The effect of cyclic nucleotides on secretin secretion in canine duodenal mucosa in vitro

Modulation of secretin release by neuropeptides in secretin-producing cells

Nerve fibers containing bombesin (BB)/gastrin-releasing polypeptide (GRP), pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal polypeptide (VIP), or galanin are known to innervate the mucosa of the upper small intestine. Both BB/GRP and PACAP have been shown to elicit secretin secretion in vivo. We studied whether the above-mentioned neuropeptides can act directly on secretin-producing cells, including the murine neuroendocrine cell line STC-1 and a secretin cell-enriched preparation isolated from rat upper small intestinal mucosa. Secretin release from both cell types was stimulated by various agents known to elicit secretin release and by the neuropeptides BB, GRP, and PACAP, suggesting a comparable response between the two cell preparations. The effects of neuropeptides were further studied in STC-1 cells. BB, GRP, and PACAP stimulated secretin release time and concentration dependently. VIP also stimulated secretin release concentration dependently. Stimulation by BB/GRP or PACAP was accompanied by elevation of inositol-1,4,5-trisphosphate (IP3) or cAMP, respectively. The stimulatory effect of PACAP on secretin release was synergistically enhanced by BB without any synergistic increase in IP3 or cAMP production, suggesting cross talk between different signal transduction pathways downstream of the production of these two second messengers. The L-type Ca2+ channel blocker diltiazem (10 microM) and the Ca2+ chelator EGTA (1 mM) significantly inhibited BB-stimulated secretin release by 64% and 59%, respectively, and inhibited PACAP-stimulated release by 75% and 55%, respectively. The protein kinase A-specific inhibitor Rp-cAMPS (100 microM) also inhibited both BB- and PACAP-stimulated secretin release by 30% and 62%, respectively. Galanin inhibited BB- and PACAP-stimulated secretin release and production of second messengers in a concentration-dependent and pertussis toxin-sensitive manner. These results suggested that the neuropeptides BB/GRP, PACAP, VIP, and galanin can modulate secretin release in secretin-producing cells and that STC-1 cells can serve as a useful model for studying the cellular mechanism of secretin secretion elicited by luminal secretagogues and neuropeptides.

Electrophysiology of intestinal cholecystokinin secretion

The control of gastrointestinal CCK secretion may occur at several points. A decrease in potassium conductance may depolarize the plasma membrane, activating calcium channels with a resultant stimulation in CCK release. The opposite may occur with increases in potassium conductance. The model presented in Fig. 8 suggests that the key control point in the understanding of CCK secretion is the regulation of L-type calcium channels. Whether a single second messenger pathway may regulate calcium channels stimulated by agents such as glucose, phenylalanine, bombesin and cAMP is an important area to pursue.

Characterization of secretin release in secretin cell-enriched preparation isolated from canine duodenal mucosa

The release of secretin was studied in secretin cell-enriched preparations isolated from canine duodenal mucosa. The crude enterocytes were isolated by treating the duodenal mucosa sequentially with collagenase and ethylenediaminetetraacetic acid. Secretin cell-enriched fraction was prepared by centrifugation of the crude enterocytes in a counterflow elutriation rotor to obtain a final preparation containing 3.2 +/- 0.3 pmol/10(6) cell of immunoreactive secretin, which was 13-fold greater than the crude cell preparation (N = 5). The cells were incubated in Hanks' balanced salt solution for 20 min at 37 degrees C under 95% O2/5% CO2 before adding various agents and further incubated for various periods of time. The amounts of secretin released into the medium and retained by the cells were then determined by a specific radioimmunoassay. The release of immunoreactive secretin was increased dose-dependently over the control by dibutyryl cyclic-3',5'-adenosine monophosphate, forskolin, 4 beta-12-O-tetradecanoylphorbol-13-acetate, the synthetic serine protease inhibitor, camostat, and the calcium ionophore, A23187. The effects of forskolin, the phorbol ester, and A23187 were time-dependent and not observed at 4 degrees C. The release of immunoreactive secretin was also stimulated by KCl in high concentration and by sodium oleate. The effect of A23187 was abolished in a Ca(2+)-free medium, while those of dibutyryl cyclic-3',5'-adenosine monophosphate and forskolin were potentiated by 3-isobutyl-1-methylxanthine, which did not have a significant effect when added alone. These results indicate that the release of secretin is regulated by both Ca(2+)- and cyclic-3',5'-adenosine monophosphate-dependent mechanisms.2+ release.

Effect of the secretin family of peptides on gastric emptying and small intestinal transit in rats

We have compared the effects of the secretin family of peptides and their synthetic fragments on gastric emptying (GE) and small intestinal transit (SIT) using an unanesthetized rat model which simultaneously measures the GE and SIT of both solids and liquids. The meal consisting of 5% polyethylene glycol w/v, 5% Indian ink v/v and 20 non-digestible plastic beads was given intragastrically 10 minutes after the intraperitoneal injection of 0.5 ml of saline or peptides (2 and 5 micrograms/kg). Plasma secretin and the immunospecificity of secretin fragments were determined. In control rats, the t1/2 for the GE of both solids and liquids were 56 +/- 3.8 and 19 +/- 2.3 minutes, respectively. Liquids emptied faster than the solids and liquids travelled ahead of the solids in the intestine. Secretin (5 micrograms/kg) inhibited GE of both solids and liquids by 33-37%. Secretin delayed the SIT of the meal by approximately 35%. Fragments of secretin and of VIP had no effect on GE and SIT of both solids and liquids. The whole molecule of secretin was required to inhibit GE and to delay SIT of solids and liquids. Glucagon, PHI and growth hormone releasing factor (GHRF1-44) inhibited GE and SIT of both solids and liquids. For all peptides tested, the inhibition of SIT was proportional to the inhibition of GE suggesting that the prolongation of SIT was secondary to delayed GE. These observations indicate that the peptides of the secretin family inhibit GE and prolong SIT. Thus, the structural requirement required for the secretin family of peptides to effect their motor actions on the stomach is similar to that required for pancreatic enzyme secretion.

Cationic environment and secretin secretion in canine duodenal mucosa in vitro

We examined the effects of the divalent cation calcium (Ca2+) and the monovalent cations potassium (K+) and sodium (Na+) and different modalities that affect the fluxes of these cations on immunoreactive secretin (IRS) secretion from canine duodenal mucosa in vitro. In the absence of extracellular Ca2+, the basal IRS secretion was inhibited by 29%. Increased uptake of Ca2+ by passive diffusion and facilitated uptake of Ca2+ by ionophore A23187 had no effect on basal IRS secretion. However, Ca2+ channel blocking agents, LaCl3 and verapamil, inhibited basal IRS secretion by 18 and 33% respectively. High concentrations of extracellular K+ caused a dose-dependent but delayed stimulation of IRS secretion. K+-stimulated IRS secretion was not neurally mediated. Similarly, 9-aminoacridine, which blocks K+ exchange, mimicked the effects of K+. Valinomycin (10(-6) M) inhibited both K+-stimulated and 9-aminoacridine-stimulated IRS secretion. In the absence of extracellular Na+, there was a delayed inhibition of both basal and K+-stimulated IRS secretion. These results suggest that changes in cationic environment are associated with alterations in the secretion of IRS. High extracellular K+ concentration is conducive, and the absence of extracellular Na+ and Ca2+ is inhibitory to IRS secretion.