Pancreatic amylase messenger RNA progressively decreases in rats rendered diabetic with streptozotocin. Insulin reverses this effect, inducing a selective decrease in amylase messenger RNA in the pancreas. Parotid amylase messenger RNA is not significantly affected by either diabetes or insulin.

Pancreatic islet-acinar cell interaction: amylase messenger RNA levels ar determined by insulin

SUMMARY
1After an outline description of pancreatic structure and function, and a more detailed account of acinar cell morphology, this review traces the pathway of amino acids as they are taken up by the acinar cell, incorporated into digestive hydrolases, transported through the cell and finally discharged from the cell, and considers the mechanisms by which these steps are controlled. At all stages comparisons are made with other secretory cells.
2The use of radioautography and cell-fractionation techniques in determining this pathway in the pancreas are described. The route and kinetics of the process in pancreas are compared with those in other cells.
3Amino-acid entry is by an active mechanism. However the intracellular pool of accumulated amino acids may not be used directly in protein synthesis. Selection of amino acids for incorporation into proteins may occur whilst they are associated with carrier systems within the plasma membrane. There is no convincing evidence that amino-acid entry can be influenced by the pancreatic secretagogues, cholecystokin-pancreazymin (CCK-PZ) or acetylcholine.
4Secretory proteins are synthesized on ribosomes bound to the endoplasmic reticulum (ER) and the nascent proteins vectorially transferred across the ER membrane into the ER cisternae. All messenger RNA molecules which are templates for secretory proteins appear to possess an initial sequence of codons whose translation produces a ‘signal’ sequence of amino acids. This signal sequence somehow triggers attachment of the ribosomes to the ER, thereby automatically determining that the final translation product is destined for the ER cisternae.
5The effects of CCK-PZ and acetylcholine on pancreatic protein synthesis are controversial. Whereas stimulation can be observed in vivo, this has not been convincingly demonstrated in vitro. I conclude that while CCK-PZ and acetylcholine may accelerate protein synthesis, the physiological significance of this effect remains to be clarified. Long-term stimulation can modify pancreatic enzyme synthesis and this, together with other factors, may be the means of dietary adaptation by the gland.
6Newly synthesized proteins travel from the ER cisternae via the peripheral Golgi components to the Golgi cisternae. Transport from ER to Golgi cisternae may occur by a vesicle shuttle service or by direct tubular connexions. Although sustained stimulation with CCK-PZ analogues can accelerate this intracellular transport step, pancreatic secretagogues have not yet been shown to accelerate transport under physiological conditions.
7The Golgi complex has a number of functions including: glycosylation and, where appropriate, sulphation of glycoprotein and mucopolysaccharide components of the zymogen granules (ZG) and granule membranes; sequestration of divalent cations which bind to secretory proteins; the formation of condensing vacuoles (CV) from the inner Golgi cisternae.
8Aggregation of proteins occurs passively within CV so as to form osmotically inert complexes, thereby reducing internal osmotic activity and causing water to diffuse out. This condensation imparts a gel-like consistency to the mature ZG so formed.
9Discharge of ZG occurs by a process of exocytosis involving fusion of the ZG membrane with the apical plasma membrane, release of the ZG contents, and retrieval of the ZG membrane from the plasma membrane by endocytotic mechanisms. The mechanisms responsible for migration of ZG towards the cell apex and for exocytosis remain unknown but may involve the participation of microtubules and/or microfilaments. Although there is a small, basal discharge of ZG at all times, stimulation with CCK-PZ or acetylcholine greatly accelerates the process.
10The basic tenet of the secretory mechanism summarized above is that, following synthesis, secretory proteins are confined within an intracellular organelle at all times. This ‘segregation’ hypothesis has been challenged by the ‘equilibrium’ hypothesis in which secretory proteins are suggested to move across cellular membranes and are therefore at equilibrium within the various compartments of the cell. While many of the observations on which the equilibrium hypothesis are based are tenuous, some others cannot readily be explained by the segregation model. Proponents of the equilibrium hypothesis therefore suggest that preferential release of individual hydrolases from ZG occurs, followed by their separate transport across the apical cell membrane. The claims of this alternative model are discussed.
11In the final section are discussed the intracellular mechanisms by which CCK-PZ and acetylcholine act on the acinar cell to cause discharge. The overall membrane perturbations brought about by CCK-PZ and acetylcholine appear to be the same and include cell depolarization, and perhaps increased phospholipid turnover. Both events may be related to an altered membrane permeability to cations. CCK-PZ, but not acetylcholine, will activate adenylate cyclase, but cyclic AMP does not appear to be involved in regulating enzyme discharge. Instead, Ca2+ is the major intracellular second messenger. However, rather than increase Ca2+ uptake into the cell, CCK-PZ and acetylcholine appear to raise the intracellular Ca2+ concentration by causing release of Ca2+ from intracellular stores. The mechanism by which they do this, and the role of Ca2+ in the discharge process remain unknown.

