The control by internal calcium of membrane permeability to sodium and potassium

Nonlinear relationship between dietary calcium and magnesium intake and peripheral neuropathy in the general population of the United States

Background

Calcium and magnesium are essential minerals that have significant roles in nerve function and regulation. There may be a correlation between dietary calcium and magnesium intake and peripheral neuropathy. However, this relationship remains unclear and requires further study.

Methods

Data from 7,726 participants in the National Health and Nutrition Examination Survey (NHANES) from 1999 to 2004 were analyzed in this study. The relationship between total dietary calcium and magnesium intake, as well as each quantile, and peripheral neuropathy was analyzed using a multifactor logistic regression model. To illustrate the dose-response relationship between calcium and magnesium intake and peripheral neuropathy, we utilized a restricted cubic spline (RCS) plot.

Results

Our analysis found a positive correlation between dietary intake of calcium and magnesium and peripheral neuropathy (calcium: OR 1.000, 95% CI 1.000-1.000; magnesium: OR 1.001, 95% CI 1.00-1.002). Participants in the first and third quantiles of dietary calcium intake had a significantly higher incidence of peripheral neuropathy than those in the second quantile (OR 1.333, 95% CI 1.034-1.719, OR 1.497, 95% CI 1.155-1.941). Those in the first and third quantiles of dietary magnesium intake also had a significantly higher incidence of peripheral neuropathy than those in the second quantile (OR 1.275, 95% CI 1.064-1.528, OR 1.525, 95% CI 1.231-1.890). The restricted cubic spline analysis revealed a U-shaped nonlinear relationship between dietary intake of calcium and magnesium and peripheral neuropathy.

Conclusion

The study found a U-shaped non-linear relationship between dietary calcium and magnesium intake levels and peripheral neuropathy, indicating that both excessive and insufficient intake of calcium and magnesium can increase the incidence of peripheral neuropathy.

A mechanism for spike frequency adaptation

1. Spike frequency adaptation was studied in large neurones of the marine molluscs Archidoris montereyensis and Anisodoris nobilis. These cells respond to a current step with a rapid rise in spike frequency followed by a gradual decline to a new steady level.2. An exponentially declining current, I(s), was measured when the cell was voltage clamped following an adapting spike train. The initial amplitude of this current depended on the preceding number of spikes and on the voltage to which the cell was clamped. A reversal potential (V(s)) for this current was obtained by clamping to various potentials following a spike train. The time constant (tau(s)) of decay of the current was dependent upon the clamping potential.3. Clamping the membrane potential to a constant test level from various initial levels initiates an exponentially decaying current of similar time constant. The voltage dependence of the steady-state conductance (g(s)a(s)(V, infinity)) associated with this current was determined using this technique.4. Equations for neural repetitive firing (Connor & Stevens, 1971c) were modified by the addition of a term describing these slow membrane currents: [Formula: see text]. The solution to the modified equation was in good agreement with the spike frequency adaptation observed in these cells.

Caffeine activates a mechanosensitive Ca(2+) channel in human red cells

Caffeine is known to activate influx of both mono- and divalent cations in various cell types, suggesting that this xanthine opens non-selective cation channels at the plasma membrane. This possibility was investigated in human erythrocytes, studying the caffeine action on net Ca(2+), Na(+) and K(+) movements in ATP-depleted cells. Whole populations and subpopulations of young and old erythrocytes were employed. Caffeine was tested in the presence of known mechanosensitive channel blockers (Gd(3+), neomycin and amiloride) and ruthenium red as a possible inhibitor. Caffeine enhanced net cation fluxes in a concentration-dependent way. In whole populations, the Ca(2+) entry elicited by 20 mM caffeine was fully suppressed by Gd(3+) (5 microM), amiloride (250 microM) and ruthenium red (100 microM) and partially blocked by neomycin (100 microM). The above blockers also inhibited caffeine-dependent Na(+) entry whilst showing antagonistic effects on the corresponding K(+) efflux. These compounds fully suppressed hypotonically-induced (-35 mOsm/kg) Ca(2+) influx at nearly the same concentrations completely blocking caffeine-stimulated Ca(2+) entry. The effect of inhibitors on Ca(2+) influx in young cells exceeded that in old cells at similar concentrations. The results clearly show that caffeine stimulates a stretch-activated Ca(2+) channel in human red cells and that aged cells are less susceptible to mechanosensitive channel blockers.

Differences in Ca2+ pumping activity between sub-populations of human red cells

It has been shown recently that the free Ca2+ content of human red cells rises during ageing in vivo. With the aim of determining the mechanisms involved in such a change, we have investigated some aspects of Ca2+ homeostasis. Both the initial rate of Ca2+ influx and some kinetic parameters of the Ca2+ pump of human red cells were studied in light and dense sub-populations obtained through stringent, self-formed Percoll gradients. At 37 degrees C and pH 7.4, no differences in Ca2+ entry were found. By contrast, either at pH 7.0 or 7.4, the maximal Ca2+ extrusion rate of the approximately 10% heaviest cells was one-half of the corresponding lighter ones. The results demonstrate that the elevated free Ca2+ concentration distinctive of senescent cells, arises from a reduction in Ca2+ extrusion capacity during ageing. The possible physiological significance of this finding is discussed.

The effects of gossypol and chloroquine interaction on serum electrolytes of protein-malnourished rats

Serum sodium (Na), potassium (K), chloride (Cl), phosphate (Po), and calcium (Ca) were assayed in adult male Wistar rats fed a low protein diet (LP) and a normal protein diet (control, NP) and administered gossypol (GP) and chloroquine (CQ) alone and in combination (GPCQ), for 4 and 8 weeks. Gossypol was administered at 20 mg/kg body wt/rat/day in Goya oil and chloroquine at 5 mg/kg body wt/rat for 3 days/week in distilled water. The data, analyzed by two-way ANOVA, showed that the interaction had significant effects on the serum levels of Na, K and Po (P < 0.001) but not on chloride and calcium (P > 0.05). Serum levels of sodium, potassium and phosphate in LP rats were significantly lower than in NP rats. In LP and NP rats, the levels of Na, K, and Po at 8 weeks were significantly higher than at 4 weeks; and GPCQ treatment produced significantly lower sodium levels in LP and NP rats, lower phosphate in LP rats and higher phosphate in NP-fed rats than GP or CQ administered alone. Potassium level in GPCQ was between levels in Gp- and CQ-treated rats. The administration of gossypol and chloroquine together may not adversely affect serum electrolytes in protein-malnutrition state any more than gossypol or chloroquine administered alone.

