Permeability of reconstituted sarcoplasmic reticulum vesicles. Reconstitution of the K+, Na+ channel

Permeability properties of reconstituted rabbit skeletal muscle sarcoplasmic reticulum vesicles were characterized by measuring efflux rates of [3H]inulin, [3H]choline+, 86Rb+, and 22Na+, as well as membrane potential changes using the voltage-sensitive probe, 3,3'-dipentyl-2,2'-oxacarbocyanine. Native vesicles were dissociated with deoxycholate and were reconstituted by dialysis. Energized Ca2+ accumulation was partially restored. About 1/2 of the reconstituted vesicles were found to be 'leaky', i.e., permeable to choline+ of Tris+ but not to inulin. The remaining reconstituted vesicles were 'sealed', i.e., impermeable to choline+, Tris+ and inulin. Sealed reconstituted vesicles could be further subdivided according to their K+, Na+ permeability. About 1/2, previously designated Type I, were readily permeable to K+ and Na+, indicating the presence of the K+, Na+ channel of sarcoplasmic reticulum. The remaining sealed vesicles (Type II) formed a permeability barrier to K+ and Na+, suggesting that they lacked the K+, Na+ channel. These studies show that the K+, Na+ channel of sarcoplasmic reticulum can be solubilized with detergent and reconstituted with retention of activity. Furthermore, our results suggest that part or all of the decreased Ca2+-loading efficiency of reconstituted vesicles may be due to the presence of a significant fraction of leaky vesicles.