Simulation of atrial activity by a phase response curve based model of a two-dimensional pacemaker cells array: the transition from a normal activation pattern to atrial fibrillation

In this paper, we present an original model of the atria, based on our hypothesis that atrial cells have features of pacemaker cells, characterized by their normally longer intrinsic cycle lengths and different type of connection (stronger) than the, sino-atrial (SA) node pacemaker cells. The atrium is simulated by a two-dimensional array of pacemaker cells (25 × 25), composed of a region of SA node pacemaker cells (11 × 11) surrounded by atrial pacemaker cells. All pacemakers cells are characterized by only the most relevant functional properties, those which play the most direct role in the determination of the cardiac rate and in the mechanism of arrhythmias. These properties are: the intrinsic cycle length, τ, an `internal' feature of each pacemaker cell, and the phase-response curve (PRC), an `overall collective' function. The PRC embodies the interactions of each pacemaker cell with its neighboring cells, and thus represents the type of connection (strong, weak, etc.) of the pacemaker cell with its surroundings. In our model, the SA node region differs from the atrial region by cycle length distribution and PRCs. We studied the spatial interaction between SA node pacemaker cells and atrial pacemaker cells as a function of the regional variation of cells properties and as a function of the “electrical” coupling between cells (the PRC), in the SA node region, in the atrial region, and in a border zone between them. We investigated the influence of those parameters on the activation pattern, on the conduction time of the array, and on a pseudo-ECG signal. This study demonstrates that by representing the atrial cells as a population of `pacemaker-like' cells, similar to the SA node pacemaker cells, but differing markedly in their cycle lengths and cell-to-cell interaction (PRC), we can create a global picture of the atrial system by applying a simple physical-mathematical model. This approach enables us to explore physiological phenomena related to the genesis and maintenance of atrial activity. It also reveals the conditions which predispose to atrial arrhythmias and conduction disturbances (e.g. tachycardia, pacemaker shift, re-entry, fibrillation). In particular, it yields insight into the mechanism of transition from normal atrial activity to the disordered state of atrial fibrillation. Therefore, this study suggests a new way of looking at the development of cardiac arrhythmias of atrial origin. Received: 8 September 1997 / Accepted in revised form: 6 October 1998     

A natural surfactant from pig lungs

An exogenous natural lung surfactant obtained from minced pig lungs can be produced by a technology using a low cost, DEAE-cellulose adsorbent. This surfactant is composed mainly with phospholipids and the two hydrophobic polypeptides, SP-B and SP-C, both of which are necessary for optimal function of surfactants used for treatment of respiratory distress syndrome.     

Stomatal characteristics in different habitat forms of Brazilian species of Epidendrum (Orchidaceae)

Brazilian species of the genus Epidendrum , distributed over various habitats, were analysed for epidermal structure to test correlations between habitat form and variation in stomatal parameters. This study confirmed the tendency for stomatal index values to increase in the apical direction of the foliar surface in this genus. The generally held view that there are a smaller number of stomata on the adaxial surface in amphistomatic leaves was contradicted for E. vesicatum , whose pendent sympodia showed that the adaxial foliar surfaces assume the abaxial position in relation to light incidence.     

X-ray structure of plasmepsin II complexed with a potent achiral inhibitor

The malaria parasite Plasmodium falciparum degrades host cell hemoglobin inside an acidic food vacuole during the blood stage of the infectious cycle. A number of aspartic proteinases called plasmepsins (PMs) have been identified to play important roles in this degradation process and therefore generated significant interest as new antimalarial targets. Several x-ray structures of PMII have been described previously, but thus far, structure-guided drug design has been hampered by the fact that only inhibitors comprising a statine moiety or derivatives thereof have been published. Our drug discovery efforts to find innovative, cheap, and easily synthesized inhibitors against aspartic proteinases yielded some highly potent non-peptidic achiral inhibitors. A highly resolved (1.6 A) x-ray structure of PMII is presented, featuring a potent achiral inhibitor in an unprecedented orientation, contacting the catalytic aspartates indirectly via the "catalytic" water. Major side chain rearrangements in the active site occur, which open up a new pocket and allow a new binding mode of the inhibitor. Moreover, a second inhibitor molecule could be located unambiguously in the active site of PMII. These newly obtained structural insights will further guide our attempts to improve compound properties eventually leading to the identification of molecules suitable as antimalarial drugs.     

