Chloride-induced release of actively loaded calcium from light and heavy sarcoplasmic reticulum vesicles

Kevin P Campbell; Adil E Shamoo

doi:10.1007/BF01875378

Light and heavy sarcoplasmic reticulum vesicles (LSR, HSR) isolated from rabbit leg muscle have been used in a study of chloride-induced Ca2+ release. The biochemical and morphological data indicate that LSR is derived from the longitudinal reticulum and HSR is derived from the terminal cisternae of the sarcoplasmic reticulum. LSR and HSR were both able to accumulate Ca2+ in the presence of ATP to amounts greater than 100 nmol Ca2+/mg of protein in less than 1 min. LSR and HSR each had a biphasic time course of Ca2+ uptake. The initial uptake was followed by a rapid release, after approximately 1 min, of 30–40% of the accumulated Ca2+, which was then followed by a slower phase of Ca2+ accumulation. Ca2+ taken up by the SR vesicles could be released from both the LSR and HSR by changing the anion outside the vesicles from methanesulfonate to chloride. Due to the difference in permeability between methanesulfonate and chloride, this change should result in a decreased positivity inside the vesicles with respect to the exterior. It could also result in osmotic swelling of the vesicles. Changing the ionic medium from chloride to methanesulfonate caused no release of Ca2+. The amount of accumulated Ca2+ released in 6 sec by changing the anion outside the vesicles from methanesulfonate to chloride was 30–35 nmol/mg membrane protein for LSR and HSR, respectively. Osmotic buffering with 200MM sucrose caused a slight inhibition of chloride-induced Ca2+ release from HSR (17%→15%) but it greatly reduced the release of Ca2+ from LSR (32%→15%). The specificity of Ca2+ release was measured using SR vesicles which were passively loaded with 10MM 22Na+. LSR released five times more22Na+ than HSR under same conditions as chloride-induced Ca2+ release occurred. Na dantrolene (20 μm) had no effect on the release of Ca2+ from LSR but it inhibited the chloride-induced Ca2+ release from HSR by more than 50%. Na dantrolene also increased the Ca2+ uptake in the HSR by 20% while not affecting LSR Ca2+ uptake. Our results indicate the presence of a chloride-induced, Na dantrolene inhibited, Ca2+ release from HSR, which is not due to osmotic swelling.

Chloride-induced release of actively loaded calcium from light and heavy sarcoplasmic reticulum vesicles

View Online

Abstract

Details

Metrics