Mechanisms underlying mechanosensitivity of mesenteric afferent fibers to vascular flow

Spinal afferent neurons, with endings in the intestinal mesenteries, have been shown to respond to changes in vascular perfusion rates. The mechanisms underlying this sensitivity were investigated in an in vitro preparation of the mesenteric fan devoid of connections with the gut wall. Afferent discharge increased when vascular perfusion was stopped ("flow off"), a response localized to the terminal vessels just prior to where they entered the gut wall. The flow-off response was compared following pharmacological manipulations designed to determine direct mechanical activation from indirect mechanisms via the vascular endothelium or muscle. Under Ca(2+)-free conditions, responses to flow off were significantly augmented. In contrast, the myosin light chain kinase inhibitor wortmannin (1 microM, 20 min) did not affect the flow-off response despite blocking the vasoconstriction evoked by 10 microM l-phenylephrine. This ruled out active tension, generated by vascular smooth muscle, in the response to flow off. Passive changes caused by vessel collapse during flow off were speculated to affect sensory nerve terminals directly. The flow-off response was not affected by the N-, P-, and Q-type Ca(2+) channel blocker omega-conotoxin MVIIC (1 muM intra-arterially) or the P2X receptor/ion channel blocker PPADS (50 microM). However, ruthenium red (50 microM), a blocker of nonselective cation channels, greatly reduced the flow-off response and also abolished the vasodilator response to capsaicin. Our data support the concept that mesenteric afferents sense changes in vascular flow during flow off through direct mechanisms, possibly involving nonselective cation channels. Passive distortion in the fan, caused by changes in blood flow, may represent a natural stimulus for these afferents in vivo.     

Adrenomedullin release in the rat mesenteric resistance artery

Adrenomedullin (AM) is a potent vasodilator peptide whose major source is the vascular wall. In the present study, the mechanism of release of AM was investigated in the rat mesenteric resistance artery. The isolated mesenteric vascular bed was perfused with Krebs solution at a constant flow rate (5 ml/min) and AM in the perfusate was measured by a highly sensitive enzyme immunoassay (Immunoenzymometric assay; IEMA) method. In preparations without endothelium, spontaneous release of AM was detected in the perfusate (68.7+/-5.8 fmol/ml, n=45). Periarterial nerve stimulation (PNS, 4 and 8 Hz) caused 11.4+/-3.9% (4 Hz) and 9.1+/-3.5% (8 Hz) decreases in the spontaneous release of AM. Removal of Ca2+ from the medium did not affect the spontaneous AM release, but abolished the PNS-induced inhibition of spontaneous AM release. Perfusion of 10nM calcitonin gene-related peptide (CGRP) or 0.1 microM capsaicin (inducer of CGRP release) inhibited significantly the spontaneous AM release. PNS (8 Hz)-induced inhibition of spontaneous AM release was antagonized by CGRP(8-37) (CGRP receptor antagonist). These results suggest that AM is mainly released from vascular smooth muscle cells of the rat mesenteric artery and endogenous or exogenous CGRP inhibits AM release.     

Effect of mesenteric vascular congestion on reflex control of renal blood flow

Portal hypertension initiates a splenorenal reflex, whereby increases in splenic afferent nerve activity and renal sympathetic nerve activity cause a decrease in renal blood flow (RBF). We postulated that mesenteric vascular congestion similarly compromises renal function through an intestinal-renal reflex. The portal vein was partially occluded in anesthetized rats, either rostral or caudal to the junction with the splenic vein. Portal venous pressure increased (6.5 +/- 0.1 to 13.2 +/- 0.1 mmHg; n = 78) and mesenteric venous outflow was equally obstructed in both cases. However, only rostral occlusion increased splenic venous pressure. Rostral occlusion caused a fall in RBF (-1.2 +/- 0.2 ml/min; n = 9) that was attenuated by renal denervation (-0.5 +/- 0.1 ml/min; n = 6), splenic denervation (-0.2 +/- 0.1 ml/min; n = 11), celiac ganglionectomy (-0.3 +/- 0.1 ml/min; n = 9), and splenectomy (-0.5 +/- 0.1 ml/min; n = 6). Caudal occlusion induced a significantly smaller fall in RBF (-0.5 +/- 0.1 ml/min; n = 9), which was not influenced by renal denervation (-0.2 +/- 0.2 ml/min; n = 6), splenic denervation (-0.1 +/- 0.1 ml/min; n = 7), celiac ganglionectomy (-0.1 +/- 0.3 ml/min; n = 8), or splenectomy (-0.3 +/- 0.1 ml/min; n = 7). Renal arterial conductance fell only in intact animals subjected to rostral occlusion (-0.007 +/- 0.002 ml.min(-1).mmHg(-1)). This was accompanied by increases in splenic afferent nerve activity (15.0 +/- 3.5 to 32.6 +/- 6.2 spikes/s; n = 7) and renal efferent nerve activity (32.7 +/- 5.2 to 39.3 +/- 6.0 spikes/s; n = 10). In animals subjected to caudal occlusion, there were no such changes in renal arterial conductance or splenic afferent/renal sympathetic nerve activity. We conclude that the portal hypertension-induced fall in RBF is initiated by increased splenic, but not mesenteric, venous pressure, i.e., we did not find evidence for intestinal-renal reflex control of the kidneys.     

