Dielectric Relaxation Processes at Apical Membranes of Frog Skin: Changes of Capacitance and Sodium Ion Channel Density
Awayda, Mouhamed Sobhi
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https://hdl.handle.net/2142/72570
Description
Title
Dielectric Relaxation Processes at Apical Membranes of Frog Skin: Changes of Capacitance and Sodium Ion Channel Density
Author(s)
Awayda, Mouhamed Sobhi
Issue Date
1993
Doctoral Committee Chair(s)
Helman, S.,
Department of Study
Physiology and Biophysics
Discipline
Physiology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Cell
Biology, Animal Physiology
Biophysics, General
Abstract
Impedance measurements were carried out on isolated epithelia of frog skin, to characterize the capacitive properties of their apical plasma membranes and to determine whether vesicle fusion is the likely mechanism for the regulation of apical Na$\sp+$-channel density. Experiments were done in the presence of 100 $\mu$M amiloride and Na$\sp+$-free apical Ringer solution. This procedure reduced this epithelium to an electrical equivalent of an apical membrane capacitance in parallel with the extracellular shunt resistance. Under these conditions the capacitive properties of the apical membrane were invariably found to be frequency-dependent at low-audio and very low-audio frequencies. The capacitance spectrum of apical membranes exhibited multiple relaxation processes, where some membranes exhibited two relaxation processes while others exhibited three relaxation processes. Each relaxation process could be described by the Cole-Cole equation (1941), consisting of a static capacitance (C$\sb{\rm i}$), a relaxation frequency (f$\sb{\rm ri}$), and a power law-coefficient ($a\sb{\rm i}$). The dc or static capacitance of the apical membrane (C$\sb{\rm a}$(dc)) averaged 1.90 $\mu$F/cm$\sp2$, while the infinite frequency capacitance averaged 0.15 $\mu$F/cm$\sp2$ (n = 33). Assessment of the capacitance components during the control and some experimental periods, indicated the possibility of a dynamic equilibrium between these components. C$\sb{\rm a}$(dc) was assessed after stimulation of Na$\sp+$-channel density with forskolin (Els and Helman 1991) which caused a reversible increase of C$\sb{\rm a}$(dc) by $\approx$ 0.18 $\mu$F/cm$\sp2$. Inhibition of Na$\sp+$-channel density with quinine (Kizer, 1990) caused a decrease of C$\sb{\rm a}$(dc) by $\approx$ 0.32 $\mu$F/cm$\sp2$. The time course of the changes of C,(dc) with both forskolin and quinine followed exponential kinetics and were similar to the time course of changes of Na$\sp+$-channel density. The changes of C$\sb{\rm a}$(dc) with quinine, over a 2 hr period, were not mediated via any selective changes of C$\sb1$, C$\sb2$, and C$\sb{3/4}$. However, in 11 out of 19 tissues, where the exponential increases of C$\sb{\rm a}$(dc) caused by forskolin were paralleled by exponential increases of the components, the changes of C$\sb{\rm a}$(dc) were selectively mediated via changes of C$\sb2$. These data provide evidence that the regulation of Na$\sp+$-channel density occurs by vesicle fusion, and that the membranes of these vesicles are likely to contain only C$\sb2$.
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