Participation of a transmembrane proton gradient in 5-hydroxytryptamine transport by platelet dense granules and dense-granule ghosts

Essential role of vesicular nucleotide transporter in vesicular storage and release of nucleotides in platelets

Nucleotides are stored in the dense granules of platelets. The release of nucleotides triggers one of the first steps in a series of cascades responsible for blood coagulation. However, the mechanism of how the nucleotides are accumulated in the granules is still far less understood. The transporter protein responsible for storage of nucleotides in the neuroendocrine cells has been identified and characterized. We hypothesized that the vesicular nucleotide transporter (VNUT) is also involved in the vesicular storage of nucleotides in platelets. In this article, we present three lines of evidence that VNUT is responsible for the vesicular storage of nucleotides in platelets and that vesicular ATP transport is crucial for platelet function, detection and characterization of VNUT activity in platelets isolated from healthy humans and MEG‐01 cells, RNA interference experiments on MEG‐01 cells, and studies on nucleotide transport and release with a selective inhibitor.

VNUT is highly expressed and associated with dense granules in platelets. VNUT plays an essential role in vesicular storage of nucleotide in platelets.

Porcine platelets in vitro and in vivo studies: relevance to human thrombosis research

This review summarizes present knowledge on porcine platelets in vitro and recent studies on in vivo activation of platelets in the pig. There are certain differences compared to human platelets: Platelet aggregation and secretion cannot be achieved by epinephrine, and the arachidonate pathway seems poorly developed in porcine platelets. Genetic models for von Willebrand disease (vWD) and storage pool deficiency (SPD) have been developed in the pig. Several models for the study of in vivo platelet deposition and early thrombus formation have been developed. Platelet radio-labeling techniques (with 111In) have been used extensively. We conclude that the pig seems to be a good choice for the investigation of in vivo platelet activation and deposition based on present knowledge of porcine platelets and on already established animal models.

Human platelet dense granules: improved isolation and preliminary characterization of [3H]-serotonin uptake and tetrabenazine-displaceable [3H]-ketanserin binding

An improved method for the isolation of human platelet dense granules was developed. A good yield (45%) of highly enriched (69-fold, based on serotonin content) dense granules was obtained after mild sonication and Percoll gradient centrifugation. The method has facilitated characterization of the granule, permitting the first report of Km and Vmax values for [3H]-serotonin uptake, as well as the first determination of Kd and Bmax values for tetrabenazine-displaceable [3H]-ketanserin binding, in the human platelet dense granule. The rates and affinities (Vmax 1.45 nmol/mg/min, Km 0.93 uM) of [3H]-serotonin uptake were similar to those previously reported for porcine dense granules. Tetrabenazine-displaceable [3H]-ketanserin binding was observed with a Kd (9.4 nM) similar to, and a Bmax (5.4 pmol/mg) approximately 10-fold lower than, that previously seen in bovine chromaffin granules.

Amine uptake into intact mast cell granules in vitro

Histamine, the principal amine of rat peritoneal mast cells, is taken up into isolated granules with intact membranes. Uptake is pH- and concentration-dependent and is not stimulated by the addition of Mg2+-ATP. The saturable uptake has a Km of 91.1 microM and a Vmax of 95.4 pmol (mg of protein)-1 min-1. Uptake is abolished by 5 mM ammonium ion. 5-HT, the other endogenous amine of the granules, and dopamine and tyramine, which do not occur naturally in rat mast cells, each competitively inhibits [3H]-histamine uptake with Ki's close to 1 microM. Reserpine, a putative amine carrier blocker, inhibits uptake at nanomolar concentrations. At high concentrations, uptake of [3H]-5-HT is nonsaturable; at low concentrations, a saturable component is observed with a Km of 1.6 microM. Uptake of [3H]-5-HT is not enhanced by Mg2+-ATP. It is pH-dependent but with a lower apparent pKa than that of histamine. [3H]-5-HT uptake can be completely inhibited by ammonium ions. Amine inhibition of [3H]-5-HT gives nonlinear Dixon plots, and high concentrations of the competing amines or reserpine cannot completely block uptake. We propose a model consistent with these results in which amine uptake occurs by several distinct saturable transport systems. According to the model, histamine is transported by a single system, which also transports 5-HT and dopamine. 5-HT and dopamine are transported by one or more other systems.

