The importance of nervous and humoral factors in the control of vascular resistance, blood flow distribution and net fluid absorption in the cat small intestine during hemorrhage

Isoflurane Inhibits Compensatory Intravascular Volume Expansion After Hemorrhage in Sheep

After hemorrhage, blood volume is partially restored by transcapillary refill, a process of spontaneous compensatory intravascular volume expansion that we hypothesized would be inhibited by anesthesia. Six chronically instrumented sheep were subjected to four randomly ordered experiments while conscious or during anesthesia with isoflurane. After plasma volume measurement (indocyanine green), 15% or 45% of the blood volume was withdrawn. To quantify transcapillary refill, mass balance and kinetic calculations utilized repeated measurements of hemoglobin concentration, assuming that transcapillary refill would dilute hemoglobin concentration. After 15% hemorrhage, mean arterial blood pressure remained stable in both conscious and isoflurane-anesthetized sheep (normotensive hemorrhage) but decreased after 45% hemorrhage (hypotensive hemorrhage). After either normotensive or hypotensive hemorrhage, transcapillary refill occurred more rapidly during the first 40 min than during the next 140 min (P < 0.001). In conscious sheep, at 180 min, 57% and 42% of the bled volume had been restored after normotensive and hypotensive hemorrhage, respectively, in contrast to only 13% and 27% (P < 0.001) in isoflurane-anesthetized sheep. A novel kinetic model implicated hemodynamic factors in rapid, early transcapillary refill and decreased plasma oncotic pressure in subsequent slower filling. We conclude that isoflurane inhibits transcapillary refill after both normotensive and hypotensive hemorrhage in sheep.

The effect of the transfusion of stored RBCs on intestinal microvascular oxygenation in the rat

BACKGROUND

Although it is known that the transfusion of stored RBCs does not always improve tissue O(2) consumption under conditions of limited tissue oxygenation, the efficiency of O(2) delivery to the microcirculation by stored RBCs has never been determined.

STUDY DESIGN AND METHODS

In a rat hemorrhagic shock model, the effects of resuscitation with fresh or 28-day-old RBCs stored in CPD plasma, saline-adenine-glucose-mannitol, and CPDA-1 plasma were investigated. Systemic hemodynamic and intestinal oxygenation measures were monitored. Intestinal microvascular PO(2) was determined with the O(2)-dependent quenching of palladium-porphyrin phosphorescence, and the RBC deformability was measured with a Laser-assisted optic rotational cell analyzer.

RESULTS

Hemodynamic and oxygenation measures were significantly decreased during hemorrhagic shock. Intestinal oxygen consumption and mesenteric venous pO(2) were restored with the transfusion of both fresh and stored RBCs, except for CPD-stored RBCs. The intestinal microvascular pO(2) improved only with the transfusion of fresh RBCs. Deformability of the stored RBCs was significantly decreased.

CONCLUSION

In contrast to that of fresh RBCs, the transfusion of stored RBCs did not restore the microcirculatory oxygenation, possibly because of impaired O(2) unloading, but, except for CPD-stored RBCs, the storage-induced changes were not enough to impair intestinal VO(2) and mesenteric venous pO(2).

Angiotensin II mesenteric and renal vasoregulation: dissimilar modulatory effects with nitroprusside

BACKGROUND

The role of systemic arterial pressure for the vascular effects of angiotensin II (Ang II) and the interactions between Ang II and perfusion pressure-dependent local vascular control mechanisms are not well understood. This study addresses these aspects of exogenous Ang II in the mesenteric and renal regional circulations.

METHODS

Ang II was infused in incremental doses (0-200 microg/h) in anesthetized instrumented pigs (n=10). Renal and portal blood flows were measured by perivascular ultrasound. In the second part of the study, sodium nitroprusside (SNP) was infused at doses titrated to keep mean arterial pressure constant, in spite of concurrent Ang II administration.

