Characterization of a high-affinity Mg2+-independent Ca2+-ATPase from rat brain synaptosomal membranes

Structural and functional comparison of magnesium transporters throughout evolution

Magnesium (Mg²⁺) is the most prevalent divalent intracellular cation. As co-factor in many enzymatic reactions, Mg²⁺ is essential for protein synthesis, energy production, and DNA stability. Disturbances in intracellular Mg²⁺ concentrations, therefore, unequivocally result in delayed cell growth and metabolic defects. To maintain physiological Mg²⁺ levels, all organisms rely on balanced Mg²⁺ influx and efflux via Mg²⁺ channels and transporters. This review compares the structure and the function of prokaryotic Mg²⁺ transporters and their eukaryotic counterparts. In prokaryotes, cellular Mg²⁺ homeostasis is orchestrated via the CorA, MgtA/B, MgtE, and CorB/C Mg²⁺ transporters. For CorA, MgtE, and CorB/C, the motifs that form the selectivity pore are conserved during evolution. These findings suggest that CNNM proteins, the vertebrate orthologues of CorB/C, also have Mg²⁺ transport capacity. Whereas CorA and CorB/C proteins share the gross quaternary structure and functional properties with their respective orthologues, the MgtE channel only shares the selectivity pore with SLC41 Na⁺/Mg²⁺ transporters. In eukaryotes, TRPM6 and TRPM7 Mg²⁺ channels provide an additional Mg²⁺ transport mechanism, consisting of a fusion of channel with a kinase. The unique features these TRP channels allow the integration of hormonal, cellular, and transcriptional regulatory pathways that determine their Mg²⁺ transport capacity. Our review demonstrates that understanding the structure and function of prokaryotic magnesiotropic proteins aids in our basic understanding of Mg²⁺ transport.

Trypanosoma evansi infection impairs memory, increases anxiety behaviour and alters neurochemical parameters in rats

The aim of this study was to investigate neurochemical and enzymatic changes in rats infected with Trypanosoma evansi, and their interference in the cognitive parameters. Behavioural assessment (assessment of cognitive performance), evaluation of cerebral L-[3H]glutamate uptake, acetylcholinesterase (AChE) activity and Ca+2 and Na+, K+-ATPase activity were evaluated at 5 and 30 days post infection (dpi). This study demonstrates a cognitive impairment in rats infected with T. evansi. At 5 dpi memory deficit was demonstrated by an inhibitory avoidance test. With the chronicity of the disease (30 dpi) animals showed anxiety symptoms. It is possible the inhibition of cerebral Na+, K+-ATPase activity, AChE and synaptosomal glutamate uptake are involved in cognitive impairment in infected rats by T. evansi. The understanding of cerebral host–parasite relationship may shed some light on the cryptic symptoms of animals and possibly human infection where patients often present with other central nervous system (CNS) disorders.

Effect of hyperoxia on cortical neuronal nuclear function and programmed cell death mechanisms

There is growing concern over detrimental neurologic effects to human newborns caused by increased inspired oxygen concentrations. We hypothesize that hyperoxia (FiO(2)>0.95) results in increased high-affinity Ca(2+)-ATPase activity, Ca(2+)-influx, and proapoptotic protein expression in cortical neuronal nuclei of newborn piglets. Neuronal cerebral energy metabolism was documented by determining ATP and phosphocreatine levels. Neuronal nuclear conjugated dienes and fluorescent compounds were measured as indices of lipid peroxidation. High-affinity Ca(2+)-ATPase activity and ATP-dependent Ca(2+)-influx were determined to document neuronal nuclear membrane function. Hyperoxia resulted in increases in lipid peroxidation, high-affinity Ca(2+)-ATPase activity, ATP-dependent Ca(2+)-influx, and Bax/Bcl-2 ratio in the cortical neuronal nuclei of newborn piglets. We conclude that hyperoxia results in modification of neuronal nuclear membrane function leading to increased nuclear Ca(2+)-influx, and propose that hyperoxia-induced increases in intranuclear Ca(2+) activates the Ca(2+)/calmodulin-dependent protein kinase pathway, triggering increased CREB protein-mediated apoptotic protein expression in hyperoxic neurons.

Stimulation of Mg2+-independent form of Ca2+-ATPase by a low molecular mass protein purified from goat testes cytosol

A low molecular mass protein purified from goat (Capra hircus) testes cytosol following gel filtration and anion exchange chromatographic separation stimulates Mg(2+)-independent Ca(2+)-ATPase activity without any significant effect on Mg(2+)-dependent Ca(2+)-ATPase. Stimulation of the ATPase is due to an increase in the rate of dephosphorylation of the overall reaction step of the enzyme. Binding of the stimulator increases the affinity of Ca(2+)-ATPase for Ca(2+). An analysis of enzyme kinetics reveals a reversible type of binding of the stimulator to the ATPase and non-competitive type of stimulation with respect to the substrate. Stimulation seems due to binding of the protein at a single site following Michaelis-Menten model. The protein can also counter the effect of calcium antagonists exerted on the ATPase. The pI of the protein is 6.2 and its molecular mass has been determined to be 13, 961 by Q-TOF-MS.