Capsaicin-sensitive nerves modulate reactive hyperemia in rat gut.

Reactive hyperemia (RH) is a local, vascular response that occurs following release from mechanical occlusion of an artery, with restoration of intra-arterial pressure. The mechanism of this postocclusion hyperemia in the gut has not been identified, although metabolic, myogenic, and neurogenic mediators of this response have been proposed. The present study was conducted to evaluate a possible modulatory role for sensory innervation of the intestinal vasculature in RH, using acute and chronic treatment with capsaicin applied in different ways. In anesthetized rats, the velocity of flowing blood in the gut was determined continuously with a pulsed Doppler velocimeter, and arterial pressure was determined with a transducer. The increase in calculated intestinal vascular conductance at the height of RH (Ch), the excess volume of blood accumulating during RH, and the duration of the hyperemia were also used to quantify RH after occluding the anterior mesenteric artery for 30, 60, and 120 sec. In the initial control group of rats, the maximal increases in the velocity of flowing blood during RH were 61 +/- 4%, 90 +/- 7%, and 129 +/- 10% of control, conductances were increased to 192 +/- 5%, 222 +/- 12%, and 267 +/- 15% of control, volumes were 3.5 +/- 0.6 ml, 7.2 +/- 0.4 ml, and 16.2 +/- 1.8 ml, and durations of hyperemia were 78 +/- 5 sec, 93 +/- 6 sec, and 178 +/- 7 sec, respectively, after each elapsed period of occlusion. Acute treatment with periarterial capsaicin significantly decreased peak conductances in RH by 15-35% for all occlusions tested and reduced both volume and duration values. Rats treated with capsaicin in neonatal life exhibited reduced Ch values, as did adult rats treated chronically with capsaicin. Both periarterial and intrajejunal treatment with capsaicin decreased the duration of RH. Hexamethonium increased both Ch and the duration of RH and tended to reverse reductions in these parameters caused by capsaicin. These results suggest that sensory innervation of the intestinal vasculature exerts a modulatory influence in the regulation of intestinal RH.     

Lipopolysaccharide-induced changes in mesenteric afferent sensitivity of rat jejunum in vitro: role of prostaglandins

Bacterial translocation across the intestinal mucosal barrier leads to a macrophage-mediated inflammatory response, visceral hyperalgesia, and ileus. Our aim was to examine how mediators released into mesenteric lymph following LPS treatment influence intestinal afferent sensitivity and the role played by prostanoids in any sensitization. Intestinal lymph was collected from awake rats following treatment with either saline or LPS (5 mg/kg ip). Extracellular multiunit afferent recordings were made from paravascular mesenteric nerve bundles supplying the rat jejunum in vitro following arterial administration of control lymph, LPS lymph, and LPS. Mesenteric afferent discharge increased significantly after LPS lymph compared with control lymph. Peak discharge occurred within 2 min and remained elevated for 5 to 8 min. This response was attenuated by pretreatment with naproxen (10 microM), and restored upon addition of prostaglandin E(2) (5 microM) in the presence of naproxen, but AH6809 (5 microM), an EP(1)/EP(2) receptor(s) antagonist, failed to decrease the magnitude of LPS lymph-induced response. LPS itself also stimulated mesenteric afferent discharge but was unaffected by naproxen. TNF-alpha was significantly increased in LPS lymph compared with control lymph (1,583 +/- 197 vs. 169 +/- 38 pg/ml, P < 0.01) but exogenous TNF-alpha failed to evoke any afferent nerve discharge. We concluded that inflammatory mediators released from the gut into mesenteric lymph during endotoxemia have a profound effect on afferent discharge. These mediators influence afferent firing via the release of local prostaglandins.     

