Electrophysiological-anatomic correlates of ATP-triggered vagal reflex in the dog. V. Role of purinergic receptors

The mechanism of extracellular ATP-triggered vagal depressor reflex was further studied in a closed-chest canine model. Adenosine and ATP were administered individually in equimolar doses (0.01-1.0 mumol/kg) into the right coronary artery (RCA) and left circumflex coronary artery (LCA). When administered into the RCA, adenosine and ATP exerted an identical and relatively small negative chronotropic effect on sinus node automaticity; the time to peak negative chronotropic effect was >/=7 s. When administered into the LCA, adenosine had no effect on sinus node automaticity, whereas ATP markedly suppressed sinus node automaticity. This effect of ATP 1) reached its peak in <2 s after its administration, 2) was short lasting, and 3) was completely abolished by either intravenous administration of the muscarinic cholinergic blocker atropine (0.2 mg/kg) or intra-LCA administration of 2',3'-O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP), a potent P2X(2/3) purinergic receptor (P2X(2/3)R) antagonist, but not by diinosine pentaphosphate (Ip(5)I), a potent inhibitor of P2X(1)R and P2X(3)R. Repetitive administrations of ATP were not associated with reduced effects, indicative of receptor desensitization, thereby excluding the involvement of the rapidly desensitized P2X(1)R in the action of ATP. It was concluded that ATP triggers a cardio-cardiac vagal depressor reflex by activating P2X(2/3)R located on vagal sensory nerve terminals localized in the left ventricle. Because these terminals mediate vasovagal syncope, these data could suggest a mechanistic role of extracellular ATP in this syndrome and, in addition, give further support to the hypothesis that endogenous ATP released from ischemic myocytes is a mediator of atropine-sensitive bradyarrhythmias associated with left ventricular myocardial infarction.     

The structure of the excisionase (Xis) protein from conjugative transposon Tn916 provides insights into the regulation of heterobivalent tyrosine recombinases

Heterobivalent tyrosine recombinases play a prominent role in numerous bacteriophage and transposon recombination systems. Their enzymatic activities are frequently regulated at a structural level by excisionase factors, which alter the ability of the recombinase to assemble into higher-order recombinogenic nucleoprotein structures. The Tn916 conjugative transposon spreads antibiotic resistance in pathogenic bacteria and is mobilized by a heterobivalent recombinase (Tn916Int), whose activity is regulated by an excisionase factor (Tn916Xis). Unlike the well-characterized (lambda)Xis excisionase from bacteriophage lambda, Tn916Xis stimulates excision in vitro and in Escherichia coli only modestly. To gain insights into this functional difference, we have performed in vitro DNA-binding studies of Tn916Xis and Tn916Int, and we have solved the solution structure of Tn916Xis. We show that the heterobivalent Tn916Int protein is capable of bridging the DR2-type and core-type sites on the left arm of the tranpsoson. Consistent with the notion that Tn916Int is regulated only loosely, we find that Tn916Xis binding does not alter the stability of DR2-Tn916Int-core bridges or the ability of Tn916Int to recognize the arms of the transposon in vitro. Despite a high degree of divergence at the primary sequence level, we show that Tn916Xis and (lambda)Xis adopt related prokaryotic winged-helix structures. However, they differ at their C termini, with Tn916Xis replacing the flexible integrase contacting tail found in (lambda)Xis with a positively charged alpha-helix. This difference provides a structural explanation for why Tn916Xis does not interact cooperatively with its cognate integrase in vitro, and reveals how subtle changes in the winged-helix fold can modulate the functional properties of excisionase factors.     

Search for critical loading condition of the spine--a meta analysis of a nonlinear viscoelastic finite element model

The relative vulnerability of spinal motion segments to different loading combinations remains unknown. The meta-analysis described here using the results of a validated L2-L3 nonlinear viscoelastic finite element model was designed to investigate the critical loading and its effect on the internal mechanics of the human lumbar spine. A Box-Behnken experimental design was used to design the magnitude of seven independent variables associated with loads, rotations and velocity of motion. Subsequently, an optimization method was used to find the primary and secondary variables that influence spine mechanical output related to facet forces, disc pressure, ligament forces, annulus matrix compressive/shear stresses and anulus fibers strain. The mechanical responses with respect to the two most-relevant variables were then regressed linearly using the response surface quadratic model. Axial force and sagittal rotation were identified as the most-relevant variables for mechanical responses. The procedure developed can be used to find the critical loading for finite element models with multi input variables. The derived meta-models can be used to predict the risk associated with various loading parameters and in setting safer load limits.     