RESULTS

Powerful dose-dependent vasoconstrictions by Ang II were found in renal and mesenteric vascular beds (at highest Ang II doses vascular resistances increased by 109% and 88% respectively). Ang II-induced vasoconstriction was fully inhibited in the mesenteric, but not in the renal circulation, during conditions of constant mean arterial pressures achieved by SNP infusion.

CONCLUSIONS

Mesenteric, but not renal, vasoconstriction by Ang II was inhibited by pharmacological maintenance of perfusion pressure. This could reflect differences between these vascular beds as regards the importance of co-acting myogenic pressure-dependent vasoconstriction. Alternatively, as the drug chosen for pressure control, sodium nitroprusside, serves as a nitric oxide donor, the relative balance between nitric oxide-mediated vasodilation and Ang II-induced vasoconstriction could have regional differences.

Intestinal transplantation: effects on ileal enteric absorptive physiology

BACKGROUND

The effects of small intestine transplantation on enteric physiology are poorly understood. After orthotopic jejunoileal autotransplantation, dogs develop a severe watery diarrhea and lose up to 15% of their body weight. The cause of these changes has not been explained. Our aim was to determine the influence of jejunoileal autotransplantation on ileal absorption of water, electrolytes, and bile salts and the effects of proabsorptive and prosecretory agents on ileal transport.

METHODS

Seven dogs were studied before and at 2 and 8 weeks after in situ jejunoileal neural and lymphatic isolation (a model of small intestine autotransplantation). With a triple-lumen perfusion technique, net ileal fluxes of water, electrolytes, and bile salts were measured before and at 2 and 8 weeks after this model of jejunoileal autotransplantation. In addition, the effects of an intravenous infusion of vasoactive intestinal polypeptide (a prosecretory agent) and norepinephrine (a proabsorptive agent) on net transport were evaluated.

RESULTS

Dogs developed a profuse diarrhea after this model of autotransplantation. Ileal absorption of water and electrolytes decreased immediately (measured during operation), remained decreased for 2 weeks, and returned toward baseline by 8 weeks. A similar decrease in net flux of bile salts was shown at 2 weeks after transplantation and returned toward baseline by 8 weeks. The prosecretory response of vasoactive intestinal polypeptide on ileal fluxes of water and electrolytes was unchanged, whereas the proabsorptive response to norepinephrine increased after this model of autotransplantation.

CONCLUSIONS

Jejunoileal autotransplantation decreases ileal absorption of water, electrolytes, and bile salts. The profuse watery diarrhea observed in dogs after small intestine autotransplantation may be a secretory and/or a bile salt-induced diarrhea related to the effects of jejunoileal denervation.

Acetate absorption in the normal and secreting rat jejunum

Acetate absorption was studied in rat jejunum using steady state perfusion in vivo. Absorption conformed to apparent saturation kinetics and was similar in magnitude to glucose absorption. When compared with normal saline, acetate perfusion was associated with luminal alkalinisation. There was no difference in total CO2 secretion when similar rates of acetate and glucose absorption were compared, suggesting that total CO2 secretion was the result of mucosal metabolism. Absorption of acetate and propionate were mutually inhibitory. Acetate absorption was also inhibited by Tris-Hepes pH 7.0. When the gut was pretreated with cholera toxin to induce a secretory state, acetate absorption was reduced by 41.9%. This effect could be reproduced if similar water secretion was osmotically induced by the addition of mannitol. These data suggest that acetate is absorbed, at least, partially by non-ionic diffusion in the rat jejunum and that its absorption is reduced in the secreting intestine by solvent drag.

