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ABSTRACT
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.
Key Words: excitation–contraction coupling • Ca release • skeletal muscle • sarcoplasmic reticulum
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