In vitro effect of zinc and cadmium on acetylcholinesterase and ectonucleotidase activities in zebrafish (Danio rerio) brain

Zinc and cadmium are environmental contaminants that induce a wide range of effects on CNS. Here we tested the in vitro effect of these metals on acetylcholinesterase (AChE) and ectonucleotidase (NTPDase and ecto-5'-nucleotidase) activities in zebrafish brain. Both zinc and cadmium treatments did not alter significantly the zebrafish brain AChE activity. ATP hydrolysis presented a significant increase at 1 mM zinc (17%) and the AMPase activity had a dose-dependent increase at 0.5 and 1 mM zinc exposure (188% and 199%). After cadmium treatment, ATPase activity was significantly increased (53% and 48%) at 0.5 and 1 mM, respectively. Cadmium, in the range 0.25-1 mM, inhibited ADP hydrolysis in a dose-dependent manner (13.4-69%). Ecto-5'-nucleotidase activity was only inhibited (38%) in the presence of 1 mM cadmium. It is possible to suggest that changes on NTPDase and ecto-5'-nucleotidase activities can be an important mechanism involved in neurotoxic effects promoted by zinc and cadmium.

Hypoxia modifies nuclear calcium uptake pathways in the cerebral cortex of the guinea-pig fetus

Nuclear Ca2+ signals are thought to play a critical role in the initiation and progression of programmed cell death. The present study tests the hypothesis that hypoxia alters nuclear Ca2+ transport pathways and leads to an increase in nuclear Ca(2+)-influx in cerebral cortical neuronal nuclei. To test this hypothesis the effect of tissue hypoxia on high affinity Ca(2+)-ATPase activity and the binding characteristics of inositol 1,4,5-triphosphate (IP3) and inositol 1,3,4,5-tetrakisphosphate (IP4) receptors were studied in neuronal nuclei from the cerebral cortex of guinea-pig fetuses. Results show increased high-affinity Ca(2+)-ATPase activity (nmol/mg protein/h) in the hypoxic group 969.7+/-79 as compared with 602.4+/-90.9 in the normoxic group, P<0.05. The number of IP3 receptors (Bmax, fmol/mg protein) increased from 61+/-21 in the normoxic group to 164+/-49 in the hypoxic group, P<0.05. K(d) values did not change following hypoxia. In contrast, IP4 receptor Bmax (fmol/mg protein) and K(d) (nM) values increased from 360+/-32 in the normoxic group to 626+/-136 in the hypoxic group (P<0.001) and, from 26+/-1 in the normoxic group to 61+/-9 in the hypoxic group (P<0.001), respectively. 45Ca(2+)-influx (pmol/mg protein) significantly increased from 6.3+/-1.9 in the normoxic group to 10.9+/-1.1 the hypoxic group (P<0.001). The data show that hypoxia modifies nuclear Ca2+ transport pathways and results in increased nuclear Ca(2+)-influx. We speculate that hypoxia increases nuclear Ca2+ uptake from the cytoplasm to the nucleoplasm, resulting in increased transcription of proapoptotic genes and subsequent activation of programmed cell death pathways.

ATP and ADP hydrolysis in brain membranes of zebrafish (Danio rerio)

Nucleotides, e.g. ATP and ADP, are important signaling molecules, which elicit several biological responses. The degradation of nucleotides is catalyzed by a family of enzymes called NTPDases (nucleoside triphosphate diphosphohydrolases). The present study reports the enzymatic properties of a NTPDase (CD39, apyrase, ATP diphosphohydrolase) in brain membranes of zebrafish (Danio rerio). This enzyme was cation-dependent, with a maximal rate for ATP and ADP hydrolysis in a pH range of 7.5-8.0 in the presence of Ca(2+) (5 mM). The enzyme displayed a maximal activity for ATP and ADP hydrolysis at 37 degrees C. It was able to hydrolyze purine and pyrimidine nucleosides 5'-di and triphosphates, being insensitive to classical ATPase inhibitors, such as ouabain (1 mM), N-ethylmaleimide (0.1 mM), orthovanadate (0.1 mM) and sodium azide (0.1 mM). A significant inhibition of ATP and ADP hydrolysis (68% and 34%, respectively) was observed in the presence of 20 mM sodium azide, used as a possible inhibitor of ATP diphosphohydrolase. Levamisole (1 mM) and tetramisole (1 mM), specific inhibitors of alkaline phosphatase and P1, P(5)-di (adenosine 5'-) pentaphosphate, an inhibitor of adenylate kinase did not alter the enzyme activity. The presence of a NTPDase in brain membranes of zebrafish may be important for the modulation of nucleotide and nucleoside levels, controlling their actions on specific purinoceptors in central nervous system of this specie.

