Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UNIPROT:P50583 (asymmetrical)
 12,197 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

1. Single Na+-activated K+ channels (K(Na)) were investigated by means of the inside-out patch clamp technique in ventricular myocytes isolated from the guinea-pig heart. 2. Na+-activated K+ channels were observed at very low density (< 9% of patches). In symmetrical (60/60 mM) K+ solutions, K(Na) channels had a mean slope conductance of 75 pS and in asymmetrical (150/70 mM; outside/inside) K+ solutions, they had a mean slope conductance of 220 pS. The reversal potentials obtained under these two ionic conditions were close to the equilibrium potential for K+, suggesting K+ selectivity. 3. In high (98 mM) [Na+]i, the channel showed two open states and up to four closed states, and K(Na) channels also displayed long closures (of the order of seconds). The opening probability (Po) was not voltage dependent. Transient sublevels between 8 and 86% of the main state were identified and appeared to be a common feature of K(Na) channels. 4. Decreasing the activating [Na+]i, reduced Po and this was associated with both an increase in mean closed times and a decrease in mean open times. Lowering [Na+]i also increased the longer closed-time constants and their relative proportions. The first open-time constant was more sensitive to alterations in [Na+]i. 5. Distributions of burst duration, between burst duration and openings within bursts were best described by the sum of two exponentials. Lowering [Na+]i decreased the burst duration and the duration of openings within burst. 6. These observations show that the Na+-activated K+ channel from guinea-pig ventricular myocytes has complex gating and bursting behaviour.
 ...
PMID:Kinetic properties of unitary Na+-dependent K+ channels in inside-out patches from isolated guinea-pig ventricular myocytes. 909 31

We obtained a membrane fraction enriched in the contractile vacuole by aqueous-polymer two-phase partitioning and its channel activities were analysed by incorporating it into artificial planar lipid bilayers. In asymmetrical KCl solutions (cis, 300 mM/100 mM, trans), we observed single-channel currents of a highly K(+)-selective channel with slope conductance of 102 pS and reversal potential of -20.4 mV, which corresponded to PK+/PCl- = 7. They showed bursts separated by infrequent quiescent periods. At 0 mV the mean open time was 2.0 ms. Among monovalent cations, Na+ and Li+ were impermeable, whereas Rb+ showed permeability equivalent to that of K+, although the unitary conductance was apparently reduced when the current flowed from the Rb+ containing side, suggesting that Rb+ is a permeant blocking ion. The open probability within bursts remained constant at approx.0.6 as long as the holding potential was positive on the cis side with respect to the trans side, but it decreased to 0 at negative potential. This channel was blocked by submillimolar concentrations of quinine and 30 mM TEA+. The open probability-voltage relationship showed a striking dependency on the KCl concentration on either side. This channel may play a role in water transport in this organelle.
 ...
PMID:A voltage- and K+-dependent K+ channel from a membrane fraction enriched in contractile vacuole of Dictyostelium discoideum. 916 43

In the cortical collecting duct of the rat two Ca(2+)-dependent K+ channels have been described so far. In the luminal membrane a maxi K+ channel with a single channel conductance of 139 +/- 3 pS in excised membrane patches (n = 91) at 0 mV clamp voltage and asymmetrical KCl-concentrations in pipette and bath was found, while in the basolateral membrane an intermediate conductance K+ channel (85 +/- 1 pS, n = 53) and a small K+ channel (28 +/- 2 pS, n = 15) was described. All these K+ channels had similar pharmacological properties since all could be blocked by the K+ channel inhibitors Ba2+, TEA+, and charybdotoxin. Verapamil, known as a L-type Ca2+ channel blocker, was also capable of inhibiting these K+ channels. While the maxi K+ channel from the luminal membrane was upregulated by intracellular Ca2+ (EC50: 5 microM), the small and the intermediate K+ channel from the basolateral membrane were downregulated (IC50: 10 microM). When the cytosolic Ca(2+)-activity was in the physiological range below 1 microM the activity of the maxi K+ channel was low and regulated via intracellular pH and ATP. Furthermore, when CCD cells were strongly depolarized and under hypoosmotic stress, Ca2+ rose and activated this K+ channel, indicating that this channel is involved in volume regulation. Like the maxi K+ channel the intermediate conductance K+ channel from the basolateral membrane was also sensitive to intracellular changes of pH where acidic pH inhibited while alkaline pH activated this channel. But unlike the K+ channels from the luminal membrane the K+ channel from the basolateral membrane is not regulated by ATP up to 5 mM. The activity of the K+ channels from the basolateral membrane decreased steadily after excision of the membrane. This decrease could be prevented by applying cGMP and MgATP to the bath and thus, activating a membrane-bound cGMP-dependent protein kinase (PKG). The activation of the PKG could be reversed by its specific inhibitor KT5823 (1 microM). Due to the opposite regulation via intracellular Ca2+ and the involvement of different protein kinases a specific and independent regulation of K+ secretion and Na+ reabsorption is possible in the CCD of the rat.
 ...
PMID:Ca(2+)-dependent K+ channels in the cortical collecting duct of rat. 926 90

