Nutrient control of energy homeostasis via gut-brain neural circuits.

Intestinal gluconeogenesis is a recently described function in intestinal glucose metabolism. In particular, the intestine contributes around 20-25% of total endogenous glucose production during fasting. Intestinal gluconeogenesis appears to regulate energy homeostasis via a neurally mediated mechanism linking the enterohepatic portal system with the brain. The periportal neural system is able to sense glucose produced by intestinal gluconeogenesis in the portal vein walls, which sends a signal to the brain to modulate energy and glucose homeostasis. Dietary proteins mobilize intestinal gluconeogenesis as a mandatory link between the sensing of these proteins in the portal vein and their well-known effect of satiety. Comparably, dietary soluble fibers exert their antiobesity and antidiabetic effects via the induction of intestinal gluconeogenesis. Finally, intestinal gluconeogenesis might be involved in the rapid metabolic improvements in energy homeostasis induced by gastric bypass surgeries of obesity.

Metabolic effects of portal vein sensing.

The extrinsic gastrointestinal nerves are crucial in the sensing of nutrients and hormones and its translation in terms of control of food intake. Major macronutrients like glucose and protein are sensed by the extrinsic nerves located in the portal vein walls, which signal to the brain and account for the satiety phenomenon they promote. Glucose is sensed in the portal vein by neurons expressing the glucose receptor SGLT3, which activate the main regions of the brain involved in the control of food intake. Proteins indirectly act on food intake by inducing intestinal gluconeogenesis and its sensing by the portal glucose sensor. The mechanism involves a prior antagonism by peptides of the µ-opioid receptors present in the portal vein nervous system and a reflex arc with the brain inducing intestinal gluconeogenesis. In a comparable manner, short-chain fatty acids produced from soluble fibre act via intestinal gluconeogenesis to exert anti-obesity and anti-diabetic effects. In the case of propionate, the mechanism involves a prior activation of the free fatty acid receptor FFAR3 present in the portal nerves and a reflex arc initiating intestinal gluconeogenesis.

Enhanced insulin clearance in mice lacking TRPM8 channels.

Blood glucose concentration is tightly regulated by the rate of insulin secretion and clearance, a process partially controlled by sensory neurons serving as metabolic sensors in relevant tissues. The activity of these neurons is regulated by the products of metabolism which regulate transmitter release, and recent evidence suggests that neuronally expressed ion channels of the transient receptor potential (TRP) family function in this critical process. Here, we report the novel finding that the cold and menthol-gated channel TRPM8 is necessary for proper insulin homeostasis. Mice lacking TRPM8 respond normally to a glucose challenge while exhibiting prolonged hypoglycemia in response to insulin. Additionally, Trpm8-/- mice have increased rates of insulin clearance compared with wild-type animals and increased expression of insulin-degrading enzyme in the liver. TRPM8 channels are not expressed in the liver, but TRPM8-expressing sensory afferents innervate the hepatic portal vein, suggesting a TRPM8-mediated neuronal control of liver insulin clearance. These results demonstrate that TRPM8 is a novel regulator of serum insulin and support the role of sensory innervation in metabolic homeostasis.

Vagal innervation of the hepatic portal vein and liver is not necessary for Roux-en-Y gastric bypass surgery-induced hypophagia, weight loss, and hypermetabolism.

