Article Information

• PDF (1129 kb)

PubMed

• Articles by Bert A. Mobley
• Articles by G. Eidt

Related Articles

• Frequency-Dependent Capacitance of the Apical Membrane of Fr...

  Frequency-Dependent Capacitance of the Apical Membrane of Frog Skin: Dielectric Relaxation Processes Biophysical Journal, Volume 76, Issue 1, 1 January 1999, Pages 219-232 Mouhamed S. Awayda, Willy Van Driessche and Sandy I. Helman Abstract Impedance analysis of the isolated epithelium of frog skin (northern ) was carried out in the frequency range between 0.1Hz and 5.5kHz while Na transport was abolished. Under these conditions, the impedance is determined almost completely by the dielectric properties of the apical membranes of the cells and the parallel shunt resistance. The modeling of the apical membrane impedance function required the inclusion of dielectric relaxation processes as originally described by Cole and Cole (1941. 9:341–351), where each process is characterized by a dielectric increment, relaxation frequency, and power law dependence. We found that the apical plasma membrane exhibited several populations of audio frequency dielectric relaxation processes centered at 30, 103, 2364, and 6604Hz, with mean capacitive increments of 0.72, 1.00, 0.88, and 0.29F/cm, respectively, that gave rise to dc capacitances of 1.95±0.06F/cm in 49 tissues. Capacitance was uncorrelated with large ranges of parallel shunt resistance and was not changed appreciably within minutes by K depolarization and hence a decrease in basolateral membrane resistance. A significant linear correlation existed between the dc capacitance and Na transport rates measured as short-circuit currents (=0.028 +1.48; between 4 and 35A/cm) before inhibition of transport by amiloride and substitution of all Na with NMDG (-methyl--glucamine) in the apical solution. The existence of dominant audio frequency capacitive relaxation processes complicates and precludes unequivocal interpretation of changes of capacitance in terms of membrane area alone when capacitance is measured at audio frequencies. Abstract | Full Text | PDF (270 kb)

• Continuous Vesicle Cycling in the Synaptic Terminal of Retin...

  Continuous Vesicle Cycling in the Synaptic Terminal of Retinal Bipolar Cells Neuron, Volume 17, Issue 5, 1 November 1996, Pages 957-967 Leon Lagnado, Ana Gomis and Christy Job Summary Endocytosis and exocytosis were investigated in the synaptic terminal of retinal bipolar cells by monitoring the uptake and loss of the fluorescent dye FM1-43. Depolarization in the presence of Ca stimulated a continuous cycle of exocytosis and endocytosis that was approximately balanced at rates up to 3800 vesicles per s. Vesicles became available for exocytosis within 1 min of endocytosis, and about 700,000 releasable vesicles were specifically localized to a region within 2 μm of the plasma membrane. Release of caged Ca using NP–EGTA while simultaneously monitoring cytosolic Ca with Fura-2 indicated that continuous exocytosis was stimulated by sub-micromolar levels of Ca. It has been suggested that the ribbon synapse of bipolar cells only supports transient exocytosis, but our results demonstrate that this synapse is specialized for the continuous secretion of neurotransmitter. Summary | Full Text | PDF (460 kb)

• The concept of chemical capacitance, A critique

  The concept of chemical capacitance, A critique Biophysical Journal, Volume 33, Issue 2, 1 February 1981, Pages 233-242 H.W. Trissl Abstract The concept of chemical capacitance as introduced by Hong and Mauzerall (Proc. Natl. Acad. Sci. U.S.A. 1974. 71:1564) is critically reexamined. This novel capacitance was introduced to explain the time-course of flash-induced photocurrents observed in lipid bilayer membranes containing porphyrins. According to Hong and Mauzerall, the chemical capacitance results from a combination of three fundamental capacitances: the geometric membrane capacitance and the two interfacial double layer capacitances. The concept of chemical capacitance is questioned for the following reasons: (i) The system analysis is insufficiently determinate. (ii) The measured chemical capacitance is approximately 0.16% of that predicted by the theory. (iii) The fact that only 20% of the membrane area is illuminated was not considered in the analysis. The latter point offers an alternative explanation of the capacitance in question: this capacitance may reflect that fraction of the total membrane capacitance that is photochemically active. If so, the concept of chemical capacitance lacks general significance. Abstract | PDF (596 kb)

• …more

Abstract

The transverse electrical impedance of single frog skeletal muscle fibers was measured at 31 frequencies that ranged from 1 to 100,000 Hz. Each fiber was bathed entirely in Ringer's solution, but it was positioned so that a central length of 5 mm was in a hollow plastic disk and was electrically isolated from the ends of the fiber. The diameter of the segment of the fiber in the disk was measured and then the segment was pressed from opposite sides by two insulating wedges. Electrical current was passed transversely through the segment between two platinum-platinum black electrodes that were located in the pools of Ringer's solution within the disk. The results were corrected for stray parallel capacitance, series resistance of the Ringer's solution between the fiber and the electrodes, parallel shunt resistance around the fiber, and the phase shift of the measuring apparatus. A nonlinear least-squares routine was used to fit a lumped equivalent circuit to the data from six fibers. The equivalent circuit that was chosen for the fibers contained three parallel branches; each branch was composed of a resistor and a capacitor in series. The model also included a seventh adjustable parameter that was designed to account for the degree of compression of the fibers by the insulating wedges. The branches of the equivalent circuit were assumed to represent the electrical properties of: (a) the myoplasm in series with the membrane capacitance that was exposed directly to the pools of Ringer's solution; (b) the capacitance and series resistance of the transverse tubules that were exposed directly to the pools of Ringer's solution; (c) the membrane capacitance in series with the shunt resistance between the fibers and the insulating wedges. The results gave no indication that current entered the sarcoplasmic reticulum.