Sarcoplasmic reticulum Ca2+-ATPase of sea cucumber smooth muscle:: regulation by K+ and ATP

Although several Ca2+-ATPase isoforms have been described in vertebrates, little is known about Ca2+-transport in the muscle of invertebrates. In the microsomal fraction obtained from the sea cucumber (Ludwigothurea grisea) longitudinal body wall smooth muscle, we identified a Ca2+-transport ATPase that is able to transport Ca2+ at the expense of ATP hydrolysis. This enzyme has a high affinity for both. Ca2+ and ATP, an optimum pH around 7.0, and - different from the vertebrate sarcoplasmic reticulum Ca2+-ATPases isoforms so far described - is activated 3- to 5-fold by K+ but not by Li+, at all temperatures, Ca2+ and ATP concentrations tested. Calcium accumulation by the sea cucumber microsomes is inhibited by Mg/ATP concentrations > 1 mM and the accumulated Ca2+ is released to the medium when the ATP concentration is raised from 0.1 to 4.0 mM. (C) 2000 Elsevier Science Inc. All rights reserved.