OBJECTIVE: To determine the role of the common hepatic branch of the abdominal vagus on the beneficial effects of Roux-en-Y gastric bypass (RYGB) on weight loss, food intake, food choice, and energy expenditure in a rat model. BACKGROUND: Although changes in gut hormone patterns are the leading candidates in RYGB's effects on appetite, weight loss, and reversal of diabetes, a potential role for afferent signaling through the vagal hepatic branch potentially sensing glucose levels in the hepatic portal vein has recently been suggested in a mouse model of RYGB. METHODS: Male Sprague-Dawley rats underwent either RYGB alone (RYGB; n = 7), RYGB + common hepatic branch vagotomy (RYGB + HV; n = 6), or sham procedure (sham; n = 9). Body weight, body composition, meal patterns, food choice, energy expenditure, and fecal energy loss were monitored up to 3 months after intervention. RESULTS: Both RYGB and RYGB + HV significantly reduced body weight, adiposity, meal size, and fat preference, and increased satiety, energy expenditure, and respiratory exchange rate compared with sham procedure, and there were no significant differences in these effects between RYGB and RYGB + HV rats. CONCLUSIONS: Integrity of vagal nerve supply to the liver, hepatic portal vein, and the proximal duodenum provided by the common hepatic branch is not necessary for RYGB to reduce food intake and body weight or increase energy expenditure. Specifically, it is unlikely that a hepatic portal vein glucose sensor signaling RYGB-induced increased intestinal gluconeogenesis to the brain depends on vagal afferent fibers.

Portal sensing of intestinal gluconeogenesis is a mechanistic link in the diminution of food intake induced by diet protein.

Protein feeding is known to decrease hunger and subsequent food intake in animals and humans. It has also been suggested that glucose appearance into portal vein, as occurring during meal assimilation, may induce comparable effects. Here, we connect these previous observations by reporting that intestinal gluconeogenesis (i.e., de novo synthesis of glucose) is induced during the postabsorptive time (following food digestion) in rats specifically fed on protein-enriched diet. This results in glucose release into portal blood, counterbalancing the lowering of glycemia resulting from intestinal glucose utilization. Comparable infusions into the portal vein of control postabsorptive rats (fed on starch-enriched diet) decrease food consumption and activate the hypothalamic nuclei regulating food intake. Similar hypothalamic activation occurs on protein feeding. All these effects are absent after denervation of the portal vein. Thus, portal sensing of intestinal gluconeogenesis may be a novel mechanism connecting the macronutrient composition of diet to food intake.

Glucagon-like peptide-1 (GLP-1) receptors expressed on nerve terminals in the portal vein mediate the effects of endogenous GLP-1 on glucose tolerance in rats.

Glucagon-like peptide-1 (GLP-1) is an intestinal hormone that is secreted during meal absorption and is essential for normal glucose homeostasis. However, the relatively low plasma levels and rapid metabolism of GLP-1 raise questions as to whether direct endocrine action on target organs, such as islet cells, account for all of its effects on glucose tolerance. Recently, an alternative neural pathway initiated by sensors in the hepatic portal region has been proposed to mediate GLP-1 activity. We hypothesized that visceral afferent neurons in the portal bed express the GLP-1 receptor (GLP-1r) and regulate glucose tolerance. Consistent with this hypothesis, GLP-1r mRNA was present in the nodose ganglia, and nerve terminals innervating the portal vein contained the GLP-1r. Rats given an intraportal infusion of the GLP-1r antagonist, [des-His(1),Glu(9)] exendin-4, in a low dose, had glucose intolerance, with a 53% higher glucose excursion compared with a vehicle-infused control group. Infusion of [des-His(1),Glu(9)] exendin-4 at an identical rate into the jugular vein had no effect on glucose tolerance, demonstrating that this dose of GLP-1r antagonist did not affect blood glucose due to spillover into the systemic circulation. These studies demonstrate that GLP-1r are present on nerve terminals in the hepatic portal bed and that GLP-1 antagonism localized to this region impairs glucose tolerance. These data are consistent with an important component of neural mediation of GLP-1 action.

Acetylcholine exerts additive and permissive but not synergistic effects with insulin on glycogen synthesis in hepatocytes.

Parasympathetic (cholinergic) innervation is implicated in the stimulation of hepatic glucose uptake by portal vein hyperglycaemia. We determined the direct effects of acetylcholine on hepatocytes. Acute exposure to acetylcholine mimicked insulin action on inactivation of phosphorylase, stimulation of glycogen synthesis and suppression of phosphoenolpyruvate carboxykinase mRNA levels but with lower efficacy and without synergy. Pre-exposure to acetylcholine had a permissive effect on insulin action similar to glucocorticoids and associated with increased glucokinase activity. It is concluded that acetylcholine has a permissive effect on insulin action but cannot fully account for the rapid stimulation of glucose uptake by the portal signal.

