Hyperosmolality and pancreatic blood flow

Glucose-induced time-dependent potentiation of insulin release, but not islet blood perfusion, in anesthetized rats

BACKGROUND

Repeated administration of glucose in vivo leads to a time-dependent potentiation of insulin release. Glucose is also known to stimulate pancreatic islet blood flow, but whether this is associated with a time-dependent potentiation is unknown. We therefore repeatedly administered glucose to anesthetized rats and evaluated effects on insulin release and islet blood flow.

METHODS

Male Wistar-Furth rats, anesthetized with thiobutabarbital, were injected intravenously with 1 ml of saline or glucose at times 0, 30 and 60 min. The combinations used were saline + saline + saline (SSS), glucose + saline + saline (GSS), saline + saline + glucose (SSG) and glucose + glucose + glucose (GGG). Regional organ blood flow values were measured 3 min after the final injection with a microsphere technique, and at this time also serum insulin concentrations were determined with ELISA.

RESULTS

Serum insulin concentrations as well as total pancreatic, pancreatic islet and duodenal blood flow were higher in SSG and GGG-treated rats when compared to those given SSS and GSS. However, only insulin concentrations, not blood flow values, were higher in GGG rats when compared to SSG animals.

CONCLUSIONS

Glucose-induced time-dependent potentiation of insulin release occurs in vivo in thiobutabarbital-anesthetized rats, but is not associated with a further increase in islet blood flow.

Secretagogue-stimulated pancreatic secretion is differentially regulated by constitutive NOS isoforms in mice

Nitric oxide (NO) and NO synthase (NOS) play controversial roles in pancreatic secretion. NOS inhibition reduces CCK-stimulated in vivo pancreatic secretion, but it is unclear which NOS isoform is responsible, because NOS inhibitors lack specificity and three NOS isoforms exist: neuronal (nNOS), endothelial (eNOS), and inducible (iNOS). Mice having individual NOS gene deletions were used to clarify the NOS species and cellular interactions influencing pancreatic secretion. In vivo secretion was performed in anesthetized mice by collecting extraduodenal pancreatic duct juice and measuring protein output. Nonselective NOS blockade was induced with N(omega)-nitro-L-arginine (L-NNA; 10 mg/kg). In vivo pancreatic secretion was maximal at 160 pmol.kg(-1).h(-1) CCK octapeptide (CCK-8) and was reduced by NOS blockade (45%) and eNOS deletion (44%). Secretion was unaffected by iNOS deletion but was increased by nNOS deletion (91%). To determine whether the influence of NOS on secretion involved nonacinar events, in vitro CCK-8-stimulated secretion of amylase from isolated acini was studied and found to be unaltered by NOS blockade and eNOS deletion. Influence of NOS on in vivo secretion was further examined with carbachol. Protein secretion, which was maximal at 100 nmol.kg(-1).h(-1) carbachol, was reduced by NOS blockade and eNOS deletion but unaffected by nNOS deletion. NOS blockade by L-NNA had no effect on carbachol-stimulated amylase secretion in vitro. Thus constitutive NOS isoforms can exert opposite effects on in vivo pancreatic secretion. eNOS likely plays a dominant role, because eNOS deletion mimics NOS blockade by inhibiting CCK-8 and carbachol-stimulated secretion, whereas nNOS deletion augments CCK-8 but not carbachol-stimulated secretion.

