Demonstration of an “Excitation-Contraction Recoupling” Mechanism in Mammalian Ventricular Myocardium

A PROCESS called “excitation-contraction coupling” has been generally accepted to take place only in the direction of excitation to contraction. Through this mechanism a propagated action potential initiates an active state in skeletal or cardiac muscle and the muscle contracts. We propose that, in the mammalian ventricular myocardium at least, the process is not unidirectional and an important reverse coupling between the contractile system and the excitable plasma membrane has been overlooked. Through this feedback interaction the mode of contraction (that is, isotonic or isometric) not only determines the instantaneous electrical state of the plasma membrane, but also influences the mechanical events of the subsequent beats. Thus when Kaufmann et al.¹ recorded intracellular action potentials from cat papillary muscle, the time course of the repolarization was altered depending on the mode of contraction. Some kind of contraction-excitation feedback has also been suggested by Stauch² and Lab^3,4. They showed a difference in the shape of the monophasic action potential, as recorded by a suction electrode, when comparing isotonic and isovolumic contraction of the intact ventricle. But their experimental conditions did not allow satisfactory analysis of the phenomenon.     

Function of the <Emphasis Type="Italic">rel</Emphasis> Gene in <Emphasis Type="Italic">Escherichia coli</Emphasis>

Sokawa et al. suggest that rel^- strains of Escherichia coli possess abnormal protein synthesizing machinery, which cannot carry out normal protein synthesis when the supply of amino-acids is limited.     

“Pleiotypic Response”

A hypothesis has been developed to relate stringent control in bacteria to a set of interactions involved in the regulation of growth of transformed and untransformed mammalian cells.     

Inhibition of DNA Synthesis but not of Poly-dAT Synthesis by the Arabinose Analogue of Cytidine <Emphasis Type="Italic">in vitro</Emphasis>

Kimball and Wilson¹ reported that the arabinose analogue of cytidine (ara-C) inhibited DNA polymerase in a crude extract prepared from Ehrlich ascites cells. Furth and Cohen² observed cytosine arabinoside triphosphate (ara-CTP) inhibited DNA polymerase in extracts from either calf thymus or bovine lymphosarcoma tissue, although these investigators³ had already found no effect of ara-CTP on DNA polymerase from Escherichia coli. The inhibition in both of these cases could be substantially reversed by dCTP; but incorporation of the arabinose nucleotide (ara-CMP) into DNA could not be unequivocally demonstrated. Graham and Whitmore⁴ reported the incorporation of ara-C into DNA in vivo and the inhibition of a DNA polymerase from L cells by ara-CTP. They found that ara-CMP was initially incorporated into small DNA strands but subsequently appeared in long strands. Momparler⁵ has presented evidence that, in vitro, ara-C incorporation was limited to the 3′-hydroxyl end of DNA chains. Such incorporation might be expected to block further chain elongation but this expectation was not supported by the evidence presented by Graham and Whitmore.     

Endopeptidase Activity of Phage λ-Endolysin

JACOB and Fuerst^1,2 demonstrated the presence of a bacteriolytic enzyme (λ-endolysin) in the induced cultures of lysogenic Escherichia coli K12 (λ). The enzyme was later identified as the product of gene R; of phage λ³ which is involved in bacterial lysis at the end of a latent period. The enzyme is apt to form spheroplast-like structures in E. coli² and one would therefore expect its substrate to be murein.     

The role of a conserved dodecamer sequence in yeast mitochondrial gene expression

All mRNAs on the yeast mitochondrial genome terminate at a conserved dodecamer sequence 5'-AAUAAUAUUCUU-3'. We have characterized two mutants with altered dodecamers. One contains a deletion of the dodecamer at the end of the var1 gene, and the other contains two adjacent transversions in the dodecamer at the end of the reading frame of fit1, a gene within the omega+ allele of the 21S rRNA gene. In each mutant, expression of the respective gene is blocked completely. A dominant nuclear suppressor, SUV3-1, was isolated that suppresses the var1 deletion but is without effect on the fit1 dodecamer mutations. Unexpectedly, however, we found that SUV3-1 blocks expression of the wild-type fit1 allele by blocking processing at its dodecamer. SUV3-1 has pleiotropic effects on mitochondrial gene expression, affecting RNA processing, RNA stability, and translation. Our results suggest that RNA metabolism and translation may be part of a multicomponent complex within mitochondria.     

Location of a synergistic factor in the capsule of a granulosis virus of the armyworm, Pseudaletia unipuncta

A synergistic factor (SyF), which enhanced the infection of nuclear polyhedrosis viruses, was purified from capsules of a Pseudaletia unipuncta granulosis virus (Hawaiian strain) by immune affinity chromatography. The isolated SyF consisted primarily of a protein with molecular mass 98 kDa. The recovery rate depended on the alkali used to dissolve the capsules: the highest rate occurred with 0.05 M Na2CO3-0.05 M NaCl, followed in turn with 0.02-0.05 M NaOH and 0.04 M NaOH-0.05 M glycine. The solubilized components from untreated capsules contained 98- and 100-kDa proteins in addition to the matrix protein (29 kDa) and its decomposed products, while those from heat-treated capsules contained only the 100-kDa protein. Virons liberated from the capsules with the glycine buffer contained three proteins (33, 98, and 100 kDa) serologically related to the SyF. Immunoelectron microscopy of infected tissue and purified virions revealed the localization of the SyF antigens on the viral envelope.     

Bactericidal activity of peroxynitrite.

Peroxynitrite is a strong oxidant formed by macrophages and potentially by other cells that produce nitric oxide and superoxide. Peroxynitrite was highly bactericidal, killing Escherichia coli in direct proportion to its concentration with an LD50 of 250 microM at 37 degrees C in potassium phosphate, pH 7.4. The apparent bactericidal activity of a given concentration peroxynitrite at acidic pH was less than that at neutral and alkaline pH. However, after taking the rapid pH-dependent decomposition of peroxynitrite into account, the rate of the killing was not significantly different at pH 5 compared to pH 7.4. Metal chelators did not decrease peroxynitrite-mediated killing, indicating that exogenous transition metals were not required for toxicity. The hydroxyl radical scavengers mannitol, ethanol, and benzoate did not significantly affect toxicity while dimethyl sulfoxide enhanced peroxynitrite-mediated killing. Dimethyl sulfoxide is a more efficient hydroxyl radical scavenger than the other three scavengers and increased the formation of nitrogen dioxide from peroxynitrite. In the presence of 100 mM dimethyl sulfoxide, 60.0 +/- 0.3 microM nitrogen dioxide was formed from 250 microM peroxynitrite as compared to 2.0 +/- 0.1 microM in buffer alone. Thus, formation of nitrogen dioxide may have enhanced the toxicity of peroxynitrite decomposing in the presence of dimethyl sulfoxide.