We have used a combination of biochemical and pharmacological techniques to investigate the role of the cyclic nucleotides, 3′, 5′-cyclic adenosine monophosphate (cyclic AMP) and 3′,5′-cyclic guanosine monophosphate (cyclic GMP), in mediating the cardioregulatory effects of FMRFamide and other neuropeptides encoded on exon II of the FMRFamide gene of Lymnaea stagnalis. The ‘isoleucine’ peptides (EFLRIamide and pQFYRIamide) produced complex biphasic effects on the frequency, force of contraction and tonus of the isolated heart of L. stagnalis, which were dependent on adenylate cyclase (AC) activity of the heart tissue. At a control rate of cyclic AMP production of less than or equal to 10 pmoles min(−)(1)mg(−)(1) protein, the ‘isoleucine’ peptides produced a significant increase in AC activity in heart membrane preparations. This suggested that the enhanced AC activity is responsible for the stimulatory effects of the ‘isoleucine’ peptides on frequency and force of contraction of heart beat. This excitation sometimes followed an initial ‘inhibitory phase’ where the frequency of beat, force of contraction and tonus of the heart were reduced by the ‘isoleucine’ peptides. Hearts that showed the inhibitory phase of the ‘isoleucine’ response, but characteristically lacked the delayed excitatory phase, were found to have high levels of membrane AC activity (breve)10 pmoles min(−)(1)mg(−)(1) protein in controls. Application of the ‘isoleucine’ peptides to membrane homogenate preparation from these hearts failed to increase AC activity. The addition of FMRFamide produced significant increases in the rate of cyclic AMP production in the heart membrane preparations, which could account, at least in part, for the cardioexcitatory effects of this peptide in the isolated whole heart. A membrane-permeable cyclic AMP analogue (8-bromo-cyclic AMP) and an AC activator (forskolin) were also cardioexcitatory. The peptide SEEPLY had no effects on the beat properties of the isolated heart and did not alter AC activity. The activity of the membrane-bound (particulate) guanylate cyclase (GC) was not significantly affected by any of the peptides.

REFERENCES

Baines
R. A.
,
Thompson
K. S. J.
,
Rayne
R. C.
,
Bacon
J. P.
(
1995
).
Analysis of the peptide content of the locust vasopressin-like immunoreactive (VPLI) neurons
.
Peptides
16
,
799
–.
Benjamin
P. R.
,
Burke
J. F.
(
1994
).
Alternative mRNA splicing of the FMRFamide gene and its role in neuropeptidergic signaling in a defined neural network
.
BioEssays
16
,
335
–.
Chin
G. J.
,
Payza
K.
,
Price
D. A.
,
Greenberg
M. J.
,
Doble
K. E.
(
1994
).
Characterization and solubilization of the FMRFamide receptor of squid
.
Biol. Bull
187
,
185
–.
Colombaioni
L.
,
Paupardin-Tritsch
D.
,
Vidal
P. P.
,
Gerschenfeld
H. M.
(
1985
).
The neuropeptide FMRF-amide decreases both the Ca2+condutance and a cyclic 3,5 -adenosine monophosphate-dependent K+conductance in identified molluscan neurons
.
J. Neurosci
5
,
2533
–.
Cooper
D. M. F.
,
Mons
N.
,
Karpen
J. W.
(
1995
).
Adenylyl cyclases and the interaction between calcium and cyclic AMP signalling
.
Nature
374
,
421
–.
Difrancesco
D.
,
Tortora
P.
(
1991
).
Direct activation of cardiac pacemaker channels by intracellular cyclic AMP
.
Nature
351
,
144
–.
Freedman
N. J.
,
Lefkowitz
R. J.
(
1996
).
Desensitization of G-protein coupled receptors
.
Recent Prog. Horm. Res
51
,
319
–.
Gilbert
J.
,
Moses
C.
,
Pfenning
M.
,
Richelsen
E.
(
1986
).
Neurotensin and its analoguescorrelation of specific binding with with stimulation of cyclic GMP formation in neuroblastoma clone N1E-115
.
Biochem. Pharmacol
35
,
391
–.
Green
K. A.
,
Falconer
W. P.
,
Cottrell
G. A.
(
1994
).
The neuropeptide Phe-Met-Arg-Phe-NH2(FMRFamide) directly gates two ion channels in an identified Helix neurone
.
