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Centre National de la Recherche Scientifique Enseignement
Supérieur Associé 5017, Physiopathologie et Pharmacologie
Vasculaire, Faculté de Pharmacie, Université de Bordeaux
II, 33076 Bordeaux, France (J.-L.M., J.M., J.-L.L., M.H.),
Institut
Pasteur de Lille, Service de Chimie des Biomolécules, 59019 Lille, France (H.D., P.S., A.T.), and
Department of Biological Science,
Faculty of Sciences, Yarmouk University, Irbid, Jordan (J.Q.)
Marine sponges are synthesizing a wide variety of peptidic and organic
molecules with biological activities. Multiple-step purification of
Cliona vastifica extract led to a new dimeric peptide
(mapacalcine; Mr = 19,064) that
is composed of two homologous chains, each containing nine cysteins.
This protein has been found to selectively block a new calcium
conductance characterized in mouse duodenal myocytes with an
IC50 value of ~0.2 µM. The
mapacalcine-sensitive current was a non-L-type calcium current
activated from a holding potential of 
80 mV that persisted during
stimulation of the cell at high frequencies (0.1-0.2 Hz) within 5-10
min. Time constants of inactivation were similar for both L-type and
non-L-type calcium currents. The non-L-type calcium current of duodenal
myocytes was not blocked by the pharmacological agents specific for N-,
L-, P-, or Q-type calcium channels. Mapacalcine was unable to block
T-type calcium current in portal vein myocytes as well as
voltage-dependent potassium currents and calcium-activated chloride
currents in duodenal and portal vein cells. Mapacalcine did not affect
caffeine-induced calcium responses, indicating that it did not
interfere with intracellular calcium stores. Competition experiments on
mouse intestinal membranes showed that mapacalcine did not interact
with dihydropyridines receptors. These data suggest that mapacalcine
may be a specific inhibitor of a new type of calcium current, first
identified in duodenal myocytes.
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