The peptide FMRFamide activates a divalent cation-conducting channel in heart muscle cells of the snail Lymnaea stagnalis.

1. Isolated Lymnaea stagnalis heart ventricle cells contain cation-conducting channels with properties characteristic of Ca2+ channels. These channels, which carry inward Na+ currents in the absence of Ca2+, are activated by the molluscan cardioactive peptides FMRFamide and FLRFamide, and are blocked by Co2+ ions. 2. FMRFamide also activated inward Ba2+ currents at the cell's resting potential. These currents, which were also blocked by Co2+ ions, reversed at a membrane potential of +70 mV. 3. Both sodium and barium currents were initiated when the peptides were applied to the cell outside of the patch pipette indicating that a secondary messenger is likely to be involved in the FMRFamide response.