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Sarcoplasmic reticulum Ca(2+)-ATPase of sea cucumber smooth muscle: regulation by K(+) and ATP
A M Landeira-Fernandez1, A Galina, P Jennings
1Departamento de Bioquímica Médica, Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro, Cidade Universitária, 21941-590, Rio de Janeiro, Brazil.
Abstract:
Although several Ca(2+)-ATPase isoforms have been described in vertebrates, little is known about Ca(2+)-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 Ca(2+)-transport ATPase that is able to transport Ca(2+) at the expense of ATP hydrolysis. This enzyme has a high affinity for both Ca(2+) and ATP, an optimum pH around 7.0, and - different from the vertebrate sarcoplasmic reticulum Ca(2+)-ATPases isoforms so far described - is activated 3- to 5-fold by K(+) but not by Li(+), at all temperatures, Ca(2+) and ATP concentrations tested. Calcium accumulation by the sea cucumber microsomes is inhibited by Mg/ATP concentrations >1 mM and the accumulated Ca(2+) is released to the medium when the ATP concentration is raised from 0.1 to 4.0 mM.