Pflugers. Arch
428
,
232
–.
Hartzell
H. C.
,
Fischmeister
R.
(
1986
).
Opposite effects of cyclic GMP and cyclic AMP on Ca2+current in single heart cells
.
Nature
323
,
273
–.
Higgins
W. J.
(
1974
).
Intracellular actions of 5-hydroxytryptamine on the bivalve myocardium. I. Adenylate and guanylate cyclase
.
J. Exp. Zool
190
,
99
–.
Higgins
W. J.
,
Price
D. A.
,
Greenberg
M. J.
(
1978
).
FMRFamide increases the adenylate cyclase activity and cyclic AMP level of molluscan heart
.
Eur. J. Pharmacol
48
,
425
–.
Irisawa
H.
,
Brown
H. F.
,
Giles
W.
(
1993
).
Cardiac pacemaking in the sinoatrial node
.
Physiol. Rev
73
,
197
–.
Kellett
E.
,
Saunders
S. E.
,
Li
K. W.
,
Staddon
J. W.
,
Benjamin
P. R.
,
Burke
J. F.
(
1994
).
Genomic organization of the FMRFamide gene in Lymnaea: Multiple exons encoding novel neuropeptides
.
J. Neurosci
14
,
6564
–.
Lundberg
J. M.
(
1996
).
Pharmacology of cotransmission in the autonomic nervous system: Integrative aspects of amines, neuropeptides, adenosine triphosphate, amino acids and nitric oxide
.
Pharmacol. Rev
48
,
113
–.
Prier
K. R.
,
Beckman
O. H.
,
Tublitz
N. J.
(
1994
).
Modulating the modulator: biogenic amines at subthreshold levels potentiate peptide-mediated cardioexcitation of the heart of the tobacco hawkmoth Manduca sexta
.
J. Exp. Biol
197
,
377
–.
Reich
G.
,
Doble
K. E.
,
Price
D. A.
,
Greenberg
M. J.
(
1997
).
Effects of cardioactive peptides on myocardial cyclic AMP levels in the snail Helix aspersa
.
Peptides
18
,
355
–.
Reiser
G.
,
Walter
V.
,
Hamprecht
B.
(
1984
).
Bradykinin regulates the level of guanosine 3,5 -cyclic monophosphate (cyclic GMP) in neural cell lines
.
Brain Res
290
,
367
–.
Tohse
N.
,
Nakaya
H.
,
Tadeka
Y.
,
Kanno
M.
(
1995
).
Cyclic GMP-mediated inhibition of L-type Ca2+channel activity by human natriuretic peptide in rabbit heart cells
.
Br. J. Pharmacol
114
,
1076
–.
Wang
Z.
,
Lange
A. B.
,
Orchard
I.
(
1995
).
Coupling of a receptor to two different G-proteins in the signal transduction of FMRFamide-related peptides
.
Biochem. Biophys. Res. Comm
212
,
531
–.
Weiss
K. R.
,
Brezina
V.
,
Cropper
E. C.
,
Hooper
S. L.
,
Miller
M. W.
,
Probst
W. C.
,
Vilim
F. S.
,
Kupfermann
I.
(
1992
).
Peptidergic co-transmission in Aplysia: functional implications for rhythmic behaviors
.
Experientia
48
,
456
–.
Willoughby
D.
,
Yeoman
M. S.
,
Benjamin
P. R.
(
1999
).
Inositol-1,4,5-trisphosphate and inositol-1,3,4,5-tetrakisphosphate are second messenger targets for cardioactive peptides encoded on the Lymnaea FMRFamide gene
.
J. Exp. Biol
202
,
2581
2593
.
Worster
B. M.
,
Yeoman
M. S.
,
Benjamin
P. R.
(
1999
).
Maldi-Tof MS analyses of the pattern of peptide expression in single neurones resulting from alternative mRNA splicing of the FMRFamide gene
.
Eur. J. Neurosci
10
,
3498
–.
Xiao
R. P.
,
Ji
X. W.
,
Lakatta
E. G.
(
1995
).
Functional coupling of the beta(2)-adrenergic receptor to a pertussis toxin-sensitive G protein in cardiac myocytes
.
Mol. Pharmacol
47
,
322
–.
This content is only available via PDF.