The Ca2+ permeability of sarcoplasmic reticulum vesicles I. Ca2+ outflow in the non-energized state of the calcium pump |
| |
Authors: | Ulrik Gerdes Atif M. Nakhla Jesper V. Møller |
| |
Affiliation: | Institute of Medical Biochemistry, Aarhus University, 8000 Aarhus C Denmark |
| |
Abstract: | Passive Ca2+ permeability of sarcoplasmic reticulum vesicles has been studied after maximal loading with Ca2+ (150–200 nmol/mg protein) in the presence of Ca2+, MgATP and an ATP generating system of limited capacity. Outflow of accumulated Ca2+ in the non-energized state of the system was studied by depletion of the medium of one of the substrates, either MgATP (by complete consumption) or Ca2+ (by complexation with EGTA). It was found that Ca2+ outflow under these conditions is relatively slow and independent of the medium concentration of Ca2+ (5·10?9–5·10?5 M) or MgATP (0.7–730 μM). Outflow curves were steep at the beginning of the outflow phase (30–60 nmol/min per mg protein), and outflow proceeded at a much lower rate below 100 nmol Ca2+/mg protein. Outflow could be completely inhibited by La3+. The Ca2+ release curves are not compatible with simple diffusion, and cannot be accounted for by Ca2+ binding inside the vesicles. Neither are our observations consistent with permeation mediated via the Ca2+ translocation sites involved in active transport. We suggest that non-energized Ca2+ outflow may proceed by a process of ion-exchange through negatively charged, water-filled channels in the membrane, the properties of which are altered by a high intravesicular concentration of Ca2+. |
| |
Keywords: | ATP depletion Sarcoplasmic reticulum vesicle (Rabbit skeletal muscle) |
本文献已被 ScienceDirect 等数据库收录! |