Portal vein afferents are critical for the sympathoadrenal response to hypoglycemia.

We sought to elucidate the role of the portal vein afferents in the sympathetic response to hypoglycemia. Laparotomy was performed on 27 male Wistar rats. Portal veins were painted with either 90% phenol (denervation group [PDN]) or 0.9% saline solution (sham-operated group [SHAM]). Rats were chronically cannulated in the carotid artery (sampling), jugular vein (infusion), and portal vein (infusion). After a recovery period of 5 days, animals were exposed to a hyperinsulinemic-hypoglycemic clamp, with glucose infused either portally (POR) or peripherally (PER). In all animals, systemic hypoglycemia (2.48+/-0.09 mmol/l) was induced via jugular vein insulin infusion (50 mU x kg(-1) x min(-1)). Arterial plasma catecholamines were assessed at basal (-30 and 0 min) and during sustained hypoglycemia (60, 75, 90, and 105 min). By design, portal vein glucose concentrations were significantly elevated during POR versus PER (4.4+/-0.14 vs. 2.5+/-0.07 mmol/l; P<0.01, respectively) for both PDN and SHAM. There were no significant differences in arterial glucose or insulin concentration between the four experimental conditions at any point in time. When portal glycemia and systemic glycemia fell concomitantly (SHAM-PER), epinephrine increased 12-fold above basal (3.75+/-0.34 and 44.56+/-6.1 nmol/l; P<0.001). However, maintenance of portal normoglycemia (SHAM-POR) caused a 50% suppression of the epinephrine response, despite cerebral hypoglycemia (22.2+/-3.1 nmol/l, P<0.001). Portal denervation resulted in a significant blunting of the sympathoadrenal response to whole-body hypoglycemia (PDN-PER 27.6+/-3.8 nmol/l vs. SHAM-PER; P<0.002). In contrast to the sham experiments, there was no further suppression in arterial epinephrine concentrations observed during PDN-POR versus PDN-PER (P = 0.8). These findings indicate that portal vein afferent innervation is critical for hypoglycemic detection and normal sympathoadrenal counterregulation.

Portal glucose infusion in the mouse induces hypoglycemia: evidence that the hepatoportal glucose sensor stimulates glucose utilization.

To analyze the role of the murine hepatoportal glucose sensor in the control of whole-body glucose metabolism, we infused glucose at a rate corresponding to the endogenous glucose production rate through the portal vein of conscious mice (Po-mice) that were fasted for 6 h. Mice infused with glucose at the same rate through the femoral vein (Fe-mice) and mice infused with a saline solution (Sal-mice) were used as controls. In Po-mice, hypoglycemia progressively developed until glucose levels dropped to a nadir of 2.3 +/- 0.1 mmol/l, whereas in Fe-mice, glycemia rapidly and transiently developed, and glucose levels increased to 7.7 +/- 0.6 mmol/l before progressively returning to fasting glycemic levels. Plasma insulin levels were similar in both Po- and Fe-mice during and at the end of the infusion periods (21.2 +/- 2.2 vs. 25.7 +/- 0.9 microU/ml, respectively, at 180 min of infusion). The whole-body glucose turnover rate was significantly higher in Po-mice than in Fe-mice (45.9 +/- 3.8 vs. 37.7 +/- 2.0 mg x kg(-1) x min)-1), respectively) and in Sal-mice (24.4 +/- 1.8 mg x kg(-1) x min(-1)). Somatostatin co-infusion with glucose in Po-mice prevented hypoglycemia without modifying the plasma insulin profile. Finally, tissue glucose clearance, which was determined after injecting 14C-2-deoxyglucose, increased to a higher level in Po-mice versus Fe-mice in the heart, brown adipose tissue, and the soleus muscle. Our data show that stimulation of the hepatoportal glucose sensor induced hypoglycemia and increased glucose utilization by a combination of insulin-dependent and insulin-independent or -sensitizing mechanisms. Furthermore, activation of the glucose sensor and/or transmission of its signal to target tissues can be blocked by somatostatin.

