Storage of 5-hydroxytryptamine in megakaryocytes

Platelet dense granules begin to selectively accumulate mepacrine during proplatelet formation

Platelet dense granules (DGs) are storage organelles for calcium ions, small organic molecules such as ADP and serotonin, and larger polyphosphates that are secreted upon platelet stimulation to enhance platelet activation, adhesion, and stabilization at sites of vascular damage. DGs are thought to fully mature within megakaryocytes (MKs) prior to platelet formation. Here we challenge this notion by exploiting vital fluorescent dyes to distinguish mildly acidic DGs from highly acidic compartments by microscopy in platelets and MKs. In isolated primary mouse platelets, compartments labeled by mepacrine - a fluorescent weak base that accumulates in DGs - are readily distinguishable from highly acidic compartments, likely lysosomes, that are labeled by the acidic pH indicator, LysoTracker, and from endolysosomes and alpha granules labeled by internalized and partially digested DQ™ BSA. By contrast, in murine fetal liver- and human CD34⁺ cell-derived MKs and the megakaryocytoid cell lines, MEG-01 and differentiated G1ME2, labeling by mepacrine overlapped nearly completely with labeling by LysoTracker and partially with labeling by DQ™ BSA. Mepacrine labeling in G1ME2-derived MKs was fully sensitive to proton ATPase inhibitors, but was only partially sensitive in platelets. These data indicate that mepacrine in MKs accumulates as a weak base in endolysosomes but is likely pumped into or retained in separate DGs in platelets. Fluorescent puncta that labeled uniquely for mepacrine were first evident in G1ME2-derived proplatelets, suggesting that DGs undergo a maturation step that initiates in the final stages of MK differentiation.

Electron opaque structures in human platelets: which are or are not dense bodies?

The dense bodies, also referred to as delta (delta) granules, present in human platelets are the storage sites for adenine nucleotides and serotonin. Stored products released following activation are important for platelet aggregation during hemostasis. Dense bodies are easily detected in thin sections of properly fixed platelets and in unfixed, unstained whole mount preparations. It is important to determine their presence and frequency with accuracy because they are absent or markedly reduced in platelet storage pool deficiency disorders. The present study has demonstrated that identification of dense bodies as not a simple matter. There are electron dense structures, including dense rings, glycosomes, "fuzzy" balls, chains, clusters and other dense elements, that may confuse the determination of true dense bodies. Even some alpha granules are sufficiently electron dense to be confused with delta granules when using densitometric techniques. The present work may prevent investigators from making diagnostic errors.

Advances in molecular diagnostics of myeloproliferative disorders

Incremental advances in the molecular pathogenesis of myeloproliferative disorders (MPDs) have had a substantial impact on clinical practice in terms of both diagnosis and treatment. An array of novel molecular methods are being developed and integrated into the current battery of tests for diagnosis and monitoring of treatment response. Primarily, subjective clinico-histologic approaches to diagnosis are being replaced by more objective semimolecular diagnostic algorithms. Furthermore, identification of disease-specific molecular markers has facilitated the development of small-molecule drugs for targeted therapy. This review provides an overview of MPDs with emphasis on molecular diagnostic tests and their incorporation into contemporary diagnostic and therapeutic algorithms.

The diagnosis of polycythemia vera: New tests and old dictums

Pathogenetically fundamental observations have identified polycythemia vera (PV) as a clonal stem cell disease with bone marrow histological and other biological features that distinctly differentiate it from other causes of 'increased' hematocrit. However, relatively little attention has been given to the effective utilization of pathology and laboratory markers of clonal myeloproliferation as diagnostic tools in PV. In contrast, the diagnostic use of red cell mass (RCM) measurement in PV stemmed from the accidental endorsement, as 'diagnostic criteria', of 'study eligibility criteria' that were formulated for clinical trials. It has since become evident that RCM measurement is a tedious procedure that is fraught with multiple-level imprecision, as well as suboptimal diagnostic accuracy. Therefore, it is reasonable to consider dispensing with RCM measurement as a diagnostic test for PV and instead utilize a diagnostic algorithm that combines clinical information with easily accessible laboratory data, including serum erythropoietin level and bone marrow histology. Recent discoveries of myeloproliferative-disease-specific molecular markers, including the JAK2 V617F tyrosine kinase mutation that is found in the majority of patients with PV, provide further support for such a measure.

