Biomedical subjects

Gerald H Pollack

Publications and source records attributed to Gerald H Pollack.

6 recordsLinked to original sources

PubMed2003

Dynamics of individual sarcomeres during and after stretch in activated single myofibrils.

It is generally assumed that sarcomere lengths (SLs) change in isometric fibres following activation and following stretch on the descending limb of the force-length relationship, because of an inherent instability. Although this assumption has never been tested directly, instability and SL non-uniformity have been associated with several mechanical properties, such as 'creep' and force enhancement. The aim of this study was to test directly the hypothesis that sarcomeres are unstable on the descending limb of the force-length relationship. We used single myofibrils, isolated from rabbit psoas, that were attached to glass needles that allowed for controlled stretching of myofibrils. Images of the sarcomere striation pattern were projected onto a linear photodiode array, which was scanned at 20 Hz to produce dark-light patterns corresponding to the A- and I-bands, respectively. Starting from a mean SL of 2.55 +/- 0.07 microm, stretches of 11.2 +/- 1.6% of SL at a speed of 118.9 +/- 5.9 nm s(-1) were applied to the activated myofibrils (pCa(2+) = 4.75). SLs along the myofibril were non-uniform before, during and after the stretch, but with few exceptions, they remained constant during the isometric period before stretch, and during the extended isometric period after stretch. Sarcomeres never lengthened to a point beyond thick and thin filament overlap. We conclude that sarcomeres are non-uniform but generally stable on the descending limb of the force-length relationship.

Animals↗

PubMed2003

The role of aqueous interfaces in the cell.

The cell is rich with biopolymeric surfaces. Yet, the role of these surfaces and attendant surface-water interfaces has received little attention among biologists, most of whom consider water as a neutral carrier. This review aims to begin bridging the gap between biology and interface science-to show that a surface-oriented approach has power to bring fresh insights into an otherwise impenetrably complex maze. In this approach the cell is treated as a polymer gel. If the cell is a gel, then a logical approach to the understanding of cell function is through an understanding of gel function. Great strides have been made recently in understanding the principles of polymer-gel dynamics, and particularly the role of the polymer-water interface. It has become clear that a central mechanism in biology is the phase-transition-a major structural change prompted by a subtle change of environment. Phase-transitions are capable of doing work and such work could be responsible for much of the work of the cell. Here, we pursue this approach. We set up a polymer-gel-based foundation for cell behavior, and explore the extent to which this foundation explains how the cell achieves its everyday tasks.

Animals↗

PubMed2003

Labview virtual instruments for calcium buffer calculations.

Labview VIs based upon the calculator programs of Fabiato and Fabiato (J. Physiol. Paris 75 (1979) 463) are presented. The VIs comprise the necessary computations for the accurate preparation of multiple-metal buffers, for the back-calculation of buffer composition given known free metal concentrations and stability constants used, for the determination of free concentrations from a given buffer composition, and for the determination of apparent stability constants from absolute constants. As implemented, the VIs can concurrently account for up to three divalent metals, two monovalent metals and four ligands thereof, and the modular design of the VIs facilitates further extension of their capacity. As Labview VIs are inherently graphical, these VIs may serve as useful templates for those wishing to adapt this software to other platforms.

Buffers↗

PubMed2002

Mechanics of F-actin characterized with microfabricated cantilevers.

In this report we characterized the longitudinal elasticity of single actin filaments manipulated by novel silicon-nitride microfabricated levers. Single actin filaments were stretched from zero tension to maximal physiological tension, P(0). The obtained length-tension relation was nonlinear in the low-tension range (0-50 pN) with a resultant strain of approximately 0.4-0.6% and then became linear at moderate to high tensions (approximately 50-230 pN). In this region, the stretching stiffness of a single rhodamine-phalloidin-labeled, 1-microm-long F-actin is 34.5 +/- 3.5 pN/nm. Such a length-tension relation could be characterized by an entropic-enthalpic worm-like chain model, which ascribes most of the energy consumed in the nonlinear portion to overcoming thermal undulations arising from the filament's interaction with surrounding solution and the linear portion to the intrinsic stretching elasticity. By fitting the experimental data with such a worm-like chain model, an estimation of persistence length of approximately 8.75 microm was derived. These results suggest that F-actin is more compliant than previously thought and that thin filament compliance may account for a substantial fraction of the sarcomere's elasticity.

Actins↗

PubMed2002

Fluorescence anisotropy near-field scanning optical microscopy (FANSOM): a new technique for nanoscale microviscometry.

A near-field scanning optical microscope system was implemented and adapted for nanoscale steady-state fluorescence anisotropy measurement. The system as implemented can resolve approximately 0.1 cP microviscosity variations with a resolution of 250 nm laterally in the near field, or approximately 10 microm when employed in a vertical scanning mode. The system was initially used to investigate the extent of microviscous vicinal water over surfaces of varying hydrophilicity. Water above a cleaved mica surface was found to have a decreased microviscosity, while water above a hydrophobic surface showed no change (detection limit approximately 0.1 cP at approximately 30 + nm from the surface).

Fluorescein↗

PubMed2002

Fundamental step size in single cardiac and skeletal sarcomeres.

In attempting to deduce the size of the elementary molecular translation step, recent experiments using single myosin molecules translating over actin filaments have shown a consistent step size of 5.4 nm (10, 21). We have carried out parallel measurements on single myofibrils from rabbit cardiac muscle and bumblebee flight muscle. Activated specimens were released or stretched with a motor-imposed ramp, and the time course of length of individual sarcomeres was measured by projecting the image of the striations onto a linear photodiode array and tracking the spacing between A-band centroids. We confirmed the 5.4-nm step. With subnanometer precision, however, we find that this value is two times that of a more fundamental step size of 2.7 nm. Step sizes were always integer multiples of 2.7 nm, whether the length change was positive or negative. This value is equal to the linear repeat of actin monomers along the thin filament, a result that ties dynamic events to molecular structure and places narrow constraints on any proposed molecular mechanism.

Algorithms↗