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PubMed1982

Combined effects of ATP and phosphate on rubidium exchange mediated by Na-K-ATPase reconstituted into phospholipid vesicles.

1. Phospholipid vesicles reconstituted with Na-K-ATPase show an (ATP+phosphate)-stimulated Rb-Rb exchange, with properties similar to the K-K exchange of human red cells. This includes a rate 15-20% of the rate of active ATP-dependent Na-K exchange.2. We have studied activation of this Rb-Rb exchange by ATP at fixed phosphate concentrations and by phosphate at fixed ATP concentrations. It is found for both ATP and phosphate that with low concentrations of the fixed ligand an increase in concentration of the complementary ligand produces first stimulation and then inhibition of Rb-Rb exchange. At high concentrations of the fixed ligand the complementary ligand shows only saturation behaviour.3. The pattern of activation and of inhibition by ATP and by phosphate is affected by the Rb(0) concentration in the exterior medium, in that higher concentrations of Rb(0) counteract inhibitory effects of high concentrations of ATP and phosphate.4. (ATP+phosphate)-stimulated Rb-Rb exchange is activated by Rb(0) in the exterior medium along a sigmoid curve. An increase of Rb(i) within the vesicles, which raises the maximal velocity of Rb-Rb exchange, is accompanied by a smaller increase in the Rb(0) concentration required for half-maximal stimulation of the Rb-Rb exchange.5. The data are interpreted in terms of a model similar to those proposed by Karlish & Stein (1982a,b), but extended to include simultaneous effects of ATP and phosphate. Inhibitions by high concentration of ATP or phosphate arise as a result of stabilization of E(1) ATP or E(2)-P forms respectively, in the presence of low concentrations of the complementary ligand. With high concentrations of the fixed ligand, saturation behaviour of the varying ligand is observed because the occluded Rb forms become the dominant transport intermediates. The occluded Rb forms bind both ATP and phosphate weakly and independently. The effects of ATP together with phosphate are accounted for by a simple combination of their separate effects on the Rb-Rb exchange.6. We suggest that the functional role of the occluded Rb form E(2) (Rb)(occ) in active transport is to minimize passive cation leaks through the system and allow control of the direction of cation movements by binding of physiological ligands such as ATP or phosphate.

Adenosine Triphosphate↗

PubMed1982

Block and activation of the pace-maker channel in calf purkinje fibres: effects of potassium, caesium and rubidium.

1. The effects of low concentrations of Cs(+) (0.01-3mM) on the fully activated I-V relation ī(f)(E) for the pace-maker current in calf Purkinje fibres have been investigated. The action of Cs(+) is two-fold: in the negative region of the I-V curve Cs(+) induces a channel blockade; on the other hand, at more positive potentials Cs(+) can produce the opposite effect, i.e. a current increase.2. Cs(+)-induced blockade is concentration- and voltage-dependent, as observed on other cation channels. Data in the far negative voltage range (about - 150 to - 50 mV) can be fitted by a simple block model (Woodhull, 1973), which gives a mean value of 0.71 for the fraction of membrane thickness (delta) crossed by Cs(+) ions before reaching the blocking site. The value of delta does not appear to be affected by either external Na or external K concentrations. Values for the dissociation constant of the blocking reaction at E = 0 mV (k(0)) are found in the range 0.5-3.7 mM. In the positive region of the ī(f)(E) relation the current depression caused by channel blockade vanishes. Unexpectedly, in this range the current can be observed to increase with Cs(+), and ī(f)(E) curves in different Cs(+) concentrations show cross-over.3. Changing external K(+) also produces similar cross-over phenomena. Investigation of this effect reveals that the increase in slope of the I-V curve on raising the external K(+) concentration follows Michaelis-Menten kinetics, and can be interpteted in terms of K(+)-induced channel activation. It is found that 44+/-6 mM-K(+) half-saturates the channel activating reaction.4. The Cs(+)-induced current increase is large in low-K(+) solutions and vanishes in high-K(+) solutions, suggesting a competition between Cs(+) and K(+) ions in their activating action. Increasing Na(+) also limits the Cs(+)-induced current increase.5. Rb(+) also blocks the i(f) channel, though less efficiently than Cs(+). The block caused by Rb(+) is, unlike that of Cs(+), nearly voltage-independent, and is explained by assuming that the blocking reaction occurs near the external mouth of the channel (mean value of delta is 0.05). The zero-voltage dissociation constant (k(0)) of the Rb(+)-blocking reaction ranges between 1.4 and 5.4 mM, and is lower in low-Na(+), high-K(+) solutions.6. A possible characterization of the i(f) channel which explains these results includes an inner ;blocking' site, to which external Cs(+) ions bind, blocking the channel, and a more external ;activatory' site, to which K(+), Cs(+), Rb(+) and possibly Na(+) ions bind. Binding of K(+) to this site induces a current increase either by modulating the channel, or actually by opening the channel itself. A similar mechanism can apply to Cs(+) and to Rb(+) binding.

