Enzymatic Characteristics and Possible Role of Synaptosomal Ecto-Adenosine Triphosphatase from Mammalian Brain

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.

Differential expression of phospholipase C specific for inositol phospholipids at the cell surface of rat glial cells and REF52 rat embryo fibroblasts

Phosphatidylinositol(PI)-specific phospholipase C activity was detected on the surface of rat astrocytes, rat C6 glioma cells, and rat embryo (REF52) fibroblasts. The cell surface phospholipase C (ecto-PLC) activity was calcium-dependent, did not result from secreted phospholipase C, and was not released from the cell surface by bacterial PI-specific phospholipase C. Agents known to stimulate intracellular PI turnover, including carbachol, L-glutamic acid, acetylcholine, and orthovanadate, did not induce measurable alterations in the activity of the ecto-PLC. The expression of ecto-PLC activity by REF52 fibroblasts was density-dependent: subconfluent cultures of REF52 exhibited low levels of activity (less than 80 pmol of inositol phosphate formed/min/10(6) cells), whereas in confluent cultures ecto-PLC activity increased to approximately 300 pmol/min/10(6) cells. In contrast to this behavior and that exhibited by previously reported ecto-PLC-positive cell types, the ecto-PLC activity exhibited by astrocytes (approximately 1,000 pmol/min/10(6) cells) and by C6 glioma cells (approximately 100 pmol/min/10(6) cells) was independent of cell culture density up to confluence. The constitutive expression of ecto-PLC activity of astroglial cells may be related to their function as accessory cells in close association with neurons.

Ecto-ATPase/phosphatase activity in the olfactory sensilla of the spiny lobster, Panulirus argus: localization and characterization

Electrophysiological studies have shown that the olfactory organ (antennule) of the spiny lobster, Panulirus argus, has chemoreceptors that are selectively excited by adenine nucleotides in seawater. Biochemical studies have revealed that these same nucleotides can be rapidly dephosphorylated by ectoenzymes associated with the olfactory sensilla (aesthetascs). In this study the distribution of ecto-ATPase/phosphatase activity within aesthetascs was determined cytochemically and the nature of the adenine-nucleotide dephosphorylating activity was dissected biochemically. Cytochemically, the distribution of ATP-dephosphorylating activity was similar to that shown previously for AMP and beta-glycerol phosphate; i.e., cerium phosphate reaction product was specifically localized to the transitional zone where the sensory dendrites develop cilia and branch to form the outer dendritic segments. Unlike the dephosphorylation of AMP and beta-glycerol phosphate, Mg2+ or Ca2+ was required for ecto-ATPase/phosphatase activity. Biochemical measures of both AMP- and ATP-dephosphorylating activity within aesthetascs corroborated the cytochemical evidence that these activities are localized to the transitional zone. A major portion of the AMP dephosphorylation (about 67%) derives from nonspecific alkaline phosphatase activity that is insensitive to levamisole and L-bromotetramisole. In contrast, nonspecific phosphatase activity accounted for a much smaller part of the ATP dephosphorylation (about 15%). Ectoenzymatic activity in the transitional zone may be an important means of removing excitatory/inhibitory nucleotides from this region.

Adenosine 5'-monophosphate transport across the membrane of synaptosomes and myelin

Synaptosome-enriched preparations from rat and guinea pig brain tissue vigorously accumulated [3H]-adenosine 5'-monophosphate ([3H]AMP). When the accumulation of [3H]AMP was determined using incubation periods of 30 s or less, high concentrations of adenosine, dipyridamole and soluflazine did not inhibit the accumulation of label appreciably. The accumulation of [3H]AMP was saturable, temperature-dependent, osmotic-sensitive and exhibited structural specificity. Based on the kinetics of uptake by different subcellular fractions, and the inhibitory effects of other nucleotides, the uptake of AMP appeared to be mediated by three saturable systems with Kt values of approximately 0.2, 6, and 100 microM. The transport system with the highest affinity for AMP was selectively inhibited by guanosine 5'-monophosphate, and its Vmax was several fold higher in a myelin-enriched fraction than in synaptosome-enriched fractions. The transport system with the Kt approximately 6 microM was selectively inhibited by alpha, beta-methylene adenosine diphosphate, and its Vmax was several times higher in a fraction enriched in high-density synaptosomes than in fractions enriched in low-density synaptosomes or myelin. Both of these transport systems were potently inhibited by ATP and ADP. Nucleotides that were either weak or inactive as inhibitors of AMP transport included 3'-AMP, cyclic AMP, guanosine 5'-diphosphate, and the 5'-mononucleotides of cytosine, inosine, and uridine. GTP consistently enhanced uptake at concentrations greater than or equal to 1 microM. The transport of AMP was not Na(+)-dependent and was not inhibited by membrane depolarization. This transport system may mediate the release of AMP for subsequent conversion to adenosine extracellularly.

Metabolism of extracellular adenine nucleotides by cultured rat brain astrocytes

Intact astrocytes cultured from newborn rat cerebral cortex rapidly converted extracellular ATP to ADP. The ATPase responsible was apparently not saturated, even at 750 microM ATP. In contrast, the conversion of ADP to AMP was slow, and the reaction was limiting for the subsequent dephosphorylation process. Adenosine formation was the only fate for AMP. The reaction was catalyzed by 5'-nucleotidase with an apparent Km of 55 microM for AMP and appeared to be inhibited by high concentrations of ATP and ADP. Astrocytes were able to take up adenosine with an apparent Km value of 45 microM. Uptake was inhibited by dipyridamole but not by anti-5'-nucleotidase IgG. The results support the proposal that astrocytes play a role in modulating synaptic events involving ATP and adenosine.

Soluble 5'-nucleotidase activities in rat brain

5'-Nucleotidase activity was assayed in 105,000-g supernatants from rat brain by following conversion of [3H]AMP into adenosine. The effect of ATP on this process was complex and suggested the presence of at least two soluble 5'-nucleotidase activities: one inhibited by ATP and another activated by ATP. The relative proportions of these activities differed considerably among brain regions. Activity changes induced by hypothyroidism also suggested that these activities may be regulated independently. These findings may have consequences for the regional regulation of adenosine formation in the brain.

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.

ATP release from affinity-purified rat cholinergic nerve terminals

Cholinergic nerve terminals were affinity purified from rat caudate nucleus. On stimulation with both 22.6 mM KCl and 50 microM veratridine, ATP was released in a Ca2+-dependent manner. The molar ratio of released acetylcholine to ATP (9:1) was closer to that found in isolated cholinergic vesicles (7:1) than whole terminals (3:1). Extracellular [14C]ATP was rapidly metabolized by these terminals to adenosine and inosine via ectonucleotidases. The terminals had a saturable, high-affinity uptake mechanism for adenosine (Km = 16.6 microM). Veratridine stimulation also caused the Ca2+-dependent release of nucleosides in a dipyridamole-sensitive manner. Both theophylline treatment and inhibition of extracellular ATP breakdown resulted in increased ATP and nucleoside release. Extracellular adenosine was shown to inhibit acetylcholine release, probably via the A1 receptor. The role of extracellular purines at the cholinergic nerve terminal is discussed.