Glucose sensing by the hepatoportal sensor is GLUT2-dependent: in vivo analysis in GLUT2-null mice.

In the preceding article, we demonstrated that activation of the hepatoportal glucose sensor led to a paradoxical development of hypoglycemia that was associated with increased glucose utilization by a subset of tissues. In this study, we tested whether GLUT2 plays a role in the portal glucose-sensing system that is similar to its involvement in pancreatic beta-cells. Awake RIPGLUT1 x GLUT2-/- and control mice were infused with glucose through the portal (Po-) or the femoral (Fe-) vein for 3 h at a rate equivalent to the endogenous glucose production rate. Blood glucose and plasma insulin concentrations were continuously monitored. Glucose turnover, glycolysis, and glycogen synthesis rates were determined by the 3H-glucose infusion technique. We showed that portal glucose infusion in RIPGLUT1 x GLUT24-/- mice did not induce the hypoglycemia observed in control mice but, in contrast, led to a transient hyperglycemic state followed by a return to normoglycemia; this glycemic pattern was similar to that observed in control Fe-mice and RIPGLUT1 x GLUT2-/- Fe-mice. Plasma insulin profiles during the infusion period were similar in control and RIPGLUT1 x GLUT2-/- Po- and Fe-mice. The lack of hypoglycemia development in RIPGLUT1 x GLUT2-/- mice was not due to the absence of GLUT2 in the liver. Indeed, reexpression by transgenesis of this transporter in hepatocytes did not restore the development of hypoglycemia after initiating portal vein glucose infusion. In the absence of GLUT2, glucose turnover increased in Po-mice to the same extent as that in RIPGLUT1 x GLUT2-/- or control Fe-mice. Finally, co-infusion of somatostatin with glucose prevented development of hypoglycemia in control Po-mice, but it did not affect the glycemia or insulinemia of RIPGLUT1 x GLUT2-/- Po-mice. Together, our data demonstrate that GLUT2 is required for the function of the hepatoportal glucose sensor and that somatostatin could inhibit the glucose signal by interfering with GLUT2-expressing sensing units.

Potential role of the neuropeptide CGRP in the induction of differentiation of rat hepatic portal vein wall.

The media of the rat hepatic portal vein is composed of an internal circular muscular layer (CL) and an external longitudinal muscular layer (LL). These two perpendicular layers differentiate progressively from mesenchymal cells within the first month after birth. In this paper, we studied the development of calcitonin gene-related peptide (CGRP) innervation during post-natal differentiation of the vessel. We show that CGRP innervation is already present around the vessel at birth in the future adventitia but far from the lumen of the vessel. Progressively, CGRP immunoreactive fibers reached first LL then CL. CL by itself become only innervated at day 14 after birth. This corresponds to the time at which thick filaments (myosin) are visible in electron microscopy and desmin visualisable by immunocytochemistry. Furthermore, we provide evidence by autoradiography, that binding sites for CGRP are transiently expressed on the portal vein media at day 1 and 14 after birth. Vascular smooth muscle cells were transfected with constructs containing promoters for desmin or smooth muscle myosin heavy chain (smMHC). CGRP treatment of the cells significantly increased the expression of smMHC. Overall these results suggest that CGRP can potentially influence the differentiation of smooth muscle cells from the vessel wall.

[Nutrient sensing by the gastro-intestinal nervous system and control of energy homeostasis]. / Détection des nutriments par le système nerveux gastro-intestinal et contrôle de l'homéostasie énergétique.

