Effect of gadolinium on the ryanodine receptor/sarcoplasmic reticulum calcium release channel of skeletal muscle

The effect of gadolinium ions on the sarcoplasmic reticulum (SR) calcium release channel/ryanodine receptor (RyR1) was studied using heavy SR (HSR) vesicles and RyR1 isolated from rabbit fast twitch muscle. In the [(3)H]ryanodine binding assay, 5 microM Gd(3+) increased the K(d) of the [(3)H]ryanodine binding of the vesicles from 33.8 nM to 45.6 nM while B(max), referring to the binding capacity, was not affected significantly. In the presence of 18 nM[(3)H]ryanodine and 100 microM free Ca(2+), Gd(3+) inhibited the binding of the radiolabeled ryanodine with an apparent K(d) value of 14.7 microM and a Hill coefficient of 3.17. In (45)Ca(2+) experiments the time constant of (45)Ca(2+) efflux from HSR vesicles increased from 90.9 (+/- 11.1) ms to 187.7 (+/- 24.9) ms in the presence of 20 microM gadolinium. In single channel experiments gadolinium inhibited the channel activity from both the cytoplasmic (cis) (IC(50) = 5.65 +/- 0.33 microM, n(Hill) = 4.71) and the luminal (trans) side (IC(50) = 5.47 +/- 0.24 microM, n(Hill) = 4.31). The degree of inhibition on the cis side didn't show calcium dependency in the 100 microM to 1 mM Ca(2+) concentration range which indicates no competition with calcium on its regulatory binding sites. When Gd(3+) was applied at the trans side, EGTA was present at the cis side to prevent the binding of Gd(+3) to the cytoplasmic calcium binding regulatory sites of the RyR1 if Gd(3+) accidentally passed through the channel. The inhibition of the channel did not show any voltage dependence, which would be the case if Gd(3+) exerted its effect after getting to the cis side. Our results suggest the presence of inhibitory binding sites for Gd(3+) on both sides of the RyR1 with similar Hill coefficients and IC(50) values.