In muscle cells, excitation-contraction coupling involves the translocation of Ca(2+) between intracellular compartments and the cytosol. Heat derived from the hydrolysis of ATP by the sarcoplasmic reticulum Ca(2+)-ATPase of skeletal muscle plays an important role in the thermoregulation and energy balance of the cell. Although several Ca(2+)-ATPase isoforms have been described in vertebrates, little is known about Ca(2+) transport in invertebrates. In this report, a Ca(2+)-ATPase is identified in the microsomal fraction obtained from sea cucumber (Ludwigothurea grisea) smooth muscle. The activity of this enzyme is enhanced three- to fivefold by K(+) and Na(+). During Ca(2+) transport, the ATPase can synthesise ATP from ADP and inorganic phosphate (P(i)) using the energy derived from the Ca(2+) gradient formed across the microsomal membrane (ATP<->P(i) exchange). The apparent affinity of the enzyme for P(i) is increased by more than one order of magnitude by K(+). In the presence of K(+), the fraction of ATP synthesised during the exchange reaction by sea cucumber microsomes was found to be larger than that measured in microsomes derived from either rabbit or trout muscle. Like the isoforms found in skeletal muscle, the sea cucumber Ca(2+)-ATPase can convert osmotic energy into heat. The amount of heat produced after the hydrolysis of each ATP molecule increases two- to threefold when a Ca(2+) gradient is formed across the microsomal membrane.

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