Cardiac natriuretic peptides and pancreatic islet blood flow in anesthetized rats.

The aim of the study was to evaluate effects of cardiac natriuretic peptides on splanchnic circulation, especially to the pancreatic islets. Pentobarbital-anesthetized rats were infused intravenously (0.01 ml/min for 20 min) with saline, atrial natriuretic peptide (ANP; 0.25 or 0.5 microg/kg BW/min), brain natriuretic peptide (BNP; 0.5 microg/kg BW/min) or C-type natriuretic peptide (CNP; 0.5 or 2.0 microg/kg BW/min). Splanchnic blood perfusion was then measured with a microsphere technique. Mean arterial blood pressure was decreased by ANP and BNP, but not by CNP. The animals given the highest dose of ANP became markedly hypoglycemic, whilst no such effects were seen in any of the other groups of animals. Total pancreatic blood flow was decreased by the highest dose of CNP, whereas no change was seen after administration of the other peptides. Islet blood flow was increased by the highest dose of ANP. Neither BNP nor CNP affected islet blood flow. None of the natriuretic peptides influenced duodenal, colonic or arterial hepatic blood flow. It is concluded that cardiac natriuretic peptides exert only minor effects on splanchnic blood perfusion in anesthetized rats. However, islet blood perfusion may be influenced by ANP.     

Moderate hypothermia induces a preferential increase in pancreatic islet blood flow in anesthetized rats

The aim of the study was to characterize the effects of induced moderate hypothermia on splanchnic blood flow, with particular reference to that of the pancreas and the islets of Langerhans. We also investigated how interference with the autonomic nervous system at different levels influenced the blood perfusion during hypothermia. For this purpose, hypothermia (body temperature of 28 degrees C) was induced by external cooling, whereas normothermic (37.5 degrees C) anesthetized Sprague-Dawley rats were used as controls. Some rats were pretreated with either propranolol, yohimbine, atropine, hexamethonium, or a bilateral abdominal vagotomy. Our findings suggest that moderate hypothermia elicits complex, organ-specific circulatory changes, with increased perfusion noted in the pylorus, as well as the whole pancreas and the pancreatic islets. The pancreatic islets maintain their high blood perfusion through mechanisms involving both sympathetic and parasympathetic mediators, whereas the increased pyloric blood flow is mediated through parasympathetic mechanisms. Renal blood flow was decreased, and this can be prevented by ganglionic blockade and is also influenced by beta-adrenoceptors.     

Intraportal GLP-1 infusion increases nonhepatic glucose utilization without changing pancreatic hormone levels

After a meal, glucagon-like peptide-1 (GLP-1) levels in the hepatic portal vein are elevated and are twice those in peripheral blood. The aim of this study was to determine whether any of GLP-1's acute metabolic effects are initiated within the hepatic portal vein. Experiments consisted of a 40-min basal period, followed by a 240-min experimental period, during which conscious 42-h-fasted dogs received glucose intraportally (4 mgxkg(-1)xmin(-1)) and peripherally (as needed) to maintain arterial plasma glucose levels at approximately 160 mg/dl. In addition, saline was given intraportally (CON; n = 8) or GLP-1 (1 pmolxkg(-1)xmin(-1)) was given into the hepatic portal vein (POR; n = 11) or the hepatic artery (HAT; n = 8). Portal vein plasma GLP-1 levels were basal in CON, 20x basal in POR, and 10x basal in HAT, whereas levels in the periphery and liver were the same in HAT and CON. The glucose infusion rate required to maintain hyperglycemia was significantly greater in POR (8.5 +/- 0.7 mgxkg(-1)xmin(-1), final 2 h) than in either CON or HAT (6.0 +/- 0.5 or 6.7 +/- 1.0 mgxkg(-1)xmin(-1), respectively). There were no differences among groups in either arterial plasma insulin (24 +/- 2, 23 +/- 3, and 23 +/- 3 microU/ml for CON, POR, and HAT, respectively) or glucagon (23 +/- 2, 30 +/- 3, and 25 +/- 2 pg/ml) levels during the experimental period. The increased need for glucose infusion reflected greater nonhepatic as opposed to liver glucose uptake. GLP-1 infusion increased glucose disposal independently of changes in pancreatic hormone secretion but only when the peptide was delivered intraportally.     