Animals↗

PubMed1983

Actions of barium and rubidium on membrane currents in canine Purkinje fibres.

The actions of Ba2+ and Rb+, two blockers of background K+ conductance, were investigated. Recent studies performed on ungulate Purkinje fibres have suggested that the pace-maker current is an inward current activated by hyperpolarization. This hypothesis is based on the assumption that Ba2+ reduces the inwardly rectifying background K+ conductance without affecting the pace-maker current. Addition of 5 mM-BaCl2 to the bathing Tyrode solution decreases background K+ permeability and eliminates the reversal of the pace-maker current. The reversal reappears on return to Ba2+-free Tyrode solution. 5 mM-BaCl2 also reduces the time-dependent current at pace-maker potentials positive to about -95 mV in 4 mM-K+ Tyrode solution. The pace-maker current in Ba2+ Tyrode solution usually does not have an exponential time course, and often decays non-monotonically. It can take more than two minutes to reach a steady state. The fast initial component of membrane current, which is observed on hyperpolarizing in the pace-maker potential range in Purkinje fibres and which has been called the 'depletion current', is still present in Ba2+ Tyrode solution, but is reduced or eliminated if 10 mM-CsCl is added to the Ba2+ Tyrode solution. The addition of Cs+ is accompanied by an outward shift in membrane current in Ba2+ Tyrode solution. Ba2+ reduces the background K+ permeability in a dose-dependent manner. Addition of between 0.5 and 1 mM-BaCl2 achieves a maximum effect. Raising the amount of BaCl2 above this level reduces the time-dependent current even when no further effect on background permeability is observed. Rb+ substitution for K+ reduces the magnitude of the pace-maker current at potentials positive to -100 mV, eliminates the reversal of the pace-maker current, shifts the activation range to more negative potentials, and decreases the voltage dependence of pace-maker current kinetics. Rb+ addition to Tyrode solution has little effect on pace-maker current magnitude or time course positive to -90 mV, but does shift the reversal to more negative potentials. The available evidence suggests that the pace-maker current in Ba2+ Tyrode solution is an inward current activated by hyperpolarization. However, Ba2+ blocks an unknown fraction of the pace-maker current in a dose-dependent, and possibly voltage-dependent manner. Also, the presence of a slow component of pace-maker decay suggests that the standard Hodgkin-Huxley formalism for the analysis of pace-maker currents is inappropriate.

Action Potentials↗

PubMed1986

The effects of rubidium ions on components of the potassium conductance in the frog node of Ranvier.

The effects of replacement of external and internal K+ ions by Rb+ ions on the two fast components (gf1 and gf2) and slow component (gs) of the K+ conductance (gK) in frog nodes of Ranvier were investigated under voltage- and current-clamp conditions. Fast and slow components of gK were separated by double exponential fits to tail currents following long depolarizing pre-pulses, or by the use of short pre-pulses which activate little gs X gs was also isolated by 1 mM-4-aminopyridine (4-AP). gf1 and gf2 were distinguished in the fast conductance-voltage curve by their different voltage dependences, gf1 activating at more negative potentials. Reversal potential measurements indicated that Rb+ is less permeant than K+, and measurements in 4-AP indicated that the slow component has a lower Rb+ permeability than the fast. In a 50% K+, 50% Rb+ mixture PRb/PK was less than that in 100% Rb+ suggesting that PRb/PK is mole-fraction dependent. With external Rb+ the current-voltage relation was shifted by ca.-10 mV compared to that in K+, an effect on gf ( = gf1 + gf2). The slow conductance (gs) and, under similar conditions, the Na+ current-voltage relation were not shifted. gf, calculated from inward tail currents, was reduced with external Rb+ at potentials where gf2 was activated. Instantaneous current-voltage relations following pre-pulses which activate different components of gf confirmed these observations. In K+ the instantaneous current-voltage relation showed some inward rectification which was largely abolished with Rb+. Comparison of gf calculated from outward (go) and inward (gi) currents confirmed this, and showed that inward gf2 was reduced with Rb+ such that go = gi. Outward currents were little affected by external Rb+. External Rb+ slowed the fast inward tail current following all pre-pulses which activate gf, but had no effect on the time course of the slow component of the tail current. Regenerative responses, which occur in high [K+] (+300 nM-tetrodotoxin) solutions in current clamp did not repolarize in Rb+. Voltage-clamp experiments showed that inactivation of inward currents is slowed when Rb+ is the charge carrier. Replacement of internal K+, by application of Rb+ to the cut ends of the fibre, shifted the reversal potential to more positive potentials but had no effect on the conductance or kinetics. External Rb+ has a large number of effects on inward currents, but little effect on outward currents. Internal Rb+ had little effect on outward or inward currents.(ABSTRACT TRUNCATED AT 400 WORDS)

4-Aminopyridine↗

PubMed1989

Rubidium and sodium permeability of the ATP-sensitive K+ channel in single rat pancreatic beta-cells.