The gastrointestinal nerves are crucial in the sensing of nutrients and hormones and its translation in terms of control of food intake. Major macronutrients like glucose and proteins are sensed by the extrinsic nerves located around the portal vein walls, which signal to the brain and account for the satiety phenomenon they promote. Glucose is sensed in the portal vein by neurons expressing the glucose receptor SGLT3, which activates the main regions of the brain involved in the control of food intake. Proteins indirectly act on food intake by inducing intestinal gluconeogenesis and its sensing by the portal glucose sensor. The mechanism involves a prior antagonism by peptides of the µ-opioid receptors present in the portal vein nervous system and a reflex arc with the brain inducing intestinal gluconeogenesis. In a comparable manner, short chain fatty acids produced from soluble fibers act via intestinal gluconeogenesis to exert anti-obesity and anti-diabetic effects. In the case of propionate, the mechanism involves a prior activation of the free fatty acid receptor FFAR3 present in the portal nerves and a reflex arc initiating intestinal gluconeogenesis.

Noradrenergic transmission in the isolated portal vein of the spontaneously hypertensive rat.

The effect of electrical field stimulation (1, 2, 5, 10 Hz for a total of 480 pulses at 15-minute intervals) on the release of 3H-norepinephrine from the superfused portal vein of spontaneously hypertensive rats (SHR) or Wistar-Kyoto rats (WKY) of various ages was studied. The ages of the animals were (in weeks) 5-6 (prehypertensive), 8-10 (young hypertensives), 16-18 (older hypertensives), and 28 (mature hypertensives). There was no difference in the release of 3H-norepinephrine or developed tension of the portal vein to any frequency of field stimulation of SHR or WKY at 5-6 weeks of age. However, there was a significantly greater release of 3H-norepinephrine and developed tension of veins of SHR in response to low (1 or 2 Hz) but not high frequencies (5 or 10 Hz) at 8-10, 16-18, and 28 weeks of age. Vessels from hypertensive animals also developed greater resting tension and spontaneous activity, which was reduced to that of WKY in the presence of an alpha-adrenergic antagonist. The alpha 2 selective adrenergic antagonist yohimbine produced the same degree of enhancement of release of 3H-norepinephrine to field stimulation of veins obtained from both SHR and WKY at 5-6, 8-10 and 16-18 weeks of age. However, the facilitory effect of yohimbine was significantly attenuated in portal veins obtained from SHR at 28 weeks of age compared to age-matched WKY.(ABSTRACT TRUNCATED AT 250 WORDS)

Spinal substance P transmits bradykinin but not osmotic stimuli from hepatic portal vein to hypothalamus in rat.

Peripheral osmo - and bradykinin-sensitive receptors which have been previously localised within the hepatic portal vein area, activate the hypothalamo-neuro-hypophysial system through a neural pathway projecting to the lower thoracic spinal cord. In this paper we attempted to identify the spinal transmitter(s) involved and to answer the question whether osmoreceptors are in fact chemosensitive nociceptors. The portal vein of anesthetised rats was superfused with 0.2 ml of 4% NaCl or 1 microM bradykinin, and hypothalamo-neurohypophysial responses were measured either electrophysiologically or by radioimmunoassay of arginine vasopressin. Responses to bradykinin, but not to hypertonic saline, were abolished in rats pretreated 2 wks previously with capsaicin s.c., and immunocytochemistry for substance P in these animals showed that substance P was strongly depleted both in the dorsal thoracic spinal cord and in the portal vein. The spinal injection of 8 microliter 0.1 mM capsaicin at T8-T9 elicited a pronounced hypothalamo-neurohypophysial response, and diminished reversibly the response to bradykinin superfusion of the portal vein. Spinal capsaicin had no effect on responses to hypertonic saline. Similarly, the spinal (T8-T9) injection of 8 micrograms substance P antagonist, the [D-Pro4, D- Trp7 ,9,10, Val8 ]substance P (4-11), reduced reversibly the responses to bradykinin by about 50% without affecting those to hypertonic saline. The spinal injection of 8 micrograms substance P, at the same site where substance P antagonist was applied, elicited within 4 s a prolonged response (several min). A slightly longer delay between stimulus and neurophysiological response was observed for spinal capsaicin and for bradykinin superfusion. Responses to hypertonic saline superfusion of the portal vein, however, occurred within 1-2 s. The results show that portal vein osmoreceptors are distinct from chemo-sensitive nociceptors, and suggest that substance P may be a spinal mediator for chemo-sensitive portal vein nociceptors. The spinal transmitter for osmosensitive afferents, and the physiological importance of the portal vein area in chemosensation remain to be established.

