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J. Gen. Physiol.,
Volume 112, Number 2, August 1, 1998 161-179
From the Département de physiologie et biophysique, Université de Sherbrooke Faculté de Médecine, Sherbrooke, Québec J1H5N4,
Canada
Cut muscle fibers from Rana temporaria (sarcomere length, 3.5-3.9 µm; 14-16°C) were mounted in a
double Vaseline-gap chamber and equilibrated with an external solution that contained tetraethyl ammonium-
gluconate and an internal solution that contained Cs as the principal cation, 20 mM EGTA, and 0 Ca. Fibers were
stimulated with a voltage-clamp pulse protocol that consisted of pulses to 
70, 65, 60, 45, and 20 mV, each
separated by 400-ms periods at 90 mV. The change in total Ca that entered into the myoplasm (
[CaT]) and the
Ca content of the SR ([CaSR]) were estimated with the EGTA/phenol red method (Pape, P.C., D.-S. Jong, and
W.K. Chandler. 1995. J. Gen. Physiol. 106:259-336). Fibers were stimulated with the pulse protocol, usually every
5 min, so that the resting value of [CaSR] decreased from its initial value of 1,700-2,300 µM to values near or below
100 µM after 18-30 stimulations. Three main findings for the voltage pulses to 70, 65, and 60 mV are: (a)
the depletion-corrected rate of Ca release (release permeability) showed little change when [CaSR] decreased
from its highest level (>1,700 µM) to ~1,000 µM; (b) as [CaSR] decreased below 1,000 µM, the release permeability increased to a maximum level when [CaSR] was near 300 µM that was on average about sevenfold larger than
the values observed for [CaSR] > 1,000 µM; and (c) as [CaSR] decreased from ~300 µM to <100 µM, the release
permeability decreased, reaching half its maximum value when [CaSR] was ~110 µM on average. It was concluded
that finding b was likely due to a decrease in Ca inactivation, while finding c was likely due to a decrease in Ca-induced Ca release.
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