The structure of three-way DNA junctions with and without extrahelical adenine nucleotides in one strand at the branch point of the junction (i.e., An bulges with n = 0, 1, 2, and 3) has been investigated by fluorescence resonance energy transfer. The structure of the junction without bulged nucleotides was found to have a symmetric trigonal geometry. With bulges, the arrangement of the arms becomes asymmetrical. The energy transfer results suggest a model of bulged junctions where the angle between two of the arms is significantly smaller than between the other two pairs of arms. The acute angle becomes smaller as the number of nucleotides in the bulge increases. The FRET efficiencies of the junctions are the same in the presence of Mg++ and Na+ ions.
 ...
PMID:Global structure of three-way DNA junctions with and without additional unpaired bases: a fluorescence resonance energy transfer analysis. 935 21

1. The nature of the membrane channels underlying the membrane conductance changes induced by the nitric oxide (NO) donors, S-nitroso-L-cysteine (NOCys) and sodium nitroprusside (SNP) were investigated in single myocytes isolated from the circular muscle layer of the guinea-pig proximal colon, by use of standard whole-cell and single channel recording techniques. 2. Under voltage clamp, depolarizing steps from -60 mV elicited a rapidly-developing, little-inactivating outward K+ current (IK) at potentials positive to -40 mV (at 20-25 degrees C). The steady-state level (ISS) of this K current increased in amplitude as the step potential was made to more positive potentials. If the depolarizing steps were made from a holding potential of -80 mV an additional rapidly activating and inactivating outward K+ current was also elicited, superimposed on IK. 3. At 20-25 degrees C, NOCys (2.5 microM), SNP (100 microM) and 8-bromo-cyclic GMP (500 microM) increased the amplitude of ISS of IK elicited from a holding potential of -60 mV. In contrast, NOCys (2-5 microM) had little effect on ISS at 35 degrees C. Higher concentrations (> or = 5 microM at 20-25 degrees C and > or = 10 microM at 35 degrees C) of NOCys decreased the peak amplitude (I[Peak]) and ISS of IK in a concentration-dependent manner. This blockade of IK with NOCys was always associated with an increase of the holding current (IHold), due to the activation of a membrane conductance with a reversal potential between 0 and + 30 mV and which was reduced approximately 50% upon the addition of Cd2+ (1 mM). 4. NOCys (2.5 to 10 microM) or SNP (100 microM) increased the activity of large conductance Ca2+-activated (BK) K' channels in both cell-attached and excised inside-out patches, bathed in either a symmetrical high K+ (130 mM) or an asymmetrically K+ (6 mMout: 130 mMin) physiological saline. Increases in BK channel activity in NOCys (10 microM) or SNP (100 microM) were associated with an increase in the probability of BK channel opening (N.Po), and with a negative shift of the plots of ln(N.Po) against the patch potential, with little change in the slopes of these plots. In cell-attached patches, the increase in N.Po with NOCys was often associated with a decrease in the BK single channel conductance. 5. In both cell-attached and excised patches, NOCys (2.5 to 10 microM) also activated an additional population of channels which allowed inward current flow at potentials positive to EK. In excised inside-out patches bathed in asymmetrical K+ physiological saline, these single channel currents were 2-3 pA in amplitude at -30 mV and reversed in direction near + 10 mV, even if the NaCl (126 mM) concentration in the pipette solution had been replaced with an equimolar concentration of Na gluconate. 6. Under current clamp, NOCys (2.5 microM) and SNP (100 microM) had variable effects on the membrane potential of colonic myocytes, inducing either a small membrane hyperpolarization of <5 mV, or a slowly-developing membrane depolarization of about 5 mV. In contrast, NOCys (5 microM) produced a transient membrane hyperpolarization which was followed by a large depolarization of the membrane potential to positive potentials. The electrotonic potentials elicited in response to an injection of constant hyperpolarizing current (10 pA for 400 ms) were little changed during the NOCys (5 PM)-induced membrane hyperpolarization, but significantly reduced (to 61% of control) during the periods of membrane depolarization. 7. It was concluded that NOCys and SNP, directly increased the number of active BK channels in the membrane of colonic myocytes which leads to a small rapidly oscillating membrane hyperpolarization. The following rebound depolarization in NOCys arises from both the direct opening of a population of cationic channels and the blockade of voltage- and Ca-activated K+ conductances. Finally, the apamin-sensitive K+channels underlying the initial transient hyperpolarization recorded in the intact proximal colon, in response to nerve-released or directly-applied NO, have yet to be identified at the single channel or whole-cell current level.
 ...
PMID:Effects of nitric oxide donors, S-nitroso-L-cysteine and sodium nitroprusside, on the whole-cell and single channel currents in single myocytes of the guinea-pig proximal colon. 950 92

