Sensitivity of cardiac background inward rectifying K+ outward current (IK1) to the alkaloids lepadiformines A, B, and C

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 mu M) blocked I-K1 dose-dependently with an apparent dissociation constant (K-D) equal to 1.42 mu M 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 mu M) 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.