Comparative aspects of Na(+)-K+ and Ca(2+)-Mg2+ ATPase in erythrocyte membranes of various mammals

This work is a comparative study of Na(+)-K+ and Ca(2+)-Mg2+ ATPase associated with the erythrocyte plasma membranes in different mammals. The method used to test the activity of these enzymes is based on quantitative measurements of ADP released during the reaction with HPLC: the chromatographic type is an Ion-Pair Reversed Phase. We have found that the levels of Ca2+ stimulated ATPase are higher than those of Na(+)-K+ ATPase in red blood cells of all the different mammalian species, with the only exception being lamb erythrocytes where the values of both the ATPase activities are almost equal. The results obtained have shown different levels of the Na(+)-K+ ATPase as well as of the Ca2+ stimulated ATPase activity. Furthermore, we have made a comparative study of the ATPase activities in different red blood cells with specific reference to the optimum pH, thermostability, kinetic characteristics and the inhibitory effects of ouabain and vanadate.

Metabolic control of the K+ channel of human red cells

The effects of cAMP, ATP and GTP on the Ca2(+)-dependent K+ channel of fresh (1-2 days) or cold-stored (28-36 days) human red cells were studied using atomic absorption flame photometry of Ca2(+)-EGTA loaded ghosts which had been resealed to monovalent cations in dextran solution. When high-K+ ghosts were incubated in an isotonic Na+ medium, the rate constant of Ca2(+)-dependent K+ efflux was reduced by a half on increasing the theophylline concentration to 40 mM. This effect was observed in ghosts from both fresh and stored cells, but only if they were previously loaded with ATP. The inhibition was more marked when Mg2+ was added together with ATP, and it was abolished by raising free Ca2+ to the micromolar level. Like theophylline, isobutyl methylxanthine (10 mM) also affected K+ efflux. cAMP (0.2-0.5 mM), added both internally and externally (as free salt, dibutyryl or bromide derivatives), had no significant effect on K+ loss when the ghost free-Ca2+ level was below 1 microM, but it was slightly inhibitory at higher concentrations. The combined presence of cAMP (0.2 mM) plus either theophylline (10 mM), or isobutyl methylxanthine (0.5 mM), was more effective than cAMP alone. This inhibition showed a strict requirement for ATP plus Mg2+ and it was not overcome by raising internal Ca2+. Ghosts from stored cells seemed more sensitive than those from fresh cells, to the combined action of cAMP and methylxanthines. Loading ATP into ghosts from fresh or stored cells markedly decreased K+ loss. Although this effect was observed in the absence of added Mg2+ (0.5 mM EDTA present), it was potentiated upon adding 2 mM Mg2+. The K+ efflux from ATP-loaded ghosts was not altered by dithio-bis-nitrobenzoic acid (10 mM) or acridine orange (100 microM), while it was increased two- to fourfold by incubating with MgF2 (10 mM), or MgF2 (10 mM) + theophylline (40 mM), respectively. By contrast, a marked efflux reduction was obtained by incorporating 0.5 mM GTP into ATP-containing ghosts. The degree of phosphorylation obtained by incubating membranes with (gamma-32P)ATP under various conditions affecting K+ channel activity, was in direct correspondence to their effect on K+ efflux. The results suggest that the K+ channel of red cells is under complex metabolic control, via cAMP-mediated and nonmediated mechanisms, some which require ATP and presumably, involve phosphorylation of the channel proteins.

Distinction of two components of passive Ca2+ transport into human erythrocytes by Ca2+ entry blockers

The nature of downhill Ca2+ net-transport into human erythrocytes was investigated using the experimental models of Ca2+ pump inhibition by vanadate and of intracellular chelation of Ca2+ by quin2. Ca2+ uptake by erythrocytes loaded with 0.5 mM vanadate and suspended in 145 mM Na+ -5 mM K+ media was reduced by about 60% when medium K+ was raised to 80 mM. Organic and inorganic Ca2+ entry blockers such as nifedipine (10(-5) M), verapamil (10(-4) M), diltiazem (10(-4) M), Co2+ (1.5 mM) and Cu2+ (0.1 mM) as well as the K+ channel blocker quinidine (1mM) inhibited Ca2+ uptake in 145 mM Na+ -5 mM K+ media by 60-75%. Flunarizine was less effective. In vanadate-loaded cells suspended in 70 mM Na+ -80 mM K+ media, in contrast, flunarizine exerted a dose-dependent inhibition of Ca2+ uptake by up to 80% at 10(-5) M, the other blockers being ineffective (except for verapamil at 10(-4) M). A similar pattern of inhibition was seen in quin2-loaded erythrocytes. The different susceptibility towards inhibitors may indicate that passive Ca2+ uptake by vanadate-loaded erythrocytes suspended in 145 mM Na+ -5 mM K+ media, on the one hand, and by vanadate-loaded erythrocytes suspended in 70 mM Na+ -80 mM K+ media as well as by quin2-loaded erythrocytes, on the other hand, is mediated by two different transport components.

Asymmetric effects of divalent cations and protons on active Ca2+ efflux and Ca2+-ATPase in intact red blood cells

The influence of the asymmetric addition of various divalent cations and protons on the properties of active Ca2+ transport have been examined in intact human red blood cells. Active Ca2+ efflux was determined from the initial rate of 45Ca2+ loss after CoCl2 was added to block Ca2+ loading via the ionophore A23187. Ca2+-ATPase activity was measured as phosphate production over 5 min in cells equilibrated with EGTA-buffered free Ca2+ in the presence of A23187. The apparent Ca affinity of active Ca2+ efflux (K0.5 = 30-40 mumol/liter cells) was significantly lower than that measured by the Ca2+-ATPase assay (K0.5 = 0.4 microM). Possible reasons for this apparent difference are considered. Both active Ca2+ efflux and Ca2+-ATPase activity were reduced to less than 5% of maximal levels (20 mmol/liter cells.hr) in Mg2+-depleted cells, and completely restored by reintroduction of intracellular Mg2+. Active Ca2+ efflux was inhibited almost completely by raising external CaCl2 (but not MgCl2) to 20 mM, probably by interaction of Ca2+ at the externally oriented E2P conformation of the pump. Cd2+ was more potent than Ca2+ in this inhibition, while Mn2+ was less potent and 10 mM Ba2+ was without effect. A Ca2+: proton exchange mechanism for active Ca2+ efflux was supported by the results, as external protons (pH 6-6.5) stimulated active Ca2+ efflux at least twofold above the efflux rate at pH 7.8 Ca2+ transport was not affected by decreasing the membrane potential across the red cell.

