LIGAND-GATED CHANNEL OF THE SARCOPLASMIC-RETICULUM CA2+ TRANSPORT ATPASE

In resting muscle, cytoplasmic Ca2+ concentration is maintained at a l ow level by active Ca2+ transport mediated by the Ca2+ ATPase from sar coplasmic reticulum. The region of the protein that contains the catal ytic site faces the cytoplasmic side of the membrane, while the transm embrane helices form a channel-like structure that allows Ca2+ translo cation across the membrane. When the coupling between the catalytic an d transport domains is lost, the ATPase mediates Ca2+ efflux as a Ca2 channel. The Ca2+ efflux through the ATPase channel is activated by d ifferent hydrophobic drugs and is arrested by ligands and substrates o f the ATPase at physiological pH. At acid pH, the inhibitory effect of cations is no longer observed. It is concluded that the Ca2+ efflux t hrough the ATPase may be sufficiently fast to support physiological Ca 2+ oscillations in skeletal muscle, that occur mainly in conditions of intracellular acidosis.