Inorganic nitrite stimulates pancreatic islet blood flow and insulin secretion

Reactive nitrogen and oxygen species have been proposed to be involved in control of insulin release from the pancreatic β cell. Recent evidence suggests that the supposedly inert anions nitrate and nitrite are metabolized in blood and tissues to form nitric oxide (NO) and other bioactive nitrogen oxides. Here we present evidence for a novel stimulatory role of nitrite in influencing pancreatic islet physiology via a dual mechanism, involving both indirect enhancement (through microcirculation redistribution) and direct insulinotropic effects on the β cell. In rats, intraperitoneal injection of sodium nitrite increased pancreatic islet blood flow by 50% and serum insulin concentrations by 30%, while whole pancreatic blood flow and glycemia remained unaffected. Nitrite also dose dependently enhanced insulin secretion from rat β cells in vitro under nonstimulatory glucose concentrations. This effect was not mimicked by nitrate and was abolished by the guanylyl cyclase (GC) inhibitor ODQ and the NO scavenger cPTIO. It was also mimicked by a cyclic GMP agonist (8-CPT-cGMP) and a classical NO donor (NONOate). Interestingly, a reactive oxygen species scavenger (vitamin E analog, Trolox) abolished the insulin secretion induced by nitrite. We conclude that nitrite exerts dual stimulatory effects on pancreatic islet function, including enhancement of islet blood flow and subsequent insulin secretion in vivo and direct stimulation of insulin release in vitro. The insulinotropic effect of nitrite is cGMP-dependent and involves formation of reactive nitrogen and oxygen species.     

The effects of ethanol on pancreatic blood flow in awake and anesthetized dogs

The pathophysiology of alcohol-induced acute pancreatitis is not clear. Ischemic injury has been suggested as a possible mechanism. To examine the effects of ethanol on pancreatic and splanchnic blood flow, measurements were made in fasted, conditioned awake dogs before and after iv infusion of ethanol (1.7 g/kg). At 30 min blood ethanol concentration ranged between 60 and 150 mg/dl and at 60 min between 166 and 350 mg/dl. Although cardiac output, aortic pressure, left atrial pressure, and arterial pH did not change, pancreatic flow declined by 39 +/- 12 ml/min/100 g, P less than 0.05 (from 173 +/- 10 ml/min/100 g) at 30 min and was still depressed (by 27 +/- 12 ml/min/100 g, P less than 0.05) at 60 min. Concomitantly, hepatic arterial flow increased. While hepatic and pancreatic flow changed inversely, the correlation (r = -0.17) of these changes was not significant. At comparable blood ethanol concentrations in pentobarbital-anesthetized dogs hepatic arterial flow increased by 11 +/- 3 ml/min/100 g, P less than 0.01 (from 24 +/- 5 ml/min/100 g), but pancreatic flow did not change. Thus, in the awake dog at blood levels that would produce mild to moderate alcoholic intoxication in man, ethanol reduces pancreatic flow. Although hepatic flow increases concomitantly, the relationship of these changes appears to be independent.     

Age-induced changes in pancreatic islet blood flow: evidence for an impaired regulation in diabetic GK rats

The present study aimed to compare longitudinal variations in islet blood perfusion in rats with different degrees of impairment of glucose metabolism. For this purpose, mildly diabetic Goto-Kakizaki (GK) rats, glucose intolerant F(1) hybrids of GK and Wistar (W) rats (H), and control W rats were examined at 5 wk, 12 wk, or 1 yr of age, using the microsphere technique for blood flow measurements. W rats showed progressively increasing islet blood flow (IBF) throughout the experiment. Both GK and H rats demonstrated increasing IBF between 5 and 12 wk. However, H rats showed no further increment in IBF at 1 yr, whereas GK rats displayed a pronounced decrease in IBF between 12 wk and 1 yr of age. The augmented IBF seen in older W rats may constitute an adaptation to the increasing demand for insulin secretion in aging rats. The inability to adapt to the increased demand for insulin secretion by upregulation of islet blood flow could contribute to the progressive deterioration of glucose metabolism seen in the aging GK rat.     

