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PubMed1986

Rubidium uptake by mouse pancreatic islets exposed to 6-hydroxydopamine, ninhydrin, or other generators of hydroxyl radicals.

The purpose was to study the toxicity of drugs known to generate free radicals on isolated pancreatic islets. The accumulation of 86Rb+ by mouse pancreatic islets was measured in vitro. Exposing the islets to 6-hydroxydopamine, ninhydrin, or phenazine methosulphate+NADH inhibited the Rb+ uptake, whereas paraquat or acetylphenylhydrazine had no effect. This effect of 6-hydroxydopamine was prevented by either of the hydroxyl radical scavengers, sodium benzoate and mannitol, but not by the non-scavenger, urea; ninhydrin was partially protected against by mannitol but not by benzoate. Protection against 6-hydroxydopamine was also afforded by D-glucose but not by L-glucose or 3-O-methyl-D-glucose; none of the sugars protected against ninhydrin. In damaging islet beta-cells and in being protected against by D-glucose, 6-hydroxydopamine closely resembles the diabetogenic drug, alloxan. It is suggested that protection against alloxan may involve both glucose metabolism and the interaction of glucose with its membrane-located carrier, while protection against 6-hydroxydopamine appears to be unrelated to the hexose carrier mechanism.

Animals↗
PubMed1980

Effects of rubidium, caesium, strontium, barium and lanthanum on ionic currents in myelinated nerve fibres from Xenopus laevis.

A comparative analysis of the effects of externally applied Rb+, Cs+, Ba2+ and La3+ on myelinated fibres under potential clamp conditions was made. The ions Cs+, Ba2+ and La3+ (belonging to groups 1a, 2a and 3a of period 6 in the periodic system) were found to affect the K system in specific ways. Ba2+ and La3+ reduced the outward K+ current associated with positive potential steps; the effect of La3+ being larger than that of Ba2+. The effect of Ba2+ depended on the K+ concentration. Cs+, Ba2+ and La3+ reduced the inward K+ tail current at repolarisation in solutions with high K+ concentration. Cs+ and Ba2+ caused a rectification of the K+ current, i.e. they reduced the inward tail current proportionally more than the outward steady state current. The effect of Ba2+ could not be described as a simple reduction of the permeability constant PK. Rb+ was found to affect the time course of the K system by increasing the rate constant beta n. Sr2+, Ba2+ and La3+ reduced the permeability constant PNa. These ions also shifted potential dependent parameters in general along the potential axis. The effect of La3+ was larger than that of Sr2+ and Ba2+. Cs+, Ba2+ and La3+ reduced the p-current, Ba2+ and La3+ the leak conductance.

Action Potentials↗
PubMed1983

Effects of alpha-adrenoceptor agonists and antagonists on insulin secretion, calcium uptake, and rubidium efflux in mouse pancreatic islets.

Epinephrine, norepinephrine or the more selective alpha-2 adrenoceptor agonist, clonidine, inhibited insulin release from isolated pancreatic islets of lean mice or obese mice homozygous for the gene ob. Clonidine was highly effective at 0.1 mumol/l. In contrast, the preferential alpha-1 adrenoceptor agonist, phenylephrine, had no or only a modest effect at 10 mumol/l. The effects of norepinephrine or clonidine were counteracted by yohimbine, a preferential blocker of alpha-2 receptors, but not by prazosine, an alpha-1 receptor blocker. The glucose-stimulated uptake of 45Ca2+ in the islets was only consistently inhibited by epinephrine. This effect was counteracted by yohimbine. Clonidine had no effect on the release of 86Rb+ from preloaded islets. It is concluded that insulin secretion is suppressed by alpha-2 receptor agonism in the pancreatic beta-cells and that this effect is mediated by mechanisms other than the transmembrane fluxes of calcium or potassium ions.

Adrenergic alpha-Agonists↗
PubMed1986

Potassium and rubidium permeability and potassium conductance of the beta-cell membrane in mouse islets of Langerhans.

The efflux of 42K+ and 86Rb+ has been studied in collagenase-isolated normal mouse islets of Langerhans. In double-labelled experiments, the ratio of efflux rate constants of Rb+ and K+ (kRb/kK) was 0.80 in 5 mM-K+ and 0 glucose. The ratio was unaffected by glucose concentrations up to 22.2 mM. In the presence of 50 mM-K+, 0 glucose, the ratio increased to 0.91 and in 50 mM-Rb+, 0 glucose and 0 K+, the ratio was 1.06. With these limitations, the results indicate that 86Rb+ is an acceptable isotope for K+. Using the Goldman model, K+ and Rb+ permeability coefficients and K+ slope and chord conductances were computed. The permeabilities decreased in glucose and in high K+ or high Rb+. In the case of high external K+, the K+ conductances increased. Also there may be more than one type of K+ channel with differing selectivities to K+ and Rb+. The addition of glucose in the presence of 50 mM-K+ had no further effect on Rb+ permeability. It is suggested that there are about ten small K+ channels open in the resting beta-cell and that progressive closure of these channels is involved in the depolarization of the cell membrane that initiates spike activity.

