Original Publication Date
The Biophysical Journal
DOI of Original Publication
Date of Submission
It has been suggested that potentials measured with conventional microelectrodes in chemically or mechanically skinned muscle fibers arise from a Donnan equilibrium due to myofilament fixed charges. This hypothesis was tested in mechanically skinned frog (Rana pipiens) semitendinosus fibers by measuring the distribution potential (Ed) between fiber and bath with 3 M KCI-filled microelectrodes and the K+ activity gradient (a'K/a) with K+ ion-selective microelectrodes (KISE). If skinned fibers are a Donnan system, Ed should become more positive as pH is decreased, altering the fixed charge on the myofilaments. Consistent with this expectation, Ed was -4.4, -0.6, and +4.8 mV in ATP-containing solutions and -6.5, -2.2, and +8.4 mV in ATP-free solutions at pH 7, 6, and 5, respectively. Donnan equilibrium also requires that all mobile ionic species be in electrochemical equilibrium. In ATP-containing solutions, this was true for K+ at pH 7. At pH 5, however, KISE indicated that K+ was not in equilibrium; average Ed was 5.9 mV positive to the K+ equilibrium potential, and a'K/ao was 1.04, while the Donnan prediction was 0.83. In contrast, KISE measurements in ATP-free solutions indicated that K+ was in equilibrium at all pH studied. Skinned fibers in ATP-containing media are not equilibrium systems because ATPase reactions occur. Under our conditions, frog myofibrils hydrolyze 0.4 and 0.08 ,mol ATP/min - mg myofibrillar protein at pH 7 and 5, respectively. It is suggested that in the presence of ATP, Ed is a superposition of Donnan and diffusion potentials, the latter arising from differences in the mobilities of anionic substrate and products that diffuse through the charged myofilament lattice. A coupling to diffusion of K+, the predominant counter ion, is required for macroscopic electroneutrality. This coupling may be the origin of the nonequilibrium K+ distribution.
From The Biophysical Journal, Godt, R.E. and Baumgarten, C.M., Potential and K+ activity in skinned muscle fibers. Evidence against a simple Donnan equilibrium, Vol. 45, Page 375. Copyright © 1984 The Biophysical Society. Published by Elsevier Inc. Reprinted with permission.
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VCU Physiology and Biophysics Publications