Note on the blood supply of the ducts of the rabbit pancreas

The Foundation for Engineering a Pancreatic Islet Niche

Progress in diabetes research is hindered, in part, by deficiencies in current experimental systems to accurately model human pathophysiology and/or predict clinical outcomes. Engineering human-centric platforms that more closely mimic in vivo physiology, however, requires thoughtful and informed design. Summarizing our contemporary understanding of the unique and critical features of the pancreatic islet can inform engineering design criteria. Furthermore, a broad understanding of conventional experimental practices and their current advantages and limitations ensures that new models address key gaps. Improving beyond traditional cell culture, emerging platforms are combining diabetes-relevant cells within three-dimensional niches containing dynamic matrices and controlled fluidic flow. While highly promising, islet-on-a-chip prototypes must evolve their utility, adaptability, and adoptability to ensure broad and reproducible use. Here we propose a roadmap for engineers to craft biorelevant and accessible diabetes models. Concurrently, we seek to inspire biologists to leverage such tools to ask complex and nuanced questions. The progenies of such diabetes models should ultimately enable investigators to translate ambitious research expeditions from benchtop to the clinic.

Pancreatic Blood Flow with Special Emphasis on Blood Perfusion of the Islets of Langerhans

The pancreatic islets are more richly vascularized than the exocrine pancreas, and possess a 5- to 10-fold higher basal and stimulated blood flow, which is separately regulated. This is reflected in the vascular anatomy of the pancreas where islets have separate arterioles. There is also an insulo-acinar portal system, where numerous venules connect each islet to the acinar capillaries. Both islets and acini possess strong metabolic regulation of their blood perfusion. Of particular importance, especially in the islets, is adenosine and ATP/ADP. Basal and stimulated blood flow is modified by local endothelial mediators, the nervous system as well as gastrointestinal hormones. Normally the responses to the nervous system, especially the parasympathetic and sympathetic nerves, are fairly similar in endocrine and exocrine parts. The islets seem to be more sensitive to the effects of endothelial mediators, especially nitric oxide, which is a permissive factor to maintain the high basal islet blood flow. The gastrointestinal hormones with pancreatic effects mainly influence the exocrine pancreatic blood flow, whereas islets are less affected. A notable exception is incretin hormones and adipokines, which preferentially affect islet vasculature. Islet hormones can influence both exocrine and endocrine blood vessels, and these complex effects are discussed. Secondary changes in pancreatic and islet blood flow occur during several conditions. To what extent changes in blood perfusion may affect the pathogenesis of pancreatic diseases is discussed. Both type 2 diabetes mellitus and acute pancreatitis are conditions where we think there is evidence that blood flow may contribute to disease manifestations. © 2019 American Physiological Society. Compr Physiol 9:799-837, 2019.

Pancreatic islet blood flow and its measurement

Pancreatic islets are richly vascularized, and islet blood vessels are uniquely adapted to maintain and support the internal milieu of the islets favoring normal endocrine function. Islet blood flow is normally very high compared with that to the exocrine pancreas and is autonomously regulated through complex interactions between the nervous system, metabolites from insulin secreting β-cells, endothelium-derived mediators, and hormones. The islet blood flow is normally coupled to the needs for insulin release and is usually disturbed during glucose intolerance and overt diabetes. The present review provides a brief background on islet vascular function and especially focuses on available techniques to measure islet blood perfusion. The gold standard for islet blood flow measurements in experimental animals is the microsphere technique, and its advantages and disadvantages will be discussed. In humans there are still no methods to measure islet blood flow selectively, but new developments in radiological techniques hold great hopes for the future.

The complex exocrine-endocrine relationship and secondary diabetes in exocrine pancreatic disorders

The pancreas is a dual organ with exocrine and endocrine functions. The interrelationship of the endocrine-exocrine parts of the pancreas is a complex one, but recent clinical and experimental studies have expanded our knowledge. Many disorders primarily of the exocrine pancreas, often solely in the clinical realm of gastroenterologists are associated with diabetes mellitus (DM). Although, the DM becoming disorders are often grouped with type 2 diabetes, the pathogenesis, clinical manifestations and management differ. We review here data on the association of exocrine-endocrine pancreas, the many hormones of the pancreas and their possible effects on the exocrine functions followed by data on the epidemiology, pathogenesis, and management of DM in chronic pancreatitis, cystic fibrosis, pancreatic cancer, and clinical states after pancreatic surgery.

Islet Vasculature as a Regulator of Endocrine Pancreas Function

The islets of Langerhans consist of endocrine cells embedded in a network of specialized capillaries that regulate islet blood flow. Despite evidence for a critical role of islet perfusion in endocrine pancreas function, there is information to support no fewer than three models of endocrine cell perfusion, emphasizing the lack of a universally accepted physiological theory. Islet blood flow is regulated by signals, such as hormones and nutrients that reach the islet vasculature from distant tissues via the bloodstream. In addition, islet perfusion determines communication between endocrine and exocrine cells and between different types of endocrine cells within islets. Interest in islet microcirculation has increased after improvements in islet transplantation, a therapy for diabetes mellitus that requires revascularization of grafted islets in a new host organ. Abnormal revascularization is thought to be partly responsible for differences in graft and native islet function. Similarly, angiogenesis has been shown to be a critical step in the transformation of islet hyperplasia to neoplasia.

