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Biomedical subjects

D E MacIntyre

Publications and source records attributed to D E MacIntyre.

At least 73 records · Page 4Linked to original sources
PubMed1985

Angiotensin II effects on platelet function.

The effects of angiotensin II (ANG II) alone and in combination with other agonists on platelet aggregation, thromboxane B2 (TxB2) synthesis and cytosolic [Ca2+]i were investigated. Angiotensin II (10(-11)-10(-7) mol/l) alone had no direct effect on aggregation, TxB2 production or [Ca2+]i after short- (< 2 min) or long-term (30 min) incubation. In contrast, low concentrations of ANG II (10(-11) mol/l) enhanced adrenaline-induced platelet aggregation but high concentrations (10(-7) mol/l) had an inhibitory effect. Moreover, ANG II (10(-11)-10(-7) mol/l) augmented platelet responses to the TxA2 mimetic U44069. The facilitatory effect of ANG II on adrenaline-induced platelet aggregation was abolished by pretreatment of platelets with flurbiprofen. Thromboxane B2 synthesis by adrenaline-treated platelets was inhibited by ANG II. The results indicate that ANG II stimulation of agonist-induced platelet activation is due to potentiation of the effects rather than the synthesis of TxA2.

Angiotensin II↗
PubMed1985

Endogenous and pharmacological mechanisms for the regulation of human platelet cytosolic free Ca2+.

Because they inhibit the processes that promote elevation of [Ca2+]i and augment the processes that promote removal of Ca2+ from the cytosol, receptor antagonists, agents that mimic or elevate cAMP, cGMP or 1,2-Diacylglycerol (DG), and both inorganic and organic Ca2+ channel blockers can be considered to act as 'Ca2+ antagonists' on human platelets. Agonist-induced elevation of [Ca2+]i is associated with phosphoinositide hydrolysis. Unlike agents that mimic or elevate cAMP, cGMP or DG, receptor antagonists and organic Ca2+ influx, mobilisation of internal Ca2+ and inositol lipid hydrolysis. Lanthanides apparently inhibit only Ca2+ influx. Thus La3+ but not Verapamil or Diltiazem block receptor-operated Ca2+ channels on human platelets. The endogenous processes that promote extrusion or sequestration of cytosolic Ca2+ may be augmented by cAMP, cGMP, DG and by Ca2+. DG, via activation of protein kinase C, may serve as a bi-directional regulator of platelet reactivity.

Aminoquinolines↗
PubMed1984

Thromboxane-induced phosphatidate formation in human platelets. Relationship to receptor occupancy and to changes in cytosolic free calcium.

The inter-relationships between receptor occupancy, inositol phospholipid metabolism and elevation of cytosolic free Ca2+ in thromboxane A2-induced human platelet activation were investigated by using the stable thromboxane A2 mimetic, 9,11-epoxymethanoprostaglandin H2, and the thromboxane A2 receptor antagonist, EPO45. 9,11-Epoxymethanoprostaglandin H2 stimulated platelet phosphatidylinositol metabolism as indicated by the rapid accumulation of [32P]phosphatidate and later accumulation of [32P]phosphatidylinositol in platelets pre-labelled with [32P]Pi. These effects of 9,11-epoxymethanoprostaglandin H2 were concentration-dependent and half-maximal [32P]phosphatidate formation occurred at an agonist concentration of 54 +/- 8 nM. With platelets labelled with the fluorescent Ca2+ indicator quin 2, resting cytosolic free Ca2+ was 86 +/- 12 nM. 9,11-Epoxymethanoprostaglandin H2 induced a rapid, concentration-dependent elevation of cytosolic free Ca2+ to a maximum of 300-700 nM. Half-maximal stimulation was observed at an agonist concentration of 80 +/- 23 nM. The thromboxane A2 receptor antagonist EPO45 selectively inhibited 9,11-epoxymethanoprostaglandin H2-induced [32P]phosphatidate formation and elevation of cytosolic free Ca2+, indicating that both events are sequelae of receptor occupancy. Human platelets contain a single class of stereospecific, saturable, high affinity (KD = 70 +/- 13 nM) binding sites for 9,11-epoxymethano[3H]prostaglandin H2. The concentration-response curve for receptor occupancy (9,11-epoxymethano-[3H]prostaglandin H2 binding) is similar to that for 9,11-epoxymethanoprostaglandin H2-induced [32P]phosphatidate formation and for elevation of cytosolic free Ca2+. These observations indicate that human platelet thromboxane A2 receptor occupation is closely linked to inositol phospholipid metabolism and to elevation of cytosolic free Ca2+. Both such events may be necessary for thromboxane A2-induced human platelet activation.

