Biomedical subjects

Sang-Hyuk Lee

Publications and source records attributed to Sang-Hyuk Lee.

4 recordsLinked to original sources

PubMed2006

Giant colloidal diffusivity on corrugated optical vortices.

A single colloidal sphere circulating around a periodically modulated optical vortex trap can enter a dynamical state in which it intermittently alternates between freely running around the ringlike optical vortex and becoming trapped in local potential energy minima. Velocity fluctuations in this randomly switching state still are characterized by a linear Einstein-like diffusion law, but with an effective diffusion coefficient that is enhanced by more than 2 orders of magnitude.

Journal Article↗

PubMed2005

Flux reversal in a two-state symmetric optical thermal ratchet.

A Brownian particle's random motions can be rectified by a periodic potential-energy landscape that alternates between two states, even if both states are spatially symmetric. If the two states differ only by a discrete translation, the direction of the ratchet-driven current can be reversed by changing their relative durations. We experimentally demonstrate flux reversal in a symmetric two-state ratchet by tracking the motions of colloidal spheres moving through large arrays of discrete potential-energy wells created with dynamic holographic optical tweezers. The model's simplicity and high degree of symmetry suggest possible applications in molecular-scale motors.

Computer Simulation↗

PubMed2005

Observation of flux reversal in a symmetric optical thermal ratchet.

We demonstrate that a cycle of three holographic optical trapping patterns can implement a thermal ratchet for diffusing colloidal spheres and that the ratchet-driven transport displays flux reversal as a function of the cycle frequency and the intertrap separation. Unlike previously described ratchet models, the approach we describe involves three equivalent states, each of which is locally and globally spatially symmetric, with spatiotemporal symmetry being broken by the sequence of states.

Journal Article↗

PubMed2003

Distribution of K+-dependent Na+/Ca2+ exchangers in the rat supraoptic magnocellular neuron is polarized to axon terminals.

Neurons are polarized into compartments such as the soma, dendrites, and axon terminals, each of which has highly specialized functions. To test whether Ca2+ is differently handled in different compartments of a neuron, we investigated Ca2+ clearance mechanisms in somata of supraoptic magnocellular neurosecretory cells (MNCs) and in their axon terminals located in neurohypophyses. Using patch-clamp and microfluorometry techniques, Ca2+ transients were evoked by depolarizing pulses. Endogenous Ca2+ binding ratios (kappaS) and Ca2+ clearance rates were calculated from the decay phases of Ca2+ transients according to the single compartment model. Mean values of kappaS were 79 +/- 2.6 in somata of MNCs and 187 +/- 19 in axon terminals. Ca2+ clearance rate in axon terminals, which were calculated from time derivative of Ca2+ decay and the kappaS values, were approximately threefold higher than in somata. In response to external Na+ reduction, Ca2+ clearance rates were reduced by 65% in axon terminals, but did not change in somata. Immunohistochemical assays confirmed that K+-dependent Na+/Ca2+ exchanger (NCKX2) was specifically localized to neurohypophysial axon terminals and was not found in somata. In somata, inhibition of sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) pumps, mitochondrial Ca2+-uniporter, and plasma membrane Ca2+-ATPase (PMCA) pumps decreased Ca2+ clearance rate by 48, 27, and 21%, respectively. These results suggest that neurohypophysial axon terminals have greater Ca2+ clearance power than somata because of the specific localization of NCKX2, and that Ca2+ clearance in somata of MNCs is mediated by SERCA pumps, mitochondrial uniporter, and PMCA pumps.

Animals↗