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K+ Efflux (k+ + efflux)
Selected AbstractsDevelopment of an Improved Technique for the Perfusion of the Isolated Caudal Lobe of Sheep LiverEXPERIMENTAL PHYSIOLOGY, Issue 5 2000A. M. Ali The study was designed to develop an improved technique for perfusing the isolated caudal lobe of sheep liver. Twenty caudal lobes were perfused for 3-4 h, in a non-recirculating mode, with Krebs-Henseleit bicarbonate buffer. The perfusion system was designed to give a constant flow. The hepatic viability and functional normality of the perfused lobe were assessed by measuring the perfusion flow rate, pH, K+ efflux, O2 uptake, substrate uptake, gluconeogenesis from propionate and amino acids, and ureagenesis from ammonia and amino acids. Liver tissue was sampled for histological examination, as well as for the determination of liver glycogen and wet: dry weight ratio. The perfusion flow rate and pH were both stable throughout the perfusion. The potassium concentration in the effluent perfusate did not increase during the perfusion, suggesting that there was no loss of viability or hypoxia. The perfused lobe extracted more than 50% of the O2 supply. The rate of oxygen consumption was comparable to the rate reported in vivo. The initial glycogen content was reduced by about 40% after 4 h perfusion. The wet: dry weight ratio was 3.6, consistent with the absence of tissue oedema. Urea production was stimulated when NH4Cl (0.3 mM) was added to the medium but there was no significant increase in urea release when alanine (0.15 mM), glutamine (0.2 mM) or lysine (0.2 mM) was added. Urea production, however, increased by about 171% when a physiological mixture of amino acids was added. Propionate (0.5 mM), alanine and glutamine stimulated glucose production but not lysine or the complete amino acid mixture. Glutamine release was lower than that reported in the rat liver. Changing the direction of flow also revealed an apparent difference between livers from sheep and rats in their metabolism of ammonia. The improved technique offers a simple practical and inexpensive approach to many problems in ruminant physiology and nutritional biochemistry. [source] Membrane depolarization induces K+ efflux from subapical maize root segmentsNEW PHYTOLOGIST, Issue 1 2002Fabio F. Nocito Summary ,,The role of potassium efflux from maize (Zea mays) root segments in maintaining transmembrane electric potential difference (Em) was studied in vivo, together with the involvement of outward rectifying K+ channels (ORCs). ,,Measurements were made of the efflux of potassium (K+) from roots when its uptake was competitively inhibited by rubidium (Rb+), of the Em of the root cells by microelectrodes and of the unidirectional fluxes of monovalent cations. ,,The influx of Rb+, caesium (Cs+) or ammonium (NH4+) into the segments induced an efflux of K+. Lithium (Li+) and sodium (Na+) were not taken up and did not induce K+ efflux. The permeating cations induced membrane depolarizations, which were closely related to the values of K+ efflux. Two K+ -channel blockers, tetraethylammonium-chloride and quinidine, inhibited K+ efflux. The inhibition was accompanied by a higher membrane depolarization induced by Rb+, whose influx was not affected. ,,The results suggest that a depolarizing event caused by cation uptake increased K+ efflux from the cells, probably through the activation of ORCs involved in restoration and stabilization of Em. [source] The cyclic nucleotide-gated channel, AtCNGC10, influences salt tolerance in ArabidopsisPHYSIOLOGIA PLANTARUM, Issue 3 2008Kun-Mei Guo Cyclic nucleotide-gated channels (CNGCs) in the plasma membrane transport K+ and other cations; however, their roles in the response and adaptation of plants to environmental salinity are unclear. Growth, cation contents, salt tolerance and K+ fluxes were assessed in wild-type and two AtCNGC10 antisense lines (A2 and A3) of Arabidopsis thaliana (L.) Heynh. Compared with the wild-type, mature plants of both antisense lines had altered K+ and Na+ concentrations in shoots and were more sensitive to salt stress, as assessed by biomass and Chl fluorescence. The shoots of A2 and A3 plants contained higher Na+ concentrations and significantly higher Na+/K+ ratios compared with wild-type, whereas roots contained higher K+ concentrations and lower Na+/K+ ratios. Four-day-old seedlings of both antisense lines exposed to salt stress had smaller Na+/K+ ratios and longer roots than the wild-type. Under sudden salt treatment, the Na+ efflux was higher and the K+ efflux was smaller in the antisense lines, indicating that AtCNGC10 might function as a channel providing Na+ influx and K+ efflux at the root/soil interface. We conclude that the AtCNGC10 channel is involved in Na+ and K+ transport during cation uptake in roots and in long-distance transport, such as phloem loading and/or xylem retrieval. Mature A2 and A3 plants became more salt sensitive than wild-type plants because of impaired photosynthesis induced by a higher Na+ concentration in the leaves. [source] The K+,Cl, cotransporter KCC2 promotes GABAergic excitation in the mature rat hippocampusTHE JOURNAL OF PHYSIOLOGY, Issue 9 2010Tero Viitanen GABAergic excitatory [K+]o transients can be readily evoked in the mature rat hippocampus by intense activation of GABAA receptors (GABAARs). Here we show that these [K+]o responses induced by high-frequency stimulation or GABAA agonist application are generated by the neuronal K+,Cl, cotransporter KCC2 and that the transporter-mediated KCl extrusion is critically dependent on the bicarbonate-driven accumulation of Cl, in pyramidal neurons. The mechanism underlying GABAergic [K+]o transients was studied in CA1 stratum pyramidale using intracellular sharp microelectrodes and extracellular ion-sensitive microelectrodes. The evoked [K+]o transients, as well as the associated afterdischarges, were strongly suppressed by 0.5,1 mm furosemide, a KCl cotransport inhibitor. Importantly, the GABAAR-mediated intrapyramidal accumulation of Cl,, as measured by monitoring the reversal potential of fused IPSPs, was unaffected by the drug. It was further confirmed that the reduction in the [K+]o transients was not due to effects of furosemide on the Na+ -dependent K+ -Cl, cotransporter NKCC1 or on intraneuronal carbonic anhydrase activity. Blocking potassium channels by Ba2+ enhanced [K+]o transients whereas pyramidal cell depolarizations were attenuated in further agreement with a lack of contribution by channel-mediated K+ efflux. The key role of the GABAAR channel-mediated anion fluxes in the generation of the [K+]o transients was examined in experiments where bicarbonate was replaced with formate. This anion substitution had no significant effect on the rate of Cl, accumulation, [K+]o response or afterdischarges. Our findings reveal a novel excitatory mode of action of KCC2 that can have substantial implications for the role of GABAergic transmission during ictal epileptiform activity. [source] External K+ modulates the activity of the Arabidopsis potassium channel SKOR via an unusual mechanismTHE PLANT JOURNAL, Issue 2 2006Ingela Johansson Summary Plant outward-rectifying K+ channels mediate K+ efflux from guard cells during stomatal closure and from root cells into the xylem for root,shoot allocation of potassium (K). Intriguingly, the gating of these channels depends on the extracellular K+ concentration, although the ions carrying the current are derived from inside the cell. This K+ dependence confers a sensitivity to the extracellular K+ concentration ([K+]) that ensures that the channels mediate K+ efflux only, regardless of the [K+] prevailing outside. We investigated the mechanism of K+ -dependent gating of the K+ channel SKOR of Arabidopsis by site-directed mutagenesis. Mutations affecting the intrinsic K+ dependence of gating were found to cluster in the pore and within the sixth transmembrane helix (S6), identifying an ,S6 gating domain' deep within the membrane. Mapping the SKOR sequence to the crystal structure of the voltage-dependent K+ channel KvAP from Aeropyrum pernix suggested interaction between the S6 gating domain and the base of the pore helix, a prediction supported by mutations at this site. These results offer a unique insight into the molecular basis for a physiologically important K+ -sensory process in plants. [source] Modulation of insulin release by adenosine A1 receptor agonists and antagonists in INS-1 cells: The possible contribution of 86Rb+ efflux and 45Ca2+ uptakeCELL BIOCHEMISTRY AND FUNCTION, Issue 8 2008M. Töpfer Abstract Due to the lack of specific agonists and antagonists the role of adenosine receptor subtypes with respect to their effect on the insulin secretory system is not well investigated. The A1 receptor may be linked to different 2nd messenger systems, i.e. cAMP, K+ - and 45Ca2+ channel activity. Partial A1 receptor agonists are going to be developed in order to improve diabetes (increase in insulin sensitivity, lowering of FFA and triglycerides). In this study newly synthesized selective A1 receptor agonists and antagonists were investigated thereby integrating three parameters, insulin release (RIA), 45Ca2+ uptake and 86Rb+ efflux (surrogate for K+ efflux) of INS-1 cells, an insulin secretory cell line. The presence of A1 -receptors was demonstrated by Western blotting. The receptor nonselective adenosine analogue NECA (5,- N -ethylcarboxyamidoadenosine) at high concentration (10,µM) had no effect on insulin release and 45Ca2+ uptake which could be interpreted as the sum of effects mediated by mutual antagonistic adenosine receptor subtypes. However, an inhibitory effect mediated by A1 receptor agonism was detected at 10,nM NECA and could be confirmed by adding the A1 receptor antagonist PSB-36 (1-butyl-8-(3-noradamantyl)-3-(3-hydroxy-propyl)xanthine). NECA inhibited 86Rb+ efflux which, however, did not fit with the simultaneous inhibition of insulin secretion. The selective A1 receptor agonist CHA (N6 -cyclohexyladenosine) inhibited insulin release; the simultaneously increased Ca2+ uptake (nifedipine dependent) and inhibition of 86Rb+ efflux did not fit the insulin release data. The CHA effect (even the maximum effect at 50,µM) can be increased by 10,µM NECA indicating that CHA and NECA have nonspecific and physiologically non-relevant effects on 86Rb+ efflux in addition to their A1 -receptor interaction. Since PSB-36 did not influence the NECA-induced inhibition of 86Rb+ efflux, the NECA effect is not mediated by potassium channel-linked A1 receptors. The nonselective adenosine receptor antagonist caffeine increased insulin release which was reversed by CHA as expected when hypothesizing that both act via A1 receptors in this case. In conclusion, stimulation of A1 receptors by receptor selective and nonselective compounds reduced insulin release which is not coupled to opening of potassium channels (86Rb+ efflux experiments) or inhibition of calcium channels (45Ca2+ uptake experiments). It may be expected that of all pleiotropic 2nd messengers, the cAMP system (not tested here) is predominant for A1 receptor effects and the channel systems (K+ and Ca2+) are of minor importance and do not contribute to insulin release though being coupled to the receptor in other tissues. Copyright © 2008 John Wiley & Sons, Ltd. [source] | ||||||||||||||||||||||