Role of bradykinin in acid-induced mesenteric hyperemia and duodenal villous damage

Intestinal mucosal capsaicin-sensitive afferent nerves mediate, in part, the mesenteric hyperemia after intraduodenal acidification. The hyperemia plays a role in protecting the duodenal mucosa against acid damage. We tested the hypothesis that bradykinin contributes to this protective hyperemia. A specific antagonist of bradykinin will attenuate the hyperemia and exacerbate duodenal villous damage induced by acid. Study 1: Intravenous vehicle, or the specific bradykinin B2 receptor antagonist (HOE 140) was administered to anesthetized rats. This was followed by intraduodenal bolus administration of 160 microM capsaicin or 0.1 N HCl, and then intravenous bradykinin. Study 2: Intravenous administration of vehicle or HOE 140 was followed by duodenal perfusion with 0.1 N HCl. Superior mesenteric artery blood flow (pulsed Doppler flowmetry) (Study 1) and duodenal villous damage (histology) (Study 2) were recorded. HOE 140 significantly reduced the hyperemia induced by bradykinin and intraduodenal capsaicin or acid. Deep villous injury was significantly increased after treatment with HOE 140. These findings support the hypothesis that acid-induced and afferent nerve-mediated mesenteric hyperemia is modulated by a mechanism that involves bradykinin B2 receptor. Antagonism of bradykinin B2 receptor also increased acid-induced deep duodenal villous damage. Thus, maintenance of bradykinin-mediated mesenteric hyperemia, is a previous unrecognized mechanism associated with protection of the rat duodenal mucosa against acid-induced damage.     

Vagal modulation of intestinal afferent sensitivity to systemic LPS in the rat

The central nervous system modulates inflammation in the gastrointestinal tract via efferent vagal pathways. We hypothesized that these vagal efferents receive synaptic input from vagal afferents, representing an autonomic feedback mechanism. The consequence of this vagovagal reflex for afferent signal generation in response to LPS was examined in the present study. Different modifications of the vagal innervation or sham procedures were performed in anesthetized rats. Extracellular mesenteric afferent nerve discharge and systemic blood pressure were recorded in vivo before and after systemic administration of LPS (6 mg/kg iv). Mesenteric afferent nerve discharge increased dramatically following LPS, which was unchanged when vagal efferent traffic was eliminated by acute vagotomy. In chronically vagotomized animals, to eliminate both vagal afferent and efferent traffic, the increase in afferent firing 3.5 min after LPS was reduced to 3.2 +/- 2.5 impulses/s above baseline compared with 42.2 +/- 2.0 impulses/s in controls (P < 0.001). A similar effect was observed following perivagal capsaicin, which was used to eliminate vagal afferent traffic only. LPS also caused a transient hypotension (<10 min), a partial recovery, and then persistent hypertension that was exacerbated by all three procedures. Mechanosensitivity was increased 15 min following LPS but had recovered at 30 min in all subgroups except for the chronic vagotomy group. In conclusion, discharge in capsaicin-sensitive mesenteric vagal afferents is augmented following systemic LPS. This activity, through a vagovagal pathway, helps to attenuate the effects of septic shock. The persistent hypersensitivity to mechanical stimulation after chronic vagal denervation suggests that the vagus exerts a regulatory influence on spinal afferent sensitization following LPS.     

Capsaicin-sensitive intestinal mucosal afferent mechanism and body fat distribution

