Kinetic studies of calcium release from sarcoplasmic reticulum in vitro

Release of Ca2+ from a heavy fraction of rabbit skeletal muscle sarcoplasmic reticulum was triggered by several different methods: (a) increasing extravesicular [Ca2+] [( CaO2+] from about 0.1 microM to 10 microM), (b), adding caffeine, (c) adding quercetin, and (d) substituting a solution containing equimolar choline+ for K+-containing solution (depolarization-induced Ca2+ release). The maximal rate of Ca2+ release triggered by caffeine or quercetin in the presence of 12.5 microM [CaO2+] (21-25 nmol of Ca2+/mg/s) is similar to that of the depolarization-induced Ca2+ release (19 nmol of Ca2+/mg/s), as determined by stopped flow spectrometry of changes in [CaO2+] with arsenazo III. The release is transient and all of the released Ca2+ is reaccumulated. The rates of Ca2+ release triggered by caffeine, quercetin, or membrane depolarization sharply decrease at high [CaO2+], suggesting a negative feedback effect of the released Ca2+. Inhibition of the release pathway allows the sarcoplasmic reticulum to reaccumulate Ca2+. The rate of Ca2+ release triggered by caffeine or quercetin, but not that triggered by membrane depolarization, is also reduced upon decreasing [CaO2+] to the submicromolar range. Passive efflux of intravesicular Ca2+ in solutions containing lower [CaO2+] in the absence of Mg.ATP is attenuated at about the same time (congruent to 1 min) regardless of the amounts of Ca2+ released, indicating that the opened Ca2+ channels close spontaneously. These results suggest that kinetically identical channels are responsible for Ca2+ release independent of the methods of triggering and this in vitro release is consistent with the physiological mechanism both in terms of the rapidity and the reversibility of Ca2+ release.