Jc. Lallement et al., GASTRIN(13) AND THE C-TERMINAL OCTAPEPTIDE OF CHOLECYSTOKININ ARE DIFFERENTLY COUPLED TO G-PROTEINS IN GUINEA-PIG BRAIN MEMBRANES, European journal of pharmacology. Molecular pharmacology section, 267(3), 1994, pp. 297-305
In the course of our study concerning gastrin and cholecystokinin (CCK
) receptors, we synthesized and characterized a labelled gastrin ligan
d, [I-125]BH[Leu(15)]gastrin-(5-17) 4-hydroxyphenyl)propionyl[Leu(15)]
gastrin-(5-17)). On isolated canine fundic mucosal cells and human Jur
kat lymphoblastic cell line, known to express CCKB/gastrin receptors,
the binding experiments performed indicate that [I-125]BH[Leu(15)]gast
rin-(5-17) provides a convenient biologically active ligand for cholec
ystokinin/gastrin receptor studies. We showed in this study that, on g
uinea-pig brain membranes known to possess CCKB and CCKA receptors, [I
-125]BH[Leu(15)]gastrin-(5-17) binds to a single class of high-affinit
y binding sites in a saturable and specific manner. [I-125]BH[Leu(15)]
gastrin-(5-17) interacts with guinea-pig brain membranes with a maxima
l binding capacity that is about three-fold lower than that of [I-125]
BHCCK8 (CCK8, the C-terminal octapeptide of cholecystokinin). The appa
rent affinities of CCK analogues and selective CCK receptor antagonist
s L-365,260 and MK-329 for the sites labelled by both probes were in a
ccordance with a CCKB-like profile. Association-dissociation kinetics
of [I-125]BH[Leu(15)]gastrin-(5-17) and [I-125]BHCCK8 were performed a
nd compared. They showed that [I-125]BHCCK8 equilibrated more slowly t
han [I-125]BH[Leu(15)]gastrin-(5-17). The effects of pH, monovalent an
d divalent cations on binding of both probes were investigated. The re
sults obtained did not indicate strong differences between [I-125]BH[L
eu(15)]gastrin-(5-17) and [I-125]BHCCK8 binding. Binding experiments i
n the presence of stable analogues of GTP showed a different behaviour
between [I-125]BH[Leu(15)]gastrin-(5-17) and [I-125]BHCCK8. GTP gamma
S strongly decreased [I-125]BH[Leu(15)]gastrin-(5-17) binding whereas
it weakly affected [I-125]BHCCK8 binding. The 5'-adenylylimidodiphosp
hate was found to exert a similar effect than GTP gamma S on gastrin a
nd CCK8 binding. The results of binding studies carried out with [I-12
5]BH[Leu(15)]gastrin-(5-17) showed that gastrin binds specifically to
guinea-pig brain membranes. Furthermore, the different effects of guan
yl nucleotides on gastrin and CCK binding strongly suggested that gast
rin and CCK trigger a differential G protein coupling through the same
binding sites.