Differential Effects Of Temperature And Lipids On The Gating Of Ryr And Sr K+ Channels

The ryanodine receptor (RyR) is the pathway for release of the sarcoplasmic reticulum (SR) Ca2+ required for cardiac and skeletal muscle contraction. The opening of SR K+ channels is thought to support this process. Movements of the RyR and SR K+ channel gates required for channel opening and closing will be dependent on the biophysical properties of the SR membrane which, in turn, will be affected by factors such as temperature and membrane composition. Cardiac RyR channel opening is hypersensitive to temperature changes (Sitsapesan et al., 1991, J Physiol., 434:469-488) although single-channel recordings are invariably performed at room temperature. We have therefore incorporated mammalian cardiac and skeletal RyR and SR K+ channels into bilayers under voltage-clamp conditions to investigate the effects of changing temperature and membrane lipid composition on single-channel behaviour.Changing bilayer lipid composition from phosphatidylethanolamine (PE) only, to a more physiological composition containing a 5:4:1 ratio of PE: phosphatidylserine (PS): phosphatidylcholine (PC) at 23oC, significantly increased the open probability (Po) and single channel conductance of both channels. For example, RyR Po increased from 0.06±0.03 to 0.26±0.08 (n=8; SEM; Pu003c0.05). With bilayer lipid ratio of 5 PE: 4 PS: 1 PC, raising temperature from 10oC to 37oC caused a graded decrease in RyR Po. In contrast, SR K+ channels exhibited non-linear changes in Po with a minimum at approximately 23oC. Our results demonstrate the importance of investigating intracellular ion channel function at physiological temperatures and show that changes in SR membrane lipid composition markedly influence the behaviour of ion channels in the SR.Supported by the British Heart Foundation.