The Journal of General Physiology, Vol 75, 531-551, Copyright © 1980 by The Rockefeller University Press
Water and electrolyte content of the myofilament phase in the chemically skinned barnacle fiber
JA Hinke
Muscle fibers from the giant barnacle, Balanus nubilus, were placed inside
the lumen of a porous glass capillary and equilibrated for 48 h in an
electrolyte solution containing 2% Tween. The glass capillary prevented the
chemically "skinned" fiber from swelling with a water content beyond 80%.
Isotope exchange studies using 22Na, 42K, and 36Cl indicated the existence
of an intermediate rate constant and compartment which varied with pH. This
intermediate rate was attributed to counter-ions and co-ions in the
myofilament phase. Analysis of the electrolyte composition of the fiber at
pH 8 predicts that the myofilaments contain about 0.3 of the fiber water,
and that a -15 mV Donnan potential exists at the myofilament surface. An
open-tipped (1- micrometer) microelectrode in the skinned fiber measured a
potential (similar in magnitude to the Donnan potential), which decreased
and reversed sign as the pH was lowered. The measured cation contents of
the fiber between pH 5 and 8 were found to be similar to the cation
contents predicted from the measured Donnan potentials. The net negative
charge of the myofilaments at pH 7.5 and at ionic strength 0.56 is
estimated to be 41 eq per 10(5) g of dry weight.
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