This report summarizes clinical and experimental data in support of the hypothesis that capsaicin-sensitive intestinal mucosal afferent mechanism plays a role in regulating body fat distribution. Epidemiological data have revealed that the consumption of foods containing capsaicin is associated with a lower prevalence of obesity. Rural Thai people consume diets containing 0.014% capsaicin. Rodents fed a diet containing 0.014% capsaicin showed no change in caloric intake but a significant 24% and 29% reduction in the visceral (peri-renal) fat weight. Increase in intestinal blood flow facilitates nutrient energy absorption and decrease in adipose tissue blood flow facilitates storage of nutrient energy in adipose tissue. Stimulation of intestinal mucosal afferent nerves increases intestinal blood flow, but decreases visceral (mesenteric) adipost tissue blood flow. In in vitro cell studies capsaicin has a direct effect on adipocytes. Intravenous capsaicin produces measurable plasma level and subcutaneous capsaicin retards accumulation of adipose tissue. The data on a direct effect of oral capsaicin on adipose tissue at remote sites, however, are conflicting. Capsaicin absorbed from the gut lumen is almost completely metabolized before reaching the general circulation. Oral capsaicin significantly increases transient receptor potential vanilloid type-1 (TRPV1) channel expression as well as TRPV1 messenger ribonucleic acid (mRNA) in visceral adipose tissue. In TRPV1 knockout mice on a high fat diet the body weight was not significantly different in the absence or presence of oral capsaicin. In rodent experiments, daily intragastric administration of capsaicin for two weeks led to defunctionalization of intestinal mucosal afferent nerves, manifested by loss of acute mucosal capsaicin-induced effects; but not the corneal afferent nerves, with preservation of the paw wiping reflex of the eye exposed briefly to dilute capsaicin. The latter indicated the absence of an oral capsaicin effect at one remote site. There was an accompanying decrease and an increase in the proportion of body fat in visceral and subcutaenous compartments, respectively. Taken together, if oral capsaicin could regulate adipose tissue distribution, the process might involve the effect of intestinal mucosal afferent nerves in modulating intestinal and visceral adipose tissue blood flow. The hypothesis that the intestinal mucosal afferent mechanism is a plausible therapeutic target for abating visceral obesity deserves to be further evaluated.     

Bombesin-like immunoreactive material in the gut, and the effect of bombesin on the stomach circulatory system of an elasmobranch fish, Squalus acanthias

The distribution, nature and amount of bombesin-like immunoreactivity (IR) in the gastrointestinal canal and its afferent vessels was investigated in the spiny dogfish (Squalus acanthias) together with the in vitro effect of synthetic bombesin on perfusion flow through the vascularly perfused dogfish stomach. Nerve fibres showing bombesin-like IR frequently occurred in the walls of the anterior mesenteric and coeliac arteries and the intrinsic vessels of the gut. Chromatographic studies revealed that multiple peaks of bombesin-like IR material were present in extracts of the spiny dogfish gastrointestinal vessels. Bombesin-like IR was also present in muscle and mucosal layers of the gut with higher levels in muscle compared with mucosa, and higher levels in the stomach than in the intestine and the rectum. Exogenous bombesin increased the flow through the vasculary perfused spiny dogfish stomach in a dose-dependent manner. Studies with tetrodotoxin and atropine showed that bombesin probably exerts its effect directly on the vascular musculature. It is concluded from this study that bombesin-like material is present in nerves innervating the gut circulatory system of the spiny dogfish. Bombesin may affect the blood-flow to the gastrointestinal canal, possibly via a direct effect on vascular smooth muscle.     

A peptidergic component to vagally induced tracheal vasodilation in the dog.

The purpose of the study was to determine the extent that peptidergic afferent and efferent pathways contribute to vagally induced vasodilation in the trachea of the dog. The change in vascular resistance of the tracheal branch of the cranial thyroid artery and the trachealis responses were determined in 28 anesthetized, paralyzed, and mechanically ventilated dogs. After propranolol (2 mg/kg) and phentolamine (1.5 mg/kg), stimulation of the superior laryngeal nerves (NS; 15 Hz, 7 V, 2 ms, 30 s) caused a decrease in vascular resistance of 11.7 +/- 0.8% and a tracheal contraction of 5.2 +/- 4.7 cmH2O. Atropine (1.5 mg/kg) reduced the fall in vascular resistance to 4.7 +/- 0.8% (P less than 0.01), whereas tracheal contraction was abolished. Thiorphan (1.5 mg), a neutral endopeptidase inhibitor, augmented the decrease in vascular resistance (8.8 +/- 0.6%; P less than 0.01) to NS. After hexamethonium (0.5 mg/kg), NS still caused a small decrease in TVR (2.9 +/- 0.9%; P less than 0.05), which was abolished by capsaicin. In atropinized dogs, capsaicin reduced the fall in vascular resistance after NS; the residual vasodilation was virtually abolished by hexamethonium. Acetylcholine (10(-3) mg/kg) decreased vascular resistance (15.7 +/- 3.0%), and the effect was abolished by atropine. We conclude that there is noncholinergic nonadrenergic vagally induced tracheal vasodilation that is peptidergic. The peptidergic vasodilation appears to be mediated by both afferent and efferent pathways.     

