Exocrine pancreas from different species behaves differently in response to the presence of intact or digested nutrients in the duodenum. A failure of cholecystokinin (CCK) release after a meal has been shown among patients with exocrine pancreatic insufficiency. This abnormality could be restored by the administration of pancreatic extracts, suggesting that digested rather than intact nutrients are responsible for the release of CCK and subsequently gallbladder contraction in humans. The aim of this study was to determine the specific role of different lipidic stimuli in humans. Seven male patients (mean age, 52 years) with pancreatic insufficiency secondary to chronic pancreatitis were selected. Pancreatic insufficiency was considered severe in five of them (lipase output, <1,000 IU/ min) and moderate in another two (lipase output, >1,000 and <2,300 IU/min). Plasma CCK (by bioassay), gallbladder contraction (by ultrasound), and enzyme output (chymotrypsin) in response to duodenal administration of either oleic acid as free fatty acids or 20% Intralipid as triglycerides were measured in each patient with at least a 48-h interval between each test. In all these patients with pancreatic insufficiency, duodenal perfusion of free fatty acids generated a more pronounced (9 1 ? 1 1 vs. 49 ? 21 pM) and faster (15 vs. 30 min) ( p < 0.05) CCK release than triglycerides. Furthermore, gallbladder contraction was more efficient when free fatty acids instead of triglycerides were administered in the duodenum (86 t 5 vs. 69 * 4%) at 10 min (p < 0.05) and (73 & 8 vs. 5 1 ? 5 % ) at 15 min 0, < 0.03).

The binding of 125I-[Tyr11]somatostatin to guinea pig pancreatic acini was saturable and temperature, protein, and radioligand concentration dependent. Dissociation rate was very slow (t1/2 = 193 +/- 24 min). Scatchard analysis revealed a single class of binding sites with a Kd of 0.28 +/- 0.02 nM and a maximal binding capacity of 72 +/- 10.6 fmol/mg prot. There was a strong correlation between binding capacity and extracellular calcium concentration. Incubating acini in EGTA-containing medium with no added Ca2+ caused a 50% decrease in maximal binding capacity with a decrease in receptor affinity. Furthermore, in the absence of calcium, bound somatostatin was rapidly released (t1/2 = 14 +/- 1 min). Subcellular fractionation studies and acid treatment of acini incubated with the tracer showed that most of the somatostatin binding sites were located on the cell surface. Agents that altered cellular calcium in pancreatic acini, such as analogues of cholecystokinin and cholinergic agents, also inhibited the binding of 125I-[Tyr11]somatostatin by a calcium-dependent process. We conclude that somatostatin binds to specific plasma membrane receptors to form a slowly reversible complex that is highly reactive with calcium. Cell calcium-mobilizing agents decrease the affinity of acinar somatostatin receptors for somatostatin.

We have previously demonstrated the presence of specific binding sites for somatostatin on plasma membranes from pancreatic acinar cells. In the present study we attempted to characterize the fate of receptor-bound 125I-[Tyr11]somatostatin. Internalization of somatostatin was rapid (reaching a plateau at 20% of the cell-associated specific radioactivity) and temperature dependent. To follow the processing of bound somatostatin, acini were incubated with 125I-[Tyr11]somatostatin at 5 degrees C during 16 h then, after washing, incubated at 37 degrees C for 90 min in fresh medium. Surface-bound somatostatin decreased rapidly, whereas radioactivity increased in the cell interior and the incubation medium. Intracellular and membrane-bound radioactivity was mainly intact 125I-[Tyr11]somatostatin. Degradation occurred at the plasma membrane level and led to iodotyrosine production. After 15 min of incubation, 15% of the initially surface-bound 125I-[Tyr11]somatostatin was compartmentalized within the cell, mainly in the microsomal fraction. After 30 min, a significant increase in radioactivity appeared in the nuclear fraction. These results indicate that the major part of somatostatin cellular degradation takes place at the plasma membrane level. Within the cell, somatostatin is routed to the nucleus via particular fractions sedimenting with microsomal vesicles.

The action of natural and synthetic somatostatin-(1--28), [Nle8]somatostatin-(1--28), somatostatin-(15--28), and somatostatin-(1--14) was examined in dispersed acini from guinea pig pancreas. At high concentrations, the 28-amino acid form of somatostatin increased amylase release, outflux of 45Ca, cellular cGMP, and to a lesser extent cellular cAMP. The increase in amylase release was suppressed by dibutyryl cGMP but was not modified by theophylline or atropine. Binding of 125I-labeled [Thr28, Nle31] cholecystokinin-(25--33) was inhibited by [Nle8]somatostatin-(1--28). These effects required the entire 28-amino acid peptide and appeared to result from occupation of cholecystokinin receptors. It is postulated that they involve interactions between the C-terminal and the N-terminal sequences of the molecule with the participation of the amino acid in position 8. At low concentrations, natural and synthetic forms of somatostatin-(1--28) and somatostatin-(15--28) inhibited secretin- and vasoactive intestinal peptide (VIP)-stimulated increases in cellular cAMP concentration. No difference was found between the potency of somatostatin peptides, indicating that the tetradecapeptide somatostatin-(15--28) is sufficient to exert an inhibitory action on secretin- and VIP-stimulated cellular cAMP concentration. By contrast, the somatostatin fragment S-(1--14) was inactive on pancreatic cellular function.

This work describes a new preparation of dog pancreatic acini which were used to study amylase release in response to various secretagogues. Neither secretin nor vasoactive intestinal peptide stimulated amylase release from acini. Caerulein, carbachol and human synthetic gastrin G17 stimulated amylase release with the same efficacy, but with different potencies. Bombesin nonapeptide did not show any evidence of a direct stimulatory effect on amylase release. These species-related peculiarities stress the necessity of using the same species when comparing pancreatic cell behaviour in vivo and in vitro.