Effect of graded hypoxia on high-affinity Ca2+-ATPase activity in cortical neuronal nuclei of newborn piglets

Previous studies have shown that nuclear calcium signals control a variety of nuclear functions including gene transcription, DNA synthesis, DNA repair and nuclear envelope breakdown. The present study tested the hypothesis that the activity of the neuronal nuclear high affinity Ca2+-ATPase increases as a function of decreased energy metabolism in the cerebral cortex. Studies were performed in 11 ventilated newborn piglets, age 3-5 days, divided into normoxic (Nx, n = 4) and hypoxic (Hx, n = 7) groups. The animals were exposed to a single FiO2 in the range from 0.21 to 0.05 for one hr. Cerebral tissue hypoxia was confirmed biochemically by determining brain tissue ATP and phosphocreatine levels. Neuronal nuclei were isolated and the high-affinity Ca2+-ATPase activity determined. During graded hypoxia, cerebral tissue ATP decreased from 4.80 +/- 0.58 (normoxic) to 1.03 +/- 0.38 (ranging from 0.61-1.63) micromol/g brain (p < 0.05) and PCr decreased from 3.94 +/- 0.75 (normoxic) to 0.99 +/- 0.27 (ranging from 0.50 to 1.31) micromol/g brain (p < 0.05). The total high affinity Ca2+-ATPase activity in the hypoxic nuclei increased and ranged from 541 to 662 nmol/mg protein/hr, compared to activity in normoxic group of 327 to 446 nmol/mg protein/hr. During graded hypoxia, the level of nuclear high affinity Ca2+-ATPase activity correlated inversely with ATP (r = 0.91) and PCr levels (r = 0.82), with activity increasing as tissue high energy phosphates decreased. The results demonstrate that the decrease in cerebral energy metabolism during hypoxia is linearly correlated with an increase in activity of high affinity Ca2+-ATPase in cerebral cortical nuclei from immature brain. We propose that increased nuclear membrane high affinity Ca2+-ATPase activity, leading to increased nuclear Ca2+, will result in altered expression of apoptotic genes that could initiate programmed neuronal death.

Increase of Ca2+-ATPase activity in the brain microsomes of rats with increasing ages: involvement of protein kinase C

A possible mechanism of aging-induced increase in brain microsomal Ca2+-adenosine triphosphatase (ATPase) activity of rats was investigated. Calcium content in the brain tissues and Ca2+-ATPase activity in the brain microsomes of aging rats (50 weeks of age) increased significantly as compared with those of young rats (5 weeks of age). Brain microsomal Ca2+-ATPase activity in aging rats was decreased significantly by treatment of ethyleneglycol-bis-(aminoethylether) N,N,N',N'-tetraacetic acid (EGTA) (2.7 mM) or digitonin (10(-3)%), while such decrease was not seen in the enzyme activity of young rats. Microsomal Ca2+-ATPase activity in aging rats was markedly decreased by the presence of staurosporine (10(-8) and 10(-7) M), an inhibitor of protein kinase C, in the enzyme reaction mixture, although the enzyme activity of young rats was not inhibited. Meanwhile, dibucaine (10(-6) and 10(-5) M), an inhibitor of Ca2+/calmodulin-dependent protein kinase, did not have an effect on Ca2+-ATPase activity in the brain microsomes of young and aging rats. The addition of protein kinase C (100 and 200 mU/ml) in the reaction mixture caused a significant increase in brain microsomal Ca2+-ATPase activity of young rats. These results suggest that protein kinase C is partly involved in the elevation of brain microsomal Ca2+-ATPase activity in rats with increasing ages.

Stimulatory effect of calcium-binding protein regucalcin on phosphatase activity in the brain cytosol of rats with different ages

The effect of calcium-binding protein regucalcin on phosphatase activity in the brain cytosol of rats with different ages was investigated. The presence of regucalcin (10(-8) and 10(-7) M) in the enzyme reaction mixture caused a significant increase of neutral p-nitrophenylphosphatase activity in the brain cytosol obtained from 5- and 50-week-old rats. This increase was seen in the absence or presence of calmodulin (2 microg/mL) and calcium chloride (100 microM). Brain cytosolic phosphatase activity was not significantly altered by S-100A (10(-6) M) or calbindin (10(-7) M), which is a calcium-binding protein. Regucalcin-increased phosphatase activity was clearly decreased by N-ethylmaleimide, a modifying reagent of thiol(SH)-group, suggesting that regucalcin acts on the SH-group of the enzyme. Moreover, the presence of anti-regucalcin monoclonal antibody (50-200 ng/mL) in the reaction mixture caused a significant decrease of brain cytosolic phosphatase activity, suggesting that the endogenous regucalcin has a stimulatory effect on the enzyme activity. These results suggest that regucalcin plays a role in the regulation of protein phosphatase in rat brain cytosol.