The influence of dopamine agonists and antagonists on indomethacin lesions in stomach and small intestine in rats

Dopamine agents (saline in control groups) were coadministered with indomethacin by either single or repeated application. The ulcerogenic effect (erosions and/or ulcers) of repeated given indomethacin on gastric mucosa differed clearly from that on intestinal mucosa. The effect on intestinal mucosa was markedly greater than after a single dose. The effects of dopamine agents appeared to be more consistent. Domperidone and haloperidol, given as single or repeated doses, strongly aggravated both the gastric and intestinal lesions. Bromocriptine and amantadine had a protective effect. The adverse effects of both dopamine antagonists (increased after repeated administration) were strongly inhibited by the simultaneous administration of either bromocriptine or amantadine. The involvement of the dopamine system (central or peripheral) in the mechanisms that maintain gastric (probably related to cytoprotection also) and intestinal mucosa integrity is therefore suggested.

Correlated electrophysiological and histochemical studies of submucous neurons and their contribution to understanding enteric neural circuits

Neither submucous ganglia, nor intestinal secretomotor reflexes are mentioned in the majority of the textbooks of physiology; because it has been realized only very recently that the submucous neurons may have important influences on whole body water and electrolyte balance. In the present review, we trace the rapid progress that has been made in determining the physiological properties of submucous neurons with known chemistry and projections in the guinea-pig small intestine, and we analyze how the work relates to studies in vivo of the neuronal control of intestinal trans-epithelial fluid transport. Four types of submucous neurons, which appear to be the full complement in the guinea-pig small intestine, have been identified through electrophysiological and histochemical analysis. (1) Cholinergic secretomotor neurons contain immunoreactivity for choline-acetyltransferase (ChAT), calcitonin gene-related peptide (CGRP), cholecystokinin (CCK), neuropeptide Y (NPY), somatostatin (SOM), and in the majority of cases galanin (GAL); these neurons project to the mucosal epithelium. (2) Non-cholinergic secretomotor neurons contain dynorphin (DYN), GAL and vasoactive intestinal peptide (VIP); these neurons project to the mucosa and provide collaterals to submucous arterioles. (3) Cholinergic interneurons contain ChAT alone; these neurons connect with the secretomotor neurons. (4) Presumed sensory neurons contain ChAT and substance P (SP) and have nerve endings in the mucosa. The two groups of secretomotor neurons receive cholinergic synaptic inputs from both myenteric and submucous ganglia. In addition, the DYN/GAL/VIP neurons receive sympathetic inhibitory inputs as well as inhibitory and non-cholinergic excitatory inputs from myenteric ganglia. The ChAT/SP nerve cells in submucous ganglia receive no or very ineffective inputs. From these data, from experiments on transmission from the neurons to the intestinal epithelium, and from studies of secretomotor reflexes in vivo, a correlated functional and structural circuitry of the submucous ganglia and their connections has been deduced. It is concluded that secretomotor reflexes are stimulated by the contents of the lumen during the digestion and absorption of food and that these reflexes cause a proportion of water and electrolytes that are absorbed with nutrients such as glucose to be returned to the lumen. The balance of absorption and secretion of water and electrolytes is controlled by sympathetic inhibitory inputs to secretomotor neurons, the activity in sympathetic pathways being varied to contribute to whole body water and electrolyte balance.

Influence of right atrial stretch and atrial natriuretic factor on rat intestinal fluid content

1. Studies were made on the effects of right atrial stretch and atrial natriuretic factor (ANF) infusion on fluid movement into the intestinal tract. 2. Stimulation of the atrial volume receptors by inflation of an intracardiac balloon in the conscious, unrestrained rat did not change intestinal fluid content under normovolaemic conditions. 3. When the rat was rendered hypovolaemic by peritoneal dialysis (34% deficit in extracellular fluid volume), right atrial stretch significantly increased intestinal fluid content. Under these conditions, inflation of the balloon restored large intestinal fluid content to the pre-dialysis state, i.e. right atrial stretch completely abolished that component of fluid absorption attributable to the extracellular fluid volume deficit. 4. These data suggest that stimulation of the right atrial receptors inhibits intestinal fluid reabsorption but probably does not initiate fluid secretion. 5. There was no evidence that this might be mediated by ANF since rat ANF (twenty-eight residue peptide, Ser-99-Tyr-126), infused for 1 h at doses of 0, 0.05, 0.1 and 0.5 microgram/min, did not cause any detectable changes in the fluid content of the large or small intestine of similarly prepared hypovolaemic rats.

