Secretin and pancreatic islet blood flow in anesthetized rats: increased insulin secretion with no augmentation of blood perfusion

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.

Structure-Activity Relationship Studies of N- and C-Terminally Modified Secretin Analogs for the Human Secretin Receptor

The pleiotropic role of human secretin (hSCT) validates its potential use as a therapeutic agent. Nevertheless, the structure of secretin in complex with its receptor is necessary to develop a suitable therapeutic agent. Therefore, in an effort to design a three-dimensional virtual homology model and identify a peptide agonist and/or antagonist for the human secretin receptor (hSR), the significance of the primary sequence of secretin peptides in allosteric binding and activation was elucidated using virtual docking, FRET competitive binding and assessment of the cAMP response. Secretin analogs containing various N- or C-terminal modifications were prepared based on previous findings of the role of these domains in receptor binding and activation. These analogs exhibited very low or no binding affinity in a virtual model, and were found to neither exhibit in vitro binding nor agonistic or antagonistic properties. A parallel analysis of the analogs in the virtual model and in vitro studies revealed instability of these peptide analogs to bind and activate the receptor.

Metabolic effects of secretin

Secretin (Sct), traditionally a gastrointestinal hormone backed by a century long research, is now beginning to be recognized also as a neuroactive peptide. Substantiation by recent evidence on the functional role of Sct in various regions of the brain, especially on its potential neurosecretion from the posterior pituitary, has revealed Sct's physiological actions in regulating water homeostasis. Recent advances in understanding the functional roles of central and peripheral Sct has been made possible by the development of Sct and Sct receptor (SctR) knockout animal models which have led to novel approaches in research on the physiology of this brain-gut peptide. While research on the role of Sct in appetite regulation and fatty acid metabolism has been initiated recently, its role in glucose homeostasis is unclear. This review focuses mainly on the metabolic role of Sct by discussing data from the last century and recent discoveries, with emphasis on the need for revisiting and elucidating the role of Sct in metabolism and energy homeostasis.

Pioglitazone reverses insulin resistance and impaired CCK-stimulated pancreatic secretion in eNOS(-/-) mice: therapy for exocrine pancreatic disorders?

In mice, eNOS (endothelial nitric oxide synthase) maintains in vivo pancreatic secretory responses to carbachol or cholecystokinin octapeptide (CCK-8), maintains insulin sensitivity, and modulates pancreatic microvascular blood flow (PMBF). eNOS(-/-) mice are insulin resistant, and their exocrine pancreatic secretion is impaired. We hypothesized that the reduced exocrine pancreatic secretion in eNOS(-/-) mice is due to insulin resistance or impaired PMBF. To test this hypothesis, we gave eNOS(-/-) and wild-type (WT) mice pioglitazone (20 or 50 mg.kg(-1).day(-1)), an insulin-sensitizing peroxisome proliferator-activated receptor-gamma (PPAR-gamma) activator, and measured pancreatic protein secretion evoked by CCK-8 (160 pmol.kg(-1).h(-1), a maximal stimulus). We also measured insulin resistance, serum glucose, C-peptide, insulin, pancreatic RNA digestive enzyme expression, and PMBF (microsphere technique). In WT mice, pioglitazone did not increase CCK-8-stimulated protein output over baseline. In eNOS(-/-) mice, however, pioglitazone substantially increased the low CCK-8-stimulated protein output that is characteristic of these mutant mice (P < 0.005). Pioglitazone abolished the CCK-8-evoked hyperinsulinemia (P < 0.005) and increased insulin sensitivity of eNOS(-/-) mice (P < 0.05), the latter based on hyperinsulinemic-euglycemic clamp studies. Pioglitazone had no effect on PMBF or pancreas mRNA expression of insulin or digestive enzymes. We conclude that in hyperinsulinemic eNOS(-/-) mice, a nonobese model of insulin resistance relevant to diabetes mellitus and possibly chronic pancreatitis, reduced pancreatic secretion is caused, at least in part, by insulin resistance. Insulin-sensitizing PPAR-gamma agonists such as pioglitazone may thus simultaneously correct endocrine and exocrine pancreatic disorders.

Effect of exogenous cholecystokinin on islet blood flow in anesthetized rats

Although a number of studies have investigated the effect of cholecystokinin (CCK) on pancreatic blood flow and exocrine function, few have addressed the effect of CCK on islet blood flow. Here, we studied the effect of exogenous CCK on islet blood flow in anesthetized rats. Islet blood flow was measured by the color microsphere method. Bolus intravenous administration of CCK (10 microg/kg) significantly increased pancreatic and islet blood flow in control Long-Evans Tokushima Otsuka (LETO) rats, but not in Otsuka Long-Evans Tokushima Fatty (OLETF) rats lacking CCK-A receptors. Since fractional islet blood flow expressed as a percentage of whole pancreatic blood flow was decreased after CCK administration in LETO rats, the vasodilating effect of CCK appeared to be stronger in exocrine than endocrine tissue. Although vagotomy failed to alter the CCK-induced increase in pancreatic and islet blood flow, pretreatment with nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine completely prevented the increase in pancreatic and islet blood flow. Our results demonstrated that exogenous CCK is a potent vasodilator of exocrine as well as islet vasculature via CCK-A receptors, and that such action is mediated by a NO-dependent mechanism rather than by vagal mechanisms.

Multiple injections of coloured microspheres for islet blood flow measurements in anaesthetised rats: influence of microsphere size

We investigated if coloured microspheres could be used for repeated measurements of pancreatic islet blood flow in rats. An initial injection of 1.0-1.5 x 10(5) microspheres (black colour), with a size of 10 or 15 microm, was made into the ascending aorta, while an arterial reference sample was collected from the femoral artery. Twelve min later, 1 ml of saline or 30% D-glucose was injected intravenously. Three min after this injection a second injection of 10- or 15-microm microspheres (green colour) was given. The animals were then killed, and the pancreas and adrenals were removed and samples (150-200 mg) were secured from the duodenum, ileum, colon, right kidney and liver. The microsphere contents were determined with the aid of a freeze-thawing technique and blood flow values were calculated. Our results suggest that 10-microm microspheres, but not 15-microm microspheres, provide reproducible islet and total pancreatic blood flow measurements when repeatedly injected. Values for the blood flow to the intestines, kidney and liver were less sensitive to the size of the microspheres. We conclude that repeated administration of 15-microm microspheres induces a high risk for erroneous islet and total pancreatic blood flow measurements, whereas two such measurements can be performed if 10-microm microspheres are used.