Viscoelastic properties of the oxygenated sickle erythrocyte membrane.

Although most apparent in permanently misshapen irreversibly sickled erythrocytes (ISC), biochemical and structural alterations are present in the majority of sickle cell membranes. The relationship of membrane rigidity to cell shape and its dependence upon the internal hemoglobin cytosol are not clarified. We therefore examined the frequency dependent viscoelasticity of oxygenated, packed sickle red cell and ghost suspensions and hemoglobin solutions prepared from density gradient separated ISC and reversibly sickled cell (RSC) fractions. Low amplitude, oscillatory shear was applied in a Weissenberg cone and plate viscometer and the resultant viscoelastic signals provided a dynamic viscosity (eta') and elastic storage modulus (G') which varied with frequency of deformation. The viscoelastic response of the cell and ghost suspensions reflected the material properties of the membrane over most of the frequency range tested. Sickle erythrocyte, red ghost, and white ghost suspensions demonstrated greater viscocoelasticity than comparable normal suspensions. The viscoelastic magnitude of ISC was several-fold greater than normal, with little variation of viscoelasticity with frequency. RSC samples which were characterized by normal shape, size, and internal hemoglobin concentration were also significantly harder than normal, although similar in frequency dependence. Red ghosts prepared from ISC manifested 80% of the viscoelasticity of intact ISC despite diminution of the internal hemoglobin concentration by 90%. Under conditions of low amplitude shear, the behavior of the RSC membrane is compatible with a cytoskeleton possessing an increased number of molecular associations. The mechanical stability of the ISC membrane is related to a substantial, intrinsic reorganization of the cytoskeleton.

Viscoelasticity of packed erythrocyte suspensions subjected to low amplitude oscillatory deformation.

Concentrated adult erythrocyte suspensions were subjected to low amplitude oscillatory shear in a Weissenberg rheogoniometer equipped with a cone-and-plate assembly. The dynamic viscoelastic properties of the suspension were measured over a broad range of frequency by a numerical solution that accounted for fluid inertia. Variation of shear amplitude and cell volume percent, and comparison of buffered saline, plasma, and dextran as suspending media showed that the cellular elements had undergone small bending and shearing deformations. Studies of normal adult erythrocytes, hypotonically swollen cells, temperature-altered cells, and erythrocyte ghosts suggested that the method was evaluating membrane material properties. The normal membrane was found to exhibit a shear rate dependent elastic modulus that increased by more than a factor of 20 over a frequency range from 0.0076 Hz to 60 Hz. The membrane viscosity showed a substantial drop with frequency indicative of a frequency thinning phenomenon. At high frequency of deformation the viscous response of normal erythrocytes was no longer indicative of a membrane property due to the dominant influence of the internal hemoglobin solution. The studies generally supported the ability of the method to quantify relative membrane material properties and detect changes in membrane structure.

Prolonged absorption with development of tolerance to toxic effects after cutaneous exposure to nicotine.

This report describes a patient who developed nicotine poisoning after cutaneous application of nicotine sulfate. Measurement of nicotine and metabolite levels in the blood demonstrated prolonged absorption of nicotine despite vigorous skin decontamination. This suggests that the skin may be a reservoir for slow release of nicotine into the circulation. Despite extraordinarily high levels of nicotine, the patient had full resolution of signs and symptoms of intoxication, indicating rapid and profound development of tolerance.

Artificial heart and assist devices: directions, needs, costs, societal and ethical issues.

A Working Group appointed by the Director of the National Heart, Lung, and Blood Institute (NHBLI) has reviewed the current status of mechanical circulatory support systems (MCSS), and has examined the potential need for such devices, their cost, and certain societal and ethical issues related to their use. The media have reported the limited clinical investigative use of pneumatically energized total artificial hearts (which actually replace the patient's heart) and left ventricular assist devices (which support or replace the function of the left ventricle by pumping blood from the left heart to the aorta with the patient's heart in place). However, electrically energized systems, which will allow full implantation, permit relatively normal everyday activity, and involve battery exchange or recharge two or three times a day, are currently approaching long-term validation in animals prior to clinical testing. Such long-term left ventricular assist devices have been the primary goal of the NHLBI targeted artificial heart program. Although the ventricular assist device is regarded as an important step in the sequence of MCSS development, the Working Group believes that a fully implantable, long-term, total artificial heart will be a clinical necessity and recommends that the mission of the targeted program include the development of such systems. Past estimates of the potential usage of artificial hearts have been reviewed in the context of advances in medical care and in the prevention of cardiovascular disease. In addition, a retrospective analysis of needs was carried out within a defined population. The resulting projection of 17,000-35,000 cases annually, in patients below age 70, falls within the general range of earlier estimates, but is highly sensitive to many variables. In the absence of an actual base of data and experience with MCSS, projection of costs and prognoses was carried out using explicit sets of assumptions. The total cost of a left ventricular assist device, its implantation and maintenance for a projected average of 4 1/2 years of survival might be approximately $150,000 (in 1983 dollars). The gross annual cost to society could fall in the range of $2.5-$5 billion. Ethical issues associated with use of the artificial heart are not unique. For individual patients these relate primarily to risk-benefit, informed consent, patient selection, and privacy. However, for society as a whole, the larger concern relates to the distribution of national resources.(ABSTRACT TRUNCATED AT 400 WORDS)

Surface tension gradients: feasible model for gliding motility of Myxococcus xanthus.

