W. Schepp et al., EXENDIN-4 AND EXENDIN-(9-39)NH2 - AGONIST AND ANTAGONIST, RESPECTIVELY, AT THE RAT PARIETAL-CELL RECEPTOR FOR GLUCAGON-LIKE PEPTIDE-1-(7-36)NH2, European journal of pharmacology. Molecular pharmacology section, 269(2), 1994, pp. 183-191
Exendin-4 is a novel peptide from Heloderma suspectum venom which is 5
3% homologous with glucagon-like peptide-1 GLP-1-(7-36)NH2, a stimulan
t of cAMP-dependent H+ production in rat parietal cells. It was the ai
m of the present study to determine whether this effect of GLP-1-(7-36
)NH2 is shared by exendin-4, and whether the responses to either pepti
de are blocked by exendin-(9-39)NH2, a competitive specific exendin re
ceptor antagonist. In enriched rat parietal cells H+ production was me
asured indirectly by [C-14]aminopyrine accumulation. cAMP production w
as determined by radioimmunoassay. [I-125]GLP-1-(7-36)NH2 was prepared
using chloramine T followed by high pressure liquid chromatography (H
PLC) purification. Exendin-4 (10(-12)-10(-8) M) stimulated [C-14]amino
pyrine accumulation in a concentration-dependent manner (EC(50) = 7.6
x 10(-11) M). At the maximally effective concentration (10(-9) M) exen
din-4 was as effective as GLP-1-(7-36)NH2 reaching 70-80% of the respo
nse to 10(-4) M histamine. Likewise, exendin-4 (10(-11)-10(-7) M) stim
ulated parietal cell cAMP production up to 2.8-fold. Maximal stimulati
on by exendin-4 of [C-14]aminopyrine accumulation was not affected by
ranitidine (10(-4) M), but was concentration-dependently reduced by ex
endin-(9-39)NH2 (10(-11)-10(-7) M). At the maximal concentration, exen
din-(9-39)NH2 completely abolished the responses to 10(-9) M exendin-4
and to 10(-9) M GLP-1-(7-36)NH2 while not altering stimulation by 10(
-4) M histamine. Binding of [I-125]GLP-1-(7-36)NH2 to enriched parieta
l cells was displaced by exendin-4 (K-i = 4.6 X 10(-10) M) as well as
by exendin-(9-39)NH2 (K-i = 4.0 X 10(-9) M). Covalent cross-linking of
[I-125]GLP-1-(7-36)NH2 to parietal cell membranes revealed a single b
and at 59 kDa which was abolished by 10(-7) M unlabeled GLP-1-(7-36)NH
2, exendin-4 or exendin-(9-39)NH2. In rat parietal cells exendin-4 and
GLP-1-(7-36)NH2 are equipotent and equieffective stimuli of cAMP-depe
ndent H+-production, apparently via identical 59 kDa membrane receptor
s. Inhibition of [I-125]GLP-1-(7-36)NH2 binding requires about 1 log u
nit higher concentrations of exendin-4 than stimulation of [C-14]amino
pyrine accumulation suggesting the presence of spare receptors. Exendi
n-(9-39)NH2 is a useful pharmacological tool to specifically block the
parietal cell responses to exendin-4 as well as to GLP-1-(7-36)NH2.