Blood Platelets↗
PubMed1984

Inhibition of platelet function by cis-unsaturated fatty acids.

The uptake of free fatty acids has previously been shown to affect the capping of lymphocytes, and there is evidence that different types of fatty acids may partition into separate lipid domains in cell surface membranes. In studies of gel-filtered human platelets, we found that cis-unsaturated fatty acids (1-35 microM) inhibited platelet shape change, aggregation, and secretion of 5-hydroxytryptamine induced by thrombin, adenosine diphosphate (ADP), collagen, U46619 (a thromboxane A2 analog), or plant lectins, but not that induced by A23187, a calcium ionophore. Trans-unsaturated and saturated fatty acids had little or no inhibitory effect. The inhibitory effects of cis-unsaturated fatty acids were not affected by inhibition of adenylate cyclase or cyclooxygenase. 14C-labeled fatty acids were taken up into platelet lipids. The maximum platelet-inhibitory effect of cis-unsaturated fatty acids was seen when over 90% of the platelet label was still in the form of free fatty acids. Platelet inhibition could be reversed by washing the platelets by gel filtration. Binding of platelet agonists to the platelet was not inhibited by the fatty acids. Cis-unsaturated fatty acids, but not trans-unsaturated or saturated fatty acids, decreased fluorescence polarization of platelets or isolated platelet membranes monitored with 1,6-diphenyl- 1,3,5-hexatriene. The potency of the fatty acids as inhibitors of platelet aggregation was inversely correlated with their melting points. These data suggest that inhibition of receptor-mediated platelet responses by cis-unsaturated fatty acids results from perturbation of the platelet membrane in specific lipid domains.

Blood Platelets↗
PubMed1983

Phospholipid-induced human platelet activation: effects of calcium channel blockers and calcium chelators.

Human platelet activation (aggregation, [14C]-5HT release and TxB2 production) induced by the phospholipids, PAF and lysophosphatidic acid (LPA) was inhibited by EGTA, TMB-8 (an intracellular calcium antagonist) and by phenylalkylamine (Class II) but not 1,4-dihydropyridine (Class I) calcium channel blockers. Primary aggregation induced by PAF was selectively inhibited by phenylalkylamine (verapamil, methoxyverapamil) calcium channel blockers. Phospholipid-induced human platelet activation depends predominantly on the influx of extracellular calcium, possibly via specific receptor-operated calcium channels.

Biological Transport↗
PubMed1983

Platelet-activating factor stimulates phosphatidylinositol turnover in human platelets.

Platelet-activating factor stimulates phosphatidylinositol turnover in human platelets as indicated by [32P]phosphatidate accumulation in platelets pre-labelled with [32P]Pi, and by [3H]phosphatidate accumulation and [3H]phosphatidylinositol loss in platelets pre-labelled with [3H]arachidonate. These effects of platelet-activating factor are direct and are independent of the production and/or release of endogenous platelet agonists such as ADP, 5-hydroxytryptamine and thromboxane A2.

Arachidonic Acid↗
PubMed1983

Platelet products and vascular PGI2 production.

The effects of products synthesized and/or secreted by activated platelets on production of PGI2 by human, rat and rabbit vascular rings were investigated. Of the platelet dense body constituents, 5HT stimulated PGI2 production by vascular tissue of all three species whereas ADP was active only on rat tissue. Of the lipids produced during platelet activation, PAF stimulated PGI2 production by vascular tissue of all three species, Lysophosphatidate was less active than PAF on rabbit and human tissue and inactive in rat tissue, and the TxA2-mimetic, U46619, was inactive on vascular tissue of all three species. It is concluded that there are species variations in the effects of agonists on vascular PGI2 production and that platelet-derived products other than platelet-derived growth factor and beta-thromboglobulin could modulate PGI2 production to regulate platelet activation in vivo.

Adenosine Diphosphate↗
PubMed1982

The role of platelet membrane potential in the initiation of platelet aggregation.

