Three marine alkaloids, purified from Clavelina moluccensis, were structurally identified as lepadiformines A, B, and C and studied on frog atrial myocytes I(K1), using the patch-clamp technique. Lepadiformine A (0.4 to 3.3 microM) blocked I(K1) dose-dependently with an apparent dissociation constant (K(D)) equal to 1.42 microM and a stoichiometry of 0.77. The block is voltage-dependent, suggesting that lepadiformine A occupies a receptor site located at about two-thirds of the membrane depth. The shortening of the aliphatic chain at position C13 of lepadiformine B decreased the potency of the molecule to block I(K1) but not the affinity (K(D) = 1.56 microM) and stoichiometry (0.72). Additional deletion of the oxygenated side chain at C2 in lepadiformine C markedly decreased the inhibitory effect of the molecule. In conclusion, lepadiformine modulates I(K1) response in cardiac muscle. The oxygenated side chain in C2 is implicated in the affinity of lepadiformine, which behaved as an amine, for a receptor located near or inside the I(K1) pore, and the aliphatic chain length at position C13 is involved in the degree of I(K1) blockage.

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