Divergent effects of the malignant hyperthermia-susceptible Arg(615)-> Cysmutation on the Ca2+ and Mg2+ dependence of the RyR1

The sarcoplasmic reticulum (SR) Ca2+ release channel (RyR1) from malignant hyperthermia-susceptible (MHS) porcine skeletal muscle has a decreased sens itivity to inhibition by Mg2+. This diminished Mg2+ inhibition has been att ributed to a lower Mg2+ affinity of the inhibition (I) site. To determine w hether alterations in the Ca2+ and Mg2+ affinity of the activation (A) site contribute to the altered Mg2+ inhibition, we estimated the Ca2+ and Mg2affinities of the A- and I-sites of normal and MHS RyR1. Compared with norm al SR, MHS SR required less Ca2+ to half-maximally activate [H-3]ryanodine binding (K-A,K-Ca: MHS = 0.17 +/-0.01 muM; normal = 0.29 +/-0.02 muM) and m ore Ca2+ to half-maximally inhibit ryanodine binding (K-I,K-Ca: MHS = 519.3 +/- 48.7 muM; normal = 293.3 +/- 24.2 muM). The apparent Mg2+ affinity con stants of the MHS RyR1 A- and I-sites were approximately twice those of the A- and I-sites of the normal RyR1 (K-A,K-Mg: MHS = 44.36 +/-4.54 muM; norm al = 21.59 +/-1.66 muM, K-I,K-Mg: MHS = 660.8 +/- 53.0 muM; normal = 299.2 +/- 24.5 muM). Thus, the reduced Mg2+ inhibition of the MHS RyR1 compared w ith the normal RyR1 is due to both an enhanced selectivity of the MHS RyR1 A-site for Ca2+ over Mg2+ and a reduced Mg2+ affinity of the I-site.