Duct ligation and pancreatic islet blood flow in rats: physiological growth of islets does not affect islet blood perfusion

OBJECTIVES

The aim of this study was to evaluate islet blood-flow changes during stimulated growth of the islet organ without any associated functional impairment of islet function.

DESIGN

A duct ligation encompassing the distal two-thirds of the pancreas was performed in adult, male Sprague-Dawley rats.

METHODS

Pancreatic islet blood flow was measured in duct-ligated and sham-operated rats 1, 2 or 4 weeks after surgery. In some animals studied 4 weeks after surgery, islet blood flow was also measured also during hyperglycaemic conditions.

RESULTS

A marked atrophy of the exocrine pancreas was seen in all duct-ligated rats. Blood glucose and serum insulin concentrations were normal. An increased islet mass was only seen 4 weeks after surgery. No differences in islet blood perfusion were noted at any time point after duct ligation. In both sham-operated and duct-ligated rats islet blood flow was increased during hyperglycaemia; the response was, however, slightly more pronounced in the duct-ligated part of the gland.

CONCLUSIONS

Normal, physiological islet growth does not cause any major changes in the islet blood perfusion or its regulation. This is in contrast to findings during increased functional demands on the islets or during deteriorated islet function, when increased islet blood flow is consistently seen.

Association between endocrine pancreas and ductal system. More than an epiphenomenon of endocrine differentiation and development?

Traditional histological descriptions of the pancreas distinguish between the exocrine and the endocrine pancreas, as if they were two functionally distinct glands. This view has been proven incorrect and can be considered obsolete. Interactions between acinar and islet tissues have been well established through numerous studies that reveal the existence of anatomical and functional relationships between these compartments of the gland. Less attention, however, has traditionally been paid to the relationships occurring between the endocrine pancreas and the ductal system. Associations between islet tissue and ducts are considered by most researchers as only a transient epiphenomenon of endocrine development. This article reviews the evidence that has emerged in the last 10 years demonstrating the existence of stable, close, and systematic relationships between these two pancreatic compartments. Functional and pathophysiological implications are considered, and the existence of an "acinar-duct-islet" axis is put forward. The pancreas appears at present to be an integrated organ composed of three functionally related components of well-orchestrated endocrine and exocrine physiological responses.

Angioarchitecture of the atrophic pancreas

The pancreas has a complex vasculature which comprises both exocrine and endocrine structures. Copper deficiency induces highly selective acinar cell degeneration and progressive noninflammatory lipomatosis in pancreas while Langerhans islets, ducts, and nerves remain unaffected. Pancreatic vasculature was examined in rats that had dietary copper deficiency to characterize changes in the angioarchitecture of the gland. This model was used to assess the degree to which the vasculature of non-acinar components of the gland are potentially altered under conditions of exocrine atrophy. Ultrastructure of pancreas was examined by histology, enzyme histochemistry and immunohistochemistry, corrosion casting and scanning electron microscopy, in situ vascular staining, microsphere injection, biochemical analysis, and morphometry in copper-deficient rats. Results show that no acute angiopathic changes indicative of vascular disorganization accompany atrophy. Only a reduction in the complexity of the capillary beds, which normally vascularize the dense acinar parenchyma, was found. Microsphere quantitation also showed that blood flow to the lipomatous gland remains intact. Furthermore, analysis of the angioarchitecture of the atrophied pancreas supports a largely autonomous blood supply to islets and ducts. These observations support the hypothesis that while the vasculature of the atrophied gland is modified in vascular regions severely targeted by acinar necrosis, the overall structural features of the angioarchitecture are preserved. The atrophied gland thus provides an experimental model to study the vascular routes supplying islet and ductal blood flow within the complex pancreatic circulation.

Comparative analysis of insulo-acinar portal system in rats, guinea pigs, and dogs

The insulo-acinar portal system in the rat, guinea pig, and dog was comparatively analyzed using corrosion casting method in scanning electron microscopy and confocal laser scanning microscopy. In all animals examined, there were three types of arterioles according to their destination: 1) the arteriole which supplied the capillary glomerulus of the islet, 2) the arterioles which directly branched out into capillaries around the acini, and 3) the arterioles which supplied the duct system. In the rat, the afferent vessel usually ended in the cortical layer of the islet and its main branches ran along this layer before giving secondary capillary branches into the deeper regions, while in the dog and guinea pig, the region where the afferent arterioles branched out into secondary capillary branches varied among individual islets. There were three types of efferent vessels of the islet: 1) the insulo-acinar portal vessels that radiated from the islet to join the capillary network in the exocrine pancreas, 2) the emissary venules of the islet, leading directly into the systemic circulation, and 3) the insulo-ductal portal vessels which drained into the peri-ductal capillary network. In the rat and guinea pig, the intralobular islets possessed both the insulo-acinar portal vessels and the emissary venules, while the interlobular islets possessed emissary venules with occasionally occurring insulo-acinar portal vessels. In the dog, most of the islets were located within the lobule and possessed preferentially the insulo-acinar portal vessels. In this animal, the lobule was supplied by several microvascular units, in the center of which was located the capillary glomerulus of the islet. The peri-insular zone of the unit was mainly supplied by the insulo-acinar portal vessels, while the periphery, the tele-insular zone, was directly supplied by arterioles as well. The venules originated at the periphery of the unit. The islet in the dog had virtually no emissary venules. Confocal laser scanning microscopy of the rat islets showed that B cells occupied the core of all islets. The microvascular architecture within the rat islet appeared to be organized as to drain blood from the A and D cell area to the B cell area of the islet.

