Printed in Great Britain CALCIUM MOVEMENTS DURING CONTRACTION IN MOLLUSCAN SMOOTH MUSCLE, AND THE LOCI OF CALCIUM BINDING AND RELEASE

retractor and columella muscles revealed the presence of a fast and a slow calcium compartment. KCl-induced depolarization caused a stimulation of the *BCa efflux from the slow compartment, suggesting that this compart-ment may be related to calcium diffusion from intracellular sources. 2. Fine structural studies showed a lack of organized sarcoplasmic reti-culum in both muscles, and that submembrane vesicles and mitochondria were prominent. 3. Muscle sections examined by energy-dispersive X-ray microprobe analysis showed the presence of significant calcium accumulation in the submembrane vesicles and mitochondria. 4. Isolated submembrane vesicles and mitochondria of columella muscle showed strong powers of ATP-promoted calcium binding, while pre-loaded preparations could be induced to release **Ca under in vitro conditions. 5. Synthetic detergent dispersal of columella muscle submembrane vesicles and mitochondria, followed by polyacrylamide gel electrophoresis revealed, in both fractions, a striking similarity of protein profile with those from skeletal muscle sarcoplasmic reticulum. Two dominant protein bands were present, at 110000-112000 daltons and at 51000-53000 daltons, in a position similar to the Ca^/Mg2"1 " ATPase and calsequestrin or high affinity proteins of skeletal muscle. 6. It is suggested that, in the absence of an organized sarcoplasmic reticulum, the submembrane vesicles and mitochondria may act as the major calcium pools for contraction in these molluscan smooth muscles