Differential effects of general anesthetics on the quaternary structure of the Ca-ATPases of cardiac and skeletal sarcoplasmic reticulum

The effects of the general anesthetics hexanol, halothane, and diethyl ether on Ca-ATPase activity and on the oligomeric state of the Ca-ATPase of sarcoplasmic reticulum (SR) from cardiac and skeletal muscle were investigated. The effects of these general anesthetics on Ca-ATPase activity were similar in cardiac and skeletal SR and were characterized by stimulation of Ca-ATPase activity at lower concentrations of anesthetics and inhibition at higher concentrations. The distribution of the Ca-ATPase among its oligomeric states was estimated from the time-resolved phosphorescence anisotropy (TPA) decay of SR in which Ca-ATPase was covalently labeled with erythrosin isothiocyanate (ERITC) or with erythrosin iodoacetamide (ERIA). In contrast to the similar responses of Ca-ATPase activity, there were marked differences in the responses to general anesthetics of the TPA decay between cardiac and skeletal SR. In cardiac SR hexanol halothane, and diethyl ether caused pronounced increases in the limiting anisotropy at very long times (r∞), which indicate increases in the fraction of oligomers too large to rotate on the millisecond time scale of the experiments. In skeletal SR, by contrast, there were no significant changes in r∞ in response to the three general anesthetics. This difference between cardiac and skeletal SR in response to general anesthetics is not due to the presence of phospholamban in cardiac SR, since SR from AT-1 cells, which have the SERCA2a isoform of Ca-ATPase, but only trace levels of phospholamban, have increases in r∞ in response to the general anesthetics that resemble those in cardiac SR. Experiments with cardiac SR labeled with ERIA give similar results, showing that the results with ERITC are not an artifact of the labeling procedure. Increasing the ionic strength with LiCI diminished the proportion of large immobile oligomers of cardiac Ca-ATPase under control conditions but enhanced the formation of large oligomers in response to hexanol.