Platelet-rich plasma serotonin levels in chronic myeloproliferative disorders: evaluation of diagnostic use and comparison with the neutrophil PRV-1 assay

In a prospective study of 109 subjects, an enzyme-linked immunosorbent assay (ELISA) was used to measure platelet-rich plasma (PRP) serotonin levels in patients with polycythaemia vera (PV; n = 27), essential thrombocythaemia (ET; n = 14), myelofibrosis with myeloid metaplasia (MMM; n = 30), secondary or spurious polycythaemia (SP; n = 22) and controls (n = 16). Nine study subjects who were taking a selective serotonin reuptake inhibitor (SSRI) all displayed a markedly decreased PRP serotonin level (median, 24.2 ng/10(9) platelets; range, 0-49.3) and were therefore excluded from further analysis. Among the remaining 100 subjects, the median and range of PRP serotonin levels, in ng/10(9) platelets, was significantly lower in MMM (89.5; 0-278.3), PV (204.8; 0-496.0) and ET (385.3; 136.8-1025.7) compared with both SP (608.8; 369.0-1780.1) and controls (567.2; 359.9-1071.1). Neutrophil polycythaemia rubra vera-1 (PRV-1) expression was concurrently assayed by real-time polymerase chain reaction in 69 patients (23 PV, 17 SP, 12 ET, seven MMM, 10 controls). PRP serotonin measurement performed as well as the PRV-1 assay in distinguishing PV from SP (93% vs. 86% test accuracy). The current study suggests that PRP serotonin concentration might be considered as one of the several biological markers that complement each other for the diagnosis of PV.

Human megakaryocytes cultured in vitro accumulate serotonin but not meta-iodobenzylguanidine whereas platelets concentrate both

OBJECTIVE

Thrombocytopenia is the major toxicity of radio-iodinated meta-iodobenzylguanidine (MIBG) therapy in patients with recurrent neuroblastoma. MIBG is taken up in platelets via the serotonin transporter. Given the delayed appearance and long duration of the thrombocytopenia, it seems likely that the precursor megakaryocytes are the primary targets of [131I]MIBG radiotoxicity.

MATERIALS AND METHODS

We investigated MIBG and serotonin uptake in cultured human megakaryocytes grown in vitro from CD34(+) cells obtained from bone marrow.

RESULTS

With radio-iodinated MIBG, cell-associated radioactivity was negligible, even after prolonged incubations for up to 16 hours. In contrast, after 4 or 16 hours with 10(-8) M [3H]serotonin, 6% or 14% of the added substrate was accumulated in the megakaryocytes. This uptake approached saturation above 10(-7) M and was reduced greater than 90% by coincubation by imipramine. This indicates specific uptake, which was confirmed by fluvoxamine and citalopram. The serotonin reuptake inhibitors fluvoxamine (0.3 nM) and citalopram (1 nM) effectively reduced serotonin uptake to 44% +/- 3% and 30% +/- 9% of the controls, respectively.

CONCLUSIONS

Megakaryocytes efficiently retain serotonin in storage granules, as concluded from the consistent reductive effect of tetrabenazine on uptake, retention, and localization (micro-autoradiographic) of serotonin. Thus, serotonin, but not MIBG, is taken up by cultured megakaryocytes.