Animals↗
PubMed1968

Effect of sodium content on sodium efflux from human red cells suspended in sodium-free media containing potassium, rubidium, caesium or lithium chloride.

1. Human red cells treated with lactose solution and loaded with NaCl and BCl subsequently exchange cation with a nutrient BCl medium. B is the same in cells and medium, and is either K, Rb, Cs or Li. In these circumstances Na always moves outwards with the concentration gradient, but the efflux is largely active.2. With suspensions in media containing Ca(2+), the total Na efflux depends on the amount of Na in the cells and on the nature of cation B. Thus for any given value of mean cell Na (Na(m)) in excess of 30 m-equiv/l. cells, the effect of B on the amount of Na efflux is K > Cs > Rb > Li, while with Na(m) between 0.7 and 5 m-equiv/l. cells, the sequence is Cs > Li > K, Rb. With Na(m) between 5 and 30 m-equiv/l. cells, intermediate sequences may be demonstrated for the effects of B on Na efflux. This applies both to the efflux itself and to the flux: concentration ratio, FCR.3. FCR for passive Na efflux in these circumstances is determined by adding strophanthin G to the medium. It varies inversely with the duration of exposure to Ca(2+) in the exchange and nutrient media, but not with the nature of cation B. FCR for passive efflux is probably little affected by the value of Na(m).4. By deducting passive from total Na efflux, active Na efflux is obtained, and variations in the latter with cell Na content, and with B, result in FCR curves similar to those obtained with total Na efflux.5. Total and passive Na efflux have also been measured in Ca-free media. Here the passive efflux is considerable, and with Na + K cells in KCl media FCR increases with Na(m), but in other systems this change is not significant. However, the rate of passive efflux into LiCl media is less than that for KCl or CsCl media. Owing to the magnitude of passive flux in Ca-free systems, the total Na efflux is also increased, but FCR for active Na efflux is quantitatively and qualitatively similar to that occurring in systems containing Ca(2+).6. The effects of B and Na(m) on Na efflux give a series of sequences for B which recall some of those obtainable when chemically modified glass membranes separate solutions of salts, and which are attributable to the charge on the membrane and the hydration of the cations involved. However, certain sequences obtained with red cells do not occur with glass membranes. This difficulty is resolved if it be assumed that throughout the range of Na(m) (0-80 m-equiv/l. cells) active B influx at the external cell face modifies linked Na efflux according to the series K > Rb, Cs > Li, while with Na(m) between 0 and 30 m-equiv/l. cells (and high complementary B), cations escaping passively compete with active efflux of Na inhibiting the latter according to the series K, Rb > Li > Cs. Both these series could theoretically be explained in terms of surface charges and hydration of cations.7. Li-loaded cells in nutrient KCl or other media failed to show active Li efflux.

Biological Transport↗
PubMed1974

Transport of rubidium and sodium in pancreatic islets.

1. Fluxes of (86)Rb(+) and (22)Na(+) were measured in pancreatic islets of ob/ob-mice. The islets, which contain more than 90% beta-cells, were incubated at 37 degrees C in Krebs-Ringer bicarbonate buffer with modifications known to influence insulin release.2. In the presence of Na(+), the islets vigorously accumulated Rb(+). The Rb(+) uptake was inhibited by depletion of islet Na(+) or by 1 mm ouabain or 0.1 mm chloromercuribenzene-p-sulphonic acid. Rb(+) uptake was stimulated by 1 mm-5,5'-dithiobis (2-nitrobenzoic acid) or by depletion of islet Ca(2+), while 20 mm glucose, 5 mm theophylline, 0.1 mm iodoacetamide, or 1 mm-6,6'-dithionicotinic acid had no significant effects.3. The efflux of Rb(+) from preloaded islets followed exponential kinetics with a half-life of about 16 min. The rate of efflux was enhanced by 0.1 mm chloromercuribenzene-p-sulphonic acid and inhibited by 20 mm glucose. Omission of Na(+), K(+) or Ca(2+) from the incubation medium had no significant effects.4. The efflux of (22)Na(+) from islets preloaded with this isotope was enhanced by 0.1 mm chloromercuribenzene-p-sulphonic acid or by Ca(2+) deficiency. It was inhibited by 1 mm ouabain, 0.1 mm-2,4-dinitrophenol, or by omission of Na(+) from the incubation medium. Omission of K(+) or the addition of 20 mm glucose had no significant effects.5. It is concluded that the beta-cells are permeable to Na(+) and Rb(+) and expel Na(+) by an active mechanism similar to, or identical with, the Na(+)/K(+)-pump in other cells. The mechanisms of active and passive cation movements are discussed in relation to current hypotheses of stimulus-secretion coupling in the beta-cells depending on interactions between Na(+) and Ca(2+). In particular, the results support the hypotheses of insulin release being stimulated by ouabain through inhibition of the Na(+)/K(+)-pump and by organic mercurials through enhancement of membrane permeability to cations.