Washout kinetics of blood cells from the perfused pancreas of normoglycemic and diabetic rats

The aim of the present study was to evaluate vascular compartments within the rat pancreas with compartmental analysis of the outflow of blood cells from the perfused gland in situ. The presence of two vascular compartments requiring approximately 15 and 30 min for emptying, was noted in normoglycemic rats. The pancreas from diabetic rats, in which the islet beta-cells had been destroyed by intravenous injection of streptozotocin 1 or 6 weeks earlier, demonstrated the same outflow characteristics. It is therefore likely that these observations reflect the presence of two vascular compartments within the rat pancreas, possibly representing the islet-acinar vasculature and the ductal vasculature.

Suppressive role of the islet-acinar axis in the perfused rat pancreas

BACKGROUND

The stimulating effects of insulin on the exocrine pancreas are well known. The effects of other islet hormones, however, are controversial. The aim of the present study was to determine whether the islet-acinar axis, as a whole, is stimulatory or inhibitory. Because we have shown that retrograde perfusion reverses the islet-acinar directed microcirculation, retrograde perfusion was expected to remove the overall effects of islet hormones from the acinar tissue.

METHODS

Rat pancreata were perfused (7 mmol/L glucose plus 3 mmol/L mixed amino acids) either anterogradely or retrogradely. Pancreatic juice flow, protein output, and amylase output were measured.

RESULTS

When perfusion was switched from anterograde to retrograde, juice flow increased threefold without changes in protein and amylase output. When cerulein (10(-10) mol/L) was infused, retrograde protein and amylase responses were larger than anterograde responses (each, n = 7; 2.71 +/- 0.23 vs. 1.71 +/- 0.11 mg/40 minutes; 173 +/- 17 vs. 98 +/- 8 U/40 minutes; mean +/- SE; both, P < 0.01). Somatostatin-14 and rat pancreatic polypeptide (each, 10(-9) mol/L) reduced the retrograde protein and amylase responses, but not juice flow, to the anterograde response levels. Conversely, these peptides did not affect exocrine function during anterograde perfusion.

CONCLUSIONS

A suppressive role of the islet-acinar axis via endogenous somatostatin and/or pancreatic polypeptide is suggested.

Microvasculature of the pancreas. Relation to pancreatitis

Local or generalized alteration of microcirculation may be expected in diseases of the pancreas. Changes may range from increased permeability of capillaries to hemorrhage. Tissue necrosis may result from prolonged ischemia owing to intravascular coagulation and severely impaired blood flow. It is possible to observe early microvascular changes by intravital microscopy. Klar and coworkers have demonstrated by this method that isovolemic hemodilution improves blood flow under conditions that would otherwise lead to tissue damage. This paper presents the basic microcirculation of the pancreas and the changes that accompany pancreatic disease. It emphasizes that concentration on the changes in microcirculation that accompany the early manifestations of pancreatic diseases, particularly pancreatitis, may reveal important clues to their pathogenesis.

Glucose tolerance and pancreatic islet blood flow in rats after intraperitoneal administration of different anesthetic drugs

A comparison of the effects of different anesthetics on the pancreatic islet blood flow as measured with a microsphere technique and the blood sugar homeostasis in rats was made in rats anesthetized with an IP injection of either thiobutabarbital sodium (TB), pentobarbital sodium (PB), chloral hydrate (CH), chloral hydrate + pentobarbital (CP) or ketamine + xylazine (KX). The mean arterial blood pressure was similar (approximately 100 mm Hg) in all animals except those given KX in which a 20-30% increase was observed. The serum insulin concentrations were increased in rats given CH and CP, but not in the other groups, when compared with TB rats. An intraperitoneal glucose tolerance test (2 g glucose/kg BW 15 min after induction of anesthesia) showed a marked glucose intolerance in the KZ rats, in which the glucose concentrations were elevated for 5 h. Also animals anesthetized with CP and CH were glucose intolerant when compared with TB animals. The whole pancreatic blood flow was similar in TB, PB and CP rats, but was almost doubled in CH-rats and markedly decreased in KX rats. Islet blood flow was also increased by CH and decreased by KX when compared with TB rats, whilst PB and CP did not affect the islet blood flow. It is concluded that TB and PB are suitable anesthetics for the study of pancreatic islet blood flow.