Differential alteration of dopamine, acetylcholine, and glutamate release during anoxia and/or 3,4-diaminopyridine treatment

The potassium-stimulated release of acetylcholine (ACh), glutamate (GLU) and dopamine (DA) from mouse striatal slices was studied during anoxia and/or 3,4-diaminopyridine (DAP) treatment. Anoxia, in the presence of calcium, increased DA and GLU release, but depressed ACh release. Omission of calcium from an anoxic incubation further stimulated GLU and DA release and impaired ACh release. Under normoxic conditions, DAP (100 microM) increased the release of all three neurotransmitters; the sensitivity of the slices to DAP changed with the presence or absence of an acetylcholinesterase inhibitor in the preincubation media. During an anoxic incubation, DAP did not ameliorate the anoxic-induced, K+-stimulated impairment of ACh release, but significantly reduced the K+-stimulated release of GLU and DA. These results are consistent with the hypothesis that hypoxia induces a presynaptic deficit that may underlie postsynaptic ischemic-induced changes. Amelioration of these presynaptic alterations in neurotransmitter release may be an effective approach to preventing hypoxic-induced damage.

Cytochemical localization of Ca++-ATPase activity in the lateral cochlear wall of the guinea pig

Ca++-ATPase activity was examined cytochemically in the lateral cochlear wall of the guinea pig. The reaction products showing Ca++-ATPase activity were found along the folded plasma membrane of the strial marginal cells. In contrast, little or no reaction was seen on the apical surfaces of these cells. There were also marked reaction products on the microvilli and the endolymphatic cell surface of Reissner's membrane, and the apical and lateral plasma membranes of the spiral prominence and the external sulcus cells. These reactions completely disappeared when Ca++ or ATP was removed from the incubation medium. Our results strongly suggest that Ca++-ATPase plays an important role in Ca++ transport system for the regulation of Ca++ concentration in the cochlear endolymph.

Cytochemical studies of Ca++-ATPase activity in the vestibular epithelia of the guinea pig

We used ultracytochemistry to examine Ca++-ATPase activity in the vestibular epithelia of the guinea pig. Many reaction products were found along the basolateral plasma membrane of the vestibular dark cell. There were also marked reaction deposits on the apical and lateral cell membranes of the transitional cells, and the utricular and saccular wall cells. Both sensory and supporting cells showed Ca++-ATPase activity along their ciliary membrane and apical-lateral cell surfaces. Our findings indicate that the Ca++-ATPase activity found on the plasma membrane is closely related to Ca++-transport across the plasma membrane. When either Ca++ or ATP was omitted from the incubation medium, enzyme activity (as seen by the staining reaction present) was completely abolished. Our present results suggest that Ca++-ATPase located in the vestibular epithelia plays a significant role in the regulation of the Ca++-concentration in the vestibular endolymph.

Pyruvate kinase deficiency in dog and human erythrocytes: effects of energy depletion on cation composition and cellular hydration

Pyruvate kinase-(PK) deficient human reticulocytes incubated 4 hr under conditions where mitochondrial oxidative phosphorylation is inhibited become ATP-depleted. Concomitantly, these energy-depleted red blood cells (RBC) lose massive amounts of potassium and water. PK-deficient reticulocyte-rich RBC from a Basenji dog, incubated under the same conditions, also manifest ATP-depletion and K-loss. In contrast to the cation changes in human cells, however, K-loss in PK-deficient dog reticulocyte-rich RBC is balanced by an equivalent Na gain, and consequently these canine erythrocytes do not undergo any change in cellular hydration. Potassium depletion and dehydration, which may be related to membrane injury in human RBC, do not appear to be involved in the hemolysis of PK-deficient dog erythrocytes.

Calcium-induced transient potassium efflux in human red blood cells

Human red blood cells pretreated with low-ionic-strength solutions and resuspended in saline respond biphasically to extracellular Ca. At first, addition of Ca causes a large transient K efflux of as much as 600 mM . liter cell H2O-1 . h-1; this is followed by a decrease in K flux below control levels. The first phase (phase I) resembles the Gardos effect in several respects. It is inhibited by oligomycin, by external K, and by increased exposure time to Ca. Further, the K permeability of phase I is similar to that of the Gardos effect (5 X 10(-8)-9 X 10(-8) cm/s), and the cells hyperpolarize in a low-K medium when Ca2+ is added. However, phase I is not identical to the Gardos phenomenon. For example, La, which prevents the Gardos response, is ineffective on phase I. Moreover, external Ba prevents the development of phase I but not the Gardos response, whereas internal Ba prevents the Gardos response. Attempts to demonstrate a Ca leak or pump failure during phase I have failed; passive Ca movements of both treated and normal cells are similar. The results suggest that low-ionic-strength solution exposes Ca-sensitive sites to the external medium; these sites are maintained when the cells are returned to saline.

Subsynaptosomal calcium distribution during hypoxia and 3,4-diaminopyridine treatment

Previous results demonstrate that hypoxia (low oxygen) diminishes calcium uptake by synaptosomes. The present studies examined the effects of low oxygen on calcium homeostasis in the digitonin-resistant (mitochondrial) and the digitonin-labile (nonmitochondrial) compartments of intact synaptosomes and their relation to altered membrane potentials. A 10-min hypoxic incubation in low-potassium media reduced total (-38.3%), mitochondrial (-43.3%), and nonmitochondrial (-27.8%) calcium uptake. In high-potassium media, low oxygen reduced mitochondrial (-41.2%) and total (-34.4%) uptake whereas nonmitochondrial (+ 6%) calcium uptake was essentially unaffected. A temporal analysis of nonmitochondrial calcium uptake revealed an initial depression (0-5 min) followed by a stimulation (5-10 min). Hypoxic-induced alterations in the subsynaptosomal distribution of calcium resembled those produced by uncouplers [FCCP (carbonylcyanide-p-trifluoromethoxyphenylhydrazone) or rotenone plus oligomycin]. 3,4-Diaminopyridine partially ameliorated the hypoxic- and FCCP-induced decreases in synaptosomal calcium uptake. Low oxygen reduced the total synaptosomal membrane potential (i.e., plasma plus mitochondrial membrane potential) as measured by an increased efflux of tetraphenylphosphonium ion. This hypoxic-induced efflux of tetraphenylphosphonium was slowed by pretreatment with 3,4-diaminopyridine. Thus, both drug and membrane potential studies suggest that hypoxic-induced alterations in the subcellular distribution of calcium may be due to an uncoupling mechanism and a collapse of the synaptosomal mitochondrial membrane potential.

