Rotating-Field-Induced Rotation and Measurement of the Membrane Capacitance of Single Mesophyll Cells of Avena sativa

Abstract

Rotation of single cells (mesophyll protoplasts o f Avena sativa) induced by a planar, homogeneous rotating field has been observed at a frequency o f 2 0 -4 0 kHz (conductivity o f the external mannitol solution 6 x 10-5 ß -1 cm-1). This variation in optimum frequency is largely due to an inverse dependence on cell radius. Rotation direction is opposite to that o f the field, and can be reversed at will by reversing the field. The maximum speed o f cell spinning was a few cycles per second (and thus always much slower than that o f the field) and was proportional to the square o f the amplitude of the field.The rotation o f a single cell in a rotating field is expected on the basis o f the dipole-dipole theory developed by Holzapfel et al., (J. Membrane Biol. 67, 1 -1 4 (1982)), for multi-cell rotation. Measurements of the dependence of optimum applied field frequency on medium conductivity indicate that the dipole is generated by interfacial (Maxwell-Wagner) polarization. The required frequency is a linear function o f the conductivity of the external solution. This relationship is used to derive a value for the specific membrane capacitance. Further applications o f this technique for cell and membrane research are discussed. Introduction Materials and MethodsRotation o f a very small percentage o f cells in a linear non-uniform field (frequency range betw een 100 Hz and 120 MHz) has been described by several authors [1][2][3][4].Zimmermann et al. [4] showed that nearly all cells of a given species if in close proxim ity rotated when exposed to a linear alternating field having a characteristic narrow frequency range. F u rth er work by this group showed that the presence o f a m ini mum of two cells in close proxim ity is necessary and presented a theory to account for this based on the interaction of two induced dipoles within the field [5]. Implicit in the theory was the local production of a rotating field by the interaction o f the two induced dipoles.Rotation o f any single cell induced by the same characteristic range of frequencies as observed by Zimmermann et al. [4] has now been observed, provided that the field itself rotates rather than alternating along one axis. The experimental cham ber consists of four elec trodes mounted on a microscope slide as shown in Fig. 1. The essentials of the technique were proven in a chamber with 8 mm electrodes; a smaller chamber with 3 mm electrodes was used when higher field strengths (greater than 37.5 V cm -1) were required. The four electrodes are driven with equal voltages of four phases spaced by 90°, which are produced by the unit shown in Fig. 2. The production o f a rotating field in this manner is discussed in Appen dix A. The unit, using sinne wave drive from a Toellner TE7702 function generator, produces up to 15 volts peak per electrode from 5 Hz to 500 kHz. The function generator was calibrated against a frequency counter (Tektronix D C 503). Mesophyll cell protoplasts o f Avena sativa were prepared as by Hampp and Ziegler [6].Observations were confi...

AC electrokinetics: applications for nanotechnology

“…Teixeira-Pinto et al 1960] but was most fully explored by Arnold and Zimmermann [1982;1988], who established the technique for direct measurement of the value of K( by measuring the time taken for a particle to rotate. This application has little bearing to nanotechnology, not least due to the difficulty of seeing such small particles rotate, but electrorotation may be useful as a means of inducing rotation in molecular machinery.…”

Computer-aided analysis of conditions for optimizing practical electrorotation

“…Therefore we include this design amongst those recommended for general electrorotation measurements. In contrast to these the pin geometry (Arnold and Zimmermann 1982), often employed in experimentation due to its ease of construction, has a particularly poor value of Q owing to its small, non-uniform electric field. This would imply that it is less suitable for ROT experimentation than the other geometries discussed here.…”

Mapping the electrorotational torque in planar microelectrodes

“…To avoid such problems, researchers restrict their measurements to small regions of the electrode array. For example, within the type of quadrature electrode array described here, Arnold and Zimmermann (1982) studied only those particles "within the central volume", Zhou et al (1994), "within the central region between the electrode tips", and Chan et al (1997) restricted studies to those particles within a circle of radius defined by 1/3 of the distance from chamber centre to electrode tip. New systems are now being presented which "fix" particles in place during rotation measurement using laser tweezers (de Gasperis et al…”