Acetamides↗
PubMed1975

The mechanisms of action of chloromercuribenzene-p-sulphonic acid as insulin secretagogue: fluxes of calcium, sodium and rubidium in islets exposed to mercurial and a membrane-active antagonist.

Chloromercuribenzene-p-sulphonic acid (CMBS) is known to markedly stimulate insulin release and to enhance formation of adenosine 3':5'-cyclic monophosphate (cyclic AMP) and monovalent cation permeability in the pancreatic islet cells. The effects on insulin release and cyclic AMP can be inhibited with 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS). To elucidate the role of cationic fluxes in CMBS- induced insulin release, uptake of 22Na+ and 45Ca2+ as well as efflux of 86Rb+ were studied in islets exposed to 0-1 mM CMBS or 1-0 mM SITS or both. 2. The enhancing effect of CMBS on Na+ permeability, and probably also that on Rb+ permeability, was inhibited by SITS. 3. CMBS stimulated the rate of 45Ca2+ uptakes when the islets were incubated in a poly-anionic bicarbonate buffer but not when they were incubated in Tris buffer containing only Cl- as anion. In bicarbonate buffer, the enhancement of 45Ca2+ flux was observed both with the lanthanum method for measuring intracellular 45Ca2+ uptake and with a method estimating the total islet uptake. SITS had no significant effect on the CMBS-induced 45Ca2+ uptake. 4. Chromatography on Sephadex G-15 did not reveal any significant chemical interaction between 0-1 mM CMBS and 1 mM SITS. 5. The following hypothesis for the recognition of CMBS as insulin secretagogue is suggested: by increasing Na+ permeability more than K+ permeability, CMBS depolarizes the beta-cell, leading to initiation of insulin release by an ionic mechanism which may or may not involve a change in transmembrane Ca2+ fluxes. The marked intensity of the secretory response is due to the fact that CMBS also enhances cyclic AMP formation, potentiating the effect of the ionic mechanisms on the insulin discharge apparatus.

4-Chloromercuribenzenesulfonate↗
PubMed1981

The effects of rubidium ions and membrane potentials on the intracellular sodium activity of sheep Purkinje fibres.

1. Intracellular Na activity, aiNa, was measured in voltage-clamped sheep cardiac Purkinke fibres. 2. Increasing [Rb]0 from 0 to 4 mM in K-free solutions (at a fixed membrane potential) decreased aiNa. Further increases of [Rb]0 (up to 20 mM) had little or no effect. 3. Following exposure to Rb-free, K-free solution, the addition of a test concentration of Rb produced an exponential decrease of aiNa. The rate constant of decay of aiNa increased with increasing [Rb]0 over the measured range (0-20 mM). 4. The accompanying electrogenic Na pump current transient decayed with the same rate constant as aiNa over the range of [Rb]0 examined. During this decay the electrogenic Na pump current was a linear function of aiNa. Increasing [Rb]0 increased the steepness of the dependence of the electrogenic current on aiNa. 5. A constant fraction of the net Na efflux was electrogenic. This fraction was not affected by varying [Rb]0 over the range 0-20 mM. 6. Using a simple model, it is shown that the dependence of steady-state aiNa on [Rb]0 is half-saturated by less than 1 mM-[Rb]0. The rate constant of decay of aiNa and the slope of the relationship between electrogenic Na pump current and aiNa are, however, better fitted with a lower affinity for Rb (K0.5 = 4 mM-[Rb]0). 7. Depolarizing the membrane potential with the voltage clamp decreased aiNa; hyperpolarization increased it. These effects persisted in the presence of 10(-5) M-strophanthidin. An effect of membrane potential on the net passive Na influx can account for the observations. 8. The effects of membrane potential on the net passive Na influx were examined by measuring the maximum rate of rise of aiNa at different holding potentials after inhibiting the Na-K pump in a K-free, Rb-free solution. Depolarization decreased the Na influx. 9. Using the constant field equation, the net passive Na influx was used to estimate the apparent Na permeability coefficient, PNa. This was between 0.8 x 10(-8) and 1.5 x 10(-8) cm sec-1.

Animals↗
PubMed1982

Passive rubidium fluxes mediated by Na-K-ATPase reconstituted into phospholipid vesicles when ATP- and phosphate-free.

