Narrow escape problem for Brownian particles in a microsphere with internal circulation.

This paper considers the narrow escape problem of a Brownian particle within a two-dimensional domain with two escape windows and an internal circulation modeled by the flow within a Hill's vortex. To account for the spatially inhomogeneous flow within the domain, a Lagrangian study is undertaken using the complete equations of motion for a dense particle which is necessary to distinguish between the various regimes as the strength of the internal circulation is varied. For very low internal circulation the particle undergoes the conventional narrow escape problem, and agreement is good with the asymptotic expression. As the internal circulation is increased, regimes are identified with different scaling for the mean escape time. The potential application of this for drug delivery (were nanoparticles are encased in a microsphere) is discussed.

Form, shape and function: segmented blood flow in the choriocapillaris.

The development of fluid transport systems was a key event in the evolution of animals and plants. While within vertebrates branched geometries predominate, the choriocapillaris, which is the microvascular bed that is responsible for the maintenance of the outer retina, has evolved a planar topology. Here we examine the flow and mass transfer properties associated with this unusual geometry. We show that as a result of the form of the choriocapillaris, the blood flow is decomposed into a tessellation of functional vascular segments of various shapes delineated by separation surfaces across which there is no flow, and in the vicinity of which the transport of passive substances is diffusion-limited. The shape of each functional segment is determined by the distribution of arterioles and venules and their respective relative flow rates. We also show that, remarkably, the mass exchange with the outer retina is a function of the shape of each functional segment. In addition to introducing a novel framework in which the structure and function of the metabolite delivery system to the outer retina may be investigated in health and disease, the present work provides a general characterisation of the flow and transfers in multipole Hele-Shaw configurations.

Movement of airborne contaminants in a hospital isolation room.

We analyse the characteristics of a force-ventilated isolation room, and the contributions to transport caused by the movement of people and doors opening/closing. The spread of fine droplets and particles can be understood, to leading order, by considering the movement of passive contaminants. A scaled (1:10) model of an isolation room (with water instead of air) was used to analyse the dilution of a passive contaminant (food dye), released either instantaneously or at a constant rate. The high level of turbulence, typical of isolation rooms, ensures that the dye concentration is uniform within the model room and mixing is perfect, and the measured mean concentration can be predicted theoretically. In a second series of experiments, the exchange generated by a door opening/closing is measured for different opening angles. A dipolar vortex is generated at the tip of the door which moves into the centre of the room, with a large coherent structure moving along the wall. The exchange volume is comparable to the swept volume of the door. Larger droplets and particles do not move passively. Their movement within a turbulent flow is studied by combining a Lagrangian model of particle movement with a kinematic simulation of a pseudo turbulent flow. The results show that while the mean fall velocity of particles is largely unchanged, turbulence significantly enhances horizontal and vertical dispersion. The horizontal spread as a function of the level of turbulence and droplet properties is estimated. The conclusions from both studies are brought together and discussed in the context of the airborne spread of contaminants within a general hospital room.

Cyclic AMP-mediated phosphorylation and insolubilization of a 49-kDa cytoskeletal marker protein of lens fiber terminal differentiation.

Terminal differentiation of chick lens fiber cells has been previously characterized by the accumulation, acidification via phosphorylation, and increased membrane association of a 49-kDa cytoskeletal protein. In these studies, we examine: (1) the subcellular distribution of the 49-kDa protein with regard to ageing and isoform composition; and (2) potential mechanisms regulating 49-kDa phosphorylation and insolubilization. With conventional Western blotting techniques, the 49-kDa protein is found exclusively in insoluble form within terminally differentiated nuclear fiber cells. Cortical fibers, on the other hand, exhibit a more widespread subcellular distribution of the 49-kDa protein. On two-dimensional gels, cortical 49-kDa isoelectric variants segregate according to their ease of sedimentation. After homogenization in detergent-containing buffers, the major isoform of the 49-kDa protein found in low speed pellets (40,000 g, 20 min) exhibits an acidic pI. The 40,000 g supernate and the high speed pellet (100,000 g, 2 hr) which is sedimented from this supernate are enriched in more basic isoforms of the 49-kDa protein. The 100,000 g supernate overlying the high speed pellet is dominated by the most basic isoform. With in vitro phosphorylation assays, the 49 kDa protein is shown to be a major substrate affected by endogenous cAMP-dependent mechanisms. Both the low and high speed pellets exhibit endogenous cAMP-dependent kinase activity. An inhibitor of cAMP-dependent protein kinase activity is also found in soluble lens fractions. Conversion of the 49-kDa protein into more acidic, phosphorylated isoforms increases its insolubility and ease of sedimentation.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypothyroidism increases serotonin turnover and sympathetic activity in the adult rat.

Adult, male Sprague-Dawley rats underwent surgical thyroidectomy (Tx) or sham surgery. In all three experiments from which data are reported, a 3-week recovery period was allowed. In experiments I and II, baseline measurements of colonic temperature (Tc) and urinary norepinephrine excretion (NE) were obtained, and both variables were monitored daily for the duration of the studies. After baseline measurements, half of each surgical group was given either triiodothyronine (T3) or vehicle injections subcutaneously; in experiment I replacements continued for 1.5 days, while in experiment II T3 replacement continued for 3.5 days. Rats were decapitated at the end of each experiment and serotonin (5-HT) turnover was measured in brainstem. Serotonin turnover in rostral and caudal brainstem was increased with Tx (p less than 0.05). Increased turnover in caudal brainstem was normalized by T3 only in experiment II. Similarly, decreased Tc and elevated NE with Tx were normalized in experiment II but not in experiment I. In experiment III, NE measurements normalized on a creatinine excretion basis indicated that increased NE is evident with Tx, irrespective of normalization procedure. Significant correlations between 5-HT in caudal brainstem and metabolic correlates of sympathetic function, concurrent normalization of NE and 5-HT in caudal brainstem, plus work from other laboratories describing sympathoexcitatory serotonergic neurons located in the caudal brainstem suggest that the central and peripheral changes in the hypothyroid rat are causally related.

Pseudomonas aeruginosa: a probe to detail change with age in corneal receptor(s).

This in vitro study used Pseudomonas aeruginosa as a probe to examine the change with age of the host receptor(s) facilitating bacterial adherence to cornea in the immature eye and to pinpoint in time, when the host receptor(s) was no longer able to be recognized by the bacterium. Scanning EM was used to quantitate adherent organisms at the corneal surface of mice of select ages (5-37 postnatal days, P, and 28 months) at 15, 30 and 60 min following bacterial application. Organisms were most numerous at the surface of the 5-P eye at all time periods. In 9-21-P mice, no bacterial binding was observed until 30 min, and bacterial adherence modestly increased at 60 min after bacterial application. In the 37-P and in aged 28-month-old mice, no binding to cornea was observed until 60 min after bacterial application. Statistical analysis of these data revealed significant differences in the mean number of bacteria binding to cornea between age groups.