J. Biol. Chem., Vol. 259, Issue 2, 1073-1081, 01, 1984
Spontaneous calcium release from sarcoplasmic reticulum. Assessment of other ionic influences
RD Mitchell, P Palade and S Fleischer
A form of spontaneous Ca2+ release from purified light sarcoplasmic
reticulum has been recently described (Palade, P., Mitchell, R. D., and
Fleischer, S. (1983) J. Biol. Chem. 258, 8098-8107). It is characterized by
rapid Ca2+ efflux (1-10 mumol/min X mg protein) which begins only after a
delay following preloading and depletion of extravesicular Ca2+. In the
present study, the influences of a number of ionic factors modulating the
spontaneous calcium release phenomenon are described. The divalent metal
ions Mn2+ and Sr2+ and higher external [Mg2+] inhibit release. Mn2+ appears
to inhibit from outside, whereas Sr2+ must be taken up to inhibit.
Decreasing the phosphate concentration during preloading eliminates
spontaneous release, but it can be partially restored with salts of other
nonprecipitating anions. No such release was obtained with other
Ca2+-precipitating anions, i.e. pyrophosphate, oxalate, and fluoride. The
release characteristics are strongly dependent on the monovalent cation
present. Spontaneous Ca2+ release is optimal at pH 6.8-7.0 and decreases
sharply at higher and lower pH. Ca2+ release can be enhanced by addition,
during the lag period, of concentrated salt solutions consisting of more
permeant cations than anions, which may indicate that an inside positive
membrane potential enhances the rate of calcium release. Spontaneous Ca2+
release can take place in the presence of sucrose, and Ca2+ efflux is not
accompanied by a corresponding efflux of preloaded sucrose, demonstrating
that the vesicles remain sealed during spontaneous release. The spontaneous
Ca2+ release process is distinct from several other forms of Ca2+ release
from sarcoplasmic reticulum.
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