The mechanism and site of action of lidocaine hydrochloride in guinea pig inner ear

Lidocaine was applied to the round window (RW) in order to localize its site of action in the cochlea. Cochlear microphonic (CM), summating potential (SP), and compound action potential (CAP) input/output functions were measured to a 16 kHz tone burst to assess the functional changes of the cochlea. In separate experiments, the effect of lidocaine on the whole cell current of isolated outer hair cells (OHC) was studied. A dose of 2 microliters of 40 mM lidocaine in saline solution, when applied to the RW, caused a small change in all measured variables, indicating a passage of the drug through the RW membrane to sites of action. However, 160 mM of lidocaine further decreased CM, SP, and CAP by a total of 40% from the control. A partial recovery occurred for CM during the 30 min follow-up period. CAP and SP continued to decline. In isolated OHCs, lidocaine decreased the whole cell current in a dose-dependent fashion. The KD for lidocaine effect on OHCs was 7 mM. Our in vivo results indicate that lidocaine affects OHCs and reduces CM, causing a subsequent reduction in SP and CAP. The increased effect of lidocaine on CAP and SP, while CM is recovering, suggests an additional specific effect of lidocaine on the cochlear nerve and/or on inner hair cells. Considering that lidocaine alters OHC current (in isolated hair cells) and that lidocaine does not affect endocochlear potential [Laurikainen et al. Acta Otolaryngol (Stockh) 1991: 112: 800-9], the observed CM changes are most likely due to an in vivo effect on OHCs. Thus, the early effect of lidocaine on the cochlea appears to be due to a significant change in organ of Corti function, rather than to direct anesthesia of the cochlear nerve. Later, an independent effect of the drug may occur on neural tissues in the inner ear.

Reduced cortical ecto-ATPase activity in rat brains during prolonged status epilepticus induced by sequential administration of lithium and pilocarpine

Considerable evidence indicates that ATP, acting intracellularly of as a neurotransmitter, can influence nerve cell physiology in a variety of ways. Defects in the functioning of ATP-metabolizing enzymes could therefore lead to disturbances in neurotransmission and creation of sustained neuronal discharges characteristic of status epilepticus. In this study we investigated synaptosomal ATPase changes in rat brains during lithium/pilocarpine-induced status epilepticus. After 2 h of continuous electroencephalographic spiking, both Mg(2+)- and Ca(2+)-dependent ecto-ATPases were significantly decreased in freshly prepared synaptosomal preparations from the status rats. The intracellularly acting Ca2+Mg(2+)-ATPase (Ca-pump) was also decreased, but no changes occurred in synaptosomal Na+K(+)-ATPase activity. The difference between ecto-ATPase activities of the control and status rat brains was not affected by repeated freezing-thawing and lengthy storage. Possible involvement of reduced synaptosomal divalent cation-dependent ATPases in the pathophysiology of status epilepticus is discussed.

Characterization of calcium accumulation in the brain of rats administered orally calcium: the significance of energy-dependent mechanism

The characterization of calcium accumulation in the brain of rats administered orally calcium chloride solution was investigated. Rats received a single oral administration of calcium (15-50 mg/100 g body weight), and they were sacrificed by bleeding between 15 and 120 min after the administration. The administration of calcium (50 mg/100 g) produced a significant increase in serum calcium concentration and a corresponding elevation of brain calcium content, indicating that the transport of calcium into the brain is associated with the elevation of serum calcium levels. The increase in brain calcium content by calcium administration was not appreciably altered by the pretreatment with Ca2+ channel blockers (verapamil or diltiazem with the doses of 1.5 and 3.0 mg/100 g). In thyroparathyroidectomized rats, the administration of calcium (50 mg/100 g) caused a significant increase in brain calcium content, indicating that calcium-regulating hormones do not participate in the brain calcium transport. Now, brain calcium content was clearly elevated by fasting (overnight), although serum calcium level was not significantly altered. Calcium administration to fasted rats induced a further elevation of brain calcium content as compared with that of control (fasted) rats. The fasting-induced increase in brain calcium content was appreciably restored by refeeding. This restoration was also seen by the oral administration of glucose (0.4 g/100 g) to fasted rats. The present study demonstrates that serum calcium is transported to brain, and that the increased brain calcium is released promptly. The release of calcium from brain may be involved in energy metabolism, and this release may be weakened by the reduction of glucose supply into brain. The finding suggests a physiological significance of energy-dependent mechanism in the regulation of brain calcium.

