. The HLK3 gene is located on the short arm of human chromosome 1 (p13.3). Polymerase chain reaction was used to clone HIsK from Jurkat cDNA. The HIsK clone shares the same sequence with a previously described genomic clone (Murai, T., Kazikuka, A., Takumi, T., Ohkubo, H., and Nakanishi, S. (1989) Biochem. Biophys. Res. Commun. 161, 176-181). In Xenopus oocytes, it encodes a slowly activating, noninactivating K+ channel which cannot be recorded in Jurkat cells by conventional patch-clamp techniques. Transcripts of both clones are present at a similar level before and after activation of purified human T lymphocytes and Jurkat cells, reflecting a constitutive expression of K+ channel messages. This finding is in good agreement with the electrophysiological results for type n K+ current density on the same cells. HLK3 current is very sensitive to the scorpion toxin charybdotoxin (IC50 = 0.8 nM). HIsK current is totally insensitive to this toxin but is blocked by the antiarrhythmic clofilium (IC50 = 80 microM). While charybdotoxin has no effect on interleukin 2 mRNA induction, clofilium potently inhibits interleukin 2 mRNA expression upon mitogen-induced T cell activation. It is concluded that the HLK3 channel is not an important component of the T cell mitogenic response. Other targets for K+ channel blockers, such as the HIsK protein, could be involved in the activation process.

Cloning, expression, pharmacology and regulation of a delayed rectifier K+ channel in mouse heart

Neonatal mouse cardiac poly(A)+ mRNA microinjection into Xenopus oocytes directed the expression of a delayed rectifier K+ current. A cDNA encoding this channel, called mIsK, was cloned from a neonatal mouse heart cDNA library whose properties were studied after expression of the complementary RNA in Xenopus oocytes. Among the different known K+ channel blockers, only the class III antiarrhythmic clofilium inhibited mIsK in the 10-100 microM range. The channel was completely insensitive to other antiarrhythmics such as quinine, quinidine, sotalol or amiodarone. mIsK was enhanced by increasing intracellular Ca2+ and by microinjected Ca(2+)-calmodulin dependent protein kinase II. These stimulations were reversed by the calmodulin antagonist W7. Conversely, the phorbol ester PMA, the diacylglycerol analog OAG and microinjected purified protein kinase C inhibited mIsK. This inhibitory effect could be prevented by the protein kinase C inhibitor staurosporine. These results were consistent with the presence of consensus sequences for kinase II and kinase C in the mIsK structure. Cultured newborn mouse ventricular cardiac cells exhibited a delayed rectifier K+ current which had biophysical properties similar to those of cloned mIsK and which was inhibited by clofilium and protein kinase C activators. In situ hybridization experiments revealed that mIsK mRNA was homogeneously distributed in the cardiac tissue. Neonatal mouse heart expressed the most mIsK mRNA compared with various other rat and mouse tissues. Since this K+ channel generates a current which appears to be involved in the control of both the action potential duration and the beating rate, these results suggest an important role for the mIsK channel in cardiac cell physiology and cardiac pathology.

Two different types of channels are targets for potassium channel openers in Xenopus oocytes

K+ channel openers elicit K+ currents in follicle-enclosed Xenopus oocytes. The most potent activators are the pinacidil derivatives P1075 and P1060. The rank order of potency to activate K+ currents in follicle-enclosed oocytes was: P1075 (K0.5:5 microM) greater than P1060 (K0.5:12 microM) greater than BRL38227 (lemakalim) (K0.5:77 microM) greater than RP61410 (K0.5:100 microM) greater than (-)pinacidil (K0.5:300 microM). Minoxidil sulfate, nicorandil, RP49356 and diazoxide were ineffective. Activation by the K+ channel openers could be abolished by the antidiabetic sulfonylurea glibenclamide. It was not affected by the blocker of the Ca(2+)-activated K+ channels charybdotoxin. The various K+ channel openers failed to activate glibenclamide-sensitive K+ channels in defolliculated oocytes, but BRL derivatives (K0.5 for BRL38226 is 150 microM) and RP61419 inhibited a background current. The channel responsible for this background current is K+ permeable but not fully selective for K+. It is resistant to glibenclamide. It is inhibited by Ba2+, 4-aminopyridine, Co2+, Ni2+ and La3+.

