Vanadate inhibits uncoupled Ca efflux but not Na--Ca exchange in squid axons

Regulation of presynaptic calcium in a mammalian synaptic terminal

Ca(2+) signaling in synaptic terminals plays a critical role in neurotransmitter release and short-term synaptic plasticity. In the present study, we examined the role of synaptic Ca(2+) handling mechanisms in the synaptic terminals of mammalian rod bipolar cells, neurons that play a pivotal role in the high-sensitivity vision pathway. We found that mitochondria sequester Ca(2+) under conditions of high Ca(2+) load, maintaining intraterminal Ca(2+) near resting levels. Indeed, the effect of the mitochondria was so powerful that the ability to clamp intraterminal Ca(2+) with a somatically positioned whole cell patch pipette was compromised. The plasma membrane Ca(2+)-ATPase (PMCA), but not the Na(+)/Ca(2+) exchanger (NCX) or the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA), was an important regulator of resting Ca(2+). Furthermore, PMCA activity, but not NCX or SERCA activity, was essential for the recovery of Ca(2+) levels following depolarization-evoked Ca(2+) entry. Loss of PMCA function was also associated with impaired restoration of membrane surface area following depolarization-evoked exocytosis. Given its roles in the regulation of intraterminal Ca(2+) at rest and after a stimulus-evoked Ca(2+) rise, the PMCA is poised to modulate luminance coding and adaptation to background illumination in the mammalian rod bipolar cell.

A basic study on the biological monitoring for vanadium-effects of vanadium on Vero cells and the evaluation of intracellular vanadium contents

A high concentration of vanadium (V) has toxic effects on human and animals and is one of environmental pollutants. In the present study, we have conducted a fundamental study using cultured Vero cells from monkey kidney for the future environmental monitoring. Orthovanadate (VAN), one of V compounds, of 10(-10) and 10(-8) M did not affect the cell growth although the higher concentration of above 10(-6) M VAN inhibited the cell growth accompanied with the decrease in cell numbers and morphological changes. Given that the washing method with ice-cold Li is also effective for determination of the cellular Na content, we used this method for the determination of the V content of the Vero cells. The V distributions in Vero cell; in the 10(-3) M VAN solution, extracellular and intracellular were obtained as 1:0.564:0.036 and 1:0.662:0.098 at 60 and 120 min after the treatment of VAN. The intracellular V content was 10% of the applied concentration of VAN. Consequently, it was suggested that V concentration of 10(-7) and 10(-6) M in the tissue and environment, respectively, might become the threshold concentration; a criterion of the environmental contamination when we carry out environmental monitoring.

In the squid axon Na+/Ca2+ exchanger the state of the Ca i-regulatory site influences the affinities of the intra- and extracellular transport sites for Na+ and Ca2+

In squid axons, intracellular Mg2+ reduces the activity of the Na+/Ca2+ exchanger by competing with Ca2+ i for its regulatory site. The state of the Ca i-regulatory site (active-inactive) also alters the apparent affinity of intra- and extracellular transport sites. Conditions that hinder the binding of Ca2+ i (low pH i, low [Ca2+]i, high [Mg2+]i) diminish the apparent affinity of intracellular transport sites, in particular for Na i due to its synergism with H+ inhibition, but less noticeably for Ca2+ i because of its antagonism towards (Ha i + Na+ i) and Mg2+ i inhibitions. These are kinetic effects unrelated to the true affinity of the sites. With the Ca i-regulatory site saturated, the intracellular transporting sites are insensitive to [H+]i and to ATP. Likewise, the state of the Ca i-regulatory site (activated or inactivated) influences the affinity of the extracellular Ca o and Na o-transport sites (trans effects). In this case, the effects are opposite to those predicted by any of the transport schemes proposed for the Na+/Ca2+exchanger; i.e. its mechanism remains unexplained. In addition to their intrinsic importance for a full understanding of the properties of the Na+/Ca2+ exchanger, these findings show a new way by which the state of the Ca i-regulatory site may determine net movements of Ca2+ through this system.

Calcium regulation in photoreceptors

In this review we describe some of the remarkable and intricate mechanisms through which the calcium ion (Ca2+) contributes to detection, transduction and synaptic transfer of light stimuli in rod and cone photoreceptors. The function of Ca2+ is highly compartmentalized. In the outer segment, Ca2+ controls photoreceptor light adaptation by independently adjusting the gain of phototransduction at several stages in the transduction chain. In the inner segment and synaptic terminal, Ca2+ regulates cells' metabolism, glutamate release, cytoskeletal dynamics, gene expression and cell death. We discuss the mechanisms of Ca2+ entry, buffering, sequestration, release from internal stores and Ca2+ extrusion from both outer and inner segments, showing that these two compartments have little in common with respect to Ca2+ homeostasis. We also investigate the various roles played by Ca2+ as an integrator of intracellular signaling pathways, and emphasize the central role played by Ca2+ as a second messenger in neuromodulation of photoreceptor signaling by extracellular ligands such as dopamine, adenosine and somatostatin. Finally, we review the intimate link between dysfunction in photoreceptor Ca2+ homeostasis and pathologies leading to retinal dysfunction and blindness.

ATP-dependent Ca2+ transport is up-regulated during third trimester in human syncytiotrophoblast basal membranes

In late gestation, Ca2+ transport across the human placenta must increase in response to the demands of accelerating bone mineralization of the fetus. This is an ATP-dependent transport against a concentration gradient across the basal or the fetal-facing plasma membrane of the syncytiotrophoblast. The aims of the present study were to determine the relationship between ATP-dependent Ca2+ transport and gestational age in the third trimester and to identify the specific isoforms of plasma membrane Ca2+ ATPase (PMCA) present in human syncytiotrophoblast. Basal membrane vesicles were isolated from normal placentas and from placentas obtained from preterm deliveries with no other complications (32-37 wk of gestation). We studied the uptake of 45Ca2+ into basal membrane vesicles in the absence and presence of ATP by using rapid filtration techniques. Western blot was used to assess the protein expression of the PMCA isoforms 1-4. Isoforms 1 and 4 of PMCA were identified in basal membrane of human placenta. The ATP-dependent Ca2+ transport increased linearly during the third trimester (r = 0.571, p = 0.0015, n = 28). However, PMCA protein expression was unaltered during the same period of gestation. Our results show that PMCA in the fetal-facing plasma membrane of the human syncytiotrophoblast is markedly activated toward the end of pregnancy. We suggest that these changes are critical in supplying the rapidly growing fetus with sufficient Ca2+ for bone mineralization.

Relationship between vanadate induced relaxation and vanadium content in guinea pig taenia coli

Vanadate has been known to induce a transient increase in high K+ induced contraction, and also gradually relax the high K+ contraction itself in guinea pig taenia coli. The relationship between the rate of relaxation and ion content of Na+, K+, and V ion at the cellular level was investigated when vanadate was applied to contracted muscle. Tissue Na+ and V ion content increased linearly, depending on the time after vanadate treatment, reaching maximum levels of approximately 50 mM·kg-1 and 0.25 mM·kg-1 wet weight, respectively. There was a positive correlation between the V ion and Na+ contents, while there was a negative correlattion between both ions and the relaxed rate of the high K+ induced contraction. The uptake of V ion was affected by the external K+ concentration, and the maximum rate of V ion uptake decreased to 40% in the presence of 90 mM external K+. These results suggest that a small amount of V ion was enough to inhibit the Na+ pump activity and muscle contraction in the high K+ solution.Key words: vanadate, high K+, relaxation, taenia coli, vanadium.