Neuromuscular control of the point to point and oscillatory movements of a sagittal arm with the actor-critic reinforcement learning method

In this study, we have used a single link system with a pair of muscles that are excited with alpha and gamma signals to achieve both point to point and oscillatory movements with variable amplitude and frequency.The system is highly nonlinear in all its physical and physiological attributes. The major physiological characteristics of this system are simultaneous activation of a pair of nonlinear muscle-like-actuators for control purposes, existence of nonlinear spindle-like sensors and Golgi tendon organ-like sensor, actions of gravity and external loading. Transmission delays are included in the afferent and efferent neural paths to account for a more accurate representation of the reflex loops.A reinforcement learning method with an actor-critic (AC) architecture instead of middle and low level of central nervous system (CNS), is used to track a desired trajectory. The actor in this structure is a two layer feedforward neural network and the critic is a model of the cerebellum. The critic is trained by state-action-reward-state-action (SARSA) method. The critic will train the actor by supervisory learning based on the prior experiences. Simulation studies of oscillatory movements based on the proposed algorithm demonstrate excellent tracking capability and after 280 epochs the RMS error for position and velocity profiles were 0.02, 0.04 rad and rad/s, respectively.     

Antithrombin III phenylalanines 122 and 121 contribute to its high affinity for heparin and its conformational activation

The dissociation equilibrium constant for heparin binding to antithrombin III (ATIII) is a measure of the cofactor's binding to and activation of the proteinase inhibitor, and its salt dependence indicates that ionic and non-ionic interactions contribute approximately 40 and approximately 60% of the binding free energy, respectively. We now report that phenylalanines 121 and 122 (Phe-121 and Phe-122) together contribute 43% of the total binding free energy and 77% of the energy of non-ionic binding interactions. The large contribution of these hydrophobic residues to the binding energy is mediated not by direct interactions with heparin, but indirectly, through contacts between their phenyl rings and the non-polar stems of positively charged heparin binding residues, whose terminal amino and guanidinium groups are thereby organized to form extensive and specific ionic and non-ionic contacts with the pentasaccharide. Investigation of the kinetics of heparin binding demonstrated that Phe-122 is critical for promoting a normal rate of conformational change and stabilizing AT*H, the high affinity-activated binary complex. Kinetic and structural considerations suggest that Phe-122 and Lys-114 act cooperatively through non-ionic interactions to promote P-helix formation and ATIII binding to the pentasaccharide. In summary, although hydrophobic residues Phe-122 and Phe-121 make minimal contact with the pentasaccharide, they play a critical role in heparin binding and activation of antithrombin by coordinating the P-helix-mediated conformational change and organizing an extensive network of ionic and non-ionic interactions between positively charged heparin binding site residues and the cofactor.     

Oxidized lipids as mediators of coronary heart disease

PURPOSE OF REVIEW: To summarize the recent evidence on the physiological relevance of the view that LDL lipid oxidation may play a major role in the inflammatory reaction that leads to or amplifies atherogenesis. Oxidation of LDL phospholipids containing arachidonic acid at the sn-2 position occurs when a critical concentration of 'seeding molecules' derived from the lipoxygenase pathway is reached in LDL. This generates a series of biologically active, oxidized phospholipids that mediate the cellular events seen in the developing fatty streak. RECENT FINDINGS: We have observed that LDL from mice that are genetically predisposed to diet-induced atherosclerosis is highly proinflammatory when the mice are maintained on an atherogenic diet, when they are injected with LDL-derived oxidized phospholipids, or once they are infected with influenza A virus. Patients with coronary atherosclerosis also had highly proinflammatory LDL, despite having normal blood lipid levels or normal plasma HDL levels. SUMMARY: We and others have hypothesized that HDL and LDL-derived oxidized phospholipids may be part of a system of nonspecific innate immunity. We therefore propose that determination of HDL capacity against LDL oxidation and the detection of proinflammatory HDL may be a useful marker of susceptibility to atherosclerosis.     

