Dynamic switching of the spin circulation in tapered magnetic nanodisks.

Magnetic vortices are characterized by the sense of in-plane magnetization circulation and by the polarity of the vortex core. With each having two possible states, there are four possible stable magnetization configurations that can be utilized for a multibit memory cell. Dynamic control of vortex core polarity has been demonstrated using both alternating and pulsed magnetic fields and currents. Here, we show controlled dynamic switching of spin circulation in vortices using nanosecond field pulses by imaging the process with full-field soft X-ray transmission microscopy. The dynamic reversal process is controlled by far-from-equilibrium gyrotropic precession of the vortex core, and the reversal is achieved at significantly reduced field amplitudes when compared with static switching. We further show that both the field pulse amplitude and duration required for efficient circulation reversal can be controlled by appropriate selection of the disk geometry.

Mechanisms underlying the induction of vasorelaxation in rat thoracic aorta by sanguinarine.

In the present study, the effect of sanguinarine (SANG) on smooth muscle was investigated in thoracic aorta isolated from rats. SANG dose-dependently relaxed the phenylephrine (PE, 3 microM)-precontracted aorta; and the concentrations to produce 50% relaxation were 3.18 +/- 0.37 and 3.42 +/- 1.14 microM, respectively, in intact and denuded aorta. These results suggest that the relaxing effect of SANG was endothelium-independent. The total contraction induced by PE was inhibited in aorta pretreated with SANG at microM concentration. Both phasic and tonic contractions induced by PE were inhibited by SANG independently, which were further supported by the fact that inositol 1,4,5-trisphosphate (IP3) formation and 45Ca2+ influx induced by 3 microM PE in denuded aorta were inhibited by SANG concentration-dependently. In addition, the vasocontraction induced by high-K+ was also inhibited by SANG, however, at higher concentrations. The inhibitory effects of SANG were reversed by dithiothreitol, a thiol reducing agent, implying that the oxidation of critical sulfhydryl groups on key molecules that regulate the smooth muscle contraction were involved. These data suggested that the inhibitory effects of SANG on PE-induced vasocontraction might involve the inhibition of IP3 formation and blockade of calcium channel.

Oxygen diffusion in a network model of the myocardial microcirculation.

Oxygen supply was studied in a three-dimensional capillary network model of the myocardial microcirculation. Capillary networks were generated using one common strategy to locate the capillary branchings and segments, arterioles and venules. Flow paths developed with different capillary flow velocities. All pressure-flow relationships were linear. The model includes free diffusion of oxygen within tissue slices that are perpendicular to the main capillary orientation. Oxygen pressure distributions were calculated and correlated to oxygen delivery by small capillary segments. It was shown that intercapillary diffusion is important for reducing PO2 heterogeneity. The absence of this feature leads to an oxygen distribution that has similar heterogeneity characteristics as the capillary flow heterogeneity. Such situations may also occur during simulated, elevated metabolic activity in a network model that allows intercapillary diffusion. On the basis of our simulations of metabolic vasoactivity we concluded that the venous PO2 is a misleading quantity to indicate tissue oxygenation. The venous PO2 in our model was not a good measure for the mean tissue or capillary PO2, and for the low oxygen pressures that exist at some locations in the network model. Moreover, the venous PO2 may remain constant despite considerable changes on the tissue PO2 distribution induced by metabolic activity.