Aging-related changes in calcium oscillations in fertilized mouse oocytes

Aging of oocytes, being not fertilized after ovulation for a prolonged time, considerably affects normal development of the fertilized oocyte. We examined effects of the aging on a series of highly repetitive Ca2+ transients commonly seen in fertilized mouse oocytes (Ca2+ oscillations). Frequency of Ca2+ oscillations in the aged oocyte [20 hrs after induction of superovulation by i.p. human chorionic gonadotropin (hCG)] was significantly higher (34.1 +/- 5.8 l/hr) than the fresh oocyte (14 hr post-hCG, 21.8 +/- 7.9 l/hr). Rates of rise and fall of individual Ca2+ transient in the aged oocyte were significantly slower than the fresh oocyte, whereas durations of individual Ca2+ transients were similar. When extracellular Ca2+ was raised from 2.04 mM to 5.00 mM, aged oocytes showed significant prolongation of the duration of individual Ca2+ transient, that resulted in a sustained elevation of intracellular Ca2+ ([Ca2+]i) in 33% of the aged oocyte. Transient increase in [Ca2+]i by photolysis of a caged Ca2+, Nitr-5, injected into cytoplasm was completely restored in the fresh oocyte [fluorescence intensity of [Ca2+]i indicator dye Fluo-3 (F480) returned to 97 +/- 2% of the control level, time constant = 37 +/- 9 sec]. In contrast, in the aged oocyte, restoration of F480 following Nitr-5 photolysis was incomplete (115 +/- 12% of the control) and slow (time constant = 64 +/- 23 sec). Because inhibition of the Ca2+ pump of the endoplasmic reticulum (ER) by 5 microM thapsigargin almost completely inhibited restoration of F480 following Nitr-5 photolysis in the fresh oocyte, we conclude that the aging-related changes in Ca2+ oscillations may be accounted for by dysfunction of intracellular Ca2+ regulation, presumably of the Ca2+ pump of the ER.

Intracellular calcium and its sodium-independent regulation in voltage-clamped snail neurones

1. We have used both Ca(2+)-sensitive microelectrodes and fura-2 to measure the intracellular free calcium ion concentration ([Ca2+]i or its negative log, pCai) of snail neurones voltage clamped to -50 or -60 mV. Using Ca(2+)-sensitive microelectrodes, [Ca2+]i was found to be approximately 174 nM and pCai, 6.76 +/- 0.09 (mean +/- S.E.M.; n = 11); using fura-2, [Ca2+]i was approximately 40 nM and pCai, 7.44 +/- 0.06 (mean +/- S.E.M., n = 10). 2. Depolarizations (1-20 s) caused an increase in [Ca2+]i which was abolished by removal of extracellular Ca2+, indicating that the rise in [Ca2+]i was due to Ca2+ influx through voltage-activated Ca2+ channels. 3. Caffeine (10-20 mM) caused an increase in [Ca2+]i in the presence or absence of extracellular Ca2+. The effects of caffeine on [Ca2+]i could be prevented by ryanodine. 4. Thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2+)-ATPase, caused a small increase in resting [Ca2+]i and slowed the rate of recovery from Ca2+ loads following 20 s depolarizations. 5. Neither replacement of extracellular sodium with N-methyl-D-glucamine (NMDG), nor loading the cells with intracellular sodium, had any effect on resting [Ca2+]i or the rate of recovery of [Ca2+]i following depolarizations. 6. The mitochondrial uncoupling agent carbonyl cyanide m-chlorophenylhydrazone (CCmP) caused a small gradual rise in resting [Ca2+]i. Removal of extracellular sodium during exposure to CCmP had no further effect on [Ca2+]i. 7. Intracellular orthovanadate caused an increase in resting [Ca2+]i and prevented the full recovery of [Ca2+]i following small Ca2+ loads, but removal of extracellular sodium did not cause a rise in [Ca2+]i. We conclude that there is no Na(+)-Ca2+ exchanger present in the cell body of these neurones and that [Ca2+]i is maintained by an ATP-dependent Ca2+ pump.

Ca2+ efflux mechanisms following depolarization evoked calcium transients in cultured rat sensory neurones

1. We have used a combination of microfluorimetry and patch-clamp techniques to investigate cytoplasmic Ca2+ ([Ca2+]i) buffering in response to physiological Ca2+ loads in neurones cultured from the dorsal root ganglia of the rat. 2. In cells loaded with Indo-1 AM and using high resistance microelectrodes to initiate and record action potentials, single action potentials were associated with a measurable rise in [Ca2+]i. Short trains of action potentials evoked [Ca2+]i transients with monoexponential recovery rates with time constants of around 5 s. 3. Similar Ca2+ buffering properties were seen in cells perfused with patch-clamp pipettes in the whole-cell recording mode suggesting that the slow (seconds) Ca2+ buffering properties were not seriously perturbed by the recording technique. 4. In cells held under voltage clamp, reversal of the Na(+)-Ca2+ exchanger driving force had a small but significant effect on the rate of Ca2+ removal. 5. Increasing extracellular pH or adding vanadate (200 microM) to the internal solution dramatically slowed the rate of recovery. Addition of calmidazolium to the pipette solution also produced a significant but much less dramatic slowing of Ca2+ efflux. 6. The results demonstrate that the activity of a plasmalemmal Ca(2+)-ATPase is important for the removal of somatic Ca2+ loads of a similar amplitude to those generated by the firing of a few action potentials.

Inhibition of depolarisation-induced calcium influx into GH3 cells by arachidonic acid: the involvement of protein kinase C

The influx of 45Ca2+ induced in GH3 cells by exposure to 60 mM K+ medium was inhibited by arachidonic acid (AA) in a concentration-dependent manner. This action of AA was not prevented by inhibitors of its metabolism but was reversed by the inhibitors of protein kinase C (PKC), H7 and staurosporine but not their less active congeners HA 1004 and K252a, respectively. Presumed down-regulation of PKC by pretreatment with phorbol 12,13-dibutyrate (PDBu) also greatly diminished the effect of AA. Experiments to assess effects of AA on 45Ca2+ efflux and on cytosolic Ca2+ concentrations indicated that an additional PKC-independent action of AA involving the release of intracellularly stored calcium was present. Both direct activation of certain PKC isoform(s) by AA and the synergistic influence on PKC activity by its concomitant raising of intracellular Ca2+ concentrations, may be physiologically important in the regulation of depolarisation-induced Ca2+ entry.