Expression of a Functional N-Methyl-d-Aspartate–Type Glutamate Receptor by Bone Marrow Megakaryocytes

Better understanding of hemostasis will be possible by the identification of new lineage-specific stimuli that regulate platelet formation. We describe a novel functional megakaryocyte receptor that belongs to a family of ionotropic glutamate receptors of theN-methyl-d-aspartate (NMDA) subtype responsible for synaptic neurotransmission in the central nervous system (CNS). Northern blotting and reverse-transcriptase polymerase chain reaction (RT-PCR) studies identified expression of NMDAR1 and NMDAR2D type subunit mRNA in rat marrow, human megakaryocytes, and MEG-01 clonal megakaryoblastic cells. Immunohistochemistry and in vivo autoradiographic binding of the NMDA receptor-specific antagonist MK-801 confirmed that megakaryocytes expressed open channel-forming NMDA receptors in vivo. Western blots indicated that megakaryocyte NMDAR1 was either unglycosylated or only glycosylated to low levels, and of identical size to CNS-type NMDAR1 after deglycosylation with endoglycosidase F/peptide-N-glycosidase F. In functional studies, we demonstrated that NMDA receptor activity was necessary for phorbol myristate acetate (PMA)-induced differentiation of megakaryoblastic cells; NMDA receptor blockade by specific antagonists significantly inhibited PMA-mediated increases in cell size, CD41 expression, and adhesion of MEG-01 cells. These results provide evidence for a novel pathway by which megakaryocytopoiesis and platelet production may be regulated.

Expression of a Functional N-Methyl-d-Aspartate–Type Glutamate Receptor by Bone Marrow Megakaryocytes

Better understanding of hemostasis will be possible by the identification of new lineage-specific stimuli that regulate platelet formation. We describe a novel functional megakaryocyte receptor that belongs to a family of ionotropic glutamate receptors of theN-methyl-d-aspartate (NMDA) subtype responsible for synaptic neurotransmission in the central nervous system (CNS). Northern blotting and reverse-transcriptase polymerase chain reaction (RT-PCR) studies identified expression of NMDAR1 and NMDAR2D type subunit mRNA in rat marrow, human megakaryocytes, and MEG-01 clonal megakaryoblastic cells. Immunohistochemistry and in vivo autoradiographic binding of the NMDA receptor-specific antagonist MK-801 confirmed that megakaryocytes expressed open channel-forming NMDA receptors in vivo. Western blots indicated that megakaryocyte NMDAR1 was either unglycosylated or only glycosylated to low levels, and of identical size to CNS-type NMDAR1 after deglycosylation with endoglycosidase F/peptide-N-glycosidase F. In functional studies, we demonstrated that NMDA receptor activity was necessary for phorbol myristate acetate (PMA)-induced differentiation of megakaryoblastic cells; NMDA receptor blockade by specific antagonists significantly inhibited PMA-mediated increases in cell size, CD41 expression, and adhesion of MEG-01 cells. These results provide evidence for a novel pathway by which megakaryocytopoiesis and platelet production may be regulated.

Mediation by 5-HT1D receptors of 5-hydroxytryptamine-induced contractions of rabbit middle and posterior cerebral arteries

1. 5-Hydroxytryptamine (5-HT) receptor-mediated contraction of endothelium denuded rabbit middle (MCA) and posterior (PCA) cerebral arteries was characterized by use of selective agonists and antagonists for different 5-HT receptor subtypes. 2. 5-HT and various 5-HT receptor agonists contracted the arteries with the following rank order of potency in MCA: 5-carboxamidotryptamine (5-CT) > 5-HT > 5-methoxytryptamine (5-MeOT) > sumatriptan > alpha-methyl-5-HT (alpha-Me-5-HT) >> 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and in PCA: 5-CT > 5-HT > sumatriptan > 5-MeOT > alpha-Me-5-HT >> 8-OH-DPAT. With few exceptions, the maximal contractile responses of these agonists were similar to that induced by 5-HT. 3. The selective antagonists of 5-HT2A/2C (ketanserin), 5-HT4 (SDZ 205-557) and 5-HT1A/1B (S-(-)-propranolol) sites were devoid of inhibitory effect on 5-HT-mediated contraction in both MCA and PCA, thus excluding activation of the corresponding receptors. 4. In both arteries, the contraction-response curve to 5-HT was unaffected by the 5-HT3 receptor antagonist, ICS 205-930 (0.01 and 0.1 microM) whilst a small (3 and 6 fold displacement) was seen with MDL 72222 (0.1 and 1 microM). 5. The mixed 5-HT1-like/5-HT2A receptor antagonist, methiothepin (0.001-0.1 microM), was a potent antagonist of 5-HT-induced contractions in both arteries, giving pA2 values of 9.4 +/- 0.7 and 9.6 +/- 0.8 in MCA and PCA, respectively. 6. Rauwolscine (O.1-10 MicroM) and yohimbine (0.3, 3 MicroM) inhibited contractions to 5-HT in a competitive manner, pA2 values of 7.1 +/- 0.6 and 6.7 +/-0.6 were determined for rauwolscine in MCA and PCA,respectively. An apparent pA2 value of 6.9 +/-0.2 was calculated for yohimbine (3 MicroM) in both MCA and PCA.7. In conclusion, these results suggest that the contractile response to 5-HT in rabbit isolated MCA and PCA is predominantly mediated by the 5-HTID receptor subtype, although a small contribution by 5-HT3 receptors cannot be excluded.