Hormone-regulated K+ channels in follicle-enclosed oocytes are activated by vasorelaxing K+ channel openers and blocked by antidiabetic sulfonylureas

Follicular oocytes from Xenopus laevis contain K+ channels activated by members of the recently recognized class of vasorelaxants that include cromakalim and pinacidil and blocked by antidiabetic sulfonylureas, such as glibenclamide. These channels are situated on the adherent follicular cells and are not present in denuded oocytes. Cromakalim-activated K+ channels are also activated by increases in intracellular cAMP, and cAMP-activated K+ channels are blocked by glibenclamide. Although cromakalim and cAMP effects are synergistic, cromakalim activation of K+ channels is drastically reduced or abolished by treatments that stimulate protein kinase C (e.g., muscarinic effectors, phorbol esters). Gonadotropins, known to play an essential role in ovarian physiology, also activate cromakalim and sulfonylurea-sensitive K+ channels. Follicular oocytes constitute an excellent system for studying regulation of cromakalim-sensitive K+ channels that are important in relation to a variety of disease processes, such as cardiovascular dysfunction and asthma, as well as brain function.

Functional expression of P2Y purinoceptor in Xenopus oocyte injected with brain mRNA

Xenopus oocytes injected with embryonic guinea-pig brain mRNA expressed functional P2Y purinoceptors. Extracellular ATP stimulated in a dose-dependent manner a delayed Ca(2+)-dependent Cl- current component. Analysis of the interactions of ATP with compounds that affect Ca2+ fluxes through the plasma membrane or Ca2+ release from internal stores indicates that ATP raises [Ca2+]i by a mechanism that involves activation of voltage-dependent Ca2+ channels, which leads to influx of extracellular Ca2+ into the cells, as well as release of Ca2+ from intracellular stores. Since this phenomenon was not found in control oocytes, it is suggested that brain mRNA encoded for a newly synthesized Ca(2+)-release process stimulated by purinoceptor activation. This mechanism could be largely involved in the short-term regulation of intracellular Ca2+ level involved in ATP neuromodulation functions.

[The excitation-contraction coupling of the vascular smooth muscle cells]

Two phenomena may lead to an increase in intracellular calcium concentration of vascular smooth muscle cells: an increase in the permeability of the cell membrane to Ca2+ ions; liberation of Ca2+ ions from the intracellular reservoirs. The calcium channels of smooth muscle are varied. There are two types of voltage operated calcium channels: the fast (T) and the slow (L) channels. The calcium channels activated by extracellular membrane receptors are not voltage dependent. Only the L calcium channels are sensitive to dihydropyridines. The liberation of Ca2+ from the sarcoplasmic reticulum which is the intracellular reservoir of calcium can be controlled by two different mechanisms: a direct mechanism by the influx of Ca2+ into the cell through the voltage-operated channels; by the intermediary of a second intracellular messenger. High conductance calcium channels controlled by cytosolic Ca2+ and by IP2 have been demonstrated on the membrane of the sarcoplasmic reticulum. The contraction of smooth muscle may therefore be regulated directly through control of the phosphorylation of the contractile proteins by the intermediary of the systems of adenylate and guanylate cyclase.