Comparative effects of secretin (SEC) and cholecystokinin-octapeptide (CCK-8) on pancreatic microcirculation

Using epifluorescent microscopy, we investigated the dynamic changes in pancreatic microcirculation in vivo after bolus administration of secretin (SEC) (0.1-10.0 micrograms/100 g body wt) and cholecystokinin-octapeptide (CCK-8) (0.005-1.2 micrograms/100 g body wt) in pentobarbital-anesthetized rats. Pancreatic capillary red cell velocity as a monitor for pancreatic capillary blood flow was measured in 1-min intervals from 2 min prior to 8 min following bolus infusion of SEC or CCK-8. Physiological concentrations of SEC did not increase pancreatic capillary blood flow. However, pharmacological SEC concentrations induced a dose-dependent increase in pancreatic capillary blood flow (to 162 +/- 19% of baseline; P < 0.05), due to an increase in blood flow velocity (to 153 +/- 18% of baseline; P < 0.05). In contrast, bolus administration of physiological CCK-8 concentrations, which have been proven to stimulate enzyme secretion, induced a transient and dose-dependent increase in pancreatic capillary blood flow (to 235 +/- 24% of baseline; P < 0.05), due to an increase in blood flow velocity (to 184 +/- 13% of baseline; P < 0.05) and capillary diameters (+0.63 +/- 0.15 micron; P < 0.05).

Stimulated pancreatic exocrine secretion does not require pancreatic hyperemia in rats. Potential cholinergic role

Although blood flow and cholinergic tone influence gastric and salivary gland secretion, their role in pancreatic secretion is poorly defined. The purpose of the present study was: (1) to test the hypothesis that an increase in pancreatic blood flow accompanies stimulated pancreatic exocrine secretion, and (2) to examine the effects of cholinergic agents on basal and stimulated blood flow using hydrogen gas clearance. Stimulated pancreatic exocrine secretion (secretin 0.4, 0.8, 1.6 micrograms/kg/hr) resulted in a significant (P < 0.005) increase in secretory volume; however, pancreatic blood flow was not significantly changed, and a negative correlation between blood flow and secretion was observed. A pharmacologic dose of secretin (5.0 micrograms/kg/hr) resulted in a significant (P < 0.05) increase in pancreatic blood flow, which was inhibited by atropine (5.0 micrograms/kg/hr) infusion. Although 2-deoxyglucose caused a significant decrease (P < 0.03) in basal pancreatic blood flow, atropine had no effect on basal blood flow levels. These observations suggest that: (1) under physiologic conditions, secretin- or 2-deoxyglucose-stimulated pancreatic secretion does not require pancreatic hyperemia; (2) a pharmacologic dose of secretin does produce pancreatic hyperemia, perhaps through a local cholinergic mechanism; (3) peripheral cholinergic tone does not contribute significantly to basal pancreatic blood flow; and (4) basal pancreatic blood flow may be influenced by central mechanisms.

Stimulation by glucose of the blood flow to the pancreatic islets of the rat

Blood flow to the pancreatic islets of the rat was estimated with the microsphere technique. Experiments with microspheres of different sizes (diameter 10, 15 or 50 micron) showed that optimal results were obtained with 10-micron spheres. Localization of microspheres either within or outside the islets was accomplished by freeze-thawing of the pancreas, making it transparent, so that both islets and microspheres could be distinguished in dark field illumination. Thus, microscopic examination of the freeze-thawed pancreas allowed the microspheres to be counted separately in both the endocrine and exocrine parenchyma. Under basal conditions, pancreatic blood flow was calculated as 0.60 ml X min-1 X g-1 (w/w). The islets accounted for about 10% of the total pancreatic blood flow, corresponding to 0.069 ml/min per whole pancreas. A bolus dose of glucose increased pancreatic blood flow to 0.75 ml X min-1 X g-1 (p less than 0.05), while the fractional islet blood flow rose to 15.1% (p less than 0.001) corresponding to 0.125 ml X min-1 X pancreas-1 (p less than 0.001). The glucose-induced increase in pancreatic blood flow mainly resulted from increased flow to the pancreatic tail, while the corresponding increase in islet blood flow was uniformly distributed throughout the pancreas. Injection of the non-metabolizable glucose-derivate. 3-0-methyl-D-glucose, affected neither the pancreatic nor the islet blood flow. The data indicate that the islets receive more of the pancreatic blood flow than would be accounted for by their relative volume and that glucose preferentially stimulates blood flow to the islets.