Baroreflex control of jejunal blood flow and fluid transport in cats; effects of yohimbine, an alpha 2-adrenergic antagonist

The aim of the study was to test the hypothesis that baroreceptor unloading increases jejunal fluid absorption rate via an alpha 2-adrenergic effect on electrogenic active transport. In 13 chloralose-anaesthetized cats, the carotid sinus baroreceptors were isolated and perfused with arterial blood, and we studied the effects of a graded decrease in carotid sinus pressure on intestinal vascular resistance, net fluid absorption rate and the potential difference between the intestinal lumen and the peritoneal cavity (PD). Experiments were performed in seven control animals and in six animals pretreated with yohimbine, an alpha 2-adrenergic antagonist, at a dose of 0.1 mg kg-1 i.v. Yohimbine per se had no significant effects on systemic arterial pressure, intestinal vascular resistance, net fluid absorption rate or PD. In the control animals, baroreceptor unloading induced an increase in systemic arterial pressure, intestinal vascular resistance and net fluid absorption rate, and a decrease in the PD. Yohimbine pretreatment did not significantly affect the systemic blood pressure response to baroreceptor unloading, but abolished the effect on intestinal vascular resistance and PD. After yohimbine treatment, decreases in carotid sinus pressure still enhanced net fluid absorption rate, but this response was observed in a higher range of carotid sinus pressures than in control animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Nervous control of intestinal fluid transport: physiology and pathophysiology

1. The enteric nervous system (10(8) neurones in man) consists of the myentric plexus, the submucosal plexus and other minor plexuses. Eighteen different chemicals are candidates for the role of neurotransmitters in the ENS. 2. The ENS together with the autonomic nervous system and the hormonal system controls gut epithelial transport systems.

Small intestinal fluid absorption in the rat during haemorrhage and its importance for plasma refill

Small intestinal net fluid absorption and its importance for plasma volume restitution during a slow, stepwise haemorrhage was studied in rats. A continuous administration of a Krebs-glucose solution into the intestinal lumen increased the tolerated maximal bleeding volume from 31 to 70% of initial total blood volume. The 'small intestinal' autotransfusion rate, i.e. the rate of fluid mobilized from the intestinal lumen into the vascular compartment, was calculated to 2.3 ml h-1 100 cm-2 small intestinal serosal surface. 'Small intestinal' autotransfusion is proposed to be an important mechanism in the defence line against hypovolaemia. Haemorrhage induced a 40% increase in small intestinal net fluid absorption. This increase was abolished after pretreatment with Captopril, an angiotensin-converting enzyme blocker. Post-ganglionic denervation of the small intestine reduced, but did not abolish, the stimulatory effect of haemorrhage on net fluid absorption. These results indicate that both a direct nervous component and the renin-angiotensin system are of importance for the increase in small intestinal net fluid absorption observed after haemorrhage.

Afferent vagal control of fluid absorption in the feline jejunum

The aim of the study was to test experimentally whether vagal afferent pathways are involved in the reflex regulation of jejunal fluid absorption. Acute bilateral cervical vagotomy led to an increase in net jejunal fluid absorption rate, an effect which was abolished by previous division of the splanchnic nerves. Selective division of the right cardiac branch of the vagal nerve induced an increase in fluid absorption similar to that elicited by truncal cervical vagotomy. Afferent stimulation of the right cardiac nerve at frequencies within the physiological firing range for unmyelinated C-fibre afferents induced an inhibition of net fluid absorption. Based on these findings, we propose a reflex pathway containing a non-myelinated vagal afferent branch originating from cardiopulmonary receptor endings, and an efferent sympathetic branch reaching the jejunum via the splanchnic nerves. Such a pathway might be of physiological importance in extracellular volume control by regulating the rate of fluid transport across the intestinal mucosa.