1. The patch-clamp method has been used to study the selectivity of single ATP-sensitive potassium channels in excised membrane patches from dissociated rat pancreatic beta-cells. 2. In symmetrical K+ concentrations the current-voltage relation of this channel showed slight inward rectification. The K+ permeability coefficient was 1.05 x 10(-13) cm3/s ([K+]o = 140 mM; 20 degrees C) in the inside-out patch and somewhat smaller when measured under the same conditions in the outside-out configuration (0.86 x 10(-13) cm3/s). 3. When intracellular Rb+ replaced K+, inward K+ currents were unaffected but the outward currents carried by Rb+ were substantially smaller. The extent of the reduction in the outward currents depended on the internal Rb+/K+ ratio and increased as [Rb+]i was raised. Both inward and outward Rb+ currents were blocked by 1 mM-ATP. No currents were measurable in symmetrical 140 mM-Rb+ solutions. 4. With 140 mM [K+]o and 140 mM [Rb+]i the single-channel current-voltage relation reversed at +8 mV and the potential at which the variance of the ATP-sensitive current was least was shifted by 6 mV to more positive potentials. These data suggest a PRb/PK ratio of around 0.7. 5. Outward Rb+ currents were reduced at all potentials in inside-out patches exposed to 107 mM-Rb+ solution intracellularly and 5 mM-K+ externally. 6. Partial replacement of external K+ with Rb+ substantially reduced the inward currents recorded from outside-out patches and also decreased outward K+ currents. 7. In outside-out patches, the addition of 1 mM-Rb+ to the external solution produced a block of inward K+ currents that initially increased and then decreased again with hyperpolarization. This suggests that the Rb+ block of K+ currents is voltage dependent and that Rb+ acts as a permeant blocker of K+ currents. 8. The sodium permeability of the channel, relative to that of potassium, was 0.39 for internal Na+ and 0.007 for external Na+ ions. 9. We conclude that Rb+ serves as an acceptable tracer for K+ in efflux studies when changes in K+ flux through ATP-sensitive K+ channels are of interest but that the magnitude of such fluxes will be considerably underestimated.

Adenosine Triphosphate↗

PubMed2003

Tight coupling of rubidium conductance and inactivation in human KCNQ1 potassium channels.

KCNQ1 K+ channels in humans are important for repolarization of cardiac action potentials and for K+ secretion in the inner ear. The pore-forming channel subunits form heteromeric complexes with small regulatory subunits of the KCNE family, in particular with KCNE1 to form channels that conduct a slow delayed rectifier K+ current, IKs. This association leads to alteration of biophysical properties, including a slowing of activation, a suppression of inactivation and an increase of the apparent single-channel conductance. In addition, inward Rb+ currents conducted by homomeric KCNQ1 channels are about threefold larger than K+ currents, whereas heteromeric KCNQ1-KCNE1 channels have smaller inward Rb+ currents compared to K+ currents. We determined inactivation properties and compared K+ vs. Rb+ inward currents for channels formed by co-assembly of KCNQ1 with KCNE1, KCNE3 and KCNE5, and for homomeric KCNQ1 channels with point mutations in the pore helix S5 or S6 transmembrane domains. Several of the channels with point mutations eliminated the apparent inactivation of KCNQ1, as described previously (Seebohm et al. 2001). We found that the extent of inactivation and the ratio of Rb+/K+ currents were positively correlated. Since the effect of Rb+ on the current size has been shown previously to be related to a fast 'flickery' process, our results suggest that inactivation of KCNQ1 channels is related to a fast flicker of the open channel. A kinetic model incorporating two open states, no explicit inactivated state and a fast flicker that is different for the two open states is able to account for the apparent inactivation and the correlation of inactivation and large Rb+ currents. We conclude that an association between KCNQ1 and KCNE subunits or removal of inactivation by mutation of KCNQ1 stabilizes the open conformation of the pore principally by altering an interaction between the pore helix and the selectivity filter and with S5/S6 domains.

Action Potentials↗

PubMed1955