Alpha 2A-adrenergic receptors activate protein kinase C in human platelets via a pertussis toxin-sensitive G-protein

Evaluation of selected South African plant species for antioxidant, antiplatelet, and cytotoxic activity

The antioxidant, antiplatelet, and cytoxoxic effects of seven South African plant extracts, namely, Combretum vendae A.E. van Wyk (Combretaceae), Commiphora harveyi (Engl.) Engl. (Burseraceae), Khaya anthotheca (Welm.) C.DC (Meliaceae), Kirkia wilmsii Engl. (Kirkiaceae), Loxostylis alata A. Spreng. ex Rchb. (Anacardiaceae), Ochna natalitia (Meisn.) Walp. (Ochnaceae), and Protorhus longifolia (Bernh. Ex C. Krauss) Engl. (Anacardiaceae), were evaluated using established in vitro assays. All the extracts showed comparably low toxicity except for the extract of C. harveyi that showed high hemagluttination assay titer value, which indicates toxicity. The extracts of P. longifolia, K. wilmsii, O. natalitia, L. alata, C. harveyi, and C. vendae exhibited antioxidant properties in the qualitative assay using DPPH. In the quantification of antioxidation using ABTS, only the extracts of P. longifolia, L. alata, and C. vendae showed antioxidant activity with respective TEAC values of 1.39, 1.94, and 2.08. Similarly, in the quantitative DPPH assay, L. alata (EC50, 3.58+/-0.23 microg/mL) and K. wilmsii (EC50, 3.57+/-0.41 microg/mL) did not differ significantly (p<or=0.05) from the control. K. anthotheca showed a higher EC50 (176.40+/-26.56 microg/mL) value, and differed significantly (p<or=0.05) from all the other extracts and control. In addition, the extracts of C. vendae and C. harveyi showed significant (p<or=0.05) antiplatelet activity and did not differ from the control (aspirin) with EC50 of 0.06+/-0.01 microg/mL and 0.19+/-0.00 microg/mL, respectively. Lower EC50 values in the antioxidant and antiplatelet studies are indicative of superior activity of the plant extract against oxidation and platelet aggregation.

The convenient synthesis and evaluation of the anticancer activities of new resveratrol derivatives

In the present study we report the simple synthesis and antitumour activity of novel stilbene derivatives 13-22. The key synthetic strategies involved Wadsworth-Horner-Emmons condensation and coupling reactions in high yields. All compounds showed significant growth inhibition on human tumour cell lines, with the most potent compound (19) exhibiting an IC(50) of 5.7 microM-11.4 microM in vitro.

Chloroquine: a multipotent inhibitor of human platelets in vitro

Chloroquine inhibited human platelet aggregation in vitro both at receptor- and nonreceptor-operated stimuli. The inhibition was dose-dependent, recorded on isolated platelets as well as in platelet-rich plasma, and followed the rank order of stimuli: adrenaline (second phase)>phorbol 12-myristate 13 acetate>adenosine diphosphate>adrenaline (first phase)>thrombin>calcium ionophore A23187. In thrombin-activated platelets, chloroquine decreased in a dose-dependent manner phospholipase A(2)-induced arachidonic acid liberation from membrane phospholipids, malondialdehyde formation (a marker of membrane phospholipid peroxidation), and thromboxane generation, considered the most potent autoaggregatory agent. Chloroquine only slightly altered the arachidonic acid cascade of platelets stimulated with A23187 and phorbol 12-myristate 13 acetate. Histamine formation and liberation induced with thrombin and A23187 were not affected by chloroquine. On the other hand, thrombin-stimulated serotonin secretion was significantly decreased with chloroquine in the concentration of 10 micromol/L. This indicated that chloroquine might interfere with stimulated secretion from platelets. The results suggest that chloroquine inhibited activated platelets: first, intracellularly; second, in a close relationship to the intraplatelet Ca(2+) mobile pool; and third, most probably at the site of platelet phospholipase A(2) activation.

