Permeability of sarcoplasmic reticulum membrane. The effect of changed ionic environments on Ca2+ release.

Permeability properties and the effects of a changed membrane potential on Ca2+ release of sarcoplasmic reticulum vesicles of rabbit skeletal muscle were investigated by Millipore filtration. The relative permeability of sarcoplasmic reticulum to solutes determined under conditions of isotope exchange at equilibrium and/or under conditions of net flow of solute and water into the vesicles was as follows: sucrose, Ca2+, Mn2+ less than gluconate-, choline+, Tris+ less than methanesulfonate- less than urea, glycerol, K+, Na+,Li+, Cl-. Transient membrane potentials were induced by rapidly changing the ionic environment of the vesicles. Knowledge of the relative permeation rates of the above ions allowed prediction of the direction and extent of membrane polarization. Osmotic effects in the polarization measurements due to the rapid influx of solute and water into the vesicles were minimized by using media containing a fast (K+ or Cl-) and a relatively slow (gluconate- or choline+) penetrating ion. 45Ca2+ efflux from vesicles derived from different parts of the sarcoplasmic reticulum structure was not appreciably changed when vesicles were made more positive inside (choline chloride leads to potassium gluconate) or more negative inside (potassium gluconate leads to choline chloride). These studies suggest that part or all of the ion-induced changes in sarcoplasmic reticulum membrane permeability, previously interpreted to indicate "depolarization" -induced Ca2+ release, may be due to osmotic effects.