Beta-endorphin infusion alters pancreatic hormone and glucose levels during exercise in rats

β-Endorphin (BE) infusion at rest can influence insulin and glucagon levels and thus may affect glucose availability during exercise. To clarify the effect of BE on levels of insulin, glucagon and glucose during exercise, 72 untrained male Sprague-Dawley rats were infused i.v. with either: (1) BE (bolus 0.05 mg · kg⁻¹ +0.05 mg · kg⁻¹ · h⁻¹, n = 24); (2) naloxone (N, bolus 0.8 mg · kg⁻¹ + 0.4 mg · kg⁻¹, n = 24); or (3) volume-matched saline (S, n = 24). Six rats from each group were killed after 0, 60, 90 or 120 min of running at 22 m · min⁻¹, at 0% gradient. BE infusion resulted in higher plasma glucose levels at 60 min [5.93 (0.32) mM] and 90 min [4.16 (0.29) mM] of exercise compared to S [4.62 (0.27) and 3.41 (0.26 mM] and N [4.97 (0.38) and 3.44 (0.25) mM]. Insulin levels decreased to a greater extent with BE [21.5 (0.9) and 18.3 (0.6) uIU · ml⁻¹] at 60 and 90 min compared to S [24.5 (0.5) and 20.6 (0.6) uIU · ml⁻¹] and N [24.5 (0.4) and 21.6 (0.7) uIU · ml⁻¹] groups. Plasma C-peptide declined to a greater extent at 60 and 90 min of exercise with BE infusion compared to both S and N. BE infusion increased glucagon at all times during exercise compared to S and N. These data suggest that BE infusion during exercise influences plasma glucose by augmenting glucagon levels and attenuating insulin release. Accepted: 26 February 1997     

Pancreatic islet blood flow in conscious rats during hyperglycemia and hypoglycemia

Anesthesia affects general hemodynamics and regulation of organ perfusion. We used colored microspheres to measure pancreatic islet blood flow in conscious rats at two time points, during either hyperglycemia or hypoglycemia. This method, using black and green microspheres, was validated by comparison with previous microsphere experiments and by lack of effect of a nonmetabolizable glucose analog, 3-O-methylglucose, on islet perfusion. Basal and glucose-stimulated islet blood flow levels were similar in pentobarbital sodium-anesthetized and conscious rats. However, the basal distribution of pancreatic blood flow was altered by anesthesia (fractional islet blood flow 5.8 +/- 0.4% in conscious rats, 7.9 +/- 0.8% in pentobarbital-anesthetized rats, P < 0.05). Insulin-induced hypoglycemia significantly increased whole pancreatic blood flow in conscious rats, whereas islet blood flow remained unchanged and fractional islet blood flow was decreased (5.8 +/- 0.5% in the basal state, 4.2 +/- 0.4% during hypoglycemia, P < 0.001). Methylatropine pretreatment significantly increased islet blood flow during hypoglycemia by 181%. This result suggests that prevention of hypoglycemia-induced increase in islet perfusion may be mediated, at least in part, by a cholinergic, vagal muscarinic mechanism.     

Beneficial effects of intercellular interactions between pancreatic islet cells in blood glucose regulation

Glucose homeostasis is controlled by the islets of Langerhans which are equipped with α-cells increasing the blood glucose level, β-cells decreasing it, and δ-cells the precise role of which still needs identifying. Although intercellular communications between these endocrine cells have recently been observed, their roles in glucose homeostasis have not been clearly understood. In this study, we construct a mathematical model for an islet consisting of two-state α-, β-, and δ-cells, and analyze effects of known chemical interactions between them with emphasis on the combined effects of those interactions. In particular, such features as paracrine signals of neighboring cells and cell-to-cell variations in response to external glucose concentrations as well as glucose dynamics, depending on insulin and glucagon hormone, are considered explicitly. Our model predicts three possible benefits of the cell-to-cell interactions: First, the asymmetric interaction between α- and β-cells contributes to the dynamic stability while the perturbed glucose level recovers to the normal level. Second, the inhibitory interactions of δ-cells for glucagon and insulin secretion prevent the wasteful co-secretion of them at the normal glucose level. Finally, the glucose dose-responses of insulin secretion is modified to become more pronounced at high glucose levels due to the inhibition by δ-cells. It is thus concluded that the intercellular communications in islets of Langerhans should contribute to the effective control of glucose homeostasis.     

