R.N. Friedman

Preliminary Measurements with MicroSQUID

It is well known that for optimum sensitivity, the diameter of the pickup coil for a SQUID biomagnetometer should be no less than the source-to-coil spacing, and that this distance is the primary determinant of the spatial resolution of the magnetometer (Roth et al., 1989). As a result, SQUID systems with conventional Dewars typically have spatial resolutions of 10 to 20 mm when resolution is measured in terms of the ability either to image a current distribution (Roth et al., 1989) or to resolve two closely-spaced dipolar current sources (Tan et al., 1989). While 1 to 2 cm resolution is adequate for many studies on humans, experiments on in vitro preparations such as isolated nerves and muscles and slices of brain and cardiac tissue would benefit from 1 to 2 mm resolution. Because the fields from these sources fall off as l/r2 to l/ r3, it can be advantageous to trade absolute magnetometer sensitivity for decreased coil-to-source spacing (Roth et al. 1989; Wikswo, 1988). We describe measurements made with a 4-channel SQUID gradiometer whose 3 mm diameter pickup coils are located 1.4 mm from the outer surface of the Dewar (Buchanan, et al., 1989).

High-resolution SQUID imaging of octupolar currents in anisotropic cardiac tissue

A monopolar stimulus electrode triggered depolarization in a 1-mm-thick slice of canine ventricular myocardium, maintained in a thermally regulated, oxygenated chamber. Five hundred milliseconds of data at 2 kHz were recorded with a MicroSQUID (superconducting quantum interference device) magnetometer at 1-mm intervals over a 23-mm*23-mm area, centered on the stimulus site. Without averaging, a signal-to-noise ratio of better than 100:1 was achieved, and the field maps provided evidence of a propagating wavefront of activity. Application of an inverse Fourier filter yielded current density images that consisted of four expanding circular current paths, in agreement with predictions of the bidomain model. The ability to image action currents in a DC to 2-kHz bandwidth should prove useful for understanding the complex anisotropy of cardiac tissue and how it is altered by pharmacological interventions.<<ETX>>

The biomagnetic signature of a crushed axon. A comparison of theory and experiment.

The response of a crayfish medial giant axon to a nerve crush is examined with a biomagnetic current probe. The experimental data is interpreted with a theoretical model that incorporates both radial and axial ionic transport and membrane kinetics similar to those in the Hodgkin/Huxley model. Our experiments show that the effects of the crush are manifested statically as an elevation of the resting potential and dynamically as a reduction in the amplitude of the action current and potential, and are observable up to 10 mm from the crush. In addition, the normally biphasic action current becomes monophasic near the crush. The model reflects these observations accurately, and based on the experimental data, it predicts that the crush seals with a time constant of 45 s. The injury current density entering the axon through the crush is calculated to be initially on the order of 0.1 mA/mm2 and may last until the crush seals or until the concentration gradients between the intra- and extracellular spaces equilibrate.

Intraoperative Recording of the Magnetic Field of a Human Nerve

Accurate, quantitative assessment of nerve function during surgical procedures has been impeded by inherent limitations of extracellular electrophysiological recording techniques. Extracellular recordings are hampered by critical constraints on electrode placement, uncertainty in the conductivity of the moist layer on the outside of the nerve, and the risks involved in the essential elevation of the nerve in the air for recording. Our laboratory has developed a neuromagnetic current probe system that affords room- or body-temperature recording of magnetic signals as small as one millionth of the earth’s magnetic field strength. In addition, we have designed and constructed an ‘openable,’ toroidal magnetic probe that can be placed around a nerve to assess the magnetic field arising from the electrical current flow within the active axons and across their membranes. A mathematical deconvolution model allows determination of the types and number of axons contributing to a compound nerve signal. Wikswo (1989) lists key references.

