Anesthetic alteration of ryanodine binding by cardiac calcium release channels

Differential cardiac contractile depression by volatile anesthetics is well documented, and evidence points to differing actions on the myocardial sarcoplasmic reticulum (SR). Since the Ca(2+)-release channel (CaRC) of the SR binds ryanodine with high-affinity when opened by micromolar Ca2+ concentrations, ryanodine binding to cardiac SR membrane vesicles was employed as an assay of anesthetic modulation of CaRC activity. Canine ventricle was homogenized, centrifuged preparatively and then differentially on a sucrose gradient. A fraction enriched with CaRCs was defined by: the presence of a approximately 450 kDa protein consistent with CaRC; approximately 3-fold enhancement of vesicular 45Ca2+ uptake by ruthenium red; Ca(2+)-activated [3H]ryanodine binding. Specific binding of 10 nM ryanodine was activated by > 0.5 microM Ca2+ and was maximal at approximately 6 pmol/mg protein in > or = 20 microM Ca2+. Halothane (1.5%), but not isoflurane, shifted the Ca(2+)-dependence of ryanodine binding to lower [Ca2+]. With submaximal activation by 5 microM Ca2+, 1.5% and 0.75% halothane enhanced binding of 10-80 microM ryanodine, while 2.5% isoflurane and 3.5% enflurane did not. A plot of bound/free vs. bound ryanodine suggests that halothane causes a dose-dependent increase in ryanodine binding to a high-affinity site, while isoflurane has no such action. In intact myocardium, this effect will decrease Ca2+ retention in the SR so that less Ca2+ will be available to activate contractions, consistent with halothane's depressant action.