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, 5lmGd 3+ increased the Kd of the [ 3 H]ry-anodine binding of the vesicles from nmto nmwhileBmax, refer-ring to the binding capacity, was not affected significantly. In the presence of 18 nm[ 3 H]ryanodine and 100lmfree Ca 2+ ,Gd 3+ inhibited the binding of the radiolabeled ryanodine with an apparentKd value of a Hill coefficient of . .