Perfusion pressure manipulation in porcine sepsis: effects on intestinal hemodynamics

Limited information is available about selection of the threshold for arterial blood pressure in critically ill patients, particularly in sepsis when normal organ blood flow autoregulation may be altered. The present experimental study investigated whether increasing perfusion pressure using norepinephrine in normotensive hyperdynamic porcine bacteremia affects intestinal macro- and microcirculation. Nine pigs received continuous i.v. administration of Pseudomonas aeruginosa (PSAE) to develop hyperdynamic, normotensive (mean arterial pressure [MAP] 65 mm Hg) sepsis. Norepinephrine was used to achieve 10-15 % increase in MAP. Mesenteric arterial blood flow (Q(gut)), ileal mucosal microvascular perfusion (LDF(gut)) and ileal-end-tidal PCO(2) gap (PCO(2) gap) were measured before norepinephrine, after 60 min of norepinephrine infusion and 60 min after norepinephrine infusion had been discontinued. During a 12 h period of PSAE infusion all pigs developed hyperdynamic circulation with significantly decreased MAP. Although the mesenteric blood flow remained unchanged, infusion of PSAE resulted in a gradual fall of ileal microvascular perfusion, which was associated with progressively rising PCO(2) gap. Norepinephrine which induced a 10-15 % increase in perfusion pressure (i.e. titrated to attain near baseline values of MAP) affected neither Q(gut) nor the intestinal blood flow distribution (Q(gut)/CO). Similarly, norepinephrine did not change either LDF(gut) or PCO(2) gap. In this hyperdynamic, normotensive porcine bacteremia, norepinephrine-induced increase in perfusion pressure exhibited neither beneficial nor deleterious effects on intestinal macrocirculatory blood flow and ileal mucosal microcirculation. The lack of changes suggests that the gut perfusion was within its autoregulatory range.     

Laryngeal C-fiber afferents are not involved in the apneic response to laryngeal wood smoke in anesthetized rats

Laryngeal exposure to wood smoke in rats evokes a reflex apnea which is mediated through superior laryngeal afferents (J. Appl. Physiol. 83: 723-730, 1997). To study the role of laryngeal C-fiber afferents in eliciting this response, capsaicin aerosol (0.05 - 0.2 microg/ml) and 5 ml of wood smoke were delivered separately into a functionally isolated larynx of anesthetized Sprague-Dawley rats at a constant flow rate of 1.4 ml/s, while animals breathed spontaneously. Studies were repeated after either an intravenous injection of ruthenium red (2 mg/kg; n = 8), a perineural capsaicin treatment (200 microg/ml for 5 min; n = 8) of the superior laryngeal nerves, or a perineural sham treatment (n = 8); Ruthenium red inhibits the stimulation of afferent C-fiber nerve endings by capsaicin, whereas perineural capsaicin treatment selective blocks the conduction of C-fiber afferents. Either ruthenium red or perineural capsaicin treatment abolished the apneic response to laryngeal capsaicin, but did not significantly affect the apneic response to laryngeal wood smoke. Furthermore, the apneic responses to both types of irritants were not significantly altered by perineural sham treatment, yet were completely eliminated by a subsequent denervation of superior laryngeal nerves. Our results suggest that superior laryngeal C-fiber afferents are not involved in eliciting the reflex apneic response to laryngeal wood smoke in anesthetized rats. It is speculated that this response may result mainly from the stimulation of myelinated afferents, possibly laryngeal irritant receptors.     

Gut mucosal damage during endotoxic shock is due to mechanisms other than gut ischemia.

Whether the gut alterations seen during sepsis are caused by microcirculatory hypoxia or disturbances in cellular metabolic pathways associated with mitochondrial respiration remains controversial. We hypothesized that hypoperfusion or hypoxia and local production of nitric oxide might play an important role in the development of gut mucosal injury during endotoxic shock and investigated their roles by using differing levels of fluid resuscitation and occlusion of the superior mesenteric artery (SMA). Anesthetized New Zealand rabbits were allocated to group I (sham, n = 8); group II [low-dose endotoxin (LPS, Escherichia coli-055:B5, 150 microg/kg)/fluid resuscitation (12 ml x kg(-1) x h(-1)); n = 8]; group III [high-dose LPS (1 mg/kg)/fluid resuscitation (12 ml x kg(-1) x h(-1)); n = 8]; group IV [high-dose LPS (1 mg/kg)/hypovolemia (4 ml x kg-1 x h(-1) fluids); n = 8]; and group V [SMA ligation/fluid resuscitation (12 ml x kg(-1) x h(-1)); n = 4]. Luminal gut lactate concentrations and PCO2 gap increased in groups IV and V (P < 0.05), reflecting alterations in gut perfusion. Interestingly, significant histological alterations were observed in all LPS groups but not in group V. Blood and luminal gut nitrate/nitrite concentrations increased only in group IV. The mechanism of gut injury in endotoxic shock seems unrelated to hypoxia and release of nitric oxide. Gut dysfunction may occur as a result of so-called "cytopathic hypoxia."     