Ins(1,4,5)P3 formation and fluctuating chloride current response induced by external ATP in Xenopus oocytes injected with embryonic guinea pig brain mRNA

In voltage-clamped Xenopus oocytes injected with embryonic guinea pig mRNA, effective concentrations of extracellular ATP elicited an inward fluctuating current. This current, carried by Cl-ions, was mainly dependent upon liberation of Ca2+ ions from stores as demonstrated by experiments using intracellular EGTA loading and TMB-8 superfusion. Neomycin inhibited these fluctuating currents indicating that the transplanted purinoceptor is linked to phospholipase C activity and triggers Ins(1,4,5)P3 formation. Ins(1,4,5)P3 production evoked by external ATP was clearly demonstrated by directly measuring the water-soluble Ins(1,4,5)P3 level in injected oocytes. Finally, it is suggested that the ATP effect was mediated by a Ca2+ release from Ins(1,4,5)P3 sensitive pools since heparin blocked the ATP responsiveness. The acquired purinoceptor may be made apparent to a P2 subtype since ATP and ADP were equipotent in eliciting Cl- current while AMP and Adenosine were ineffective in injected oocytes.

An ATP-sensitive conductance in cultured smooth muscle cells from pregnant rat myometrium

The whole cell voltage-clamp technique was used to study the effects of extracellular ATP in cultured smooth muscle cells isolated from pregnant rat myometrium. An inward current was elicited by ATP (IATP) in cells held at -70 mV under voltage clamp. The amplitude of IATP was reduced by estrogen pretreatment and by the end of pregnancy. IATP not only did not undergo any desensitization but showed facilitation. The current-voltage relationship of IATP was linear and reversed close to 0 mV. Changing the sodium electrochemical gradient by decreasing extracellular or intracellular sodium resulted in a linear relationship between the reversal potential of IATP and Na equilibrium potential that, however, differed from the predicted curve for a purely sodium conductance. The conductance activated by ATP was monovalent cation selective with little discrimination between potassium, cesium, and sodium ions. IATP was depressed by divalent cations, and the rank order of potency was Co greater than Mg greater than Ca greater than Ba, suggesting that the free-acid form of ATP was the effective ligand. Adenosine, AMP, and ADP were ineffective in eliciting IATP, whereas ATP gamma S and alpha,beta-methylene ATP were capable of mimicking the effects of ATP, although they were less potent. These results are consistent with the free-acid form of ATP activating a monovalent cation-selective and estrogen-sensitive conductance in myometrium.

Calcium channel current and its sensitivity to (+) isradipine in cultured pregnant rat myometrial cells. An electrophysiological and a binding study

Action of (+) isradipine (PN 200-110), a dihydropyridine derivative, was investigated on the Ca channel current in cultured cells obtained from the longitudinal layer of the pregnant rat myometrium (18-19 days of gestation). Under our experimental conditions, the inward current was attributed to L-type inward current since: (i) equimolar replacement of Ba for Ca induced an increase in the peak current and a decrease in inactivation rate; (ii) residual inward currents were recorded at the end of the pulse; (iii) membrane potential for mid inactivation was about -40 mV; (iv) the voltage dependencies of the peak current elicited from holding potentials of -40 mV and -80 mV were similar. The inward current could be reduced with nanomolar concentrations of (+) isradipine when cells were depolarized by pulses to positive potentials. This was characterized by a pronounced initial blockade, but by no increased in blockade when pulses were repeatedly applied at a frequency of 0.05 Hz. Using the double pulse procedure we confirmed that (+) isradipine did not bind to the open-state of the Ca channels. Voltage-dependence of (+) isradipine blockade was assessed by determining the steady-state availability of the Ca channels. From the shift of the inactivation curve in the presence of (+) isradipine we calculated a (K)I value of 130 pM. Scatchard analysis of the specific binding of (+)[3H] isradipine resulted in a linear plot, thereby indicating specific binding to a single class of sites with a dissociation constant Kd of about 100 pM.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of Ba currents in Xenopus oocyte injected with pregnant rat myometrium mRNA

Using the single electrode voltage-clamp technique, stage V and VI Xenopus oocytes showed a Ba inward current (endogenous IBa) with a peak amplitude of -15 +/- 2 nA in a Cl and Na-free Ba methane sulphonate medium. When oocytes were injected with pregnant rat (18 days gestation) myometrium mRNA, an additional component of Ba current could be detected (exogenous IBa). This inward current could be distinguished from the native IBa by several means: i) peak amplitude (-75 +/- 3 nA); ii) activation voltage threshold; iii) steady-state inactivation parameter; iv) sensitivity to dihydropyridines. The features of the exogenous IBa were compared to those of the inward Ca channel current recorded with the tight-seal patch-clamp technique in single myometrial cells maintained in primary culture.

