The Journal of General Physiology, Vol 98, 249-263, Copyright © 1991 by The Rockefeller University Press
Separation of intramembrane charging components in low-calcium solutions in frog skeletal muscle
CL Huang
Physiological Laboratory, Cambridge, United Kingdom.
The inactivation of charge movement components by small (-100 to -70 mV)
shifts in holding potential was examined in voltage-clamped intact
amphibian muscle fibers in low [Ca2+], Mg(2+)-containing solutions. The
pulse protocols used both large voltage excursions and smaller potential
steps that elicited prolonged (q gamma) transients. Charge species were
distinguished through the pharmacological effects of tetracaine. These
procedures confirmed earlier observations in cut fibers and identified the
following new properties of the q gamma charge. First, q gamma, previously
defined as the tetracaine-sensitive charge, is also the component primarily
responsible for the voltage- dependent inactivation induced by conditions
of low extracellular [Ca2+]. Second, this inactivation separates a
transient that includes a "hump" component and which has kinetics and a
voltage dependence distinct from the monotonic decay that remains. Third, q
gamma, previously associated with delayed charge movements, can also
contribute significant charge transfer at early times. These findings
suggest that the parallel inhibition of calcium signals and charge
movements reported in low [Ca2+] solutions arises from influences on q
gamma charge (Brum et al., 1988a, b). They also reconcile reports that
implicate tetracaine-sensitive (q gamma) charge in excitation- contraction
coupling with evidence that early intramembrane events are also involved in
this process (Pizarro et al., 1989). Finally, they are relevant to
hypotheses of possible feedback or feed-forward roles of q gamma in
excitation-contraction coupling.
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