Structural defects in inherited and giant platelet disorders

As diverse as the group of inherited structural defects and giant platelet disorders presented in this chapter may seem, there is a common thread that ties them together. All appear to represent some form of membrane aberration. Sometimes only a small inclusion identifies the membrane defect, sometimes a massive increase in size. In others, whole populations of organelles are missing or surface membranes lack specific glycoproteins essential for their function. All of them are born in the deep recesses of a hidden cell, the bone marrow megakaryocyte. Getting the megakaryocyte out into the light of day, or at least into a culture medium, should certainly lead to the solution of many, if not all, of the disorders of platelet membranes and membrane disorders. We have not been completely successful in our efforts to study the megakaryocyte in vitro. As a result, we do not yet understand the normal megakaryocyte, much less normal platelet. The megakaryocyte presents one of the greatest of challenges to our understanding of membrane biology. As our knowledge of how its cytoplasm fills with interiorly and exteriorly derived membranes, and the mechanisms underlying their organization into platelet surfaces, channels of the OCS and DTS, membrane complexes, and five kinds of organelles become clear, our ability to define the basic nature and inheritance of defects will improve rapidly. Within the next decade most aspects of platelet molecular genetics and cell biology will be solved.

Location of sites of dopamine storage in megakaryocytes by autoradiography

We have used quantitative electron microscope autoradiography to study the subcellular sites of 3H-dopamine uptake in mouse megakaryocytes after a single intraperitoneal injection. Autoradiographic grains were found to be associated almost exclusively with the vesicles of precursors of monoamine-storage organelles. The labeling intensity (radioactivity) of the demarcation membrane system which is continuous with the plasmalemma was also significantly greater than would be expected. On the other hand, radioactivity associated with the remaining sites in the cytoplasm was not significantly different from that expected of a random distribution. In order to compare 3H-dopamine uptake during cell maturation, light microscope autoradiographic studies were also done. Immature megakaryocytes were labeled slightly, but the number of silver grains increased significantly in maturing cells. Mature megakaryocytes were 2.7 times more radioactive than the maturing cells. Our results suggest that the megakaryocytes were able to accumulate dopamine from the early stages of cell maturation and to store dopamine in precursors of monoamine-storage organelles.

Megakaryocyte function and dysfunction

More than a hundred years have passed since platelets were recognized as cells and their haemostatic functions discovered. However, the process of platelet production is still not understood. The location, the mechanism and the regulation of thrombopoiesis remain elusive. Megakaryocytes are known to be the source of platelets. Investigations of megakaryocytes have revealed their normal functions and some of the abnormalities present in various diseases which affect platelets. In recent years, new techniques of cell isolation and tissue culture have been developed and have made possible advances in characterizing megakaryocyte precursors and differentiation. The primary function of megakaryocytes is to synthesize and assemble platelet components and organelles. Although debated for a long time, new data seems to indicate that the lung may be a central locus of platelet production. The new techniques for megakaryocyte investigations have barely begun to be of use in the study of abnormal platelet production in disease.

