Magnesium permeability of sarcoplasmic reticulum vesicles monitored in terms of chlortetracycline fluorescence.

The Ca2+ and Mg2+ permeabilities of sarcoplasmic reticulum vesicles were measured by using a fluorescence chelate probe, chlortetracycline. The Mg2+ permeability was studied extensively. The data were analyzed under the assumption that the increment of fluorescence intensity is proportional to the amount of the divalent cations inside the vesicles. The validity of this assumption was confirmed by the tracer method with Ca2+. The presence of Ca2+ or Mg2+ had little effect on the Mg2+ efflux, but greatly affected the Ca2+ efflux. In the absence of extravesicular divalent cations, the permeabilities for Ca2+ and Mg2+ were almost the same, but the Ca2+ permeability was slightly reduced by the presence of Mg2+ and greatly reduced by Ca2+. In the absence of extravesicular divalent cations, the Ca2+ and Mg2+ permeabilities increased in parallel with increasing pH. For example, the permeation times for Mg2+ and Ca2+ were about 3 min at pH 6.5 and 20 s at pH 8.1 at room temperature. Kinetic analysis of the fluorescence change suggested the existence of two kinds of vesicles with different permeabilities.