Non-thermal vasodilatation by radio frequency burst-type electromagnetic field radiation in the frog

1. When the web of the anaesthetized Xenopus laevis was perfused with Ringer solution maintained at 20 degrees C, radio frequency (RF) burst-type electromagnetic (EM) field radiation not only dilated arterioles of the web which had been preconstricted with noradrenaline, but also dilated arterioles under non-stimulated conditions. The EM field-induced vasodilatation increased slowly and reached a plateau 60 min after the onset of radiation. After the cessation of radiation, vasodilatation remained for 10-20 min, then slowly subsided. 2. When a 10 MHz, 1 V (peak to peak) generator voltage induced a 7.3 milliGauss, 2.19 V cm-1 EM field, the vasodilatory effect was optimum when bursts were applied 50% of the total time at 10 kHz burst rate. 3. The vasodilatory effect was not secondary to dielectric heat in the web, because the EM field was too weak to have produced enough heat to dilate the arterioles and heat would have been constantly conducted away by the perfusion solution. 4. During perfusion with Ringer solution warmed to 30 degrees C, no vasodilatation was found, but perfusion with Ringer solution warmed to 35 degrees C induced only 11% vasodilatation. Perfusion with Ringer solution warmed to 37 degrees C induced irreversible vasoconstriction. The pattern of vasodilatation induced by warm Ringer solution was different from the vasodilatory effect of weak EM field radiation. 5. The extent of the vasodilatory effect was influenced by Ca2+ concentration of the perfusion medium. Under normal Ca2+ conditions arterioles dilated to 126% of the control diameter, while under Ca(2+)-free conditions arterioles dilated to 131% of the control value and under high-Ca2+ conditions (twice the normal level) arterioles dilated to 111% of the control value. This suggests that the vasodilatory effect may be caused by facilitation of Ca2+ outflow, and the extent of this flow may settle down to the equilibrium level of countercurrent flux between Ca2+ influx and outflow. 6. The vasodilatory effect was not inhibited under perfusion with Na(+)-free Ringer solution, suggesting that Na(+)-Ca2+ exchange system may not be involved in the vasodilatory effect. The vasodilatory effect was inhibited by vanadate, an inhibitor of Ca(2+)-ATPase, and was abolished by Methylene Blue, an inhibitor of guanylate cyclase. The evidence suggests that the mechanism of the vasodilatory effect may depend on an increase in Ca2+ outflow through the plasma membrane of the smooth muscle and/or an increase in Ca2+ influx into the sarcoplasmic reticulum.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhancement of capsaicin-induced contraction of guinea-pig tracheal smooth muscle by vanadate

Contractions of guinea-pig isolated tracheal smooth muscle by submaximal capsaicin (0.1 microM) were enhanced by 43% (P less than 0.01) after vanadate (10 microM for 10 min) preincubation. Contractile responses to acetylcholine (0.1-100 microM), histamine (1-100 microM) or substance P (0.01-1 microM) were, in contrast, not affected by prior vanadate exposure. It is suggested that tachykinin release from capsaicin-sensitive afferent nerve endings within the airways was enhanced by vanadate while airway smooth muscle reactivity remained unchanged.

Gestational changes in the utilization of intracellularly stored calcium in the myometrium of guinea-pigs

1. The ability of oxytocin and prostaglandin F2 alpha (PGF) to induce contraction in guinea-pig myometrium in calcium-free solution was studied in an attempt to assess the extent to which intracellular calcium stores could be released by these two agonists. Both longitudinal and circular muscle layers were studied separately and the effects of gestational age were also examined. 2. In longitudinal strips, the responses to oxytocin and PGF in the absence of external calcium decreased progressively throughout gestation. Responses of circular strips to both agonists were unchanged throughout pregnancy, until day 64, when no response to PGF could be elicited. 3. Pre-treatment with high potassium (and normal calcium) increased the responses to the agonists in calcium-free medium while pre-treatment with beta-adrenoceptor agonists had no effect on responses to oxytocin or PGF. 4. Responses to both agonists decreased with time in calcium-free solution suggesting a loss of calcium from stores with a half-time of 3 min. The rate of the decline in the responses was the same in both muscle layers and did not change with gestational age. 5. In the presence of lanthanum contractions evoked by oxytocin, but not PGF, were augmented 2-3-fold. This potentiation of the response to oxytocin occurred in both muscle layers and throughout gestation. 6. Each agonist evoked only one response in calcium-free solution containing EGTA. The response to PGF in longitudinal strips following a challenge with oxytocin was reduced, compared with the response to PGF when applied first while the response to oxytocin in these strips was unchanged following exposure to PGF. In circular strips neither oxytocin- nor PGF-induced contractions were altered following prior exposure to the other agonist. 7. It is concluded that oxytocin and PGF operate via two distinct mechanisms to release intracellularly stored calcium in both longitudinal and circular components of the guinea-pig myometrium and a hypothesis to explain the results is presented.

The influence of vanadate on calcium uptake in maturing enamel of the rat incisor

The influence of vanadate, a potent inhibitor of Ca2+-ATPase and Na+-K+-ATPase, on 45Ca uptake in maturing enamel of the rat incisor was investigated by a vascular perfusion method combined with 45Ca autoradiography. The morphological integrity of the maturation-stage enamel organ was well-retained during vascular perfusion under all the experimental conditions. Distinct patterns of 45Ca labeling, comparable with those found in previous in vivo 45Ca autoradiographic studies, appeared in the maturing enamel after vascular perfusion with a standard perfusate. One mmol/L vanadate added to the standard perfusate caused a drastic decrease in 45Ca uptake in the maturing enamel, corresponding to the ruffle-ended ameloblasts, leaving narrow peaks of moderate intensity corresponding to the bands of the overlying smooth-ended ameloblasts. The in vitro labeling of exposed enamel surfaces with 45Ca revealed blackening of autoradiographic emulsion in wide bands separated by unlabeled or slightly labeled narrow ones resembling the distribution of smooth-ended ameloblasts in both control and vanadate-treated incisors. Our observations indicate that the ruffle-ended ameloblasts of the rat incisor serve as an efficient diffusion barrier to calcium ions and regulate transcellular calcium transport to the maturing enamel, at least in part, by a vanadate-sensitive mechanism.

A comparison of uterine contraction induced by PGE1 and oxytocin in Ca-free solution

The contraction of the rat uterus incubated in Ca-free EDTA-containing solution in response to PGE1, oxytocin and vanadate has been investigated in order to examine the mechanism of the release of Ca from intracellular stores. The results obtained show that PGE1 evoked a sustained contraction the magnitude of which diminishes slightly after successive additions of PGE1 but not after long exposure to Ca-free medium. Oxytocin induced two different contractions: one of them was transient and observed only after incubating for 5 min in Ca-free solution; the other remained constant during prolonged incubation in Ca-free medium. Vanadate, an inhibitor of Ca-ATPase, induced sustained contraction after prolonged exposure to Ca-free medium, and isoprenaline, which stimulates Ca re-uptake by intracellular organelles, counteracted the sustained contractile response induced by the three agonists.

Effects of magnesium chloride on the contractile response of uterus to several agonists in Ca-free solution

The effects of MgCl2 on the oestrogen-dominated rat uterus have been examined. Tissues were preincubated in a Ca2+- and Mg2+-free medium containing 3 mM EDTA. Most experiments were subsequently performed in a similar medium containing either no EDTA or EDTA (1 mM). When MgCl2 was added cumulatively (1-32 mM) no contractile responses were obtained in Ca,Mg-free medium or in Ca,Mg-free high K+ solution. When 2 mM CaCl2 was added, a sustained contraction was obtained. Subsequent addition of cumulative concentrations of MgCl2 caused concentration-dependent relaxation. Oxytocin, 2 microM, produced a small and sustained contraction in a Ca,Mg-free medium. Addition of MgCl2, 2 mM, increased this contraction. In a Ca,Mg-free medium vanadate (8 X 10(-5)M) did not evoke spasm of uterine smooth muscle. After addition of MgCl2 in cumulative amounts (1-32 mM) in the presence of vanadate, a concentration-dependent contraction was obtained. The present work shows that in Ca-free solution, maintained contractions induced by oxytocin and vanadate are augmented by Mg2+.

