When incubated with a secretagogue such as cholecystokinin (CCK), dispersed acini prepared from guinea pig pancreas released substantially more amylase than did dispersed single acinar cells. With CCK the rate of amylase release from dispersed acini decreased after 5 min of incubation and remained constant for the subsequent 25 min. The magnitude of the reduction in the rate of amylase release after 5 min was greater with higher concentrations of CCK. With vasoactive intestinal peptide (VIP), the rate of amylase release remained constant for at least 30 min. With CCK plus VIP, potentiation of the rate of amylase release occurred only during the first 15 min of incubation. After 15 min of incubation, the effects of the two peptides were additive. When dispersed acini were first incubated with CCK, potentiation of amylase release occurred only when VIP was added during the initial 10 min of incubation. In contrast, when cells were first incubated with VIP, potentiation of amylase release occurred when CCK was added as long as 30 min after VIP.
In dispersed acini prepared from guinea pig pancreas, peptides isolated from amphibian skin (caerulein, bombesin, litorin, and physalaemin) as well as eledoisin, a peptide isolated from the posterior salivary gland of a Mediterranean octopod, caused a significant increase in amylase release. Each amphibian peptide potentiated the stimulation of amylase release caused by vasoactive intestinal peptide or secretin in that the increase in amylase release caused by an amphibian peptide plus vasoactive intestinal peptide or secretin was significantly greater than the sum of the increase caused by each secretagogue acting alone. Potentiation of amylase secretion did not occur with an amphibian peptide plus cholecystokinin or carbachol.
SynopsisTo explore the importance of the charge of the amino acid in position 15 in influencing the apparent affinities of VIP and secretin for their receptors on pancreatic acinar cells, we tested the synthetic C-terminal 23-peptide fragment of secretin (S5-27) and two analogues containing substitutions in position 15 for their abilities to interact with secretin-preferring and VIP-preferring receptors on pancreatic acinar cells. In inhibiting the increase in cyclic AMP caused by VIP acting through the VIP-preferring receptors, 15-Lys-55-27 was equipotent with 15-Asn-S5_27, and these analogues were significantly more potent than S5-27. In inhibiting the increase in cyclic AMP caused by secretin acting through the secretin-preferring receptors, S5-27 was equipotent with 15-Asn-55-27, and these peptides were significantly more potent than 15-Lys-S5-27. These findings indicate that the affinities of these 23-peptides for the VIPpreferring receptors and for the secretin-preferring receptors were influenced primarily by the absence of a particular charge in position 15 but not by the presence of the opposite charge.
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