Dual effects of nimesulide, a COX-2 inhibitor, in human platelets

Nimesulide (CAS 51803-78-2) has been shown to exert marked anti-inflammatory effect in several in vivo models of inflammation. Since nimesulide is considered to be a selective inhibitor of COX-2, it has not been studied in detail in relation to its mechanistic effects on platelets, which express COX-1. This study was conducted to investigate the effects of nimesulide in platelet aggregation. We show that nimesulide (1-100 microM) inhibited platelet aggregation induced by adrenaline (20-200 microM). It also inhibited thromboxane A2 (TXA2) formation by platelets at low concentration (IC50; 1 microM). However, much lower concentrations of nimesulide (0.01-0.1 microM) potentiated the aggregatory response of subthreshold concentrations of adrenaline (0.2-2 microM). Such an effect was blocked by Ca2+-channel blockers, verapamil and diltiazem (IC50: 7 and 46 microM, respectively), nitric oxide donor, SNAP (IC50; 2 microM) and cinchonine (10 nM) but not by genistein (up to 10 microM). These results are indicative of the concentration-dependent dual effects of nimesulide on human platelet aggregation. The synergistic effect of low doses of nimesulide and adrenaline seems to be mediated through inhibition of multiple signalling pathways.

Effect of alpha2-adrenoceptor stimulation on isolated canine Purkinje fiber contraction

We have recently identified the presence of postjunctional alpha2-adrenoceptors in canine Purkinje fibers. In this study, we examined the effects of alpha2-adrenoceptor stimulation on the contraction strength of isolated Purkinje fibers. Exposure to the alpha2-adrenoceptor specific agonist and antagonist, UK 14,304 (5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine) and yohimbine (17-hydroxyyohimban-16-carboxylic acid methyl ester hydrochloride) alone at 0.1 microM respectively, did not produce any significant effect on Purkinje contraction strength. Purkinje contraction strength was augmented by isoproterenol (0.1 microM), forskolin (0.1 microM), or 8-bromo-adenosine cyclic 2',3'-monophosphate (8-bromo-cAMP, 10 microM). UK 14,304 significantly reversed the effects of isoproterenol and forskolin but not those of 8-bromo-cAMP on Purkinje contraction strength. After incubation with pertussis toxin, the positive inotropic effect of forskolin on Purkinje contraction strength remained intact, but the forskolin effect could no longer be reversed by UK 14,304. These results suggest that the postjunctional alpha2-adrenoceptors in canine Purkinje fibers are coupled to a pertussis toxin-sensitive G protein, probably Gi. Stimulation of the alpha2-adrenoceptor antagonizes the effect of beta-adrenoceptor stimulation on Purkinje contraction strength in an accentuated antagonism manner.

Phorbol ester and okadaic acid regulation of Na-2Cl-K cotransport in rabbit tracheal epithelial cells

We evaluated a role for protein kinase C (PKC) in the regulation of rabbit tracheal epithelial Na-Cl(K) cotransport. Short-term treatment with phorbol 12-myristate 13-acetate (PMA) dose dependently increased bumetanide-sensitive Na and Cl efflux and elevated staurosporine- and bumetanide-sensitive Na, Cl, and K uptake. PMA and the alpha 2A-adrenergic agonist guanabenz both induced contransport with a stoichiometry of 2 Cl:1 Na and 2 Cl:1 Rb and elevated staurosporine-sensitive PKC activity in cytosolic and particulate fractions. Prolonged PMA treatment did not sustain bumetanide-sensitive 2 Cl:1 Na and 2 Cl:1 Rb transport but did block stimulation of bumetanide-sensitive transport by PMA or guanabenz and elevation of PKC activity by PMA and guanabenz in a particulate fraction. Cells treated with okadaic acid exhibited a staurosporine- and bumetanide-sensitive 2 Cl:1 Na and 2 Cl:1 Rb uptake. In cultured monolayers, basolateral perfusion with epinephrine, isoproterenol, or PMA increased short-circuit current (Isc). Basolateral application of bumetanide reduced elevated Isc to baseline levels, indicating a role for Cl secretory cells in a reconstituted tracheal epithelium. Pretreatment of transmonolayer cultures with PMA diminished the stimulatory response to epinephrine. These results indicate that, in rabbit tracheal epithelial cells, alpha-adrenergic stimulation activated Na-2Cl-K cotransport and that PKC is a critical effector in this process.