The action of two classes of pyrethroids on the inhibition of brain Na-Ca and Ca + Mg ATP hydrolyzing activities of the American cockroach

Ca2+-stimulated ATP hydrolyzing activities (i.e. Na-Ca ATP hydrolysis and Ca + Mg ATP hydrolysis) measured in cockroach brain tissue were highly sensitive to the action of pyrethroid insecticides under in vitro conditions. Non-cyano-containing pyrethroids inhibited Na-Ca ATP hydrolysis to a greater extent than their cyano-containing counterparts. The reverse is true for pyrethroid action on Ca + Mg ATP hydrolysis. Nonmitochondrial Ca + Mg ATP hydrolysis of disrupted synaptosomes was the most sensitive activity examined. Ca2+-stimulated ATP hydrolyzing activities were inhibited in cockroaches poisoned by permethrin in vivo. In vivo poisoning occurred in the presence of a similar amount of bound [14C]permethrin which had been determined to cause a substantial amount of inhibition to Ca2+-stimulated ATP hydrolyzing activities in vitro.

Characterization of the ATP-promoted aspect of Na+-Ca2+ exchange present in squid retinal nerve axolemma

Using an in vitro system which consists of an axolemma-rich vesicle fraction prepared from squid retinal nerve fibers, an Na+-Ca2+ exchange process has been characterized and appears identical with that reported in squid giant axon. This exchange is absolutely dependent on the establishment of an Na+ gradient, shows monovalent and divalent cation specificity and is highly sensitive to monensin, A23187 and valinomycin but not to ouabain, digitoxigenin, vanadate, pentylenetetrazole, tetrodotoxin or tetraethylammonium. Furthermore, it was found that the exchange process is enhanced by the addition of ATP. This ATP-promoted aspects of Na+-Ca2+ exchange shares many similar characteristics with Na+-Ca2+ ATP hydrolysis and may indicate a common mechanism for both activities via a protein phosphorylation-dephosphorylation event.

Noradrenaline release induced by ouabain and vanadate in cat cerebral and peripheral arteries

The effects of 10(-4) M ouabain and 10(-3) M vanadate (Na3VO4) on [3H]noradrenaline release from cat cerebral and femoral arteries was studied. Ouabain induced tritium secretion in cerebral arteries, but not in femoral ones, which was reduced by Ca suppression and potentiated by extracellular Na reduction to 11.9 mM. However, vanadate evoked tritium release from both kinds of vessels was unaffected under these experimental conditions. These data suggest: ouabain elicited secretion from adrenergic nerve endings is likely due to inhibition of the Na, K-ATPase and subsequent Ca influx through Na-Ca exchange, and vanadate action is mediated by another mechanism different to the Na pump blockade.

Calcium transport mechanisms in dog red blood cells studied from measurements of initial flux rates

Ca2+ efflux from dog red blood cells loaded with Ca2+ using the A23187 ionophore could be separated into two main components: (1) Mg- and ATP-dependent (active transport) and (2) dependent on external Na (K1/2 around 15 mM); at 80 microM internal free Ca the relative magnitudes of these fluxes were 70% and 30% respectively. The Na-dependent Ca2+ efflux had the following additional properties: (i) it was partially inhibited by ATP depletion or preincubation with vanadate, but it was not affected by Mg2+ depletion; (ii) it failed to be stimulated by external monovalent cations other than Na: (iii) it was stimulated by reduction in the internal Na+ concentration. Both active and Na-dependent Ca2+ efflux remained unchanged in hypotonic solutions or in solutions with alkaline pH (8.5). In cells containing ATP and Mg2+, external Ca2+ inhibited Ca2+ efflux (K1/2 around 1 mM); on the other hand, in Mg-free dog red cells external Ca2+ stimulated Ca2+ efflux (K1/2 about 30 microM). In Mg-depleted red cells incubated in the absence of external Na2+, Ca2+ influx as a function of external Ca2+ followed a monotonically saturable function (K1/2 around 20 microM): addition of Na resulted in (i) inhibition of Ca2+ influx and (ii) a sigmoid relationship between flux and external Ca2+. Intracellular Ca2+ stimulated the external Na-dependent Ca2+ efflux along a sigmoid curve (K1/2 around 30 microM); on the other hand the Ca pump had a biphasic response to internal Ca2+: stimulation at low internal Ca2+ (K1/2 between 1 and 10 microM), followed by a decline at internal Ca2+ concentrations higher than 50 microM.

Forskolin mimics the hydrosmotic action of vasopressin in the urinary bladder of toads Bufo marinus

Net water flow JW was measured across the urinary bladder of toads Bufo marinus and averaged over periods of 1 min by means of a volumetric, automatic technique. The diterpene forskolin, an activator of adenylate cyclase bypassing the hormonal receptor subunit, induced a rapid, reversible, dose-dependent increase in osmotic water permeability, Pf, very similar to that induced by vasopressin. At 1.1 microM, forskolin induced a half-maximal response. At 5 microM forskolin caused a near maximal response and Pf increased from 1.66 +/- 0.15 to 66.6 +/- 2.99 microns s-1. In bladders pre-exposed to 5 microM-forskolin, further significant increases in Pf were obtained by their subsequent exposure to vasopressin, cyclic AMP, theophylline or serosal hypertonicity. The similarity of the forskolin and vasopressin actions was further demonstrated by the finding that substances causing enhancement (quercetin) or inhibition (trifluoperazine, vanadate, silver, cobalt, manganese and Ca2+-free Ringer solution) of the vasopressin response, induced parallel changes in the forskolin response. Three agents, however, induced dissimilar effects on vasopressin and forskolin: high K+ potentiated vasopressin but inhibited forskolin; methohexital and diamide inhibited vasopressin but had no effect on forskolin. The forskolin-induced hydrosmotic response can be viewed as a new criterion for ascertaining the messenger role of cycle AMP in the the hydrosmotic effect of vasopressin.

The influence of chemical agents on the level of ionized [Ca2+] in squid axons

Squid giant axons injected with either aequorin or arsenazo III and bathed in 3 mM Ca (Na) seawater were transferred to 3 mM Ca (K) seawater and the response of the aequorin light or the change in the absorbance of arsenazo III was followed. These experimental conditions were chosen because they measure the change in the rate of Na/Ca exchange in introducing Ca into the axon upon depolarization; [Ca]o is too low to effect a channel-based system of Ca entry. This procedure was applied to axons treated with a variety of compounds that have been implicated as inhibitors of Na/Ca exchange. The result obtained was that the substances tested could be placed in three groups. (a) Substances that were without effect on Ca entry effected by Na/Ca exchange were: D600 at 10-100 microM, nitrendipine at 1-5 microM, Ba2+ and Mg2+ at concentrations of 10-50 mM, lidocaine at 0.1-10 mM, cyanide at 2 mM, adriamycin at a concentration of 3 microM, chloradenosine at 35 microM, 2,4-diaminopyridine at 1 mM, Cs+ at 45-90 mM, and tetrodotoxin at 10(-7). (b) Substances that had a significant inhibitory effect on Na/Ca exchange were: Mn2+, Cd2+, and La3+ at 1-50 mM, and quinidine at 50 microM. (c) There were also blocking agents and biochemical inhibitors whose action appeared to be the inhibition of nonmitochondrial Ca buffering in axoplasm rather than an inhibition of Na/Ca exchange. These were the general anesthetic l-octanol at 0.1 mM and 1 mM orthovanadate plus apyrase.