5-HT1-like receptors mediate 5-hydroxytryptamine-induced contraction of human isolated basilar artery

1. The 5-hydroxytryptamine (5-HT) receptor mediating contraction of endothelium denuded human basilar artery has been characterized in vitro. 2. 5-HT and a variety of 5-HT agonists contracted human isolated basilar artery with a rank order of agonist potency, 5-carboxamidotryptamine (5-CT) greater than 5-HT identical to methysergide greater than GR43175 much greater than 8-OHDPAT much greater than 2-methyl-5-HT. The maximum response produced by these agonists differed. 3. None of the agonists relaxed human basilar artery when tone was elevated with prostaglandin F2 alpha, indeed further contraction was seen. 4. The contractile responses of human basilar artery to 5-HT and the selective 5-HT1-like agonist GR43175 were highly reproducible whilst those to 5-CT were not. 5. The contractile response to both 5-HT and GR43175 was resistant to antagonism by ketanserin and GR38032, thus excluding activation of 5-HT2 and 5-HT3 receptors. The contractile action of 5-HT and GR43175 was also not antagonized by (+/-)-cyanopindolol, excluding the activation of receptors similar to 5-HT1A and 5-HT1B recognition sites identified in ligand binding studies. 6. In marked contrast, methiothepin was a potent antagonist of the contractile actions of both 5-HT and GR43175, with a pA2 value of 8.8 against both agonists. Methiothepin (100 nM) had no effect on the contractile response to the thromboxane A2-mimetic U46619. 7. We conclude that 5-HT and GR43175 contract the human isolated basilar artery by activating the same receptor type. This receptor appears identical to the 5-HT1-like receptor causing contraction of the dog isolated saphenous vein and cerebral blood vessels from the dog and primate.

Platelets as a model for neurones?

The multiple biochemical and pharmacological similarities existing between blood platelets and 5-hydroxytryptamine (5-HT)-containing neurones of the CNS point to the platelets as a reliable model for the biochemical characterization of 5-HT releasers and uptake blockers which interfere with the storage and the active carrier mechanism of 5-HT in the neurones, respectively. In addition, the affinity displayed by dopamine and by dopaminergic neurotoxin MPP+ for the platelet 5-HT transport and storage indicates also some similarities between platelets and the dopaminergic system of the CNS. Since human platelets contain almost exclusively monoamine oxidase type B (MAO-B), they can be used as a source for the purification and characterization of this human enzyme. Human platelets thus offer an excellent peripheral model to indirectly assess the degree and duration of MAO-B inhibition occurring in the CNS. To date, knowledge of the many biochemical mechanisms underlying platelet physiology is still fragmentary. In fact, the functional role of binding sites located on the platelet cytoplasmic membrane, i.e. their coupling to a specific transmembrane signalling mechanism, is still in need of a precise biochemical and physiological characterization.

Uptake, release, and subcellular localization of 1-methyl-4-phenylpyridinium in blood platelets

The neurotoxic compound 1-[methyl-3H]-4-phenylpyridinium ([3H]MPP+) was actively taken up by human, rabbit, and guinea pig platelets incubated in plasma. In human platelets, the apparent Km of this uptake (22.6 microM) was 50 times higher than that for serotonin [5-hydroxytryptamine (5-HT]). The uptake of [3H]MPP+ by human platelets was inhibited by selective 5-HT uptake blockers [cianopramine, (-)-paroxetine, and clomipramine], by metabolic inhibitors (KCN and ouabain), and by drugs that interfere with amine storage in the 5-HT organelles (reserpine, mepacrine, and Ro 4-1284). Impairment of the transmembrane proton gradient by ionophores (monensin and nigericin) induced a marked release of radioactivity from platelets preincubated with [3H]MPP+. Fractionation of homogenates of rabbit platelets preincubated with [3H]MPP+ showed that the drug was concentrated to a great extent in the 5-HT organelle fraction. MPP+ competitively inhibited [14C]5-HT uptake by human platelets and reduced the endogenous 5-HT content of human, rabbit, and guinea pig platelets. These investigations show that MPP+ is transported into the platelets via the 5-HT carrier and is accumulated predominantly in the subcellular organelles that store 5-HT and other monoamines. It is suggested that an accumulation of MPP+ in amine storage vesicles of neurons may be involved in the effects of the drug in the CNS, e.g., by protecting other subcellular compartments from exposure to high concentrations of MPP+, by sustaining a gradual release of the toxin, or both.