Organo-vanadium compounds are potent activators of the protein kinase B signaling pathway and protein tyrosine phosphorylation: mechanism of insulinomimesis

Organo-vanadium compounds (OVC) have been shown to be more effective than inorganic vanadium compounds in ameliorating glucose homeostasis and insulin resistance in rodent models of diabetes mellitus. However, the precise molecular mechanism of OVC efficiency remains poorly defined. Since inorganic vanadium compounds have been found to activate several key components of the insulin signaling cascade, such as protein kinase B (PKB), the objective of the present study was to investigate if stimulation of PKB and its downstream target glycogen synthase kinase-3 (GSK-3), are responsible for the more potent insulinomimetic effects of OVC. Among several vanadium compounds tested, vanadium (IV) oxo bis (acetylacetonate) and vanadium (IV) oxo bis(maltolato) markedly induced the phosphorylation of PKB as well as GSK-3beta compared to vanadyl sulfate (VS), an inorganic vanadium salts in Chinese hamster ovary cells overexpressing the insulin receptor (IR). Furthermore, the OVC were stronger inhibitors of protein tyrosine phosphatase (PTPase) activity than VS. The higher PTPase inhibitory potential of the OVC was associated with more robust tyrosine phosphorylation of several cellular proteins, including the IRbeta subunit and insulin receptor substrate-1 (IRS-1). In addition, greater IRS-1/p85alpha interaction was elicited by the OVC than by VS. These data indicate that the higher PTPase inhibitory potential of OVC translates into greater phosphorylation of PKB and GSK-3beta, which, in turn, may contribute to a more potent effect of OVC on glucose homeostasis.     

The oxidation hypothesis of atherogenesis: the role of oxidized phospholipids and HDL

For more than two decades, there has been continuing evidence of lipid oxidation playing a central role in atherogenesis. The oxidation hypothesis of atherogenesis has evolved to focus on specific proinflammatory oxidized phospholipids that result from the oxidation of LDL phospholipids containing arachidonic acid and that are recognized by the innate immune system in animals and humans. These oxidized phospholipids are largely generated by potent oxidants produced by the lipoxygenase and myeloperoxidase pathways. The failure of antioxidant vitamins to influence clinical outcomes may have many explanations, including the inability of vitamin E to prevent the formation of these oxidized phospholipids and other lipid oxidation products of the myeloperoxidase pathway. Preliminary data suggest that the oxidation hypothesis of atherogenesis and the reverse cholesterol transport hypothesis of atherogenesis may have a common biological basis. The levels of specific oxidized lipids in plasma and lipoproteins, the levels of antibodies to these lipids, and the inflammatory/anti-inflammatory properties of HDL may be useful markers of susceptibility to atherogenesis. Apolipoprotein A-I (apoA-I) and apoA-I mimetic peptides may both promote a reduction in oxidized lipids and enhance reverse cholesterol transport and therefore may have therapeutic potential.     

Welcome To <Emphasis Type="Italic">International Seminars In Surgical Oncology</Emphasis>

This editorial marks the launch of a new online journal for surgical and medical oncology. The internet has produced a profound shift in the way in which clinicians, researchers and patients seek, interpret and utilise medical information and research. The launch of our journal comes at a time when these changes are in a rapid phase of development and consolidation.     

Effects of irradiance and spectral quality on leaf structure and function in seedlings of two Southeast Asian Hopea (Dipterocarpaceae) species

We studied the development of leaf characters in two Southeast Asian dipterocarp forest trees under different photosynthetic photon flux densities (PFD) and spectral qualities (red to far-red, R:FR). The two species, Hopea helferi and H. odorata, are taxonomically closely related but differ in their ecological requirements; H. helferi is more drought tolerant and H. odorata more shade tolerant. Seedlings were grown in replicated shadehouse treatments of differing PFD and R:FR. We measured or calculated (1) leaf and tissue thicknesses; (2) mesophyll parenchyma, air space, and lignified tissue volumes; (3) mesophyll air volumes (V(mes)/A(surf)) and surfaces (A(mes)/A(surf)); (4) palisade cell length and width; (5) chlorophyll/cm and a/b; (6) leaf absorption; and (7) attenuance/absorbance at 652 and 550 nm. These characters varied in response to light conditions in both taxa. Characters were predominantly affected by PFD, and R:FR slightly influenced many characters. Leaf characters of H. odorata were more plastic in response to treatment conditions. Characters were correlated with each other in a complex fashion. Variation in leaf anatomy is most likely a consequence of increasing leaf thickness in both taxa, which may increase mechanical strength and defense against herbivory in more exposed environments. Variation in leaf optical properties was most likely affected by pigment photo-bleaching in treatments of more intense PFD and was not correlated with A(max). The greater plasticity of leaf responses in H. odorata helps explain the acclimation over the range of light conditions encountered by this shade-tolerant taxon. The dense layer of scales on the leaf undersurface and other anatomical characters in H. helferi reduced gas exchange and growth in this drought-tolerant tree.