Cooling-induced supersensitivity to acetylcholine in the isolated airway smooth muscle of the rat

Isolated tracheal and bronchial strip-chain preparations of the rat were used to study the effect of temperature on electrically or acetylcholine-induced contraction. The preparations were suspended in the organ bath containing Krebs bicarbonate solution for isometric tension recording. A decrease of bath temperature from 37 degrees C to 20 degrees C (cooling) had no effect on basal tone but augmented the contractile responses of the trachea and bronchus caused by stimulation of intramural cholinergic nerves (0.5-5 Hz) or acetylcholine (3 mumol/l-0.3 mmol/l). Cooling-induced augmentation of the contractile response to acetylcholine was not affected by pretreatment of the tissue with physostigmine (0.1 mumol/l) or tetrodotoxin (0.3 mumol/l). The affinity of acetylcholine for the tracheal muscarinic receptors at 20 degrees C, determined from its dissociation constant (KA), was not significantly different from that at 37 degrees C. On the other hand, acetylcholine-induced contraction of trachea which was incubated with isosmotic K+- rich Krebs solution and with Ca-free, EGTA (0.1 mmol/l) containing Krebs solution were both augmented at 20 degrees C. Caffeine or vanadate, each at a lower concentration than the threshold for causing contraction by itself, augmented the contractile responses of the trachea to acetylcholine (1 mumol/l-0.3 mmol/l). These potentiating effects of caffeine and vanadate were greater at 20 degrees C then 37 degrees C. From these observations, it is concluded that increased responsiveness of the rat airway smooth muscle to acetylcholine with lowered temperature may involve the acceleration of Ca release from intracellular storage sites, inhibition of Ca extrusion from the cell and or the inhibition of Ca reuptake by intracellular storage sites.

ATP-dependent calcium accumulation by non-mitochondrial organelles of axoplasm isolated from Myxicola giant axons

Axoplasm from freshly isolated Myxicola giant axons was mixed with small volumes of 'artificial axoplasm' containing 45Ca and either CaEGTA/EGTA or CaDTPA/DTPA buffers giving various nominal values of [Ca2+]. The axoplasm samples were centrifuged at 100 000 X g for 30 min to form a pellet and the percentage of 45Ca bound to the pellet was determined. The fraction of bound calcium rose with increasing values of [Ca2+] along an S-shaped curve. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) was used to reveal the presence of mitochondrial Ca uptake. At physiological values of [Ca2+], around 100 nM, Ca uptake was insensitive to FCCP. As [Ca2+] was elevated, increasing sensitivity to FCCP was noted above [Ca2+] = 0.5 microM. At low values of [Ca2+], including the physiological range, Ca binding was significantly reduced by vanadate and quercetin, agents known to inhibit Ca uptake mediated by Ca2+-activated ATPase reactions. Inhibition of Ca binding by these agents was approximately 50% at physiological values of [Ca2+]. ATP depletion decreased the percentage of Ca binding by the pellet at physiological [Ca2+]. The results suggest that about 50% of the Ca buffering by particulate matter in axoplasm is via organelles requiring intact Ca2+-ATPase reaction at physiological values of [Ca2+].

Modulation of apical Na permeability of the toad urinary bladder by intracellular Na, Ca, and H

The Na conductance of the apical membrane of the toad urinary bladder was measured at different concentrations of Na both in the external medium and in the cell. Bladders were bathed in high K-sucrose medium to reduce basal-lateral resistance and voltage, and the transepithelial currents measured under voltage-clamp conditions. Amiloride was used as a specific blocker of the apical Na channel. At constant external Na, the internal Na concentration was increased by blocking the basal-lateral Na pump with ouabain. With high Na activity in the mucosal medium (86 mM), increases in intracellular Na activity from 10 to over 40 mM increased the amiloride-sensitive slope conductance at zero voltage while apical Na permeability, estimated from current-voltage plots using the constant field equation, decreased by less than 20%. Lowering the serosal Ca concentration from 1 to 0.1 mM had no effect on the change in PNa with increasing Nac, but increasing serosal Ca to 5 mM enhanced the reduction in PNa with increasing Nac, presumably by increasing Ca influx into the cell. PNa was also reduced by serosal vanadate (0.5 mM), a putative blocker of ATP-dependent Ca extrusion from the cell, and by acute exposure to CO2, which presumably acidifies the cytoplasm. Current-voltage relationships of the amiloride-sensitive transport pathway were also measured in the absence of a Na gradient across the apical membrane. These plots show that outward current passes through the channels somewhat less easily than does inward current. The shape of the I-V relationships was not significantly altered by changes in cellular Na, Ca or H, indicating that the effects of these ions on PNa are voltage independent.

Interactions of physiological ligands with the Ca pump and Na/Ca exchange in squid axons

We have studied the interaction of physiological ligands other than Nai and Cai with the Ca pump and Na/Ca exchange in internally dialyzed squid axons. The results show the following. (a) Internal Mg2+ is an inhibitor of the Nao-dependent Ca efflux. At physiological Mg2+i (4 mM), the inhibition amounts to approximately 50%. The inhibition is partial and noncompetitive with Cai, and is not affected by Nai or ATP. The ATP-dependent uncoupled efflux is unaffected by Mgi up to 20 mM. Both components of the Ca efflux require Mg2+i for their activation by ATP. (b) At constant membrane potential, Ki is an important cofactor for the uncoupled Ca efflux. (c) Orthophosphate (Pi) activates the Nao-dependent Ca efflux without affecting the uncoupled component. Activation by Pi occurs only in the presence of Mg-ATP or hydrolyzable ATP analogues. Pi under physiological conditions has no effect on the uncoupled component; nevertheless, at alkaline pH, it inhibits the Ca pump, probably by product inhibition. (d) ADP is a potent inhibitor of the uncoupled Ca efflux. The Nao-dependent component is inhibited by ADP only at much higher ADP concentrations. These results indicate that (a) depending on the concentration of Ca2+i, Na+i Mg2+i, and Pi, the Na/Ca carrier can operate under a low- or high-rate regime; (b) the interactions of Mg2+i, Pi, Na+i, and ATP with the carrier are not interdependent; (c) the effect of Pi on the carrier-mediated Ca efflux resembles the stimulation of the Nao-dependent Ca efflux by internal vanadate; (d) the ligand effects on the uncoupled Ca efflux are of the type seen in the Ca pump in red cells and the sarcoplasmic reticulum.

Inhibition by orthovanadate of ATP-dependent Ca2+ transport in microsomes isolated from rat liver

A technique employing sucrose-density centrifugation for the enrichment of rat liver microsomes and rat liver plasma membranes in separate subcellular fractions is described. The fractions are enriched in glucose 6-phosphatase and 5'-nucleotidase, respectively, and are free of cytochrome oxidase activity. Vanadate-sensitive Ca2+ transport activity (half-maximal inhibition at approximately 10 microM vanadate, corresponding to approximately 12 nmol/mg of protein) was detected in only that fraction enriched in microsomal membranes. Inhibition by vanadate of ATP-dependent Ca2+ transport is noncompetitive with respect to added Ca2+ but competitive with respect to added ATP. Because it inhibits ATP-dependent Ca2+ transport in rat liver microsomes but not in rat liver plasma membranes, vanadate becomes a useful tool to distinguish in vitro between these two transport systems.