The effect of secretin on pancreatic blood flow in the awake and anesthetized dog

The canine pancreatic blood flow was studied after iv secretin (resulting in a plasma level commensurate with the postcibal state), and also after larger iv doses and after duodenal acidification. We found that blood flow was unaffected by "physiological" doses of secretin, or by perfusion of duodenum at a pH as low as 2.0, but increased by bolus doses (0.5 CU/kg and above), and by acidification to pH 1.4. Anesthesia does not affect the blood flow response although the bicarbonate response appeared to be blunted under anesthesia. We conclude that increase in pancreatic blood flow is not a physiological effect of secretin.     

Glucose-induced islet blood flow increase in rats: interaction between nervous and metabolic mediators

This study investigated the mechanisms for glucose-induced islet blood flow increase in rats. The effects of adenosine, adenosine receptor antagonists, and vagotomy on islet blood flow were evaluated with a microsphere technique. Vagotomy prevented the islet blood flow increase expected 3, 10, and 20 min after injection of glucose, whereas theophylline (a nonspecific adenosine receptor antagonist) prevented the islet blood flow increase from occurring 10 and 20 min after glucose administration. Administration of selective adenosine receptor antagonists suggested that the response to theophylline was mediated by A1 receptors. Exogenous administration of adenosine did not affect islet blood flow, but local accumulation of adenosine, induced by the adenosine uptake inhibitor dipyridamole, caused a doubling of islet blood flow. In conclusion, the increased islet blood flow seen 3 min after induction of hyperglycemia is caused by the vagal nerve, whereas the increase in islet blood perfusion seen at 10 and 20 min after glucose administration is caused by both the vagal nerve and adenosine.     

Effect of central thyrotropin-releasing hormone on pancreatic blood flow in rats

Central neuropeptides play a role in physiological regulation through the autonomic nervous system. Thyrotropin-releasing hormone (TRH) is a neuropeptide distributed throughout the central nervous system and acts as a neurotransmitter to regulate gastric and hepatic functions through vagal-cholinergic pathways. In this study, the central effect of TRH on pancreatic blood flow was investigated in urethane-anesthetized rats. Pancreatic blood flow was determined by laser Doppler flowmetery. After measurement of basal blood flow, a stable TRH analog, RX 77368 (1-50 ng) or saline was injected intracisternally. Pancreatic blood flow was observed for 120 min thereafter. In some experiments, pretreatment with atropine methyl nitrate (0.15 mg/kg, i.p.), NG-nitro-L-arginine-methyl ester (10 mg/kg, i.v.), or 6-hydroxydopamine (6-OHDA;180 mg/kg, i.p.), or subdiaphragmatic vagotomy was performed. Intracisternal injection of TRH analog dose-dependently increased pancreatic blood flow with a peak response occurring 30 min after injection. The stimulatory effect of TRH analog on pancreatic blood flow was blocked by vagotomy, atropine, and NG-nitro-L-arginine-methyl ester, but not by 6-hydroxydopamine. Intravenous administration of the TRH analog did not influence pancreatic blood flow in the same animal model. These results indicate that TRH acts in the central nervous system to stimulate pancreatic blood flow through vagal-cholinergic and nitric oxide-dependent pathways.     

Pulmonary blood flow distribution in anesthetized ponies.

Results of recent investigations in humans and dogs indicate that gravity-independent factors may be important in determining the distribution of pulmonary blood flow. To further evaluate the role of gravity-independent factors, pulmonary blood flow distribution was examined using 15-microns radionuclide-labeled microspheres in five prone ponies over 5 h of pentobarbital sodium anesthesia. The ponies were killed, and the lungs were excised and dried by air inflation (pressure 45 cmH2O). The dry lungs were cut into transverse slices 1-2 cm thick along the dorsal-ventral axis, parallel to gravity. Radioactivity of pieces cut from alternate slices was measured with a gamma well counter. The main finding was a preferential distribution of pulmonary blood flow to dorsal-caudal regions and higher flow in the center of each lung slice when compared with the slice periphery. Flow was lowest in cranial and ventral areas. Differences of +/- 2 SD were observed between core and peripheral blood flow. No medial-lateral differences were found. Pulmonary blood flow distribution did not change over 5 h of anesthesia, and the basic flow pattern was not different in the left vs. right lung. These results suggest that in the intact prone mechanically ventilated pony (inspired O2 fraction greater than or equal to 0.95) factors other than gravity are primary determinants of pulmonary blood flow.     