The 5-hydroxytryptamine system of blood platelets: physiology and pathophysiology

The 5-hydroxytryptamine (5HT)-system of human blood platelets consists of a relatively specific uptake mechanism for 5HT at the plasma membrane, intracellular storage organelles (dense bodies), a metabolizing enzyme (monoaminoxidase B) and a 5HT2-receptor whose stimulation leads to activation of the phosphatidylinositide turnover, a rise in free cytoplasmic Ca2+, phosphorylation of proteins and a shape change reaction. There is neither a relevant 5HT-biosynthesis nor a marked physiological 5HT-turnover in platelets. Under physiological conditions the platelet 5HT-system may have a role as a scavenger for free extracellular 5HT and in hemostasis. Disturbances which have been described in pathophysiological states include impairment of 5HT-uptake (hypertension, migraine), impairment of 5HT-storage (storage pool deficiencies, thromboembolic disorders, hypertension) and increased sensitivity to activating agents like 5HT (cardiovascular disorders, diabetes). Besides their role in physiology and pathophysiology platelets may be useful partial models for vascular smooth muscle cells.

Incorporation of a circulating protein into megakaryocyte and platelet granules

To determine whether or not proteins circulating in plasma can be incorporated into megakaryocytes and platelets, horseradish peroxidase (HRP) was injected intravenously into guinea pigs and these cells were examined for its uptake by electron microscopy and cytochemistry. Enriched samples of megakaryocytes enabled ultrastructural analysis of large numbers of these rare cells. In megakaryocytes, 50% of alpha granules contained HRP between 75 min and 7 hr after injection. At 24 hr, 25% of the megakaryocyte granules were peroxidase-positive, less were positive by 48 hr, and there were none at 4 days. Thus, the findings demonstrate that a circulating protein can be endocytosed by megakaryocytes and rapidly packaged into alpha granules. Platelet granules also contain HRP by 7 hr after injection, and they can secrete it in response to thrombin. Unfortunately, our present studies do not allow us to distinguish between direct endocytosis by the platelet and/or shedding of new platelets from recently labeled megakaryocytes. It is concluded that while some alpha granule proteins are synthesized by megakaryocytes, others may be acquired from plasma by endocytosis. In addition to providing evidence that some of the proteins of alpha granules may be of exogenous origin, this study has allowed the definition of a pathway whereby plasma proteins may be temporarily sequestered in megakaryocytes before reentering the circulation in platelets.

Uptake of 3H-dopamine in megakaryocytes and blood platelets measured by quantitative electron-microscope autoradiography

We studied the uptake of dopamine by mature megakaryocytes and blood platelets in mouse spleen after a single intraperitoneal injection of 3H-dopamine. In order to compare the uptake of 3H-dopamine in mature megakaryocytes and blood platelets, we used quantitative autoradiography at the electron-microscope level. Dense accumulations of silver grains were observed on both mature megakaryocytes and blood platelets; all other tissue elements of the spleen exhibited considerably less dense labeling. No significant differences with regard to dopamine uptake were observed in megakaryocytes and blood platelets. This is in contrast to the previous finding of very different patterns of 3H-5-hydroxytryptamine labeling in mature megakaryocytes and blood platelets (Daimon and Uchida 1985). The results of the present study provide new evidence in favor of the hypothesis that the active uptake mechanism of dopamine through the plasma membrane is different from the uptake mechanism of 5-hydroxytryptamine.

