Browsing by Author "JAMALUDDIN, M"

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    ACTIVATION OF BLOOD-PLATELETS
    (JOURNAL OF SCIENTIFIC & INDUSTRIAL RESEARCH, 1993) JAMALUDDIN, M
    Platelets are small anucleate and excitable blood cells derived from megakaryocytes of bone marrow. Their activation by various exogenous stimuli that are exposed and engendered in vascular exigencies, plays crucial roles in haemostasis and thrombosis and is implicated in various other cardiovascular diseases. Molecular mechanisms of cause-effect relationships are not yet clearly understood but evidence for stimulus (agonist)-directed, receptor-operated, sequential activation of two major interacting biochemical pathways for activation of protein kinase C (PKC) has accumulated. One of these is linked to phospholipase C (PLC) activation and the other to phospholipase A2 (PLA2) activation and formation of cyclo-oxygenase products. These pathways provide mechanisms for the transformation of the plasma-membrane glycoprotein IIb-IIIa, the platelet-specific integrin, into an aggregation-competent form and regulation of platelet activation. Our understanding of the aggregation reaction is rudimentary. Contrary to common belief a first order rate law seems to apply. The author has proposed a sequential shape-change and interaction model of aggregation. On its basis a rate equation for platelet aggregation was derived. A spectrophotometric method of following shape-change reactions of platelets and quantitating single-platelet recruitment into aggregates has also been devised. The results of their application to the aggregation of gel-filtered calf platelets can be summarized as follows: Epinephrine did not aggregate but inhibited the aggregation induced by ADP. U-46619 and collagen induced only shape-change no aggregation but enhanced the aggregation by ADP. The kinetics of ADP-and phorbol myristate acetate (PMA)-induced aggregation showed apparent hyperbolic pattern (linear double-reciprocal plots and h=1). The kinetics of aggregation by thrombin, A 23187, Paf, arachidonic acid and H2O2 showed apparent positive co-operativity ('concave up double-reciprocal plots and h > 1). Paf has the lowest and H2O2 the highest half-maximal saturation concentrations. PMA showed the lowest and H2O2 the highest maximum rates. Arachidonate and H2O2 were found to be ligands for a dimeric haemoprotein (Mr approximately 40,000) purified from calf platelets. These ligands as well as the PGH2/TxA2 analogue U-46619 induced conformational changes in the purified protein. Similar conformational changes were found in the protein in intact platelets upon their activation. Ligand-induced conformational change of the protein has been proposed as a mechanism of platelet activation. H2O2 activates platelets by acting at two different sites one of which is the putative PGH2/TxA2 receptor, possibly its SH group(s). Adenosine and ATP have been found to be mutually exclusive modifiers of the kinetics of ADP-induced aggregation.
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    ADENOSINE AND ATP - MUTUALLY EXCLUSIVE MODIFIERS OF THE KINETICS OF ADP-INDUCED AGGREGATION OF CALF-PLATELETS
    (JOURNAL OF BIOSCIENCES, 1990) JAMALUDDIN, M; KRISHNAN, LK
    Modulations of initial rate kinetics of ADP-induced aggregation of citrated calf platelet-rich plasma by adenosine and ATP were investigated employing a spectrophotometric platelet aggregation assay. The data were analysed according to the tenets of the sequential shape-change and interaction model of aggregation. Adenosine and ATP increased the slopes and intercepts of double-reciprocal plots of ADP-aggregation kinetics. Examination of their slope and intercept effects together with their effects individually and in combination, on aggregation rates, suggested that adenosine and ATP acted at multiple, nonoverlapping, sites.
