Transient hydrodynamical behavior by dynamical nonequilibrium molecular dynamics: the formation of convective cells.

We present a method based on dynamical nonequilibrium molecular dynamics (D-NEMD) that allows one to produce rigorous ensemble averages for the transient regimes. We illustrate the method by describing the formation of convective cells within a two-dimensional fluid system of soft disks in which a gravity field and a thermal gradient are present. We analyze two different physical settings, with the thermal gradient orthogonal or parallel to the gravity field. In both settings, we follow the formation of the convective flows from the initial time, when the perturbation is turned on, to the steady state. In the first setting (orthogonal fields) we investigate several different cases, varying the initial stationary ensemble and the perturbing field. We find that the final steady-state convective cell is independent of the specific sequence of perturbation fields, which only affects the transient behavior. In all cases, we find that the convective roll is formed through a sequence of damped oscillations of the local fields (density, temperature, and velocity), superimposed to an overall relaxation toward the local steady-state values. Then, we show how D-NEMD can be applied to the Rayleigh-Bénard (RB) setting (parallel fields). In these conditions, the convective flow only establishes above a threshold, without a preferred verse of rotation. We analyze only the response to the ignition of the gravity field in a stationary system under the action of a vertical thermal gradient. Also in this case we characterize the transient response by following the evolution of the density, temperature, and velocity fields until the steady-state RB convective cell is formed. The observed transients are similar to those observed in the case of orthogonal fields. However, the final steady states are quite different. Finally, we briefly discuss the conditions for the general applicability of the D-NEMD method.

Genome flux in tomato auto- and auxo-trophic cell clones cultured in different auxin/cytokinin equilibria. I. DNA multiplicity and methylation levels.

An analysis of the effect of changing physiological conditions on genetic stability, in terms of epigenetic changes, such as DNA, methylation patterns, and multiplicity of repetitive DNA, was carried out on tomato cell clones grown on media supplemented with different auxin/cytokinin ratios. The effect of endogenous variation in phytohormone equilibria was also indirectly analysed through a comparison of auxotrophic or habituated (autotrophic) cell clones and the differentiated leaf tissue. The data obtained showed significant variation in methylation and multiplicity levels both between clones and between treatments, clearly suggesting a contemporary influence of exogenous hormonal treatments and of the initial/endogenous physiological state of the treated tissue on both phenomena studied.

Control of nifH transcription in Azospirillum brasilense: involvement of NifA and of cis-acting sequences.

The regulatory sequences of Azospirillum brasilense Sp7 nifH gene were fused with the cam reporter gene and used for studying the factors controlling nifH transcription. A DNA sequence, downstream the ATG codon of nifH, that could be involved in the negative regulation of nifH transcription, was identified. The effect of 1 and 2 mM of ammonium on the transcription of the A. brasilense nifH gene and on the nitrogenase activity, in the presence of the Klebsiella pneumoniae NifA protein, was examined.

Histamine-induced bronchial response after administration of placebo, salbutamol and a combination of a beta-2-adrenergic drug (fenoterol) with an anticholinergic agent (ipratropium bromide) in asymptomatic asthma patients.

We examined 12 non-smoking patients affected by extrinsic bronchial asthma in steady state. Histamine PD20 FEV1 was assessed 4 h after the administration of salbutamol 200 micrograms, a combination of fenoterol 200 micrograms and ipratropium bromide 80 micrograms, and placebo. Despite the absence of any relevant bronchodilator effect, both salbutamol and the combination fenoterol + ipratropium bromide showed a strong protective effect against the bronchial response to histamine compared to placebo, the combination being more effective than salbutamol (p less than 0.05). Our data suggest that, when bronchial challenges with histamine are performed, airway response may be a consequence of both a direct action of such an agent on H1 receptors and of an associated cholinergic reflex.

Bronchoprovocation tests in bronchial asthma and other respiratory disorders: a short review and some personal contributions.

The purpose of this paper, after a brief review of the main features of bronchoprovocation challenges, was to determine whether the administration of an ultrasonic mist of distilled water can influence the bronchial tone of normal subjects and patients affected by various respiratory disorders, in order to assess the effectiveness of this bronchoprovocation test in discriminating between patients. Of the 129 subjects tested 13 were classified as normal, 60 as extrinsic asthmatics, 21 as intrinsic asthmatics, 25 as allergic rhinitics and 37 as chronic bronchitics. Ultrasonic mist, was administered for 5 min and the bronchial response was evaluated in terms of difference between that after the mist minus baseline values of specific airways conductance. The data suggest that the bronchial challenge with ultrasonic mist, a safe, non-pharmacological and simple test, allows a good discrimination between asthma patients and those affected by other respiratory disorders.