Histamine uptake in pig platelets and isolated dense granules

Histamine, a major constituent of the amine-storage organelles in pig platelets, is taken up by intact platelets in only trace amounts under conditions where 70% of 14C-serotonin is accumulated. Thrombin caused the release of 70-90% of endogenous histamine but only 5-10% of the newly absorbed 3H-amine; however, after 18 hr 30% of the 3H-amine could be specifically released by thrombin. Isolated storage organelles accumulated histamine in a reserpine-sensitive, ATP-dependent manner but at a rate 80-100-fold less than serotonin uptake. Incubation of intact platelets with 1 mM serotonin until amine uptake was saturated caused no changes in platelet histamine content. Similarly, loading of isolated storage organelles with 1 mM histamine or 1 mM serotonin did not affect the levels of the other amine. These results suggested that the storage of each amine is independent of the other. Histidine decarboxylase was not detected in platelet lysates. Since platelets have a short half-life (1-2 weeks) and pig plasma levels of histamine are higher than in other animals, it is concluded that most of the histamine in the storage organelles is probably accumulated in the platelet precursor, the megakaryocyte, either by slow uptake or by synthesis.

Increase in density and accumulation of serotonin by human aging platelets

51Cr-labeled autologous platelets were infused into splenectomized subjects and the specific radioactivities of high-density (HD) and low-density (LD) platelet subpopulations were determined sequentially in postinfusion samples. A rapid decrease in the specific radioactivity of LD cohorts (T1/2 = 2.5 days) was observed, but the specific radioactivity of HD platelets remained constant or increased slightly during the first 4 days and then gradually declined for the next 5 days. No experimental artifacts during the platelet-labeling steps that could account for these results were demonstrated. These findings confirm previous observations in eusplenic individuals and support the hypothesis that human LD platelets are, on the average, younger than HD platelets. LD platelets contain 33.8 +/- 13.5 ng serotonin (5HT)/10(8) platelets and HD platelets 76.8 +/- 9.5 ng 5HT/10(8) platelets (P less than 0.001). Sequential measurements of 5HT in PRP platelets were performed during the recovery phase of thrombocytopenia following splenectomy in patients with idiopathic thrombocytopenic purpura (ITP), a condition associated with aging of platelets in circulation. Presplenectomy platelet 5HT was 17.7 ng/10(8) platelets and on days 1, 6, and 12 after surgery it increased to 18.1, 37.8, and 61.0 ng/10(8) platelets (n = 7). When three healthy volunteers were given aspirin (500 mg/day) for up to 15 days, no significant change in the 5HT content of circulating platelets was observed. If aspirin blocks, at least partially, the secretory process in vivo without interfering the 5HT uptake by the platelets, this finding stands against the possibility that a net depletion of 5HT occurs during the life-span of normal human platelets. The observation that human HD platelets, enriched with older cells, contain more 5HT than LD platelets taken together with the parallel increase in platelet 5HT and age during the recovery from thrombocytopenia in ITP patients and the lack of effect of aspirin on platelet 5HT content, provides initial evidence that human platelets accumulate 5HT during their life-span in circulation.

Precursors of monoamine-storage organelles in developing megakaryocytes of the rat

Identification and distribution of the precursors of amine-storage organelles in rat megakaryocytes during cell maturation were studied, using the uranaffin reaction for adenine nucleotide. The precursors of the amine-storage organelles appeared as 200-300 nm vesicles having an uranaffin electron dense granule, whereas they appeared as empty vesicles by conventional glutaraldehyde-OsO4 fixation. X-ray probe microanalysis confirmed the existence of U and P in the uranaffin reaction positive vesicles. The precursors appeared in the immature megakaryocytes, especially at the trans(mature) face of the Golgi apparatus, and rapidly increased in number in the maturing cells. The size of the uranaffin granules in the precursor organelles increased gradually during cell maturation and became almost equivalent to the dense body of blood platelets in the final stage of cell maturation.

Ultrastructure of resting and activated storage pool deficient platelets from animals with the Chédiak-Higashi syndrome

The ultrastructural of platelets from Chédiak-Higashi (CH) and normal cattle, mink, and cats at rest was studied. Platelets from CH animals had a virtual absence of platelet dense granules. Alpha granules, amorphous membrane-surrounded structures, mitochondria, and microtubules of CH bovine platelets were similar in number and appearance to those in normal bovine platelets. Giant CH granules, present in other cells and considered diagnostic of the syndrome, could not be identified in platelets from CH animals. The open canalicular system and dense tubule system were not readily identifiable in resting bovine platelets. The ultrastructure of normal and CH cattle platelets was evaluated at various stages of ADP-induced aggregation. After platelets changed shape during the first phase of aggregation, the ultrastructural appearance of CH platelets was similar to that of normal platelets. The CH platelets composing the aggregates during irreversible aggregation did not appear as activated as did normal platelets, even though the aggregation tracings were similar. Normal and CH cattle platelets treated with thrombin appeared morphologically similar and were characterized by centrifugal movement of granules.

