Disorder in Ca2+ Release Unit Locations Confers Robustness but Cuts Flexibility of Heart Pacemaking

Excitation-contraction coupling kinetics are dictated by the rate and rhythm of the excitations generated by sinoatrial-nodal cells. These cells generate local Ca releases (LCRs) that activate Na/Ca exchanger current, which accelerates diastolic depolarization and determines the rate and rhythm of the excitations. The LCRs are generated by clusters of ryanodine receptors, Ca release units (CRUs), residing in the sarcoplasmic reticulum. While the spatial CRU distribution in pacemaker cells exhibits substantial heterogeneity, it remains unknown if it has any functional importance. Using numerical modeling, here we showed that with a square lattice distribution of CRUs, Ca-induced-Ca-release propagation during diastolic depolarization is insufficient for pacemaking within a broad lower range of realistic ICaL densities. Allowing each CRU to deviate from its original lattice position fundamentally changes the model behavior: during diastolic depolarization sparks propagate, forming LCRs observed experimentally. As disorder in the CRU positions increases, the CRU distribution exhibits larger empty spaces but simultaneously CRU clusters, as in Poisson clumping. Propagating within the clusters, Ca release becomes synchronized, increasing AP firing rate and reviving pacemaker function within lower ICaL densities at which cells with lattice CRU distribution were dormant/non-firing. However, cells with fully disordered CRU positions cannot reach low firing rates and their β-adrenergic receptor stimulation effect was substantially decreased. Thus, order/disorder in CRU locations regulates Ca release propagation and could be harnessed by pacemaker cells to regulate their function. Excessive disorder is expected to limit heart rate range that may contribute to heart rate range decline with age and in disease.

Suppression of Reaction between Si Substrate and Obliquely Deposited Fe Atoms

ABSTRACTWe have investigated the initial stage of the growth of obliquely deposited Fe on a Si substrate held at 470 °C with atomic force microscopy (AFM) and high resolution Rutherford backscattering spectroscopy (HRBS). During the deposition from the normal direction, many Si atoms are displaced from their lattice position due to the reaction with the deposited Fe. On the contrary, the number of displaced Si atoms decreases significantly, and the nanoislands of a few 10 nm in diameter grow selectively when Fe is deposited at 85°. This is clear evidence that the local nucleation processes for the Fe silicide formation is modified by the geometrical deposition conditions.

Incorporation of Highly Concentrated Iron Impurities in InP by High Temperature Ion Implantation

Iron was introduced in InP by ion implantation with the aim of obtaining a high concentration of substitutional, electrically active, deep level impurities. A substrate temperature higher than 200 °C was maintained during implantation in order to reduce damage accumulation and Fe defect reactions. The lattice position of the implanted Fe atoms and its modification during annealing treatments was studied by means of Proton Induced X-ray Emission (PIXE) in channeling conditions and correlated with the ion induced damage measured by different techniques. The results show that a high fraction of substitutional Fe atoms is present after the implantation. This fraction is progressively reduced during thermal treatments by increasing the annealing temperature, with the formation of inactive Fe aggregates, probably in the form of small Fe-P complexes.

Diffuse scattering anisotropy and theP21/a↔A2/aphase transition in titanite, CaTiOSiO4

Diffuse scattering in titanite has been measured at three temperatures, 0.951Tc, 1.053Tcand 1.177Tc, using synchrotron radiation.Tc= 487 K is the temperature of the antiferroelectric paraelectric phase transition. Charge-coupled device (CCD) intensity measurements were centred about the \bar{4}01 reciprocal-lattice position (space-group settingP21/a) and extended beyond the neighbouring \bar{4}11 and \bar{4}\bar{1}1 fundamental positions. Planar diffuse scattering intensity is observed normal to [100] with a lens-shaped maximum centred at \bar{4}01. On approachingTcfrom above, the intensity of the maximum at \bar{4}01 increases, while intensity scattered into the planes decreases at large distances to the critical zone-boundary position. The intensity distribution is interpreted in terms of a two-dimensional spin-1/2 Ising model, in which individual spin states represent the collective displacement of octahedrally coordinated Ti atoms parallel to [100].

X-Ray Characterization of Buried δ Layers

A combination of measurements of crystal truncation roda and X-ray standing waves has been used for a detailed characterization of buried Ge δ layers on Si(001). Measurements of crystal truncation rods reveal the interface roughness, the δ layer lattice constant and the δ layer concentration. From measurements with X-ray standing waves the dopant lattice position and crystallinity of the δ layer are determined. We find a linear dependence of the local tetragonal distortion of the Ge bonding in the δ layer on the Ge concentration in the layer.