Effects of glucose infusion in exercising rats

To determine whether feedforward control of liver glycogenolysis during exercise is subject to negative feedback by elevated blood glucose, glucose was infused into exercising rats at a rate that elevated blood glucose greater than 10 mM. Liver glycogen content decreased 22.4 mg/g in saline-infused rats compared with 13.6 mg/g in glucose-infused rats during the first 40 min of treadmill running (21 m/min, 15% grade). Liver adenosine 3',5'-cyclic monophosphate (cAMP) concentration was significantly lower in the glucose-infused rats during the exercise bout. The concentration of hepatic fructose 2,6-bisphosphate remained elevated throughout the exercise bout in glucose-infused rats but decreased markedly in saline-infused rats. Plasma insulin concentration was higher and plasma glucagon concentration lower in glucose-infused rats than in saline-infused rats during exercise. Early in exercise, liver glycogenolysis proceeds in the glucose-infused rats despite the fact that glucose and insulin concentrations are markedly elevated and liver cAMP is unchanged from resting values. These observations suggest the existence of a cAMP-independent feedforward system for activation of liver glycogenolysis that can override classical negative feedback mechanisms during exercise.     

Role of the hypothalamus and pituitary gland in the development of pancreatic islet sensitivity to glucose in fetal rats

Differences in the pattern of the development of three enzymes of the plasma membrane have been established. The activity of Na, K-ATPase progressively increases, that of adenylate cyclase decreases, whereas the activity of 5-nucleotidase undergoes only slight changes during embryogenesis. Differences between these enzymes were also found with respect to the development of their sensitivity to the regulatory effects of catecholamines. Adrenaline reactivity of adenylate cyclase may be detected already in embryogenesis; it is lower than that in definite muscle tissue increasing during further ontogenesis. Catecholamine reactivity was not found in Na, K-ATPase and 5-nucleotidase up to the 17th day of incubation of chick embryos. The effect of adrenalin was observed at later stages of ontogenesis, it may be initiated by exogeneous cAMP and protein kinase. At postembryonic stages, similarity in the behavior of these enzymes was found with respect to the presence and pattern of their reaction to adrenalin (stimulation), as well as with respect to temporal dynamics of the effect. The data obtained indicate the existence of close connections between these enzymes, which are realized in the sequence adrenoreceptor-adenylate cyclase-cAMP-protein kinase-effector proteins.     

Alterations in pancreatic islet phosphate content during secretory stimulation with glucose

Isolated rat pancreatic islets were perifused and analyzed for phosphate content immediately following the transient increase in the efflux of orthophosphate which occurs when insulin secretion is stimulated by glucose. In some instances, islets were perifused directly following isolation to minimize preparative delay; in others, islets were prelabeled during incubation with [³²P]orthophosphate for 90 min prior to perifusion. In both experimental situations, total islet phosphate content declined 40–50% following exposure to stimulating concentrations of glucose and initiation of enhanced insulin release. In the experiments with prelabeled islets, tissue content of [³²P]orthophosphate fell to a similar extent so that the specific radioactivity of islet orthophosphate was unaffected. Inhibition of heightened insulin release with Ni²⁺ did not modify the decrements in total or radioactive tissue orthophosphate, thus indicating that these responses to islet stimulation reflect events which are proximal to activated exocytosis. Simultaneous analyses for tissue ATP and ADP demonstrated that the efflux in orthophosphate and reduction in tissue orthophosphate content were not mediated via net changes in islet adenine nucleotides. The observations represent the first documentation that a net reduction of tissue inorganic phosphate is one of the early components of stimulus-secretion coupling in isolated pancreatic islets.