Experimental determination of ultrasonic wave velocities in plastics as functions of temperature. III. Rigid epoxy foam

The velocity and attenuation of longitudinal bulk waves in a solid epoxy foam were measured by an acoustic pulse technique in the frequency range of 0.667–4.0 Mc./sec. and in the temperature range from ambient to 150°C. The measurements are reported with the density of the solid epoxy and with aluminum impurity loading as parameters. Over the indicated temperature and frequency ranges, complete attenuation and velocity measurements are reported for one foam corresponding to a density of 0.325 g./cc. In the density range of 0.088–0.325 g./cc. for the unloaded foams, attenuation is reported at room temperature. It is observed that the longitudinal velocities for all the densities decrease with temperature by about 40% in a span of 100°C. and that an approximately linear relation exists with temperature. The velocities in the foams loaded with small percentages of aluminum and heat-treated at 250°C. exhibit temperature behavior which is dependent upon the combined effects of loading, density change, and epoxy properties. For the loaded foam with the highest density (1.068 g./cc.), velocity is reported to a temperature of about 250°C. The velocities of all the various density samples with the exception of the loaded foams exhibit inflections at a temperature of about 110°C. The attenuation–temperature measurements on the 0.325 g./cc. sample show similar behavior at this temperature except that the effect is much more pronounced than the velocity inflection, hence a better indication of the transition. The precision of the measurement is about 2% for the relative longitudinal velocities and about 20% for the attenuation.     

A flow metamodel for delivery agents over a spatial area

This article studies fuzzy rules that were used to model autonomous entities and discusses the development of a metamodel to describe the traffic flow of autonomous agents over a spatial area. The agents deliver goods (and themselves) to prespecified locations on the given spatial area. The metamodel is defined as a set of equations, obtained through regression analysis, that are functions of several variables that affect the agent system. The performance measures are effective speed and collision probability. The fuzziest rule set consistently gave the best results in terms of collision avoidance for this study. The improved collision avoidance is gained at the cost of a lower effective speed. A “traffic control” policy may be desirable when congestion reaches a limit. The flow metamodel equations describe the emergent behavior of the system, or system-level behavior resulting from interactions at the level of the parts of the system. The equations can be used to determine whether congestion will be a problem before designing a system containing entities that navigate among themselves. The flow metamodel curves suggest combinations of system parameters that will alleviate congestion. The results have implications for several practical applications.     

Safety aspects of switched gradient fields

Examination of the first crystal structures of proteins from a halophilic organism suggests that an abundance of acidic residues distributed over the protein surface is a key determinant of adaptation to high-salt conditions. Although one extant theory suggests that acidic residues are favored because of their superior water-binding capacity, it is clear that extensive repulsive electrostatic interactions will also be present in such proteins at physiological pH. To investigate the magnitude and importance of such electrostatic interactions, we conducted a theoretical analysis of their contributions to the salt and pH-dependence of stability of two halophilic proteins. Our approach centers on use of the Poisson-Boltzmann equation of classical electrostatics, applied at an atomic level of detail to crystal structures of the proteins. We first show that in using the method, it is important to account for the fact that the dielectric constant of water decreases at high salt concentrations, in order to reproduce experimental changes in pKa values of small acids and bases. We then conduct a comparison of salt and pH effects on the stability of 2Fe-2S ferredoxins from the halophile Haloarcula marismortui and the non-halophile anabaena. In both proteins, substantial upward shifts in pKa accompany protein folding, though shifts are considerably larger, on average, in the halophile. Upward shifts for basic residues occur because of favorable salt-bridge interactions, whilst upward shifts for acidic residues result from unfavorable electrostatic interactions with other acidic groups. Our calculations suggest that at pH 7 the stability of the halophilic protein is decreased by 18.2 kcal/mol on lowering the salt concentration from 5 M to 100 mM, a result that is in line with the fact that halophilic proteins generally unfold at low salt concentrations. For comparison, the non-halophilic ferredoxin is calculated to be destabilized by only 5.1 kcal/mol over the same range. Analysis of the pH stability curve suggests that lowering the pH should increase the intrinsic stability of the halophilic protein at low salt concentrations, although in practice this is not observed because of aggregation effects. We report the results of a similar analysis carried out on the tetrameric malate dehydrogenase from H. marismortui. In this case, we investigated the salt and pH dependence of the various monomer-monomer interactions present in the tetramer. All monomer-monomer interactions are found to make substantial contributions to the salt-dependence of stability of the tetramer. Excellent agreement is obtained between our calculated results for the stability of the tetramer and experimental results. In particular, the finding that at 4 M NaCl, the tetramer is stable only between pH 4.8 and 10 is accurately reproduced. Taken together, our results suggest that repulsive electrostatic interactions between acidic residues are a major factor in the destabilization of halophilic proteins in low-salt conditions, and that these interactions remain destabilizing even at high salt concentrations. As a consequence, the role of acidic residues in halophilic proteins may be more to prevent aggregation than to make a positive contribution to intrinsic protein stability.     

