Monovalent ion and calcium ion fluxes in sarcoplasmic reticulum

The ion permeability of sarcoplasmic reticulum vesicles from skeletal and heart muscle has been characterized by radioisotope flux, osmotic and membrane potential measurements, and by incorporating vesicles into planar phospholipid bilayers. The sarcoplasmic reticulum membrane is uniquely permeable to various biologically relevant monovalent ions. At least two and possibly three separate passive permeation systems for monovalent ions have been identified: 1) a K+, Na+ channel, 2) an anion channel, and 3) a H+ (OH-) permeable pathway which may or may not be synonymous with the anion channel. A possible physiological function of these monovalent ion permeation systems is to permit rapid movement of K+, Na+, H+ and Cl- across the membrane counter electrogenic Ca2+ fluxes during Ca2+ release and uptake by sarcoplasmic reticulum.