Magnetic field of a single muscle fiber. First measurements and a core conductor model

We present the first measurements of the magnetic field from a single muscle fiber of the frog gastrocnemius, obtained by using a toroidal pickup coil coupled to a room-temperature, low-noise amplifier. The axial currents associated with the magnetic fields of single fibers were biphasic and had peak-to-peak amplitudes ranging between 50 and 100 nA, depending primarily on the fiber radius. With an intracellular microelectrode, we measured the action potential of the same fiber, which allowed us to determine that the intracellular conductivity of the muscle fiber in the core conductor approximation was 0.20 +/- 0.09 S/m. Similarly, we found that the effective membrane capacitance was 0.030 +/- 0.011 F/m2. These results were not significantly affected by the anisotropic conductivity of the muscle bundle. We demonstrate how our magnetic technique can be used to determine the transmembrane action potential without penetrating the membrane with a microelectrode, thereby offering a reliable, stable, and atraumatic method for studying contracting muscle fibers.

First Magnetic Measurements Of Smooth Muscle In Vitro Using A High-resolution DC-squid Magnetometer

A four-channel, high-resolution, DC-Superconducting Quantum Interference Device (SQUID) magnetometer was used to measure biomagnetic fields resulting from currents in prairie dog (Cynomys ludovicianus) gastrointestinal smooth muscle. We report the first high-resolution magnetic measurements of gastrointestinal smooth muscle jn vitrQ. Example recordings of spike activity and slow waves are presented. Typical values for field strength, conduction velocity and frequency of slow waves were on the order of 20 to 40 pT, 2.5 cm/sec, and 15 pulses/min, respectively.

Abstract 3900: Alpha2beta1 integrin regulation of endothelial notch signaling in the retina.

Angiogenesis expands the vascular network during normal development and in response to angiogenic stress. Dysregulation of this dynamic process contributes to tumor progression and to the pathogenesis of many diseases. Evidence suggests that the alpha2beta1 integrin, a collagen and laminin receptor, plays an important role in angiogenesis. In the wound-healing and tumor microenvironment, deletion of the alpha2beta1 integrin has been reported to increase neoangiogenesis. In contrast, small molecule inhibitor (SMI) targeting of the alpha2 integrin blocks sprouting angiogenesis. To reconcile these divergent findings and gain a fuller understanding of alpha2beta1 integrin9s role in angiogenesis we turned to the retina. The retinal model is uniquely suited for angiogenesis investigations as the retinal vasculature develops postnatally in a 2-dimensional plane in a well-characterized manner. Evaluation of the alpha2-null retina reveals a constellation of defects and delays in vascular development, including delayed vessel outgrowth, and increased vessel irregularity and decreased plexus density at the vascular front. Additionally we determined that alpha2 integrin-deletion has a protective effect in an oxygen-induced retinopathy model of retinopathy of prematurity (ROP) in mice by inhibiting both hyperoxia-induced vaso-obliteration and hypoxia-induced pathologic neovascularization. Confirming this result, our analysis of human microarray data shows, for the first time, that preterm infants with lower ITGA2 expression are less likely to suffer from ROP. This work clarifies the role of alpha2beta1 integrin in sprouting angiogenesis and raises the intriguing possibility of alpha2 integrin targeted therapies for prevention of ROP. These changes are reminiscent of changes observed in other models with dysregulated notch signaling. Recent studies reported that the alpha2beta1 integrin regulates sprouting angiogenesis by inducing DLL4 in ‘tip cells’. We show, for the first time, notch induced downregulation of alpha2beta1 integrin expression in ‘stalk cells’. Together these results suggest that the alpha2beta1 integrin coordinates endothelial notch signaling by stabilizing tip-stalk status. The apparent discrepancy between the effects of the alpha2 integrin inhibition and integrin-deletion may reflect differences between acute and chronic upregulation of notch signaling. We propose that synergistic use of notch and alpha2 integrin targeted therapies may provide enhanced anti-tumor angiogenesis. Citation Format: Aasakiran Madamanchi, Megan Capozzi, Ling Geng, Zhengzhi Li, Zhonghua Zhang, Richard Friedman, Kent Dickeson, John Penn, Mary Zutter. Alpha2beta1 integrin regulation of endothelial notch signaling in the retina. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3900. doi:10.1158/1538-7445.AM2013-3900