AJP - Cell Physiology, Vol 244, Issue 3 297-C302, Copyright © 1983 by American Physiological Society
Effects of calcium on membrane potential and sodium influx in barnacle muscle fibers
S. S. Sheu and M. P. Blaustein
The influence of internal and external Ca2+ on membrane potential and 22Na
influx were tested in internally perfused giant barnacle muscle fibers. The
fibers depolarized by about 2-3 mV, and Na+ influx increased when external
Ca2+ was removed. These effects were inhibited and reversed by adding 2 mM
La3+ externally but not by tetrodotoxin (TTX). Ca2+ channel blockers did
not prevent the depolarization. Increasing internal free Ca2+ ([Ca2+]i)
from 10(-7) to 10(-5) M also stimulated Na+ influx and depolarized the
fibers by a few millivolts. Neither external La3+ nor TTX prevented the
effects of raising [Ca2+]i; however, internal tetrabutylammonium ions
depolarized the fibers and attenuated the internal Ca2+-dependent effects.
These data are consistent with the idea that removal of external Ca2+
activates a La3+-sensitive channel that is permeable to Na+; raising
[Ca2+]i activates a La2+-insensitive, Na+-permeable channel that may be
similar to the internal Ca2+-activated nonselective cation channels
observed in cardiac muscle. The results demonstrate that all Na+ (and Ca2+)
fluxes that do not contribute to Na-Ca exchange must be carefully
identified before the exchange stoichiometry can be determined from Na+ and
Ca2+ flux measurements.
