REGULATION OF THE SARCOPLASMIC-RETICULUM RYANODINE RECEPTOR BY INORGANIC-PHOSPHATE

To better understand the mechanisms regulating myoplasmic Ca2+ during muscle activity, we have examined the effect of inorganic phosphate (P(i)) on the ryanodine receptor (RyR) Ca2+ release channel of the sarcoplasmic reticulum (SR). We report that P(i) at concentrations reached in exercising skeletal muscle (3-30 mM) produced a dose-dependent stimulation of ryanodine binding to skeletal muscle SR. Ryanodine binding was increased by 84% in the presence of 30 mM P(i) with half-maximal stimulation at 4 mM P(i). In contrast to its effect on skeletal muscle SR, ryanodine binding to cardiac muscle SR was not stimulated by P(i) (3-30 mM). Stimulation of ryanodine binding to skeletal muscle SR was maximal in the presence of micromolar Ca2+ and was associated with an increased affinity of the RyR for ryanodine (K(d) = 204 nM in the absence, versus 107 nM in the presence of 10 mM P(i)). P(i) (10 mM) also increased the rate of Ca2+ release from Ca-45(2+)-filled skeletal muscle SR vesicles by 50% in the presence of micromolar Ca2+. Conversely, arsenate and sulfate (10 mM) had no effect on either ryanodine binding or Ca2+-induced Ca2+ release, demonstrating the specificity of the P(i) effect. Single-channel recordings of purified skeletal muscle SR RyR incorporated into planar lipid bilayers showed that addition of 10 mM P(i) to the cis chamber increased the open probability of the channel by 91%. These results demonstrate that concentrations of P(i) which occur in vivo during exercise significantly stimuIate the in vitro activity of the skeletal muscle RyR Ca2+ release channel.