A computer program was developed to allow easy derivation of steady-state velocity and binding equations for multireactant mechanisms including or without rapid equilibrium segments. Its usefulness is illustrated by deriving the rate equation of the most general sequential iso ordered ter ter mechanism of cotransport in which two Na+ ions bind first to the carrier and mirror symmetry is assumed. It is demonstrated that this mechanism cannot be easily reduced to a previously proposed six-state model of Na+-D-glucose cotransport, which also includes a number of implicit assumptions. In fact, the latter model may only be valid over a restricted range of Na+ concentrations or when assuming very strong positive cooperativity for Na+ binding to the glucose symporter within a rapid equilibrium segment. We thus propose an equivalent eight-state model in which the concept of positive cooperativity is best explained within the framework of a polymeric structure of the transport protein involving a minimum number of two transport-competent and identical subunits. This model also includes an obligatory slow isomerization step between the Na+ and glucose-binding sequences, the nature of which might reflect the presence of functionally asymmetrical subunits.
 ...
PMID:Reduction of an eight-state mechanism of cotransport to a six-state model using a new computer program. 953 94

Brownian dynamics simulations have been carried out to study ionic currents flowing across a model membrane channel under various conditions. The model channel we use has a cylindrical transmembrane segment that is joined to a catenary vestibule at each side. Two cylindrical reservoirs connected to the channel contain a fixed number of sodium and chloride ions. Under a driving force of 100 mV, the channel is virtually impermeable to sodium ions, owing to the repulsive dielectric force presented to ions by the vestibular wall. When two rings of dipoles, with their negative poles facing the pore lumen, are placed just above and below the constricted channel segment, sodium ions cross the channel. The conductance increases with increasing dipole strength and reaches its maximum rapidly; a further increase in dipole strength does not increase the channel conductance further. When only those ions that acquire a kinetic energy large enough to surmount a barrier are allowed to enter the narrow transmembrane segment, the channel conductance decreases monotonically with the barrier height. This barrier represents those interactions between an ion, water molecules, and the protein wall in the transmembrane segment that are not treated explicitly in the simulation. The conductance obtained from simulations closely matches that obtained from ACh channels when a step potential barrier of 2-3 kTr is placed at the channel neck. The current-voltage relationship obtained with symmetrical solutions is ohmic in the absence of a barrier. The current-voltage curve becomes nonlinear when the 3 kTr barrier is in place. With asymmetrical solutions, the relationship approximates the Goldman equation, with the reversal potential close to that predicted by the Nernst equation. The conductance first increases linearly with concentration and then begins to rise at a slower rate with higher ionic concentration. We discuss the implications of these findings for the transport of ions across the membrane and the structure of ion channels.
 ...
PMID:Study of ionic currents across a model membrane channel using Brownian dynamics. 967 81