Tubular gelatinase A (MMP-2) and its tissue inhibitors in polycystic kidney disease in the Han:SPRD rat

Lactation in mammals is characterized by a marked hyperphagia and significantly elevated levels of prolactin (PRL). Several recent experiments in our laboratory have provided evidence for a causal relationship between PRL and hyperphagia. The present series of studies revealed that PRL injected intracerebroventricularly (i.c.v.) twice daily for ten days to free feeding virgin female rats produced a dose-dependent increase in food intake without disrupting vaginal cyclicity, that the hyperphagic effect of centrally administered PRL was not dependent on the presence of ovarian hormones, that it did not selectively potentiate feeding during the light or dark phase of the daily light cycle and that the latency of the feeding response to i.c.v. PRL administration was shorter in food restricted animals. Taken together, these results support the idea that PRL acts centrally to potentiate food intake.     

Spectrophotometric determination of the styrene content of alpha-methylstyrene — styrene copolymers

Summary The electronic absorption spectra of polystyrene and poly (alpha-methylstyrene) show a great difference in the extinction coefficient values at 226nm. On basis of this fact, it is shown that the styrene content of styrene — alpha-methylstyrene copolymers can be determined by ultraviolet spectrophotometry. The relationship between the extinction coefficient at 226nm and the polystyrene content in the mixture shows a linear dependence.     

Normal short-term renal response to acute volume expansion in heart transplant recipients: a role for atrial natriuretic peptide?

BACKGROUND: The breakdown of blood pressure and body fluid homeostasis observed in heart transplant (Htx) recipients may partly be due, as in heart failure, to a blunted renal response to elevated atrial natriuretic peptide (ANP). METHOD: This possibility was addressed through determination of the relationship between ANP, the urinary cyclic guanosine monophosphate (cGMP), a biologic marker of ANP renal activity, and the early renal responses to 10 mL/kg isotonic saline infusion over 30 minutes in 8 control subjects and 8 matched Htx recipients. RESULTS: Urine flow, natriuresis, and urinary cGMP excretion increased similarly in both groups, resulting in elimination of, respectively, 1/2 and 2/3 of the sodium and the water load during the experiment that lasted 4 hours and 30 minutes. Plasma renin and aldosterone decreases were similar in both groups. Elevated ANP further increased in Htx after saline infusion (from 19.5 +/- 3.7 to 33.8 +/- 5.6 pmol/L, P < .001). Plasma cGMP paralleled ANP in both groups (r = 0.81; P < .001). Significant correlations were observed between plasma ANP and urinary cGMP excretion (r = 0.48, P < .025 and r = 0.43, P < .05 in Htx recipients and control subjects) and between plasma ANP and urinary sodium excretion (r = 0.64, P < .001 in Htx recipients). CONCLUSION: In spite of a relative renal hyporesponsiveness to the cardiac hormone, with higher plasma ANP being not associated with increased renal excretions in Htx recipients, ANP is likely to participate in the appropriate short-term renal response to acute volume expansion in Htx recipients.