Two modes of inhibition of the Ca2+ pump in red cells by Ca2+

Two different and independent modes of inhibition of the Ca2+ pump by Ca2+ can be detected measuring active Ca2+ extrusion from resealed ghosts of human red cells: one requires extracellular and the other requires intracellular Ca2+. Ki for inhibition by extracellular Ca2+ is about 10 mM. Extracellular Mg2+ replaces Ca2+ in inhibiting Ca2+ transport but with an apparent affinity for inhibition about 3-times less than that for Ca2+. Inhibition by external Ca2+ is not affected by Na+ or K+ at both surfaces of the cell membrane, external EGTA, internal Ca2+ or ATP. The apparent affinity for external Ca2+ progressively raises as pH increases. The effects of extracellular Ca2+ and Mg2+ are consistent with the idea that for Ca2+ pumping to proceed, external sites in the pump must be protonated and not occupied by extracellular Ca2+ or Mg2+. Inhibition by intracellular Ca2+ takes place with a Ki of about 1 mM and is independent of external Ca2+. The inhibitory effects of intracellular Ca2+ can be accounted for if Ca2+ and CaATP were competitive inhibitors of the activation of the pump by Mg2+ and MgATP, respectively.

Effects of intracellular calcium on lens membrane permeability

The present investigation was designed to assess whether lens membrane permeability is affected by changes in levels of intracellular calcium. Lanthanum, an inhibitor of Ca-ATPase, affected an increase in the concentration of intracellular calcium (Cai) measured in cortical fiber cells. Preculture of lenses in lanthanum (1.0mM) caused an accumulation of 36Cl during subsequent culture at a rate three-fold higher than control lenses. Changes in calcium levels, however, were not responsible for the observed flux changes because a 40mV depolarization was observed to occur prior to a significant increase in calcium levels. The non-specific effects of lanthanum and other potential inhibitors of calcium transport were avoided by preculturing lenses in an ion-HEPES medium containing 20mM calcium chloride. In lenses with a six-fold increase in calcium levels there resulted only a 10% increase in 36Cl uptake over a 3 hr period. 86Rb efflux was also measured and the rate constant was unchanged compared to control lenses. Calcium accumulation did lead to a small (8mV) depolarization which may account for the small increase in chloride accumulation. By light microscopy, morphology of cortical lens fibers and the epithelium appeared unchanged in the calcium-loaded lens. The results provide little evidence that an increase in Cai leads to acute changes in lens membrane permeability.

Sodium butyrate-induced changes in cultured rabbit articular chondrocyte transmembrane electrical potentials. Relationship between these changes and the proliferative response

Sodium butyrate at 5mM reversibly induced a significant increase of transmembrane potentials (Em) in normal chondrocytes (24 hours after seeding) and arrested their proliferation. This increase in Em levels, which could be temporarily abolished by Tetra-ethyl Ammonium (TEA 5mM), was related to an increase in membrane permeability to K+. This hyperpolarization was correlated with the reversible inhibition of growth in G1 induced by the sodium butyrate.

The modulation of the calcium pump of human red cells by Na+ and K+

The sidedness of Ca2+-pump activation by Na+ and K+ was studied by atomic absorption spectrophotometry in human erythrocyte ghosts, which had been prepared in dextran solutions and resealed to alkali cations. When ghosts were incubated in an all-choline medium, the increase in Na+i elicited an inhibitory-stimulatory effect on Ca2+ extrusion. By contrast, only a stimulatory action was induced when choline was replaced by Na+o. A dual effect on active Ca2+ efflux was also produced by increasing K+i or K+o. The biphasic response to the latter, however, was absent from high-K+ ghosts. Furthermore, the stimulation obtained at high K+o was additive to that elicited by K+i. The results suggest that Na+ and K+ stimulate the Ca2+ pump of human red cells through two different mechanisms. The first one appears to be an electric coupling between Ca2+ efflux and the external activating cation. The other seems associated with the molecular reactions of the Ca2+-pump protein.

Modulation of Ca2+-mediated K+-gating of erythrocyte ghosts by external Ca-EGTA

Using 86Rb+ as a marker for K+ permeability, we find that extracellular Ca-EGTA influences the rate of 86Rb+ efflux from erythrocyte ghosts preloaded with 86Rb+ and "buffered" Ca2+. At an internal free Ca2+, where the rate of 86Rb+ efflux is minimal and uninfluenced by either external EGTA or external Ca2+, external Ca-EGTA at 0.2-0.5 mM can raise the flux rate to as high as can be attained by raising internal Ca2+, in the presence of an excess externally either of Ca2+ or of EGTA. Higher concentrations of Ca-EGTA (up to 1-2 mM) diminish the flux rate. External Ca-EDTA or Mg-EDTA can substitute for Ca-EGTA in enhancing and suppressing flux rate. The peak rate is insensitive to external free Ca2+ but depends on internal Ca2+; internal Mg-EDTA does not substitute for internal Ca-EGTA. Thus, the erythrocyte membrane is asymmetric with respect to its interaction with Ca2+ and Ca-EGTA. Also, 22Na+ does not substitute for 86Rb+. The peak rate of 86Rb+ flux produced by external Ca-EGTA is diminished by chlorpromazine (0.1 mM) and augmented by 1-propranolol (25 microM), in the same way as the rate produced by increasing internal Ca2+. The results suggest that external Ca-EGTA enhances the affinity of internal Ca2+ for its receptor(s) which operate the K+-gate at the inner surface of the membrane. At external concentrations of Ca-EGTA above 1-2 mM, 86Rb+ flux rate again rises with increase of Ca-EGTA. This phenomenon does not depend upon internal Ca2+, is not affected by chlorpromazine or by 1-propranolol, and is associated with an enhanced permeability to 22Na+, inulin, and haemoglobin.

Ultracytochemical study of Ca++-ATPase and K+-NPPase activities in retinal photoreceptors of the guinea pig

Ca++-ATPase activity was demonstrated histochemically at light- and electron-microscopic levels in inner and outer segments of retinal photoreceptor cells of the guinea pig with the use of a newly developed one-step lead-citrate method (Ando et al. 1981). The localization of ouabain-sensitive, K+-dependent p-nitrophenylphosphatase (K+-NPPase) activity, which represents the second dephosphorylative step of the Na+-K+-ATPase system, was studied by use of the one-step method newly adapted for ultracytochemistry (Mayahara et al. 1980). In retinal photoreceptor cells fixed for 15 min in 2% paraformaldehyde the electron-dense Ca++-ATPase reaction product accumulated significantly on the inner membranes of the mitochondria but not on the plasmalemma or other cytoplasmic elements of the inner segments. The membranes of the outer segments remained unstained except the membrane arrays in close apposition to the retinal pigment epithelium. The cytochemical reaction was Ca++- and substrate-dependent and showed sensitivity to oligomycin. When Mg++-ions were used instead of Ca++-ions, a distinct reaction was also found on mitochondrial inner membranes. In contrast to the localization of the Ca++-ATPase activity, the K+-NPPase activity was demonstrated only on the plasmalemma of the inner segments, but not on the mitochondria, other cytoplasmic elements or the outer segment membranes. This reaction was almost completely abolished by ouabain or by elimination of K+ from the incubation medium.

