Phosphate ion channels in sarcoplasmic reticulum of rabbit skeletal muscle

1. Phosphate ions (P,) enter intracellular Ca stores and precipitate Ca. Si nce transport pathways for Pi across the membrane of intracellular calcium stores have not been identified and anion channels could provide such a pat hway, we have examined the Pi conductance of single anion channels from the sarcoplasmic reticulum (SR) of rabbit skeletal muscle using the lipid bila yer technique. 2. Two anion channels in skeletal muscle SR, the small conductance (SCI) an d big conductance (BCl) chloride channels, were both found to have a Pi con ductance of 10 pS in 50 mM P-i. The SCl channel is a divalent anion channel which can pass HPO42- as well as SO42- (60 pS in 100 nM free SO42-). The B Cl channel is primarily a monovalent anion channel. The SCl and BCl channel s are permeable to a number of small monovalent anions, showing minor selec tivity between Cl-, I- and Br- (Cl- > I- > Br-) and relative impermeability to cations and large polyatomic anions (Cs+, Na+, choline(+), Tris(+), Hep es(-) and CH3O3S-). 3. The P-i conductance of SCl and BCl channels suggests that both channel t ypes could sustain the observed P-i fluxes across the SR membrane. Comparis on of the blocking effects of the phosphonocarboxylic acids, ATP and DIDS, on the anion channels with their effects on P-i transport suggests that the SCl channel is the more likely candidate for the SR P-i transport mechanis m. 4. The SCl channel, with previously unknown function, provides a regulated path-way for P-i across the SR membrane which would promote P-i entry and t hereby changes in the rapidly releasable Ca2+ store during onset and recove ry from muscle fatigue. Anion channels may provide a pathway for P-i moveme nt into and out of Ca2+ stores in general.