Changes in the distribution of a calcium-dependent ATPase during demyelination and remyelination in the central nervous system

A calcium-adenosine triphosphatase (Ca(2+)-ATPase) activity expressed by CNS nerve fibres has been examined during demyelination and remyelination in rats, 21-26 days after an intraspinal injection of ethidium bromide. The Ca(2+)-ATPase distribution was determined cytochemically, using a technique believed primarily to reflect the presence of ecto-ATPases. We confirm that in normal nerve fibres Ca(2+)-ATPase activity was present on the external surface of the myelin sheath, and on the axolemma at the nodes of Ranvier. Labelling of the internodal axolemma was restricted to small, scattered, punctate regions. However, following demyelination the Ca(2+)-ATPase activity was expressed continuously along both the exposed, previously internodal axolemma of entirely naked axons, and it was particularly prominent at sites of contact between axons and glial-cell processes. During remyelination (which in this lesion is accomplished predominantly by Schwann cells) the proportion of the axonal surface exhibiting Ca(2+)-ATPase activity decreased in concert with the progressive thickening of the new myelin sheath. The non-myelin forming plasmalemma of Schwann cells was positive for the Ca(2+)-ATPase activity, but activity was abruptly lost at the site of compaction between the inner and outer leaflets of the forming myelin sheath. Ecto-ATPase activity is a property of some cell adhesion molecules, and it follows that the changes observed in the distribution of ATPase activity in this study may reflect changes in the axolemma which are important for the successful repair of the lesion by remyelination. The ATPase activity may, for example, reflect the changing distribution of molecules important in aiding axo-glial recognition and the establishment of axo-glial contacts.

Ethanol and nerve growth factor effects on calcium homeostasis in cultured embryonic rat medial septal neurons before and during depolarization

Ethanol and nerve growth factor (NGF) affect the survival of cholinergic neurons in the rat medial septum. To investigate whether calcium (Ca2+) homeostasis in these neurons is affected by ethanol or NGF treatment, changes in intracellular free Ca2+ concentration ([Ca2+]i) were studied in embryonic (E21) cultured medial septal neurons before stimulation (basal) and during stimulation with high potassium (K+). Changes in [Ca2+]i across time were measured in cultures of neurons treated without ethanol or with 100, 200, 400, or 800 mg% ethanol with NGF (+NGF) or without NGF (-NGF). Changes in [Ca2+]i were analyzed from fluorescence images, using indo-1. The effect of ethanol or NGF treatment was to reduce the rise in basal [Ca2+]i. The combination of ethanol and NGF treatment in +NGF neurons led to increases in basal [Ca2+]i with the greatest increase in basal [Ca2+]i occurring with 200 mg% ethanol. The effect of ethanol or NGF was to increase [Ca2+]i during stimulation with high K+. The greatest increases in [Ca2+]i occurred with 100 and 800 mg% ethanol. Together, ethanol and NGF treatment in +NGF-treated neurons led to significantly greater increases or decreases in K+ stimulated changes in [Ca2+]i compared to similarly treated -NGF neurons. We conclude that in medial septal neurons (before and during depolarization) changes in Ca2+ homeostasis occur in the presence of ethanol or NGF. The changes in [Ca2+]i following ethanol treatment are greater when NGF is present.

Ecto-ATPases: identities and functions

Ecto-ATPases are ubiquitous in eukaryotic cells. They hydrolyze extracellular nucleoside tri- and/or diphosphates, and, when isolated, they exhibit E-type ATPase activity, (that is, the activity is dependent on Ca2+ or Mg2+, and it is insensitive to specific inhibitors of P-type, F-type, and V-type ATPases; in addition, several nucleotide tri- and/or diphosphates are hydrolysed, but nucleoside monophosphates and nonnucleoside phosphates are not substrates). Ecto-ATPases are glycoproteins; they do not form a phosphorylated intermediate during the catalytic cycle; they seem to have an extremely high turnover number; and they present specific experimental problems during solubilization and purification. The T-tubule Mg2+-ATPase belongs to this group of enzymes, which may serve at least two major roles: they terminate ATP/ADP-induced signal transduction and participate in adenosine recycling. Several other functions have been discussed and identity to certain cell adhesion molecules and the bile acid transport protein was suggested on the basis of cDNA clone isolation and immunological work.

