Volatile anaesthetic effects on calcium conductance of planar lipid bilayers formed with synthetic lipids or extracted lipids from sarcoplasmic reticulum.

Volatile anaesthetics are known to increase leakage of calcium from the light fraction of skeletal sarcoplasmic reticulum (L-SR) which has no calcium release channels. To explore the role of the lipid environment, we have examined the effect of volatile anaesthetics on calcium conductance (gCa) of lipid membranes. Planar lipid bilayers were formed with a mixture of synthetic phospholipids and cholesterol, resembling the composition of SR membranes, or with lipids extracted from skeletal L-SR, gCa was estimated by calculating the calcium transference number (tCa) using diffusion potential measurements. Membranes formed with L-SR-extracted lipids had a higher gCa than membranes formed with synthetic lipids. Volatile anaesthetics increased total conductance and gCa in a dose-dependent manner, but did not affect tCa or membrane specific capacitance. In membranes formed with L-SR-extracted lipids, isoflurane induced the largest increase in gCa (1260 (SEM 304) % increase, n = 4, 0.94 mmol litre-1), followed by enflurane (264 (75)%, n = 5, 1.88 mmol litre-1) and halothane (53 (33)%, n = 5; 1.54 mmol litre-1). In membranes formed with synthetic lipids, volatile anaesthetic-induced increases in gCa followed the same trend but were larger. Volatile anaesthetics increased gCa without changing the ionic selectivity of membranes. However, the magnitude of the increase in gCa in the presence of volatile anaesthetics cannot account for the previously observed calcium leakage from L-SR vesicles. Therefore, the volatile anaesthetic-induced increase in calcium leakage in L-SR vesicles must be mediated via other pathways involving membrane proteins.