1. Phospholipid vesicles reconstituted with Na-K-ATPase from pig kidney, show slow passive pump-mediated (86)Rb fluxes in the complete absence of ATP and phosphate.2. The Rb fluxes are inhibited in vesicles prepared from enzyme pre-treated with either ouabain or vanadate ions. Rb fluxes through Na-K pumps oriented inside-out or right-side out by comparison with the normal cellular orientation can be distinguished by effects of vanadate on one or both sides of the vesicle.3. (86)Rb uptake into Rb-loaded vesicles represents a (86)Rb-Rb exchange. The maximal rate of exchange through inside-out and right-side out oriented pumps is equal, suggesting a random arrangement of the pumps across the vesicle membrane. This Rb-Rb exchange is half-saturated on inside-out and right-side out pumps at about 0.6 and 0.2 mM-external Rb respectively.4. (86)Rb uptake into Rb-free vesicles represents a net Rb flux. The Rb uptake through inside-out pumps has a maximal rate about equal to the Rb-Rb exchange, half-saturates at an external Rb concentration of roughly 0.5 mM, and shows evidence for co-operativity. Net Rb uptake through right-side out pumps is very slow, and half-saturates at roughly 0.1 mM external Rb.5. K ions at low concentrations in the exterior medium stimulate (86)Rb uptake, but at high concentrations, inhibit. Na ions in the exterior medium always inhibit (86)Rb uptake. The result suggests that K ions are transported in co-operative fashion together with Rb ions, while Na ions block the Rb fluxes.6. The presence of Rb congeners at the vesicle interior raises the (86)Rb uptake through inside-out pumps with the decreasing order of effectiveness: Li > Na > Cs > K > Rb. Stimulation by Na ions involves a Rb-Na exchange.7. Turnover numbers were estimated from parallel measurement of Na/K pump mediated fluxes and amount of covalent phosphoenzyme. In units of moles of ion per mole of phosphoenzyme per second at 20 degrees C the following values were obtained: ATP-dependent Na-Rb exchange, 43; (ATP+phosphate)-stimulated Rb-Rb exchange, 7. For (ATP+phosphate)-independent fluxes: Rb-Rb exchange 0.25; net Rb uptake 0.15 and Rb-Na exchange 0.65.8. Mg ions in the exterior medium inhibited both net and exchange Rb fluxes through inside-out pumps in a manner antagonistic with respect to Rb. Mg and vanadate ions inhibit the Rb fluxes in a synergistic fashion.9. The results are interpreted in terms of a model in which net and exchange (86)Rb fluxes occur via conformational transitions between form E(1) which binds Rb at the cytoplasmic face of the protein, the form E(2) (Rb)(occ) containing occluded Rb ions and a form E(2) which binds Rb at the extracellular face of the protein. A kinetic analysis allows us to identify rate-limiting steps of the transport cycle by making use of our transport data in combination with values of rate-constants for conformational transitions observed directly in isolated Na-K-ATPase.

Animals↗
PubMed1982

Effects of atp or phosphate on passive rubidium fluxes mediated by Na-K-ATPase reconstituted into phospholipid vesicles.

1. The passive Rb fluxes mediated by the Na-K pump in reconstituted vesicles, described by Karlish & Stein (1982), are affected by ATP or by phosphate acting separately.2. Rb-Rb exchange through inside-out pumps is stimulated by ATP at low concentrations and is inhibited at high concentrations. There are mutual effects of Rb at cytoplasmic sites and ATP. The higher is the Rb concentration, the greater is the degree of stimulation and the less is the inhibition of exchange by ATP, and the higher are the concentrations of ATP required to produce effects. ATP stimulates Rb-Rb exchange maximally by about 5-fold.3. There are similar effects of ATP on zero-trans net Rb uptake through inside-out pumps. However, much lower degrees of stimulation and greater inhibition of the net flux by ATP are observed, and much lower concentrations of ATP are required for these effects, by comparison with those on Rb-Rb exchange.4. Rb uptake on inside-out pumps in Na-loaded vesicles shows only inhibition by ATP.5. Phosphate effects require the presence of Mg(0) ions. At low Mg(0) concentrations (up to 100 muM) phosphate moderately stimulates Rb uptake into Rb-free or Rb-loaded vesicles (about 50%), but has no effect on Rb uptake into Na-loaded vesicles. At millimolar concentrations of Mg(0) ions, phosphate strongly inhibits the Rb uptake into Rb-free or Na-loaded vesicles but has no effect on Rb uptake into Rb-loaded vesicles.6. The separate effects of ATP and of phosphate are explained in terms of the model proposed by Karlish & Stein (1982), modified to take into account stimulation of the conformational transition E(2)(Rb)(occ) --> E(1) Rb by ATP, and stimulation of the conformational transition E(2)(Rb)(occ) --> E(2) Rb by phosphate due to phosphorylation of the protein.

Adenosine Triphosphate↗