Disulfiram lowers Ca2+, Mg(2+)-ATPase activity of rat brain synaptosomes

The chronic administration of disulfiram (DS) to rats resulted in significant decrease of synaptosomal Ca2+, Mg(2+)-ATPase activity. In vitro studies indicated that DS (ID50 = 20 microM) produced a dose-dependent inhibition of Ca2+, Mg(2+)-ATPase. However, diethyldithio-carbamate, a metabolite of DS, failed to modify Ca2+, Mg(2+)-ATPase activity, implying that the decrease in ATPase activity in DS administered rats was due to the effect of parent compound. The DS-mediated inhibition (48%) of ATPase activity was comparable with a similar degree of inhibition (49%) achieved by treating the synaptosomal membranes with N-ethylmaleimide (ID50 = 20 microM) in vitro. Furthermore, the inhibition by DS was neither altered by washing the membranes with EGTA nor reversed by treatment with sulfhydryl reagents such as GSH or dithiothreitol. About 74% and 68% decrease of synaptosomal Ca2+, Mg(2+)-ATPase specific activity was observed when treated with DS (30 microM) and EGTA (100 microM) respectively. The remaining 25-30% of total activity is suggested to be of Mg(2+)-dependent ATPase activity. This indicates that both these drugs may act on a common target, calmodulin component that represents 70-75% of total Ca2+, Mg(2+)-ATPase activity. Therefore, DS-mediated modulation of synaptosomal Ca2+, Mg(2+)-ATPase activity could affect its function of maintaining intracellular Ca2+ concentration. This could contribute to the deleterious effects on CNS.

A membrane potential model with counterions for cytosolic calcium oscillations

An initial model has been proposed to describe a mechanism for cytosolic calcium oscillations [Jafri MS. Vajda S. Pasik P. Gillo B. (1992) A membrane model for cytosolic calcium oscillations: a study using Xenopus oocytes. Biophys. J., 63, 235-246]. In this paper we extend our original model to include the effects of counterion movement into the ER in response to calcium release. This produces smoother oscillations over a wider parameter range. We have lowered the endoplasmic reticulum (ER) intraluminal free calcium concentration and shown that the oscillations can occur at lower ER membrane potentials, consistent with physiological values. The improved model is then tested with two representative paradigms that are currently under investigation by many researchers. The model predicts that the reduction of the ER calcium pump (Ca-ATPase) rate can cause the termination of cytosolic calcium oscillations in an active cell, and induce oscillations in a resting cell. This result is consistent with experiments with thapsigargin, a Ca-ATPase activity inhibitor. In addition, we simulate the latency period for the response to the application of agonist and offer a plausible explanation for it. Our mathematical model is currently the only model that formulates the contributions of calcium binding proteins, ER membrane potential, ER counterion movements, and distinct calcium pump populations, and describes their effects on cytosolic calcium oscillations.

Experimental basis for lidocaine therapy in cochlear disorders

In order to further our basic understanding of the effects of lidocaine hydrochloride in the inner ear, cochlear potentials and blood flow (CBF) were assessed after intravenous (i.v.), anterior inferior cerebellar artery (AICA), and local round window (RW) lidocaine administrations in guinea pigs and rats. Lidocaine RW applications produced a dose dependent decrease in compound action potentials (CAP) and cochlear microphonics (CM). The sensitivity changes were more pronounced at high frequencies. These findings suggest that lidocaine has specific pharmacological action in the inner ear other than simple anesthesia of the auditory nerve. The basal turn endocochlear potentials (EP) were not altered by topical lidocaine, implicating altered organ of Corti function following local application of lidocaine. RW applications of lidocaine had no effect on CBF or systemic blood pressure (BP). I.v. infusions caused substantial reductions in BP. In the case of systemic infusions the percent changes in CBF were equal to and accountable by the BP changes. The microinfusions (50 mg/ml, 100 nl/min) through AICA produced a 30%, long lasting increase in CBF. However, neither systemic lidocaine nor AICA infusions had an effect on CAP or CM. These findings indicate that systemically given lidocaine may not cross the blood-cochlear barrier and that the cochlear electrophysiological effects due to lidocaine when given locally are partly mediated by direct influence on cochlear hair cell function; they also suggest that lidocaine-induced interference with active ion transport in the lateral wall or an influence on CBF are not contributing factors.

A membrane model for cytosolic calcium oscillations. A study using Xenopus oocytes

Cytosolic calcium oscillations occur in a wide variety of cells and are involved in different cellular functions. We describe these calcium oscillations by a mathematical model based on the putative electrophysiological properties of the endoplasmic reticulum (ER) membrane. The salient features of our membrane model are calcium-dependent calcium channels and calcium pumps in the ER membrane, constant entry of calcium into the cytosol, calcium dependent removal from the cytosol, and buffering by cytoplasmic calcium binding proteins. Numerical integration of the model allows us to study the fluctuations in the cytosolic calcium concentration, the ER membrane potential, and the concentration of free calcium binding sites on a calcium binding protein. The model demonstrates the physiological features necessary for calcium oscillations and suggests that the level of calcium flux into the cytosol controls the frequency and amplitude of oscillations. The model also suggests that the level of buffering affects the frequency and amplitude of the oscillations. The model is supported by experiments indirectly measuring cytosolic calcium by calcium-induced chloride currents in Xenopus oocytes as well as cytosolic calcium oscillations observed in other preparations.