Reversal by verapamil of defect in sodium transport in leucocytes in essential hypertension

The effect of treatment with verapamil on cell sodium transport was studied in the leucocytes of patients with essential hypertension. Previously described abnormalities of sodium efflux rate constant and intracellular sodium content were confirmed, the component of the sodium efflux rate constant sensitive to ouabain being lower and the intracellular sodium content higher in the patients compared with controls. Verapamil reversed these abnormalities and reduced blood pressure.

Ultracytochemical localization of Ca++-ATPase activity in the paraphyseal epithelial cells of the frog, Rana esculenta

Ca++-ATPase activity was studied ultracytochemically (cf. Ando et al. 1981) in the paraphysis cerebri of the frog. An intense reaction was demonstrated on the plasmalemma of the microvilli at the apical pole of paraphyseal cells; in contrast, the basolateral plasmalemma showed only a slight staining. In addition, mitochondria, gap junctions, cilia, and cytoplasmic elements (e.g., microfilaments) displayed Ca++-ATPase activity. Variation of the Ca++-concentration in the incubation medium from 0.1 mM to 100 mM altered the Ca++-ATPase activity of the cell organelles. The substitution of Ca- by Mg-ions resulted in a conspicuous decrease in the enzyme activity, especially on the apical plasmalemma. Ca++-ATPase activity is claimed to be involved in a number of extra- and intracellular functions. In comparison to the epithelium of the adjacent choroid plexus the paraphyseal epithelial cell is thought to be a principal Ca-ion regulator of the cerebrospinal fluid in frogs.

Ca2+ and pH affect the neurite formation in cultured mollusc isolated neurones

Neurite formation in neurones isolated from adult molluscs in culture has been shown to depend on the total content of Ca in the cells, intracellular Ca2+ concentration, intracellular acid-alkaline balance, extracellular pH, and the capacity and composition of buffers. The neurones with a low total Ca content prior to cultivating (1.2 mmol/kg) and low buffer capacity of cytoplasm (pH artificially shifted to the acidic level) possess the most pronounced capability of neurite regeneration. Optimal media for neurite regeneration appear to contain sodium bicarbonate as a buffer either alone or with small additions of organic buffers (1.5-3 mM) at pHs increasing from 7.6 to 8.2 under equilibrium with air. In the absence of sodium bicarbonate, when only organic buffers are used (Tris-HCl; HEPES-Na2CO3), at constant pH values ranging from 7.5 to 8.2, no neurites are formed. Artificial enhancement of intracellular Ca2+ concentration at the beginning of culture completely inhibits neurite outgrowth, and when applied on the third to fifth days of culture, it causes retraction of the neurites already formed. Neurones loaded with calcium (10 mmol/kg) form no neurites regardless of medium composition and concentrations of buffers used at pHs ranging from 7.5 to 8.2. The results obtained allow to suggest that neurite regeneration is controlled by Ca2+- and pH-regulating intracellular systems.

The effect of intracellular calcium on the sodium pump of human red cells

The inhibitory effect of cytoplasmic Ca on Na-pump-mediated Na-K exchange was investigated in intact red cells under conditions of constant cell volume, membrane potential and inorganic ion composition. The ionized cytoplasmic Ca concentration ( [Ca2+]i) was controlled using the ionophore A23187. In normal cells, ouabain-sensitive 24Na efflux was inhibited with an apparent affinity for [Ca2+]i which depended on the concentration of A23187; 50% inhibition required 20-40 microM and 160-300 microM-cytoplasmic Ca2+ with 10 microM and 0.63 microM-A23187 respectively. Cytoplasmic Ca also affected cell ATP content which fell rapidly on addition of A23187 and subsequently increased, steadied or continued to fall more slowly depending on the Ca and A23187 concentrations. Half-maximal fall required 5-15 microM and 110-170 microM-cytoplasmic Ca2+ at 10 microM and 0.63 microM-A23187 respectively. Removal of Ca from the cells failed to reverse either the Na pump inhibition or the fall in cell ATP. In ATP-enriched cells cytoplasmic Ca caused inhibition of ouabain-sensitive 24Na efflux in an A23187-dependent manner with apparent affinities for [Ca2+]i similar to those observed in the normal cells. Inhibition was complete at high [Ca2+]i. As in the normal cells, the ATP content of the cells fell in the presence of cytoplasmic Ca, but always remained above 1.2 m-mole/l. cells. This was higher than the ATP content of Ca-free normal intact cells. A23187 had no effect on the inhibition by Ca of ouabain-sensitive ATPase activity in isolated red cell membrane preparations. Both under conditions near optimal for Na-K-ATPase activity and under conditions resembling those in the cytoplasm, inhibition was half-maximal at about 25 microM-Ca2+ and in the latter case complete at below 400 microM-Ca2+. The apparent ATP-dependence of ouabain-sensitive Na efflux in the presence of cytoplasmic Ca was distinctly different in the normal and ATP-enriched cells but in both groups of cells it was similar for data obtained with high and low concentrations of A23187. The data for Na pump inhibition by cytoplasmic Ca in the intact cells were well fitted by several kinetic models involving either [Ca2+]i or CaATP as the inhibitory species and a low affinity dependence of pump activity on MgATP or total ATP. However, for any model, the apparent affinities for CaATP or for Ca2+ required to fit the ATPase data were 2.5-10 times higher than those required to fit the data for Na efflux.(ABSTRACT TRUNCATED AT 400 WORDS)

On the mechanisms of ATP-induced and succinate-induced redistribution of cations in isolated rat liver cells

1. The ability of external ATP to induce calcium uptake in isolated rat liver cells was further characterized. Stimulation of calcium uptake was specific for ATP, other nucleotides or ATP metabolites had no comparable effect. ATP was dephosphorylated while stimulating calcium uptake, but there was no stoichiometry between ATP hydrolysis and calcium uptake nor did dephosphorylation depend on calcium concentration. ATP acted from outside and was dephosphorylated by an ecto-ATPase of the cells. 2. In addition to its direct action, ATP enhanced succinate-dependent calcium uptake in a cooperative fashion. This is best explained by different sites of action. ATP increases cell membrane permeability while succinate stimulates uptake into mitochondria. 3. ATP was able to lower Na+ and K+ gradients and the pH gradient between cells and incubation medium. Increasing calcium concentration counteracted this effect though calcium uptake was then stimulated. 4. Succinate alone did not affect monovalent cation gradients but raised the pH gradient. It partially counteracted the ATP effects on these gradients. 5. Since catecholamine-like actions of ATP may be mediated by an increase in cytoplasmic calcium concentration, the action of extracellular ATP can be taken as a model to study the role of calcium as a transmitter of hormone actions. From interdependence between ATP-stimulated and succinate-stimulated calcium uptake, conclusions can be drawn on the resulting cytoplasmic calcium concentration and its effect on plasma membrane permeability.