Role of anion exchange and thiol groups in the regulation of potassium efflux by lead in human erythrocytes

Pb2+ is thought to enter erythrocytes through anion exchange (AE) and to remain in the cell by binding to thiol groups. To define the role of AE mechanisms and thiol groups in Pb2+ toxicity, we studied the effects of drugs and conditions that modify AE and that modify thiol groups on the ability of Pb2+ to stimulate potassium efflux as measured with 86Rb. The most potent stimulation of 86Rb efflux by Pb2+ occurred when conditions were optimal for the AE mechanism--that is, when bicarbonate was included in the buffer or a buffer made with Nal or NaCl rather than NaClO4 or NaNO3 was used. Furthermore, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfuonic acid, potent inhibitors of the AE mechanism, completely inhibited stimulation of the 86Rb efflux by Pb2+. These conditions or inhibitors did not affect stimulation of the 86Rb efflux by ionomycin plus Ca2+. A role for Ca2+ channels was dismissed because the inorganic Ca2+ channel blockers, Cd2+ or Mn2+, did not prevent stimulation of 86Rb efflux by Pb2+ but did inhibit stimulation by ionomycin plus Ca2+. 86Rb efflux was more sensitive to Pb2+ if erythrocytes were treated for 15 min with thiol-modifying reagents that enter cells, such as iodoacetamide, N-ethylmaleimide, or dithiothreitol, than to reduced glutathione, a thiol-modifying reagent that is not permeable to the cell. Thus, in erythrocytes the AE mechanism and internal thiol groups are critical factors that affect the stimulation of a Ca(2+)-dependent process by Pb2+.

Platelet activation

This review article describes the different receptors, second-messengers and mechanisms involved in platelet activation. Several platelet agonists have well-defined receptors at the platelet membrane of which a number are single polypeptides with 7 hydrophobic transmembrane domains. These receptors are connected, via GTP regulatory proteins, with cytoplasmic second-messenger-generating enzymes. Phospholipase C and adenylate cyclase are the two best-known enzymes, generating inositol triphosphate (IP3) and diacyl glycerol from phosphatidylinositol biphosphate and cyclic AMP from ATP respectively. The intraplatelet free calcium level, which is critical for the activation status of the platelet, is increased by IP3 and is lowered in the presence of rising cyclic AMP concentrations. Shape-change occurs with small increases in intraplatelet calcium, while aggregation and secretion of granules take place at higher calcium, levels. The role of myosin and actin filaments and of transmembrane glycoproteins is further discussed.

The mechanism of action of alpha 2-adrenoceptors in human isolated subcutaneous resistance arteries

1. The effect of noradrenaline and the selective alpha 2-adrenoceptor agonist, azepexole, on tone and intracellular Ca2+ ([Ca2+]i) was examined in human isolated subcutaneous resistance arteries. Isolated arteries were mounted on an isometric myograph and loaded with the Ca2+ indicator, fura-2, for simultaneous measurement of force and [Ca2+]i. 2. High potassium solution (KPSS), noradrenaline and azepexole increased [Ca2+]i and contracted subcutaneous arteries in physiological saline. When extracellular Ca2+ was removed and the calcium chelator, BAPTA, added to the physiological saline (PSSo), responses to noradrenaline were transient and reduced, and responses to azepexole were markedly inhibited. 3. Ryanodine, an agent which interferes with Ca2+ release from intracellular stores, had little effect on contractile responses to KPSS, noradrenaline or azepexole in physiological saline. The response to caffeine in physiological saline was inhibited by ryanodine. In PSSo, ryanodine partially inhibited contractile responses to noradrenaline and azepexole, and completely abolished the response to caffeine. 4. Noradrenaline and azepexole both significantly increased maximum force achieved by cumulative addition of Ca2+ to a Ca(2+)-free depolarizing solution and shifted the calculated relationship between [Ca2+]i and force to the left, suggesting these agents increase the sensitivity of the contractile apparatus to [Ca2+]i. 5. (-)-202 791, a dihydropyridine antagonist of voltage-operated calcium channels partially inhibited both the contractile response and the rise in [Ca2+]i induced by azepexole. Pre-treatment of arteries with pertussis toxin inhibited responses to azepexole, but had no significant effect on tone induced by KPSS or noradrenaline. ETYA, an inhibitor of phospholipase A2, lipoxygenase and cyclo-oxygenase, had no effect on azepexole-induced contraction in the presence of N omega nitro-L-arginine methyl ester.6. Azepexole, a selective alpha2-adrenoceptor agonist, contracts human subcutaneous resistance arteries by a mechanism largely dependent on the influx of extracellular Ca2", probably through voltage-operated calcium channels. This action involves a pertussis toxin-sensitive G protein, possibly Gi.