The membrane potential of human platelets, and the role of this potential in platelet aggregation, was assessed using the noncovalent, fluorescent probe DiS-C3-5. High K+ and Gramicidin depolarised the cells, whereas valinomycin in standard (4 mMK+) solution produced a hyperpolarisation. Very small changes in potential were observed when choline Cl replaced NaCl. These findings indicate that platelets possess a relatively K+-perm-selective membrane. The resting potential calculated from the "valinomycin null point" (the K+ concentration gradient at which valinomycin did not change the potential) was approximately -60 mV. Other factors that contribute to the platelet membrane potential include a significant Cl- permeability, demonstrated by replacing Cl- with methylsulphate, and an electrogenic Na+ pump, demonstrated using strophanthidin. Little or no change in potential was observed upon addition of ADP, collagen, U44069 or thrombin. Neither strong depolarisation with high K+ or gramicidin nor hyperpolarisation with valinomycin induced platelet aggregation or altered platelet responses to agonists. It is concluded that the information transduction mechanisms involved in platelet activation do not include changes in platelet membrane potential.

Benzothiazoles↗
PubMed1981

The morphological and biochemical characterization of a line of rat promegakaryoblasts.

Biochemical and morphological evidence is presented to support the characterization of a rat bone-marrow-derived cell line (RPM) as an analog of the promegakaryoblast. The conditions for in vitro growth and maturation of the RPM line are described. Rapid proliferation of the RPM line is readily achieved when cultures are supplemented with moderate levels of fetal bovine serum (FBS). The proliferative compartment is a small blast-like cell. Immunofluorescent staining demonstrates that the RPM cells contain factor VIII:antigen and fibrinogen in their cytoplasm. The cells secrete, into their conditioned medium, a potent mitogenic activity for rat aortic smooth muscle cells. When incubated under conditions of relative serum deprivation, the cells stop proliferating and undergo a process of maturation. The sequence of maturation is described as stage I (promegakaryoblast--the proliferative compartment); stage II (immature megakaryocyte or promegakaryocyte); stage III (mature megakaryocyte). The stage III cells release, from their cytoplasm, small membrane-bound vesicular bodies containing lavender granules and cytoplasmic organelles. These have been designated stage IV. The RPM line may provide a useful model for the in vitro study of megakaryocyte maturation and the synthesis of megakaryocyte-specific proteins.

Animals↗
PubMed1979

Stimulation of human platelets by carrageenans.

The rank order of four carrageenans tested as inducers of human platelet aggregation was the same (iota greater than lambda greater than gelcarin greater than kappa) as their relative inflammatory potencies in vivo. All four carrageenans caused some precipitation of plasma proteins, and induced aggregation in platelet-rich plasma or washed platelet suspensions. The second phase of aggregation was citrate-dependent and associated with secretion of 5-hydroxytryptamine and lysosomal enzymes. Platelets could provide a useful model for investigating the actions of carrageenans on cell membranes.

Blood Platelets↗
PubMed1979

Platelet-reactivity of isolated constituents of the blood vessel wall.

Collagens I and III, in fibrillar form, bound platelets equally well; both readily induced platelet aggregation. In contrast, collagens IV and V, although pretreated as collagens I and III to induce fibrillogenesis, failed to produce aggregation. No binding of platelets was detected. Lens capsule, containing collagen type IV in situ, was also inactive. Inactivity appears due to the lack of an appropriate quaternary structure since segment-long-spacing (SLS) aggregates of collagens IV and V, as of type I, induced aggregation. Elastin and its associated microfibrillar element did not aggregate platelets; some binding of platelets to elastin only was observed.

Animals↗
PubMed1979

Modulation of platelet function by prostaglandins: characterization of platelet receptors for stimulatory prostaglandins and the role of arachidonate metabolites in platelet degranulation responses.

The effects of PGG2 and PGH2 on platelets are mimicked by synthetic PG analogues in which the nature and specificity of the substituents on carbons 11 and 15 (or 16) are important determinants of reactivity. Arachidonic acid and stimulatory PGs induce secretion of platelet dense granule and alpha granule constituents, but not lysomal constituents, although arachidonate metabolism is necessary for collagen-induced release of lysosomal enzymes. NO164 acts on platelets as an endoperoxide antagonist: Trimethoquinol acts as an endoperoxide and TxA2 antagonist. PGs induce platelet aggregation by combining with a specific (endoperoxide) receptor.

Blood Platelets↗