Enhanced Ca2+ uptake by mouse erythrocytes in malarial (Plasmodium berghei) infection

Erythrocytes from Plasmodium berghei-infected mice on incubation either in plasma or artificial isotonic media showed an increase in uptake of 45Ca2+ compared with erythrocytes from uninfected mice. Infected cells (55% parasitaemia) incubated in plasma from normal or infected mice gave uptake rates of 9.8 and 8.1 nmol h-1 per 10(10) cells, assuming equilibrium between added 45Ca2+ and plasma Ca2+. Uptake rates of erythrocytes from infected mice were increased in the presence of glucose, with a rate of 15.0 nmol h-1 per 10(10) cells (52-58% parasitaemia) at 5 mM glucose, compared with 1.5 nmol h-1 per 10(10) cells in the absence of glucose. The enhancement of 45Ca2+ uptake was more pronounced with increasing parasitaemia, and in the fraction relatively enriched with erythrocytes carrying mature parasites. It is likely, therefore, that the enhancement is due to changes in membrane permeability accompanying parasite development. Enhanced haemolysis accompanied 45Ca2+ uptake of erythrocytes carrying mature parasites, but not of those carrying young parasites or uninfected erythrocytes. The possible role of an altered Ca2+ status in erythrocyte pathophysiology during malarial infection is discussed.

Ion transport in hypertension

A wide range of abnormalities of membrane sodium and potassium transport can be demonstrated in patients with essential hypertension, and in rats with genetic hypertension and with some forms of experimental hypertension. In the human red cell increased permeability to sodium and potassium, increased ouabain-sensitive sodium pumping, lithium-sodium counter-transport, and frusemide-sensitive co-transport have been described; by contrast, in the human leucocyte sodium pumping is reduced. In the spontaneously hypertensive rat and the rat with mineralocorticoid-induced hypertension, increased permeability to sodium and potassium, with increased ouabain-sensitive pumping, is shared by the red cell and the arterial smooth muscle. This abnormality is associated with decreased cell-membrane affinity for calcium and increased cell-membrane viscosity. It is proposed that in essential hypertension the decreased membrane affinity for calcium is a primary pathogenetic change giving rise to secondary changes in sodium and potassium transport.

Effects of neurotransmitter candidates on 45Ca uptake by cortical slices of rat brain: stimulatory effect of L-glutamic acid

The effects of neurotransmitter candidates and the characteristics of the stimulatory effect of L-glutamic acid (L-Glu) on 45Ca uptake by rat brain slices were investigated. 45Ca uptake was significantly stimulated by acetylcholine, serotonin and especially L-Glu, but not by other neurotransmitter candidates. L-Glu caused dose-dependent stimulation of 45Ca uptake (L-Glu-stimulated 45Ca uptake), its effect being half-maximal at 1 microM. The related compounds D-glutamic acid, D,L-alpha-aminoadipic acid and N-methyl-D,L-glutamic acid (final conc. of 10 microM) also stimulated 45Ca uptake, but less than 10 microM L-Glu. D,L-alpha-Methylglutamic acid and L-glutamic acid diethylether (final conc. of 10 microM), which are specific inhibitors of L-Glu, inhibited L-Glu-stimulated 45Ca uptake. Mg,Ca-ATPase activity was hardly affected by a concentration of 10 microM L-Glu that caused maximal stimulation of 45Ca uptake. These findings suggest that L-Glu-stimulated 45Ca uptake by brain cortical slices is linked to L-Glu receptor.

The role of calcium in overdrive suppression of canine cardiac Purkinje fibers

The role of calcium in the electrical and mechanical changes induced by overdrive was studied in canine cardiac Purkinje fibers. The following results were obtained: (1) hyperpolarization and pause induced by a "short" (1-minute) overdrive are smaller in a low and larger in a high calcium solution; (2) overdriving at a constant ratio of overdrive to spontaneous rate only reduces the difference in pause duration in different calcium concentrations; (3) after overdrive, the initial diastolic depolarization is flatter and the late diastolic depolarization is steeper the control in low calcium and the opposite changes occur in high calcium; (4) the threshold for the first beat after overdrive is more negative in low than high calcium; (5) with "long" (2-minute or more) overdrives, hyperpolarization of comparable magnitude is seen during overdrive in low and high calcium; (6) the decline of hyperpolarization after overdrive is faster in high calcium unless the fiber is driven at slow rate ("postdrive"); (7) the contractile force after overdrive is greater than before overdrive in low and normal calcium and smaller than before overdrive in high calcium, and this difference is less during postdrive; (8) hyperpolarization and pause are reduced in a low sodium solution. It is concluded that calcium modifies overdrive suppression in several respects (hyperpolarization, slope of diastolic depolarization, duration of the pause and threshold), but is only one of the several factors involved in determining overdrive hyperpolarization and suppression.

The effect of pH on Ca2+ extrusion mechanisms in dialyzed squid axons

The effect of internal and external pH, on the components of the Ca2+ efflux have been investigated in internally displayed squid axons. (1) Internal pH: a fall in intracellular pH (below 7.3) inhibited both the ATP-dependent uncoupled (Ca2+ pump) (50% at pHi 6.3) and the Na+o-dependent Ca2+ efflux (forward Na+/Ca2+ exchange) (50% at pHi 6.8). Internal alkalinization of pH 8.8 had no effect on the uncoupled component but markedly increased (4-fold) the Na+o-dependent Ca2+ efflux. (2) External pH: altering the external pH from 7.3 to 9.0 had no effect on the Na+o-dependent Ca2+ efflux mechanism. In the absence of Ca2+o, alkalinization to pHo 8.8 caused a reduction in the magnitude of the uncoupled Ca2+ pump. This inhibition is markedly enhanced by the presence of Ca2+ in the external medium. As for the case of the sarcoplasmic reticulum Ca2+-ATPase, this combined inhibitory effect of high pHo and Ca2+o is most probably related to a reversal of the cycle of the ATP driven Ca2+ pump. The marked differences in the pH dependence of the components of the Ca2+ efflux support the model of two separate mechanisms of Ca2+ extrusion in squid axons: Ca2+ pump and Na+/Ca2+ exchange.