Sodium- and adenosine-triphosphate-dependent calcium movements in membrane vesicles prepared from dog erythrocytes

Inside-out vesicles from the membranes of dog erythrocytes were obtained by the method of Lew & Seymour (1982) for study of Ca movements. In the absence of ATP, 45Ca accumulation by the vesicles was inhibited by external Na and stimulated by internal Na. The presence of either MgCl2, quinidine sulphate, or LaCl3 in the incubation medium inhibited 45Ca accumulation in the absence of ATP. The release of 45Ca from 45Ca-loaded vesicles was specifically promoted by Na+ in the absence as well as in the presence of ATP. The accumulation of 45Ca by vesicles was stimulated by ATP and the effect of ATP was entirely dependent on the presence of Mg. The Mg- and ATP-dependent 45Ca accumulation was stimulated by the presence of either K or Na in the medium, was hyperbolically activated by increasing the Ca2+ concentration in the medium, was stimulated by calmodulin and inhibited by orthovanadate (10(-4) M) or LaCl3 (10(-3) M). The data demonstrate the presence of two mechanisms for controlling Ca movements in inside-out vesicles from dog erythrocyte membranes, a Na-dependent one similar to the Na-Ca exchange described for squid axons and cardiac muscle and a Ca pump utilizing ATP with characteristics similar to those described for human erythrocytes and squid axons.

Maintained contractions of rat uterine smooth muscle incubated in a Ca2+-free solution

The effects of acetylcholine (10(-4) M), prostaglandin E2 (10(-6) M), vanadate (5 X 10(-4) M) and fluoride (10(-2) M) have been studied on the mechanical and electrical activities of rat myometrial strips perfused in Ca2+-free EGTA-containing solutions. All four substances produced maintained contractions which could be initiated repeatedly after exposure to Ca2+-free solution for more than 1 h, without a significant decrease. The largest contractions were obtained with vanadate and the smallest ones with acetylcholine. The tension was usually 7-30% of the control contraction triggered by an action potential in Ca2+ containing solution. Maintained contractions induced by fluoride were unaffected by isoprenaline while those induced by acetylcholine, prostaglandin E2 and vanadate were completely relaxed. Prostaglandin E2- and vanadate-induced contractions were slightly reduced by Na+ removal or by adding Ca2+ antagonists. In contrast, contractions induced by acetylcholine were suppressed in Na+-free solution and largely inhibited in the presence of Ca2+ antagonists. The depolarization induced by acetylcholine in Ca2+-free solution was strongly dependent on the external Na+ concentration. The relationship between the size of the acetylcholine-induced depolarization and the membrane potential (shifted by constant currents) was linear, giving an apparent reversal potential for acetylcholine close to zero potential. In Ca-free solutions and in the presence of atropine, Na+ action potentials of long duration can be evoked which produced contractions of the same order of magnitude as those initiated by acetylcholine-induced depolarizations. 7 These results are consistent with the hypothesis that the maintained contractions in Ca2+-free solutions induced by several stimulants could be related to Ca2+-independent mechanisms (fluoride) or Ca2+ release from an intracellular store. This latter mechanism would include both pharmacomechanical (prostaglandin E2, vanadate) and electromechanical (acetylcholine) coupling.

Vanadate and ouabain: a comparative study in toad skin

In this study we compare the effects of two inhibitors of the Na,K-ATPase, ouabain and vanadate, upon transport properties of the isolated short-circuited toad skin: The main conclusions are: Both inhibitors induce a similar decline in short-circuit current (SCC). They differ regarding skin electrical resistance (R). Ouabain induces an increase in resistance that, after some delay, builds up slowly after its addition to the preparation, while vanadate causes a fast increase in resistance that remains constant for most of the experimental period. Vanadate, but not ouabain, promotes an unspecific increase in skin permeability characterized by a delayed and progressive rise of 42K (JK eff) and 14C sucrose (J suc eff) effluxes. Vanadate effect upon skin permeability, as measured by JK eff, is not affected by pre-treating the skin with DIDS, a stilbene derivative, indicating that anion-exchange is not an important step for the entrance of vanadate into the epithelial cells to trigger its effect. Vanadate effect upon JK eff is also not affected by previous ouabain inhibition of the Na,K-ATPase, showing that this effect is not mediated by the inhibition of this enzyme. Vanadate action in toad skin seems to occur at junctional structures opening paracellular routes. A possible mechanism for the effect of vanadate is discussed in terms of cytosolic Ca2+ balance, cytoskeleton and their interplay with the sealing of tight junctions.

Contractions of rat uterine smooth muscle induced by acetylcholine and angiotensin II in Ca2+-free medium

The effects of acetylcholine (ACh, 10(-4)M) and angiotensin II (Ang II, 10(-6) M) have been studied on the mechanical and electrical activities of rat myometrial strips perfused in Ca2+-free EGTA-containing solutions. Both ACh and Ang II produced transient contractions, the amplitude of which can be taken as a measurement of the amount of Ca2+ present in a drug-sensitive Ca2+ store. The degree of filling of this store depended on the external Ca2+ concentration, and on the presence of contractile responses during the Ca2+ loading period. The existence of two pathways (either direct or transcytoplasmic) is suggested for Ca2+ uptake into the internal Ca2+ store. The rate of filling of the Ca2+ store in 2.1 mM-Ca2+-containing solution was faster (time to half-maximal response, t 1/2 = 29 +/- 2.2 s, n = 4) than the rate of depletion in Ca2+-free solution (t 1/2 = 3 +/- 0.3 min, n = 3). The gradual depletion of this store was much slower at 18 degrees C than at 35 degrees C, and in the presence of vanadate which is known to inhibit Ca2+-ATPases. Methoxyverapamil (D600, 10(-6)-10(-5) M) had no appreciable effect on the direct Ca2+ uptake or on the release of Ca2+ from the store by ACh and Ang II. Mn2+ (10(-3) M) completely inhibited the direct pathway to the internal Ca2+ store and also reduced the release of Ca2+. ACh and Ang II induced repetitive depolarizations close to zero potential which did not parallel the transient contractions as a function of the time of perfusion in Ca2+-free solution. Applications of 2 mM EGTA, 135 mM K+ or Ca2+ antagonists which suppressed or reduced the drug-induced depolarizations did not affect appreciably the drug-induced contractions. These results suggest that myometrial cells have an intracellular Ca2+ store sensitive to different stimulus substances. This store is not affected by depolarization of the plasma membrane and is certainly different from that described in voltage-clamp experiments.

