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

Abstract

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 μl/mg protein. Membrane potential measurements using the voltage-sensitive fluorescent probe 3,3′-dipentyl-2,2′-oxadicarbocyanine iodide (diO-C 5-(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.