Presence Of Active Transport Research Articles

Modelling the effect of myosin X motors on filopodia growth

We present a numerical simulation study of the dynamics of filopodial growth in the presence of active transport by myosin X motors. We employ both a microscopic agent-based model, which captures the stochasticity of the growth process, and a continuum mean-field theory which neglects fluctuations. We show that in the absence of motors, filopodia growth is overestimated by the continuum mean-field theory. Thus fluctuations slow down the growth, especially when the protrusions are driven by a small number (10 or less) of F-actin fibres, and when the force opposing growth (coming from membrane elasticity) is large enough. We also show that, with typical parameter values for eukaryotic cells, motors are unlikely to provide an actin transport mechanism which enhances filopodial size significantly, unless the G-actin concentration within the filopodium greatly exceeds that of the cytosol bulk. We explain these observations in terms of order-of-magnitude estimates of diffusion-induced and advection-induced growth of a bundle of Brownian ratchets.

H+, K+-ATPase is not Involved in Endolymph pH Homeostasis

In endolymph, the calculated electrochemical gradients suggest the presence of active transport of H+ and K+. The presence of H+, K+-ATPase, an active transporter that mediates the exchange of intracellular H+ for extracellular K+, was investigated using reverse transcription-polymerase chain reaction on rat microdissected inner ear tissues. Distal colon and kidney were used as positive controls. The two H+, K+-ATPase colonic or gastric isoforms were not detected in the rat cochlea and endolymphatic sac. This result suggests that H+, K+-ATPase is not involved in H+ and K+ endolymph homeostasis.

Total Protein and Immunoglobulin Concentrations in Equine Tears

Lacrimal fluid represents a major ocular surface defensive mechanism providing different concentrations of all immunoglobulin classes. In this report, four classes of immunoglobulins (IgA, IgM, IgG and IgGT) have been measured in horse tears. As in others species, IgA is the main immunoglobulin responsible for local protection and constitutes quantitatively, 50% of all lacrimal proteins. The rest of immunoglobulins studied are normally present in equine tear fluid (though in lower concentration) and contribute to ocular surface immune protection. Female and adult horses showed significant higher mean IgA concentration than male and young horses. The lack of a significant correlation between plasma and tear immunoglobulin level suggest the presence of active transport or secretion mechanisms similar to that reported for IgA in other species.

Peritubular protein modulates neutral active NaCl absorption in rabbit proximal convoluted tubule.

To assess directly the effect of increasing and decreasing peritubular protein concentration on active NaCl absorption, rabbit proximal convoluted tubules were perfused with a high-chloride solution simulating late proximal tubular fluid and bathed in a similar solution containing 0, 6, or 10 g/dl albumin. A direct examination of the active component of NaCl absorption was ensured by 1) the reduction of luminal solutes other than NaCl (no glucose or alanine and only 5 mM HCO-3), and 2) the use of perfusate and bathing solutions in approximate Donnan equilibrium, so that electrochemical driving forces responsible for passive net NaCl absorption are eliminated. When bathing solution albumin concentration was reduced from 6 to 0 g/dl, volume absorption (Jv) was reduced significantly from 0.39 +/- 0.07 to 0.22 +/- 0.08 nl X mm-1 X min-1. When bathing solution albumin concentration was increased from 6 to 10 g/dl, Jv increased significantly from 0.26 +/- 0.04 to 0.37 +/- 0.04 nl X mm-1 X min-1. When proximal convoluted tubules were bathed in 0, 6, or 10 g/dl albumin at 20 degrees C, Jv was not different from zero in all periods, confirming that changes in bathing solution albumin require the presence of active transport to influence NaCl absorption. The transepithelial potential difference attributable to active transport was not different from zero in all periods of all protocols, demonstrating that active NaCl absorption was electroneutral. In conclusion, these data show directly that peritubular protein concentration modulates neutral active NaCl absorption in the rabbit proximal convoluted tubule.

Further properties of the two-membrane model

The properties of nonlinear equations describing the solute and solvent transport across a simplified Patlak-Goldstein-Hoffman model (two membranes in series without unstirred layers) are investigated both analytically and numerically. The analysis shows that the principal coefficients measured in transport experiments in the presence of active transport are dependent on the experimental conditions. These ‘apparent’ system parameters are extensions of the corresponding parameters determined both in passive systems and in the linear Kedem-Katchalsky theory. Moreover, they are related to the local phenomenological coefficients of the single membranes of the array. Several relationships between measurable quantities and the local system parameters are indicated, allowing the planning of experiments aimed at the measurement of the latter. Data in the literature have been used to check the proposed volume flow equation.

A kinetic model for determining the consequences of electrogenic active transport in cardiac muscle

When active transport is electrogenic in a tissue that is continuously active, such as cardiac muscle, the active transport current is as important in the generation of the action potential as are the passive currents. A thermodynamically constrained kinetic model of electrogenic active transport of sodium and potassium ions has been developed in which the influences of voltage and chemical composition are explicitly defined. This model is coupled to a system of passive permeabilities, of the minimum degree of complexity, to simulate the integrated activity of active and passive ion transport in the generation of the cardiac action potential. Results of preliminary simulations indicate that electrogenic active transport provides a mechanism for slowly changing currents both within the time scale of an action potential as well as of many action potentials. The presence of active transport also complicates the interpretation of isotopic flux measurements and the separation of currents.

Ion selectivity and proximal salt reabsorption.

Electrophysiological techniques were used in isolated perfused superficial (S) and juxtamedullary (JM) rabbit proximal convoluted tubules (PCT) to examine the relative sodium-to-chloride (PNa/PCl) and bicarbonate-to-chloride (PHCO3/PCl) permeability ratios. We found that the great majority of PCT are sodium selective and that PHCO3/PCl depends on the experimental conditions. In the presence of active sodium transport, PHCO3/PCl is high and increases with PNa/PCl. When PHCO3/PCl is determined after inhibition of active sodium transport or at 25 degrees C, PHCO3/PCl approximates the free solution anion mobility ratio of 0.5 and is independent of PNa/PCl. The difference between PHCO3/PCl determined in the presence of and in the absence of active transport suggests that the lowering of bath bicarbonate concentration in the presence of active transport changes both paracellular and transcellular current flow. In addition, we found that during luminal perfusion with high chloride, low bicarbonate, organic solute-free solutions, the transepithelial electrical potential depends on PNa/PCl and PHCO3/PCl. This potential is approximately 4.0 mV in S PCT with low PNa/PCl and falls progressively to zero in JM PCT with high PNa/PCl. From these data we conclude that anion concentration gradients drive an important diffusive flux of sodium chloride through the paracellular pathway only in PCT with low PNa/PCl ratios.