Calcium channels and excitation-contraction coupling in cardiac cells. II. A pharmacological study of the biphasic contraction in guinea-pig papillary muscle

Biphasic contractions were obtained in guinea-pig papillary muscle by inducing partial depolarization in K+-rich solution (17 mM) in the presence of 0.3 microM isoproterenol. Mn2+ ions inhibited the two components of contraction in a similar way. Nifedipine and particularly Cd2+ ions specifically inhibited the second component of contraction. Isoproterenol and BAY K 8644 markedly increased the amplitude of the second component (P2) of contraction. Nevertheless, a moderate positive inotropic effect of isoproterenol was found on the first component (P1) of contraction when excitability was restored by 0.2 mM Ba instead of isoproterenol. Acetylcholine and hypoxia decreased the amplitude of the second component of contraction to a greater extent. In the presence of digoxin or Na+-free solution, P1 was strongly increased. When sarcoplasmic reticular function was hindered by 1mM caffeine or in the presence of Ca2+-free Sr2+ solution, digoxin always induced a negative inotropic effect on P2. Inversely in these conditions the transient positive inotropic effect of Na+-free solution was strongly reduced. These results are consistent with the hypothesis that the late component of contraction is triggered by the slow inward Ca2+ current and that the early component is due to Ca2+ release from the sarcoplasmic reticulum.

Calcium channels and excitation-contraction coupling in cardiac cells. I. Two components of contraction in guinea-pig papillary muscle

Biphasic contractions have been obtained in guinea-pig papillary muscle by inducing partial depolarization in K+-rich solution (17 mM) containing 0.3 microM isoproterenol; whereas in guinea-pig atria, the same conditions led to monophasic contractions corresponding to the first component of contraction in papillary muscle. The relationships between the amplitude of the two components of the biphasic contraction and the resting membrane potential were sigmoidal curves. The first component of contraction was inactivated for membrane potentials less positive than those for the second component. In Na+-low solution (25 mM), biphasic contraction became monophasic subsequent to the loss of the second component, but tetraethylammonium unmasked the second component of contraction. The relationship between the amplitude of the first component of contraction and the logarithm of extracellular Ca2+ concentration was complex, whereas for the second component it was linear. When Ca2+ ions were replaced by Sr2+ ions, only the second component of contraction was observed. It is suggested that the first component of contraction may be triggered by a Ca2+ release from sarcoplasmic reticulum, induced by the fast inward Ca2+ current and (or) by the depolarization. The second component of contraction may be due to a direct activation of contractile proteins by Ca2+ entering the cell along with the slow inward Ca2+ current and diffusing through the sarcoplasm. These results do not exclude the existence of a third "tonic" component, which could possibly be mixed with the second component of contraction.

[Effects of trimetazidine on a model of in vitro myocardial ischemia]

Cardioprotection is a new concept proposed for clinical situations as myocardial infarction and cardiac surgery. Cardioprotective effects are mainly evaluated in patients by enzymatic, electrocardiographic and sometimes histological findings. The aim of this work was to study the effects of trimetazidine on guinea-pig ventricular myocardium submitted in vitro to conditions mimicking ischaemia. A three step procedure was used: a period of stabilization of 180 minutes under control conditions, then a period of 60 minutes under ischaemic conditions, and last a replacement into control conditions perfusion (30 minutes). Trimetazidine was added in the superfusion during the last hour of stabilization and maintained during the ischaemic and the reperfusion periods. Electrical activities (micro-electrodes) and creatine phosphokinase activity in the effluent were monitorized. Trimetazidine (10(-6) M) prolonged during the ischaemic phase the duration of decremental response and improved electrical recovery during reperfusion. Moreover, trimetazidine (10(-6) M) reduced creatine phosphokinase leakage during ischaemia. The effects of trimetazidine were compared to those observed with calcium channel antagonists in the same model. Thus slowing of enzyme leakage and electrical improvement observed under trimetazidine are compatible with a so-called cardioprotective effect.