Glycoproteins of the chromaffin granule membrane: separation by two-dimensional electrophoresis and identification by lectin binding

The proteins of highly purified chromaffin-granule membranes were separated by one- or two-dimensional electrophoresis, then transferred to nitrocellulose sheets; glycosylation was investigated by binding of several different radioiodinated lectins. Over 20 different glycosylated components were identified; comparison with mitochondrial and microsomal fractions suggested that most of the major glycoproteins are genuine components of the chromaffin granule membrane, rather than contaminants originating in other organelles. Two-dimensional electrophoresis revealed heterogeneity within several of the glycoproteins, and this is ascribed to differences in the state of glycosylation, on the basis of shifts in electrophoretic mobility produced by treatment with neuraminidase. Neuraminidase treatment of chromaffin granule membranes also enhances the binding of many lectins. The identities of the lectin-binding bands are discussed: neither cytochrome b561 nor the F1-like ATPase appears to be glycosylated. Chromogranin A, although a glycoprotein, does not bind any of the lectins tested, but a number of concanavalin-A binding proteins, as well as dopamine beta-hydroxylase, are present in the chromaffin granule lysate.

Comparison of serotonin uptake by dense bodies inside and outside human platelets

A technique has been developed for quantitating the absolute number of dense bodies present in solution following isolation from human platelets. The amount of [3H]-5HT accumulated per dense body was measured following either sonication alone or sonication plus isolation utilizing a Metrizamide density gradient; the dense bodies in each case were washed and resuspended in sodium or potassium-rich buffer. Uptake per dense body following removal from the cell was less than 10% of the amount of uptake per dense body by intact platelets. It thus seems possible that residence of dense bodies inside intact platelets is required for 5HT transport into dense bodies to proceed at a maximal rate.

Proton NMR studies of nucleotide and amine storage in the dense granules of pig platelets

1H NMR measurements have been conducted at 360 MHz on isolated pig platelet dense granules. Resonances of the H8, H2 protons of the adenine ring, H1' protons of the ribose moiety, and the aromatic hydrogens of 5-hydroxytryptamine (5HT) have been identified in spectra of intact dense granules. Like the 31P resonances of the nucleotides contained in the dense granules (Uğurbil et al., 1984), the line widths and the intensities of these resonances were sensitive to sample temperature and osmolarity of the suspension medium. Their chemical shifts indicate that 5HT in the granule interior is predominantly bound to the nucleotides through ring-stacking interactions. Association of 5HT with the nucleotides was also confirmed by the presence of intermolecular nuclear Overhauser effect (NOE) between 5HT and nucleotide protons. Large and negative intermolecular NOE's observed among the nucleotide H8, H2 and H1' protons, together with upfield shifts undergone by these protons within the dense granules, demonstrate that the nucleotides form a complex where they are in close proximity of each other. The formation of this complex apparently does not require the presence of amines since removal of 5HT and histamine did not change the chemical shifts of the nucleotide protons. From T1 and T2 data, rotational correlation time of 4 ns was calculated for the nucleotides in the dense granule interior at 35 degrees C. A resonance tentatively identified as H2 of histamine was found to shift upon manipulation of the intragranular pH; it was used as an indicator of pH changes within the granule interior during 5HT uptake and showed that 5HT accumulation increases the intragranular pH. These results demonstrate that 5HT is first taken up in response to the inside acidic pH gradient across the granule membrane and is subsequently sequestered in a matrix formed by the divalent cations and the nucleotides.

