The Effect of Volatile Anaesthetics on the Cardiac Ryanodine Receptor

Abstract

Although volatile anaesthetics serve a crucial role in preventing pain they continue to have side effects such as their ability to excite Ca2+ release from the sarcoplasmic reticulum (SR) via the ryanodine receptor calcium release channels (RyR). Here we report the first detailed investigation on the effects of volatile anaesthetics on the function of cardiac RyRs.RyRs were isolated from sheep hearts and incorporated into artificial lipid bilayers and subjected to single channel recording. Clinical doses of halothane and isoflurane increased RyR activation by luminal and cytoplasmic Ca2+ by increasing channel open time and opening frequency. The Ka's for halothane and isoflurane were 1 mM and 3 mM, respectively. However, the maximal effect of halothane (5-fold increase in placePo) was ∼3-fold larger than that for isoflurane. These agents activated RyRs by interacting with cytoplasmic domains distinct from the ATP activating sites.The effects of halothane on RyR regulation by luminal and cytoplasmic Ca2+ and Mg2+ were accurately fitted by a luminal-triggered calcium feedthrough model involving four Ca2+ sensing mechanisms on each RyR subunit; two activation sites (luminal L-site, 40 μM affinity; cytoplasmic A-site, 1 μM affinity) and two cytoplasmic inactivation sites (I1-site, 10 mM affinity; I2-site, 1 μM affinity). Halothane did not appear to alter the ion binding affinities for these sites. Rather, it increased channel opening rate and decreased the channel closing rate associated with Ca2+ binding to the two activation sites.The potentiating effect of halothane on luminal Ca2+ activation of cardiac RyRs was due to 1) an increase on opening frequency because of synergistic actions of the luminal and cytoplasmic (L and A) sites and 2) an increase in open time because of Ca2+ feedthrough to the A-site.