Regulation of the Ca2+-pump by calmodulin in intact cells

ATP-enriched human red cells display high rates of Ca2+-dependent ATP hydrolysis (16 mmol . litre cells-1 . h-1) with a high Ca2+ affinity (K0.5 approximately 0.2 microM). The finding suggests a mechanism for regulation of cell Ca2+ levels, involving highly-cooperative stimulation of active Ca2+ extrusion following binding of calmodulin to the (Ca2+ +Mg2+)-ATPase.

Subcellular localization of calcium in the coronet cells and tanycytes of the saccus vasculosus of the rainbow trout, Salmo gairdneri Richardson

The intracellular localization of calcium in the saccus vasculosus of the rainbow trout, Salmo gairdneri Richardson, was studied by means of ultracytochemical and X-ray microanalytical techniques. Using a variant of the glutaraldehyde/potassium pyroantimonate-osmium tetroxide method, Ca was detected in mitochondria, smooth endoplasmic reticulum and primary vesicles of coronet cells, and in mitochondria and smooth endoplasmic reticulum of tanycytes. Mitochondria and smooth endoplasmic reticulum in both cell types are considered as general Ca-stores. The primary vesicles in the ciliary globules of coronet cells are viewed as additional Ca-reservoirs. Possible roles of these Ca-stores in the regulation of transport activities of coronet cells in the homeostasis of the CSF are discussed.

Red blood cell sodium content and permeability changes in hemorrhagic shock

Recent studies of cellular function have demonstrated significant alterations of cell homeostasis during hemorrhagic shock. The progressive depolarization of cell membranes demonstrated in several organs has been attributed to failure of energy-dependent active ion transport. Similar changes in intracellular electrolyte composition have been observed in the red blood cells of some patients in late hemorrhagic shock and in other severely ill patients. The ready availability of isolated cells that do not require extracellular fluid space assessment makes the red blood cell an attractive model for investigating the factors controlling ion movements across the cell wall. It has been suggested that the red blood cell ion abnormalities may represent a generalized cellular response to severe illness. Red blood cell sodium concentration was measured in 145 patients during severe hemorrhagic shock. The direction and magnitude of changes in red blood sodium concentration was correlated with the time course of the shock state. The initial decrease in sodium content was a function of the diminished red blood cell membrane permeability due to a decrease in plasma bicarbonate levels. The subsequent red blood cell sodium uptake above the normal range, not associated with an increase in sodium influx, indicated an impairment of energy-dependent active transport. The observed high sodium concentrations associated with a potassium decrease in the red blood cell appeared to be only one manifestation of a generalized defect in cellular function and composition during severe hemorrhagic shock.

The effect of metabolic inhibition on ion contents and sodium exchange in human lymphocytes

Lymphocytes depleted of ATP by incubation in iodoacetate (IAA) and nitrogen (N2) lost K and gained Na. Isotopic Na exchange showed a fast fraction and a slower exponential fraction, the latter conventionally assumed to reflect surface membrane properties. The gain of cell Na was not accounted for by a decrease in 22Na efflux in either the slow or the fast fraction. After 3-5 hours, Na efflux increased. These results led us to question the concept that normal cell ion levels are maintained by an ATPase pump and could not be explained by exchange diffusion, co-transport, countertransport, or other inherently dissipative mechanisms. The data are, on the other hand, consistent with the concept that cell ion contents are determined by their relative exclusion from cell water coupled with selective adsorption onto fixed macromolecular anionic sites within the cell. In this view, the IAA,N2-induced rise in cell Na is due to the occupancy of adsorption sites losing K, while the increased isotopic exchange is due to a decreased activation energy for ion-site interaction.

Active calcium transport in red cell ghosts resealed in dextran solutions

1. Human erythrocytes when lysed and resealed to Ca in the presence of dextran can be readily separated from the suspending medium by low-speed centrifugation. 2. Ghosts trapped Ca and EGTA at the same ratio as present in the haemolytic medium and remained tight to Ca after washing and subsequent incubation for up to 90 min at 37 degrees C. 3. Ca extrusion could be promoted by substrates other than ATP only from ghosts that had been loaded with low free Ca concentrations (1--22 microM). The order of activation by the various substrates employed was ATP greater than adenine + inosine greater than inosine. 4. The kinetics of extrusion depended markedly on internal free Ca. The system showed a high affinity state (KCa about 3 microM; V = 0.34 mumol Ca/ml ghosts per min) at low concentrations (1--22 microM) and a low affinity state (KCa about 250 microM; V = 0.17 mumol Ca/ml ghosts per min) at high concentrations (0.2--4.0 mM). 5. Both at low and at high free Ca, La-sensitive ATP hydrolysis was closely correlated with La-dependent Ca efflux, in keeping with an stoichiometry of 1.6. The rate of extrusion was maximal in the presence of 160 mM KCl and decreased to various extents when K was fully replaced by different cations, following the order K greater than Na = choline greater than Mg. 7. The efflux rate of high-K ghosts, resealed to alkaline cations, was stimulated by external Na, whilst Mg and choline was practically without effect. 8. The results indicate that human red cells possess a powerful Ca extrusion mechanism, the activity of which can be modulated by alkaline cations.

Effects of secretagogues on the K+ permeability of mucosal and serosal borders of rabbit colonic mucosa

(1) K+ efflux rates from the mucosal and serosal surfaces of sheets of rabbit colonic mucosa have been determined by measuring net K+ loss into K+-free Ringer solution bathing each side of the tissue. (2) Initially, there is a high rate of K+ loss from the tissue, this falls to a lower steady-state rate after 20 min. Loss of K+ from the tissue into the serosal bath is 6-8 fold faster than loss to the mucosal bath. (3) A number of intestinal secretagogues, e.g. theophylline, cyclic AMP, carbachol, ionophore A23187, as well as the laxative bisacodyl, raise the K+ efflux rate across the mucosal border by 200-300%. In the case of K+ efflux induced by carbachol the effect is shown to be dependent on raised levels of intracellular Ca2+. Ca2+-calmodulin complex does not appear to be be involved in activation of K+ efflux across the mucosal border. (4) Amiloride does not block mucosal K+ efflux, but tetraethyl-ammonium does inhibit K+ efflux across the mucosal border, induced by either bisacodyl or raised intracellular Ca2+. (5) The results suggest that laxatives may increase the rate of K+ secretion into the colonic lumen by raising the K+ permeability of the mucosal border.

