An aqueous extract of the marine sponge Ectyoplasia ferox stimulates L-type Ca2+-current by direct interaction with the Cav1.2 subunit

Marine organisms have attracted much attention as a source of pharmacological tools or potential drugs. We have produced and screened a library of sponge extracts in search of biologically active compounds that may contain useful pharmaceutical lead structures. Sponges were collected from various locations and their aqueous extracts were freeze dried. Murine right and left atria were used to screen 75 extracts for putative cardiac effects. Among seven extracts with a positive inotropic and chronotropic effect the extract C47 from Ectyoplasia ferox proved to be the most active and was chosen for further analysis. C47 also produced a beta-adrenoceptor-independent, propranolol-resistant positive inotropic effect in human atrial trabeculae. To elucidate one possible mode of action the effects of C47 on L-type Ca2+ current (I-Ca,I-L) were measured with a standard patch-clamp technique. In isolated human atrial myocytes exposure to C47 increased peak amplitude of I-Ca,I-L in a concentration-dependent manner. The threshold concentration was 15 mug/ml. In addition, voltage dependency of activation and steady-state inactivation were shifted to more negative potentials. C47 slowed the initial phase of time-dependent current inactivation and the recovery from inactivation. In cell-attached patches of HEK 293 cells expressing human Ca(v)1.2 addition of C47 to the bath solution did not affect gating properties, whereas inclusion of the extract into the pipette solution strongly increased single-channel activity, suggesting a direct effect on the pore-forming channel subunit. Despite its robust effect on I-Ca,I-L C47 enhanced cardiac force of contraction by only a fraction of the maximum increase caused by high extracellular concentrations of Ca2+ and failed to increase vascular tone. These findings suggest that the effect of C47 is restricted to the Ca2+ channel.