A comparative study of endothelin- and platelet-activating-factor-mediated signal transduction and prostaglandin synthesis in rat Kupffer cells

Endothelin-3 (ET-3) stimulated phosphoinositide metabolism and synthesis of prostaglandins in cultured rat Kupffer cells. ET-3-induced hydrolysis of phosphoinositides was characterized by the production of various inositol phosphates and of glycerophosphoinositol. The mechanism of ET-3-stimulated metabolism of phosphoinositides and synthesis of prostaglandins appeared to be distinct from the effect of platelet-activating factor (PAF) on these processes described previously [Gandhi, Hanahan & Olson (1990) J. Biol. Chem. 265, 18234-18241]. On a molar basis ET-3 was significantly more potent than PAF in stimulating phosphoinositide metabolism, e.g. ET-3-induced hydrolysis of phosphoinositides occurred at 1 pM, whereas PAF was ineffective at concentrations less than 1 nM. Upon challenging Kupffer cells with both ET-3 and PAF, an additive stimulation of phosphoinositide metabolism was observed, suggesting that the actions of these factors may be exerted on separate phosphoinositide pools. Treatment of Kupffer cells with pertussis toxin resulted in an inhibition of ET-3-induced phospholipase C activation; in contrast, cholera toxin treatment caused potentiation of ET-3-stimulated phospholipase C activity. Both toxins, however, inhibited PAF-stimulated phospholipase C activity. The present results suggest that the stimulatory effects of ET-3 and PAF on the phosphodiesteric metabolism of phosphoinositides in Kupffer cells require different guanine-nucleotide-binding proteins. Furthermore, the effects of bacterial toxins on ET-3- and PAF-induced phosphoinositide metabolism were not mediated by cyclic AMP. ET-3-induced metabolism of phosphoinositides was inhibited completely in Kupffer cells pretreated with ET-3, suggesting homologous ligand-induced desensitization of the ET-3 receptors. In contrast, similar experiments using PAF showed only a partial desensitization of subsequent PAF-induced phosphoinositide metabolism. In contrast to the increased production of prostaglandins E2 and D2 observed upon stimulation of Kupffer cells with PAF, ET-3 stimulated the biosynthesis of prostaglandin E2 only. Consistent with their additive effects on phosphoinositide metabolism, PAF and ET-3 exhibited an additive stimulation of the synthesis of prostaglandin E2.

Characterisation of Ca2+ or Mg(2+)-dependent nucleoside triphosphatase from rat mesenteric small arteries

When isolated rat mesenteric small arteries were submitted to 2 s of sonication, a nucleoside triphosphatase activity was released to the medium, mainly from the plasma membrane of the vascular smooth muscle cells. The activity was kinetically characterized: It hydrolysed ATP, UTP and GTP with the same substrate affinity and the same specific activity. CaATP, as well as MgATP were substrates for the enzyme with an apparent Km in the micromolar range. ATPase inhibitors: ouabain, vanadate, AlF4-, oligomycin and N-ethylmaleimide were without effect on the hydrolytic activity. Among other modifiers tested only N,N'-dicyclohexylcarbodiimide caused significant (greater than 30%) inhibition. In the presence of micromolecular concentrations of Ca2+ and Mg2+, small (less than 20 mM) concentrations of Na+, K+, Rb+, Cs+ and choline+, irrespective of the nature of the anion, activated the hydrolysis with an equilibrium ordered pattern, but concentrations of monovalent cation salts above 20 mM decreased the hydrolysis rate. No activation by monovalent cation salts was seen at millimolar concentrations of divalent cations and substrate. On the basis of the results a standard mixture is proposed, which allows a sensitive assay of the specific enzyme activity.

Characteristics of a presynaptic plasma membrane Ca2(+)-ATPase activity from electric organ

Ca2(+)-ATPase activity was measured in electric organ synaptosomal homogenates and their derived presynaptic plasma membranes using a low ionic strength medium, low in Ca2+ and Mg2+, and devoid of K+. The enzyme activity showed a high apparent affinity for Ca2+ (KCa:0.5 microM) and was: (1) 5-fold stimulated by 120 nM calmodulin, (2) highly sensitive to LaCl3 inhibition, and (3) not affected by 20 mM NaN3 or 0.1 mM ouabain. The addition of Mg2+ promoted the disappearance of Ca2(+)-ATPase activity. Incubation of synaptosomal homogenates in the above-mentioned assay medium with [gamma -32P]ATP resulted in the appearance of a 140 kDa band as revealed by SDS-gel electrophoresis. Labeling of this band with 32P was inhibited by 1 mM EGTA or 10 mM NH2OH, indicating that the isotope incorporation required the presence of Ca2+ and the formation of an acyl-phosphate derivative. The results indicate that the Ca2(+)-ATPase activity from synaptosomal homogenates had characteristics corresponding to those of the enzyme that catalyzes an outward transport of Ca2+ in nerve terminals. Preincubation of synaptosomes in Ca2+ plus K+, a depolarizing procedure, induced a large and rapid decrease in the Ca2(+)-ATPase activity, possibly mediated via Ca2+ entry through voltage-gated Ca2+ channels. Furthermore, the muscarinic cholinergic agonist oxotremorine (at 15 microM concentration) did not significantly affect either the enzyme activity or the intensity of the Ca2(+)-dependent 32P incorporation into the 140 kDa band, suggesting that the enzyme is not coupled to muscarinic binding sites.

