The Journal of General Physiology, Vol 99, 531-544, Copyright © 1992 by The Rockefeller University Press
Intramembrane charge movements in frog skeletal muscle in strongly hypertonic solutions
CL Huang
Physiological Laboratory, Cambridge, United Kingdom.
Intramembrane charge movements were studied in intact, voltage-clamped frog
(Rana temporaria) skeletal muscle fibers in external solutions made
increasingly hypertonic by addition of sucrose. The marked dependence of
membrane capacitance on test potential persisted with increases in
extracellular sucrose concentration between 350 and 500 mM. Charge
movements continued to show distinguishable early monotonic (q beta) decays
and the strongly voltage-dependent delayed (q gamma) charging phases
reported on earlier occasions. In contrast, a further increase to 600 mM
sucrose abolished the most steeply voltage-sensitive part of the membrane
capacitance. It left a more gradual variation with potential that closely
resembled the function that resulted when q gamma charge was abolished by
tetracaine in the presence of 500 mM sucrose. Charging transients were now
simple monotonic (q beta) decays and lacked delayed (q gamma) transients.
Furthermore, tetracaine (2 mM) altered neither the kinetic nor the
steady-state features of the charge left in 600 mM sucrose. However, Ca2+
current activation in the same fibers persisted through such tonicity
increases under identical conditions of temperature, external solution, and
holding voltage. Tonicity changes thus accomplish an independent separation
of q gamma and q beta charge as defined hitherto through their tetracaine
sensitivity. Their effects on q gamma charge correlate with earlier
observations of osmotic conditions on delta[Ca2+] signals (1987. J.
Physiol. (Lond.) 383:615-627.) and the parallel effects of other agents on
excitation-contraction coupling and q gamma charge. In contrast, they
suggest that Ca2+ current activation does not require q gamma charge
transfer whether by itself or as part of the excitation- contraction
coupling process.
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati What's this?