We propose that surface tension is the driving force for the gliding motility of Myxococcus xanthus. Our model requires that the cell be able to excrete surfactant in a polar and reversible fashion. We present calculations that (i) estimate the surface tension difference across a cell necessary to move the cell at the observed rate, which is less than 10(-5) dyn/cm, an extremely small value; (ii) estimate the rate of surfactant excretion necessary to produce the required surface tension difference, a rate that we conclude to be metabolically reasonable; (iii) predict the behavior of cells moving in close apposition to each other, and show that the model is consistent with observed behavior; and (iv) predict the behavior of cells moving in dense swarms. In an accompanying paper we present experimental evidence to support the surface tension model.

Experimental observations consistent with a surface tension model of gliding motility of Myxococcus xanthus.

We have presented experimental evidence to support the model that gliding motility of Myxococcus xanthus is driven by surface tension. (i) Motility is inhibited by the addition of sufficient exogenous, nontoxic surfactants to swamp out the cells' own surfactant gradient. (ii) M. xanthus does not move polystyrene latex beads over its surface. (iii) Motility is prevented by elimination of an interfacial surface tension either by embedding the cells in soft agar or by placing them at an agar-aqueous interface. (iv) Wild-type cells excrete surfactant, whereas two nonmotile mutants excrete reduced amounts.

Effects of random motility on growth of bacterial populations.

A spatially distributed mathematical model is developed to elucidate the effects of chemical diffusion and cell motility as well as cell growth, death, and substrate uptake on steady-state bacterial population growth in a finite, one-dimensional, nonmixed region. The situation considered is growth limited by a diffusing substrate from an adjacent phase not accessible to the bacteria. Chemotactic movement is not considered in this paper; we consider only "randomwalk"-type random motility behavior here. The following important general concepts are suggested by the results of our theoretical analysis: (a) The significance of random motility effects depends on the magnitude of the ratioµ/kL (2), whereµ is the bacterial random motility coefficient,k is the growth rate constant, andL is the linear dimension of the confined growth region. (b) In steady-state growth in a confined region, the bacterial population size decreases asµ increases. (c) The effect ofµ on population size can be great; in fact, sometimes relative population sizes of two species can be governed primarily by the relative values ofµ rather than by the relative values ofk.

A method for establishing stable concentration gradients in agar suitable for studying chemotaxis on a solid surface.

A simple technique has been developed for establishing stable gradients of a substance in agar. The technique involves the creation of a spherically symmetric concentration profile in which concentration varies inversely with the distance from the source and is independent of the diffusion coefficient of the substance. It has been shown that the gradients established with this technique are stable for at least 190 h. and, on a theoretical basis, they can be kept stable for more than 1000 h. Time-variant gradients can also be established, if desired, using the same system and limiting either the source or the agar sink. It must be emphasized that a stable gradient cannot be obtained by using a shallow agar layer as a sink. The use of such conditions (e.g. the agar in a standard petri dish) can result only in time-variant gradients. The solution to the diffusion equation in a spherically symmetric system establishes the expected concentration profile, the basis for adjusting it, and the parameters that control the behavior of the system. Some useful applications for examining chemotaxis on a solid surface as well as possible further developments are discussed.

Oxygen permeability in ultrathin and microporous membranes during gas-liquid transfer.

The apparatus described here offers a useful system for measuring membrane oxygen permeabilities with the membrane in contact with liquid on one side. Results using this system indicate that liquid migration into the pores of microporous membranes can play a significant role in reducing their permeability to oxygen.

Cell density-dependent growth of Myxococcus xanthus on casein.

When Myxococcus xanthus FB was grown on 0.2% casein it exhibited a phenomenon we call cooperative growth. That is, above 104 cells per ml, both strains that were studied exhibited increasing growth rates as a function of increasing cell numbers. Between 104 and 107 cells per ml, the mean doubling times of strains YS and TNS decreased from 15.2 to 8 h and 26 to 8.5 h, respectively. The extracellular proteinase activity of the two strains was equivalent and directly proportional to cell number. Cooperative growth was correlated with increased concentration of hydrolyzed casein in the medium, suggesting cooperative hydrolysis of casein. At low cell densities neither strain was capable of measurable growth on casein in liquid media, and we have calculated that the average concentration of hydrolyzed casein in the medium was indeed too low to support growth. At low cell densities, growth on hydrolyzed casein (Casitone) was normal and independent of cell concentration. Demonstration of cooperative growth at higher cell densities supports the suggestion that the communal behavior of myxobacteria results in more efficient feeding.

Solubility and diffusion coefficient of adenosine 3':5'-monophosphate.

Several previously unavailable parameters of adenosine 3':5'-monophosphate have been determined. The molar extinction coefficient at pH 7.0 is 1.38 X 10(-4), the aqueous solubility at pH 7.0 is 0.0236 M and the diffusion coefficient is 4.44 X 10(-6) cm2/s.

Asymmetries generated by diffusion and reaction, and their bearing on active transport through membranes.

A model is presented that suggests that a natural, stable asymmetry may arise on either side of a membrane, even when the bulk conditions on either side of the membrane are the same and the two faces of the membrane are indistinguishable. Equations are derived on the assumption of Michaelis-Menten kinetics with substrate inhibition to describe the properties of such a model membrane.