Activation of a divalent cation-conducting channel in heart ventricle muscle cells of the snail Lymnaea stagnalis by the molluscan cardioactive peptide FMRFamide

Abstract

Ion channels with characteristics of Ca2+ channels have been found in isolated heart ventricle cells of the snail Lymnaea stagnalis. Although spontaneous Ca2+ or Ba2+ currents were seen only occasionally, spontaneous inward Na+ currents were readily observed in the absence of patch pipette Ca2+ between membrane potentials of -100 mV and +20 mV. These currents were blocked by 2 mM Ni2+, 2 mM Co2+ and 10 microM Ca2+. The channels usually ceased conducting within a few minutes after seal formation with the patch pipette and could not be re-activated with depolarizing voltage steps. However, at the cell's resting potential, 10(-8) to 10(-6) M of the molluscan cardioactive peptide FMRFamide or its analogue FLRFamide2+ applied to the cell membrane away from the patch pipette, induced unitary Ba2+ currents or, in the absence of Ca2+ in the patch pipette, Na+ currents. This suggests that a secondary messenger is involved in the FMRFamide-induced activation of these channels rather than a direct activation of a channel-receptor complex by the peptide.