Do some nerve cells release more than one transmitter?

The concept that each nerve cell makes and releases only one nerve transmitter (widely known as Dale's Principle) has been re-examined. Experiments suggesting that some nerve cells store and release more than one transmitter have been reviewed. Developmental and evolutionary factors are considered. Conceptual and experimental difficulties in investigating this problem are discussed. It is suggested that the term 'transmitter' should be applied to any substance that is synthesised and stored in nerve cells, is released during nerve activity and whose interaction with specific receptors on the postsynaptic membrane leads to changes in postsynaptic activity. Expressed in this way, it seems likely that while many nerves do have only one transmitter, others in some species, during development or during hormone-dependent cycles, employ multiple transmitters.

Comparative studies of purinergic nerves

Purinergic nerves supply the gastrointestinal tract of vertebrates, including fish, amphibians, reptiles and birds, as well as mammals. Their cell bodies are located in Auerbach's plexus and their axons extend in an anal direction before innervating mainly the circular muscle coat. In the stomach they are controlled by preganglionic cholinergic fibres of parasympathetic origin. They are involved in "receptive relaxation" of the stomach, "descending inhibition" in peristalsis and reflex relaxation of oesophageal and internal anal sphincters. The terminal varicosities of purinergic nerves are characterised by a predominance of "large opaque vesicles," which can be distinguished from the "large granular vesicles" found in small numbers in both adrenergic and cholinergic nerves. Stimulation of purinergic nerves with single pulses produces hyperpolarisations of up to 25 mV (inhibitory junction potentials) in smooth muscle cells. These potentials are unaffected by atropine, adrenergic neuron blocking agents or sympathetic denervation, but are abolished by tetrodotoxin. The "rebound contraction" which characteristically follows cessation of purinergic nerve stimulation is probably due to prostaglandin. Evidence that ATP is the transmitter released from purinergic nerves includes: (1) synthesis and storage of ATP in nerves; (2) release of ATP from the nerves when they are stimulated; (3) exogenously applied ATP mimicking the action of nerve-released transmitter, both producing a specific increase in K+ conductance; (4) the presence of Mg-activated ATPase, 5'-nucleotidase and adenosine deaminase, enzymes which inactivate ATP; (5) drugs (including quinidine, some 2-substituted imidazolines, 2-2'pyridylisatogen and dipyridamole) which produce similar blocking or potentiating effects on the response to exogenously applied ATP and nerve stimulation. Speculations are made about the evolution and development of the nervous system, including the possibility that purinergic nerves are a primitive nerve type.

Ultrastructural findings in storage pool disease and aspirin-like defects of platelets

Previous studies have shown that abnormalities in collagen-induced platelet aggregation may be due to an impaired release of storage pool ADP, the agent ultimately responsible for platelet aggregation. In some patients and in normal subjects who ingest aspirin, the storage pool of ADP is present in normal amounts, but the mechanism for releasing it appears to be defective ("aspirin-like" defect). In these subjects, the centripetal reorientation of the platelet granules, which may be early structural changes of the release reaction, failed to occur. In one preleukemic patient with an aspirin-like defect, the elements of the open-channel system were also increased. In a second group of patients the impairment of aggregation is due to a deficiency of storage pool ADP. In these patients with storage pool disease, the initial ultrastructural changes associated with the collagen-induced release reaction were normal. The most striking abnormality was a marked decrease in the number of platelet dense bodies. Since the platelets of these patients are deficient in both serotonin and the storage pool of adenine nucleotides, the findings suggest that, in human platelets, these substances are normally stored in the dense bodies. A defect in the formation or function of the dense bodies may account for the abnormalities of platelet aggregation in storage pool disease.