Partial purification and characterization of the (Ca2+ + Mg2+)-ATPase from squid optic nerve plasma membrane

A membrane fraction enriched in axolemma was obtained from optic nerves of the squid (Sepiotheutis sepioidea) by differential centrifugation and density gradient fractionation. The preparation showed an oligomycin- and NaN3-insensitive (Ca2+ + Mg2+)-ATPase activity. The dependence of the ATPase activity on calcium concentration revealed the presence of two saturable components. One had a high affinity for calcium (K1 1/2 = 0.12 microM) and the second had a comparatively low affinity (K2 1/2 = 49.5 microM). Only the high-affinity component was specifically inhibited by vanadate (K1 = 35 microM). Calmodulin (12.5 micrograms/ml) stimulated the (Ca2+ + Mg2+)-ATPase by approx. 50%, and this stimulation was abolished by trifluoperazine (10 microM). Further treatment of the membrane fraction with 1% Nonidet P-40 resulted in a partial purification of the ATPase about 15-fold compared to the initial homogenate. This (Ca2+ + Mg2+)-ATPase from squid optic nerve displays some properties similar to those of the uncoupled Ca2+-pump described in internally dialyzed squid axons, suggesting that it could be its enzymatic basis.

Minireview: physiological and pharmacological properties of vanadium

Vanadium is distributed extensively in nature. It is a trace element and is present in almost all living organisms including man. Even though vanadium was originally recognized for its ability to inhibit membrane Na+-K+-ATPase, various laboratory studies now document that this element has the capacity to affect the activity of various intracellular enzyme systems and may modify their physiological functions. Vanadium may be an essential element for normal development and may play an important role in various homeostatic mechanisms, and thus vanadium deficiency may prove to be an important concern. Abnormalities in biological disposition of vanadium may be involved in the pathogenesis of certain neurological disorders or cardiovascular diseases. While the essentiality of this element for living organisms is yet to be established with certainty, vanadium has become an increasingly important element and is used extensively in various heavy industries such as steel, oil, etc.; thus, the incidence of exposure to toxic levels of vanadium to industrial workers has been an increasing concern for toxicologists. To date, little information is available on the physiological or pharmacological actions of vanadium; hence, it is difficult to reach any definitive conclusion concerning its biological significance, essentiality and its role in pathological states. An attempt has been made in this review to broadly document what is known of various biological actions of vanadium.

Kinetic properties of Na+/Ca2+ exchange in basolateral plasma membranes of rat small intestine

The presence of an Na+/Ca2+ exchange system in basolateral plasma membranes from rat small intestinal epithelium has been demonstrated by studying Na+ gradient-dependent Ca2+ uptake and the inhibition of ATP-dependent Ca2+ accumulation by Na+. The presence of 75 mM Na+ in the uptake solution reduces ATP-dependent Ca2+ transport by 45%, despite the fact that Na+ does not affect Ca2+-ATPase activity. Preincubation of the membrane vesicles with ouabain or monensin reduces the Na+ inhibition of ATP-dependent Ca2+ uptake to 20%, apparently by preventing accumulation of Na+ in the vesicles realized by the Na+-pump.l It was concluded that high intravesicular Na+ competes with Ca2+ from intravesicular Ca2+ binding sites. In the presence of ouabain, the inhibition of ATP-dependent Ca2+ transport shows a sigmoidal dependence on the Na+ concentration, suggesting cooperative interaction between counter transport of at least two sodium ions for one calcium ion. The apparent affinity for Na+ is between 15 and 20 mM. Uptake of Ca2+ in the absence of ATP can be enhanced by an Na+ gradient (Na+ inside greater than Na+ outside). This Na+ gradient-dependent Ca2+ uptake is further stimulated by an inside positive membrane potential but abolished by monensin. The apparent affinity for Ca2+ of this system is below 1 microM. In contrast to the ATP-dependent Ca2+ transport, there is no significant difference in Na+ gradient-dependent Ca2+ uptake between basolateral vesicles from duodenum, midjejunum and terminal ileum. In duodenum the activity of ATP-driven Ca2+ uptake is 5-times greater than the greater than the Na+/Ca2+ exchange capacity but in the ileum both systems are of equal potency. Furthermore, the Na+/Ca2+ exchange mechanism is not subject to regulation by 1 alpha, 25-dihydroxy vitamin D-3, since repletion of vitamin D-deficient rats with this seco-steroid hormone does not influence the Na+/Ca2+ exchange system while it doubles the ATP-driven Ca2+ pump activity.

Metabolism and transport of strontium in giant axons of Loligo

1. The strontium content of squid axons averages 2.9 mumol kg-1 axoplasm and squid blood contains 75 mumol Sr l-1. The bulk of the Sr inside axons is bound, possibly within mitochondria. Sr seems to be handled by squid axons in an essentially similar manner to Ca. 2. Sr uptake is increased by electrical activity, by lowering external Na, by depolarization with K and by poisoning with cyanide or FCCP. Uptake is unaffected by ouabain. 3. Sr efflux has a Q10 of 2.2. Efflux can be divided operationally into three components: one dependent on external Ca (or Sr), one dependent on external Na and a residual component that can be inhibited by external lanthanum or internal vanadate. 4. Poisoning an axon with cyanide or FCCP results in a large increase in Sr efflux which can be divided operationally into two components: one dependent on external Ca (or Sr) and one dependent on external Na.

Solubilization and reconstitution of membranes containing the Na+ -Ca2+ exchange carrier from rat brain

The Na+ -Ca2+ exchange carrier from brain plasmalemma was solubilized in cholate and reconstituted into asolectin vesicles by the cholate dilution method. Optimal solubilization and reconstitution required the presence of high NaCl (greater than or equal to 1.3 M). The reconstituted vesicles rapidly accumulated 45Ca2+ in the presence of an outward directed Na+ gradient. Other monovalent ion gradients (K+, Li+ or cholate+) did not drive transport. Further, Mg2+ X ATP did not drive Ca2+ uptake in the reconstituted vesicles. Uptake was temperature dependent with highest uptake occurring at 37 degrees C. Intravesicular Ca2+ accumulated by the Na+ -dependent process could be released by the Ca2+ ionophore A23187 or by extravesicular Na+ but not by external EGTA. Ca2+ uptake was inhibited by extravesicular Li+ or Na+. The Ki for Na+ inhibition was 35 mM for both the original membrane vesicles from brain plasmalemma and for the reconstituted vesicles. Ca2+ uptake was saturable with respect to extravesicular Ca2+ (Km(Ca2+) = 27 microM).

ATP-dependent calcium transport and its correlation with Ca2+ -ATPase activity in basolateral plasma membranes of rat duodenum

Isolated basolateral plasma membrane vesicles from rat duodenum epithelial cells exhibit ATP-dependent calcium-accumulation and Ca2+ -dependent ATPase activity. Calcium accumulation stimulated by ATP is prevented by the calcium ionophore A23187, inhibited 80% by 0.1 mM orthovanadate but is not effected by oligomycin. Calcium accumulation is not observed with the substrate beta-gamma-(CH2)-ATP, ADP and p-nitrophenyl phosphate. Kinetic studies reveal an apparent Km of 0.2 microM Ca2+ and a Vmax of 5.3 nmol Ca2+/min per mg protein for the ATP-dependent calcium-uptake system. Calmodulin and phenothiazines have no effect on calcium accumulation in freshly prepared membranes, but small effects are inducible after a wash with a 5 mM EGTA. The kinetic parameters of Ca2+ -ATPase are: Km = 0.25 microM Ca2+ and Vmax = 19.2 nmol Pi/min per mg protein. Three techniques, osmotic shock, treatment with Triton X-100 or the channel-forming peptide alamethicin, reveal that about 40% of the vesicles are resealed. Assuming that half of the resealed vesicles have an inside-out orientation, the Vmax of ATP-dependent calcium uptake amounts to 25 nmol Ca2+/min per mg protein and of the Ca2+ -ATPase to 23 nmol Pi/min per mg protein. The close correlation between kinetic parameters of Ca2+ -ATPase and ATP-dependent calcium-transport strongly suggests that both systems are expressions of a Ca2+ -pump located in duodenal basolateral plasma membranes.