The summer flounder (Paralichthys dentatus) undergoes a true metamorphosis during which the bilaterally symmetrical larva transforms into an asymmetrical juvenile. This study addresses the influence of manipulating thyroid status on larval development and growth. Development was assessed by categorizing larvae in stages based on the position of the translocating eye and growth was assessed as a change in total length. Larvae were raised in seawater or in seawater containing thyroxine-sodium salt (100 ppb, T4) or thiourea (30 ppm, TU). Ambient T4 raised whole-animal T4 concentration almost threefold, whereas treatment with TU reduced whole-animal T4 concentration by 95%. The whole-animal T4 concentrations of untreated larvae increased during metamorphic climax (MC) and were related to developmental stage rather than age. Altering thyroid status of larvae in different stages had different consequences. T4 treatment of late premetamorphic (late pre-M) larvae accelerated their rate of development to prometamorphosis (pro-M), early MC, and mid MC, whereas TU treatment at this stage caused developmental stasis in early MC. T4 treatment of pro-M larvae accelerated development to early MC only and, unlike the response of late pre-M larvae, some of the pro-M larvae treated with TU slowly developed to late MC. TU treatment could inhibit completion of metamorphosis in early MC and mid MC, but not in late MC. Thyroid status of larvae had no effect on growth. However, starting at an age when most untreated and T4-treated larvae were in late MC, they began to grow longer than TU-treated larvae in developmental stasis at early MC and mid MC. Thus, T4 is necessary and sufficient for metamorphosis in summer flounder and, at this single dose, has a more pronounced effect on development at earlier stages.
 ...
PMID:Metamorphosis in the summer flounder (Paralichthys dentatus): stage-specific developmental response to altered thyroid status. 967 87

The transport of L-carnitine by lactating rat mammary tissue has been examined. L-carnitine uptake by rat mammary tissue explants isolated from lactating rats, 3-4 days post partum, was via both Na+-dependent and Na+-independent pathways. The Na+-dependent pathway, the predominant route for L-carnitine uptake, was a saturable process: the Km and Vmax were, respectively, 132 microM and 201 pmol/2 h/mg of intracellular water. The Na+-independent pathway, which was non-saturable, had a coefficient of 0.26 microl/mg of intracellular water/2 h. The Na+-dependent component of L-carnitine uptake by mammary tissue explants was cis-inhibited by D-carnitine and acetyl-L-carnitine, but not by choline or taurine. In contrast, the Na+-independent component of L-carnitine uptake was not affected by any of these compounds. The uptake of L-carnitine by mammary tissue isolated from lactating rats, 10-12 days post partum, was qualitatively similar to that by mammary tissue taken from rats during the early stage of lactation. However, L-carnitine uptake was quantitatively lower: this was attributable to a reduction in the Na+-dependent component of L-carnitine uptake. L-Carnitine efflux from rat mammary tissue taken from animals 3-4 days post partum, consisted of at least two components; a fast extracellular component and a slow membrane-limited component. Reversing the trans-membrane Na+-gradient did not stimulate L-carnitine efflux suggesting that the Na+-dependent L-carnitine carrier operates with asymmetrical kinetics. A hyposmotic shock, hence cell-swelling, increased L-carnitine efflux from mammary tissue explants.
 ...
PMID:Characteristics of L-carnitine transport by lactating rat mammary tissue. 971 31

The depolarization-activated, high-conductance "maxi" cation channel in the plasma membrane of rye (Secale cereale L.) roots is permeable to a wide variety of monovalent and divalent cations. The permeation of K+, Na+, Ca2+ and Ba2+ through the pore could be simulated using a model composed of three energy barriers and two ion binding sites (a 3B2S model), which assumed single-file permeation and the possibility of double cation occupancy. The model had an asymmetrical free energy profile. Differences in permeation between cations were attributed primarily to differences in their free energy profiles in the regions of the pore adjacent to the extracellular solution. In particular, the height of the central free energy peak differed between cations, and cations differed in their affinities for ion binding sites. Significant ion repulsion occurred within the pore, and the mouths of the pore had considerable surface charge. The model adequately described the diverse current vs. voltage (I/V) relationships obtained over a wide variety of experimental conditions. It described the phenomena of non-Michaelian unitary conductance vs. activity relationships for K+, Na+ and Ca2+, differences in selectivity sequences obtained from measurements of conductance and permeability ratios, changes in relative cation permeabilities with solution composition, and the complex effects of Ba2+ and Ca2+ on K+ currents through the channel. The model enabled the prediction of unitary currents and ion fluxes through the maxi cation channel under physiological conditions. It could be used, in combination with data on the kinetics of the channel, as input to electrocoupling models allowing the relationships between membrane voltage, Ca2+ influx and Ca2+ signaling to be studied theoretically.
 ...
PMID:An energy-barrier model for the permeation of monovalent and divalent cations through the maxi cation channel in the plasma membrane of rye roots. 1005 90


<< Previous 1 2 3 4 5 6 7 8 9 10 Next >>