Identification and characterization of high-affinity Ca2(+)-ATPase associated with axonal plasma membranes of dog mesenteric nerves

The microsomal fraction isolated from dog mesenteric nerve fibres was found to contain ATPase activity stimulated by micromolar concentrations of Ca ions. Such a high-affinity Ca2(+)-ATPase (hereafter referred to as HA Ca-ATPase) followed a Michaelis-Menten kinetics with Km for Ca ions of 0.4 microM and Vmax = 12.5 +/- 2.4 mumol Pi.mg-1h-1. The examination of the subcellular origin of HA Ca-ATPase revealed that this enzyme is associated with axonal plasma membranes as documented by its co-purification with several plasma membrane marker enzymes and with tetrodotoxin-sensitive 3H-saxitoxin binding. The addition of exogenous magnesium ions (Mg) resulted in a non-competitive inhibition of HA Ca-ATPase with Ki = 0.5 mM. The reaction velocity of HA Ca-ATPase was also inhibited by other divalent ions with the order of potency Mg greater than Mn greater than Zn greater than or equal to Co greater than Ni. In contrast to low affinity (high Km) Mg- and Ca-ATPase, the HA Ca-ATPase was insensitive to the inhibition by sodium azide (10 mM) and sodium fluoride (10 mM). Similarly, the specific activity of HA Ca-ATPase was unaffected by vanadate (100 microM) and N-ethylmaleinimide (100 microM). It is concluded that axonal plasma membranes of dog mesenteric nerves contain HA Ca-ATPase which seems to be unrelated to calcium-transporting Mg-dependent, Ca-stimulated ATPase.

Ectonucleotidase activities associated with the olfactory organ of the spiny lobster

The olfactory system of the Florida spiny lobster, Panulirus argus, has olfactory receptors that are excited by the purine nucleotides AMP, ADP, and ATP. These receptors reside on chemosensory neurons that are contained within aesthetasc sensilla on the lateral filaments of the antennules. Also associated with the lobster's olfactory system are ectonucleotidase activities that dephosphorylate excitatory nucleotides, resulting in the production of the nonstimulatory nucleoside adenosine. Our studies of the 5'-ectonucleotidase, ecto-ADPase, and ecto-ATPase activities of this olfactory system showed that each activity was characterized by Michaelis-Menten kinetics; Michaelis constants ranged from 6.9 to 33.5 microM, and maximum velocities ranged from 2.5 to 28.8 fmol/sensillum/s. Evidence that AMP dephosphorylation may serve as an inactivation process was shown by the close correlation between the kinetics of 5'-ectonucleotidase activity and the periodicity of olfactory sampling. Decreased magnesium ion concentration or increased calcium ion concentration resulted in increased ecto-ATPase activity; this activity was insensitive to vanadate ion. Ectonucleotidase activities may have multiple effects on the detection of exogenous nucleotides by a chemosensory system. These effects can be either direct, such as the conversion of an odorant to an inactive compound, or indirect, such as the conversion of an odorant to another compound that can activate or inhibit either receptors or enzymes associated with the system.

Cerebrospinal fluid calcium homeostasis: evidence for a plasma membrane Ca2+-pump in mammalian choroid plexus

A major unanswered question in central nervous system physiology concerns the mechanism by which cerebrospinal fluid (CSF) Ca2+ homeostasis is maintained in the face of hypo- or hypercalcemia. To address this question, we sought and found a protein of Mr approximately 140,000 in choroid plexus plasma membranes that forms a phosphorylated intermediate with characteristics of a plasma membrane Ca2+-pump. A choroid plexus plasma membrane protein of this molecular weight also bound to a monoclonal antibody prepared against the human erythrocyte plasma membrane Ca2+-Mg2+ ATPase Ca2+-pump. When this monoclonal antibody was used for immunohistochemical localization, the plasma membrane Ca2+-pump was found primarily in the CSF-facing membranes of choroid plexus cells from rats, cats, and man. The localization of a plasma membrane Ca2+-pump in the CSF-facing membranes of the choroid plexus suggests that the choroid plexus, by mechanisms including this pump, may regulate CSF Ca2+ concentrations.