Effect of splenic extract on plasma volume and renal function in the rat

A hypotensive and natriuretic factor has recently been extracted from the rat spleen. Experiments were designed to investigate the mechanisms underlying the increase in urine output caused by splenic extract. Rat spleens were homogenized in phosphate buffered saline (PBS), centrifuged, subjected to ultrafiltration (mol. wt. cutoff 10,000), extracted on C18 affinity columns and dried. After reconstitution in isotonic saline, the extract was injected IV into conscious rats. Splenic extract caused a decrease in plasma volume (17.4+/-1.1 to 15.8+/-1.0 ml at 1 hr), and a delayed increase in urine output (1.8+/-0.2 to 4.0+/-0.4 ml/hr at 2 hr). There were no such changes in the muscle-injected control group. The increase in urine output was accompanied by an increase in glomerular filtration rate (splenic extract, 2.2+/-0.2 to 5.9+/-1.6 ml/min; muscle extract, 2.9+/-0.4 to 3.1+/-0.6 ml/min). Renal blood flow in the splenic extract-injected group fell during the course of the experiment so that, at 120 min., it was significantly lower both with respect to its baseline value and the muscle control group (splenic extract 22.1+/-0.2 to 17.5+/-2.2 ml/min; muscle extract 24.4+/-4.1 to 23.3+/-3.8 ml/min). During this same period, mean arterial pressure in the splenic extract group also fell from 98+/-2 to 91+/-4 mmHg. Renal vascular conductance therefore did not change. In conclusion, splenic extract causes a primary decrease in plasma volume and a delayed increase in urine output that is mediated, at least in part, by an increase in glomerular filtration rate. It is suggested that the splenic factor(s) probably achieves this by differential vasodilatation of the afferent glomerular arteriole and constriction of the efferent glomerular arteriole.     

Systemic and mesenteric O2 metabolism in endotoxic pigs: effect of ibuprofen and meclofenamate

The effect of two chemically dissimilar cyclooxygenase inhibitors was studied in pentobarbital-anesthetized endotoxic pigs. Animals in groups II-IV were infused with Escherichia coli lipopolysaccharide (LPS, 150 micrograms/kg) and resuscitated with normal saline (1.2 ml.kg-1.min-1). Animals in group I (n = 4) were resuscitated as above but were not infused with LPS. Animals in group II (n = 7) served as endotoxic controls. Pigs in groups III (n = 6) and IV (n = 5) were pre- and posttreated with ibuprofen (10 mg/kg bolus then 10 mg.kg-1.h-1 and meclofenamate (5 mg/kg then 5 mg.kg-1.h-1, respectively. Ileal intramucosal hydrogen ion concentration [( H+]) was estimated tonometrically. In group I, cardiac index (CI), mean arterial pressure (MAP), superior mesenteric arterial perfusion (QSMA), and mesenteric O2 delivery (DO2) increased significantly, but other variables were unchanged. After infusion of LPS in group II, MAP and systemic vascular resistance index were markedly diminished but CI was well preserved. In this group, QSMA, systemic DO2, and mesenteric DO2 decreased, whereas systemic O2 uptake (VO2) and gut [H+] increased; mesenteric VO2 was unchanged. Compared with pigs in group II, pigs treated with ibuprofen or meclofenamate manifested improved systemic and mesenteric DO2. In groups III and IV, QSMA remained normal, increased systemic VO2 was not observed, and gut intramucosal acidosis was ameliorated. Increased intramucosal [H+] in group II suggests that QSMA was inadequate. The salutary effects of ibuprofen and meclofenamate suggest that inadequate mesenteric perfusion was mediated, at least in part, by cyclooxygenase-derived metabolites or arachidonic acid.     