Molecular mechanisms of general anaesthesia

This review aims to give a balanced view of the various mechanisms which have been proposed to explain the phenomenon of general anaesthesia on both a molecular and whole animal level. An attempt is made to interrelate these and produce one cohesive model.

Influence of EGTA on the apparent Ca2+ affinity of Mg2+-dependent, Ca2+-stimulated ATPase in the human erythrocyte membrane

The apparent Ca2+ affinity of Mg2+-dependent, Ca2+-stimulated ATPase (Mg2+ + Ca2+)-ATPase) in human erythrocyte membranes increased with increasing concentrations of EGTA used to buffer free Ca2+. The shift in apparent Ca2+ affinity was seen in membranes prepared by hypotonic hemolysis and in membranes depleted of endogenous activators by EDTA treatment. The effect of EGTA differed from that of calmodulin, as it increased Ca2+ affinity without increasing V. EGTA also increased the apparent Ca2+ affinity when calmodulin was present in the assay medium. ATP-stimulated calcium binding to membranes was greater at 1 mM EGTA than at 0.1 mM EGTA. Similarly to ATPase activation, whereas binding decreased as Ca2+ was raised above 35 microM at 1.0 mM EGTA, binding progressively increased up to 100 microM or more free Ca2+ at 0.1 mM EGTA. EGTA also increased the Ca2+ affinity of Triton X-100-solubilized (Mg2+ + Ca2+)-ATPase, indicating that its effect did not depend on an intact membrane. Analysis of the kinetic data by a computerized nonlinear curve fitting procedure showed that a low Ca2+ affinity state of the enzyme was converted to a high Ca2+ affinity state in the presence of EGTA. The species associated with the enzyme interconversion appeared to be [CaEGTA]2-.

Changes in 42K efflux produced by alterations in transmembrane calcium movements in turtle cardiac pace-maker tissue

1. 42K efflux has been measured from small strips of turtle sinus venosus which were electrically paced. Three different procedures for altering transmembrane calcium influx have been utilized to test whether changes in 42K efflux may be modulated by changes in intracellular calcium levels. 2. No significant changes in the 42K fractional escape rate (FER) were observed when external calcium was reduced to O mM or increased to 4 x normal (10 mM). In these experiments extracellular divalent cation concentration was held constant by adding or removing magnesium ions. 3. Application of 10 mM-Ba2+ also failed to alter 42K FER consistently. In red blood cells and snail neurones addition of barium ions has been shown to reduce significantly the calcium-mediated potassium current. 4. A tenfold increase in pacing rate (0.5-5 Hz) resulted in an augmented 42K FER, but repetition of this rate change in O mM-Ca2+ indicated that this increase in 42K FER was not strongly dependent on the amount of calcium entry. 5. Attempts to load the pace-maker cells with calcium by using the ionophore A23187 (10 micrograms ml . -1 of 2.0 x 10(-5) M) consistently resulted in very large increases in 42K FER. However, this effect (i) was blocked by atropine and (ii) was markedly reduced by pretreating the tissues with hemicholinium, indicating that A23187-induced release of acetylcholine from the endogenous nerve terminals was responsible for the observed increase in 42K FER. 6. In summary, three different experimental tests indicate that the majority of the 42K efflux is not tightly linked to transmembrane calcium movement in sinus venosus pace-maker tissue.

The independent effects of hyperventilation, tolazoline, and dopamine on infants with persistent pulmonary hypertension

We studied the separate and combined effects of hyperventilation and administration of dopamine and tolazoline in five infants with pulmonary hypertension managed with indwelling pulmonary artery catheters. In five infants the right-to-left shunt reversed during ventilator-induced respiratory alkalosis (pH greater than 7.6). Response to drugs was variable and unpredictable. One infant could be oxygenated at normal pH during combined dopamine and tolazoline infusion. Other infants showed no response to drugs, or became worse during infusion. The ratio of pulmonary artery to systemic artery pressure averaged 1.14 with standard therapy, but decreased to 0.98 following respiratory alkalosis alone, to 0.87 following drug infusions, and to 0.70 following the combination of alkalosis and drug infusion. These changes were significant by analysis of variance at P less than 0.02, P less 0.001, and P less than 0.001, respectively. Systemic oxygenation was satisfactory in all cases when the pulmonary to systemic pressure ratio was less than 1.0.

Effects of quinine on Ca++-induced K+ efflux from human red blood cells

The Ca++-mediated increase in K+-permeability of intact red blood cells (Gardos effect) was initiated by exposing cells to know concentrations of Ca++ (using EGTA buffers) in the presence of the ionophore A23187. The potency of quinine, an inhibitor of the response, was found to depend on the external K+ concentration. In K+-free solutions the concentration of quinine to achieve 50% inhibition (K50) was 5 microM, but at 5 mM K+ the required concentration was increased 20-fold to 100 microM. An increase in internal Na+ had the opposite effect, allowing a high potency of quinine despite the presence of external K+. Alterations in the internal K+ level, on the other hand, were without effect on the K50, suggesting that the membrane potential is not a factor. This conclusion is supported by the lack of effect on quinine inhibition of substitution of Cl- by NO3-, a considerably more permeant anion. The data are consistent with the hypothesis that quinine inhibits by competitively displacing K+ from an external binding site, the reported K+-activation site for the Ca++-mediated K+-permeability.

Vanadium increases selective K+-permeability in human erythrocytes

In human erythrocytes that had been depleted of ATP by incubation with iodoacetate and adenosine, vanadate induces a 10-15-fold increase of K+-permeability. The effect is similar to that produced by calcium ions. Like the calcium-induced permeability change, the vanadate-induced effect is preceded by a lag period. Preincubation without substrates for ATP synthesis reduces the length of the lag period following the addition of either vanadate or calcium. The selective change of K+-permeability was brought about by vanadate anions (+5 oxidation state) as well as by vanadyl cations (+4 oxidation state). In both cases, the presence of EDTA prevented the permeability change. Blocking of the anion-transport system of the human erythrocytes by H2DIDS was used to discriminate between the unstable forms of vanadate anion and vanadyl cation in producing the potassium loss. The observation that H2DIDS had little if any effect on the efficiency and the previously reported fact by Cantley, L.C. and Aisen, Ph. (J. Biol. Chem., 254 (1979) 1781) that vanadate appears mostly as vanadyl in the cell interior suggests that, similar to Ca2+, Mg2+ or Pb2+, vanadyl (VO2+) can open the "potassium channel" in the erythrocyte membrane.