Calcium efflux from the rat neurohypophysis

1. Calcium efflux from isolated rat neurophypophyses has been studied. Curve fitting of the wash-out curves suggests three phases with t((1/2)) of ca. 3, 15 and 130 min.2. The slow component of the (45)Ca efflux is attributed to efflux of intracellular Ca. On the basis of the temperature sensitivity of the Ca efflux, the activation energy has been calculated to be approximately 12,000 cal/mole, corresponding to a Q(10) of ca. 2.0.3. Ca efflux decreased by approximately 32% when external Na was replaced by choline. Li(o), in the presence or absence of Ca(o), was as effective as Na(o) in stimulating the Ca efflux.4. The curve relating Ca efflux to [Na](o) or [Li](o) is sigmoid and suggests that at least two Na (or Li) ions are necessary to activate the efflux of each Ca ion. Ca(o) does not modify the absolute Na-dependent Ca efflux but decreases the affinity for Na of the site involved in Ca extrusion.5. Removal of Ca(o) decreased the Ca efflux by ca. 44% in Na-free media. The apparent affinity for Ca(o) of the Ca(o)-activated Ca efflux (K(m) (Cao) = 20 muM) is greatly decreased by the presence of 150 mM-Na (K(m) (Cao) = 0.8 mM).6. Lanthanum decreased the total Ca efflux by ca. 60% and totally abolished the Na(o)-activated and Ca(o)-activated Ca efflux.7. Vanadate reduced the Ca efflux remaining in Na-, Ca-free saline by 73%.8. Elevation of Na(i) with ouabain did not modify the rate of loss of (45)Ca.9. Increased concentration of K(o) stimulated transiently the (45)Ca loss. The time course of this increase depends on the Ca(o) concentration ([Ca](o)).10. Cyanide or CCCP (carbonyl cyanide m-chlorophenylhydrazone) increased transiently the Ca efflux. The increase induced by cyanide could only be observed when the neural lobes had been over-loaded with (45)Ca.11. Membrane destruction induced by high temperature eliminated the effect of [Na](o) and [Ca](o) on (45)Ca efflux.12. In 150 mM-Na-containing saline, half-maximum activation of (45)Ca uptake occurs in the 0.2-0.4 mM [Ca](o) range.13. The Ca efflux from isolated pituicytes was not affected by removal of Na(o).14. In conclusion we show that Ca efflux from neurosecretory nerve terminals can be subdivided into three components of approximately the same magnitude, one which is activated by Na(o), another by Ca(o) and a third component which is independent of Na(o) and Ca(o).

The effects of vanadate on calcium transport in dialyzed squid axons. Sidedness of vanadate-cation interactions

(1) Vanadate (VO3-) fully inhibits the ATP-dependent uncoupled Ca efflux (Ca pump) in dialyzed squid axons. (2) Vanadate inhibits with high affinity. The mean apparent affinity (K 1/2) obtained was 7 microM. (3) Inhibition by vanadate is dependent on Cao. External Ca lead to a release of the inhibitory effect. (K 1/2 congruent to 3 mM). This antagonistic effect can be reverted by increasing the vanadate concentration. Internal K+ increases the affinity of the intracellular vanadate binding site. External K+ has no effect on the inhibition. (4) Vanadate has no effect on the Nao-dependent Ca efflux component (forward Na-Ca exchange) in the absence of ATP. In axons containing ATP vanadate modified this component.

Ca2+ transport by intact synaptosomes: the voltage-dependent Ca2+ channel and a re-evaluation of the role of sodium/calcium exchange

The verapamil-sensitive Ca2+ channel in the synaptosomal plasma membrane is investigated. Verapamil is without effect on Ca2+ uptake or steady-state content in synaptosomes with a polarized plasma membrane, but completely inhibits the additional Ca2+ uptake following plasma-membrane depolarization by high [K+], by veratridine plus ouabain or by high concentrations of the permeant cation tetraphenylphosphonium. Verapamil-insensitive Ca2+ influx and steady-state content are identical in polarized and depolarized synaptosomes, even though the Na+ electrochemical potential is greatly decreased in the latter, indicating that Na+/Ca2+ exchange is not a significant mechanism for Ca2+ efflux under these conditions. A transient Na+-dependent Ca2+ efflux can only be observed on addition of Na+ to Na+-depleted depolarized synaptosomes. While 0.2 mM verapamil decreases the ate of 86Rb+ efflux and 22Na+ entry during depolarization induced by veratridine plus ouabain, the final steady-state Na+ accumulation is not inhibited. Ca2+ efflux from synaptosomes following mitochondrial depolarization does not occur by a verapamil-sensitive pathway.

A (Ca2+, Mg2+)-ATPase activity in plasma membrane fragments isolated from squid nerves

A (Ca2+, Mg2+)-ATPase activity and a (Ca2+, Mg2+)-dependent phosphorylation from ATP have been found in plasma membrane fragments from squid optical nerves under conditions where contamination by intracellular organelles is unlikely. The properties of this (Ca2+, Mg2+)-ATPase activity are almost identical to those of the ATP-dependent uncoupled Ca2+ efflux observed in dialyzed squid giant axons. This gives further support to the notion that the mechanism responsible for maintaining the low levels of ionized Ca concentration in nerves at rest is not a Na+-Ca2+ exchange system but an ATP-driven uncoupled Ca2+ pump.

Metabolism in the cytosol of intact isolated cattle rod outer segments as indicator for cytosolic calcium and magnesium ions

The metabolism of the chromophore of rhodopsin in the cytosol compartment of isolated intact cattle rod outer segments was used as an indicator for changes of the cytosolic Mg2+ and Ca2+ concentration upon changes of the external Mg2+ and Ca2+ concentration. The reduction of retinal to retinol upon photolysis of rhodopsin in situ in intact rod outer segments was critically dependent on the availability of cytosolic Mg2+. The latter is necessary as chelator of endogenous adenosine 5'-triphosphate (ATP). Lowering the cytosolic Ca2+ concentration beneath 10(-7) M resulted in an inhibition of the rate of retinol formation. This is presumably due to a light-activated process, which competes with retinol formation for the supply of high-energy phosphate from a common pool. These results led to the following conclusions. Changes of the external Mg2+ concentration are only followed by substantial changes of the cytosolic Mg2+ concentration when the ionophore A23187 is present. Changes of the external Ca2+ concentration are followed by parallel changes of the cytosolic Ca2+ concentration either when external Na+ is present or in the presence of A23187. Li+ and K+ could not substitute for Na+ in the former case, but K+ diminished the effectivity of Na+ at low Na+ concentrations and enhanced it at high Na+ concentrations. It is concluded that the control of cytosolic Ca2+ concentration in isolated intact rod outer segments is predominantly provided for by Na-Ca exchange, i.e., by coupled fluxes.