Na+, K+, H+ and Cl- permeability properties of rabbit skeletal muscle sarcolemmal vesicles

The ion permeability properties of rabbit skeletal muscle sarcolemmal vesicles were investigated by means of radioisotope flux, membrane potential, and light-scattering measurements. An enriched sarcolemmal fraction was obtained from the 22-27% region of sucrose gradients after isopycnic centrifugation. The presence of contaminating sarcoplasmic reticulum was assessed with the use of a purified sarcoplasmic reticulum vesicle fraction. 22Na+, 86Rb+, 36Cl-, and [3H]sucrose flux measurements indicated that the sarcolemmal fraction possessed isotope spaces ranging between 1.5 and 4 microliters/mg protein. Membrane potential measurements using the voltage-sensitive fluorescent probe 3,3'-dipentyl-2,2'-oxadicarbocyanine iodide (diO-C5-(3)) indicated that sarcolemmal vesicles were impermeable to H+ and Na+ but that 10-15% of the vesicles were permeable to K+. Light-scattering measurements indicated a small fraction of sarcolemmal vesicles were permeable to both K+ and Cl-. Whether the low permeability of sarcolemmal vesicles to Na+, K+, and Cl- is the result of a low concentration of ion channels or the inactivation of these channels during isolation is at present uncertain.