Vascular perfusion limits mesenteric lymph flow during anaphylactic hypotension in rats

To determine fluid extravasation in the splanchnic vascular bed during anaphylactic hypotension, the mesenteric lymph flow (Q(lym)) was measured in anesthetized rats sensitized with ovalbumin, along with the systemic arterial pressure (P(sa)) and portal venous pressure (P(pv)). When the antigen was injected into the sensitized rats (n = 10), P(sa) decreased from 125 ± 4 to 37 ± 2 mmHg at 10 min with a gradual recovery, whereas P(pv) increased by 16 mmHg at 2 min and returned to the baseline at 10 min. Q(lym) increased 3.3-fold from the baseline of 0.023 ± 0.002 g/min to the peak levels of 0.075 ± 0.009 g/min at 2 min and returned to the baseline within 12 min. The lymph protein concentrations increased after antigen, a finding indicating increased vascular permeability. To determine the role of the P(pv) increase in the antigen-induced increase in Q(lym), P(pv) of the nonsensitized rats (n = 10) was mechanically elevated in a manner similar to that of the sensitized rats by compressing the portal vein near the hepatic hilus. Unexpectedly, P(pv) elevation alone produced a similar increase in Q(lym), with the peak comparable to that of the sensitized rats. This finding aroused a question why the antigen-induced increase in Q(lym) was limited despite the presence of increased vascular permeability. Thus the changes in splanchnic vascular surface area were assessed by measuring the mesenteric arterial flow. The mesenteric arterial flow was decreased much more in the sensitized rats (75%; n = 5) than the nonsensitized P(pv) elevated rats (50%; n = 5). In conclusion, mesenteric lymph flow increases transiently after antigen presumably due to increased capillary pressure of the splanchnic vascular bed via downstream P(pv) elevation and perfusion and increased vascular permeability in anesthetized rats. However, this increased extravasation is subsequently limited by decreases in vascular surface area and filtration pressure.     

Impact of intrinsic blood flow regulation in cirrhosis: maintenance of hepatic arterial buffer response

The hepatic arterial buffer response (HABR) effectively controls total blood perfusion in normal livers, but little is known about blood flow regulation in cirrhosis. We therefore studied the impact of HABR on blood perfusion of cirrhotic livers in vivo. After 8-wk CCl(4) treatment to induce cirrhosis, 18 anesthetized rats (and 18 noncirrhotic controls) were used to simultaneously assess portal venous and hepatic arterial inflow with miniaturized ultrasonic flow probes. Stepwise hepatic arterial blood flow (HAF) or portal venous blood flow (PVF) reduction was performed. Cirrhotic livers revealed a significantly reduced total hepatic blood flow (12.3 +/- 0.9 ml/min) due to markedly diminished PVF (7.3 +/- 0.8 ml/min) but slightly increased HAF (5.0 +/- 0.6 ml/min) compared with noncirrhotic controls (19.0 +/- 1.6, 15.2 +/- 1.3, and 3.8 +/- 0.4 ml/min). PVF reduction caused a significant HABR, i.e., increase of HAF, in both normal and cirrhotic livers; however, buffer capacity of cirrhotic livers exceeded that of normal livers (P < 0.05) by 1. 7- to 4.5-fold (PVF 80% and 20% of baseline). Persistent PVF reduction for 1, 2, and 6 h demonstrated constant HABR in both groups. Furthermore, HABR could be repetitively provoked, as analyzed by intermittent PVF reduction. HAF reduction did not induce changes of portal flow in either group. Because PVF is reduced in cirrhosis, the maintenance of HAF and the preserved HABR must be considered as a protective effect on overall hepatic circulation, counteracting impaired nutritive blood supply via the portal vein.     

Capsaicin-sensitive vagal fibres and 5-HT3-, gastrin releasing peptide- and cholecystokinin A-receptors are involved in distension-induced inhibition of gastric emptying in the rat.

