G.J. Dockray

The secretory kinetics of the G cell in omeprazole-treated rats

Prolonged achlorhydria leads to hypergastrinemia which must be matched by increased gastrin production. The extent to which the balance between synthesis and storage or secretion is shifted in achlorhydria remains uncertain. In the present study, rats were treated for 14 days with the hydrogen-potassium-stimulated ATPase inhibitor omeprazole, and the effects on plasma and tissue gastrin concentrations and on the abundance of gastrin messenger RNA were examined. To calculate the fractional release rates of gastrin, the metabolic clearance rate of synthetic unsulfated rat heptadeca peptide gastrin in anesthetized rats was also measured. Treatment with omeprazole for 14 days led to a profound hypergastrinemia, a twofold increase in antral gastrin stores, and a tenfold increase in messenger RNA. Calculations based on the metabolic clearance rate for rat heptadecapeptide gastrin suggested that in control rats, about 0.08% of stored gastrin was released per minute compared with about 0.4% in omeprazole-treated rats. No evidence was observed to suggest that changes in the efficiency of conversion of Gly-extended gastrins to amidated peptides were of any significance in accounting for the increased production of amidated gastrin. The increased gastrin synthesis in achlorhydria is therefore attributable to increased messenger RNA levels; most of the increase in gastrin production is directly secreted as changes in the stores of gastrin appear to be of lesser importance.

Differential control of somatostatin messenger RNA in rat gastric corpus and antrum. Role of acid, food, and capsaicin-sensitive afferent neurons.

Somatostatin messenger RNA in the antrum and corpus of rat stomach was quantified by Northern and slot blotting using a probe generated by the polymerase chain reaction. Fasting for 48 h enhanced the abundance of somatostatin mRNA in the pyloric antral region, but not in the acid-secreting region of the stomach. In fasted rats, somatostatin mRNA in antrum, but not corpus, was decreased by inhibition of acid secretion with omeprazole. In contrast, in rats treated with capsaicin to lesion small diameter afferents there was a significant decrease in somatostatin mRNA abundance in the corpus but not antrum. The effects of capsaicin cannot be attributed to nonspecific changes in gastric endocrine cell gene expression, since the abundance of histidine decarboxylase mRNA (which is a functionally regulated marker for a different gastric endocrine cell type) did not change with capsaicin. Gastric capsaicin-sensitive afferents are rich in calcitonin gene-related peptide, and in rats with antibodies to this peptide there was reduced corpus somatostatin mRNA. Moreover, infusion of calcitonin gene-related peptide in control rats produced a significant increase in somatostatin mRNA in the gastric corpus. The results indicate that somatostatin mRNA abundance is controlled by the gastric luminal contents and the extrinsic afferent innervation, but the relative importance of these factors differs in antrum and corpus: luminal contents are relatively more important in antrum and primary afferents using calcitonin gene-related peptide in the corpus.

Potent stimulation of the avian exocrine pancreas by porcine and chicken vasoactive intestinal peptide.

1. The actions of chicken and porcine secretins and vasoactive intestinal peptides (VIPs) were compared on the rate of flow and rate of protein secretion from the exocrine pancreas in urethane anaesthetized turkeys and rats. 2. Chicken VIP was about twice as potent as porcine VIP and 100‐150 times as potent as chicken and porcine secretins in stimulating the flow of pancreatic juice in the turkey. 3. Porcine secretin was a strong stimulant of the flow of pancreatic juice in the rat, but chicken secretin and the two VIPs were only active in doses 20‐50 times higher than those of porcine secretin. 4. Neither the two VIPs nor the two secretins significantly stimulated the rate of pancreatic protein secretion in the turkey or rat. 5. In the turkey I.V. infusion of graded doses of chicken VIP produced graded increases in the flow of pancreatic juice; in the presence of an infusion of a low dose of CCK8 the flow of juice secreted in response to the highest dose of chicken VIP was significantly lower compared with the infusion of VIP alone, and responses to the other doses of VIP were lower but not significantly so. The infusion of chicken secretin reduced the flow of juice in response to infusions of chicken VIP, but the differences were not significant. There was no significant difference in either the rate of flow, or rate of protein secretion from the turkey pancreas in response to an infusion of chicken secretin and CCK8, compared with CCK8 alone. 6. The results cast doubt on the importance of secretin for regulation of the avian pancreas, and suggest instead that VIP might have a physiological role in regulating the flow of pancreatic juice in birds.

