FMRFamide-activated Ca2+ channels in Lymnaea heart cells are modulated by "SEEPLY," a neuropeptide encoded on the same gene.
The cell-attached, patch-clamp technique was used to investigate the modulatory role of the neuropeptide SEQPDVDDYLRDVVLQSEEPLY ("SEEPLY") on FMRFamide-activated Ca2+ channels in isolated Lymnaea heart ventricular cells. Both SEEPLY and FMRFamide are encoded on the same neuropeptide gene and are coexpressed in a pair of excitatory motor neurons that innervate the heart. FMRFamide applied alone was capable of significantly increasing the P(open) time of a Ca2+ channel in isolated heart muscle cells. However, SEEPLY applied alone did not significantly alter the basal level of Ca2+ channel activity in the same cells. Repeated applications of FMRFamide (15 s every min) resulted in a progressive reduction in the number of Ca2+ channel openings and the overall P(open) time of the channel. The fifth successive 15-s application of FMRFamide failed to cause the Ca2+ channels to open in the majority of cells tested. When FMRFamide and SEEPLY were repeatedly applied together (2-min applications every 4 min) the FMRFamide-activated Ca2+ channels continued to respond after the fifth application of the two peptides. Indeed channel activity was seen to continue after repeated 2-min applications of FMRFamide and SEEPLY for as long as the patch lasted (</=60 min). As well as preventing the loss of response to FMRFamide, SEEPLY was also capable of both up- and down-regulating the response of the Ca2+ channel to FMRFamide. The direction of the response depended on the P(open) time of the channel before the application of SEEPLY. When the P(open) time for the FMRFamide-activated channel was initially 0.004 +/- 0.002 (means +/- SE), subsequent perfusion with a mixture of FMRFamide and SEEPLY produced a statistically significant increase in Ca2+ channel activity (13 cells). In two cells where no channel activity was observed in response to an initial application of FMRFamide, superfusing the heart cells with a mixture of FMRFamide and SEEPLY induced openings of the Ca2+ channel. When the P(open) time of FMRFamide-induced Ca2+ channel openings was 0.058 +/- 0.017 the subsequent application of a mixture of SEEPLY and FMRFamide caused a statistically significant decrease in Ca2+ channel activity (8 cells). As up- and down-regulation of FMRFamide-activated Ca2+ channel openings by SEEPLY were observed in the same cells (8 cells), this suggested that corelease of the two peptides might act together to regulate the level of Ca2+ channel activity within a defined range.
Brezden
B L
BL
Ottawa-Carleton Institute of Biology, Carleton University, Ottawa, Ontario K1S 5B6, Canada.
Yeoman
M S
MS
Gardner
D R
DR
Benjamin
P R
PR
D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000378:metabolism*; D015536:Down-Regulation / Q000235:genetics; D019835:FMRFamide / Q000235:genetics* / Q000494:pharmacology; D015870:Gene Expression / Q000502:physiology; D008195:Lymnaea; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008969:Molecular Sequence Data; D009206:Myocardium / Q000737:chemistry / Q000166:cytology; D009479:Neuropeptides / Q000235:genetics* / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D015854:Up-Regulation / Q000235:genetics
D015220:Calcium Channels; D009479:Neuropeptides; C085288:SEEPLY peptide; D019835:FMRFamide; D002118:Calcium