The present study was undertaken to investigate how the activation of gastric mechanoreceptors by distension of the stomach in conscious gastric fistula rats influences gastric emptying; and the roles of capsaicin sensitive vagal afferent fibres and the 5-HT3, GRP and CCK-A receptors involved in mediating these responses. To activate mechanoreceptors by non-nutrient dependent pathways, methylcellulose in saline was used to distend the stomach (5 cm H2O) and the subsequent emptying of saline was examined immediately, and at 3, 5 and 10 min following distension. Prior distension delayed the subsequent emptying of saline instilled into the stomach compared with non-distended controls (2.28+/-0.09 ml/5 min; P < 0.001). Topical application of capsaicin, completely abolished the distension-induced inhibition of gastric emptying when compared with vehicle treated rats (2.82+/-0.09 vs. 2.38+/-0.04 ml/5 min; P < 0.001). Peripheral administration of a GRP antagonist (2258 U89UJ, 1 mg/kg), and a 5-HT3 antagonist (BRL4369UA, 50 microg/kg) significantly reversed (2.56+/-0.14 ml/5 min; P < 0.05 and 2.61+/-0.07 ml/5 min; P < 0.01; respectively) the delay in gastric emptying induced by distension. When the rats were treated with the CCK-A antagonist, gastric emptying of saline following distension was also significantly facilitated (2.56+/-0.07 ml/5 min; P < 0.001). In contrast, the CCK-B/gastrin receptor antagonist had no significant effect on the distension induced delay in gastric emptying (1.95+/-0.12 ml/5 min). The present results suggest that gastric distension in conscious gastric fistula rats delays gastric emptying by activating capsaicin-sensitive extrinsic afferent nerve fibres. Moreover, the results also indicate that distension-induced mechanisms involve GRP, 5-HT3 and CCK-A receptors, but not CCK-B receptors.     

Involvement of nitric oxide in mesenteric vascular reactivity following intraperitoneal pancreatic juice in rats

The purposes of this study were to examine the protein expressions of endothelial and inducible nitric oxide synthase (eNOS and iNOS) of the rat intestinal smooth muscle, and to elucidate the role of nitric oxide (NO) in the reactivity of the superior mesenteric artery (SMA) to vasoconstrictors following intraperitoneal (i.p.) injection of pancreatic juice. Immunohistochemistry was used to observe the protein expressions of eNOS and iNOS in the intestinal tissues 15 h after i.p. injection of pancreatic juice (1 ml/100 g body weight). To test the vascular reactiveness, SMA was isolated and perfused with Tyrode's solution at a constant flow rate of 5 ml/min. The changes in perfusion pressure as the measure of contractile responses to phenylephrine (PE) were monitored. I.P. injection of pancreatic juice induced increases of plasma levels of tumor necrosis factor α (TNFα) (P < 0.001; N = 7) and NO (P < 0.001; N = 7). Nω-nitro-L-arginine methyl ester (L-NAME) reduced the release of TNFα and NO. There were 8.3 ± 1.2-fold and 11.4 ± 2.8-fold increases in the protein expressions of eNOS and iNOS, respectively, in the intestinal tissue after pancreatic juice injection. PE (10?? ~ 10?? M) produced a dose-dependent vasoconstrictive effects on the SMA bed. Contractile responses to PE were attenuated in pancreatic juice-treated group. Addition of L-NAME (10?? M) resulted in full recovery of the responses to phenylephrine in SMA bed, while aminoguanidine (AG, 10?? M) caused only partial recovery. Our results indicate that i.p. injection of pancreatic juice results in a decrease in vascular reactivity of mesenteric vessels that is dependent on both eNOS and iNOS expressions in the intestinal vascular bed. Overproduction of NO elicits intestinal low vascular reactivity.     

Effect of vasodilator prostaglandins on the vascular renin-angiotensin system

The interaction of prostaglandin (PG) with the vascular renin-angiotensin (R-A) system was examined by studies on the effects of PGI2, PGE2 and the inhibitor of PG synthesis, indomethacin, on the release of angiotensin II (Ang II) from isolated rat mesenteric arteries. The Ang II released from the vasculature was measured after its concentration in a Sep-Pak C18 cartridge connected to the perfusion system. After perfusion with drugs, the specific vascular renin activity inhibited by anti-renin antibody was determined. The basal perfusion pressure was constant (19.6 +/- 1.1 mmHg) at a flow rate of 4.5 ml/min, and was not changed by any of these drugs. The basal levels of Ang II release and vascular renin activity were 44 +/- 5 pg/30 min and 113 +/- 8 pg Ang I/mg protein/hr, respectively. Infusion of PGI2 (10(-6) M) significantly decreased both Ang II release (p less than 0.01) and vascular renin activity (p less than 0.05) as compared with the control levels. Infusion of PGE2 (10(-6) M) decreased Ang II release significantly (p less than 0.05) and vascular renin activity slightly. Infusion of indomethacin (10(-6)M) increased vascular renin activity significantly (p less than 0.01). Pretreatment with indomethacin (10 mg/kg, ip) for 2 days also increased vascular renin activity (p less than 0.01). These results indicate that in contrast to their effects on the renal R-A system, PGs suppress the vascular R-A system and that these two local vasoactive factors interact to regulate vascular tone.