The effect of lysosomotrophic bases and inhibitors of transglutaminase on iron uptake by immature erythroid cells

The mechanism by which weak bases block iron uptake by immature erythroid cells was investigated using rabbit and rat reticulocytes and erythroblasts from the fetal rat liver. A large variety of bases was found to inhibit iron uptake but to have a much smaller or no effect on transferrin uptake by the cells. Quinacrine and chloroquine were active at the lowest concentrations. Dansylcadaverine, an inhibitor of transglutaminase, was also active at low concentration. However, the results do not indicate a role for transglutaminase in the iron uptake process. Instead they show that the major effect of the bases is to inhibit iron release from transferrin molecules on or within the cells. The possible mechanism of this effect was investigated by measurement of intracellular ATP levels, intracellular pH and by morphological studies utilizing fluorescent and electron microscopy. The bases caused little change in ATP levels, but elevated intracellular pH, probably due to accumulation within intracellular vesicles, which were shown to accumulate fluorescent weak bases, to swell under the action of the bases and to be the site of intracellular localization of transferrin. It is concluded that the bases tested in this work inhibit iron release from transferrin in intracellular vesicles by increasing their pH rather than by blocking transglutaminase and thereby restricting endocytosis. Reduction of transferrin uptake by the cells when it occurs is probably due to inhibition of recycling of transferrin receptors to the outer cell membrane.

The chromaffin vesicle and the energetics of storage organelles

In the chromaffin vesicle, energy for amine transport is provided by a proton-translocating adenosine triphosphatase. The ATPase pumps protons into the vesicle; a pair of protons is then exchanged for each catecholamine taken up. In terms of transport, the adrenal chromaffin vesicle is an important model for non-mitochondrial organelles of all types. These organelles include adrenergic synaptic vesicles, other secretory and neurotransmitter storage vesicles, and lysosomes and other intracellular organelles as well. The membranes of these organelles all possess an ATPase that pumps H+ ions into the vesicles. This proton pump presumably drives the transport of ions and molecules into the organelle and powers other energy-requiring functions of the membrane.

The internal pH and membrane potential of the insulin-secretory granule

The membrane potential (DeltaPsi) and the pH gradient (DeltapH) across the membrane of the insulin-secretory granule were determined in studies in vitro from the uptake of the permeant anion thio[(14)C]cyanate or the permeant base [(14)C]methylamine. Freshly prepared granules incubated in iso-osmotic medium containing sucrose and low concentrations of buffer salts exhibited an acidic internal pH and a DeltaPsi positive inside. Addition of MgATP(2-) under these conditions did not alter the DeltapH, but produced a marked increase in the DeltaPsi. Conversely, when a permeant anion was also included, ATP produced a marked increase in the DeltapH and a lesser increment in the DeltaPsi. NH(4) (+) salts reduced the DeltapH across granule membranes. In the presence of ATP this effect was accompanied by a reciprocal increase in the DeltaPsi. A similar reciprocity was evident when nigericin was added together with K(+) or on decreasing the medium pH, suggesting that these gradients were linked by a common electrogenic process. The effects of ATP were reversed by the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone, the combination of valinomycin, nigericin and K(+), and by the Mg(2+)-dependent ATPase inhibitor tributyltin. Uptakes of (14)C-labelled tracer molecules were also markedly reduced by cryogenic disruption of the granule membrane or hypo-osmotic incubation conditions. These results were readily interpreted within a chemiosmotic hypothesis, which proposed that the insulin granules possess an inwardly-directed electrogenic proton-translocating Mg(2+)-dependent ATPase with the additional postulate that the membrane has a low proton permeability. The intragranular pH was estimated as being between 5 and 6 in vivo. Such a value corresponds to optimal conditions for the crystallization of zinc-insulin hexamers. Several other functions related to chemiosmotic processes within insulin granules, however, may be envisaged.