Functional control of gastrin releasing peptide (GRP) mRNA in rat stomach

The gastric factors controlling abundance of mRNA encoding the important neuropeptide, gastrin releasing peptide (GRP) in rat stomach, were examined by Northern and slot blot analysis. Withdrawal of food increased antral GRP mRNA, as did treatment of fed rats with the acid inhibitory drug, omeprazole. There was no change in GRP mRNA abundance in gastric corpus. The data indicate functionally distinct populations of GRP neurons in different regions of the stomach, and control of antral neuropeptide biosynthesis by the gastric luminal contents.

Tissue specific processing of chromogranin a in bovine gut, pancreas and adrenal medulla

Chromogranin A (ChrgA) is a major endocrine secretory granule protein that is processed to a ~ariety of fragments in dif ferent endocrine cell types. We have used an antibody (l_300) to the synthetic peptide YLSKEWEDA, which corresponds to [Tyr 0] bovine Chrg A 306-313 to examine the products in bovine gut, adrenal and pancreas. lmmunofluorescent studies localized material to endocrine cells in bovine adrenal medulla, pancreas and gut. Identif ication of the immunoreactive forms in these tissues was performed by 5DS PAGE with immunoblotting, and gel f i l t rat ion with RIA. Immunoblots showed that adrenal medulla contained a major band of 51-KDa and two minor bands of 75and 62-KOa, whereas a 42-KDa band was the major form in the pancreas. In ileum, gastric corpus and pyloric antrum, a 33-KDa band predominated, although there was an additional 51-KDa band in the corpus. Sephadex (375 gel f i l t rat ion revealed two immunoreactive peaks of 8and 3-KDa apparent molecular weights in pancreas and ileum while the adrenal medulla contained an additional major immunoreactive peak emerging in the void region which corresponded to the 51-KDa band on SD5 PAGE. Conc]usion=I. YLSKEWEDA-immunreactive peptides derived from Chrg A occur in bovine adrenal medulla, pancreas and gut. 2. The processing of ChrgA is tissue specific. 8/

Enterochromaffin-like (ECL) cell function in transgenic mice expressing gastrin in pancreatic β-cells

BACKGROUND & AIMS. Raised plasma gastrin secondary to achlorhydria is associated with increased expression of several genes in ECL cells, notably histidine decarboxylase (HDC) and vesicular monoamine transporter type 2 (VMAT2). It is not clear whether endogenous gastrin regulates expression of these genes when gastric acid secretion is uninhibited. We have examined expression of these genes in ECL cells in INS-GAS transgenic mice that express the coding sequence of the human gastrin gene in pancreatic 13-cells and in which parietal cell function is uninhibited. METHODS. Plasma gastrin was measured by radioimmunoassay using antibody L2 (COOH-terminal specific for amidated gastrins) and L6 (specific for human G17). Gastrin release from 13-cells was stimulated by oral glucose. ECL cell function was assessed by Northem analysis of HDC and VMAT2 mRNAs. RESULTS. In INS-GAS mice fasted for 6hr, plasma human G17 was 67-+ 11 pM. When fasted mice were then allowed access to 20% glucose, plasma human G17 significantly increased (30min, 242 ± 33; 120min, 133 _+ 19 pM; n = 6; both p<0.01). Following glucose, corpus VMAT2 and HDC mRNA abundances increased to 119--. 6% and 147 ± 14%, relative to control (both p<0.05), respectively. In INS-GAS mice fed ab libitum with access to 20% glucose for 5 days, plasma gastrin was 283 ± 50 pM compared with 85 ± 29 pM in control INS-GAS mice (n = 6, p<0.01). Moreover, in INS-GAS mice with access to glucose for 5 days, VMAT2 mRNA abundance was increased by 255 ± 28% and HDC mRNA by 163 + 24% compared to INS-GAS mice fed ad libitum (both p<0.05). In wild type mice, human G17 was not detected in the plasma, and access to glucose did not influence plasma amidated, ie antral, gastrin (control, 82 ± 18: glucose, 86 ± I0 pM); neither did glucose influence HDC (control, 100 ± 18; glucose, 111 ± 8%) and VMAT2 (control, 100 ± 9; glucose, 95 ± 8%) mRNA abundances. CONCLUSIONS. 1. Oral glucose stimulates human G17 release in INS-GAS mice, but has no effect on antral gastrin release. 2. Moderate hypergastrinemia is associated with increases in HDC and VMAT2 mRNA in gastric corpus. 3. Hypergastrinemia-induced expression of HDC and VMAT2 in ECL ceils does not depend on achlorhydria.