Structural evidence of a passive base-flipping mechanism for AGT, an unusual GT-B glycosyltransferase

The Escherichia coli T4 bacteriophage uses two glycosyltransferases to glucosylate and thus protect its DNA: the retaining alpha-glucosyltransferase (AGT) and the inverting beta-glucosyltransferase (BGT). They glucosylate 5-hydroxymethyl cytosine (5-HMC) bases of duplex DNA using UDP-glucose as the sugar donor to form an alpha-glucosidic linkage and a beta-glucosidic linkage, respectively. Five structures of AGT have been determined: a binary complex with the UDP product and four ternary complexes with UDP or UDP-glucose and oligonucleotides containing an A:G, HMU:G (hydroxymethyl uracyl) or AP:G (apurinic/apyrimidinic) mismatch at the target base-pair. AGT adopts the GT-B fold, one of the two folds known for GTs. However, while the sugar donor binding mode is classical for a GT-B enzyme, the sugar acceptor binding mode is unexpected and breaks the established consensus: AGT is the first GT-B enzyme that predominantly binds both the sugar donor and acceptor to the C-terminal domain. Its active site pocket is highly similar to four retaining GT-B glycosyltransferases (trehalose-6-phosphate synthase, glycogen synthase, glycogen and maltodextrin phosphorylases) strongly suggesting a common evolutionary origin and catalytic mechanism for these enzymes. Structure-guided mutagenesis and kinetic analysis do not permit identification of a nucleophile residue responsible for a glycosyl-enzyme intermediate for the classical double displacement mechanism. Interestingly, the DNA structures reveal partially flipped-out bases. They provide evidence for a passive role of AGT in the base-flipping mechanism and for its specific recognition of the acceptor base.     

A base-flipping mechanism for the T4 phage beta-glucosyltransferase and identification of a transition-state analog

T4 phage beta-glucosyltransferase (BGT) modifies T4 DNA. We crystallized BGT with UDP-glucose and a 13mer DNA fragment containing an abasic site. We obtained two crystal structures of a ternary complex BGT-UDP-DNA at 1.8A and 2.5A resolution, one with a Tris molecule and the other with a metal ion at the active site. Both structures reveal a large distortion in the bound DNA. BGT flips the deoxyribose moiety at the abasic site to an extra-helical position and induces a 40 degrees bend in the DNA with a marked widening of the major groove. The Tris molecule mimics the glucose moiety in its transition state. The base-flipping mechanism, which has so far been observed only for glycosylases, methyltransferases and endonucleases, is now reported for a glucosyltransferase. BGT is unique in binding and inserting a loop into the DNA duplex through the major groove only. Furthermore, BGT compresses the backbone DNA one base further than the target base on the 3'-side.     

Regulatory volume decrease in human esophageal epithelial cells

In vivo human esophageal epithelial cells are regularly exposed to hyposmolal stress. This stress, however, only becomes destructive when the surface epithelial cell (barrier) layers are breached and there is contact of the hyposmolal solution with the basolateral cell membranes. The present investigation was designed to examine the effects of hyposmolal stress in the latter circumstance using as a model for human esophageal epithelial cells the noncancer-derived HET-1A cell line. Cell volume and the response to hyposmolal stress in suspensions of HET-1A cells were determined by cell passage through a Coulter Counter Multisizer II. HET-1A cells behaved as osmometers over the range of 280 to 118 mosmol/kg H(2)O with rapid increases in cell volume < or = 15-20% above baseline. Following swelling, the cells exhibited regulatory volume decrease (RVD), restoring baseline volume within 30 min, despite continued hyposmolal stress. With the use of pharmacologic agents and ion substitutions, RVD appeared to result from rapid activation of parallel K(+) and Cl(-) conductance pathways and this was subsequently joined by activation of a KCl cotransporter. Exposure to hyposmolal stress in an acidic environment, pH 6.6, inhibited, but did not abolish, RVD. These data indicate that human esophageal epithelial cells can protect against hyposmolal stress by RVD and that the redundancy in mechanisms may, to some extent, serve as added protection in patients with reflux disease when hyposmolal stress may occur in an acidic environment.     

A 2S albumin-homologous protein from passion fruit seeds inhibits the fungal growth and acidification of the medium by Fusarium oxysporum

Antimicrobial proteins have been isolated from a wide range of plant species. More recently, it has become increasingly clear that these types of proteins play an important role in the protection of plants. In this study, we investigate the presence of defense-related proteins from passion fruit (Passiflora edulis f. flavicarpa) seeds. Initially, seed flour was extracted for 2h (at 4 degrees C) with phosphate buffer, pH 5.5. The precipitate obtained between 0 and 70% relative ammonium sulfate saturation was re-dissolved in distilled water and heated at 80 degrees C for 15 min. The resulting suspension was clarified by centrifugation and the supernatant (F/0-70) was extensively dialyzed. A Sephadex G-50 size exclusion column was employed for further separation of proteins. The fraction with antifungal activity was pooled and submitted to CM-Sepharose cation exchange. Two proteins, named Pf1 and Pf2, were eluted in 0.1 and 0.2M of salt, respectively, and submitted to reverse-phase chromatography in HPLC. This fraction inhibited the growth, in an in vitro assay, of the phytopathogenic fungi Fusarium oxysporum and colletotrichum lindemuthianum and the yeast Saccharomyces cerevisiae and strongly inhibited glucose-stimulated acidification of the medium by F. oxysporum in a dose-dependent manner. The molecular masses of these proteins, referred to now as Pf1-RP and Pf2-RP, were obtained by MALDI-TOF spectrometry and corresponded to 12,088 Da for Pf1-RP and 11,930 Da for Pf2-RP. These proteins were also subjected to automated N-terminal amino acid sequencing. Sequence comparisons for the heavy subunit of Pf2-RP showed the presence of a protein with a high degree of homology to storage 2S albumins.