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    COMPETITIVE-INHIBITION OF HYDROGEN PEROXIDE-INDUCED AGGREGATION OF CALF PLATELETS BY PROSTAGLANDIN H-2 THROMBOXANE A2 RECEPTOR LIGANDS
    (JOURNAL OF BIOSCIENCES, 1992) JAMALUDDIN, M; THOMAS, A
    Hydrogen peroxide (H2O2)-induced aggregation of calf platelets and its modification by agents with specific properties were characterized employing a spectrophotometric assay. An Arrhenius activation energy of 20 +/- 1 kcal/mol was found in the temperature range of 25-degrees-36-degrees-C. Rate inhibition occurred on either side of this temperature range, and under anaerobic conditions. Exogenous Ca2+ ions were not required but Ca2+ ions, at 1 mM-concentration, optimally increased rates and extent of aggregation at suboptimal H2O2 concentrations but only extent of aggregation at optimal H2O2 concentrations. Ba2+, Sr2+, Cd2+, Mn2+ and Ni2+ ions (1 mM) and Zn2+, Pb2+ and Hg2+ ions (10-mu-M) were inhibitory. The cyclo-oxygenase inhibitor, indomethacin (10-30-mu-M) exerted only mild inhibition by a competitive mechanism. Another cyclooxygenase inhibitor, aspirin, functioned to increase aggregation. Ligands acting directly at the prostaglandin H2/thromboxane A2 receptor (5Z, 9, 11, 13E, 15(S) 15-hydroxy 9(11) epoxy methano prosta 5, 13-dien-1-oic acid, pinane thromboxane A2, arachidonic acid, eicosapentaenoic acid, and N-ethylmaleimide) functioned as competitive inhibitors. Another platelet-activating sulphydryl reagent, thimerosal, also inhibited competitively while the protein kinase C inhibitor, sphingosine, and the protein kinase C modulator, Zn2+ ions, inhibited by different mechanisms. The results indicate direct action of H2O2 at the prostaglandin H2/thromboxane A2 receptor, possibly its sulphydryls, to activate the protein kinase C pathway, independently of cyclo-oxygenase products. The results underscored the power of the kinetic approach for investigating mechanisms of platelet activation.
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    NEW PERSPECTIVES IN BLOOD-PLATELET AGGREGATION
    (CURRENT SCIENCE, 1991) JAMALUDDIN, M
    Aggregation of blood platelets is indispensable for haemostasis but fraught, in some circumstances, with pathological consequences whose mechanism-based prevention and treatment are a long-sought-after goal that remains elusive partly because of a lack of understanding of mechanisms. A glimmer of new insight is emerging. Aggregation appears to involve stimulus-induced, energy-dependent creation of heterogeneous platelet species which sort out partners by long-range, possibly hydrophobic, attractive forces and cell-adhesion receptor counter-receptor discriminating reactions. Dynamic extrusion of pseudopods with reactive cell-adhesion receptors on them appears to be essential. Aggregation is first order and occurs without significant compression or interdigitation of glycocalyces and cooperativity forms an important aspect of its modulation. Platelets could form a convenient model system to study intercellular interactions.
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    PLATELET ACTIVATING FACTOR-INDUCED AGGREGATION OF CALF PLATELETS - APPARENT POSITIVE COOPERATIVITY IN THE KINETICS AND NON COMPETITIVE-INHIBITION BY DILTIAZEM
    (JOURNAL OF BIOSCIENCES, 1992) JAMALUDDIN, M; THOMAS, A
    Aggregation of calf platelets by platelet activating factor was characterized by a spectrophotometric method. The aggregation kinetics of both platelet-rich plasma and purified platelets showed concave up double-reciprocal plots and linear Hill plots with h > 1 (1.7 +/- 0.2) consistent with positive cooperativity. Comparable values of maximum rates of aggregation (R) were obtained with platelet-rich plasma (0.25 +/- 0.08) and purified platelets (0.28 +/- 0.18) but the half-maximal saturation concentration (S0.5) differed greatly between platelet-rich plasma (6 +/- 3 nM) and purified platelets (0.28 +/- 0.18 nM). An Arrhenius activation energy of 21 +/- 2 kcal/mol was found for aggregation of purified platelets. Diltiazem was inhibitory with half-maximal inhibitory concentration (I0.5) of 4-mu-M but the inhibition was not competitive. Diltiazem inhibited rates but not the extent of shape-change. The receptor-antagonist and sulphydryl reagent N-ethylmaleimide and the platelet antagonistic omega-3-fatty acid, 5,8,11,14,17-eicosa pentaenoic acid, inhibited both rates and extent of shape-change reactions and inhibited aggregation competitively (I0.5 approximately 5-mu-M). Eicosa pentaenoic acid at > 25-mu-M could abolish shape-change reactions and at 50-mu-M served as an activator of platelets and the activation was enhanced by aspirin (1 mM). Although N-ethylmaleimide at > 20-mu-M could also induce platelet activation it failed to induce aggregation and aspirin had no effect on the shape-change reactions induced by it.
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    PLATELET-AGGREGATION
    (CURRENT SCIENCE, 1990) JAMALUDDIN, M
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    SPECTROPHOTOMETRIC PLATELET-AGGREGATION ASSAY TO MEASURE SINGLE PLATELET DISAPPEARANCE - AN EVIDENCE
    (CURRENT SCIENCE, 1987) SREEDEVI, C; THOMAS, A; JAMALUDDIN, M