Two components of contraction in guinea pig papillary muscle

Biphasic contractions were produced in guinea pig papillary muscle by inducing partial depolarization in a K+ -rich solution (22 mM) containing 10(-6) M isoproterenol. However, when the same conditions were applied to frog and rat, monophasic contractions were obtained. In the case of guinea pig, an increase in the beating frequency produced an increase in amplitude of the first component and a reduction of the second, while in frog and rat, only a decrease in the amplitude of contractions was recorded. Caffeine (10(-3) M) eliminated the first component and increased the second in guinea pig, while in the case of rat and frog it decreased the amplitude of contractions. Procaine (10(-3) M) suppressed the first component and decreased the second one. The contraction in frog appears to be similar to the second component of contraction in guinea pig, while in rat, the contraction is comparable with the first component in guinea pig. It is suggested that the calcium ions which activate the two components of contraction in guinea pig under the given experimental conditions may arise from two different sources.

Estrogenic response in women with amenorrhea during treatment with human menopausal gonadotropin with and without the simultaneous administration of bromocriptine

In an attempt to determine whether prolactin influences estrogen biosynthesis in the ovary, the estrogenic responses of women with amenorrhea under treatment with human menopausal gonadotropin (hMG), with and without the simultaneous administration of the prolactin inhibitor bromocriptine, were investigated in a total of 20 treatment cycles. In five of the six women studied, the addition of bromocriptine produced a urinary excretion of estrogenic compounds 79% higher than that produced by treatment with hMG alone. In one woman with a slightly increased serum prolactin level, the addition of bromocriptine necessitated halving the total hMG dosage. One woman with low endogenous levels of follicle-stimulating hormone (FSH) and a limited response to gonadotropin-releasing hormone showed no increased estrogen excretion after bromocriptine administration over that produced by hMG alone. These results suggest that (1) both elevated and normal serum prolactin levels can have a direct inhibitory effect on the ovary and (2) FSH may be necessary for the formation of prolactin receptors in the ovary.

Placenta scintigraphy using markers of the uterine cervix and pelvic skeleton

A method of placenta scintigraphy is described which provides a differential diagnosis between total and marginal placenta praevia. The uterine cervix, symphysis and 5th lumbar vertebra are included on the scintigram by means of atraumatic markers. Of 101 examined patients 6 had total placenta praevia and 12 marginal placenta praevia. No incorrect evaluation of the scans or complications occurred. The examination supplements ultrasound scanning in late pregnancy.

Changes in serum lipids during treatment with norgestrel, oestradiol-valerate and cycloprogynon

The serum concentrations of triglycerides, cholesterol, and free glycerol were determined in 23 climacteric women, before and after the administration of three different steroid drugs. Each drug was given within a period of 12 weeks (3 cycles). Period I: Norgestrel, 0.5 mg daily from the 12th to 21st day of each cycle. Period II: Oestradiol-valerate (Progynon) 2 mg daily from the 2nd to 21st day of each cycle. Period III: Oestradiol-valerate 2 mg from the 1st to 11th day followed by oestradiol-valerate 2 mg+0.5 mg dl-norgestresl from day 12 to 21 of each cycle (Cycloprogynon). A significant decrease in triglycerides was observed following the administration of norgestrel and Cycloprogynon, whereas oestradiol-valerate had no effect on the triglyceride levels. On the other hand, oestradiol-valerate, following a period of norgestrel, produced an increase in serum cholesterol levels.