Synthesis, intracellular transport and discharge of exportable proteins in the pancreatic acinar cell and other cells

Studies on the guinea pig pancreas. Parallel discharge of exocrine enzyme activities.

Recent studies indicate that insulin directly regulates the acinar pancreas. Morphologic and hemodynamic studies indicate the presence of a portal system that conveys islet blood to acinar cells. Studies both in humans with diabetes mellitus and in animals given beta cell toxins indicate that insulin is necessary for normal acinar cell function. Studies in the perfused rat pancreas indicate that endogenous insulin potentiates zymogen release. Isolated rat and mouse acini have insulin receptors, and in these cells, after binding to its receptors, insulin regulates a number of functions including: sugar transport, protein synthesis, and the number of cholecystokinin receptors. These in vivo and in vitro studies suggest, therefore, that there is an insulin-pancreatic acinar axis that plays a major role in pancreatic function.

The Insulin-Pancreatic Acinar Axis

Vascular anatomy of the pancreas In 1882, Kuhne and Lea' noticed that the pancreas of the living rabbit contained many discrete vascular regions, and they compared the vessels of these regions with glomerular tufts. It was not until 1893 that Laguesse2 suggested that the vascular regions might be secreting something into the blood which prevented the normal animal from becoming diabetic. He suggested that the vascular areas be called the islets of Langerhans, in memory of their discoverer.3 Subsequently, several authors have compared the morphology of endocrine and exocrine vessels and have demonstrated that the capillaries of the islets are wider and more densely arranged than are the exocrine capillaries surrounding the islet.4-8 These authors, with one exception, noticed that capillary connections existed between the vessels of the islets and the capillaries of the exocrine gland. The presence of these capillary connections is central to the theme of this review. As islets receive their blood supply via short arterioles6 7 and as the capillaries of the islets are often wider than those of the exocrine tissue, it is reasonable to assume that the capillary pressure is higher in the islet vessels than in the adjacent exocrine vessels. So where islet-exocrine connections exist the direction of flow should be predominantly outwards. This centrifugal flow has been demonstrated by cinematography in the pancreas of the living rabbit.9 10 The frequency of these endocrine-exocrine connecting vessels (Figure) has been shown by several authors, using a variety of injection techniques.1'-'5 Two authors have suggested that the centrifugal flow from endocrine to exocrine pancreas plays a significant role in the metabolic activity of the exocrine pancreas.16 17 This review considers the evidence for this relationship, which depends upon the presence of very high concentrations of hormones in the capillaries leaving the islet under different metabolic conditions.

https://gut.bmj.com/content/gutjnl/22/2/158.full.pdf

J.C. Palla

Papers

Effect of insulin on pancreatic amylase and chymotrypsinogen.

Action de la biosynthèse de l'amylase et de quelques autres enzymes du pancréas de rat

Comparative study of the control of amylase biosynthesis in rat pancreas and parotid glands