Polarization Mode Dispersion of Installed Fibers

Polarization mode dispersion (PMD), a potentially limiting impairment in high-speed long-distance fiber-optic communication systems, refers to the distortion of propagating optical pulses due to random birefringences in an optical system. Because these perturbations (which can be introduced through manufacturing imperfections, cabling stresses, installation procedures, and environmental sensitivities of fiber and other in-line components) are unknowable and continually changing, PMD is unique among optical impairments. This makes PMD both a fascinating research subject and potentially one of the most challenging technical obstacles for future optoelectronic transmission. Mitigation and compensation techniques, proper emulation, and accurate prediction of PMD-induced outage probabilities critically depend on the understanding and modeling of the statistics of PMD in installed links. Using extensive data on buried fibers used in long-haul high-speed links, the authors discuss the proposition that most of the temporal PMD changes that are observed in installed routes arise primarily from a relatively small number of "hot spots" along the route that are exposed to the ambient environment, whereas the buried shielded sections remain largely stable for month-long time periods. It follows that the temporal variations of the differential group delay for any given channel constitute a distinct statistical distribution with its own channel-specific mean value. The impact of these observations on outage statistics is analyzed, and the implications for future optoelectronic fiber-based transmission are discussed     

Platelet density and size: the interpretation of heterogeneity

Platelets have been separated according to buoyant density using a colloidal silica-polyvinylpyrrolidone system and subjected to electronic sizing. All density populations were found to be heterogeneous in size, the most dense platelets ranging from less than 3 fl to greater than 21 fl in both man and rat. Light platelet fractions contained no platelets greater than 13 fl in either species. Cohort labeling with [75Se]selenomethionine showed no indication of significant change in platelet buoyant density with ageing; greater specific activity found in young, dense platelets appears to be related to increased protein synthetic activity shown in vitro and likely to occur also in their precursor megakaryocytes. It is postulated that dense, intermediate and light platelets are released synchronously by the three different ploidy classes of megakaryocyte, that varying density indicates differing structural characteristics and presumably differences in function. The present findings do not deny the possibility that platelets decrease in size with ageing but if such occurs, it is not associated with a significant change in platelet buoyant density.     

On the question of an electrokinetic requirement for phospholipase C action

The phospholipase C of clostridium welchii (alpha toxin) has an absolute requirement for trace quantities of Ca2+. It attacks pure phosphatidylcholine particles (smectic mesophases) having a close-packed bilayer structure only when their surface zeta potential is made positive by the addition of certain divalent ions (e.g., Ca2+) to the aqueous phase or by the presence of low concentrations of long chain cations to the lipid. Alternatively, if the rotational freedom of individual phospholipid molecules is increased by the insertion of short n-alkanols (e.g., hexanol) into the bilayer or when a monolayer of the substrate at an air/water interface is expended, enzymic hydrolysis can occur without any requirement for a net postive charge on the surface.     

Reduction of intrinsic sinoatrial frequency and norepinephrine response of the exercised rat

Physical training is associated with a reduction of intrinsic sinoatrial activity; the present study examined the role of the parasympathetic nervous system in this reduction. Six groups of rats were studied for 10 weeks: inactive control; treadmill exercised; parasympathetic receptor blockade with atropine; exercise plus atropine; parasympathetic receptor stimulation with carbachol; and exercise plus carbachol. In vivo ISF (cardiac frequency 20 min after injection of propranolol and atropine) was measured at 3-week intervals. At the end of 10 weeks the right atrium was excised, in vitro measurements were made of ISF, and chronotropic dose-response curves to acetylcholine and norepinephrine were established. In vivo, ISF was reduced with time, the greatest reduction being found in the exercise plus atropine group; the treadmill-exercised and the atropine-treated groups also had a greater reduction than the control group. In vitro, no differences were observed in acetylcholine responses. The maximum norepinephrine chronotropic response was reduced in the treadmill-exercised and the exercise plus atropine groups. The maximum norepinephrine-induced frequency correlated with the in vitro ISF (r = 0.75). Thus, ISF was reduced with training, but this effect was independent of parasympathetic activity. The properties of the sinoatrial node which set ISF also influenced the maximum norepinephrine response.     

Correlation between the antiarrhythmic activity of acyl derivatives of phenothiazine and their affinity for phospholipid membranes

The interaction of thirteen antiarrhythmic active phenothiazine 10-acylaminopropionil derivatives with artificial phospholipid membranes was studied. Blinding constant (K) of this interaction was determined by means of a fluorescent probe. There was found a valid correlation between K and antiarrhythmic activity (A) for 10 substances; the greater the K--the greater the A. Three substances characterized by the greatest K had, however, a low A. It can be assumed that the interaction of antiarrhythmic drugs with phospholipids of target membranes in vivo is an important element of the molecular mechanism of action. At the same time a very high affinity to lipids can possibly cause a delocalization of the drug in the organism and a decrease of its antiarrhythmic activity.     

Vascular reaction following intramedullary nailing of the tibia in dogs

We have reinvestigated radioiodinated plasminogen as an agent for localizing preformed thrombi. Canine plasminogen was isolated from fresh plasma by the affinity chromatography technique on a lysine-sepharose 4B column and tagged with I-123 or I-131, at less than one iodine atom per molecule of enzyme, by the conventional ICI method. When injected into dogs more than 2 days after thrombus induction, radioiodinated plasminogen produced thrombus-to-blood activity ratios of 7.8 +/- 2.4. Thrombi as old as 6 days can be visualized in 80% of the cases. Both the weight of the thrombus and the thrombus-to-blood ratio are more variable for 1-day-old thrombi; this may be associated with plasminogen release accompanying thrombus retraction. The results suggest that radioiodinated plasminogen has potential as an imaging agent for pre-existing thrombi.     

Study of the sensitivity of different strians of the influenza A2 virus of rimantadine

The sensitivity of different influenza A2 (H3N2) virus strains to rimantadine in ovo was studied. The reference strains of influenza virus A/Hong Kong/1/68, A/England/42/72, A/Scotland/840/74 as well as new epidemic strains isolated in the USSR and Mongolia in 1974-1975 antigenically related to influenza A/Port Chalmers/1/73 virus were found to be sensitive to rimantadine.