Presynaptic autoinhibition during rest and sodium-pump inhibition in isolated rat portal vein preparation.

In the presence of cocaine and corticosterone low-frequency (2 Hz) nerve stimulation evoked release of [3H]noradrenaline measured from isolated rat portal vein preparation. In normal Krebs solution exogenously applied l-noradrenaline (3 X 10(-8)-10(-6) M) significantly reduced the nerve-evoked [3H]noradrenaline release. The IC50 value of L-noradrenaline proved to be 1.8 X 10(-7) M. Yohimbine (3 X 10(-7) M) maximally blocked the alpha 2-adrenoceptors and enhanced nerve-evoked [3H]noradrenaline release. In the presence of 5.9 mM external K+, ouabain up to 10(-4) M did not affect either the resting or the stimulation-evoked release of radioactivity from tissues. In the absence of external K+ both the resting and the nerve-evoked release of [3H]noradrenaline increased markedly. When K+ was readmitted to preparations which had been kept in K+-free solution both the resting and the stimulation-evoked [3H]noradrenaline release were greatly reduced temporarily. In K+-free solution L-noradrenaline (10(-6) M) and yohimbine (3 X 10(-7) M) failed to significantly alter the nerve-evoked release. However, 3 X 10(-6) M yohimbine in K+-free solution significantly increased the stimulation-evoked release of [3H]noradrenaline. It is concluded that presynaptic alpha 2-adrenoceptor-mediated "negative feed-back" is present in rat portal vein preparations which can be inhibited by the preferential alpha 2-adrenoceptor blocker, yohimbine. However, if the Na+-pump is inhibited (which by itself enhanced the transmitter release), presynaptic autoinhibition is more pronounced, since a high concentration of yohimbine is required to block it.

Mechanism of action of alpha-adrenoceptor activation in single cells freshly dissociated from the rabbit portal vein.

1. The action of noradrenaline was studied in freshly dispersed cells of the rabbit portal vein using microelectrode techniques. 2. In normal physiological salt solution, the ionophoretic application of noradrenaline evoked an alpha-adrenoceptor-mediated depolarization and sometimes a beta-adrenoceptor-mediated hyperpolarization. Experiments were carried out in the presence of propranolol to study the membrane mechanism associated with alpha-adrenoceptor activation. 3. In the current clamp mode of recording, the equilibrium potential of the noradrenaline-evoked depolarization was -1.9 mV. The depolarization was brought about by an increase in membrane conductance. 4. Under voltage clamp conditions, noradrenaline produced an inward current with a reversal potential of -7 +/- 3 mV (mean +/- s.e. mean). 5. The relationship between the noradrenaline-induced inward current and clamp potential was non-linear. Depolarization enhanced the conductance elicited by noradrenaline. 6. Evidence is presented which suggests that an additional conductance mechanism (probably an increase in potassium conductance) is also evoked by alpha-adrenoceptor stimulation in dispersed cells of rabbit portal vein.

Comparative pharmacological and histochemical evidence for purinergic inhibitory innervation of the portal vein of the rabbit, but not guinea-pig.

1 Intramural nerve stimulation elicited a powerful relaxation of the longitudinal muscle of the rabbit portal vein in the presence of atropine and guanethidine, but not of the guinea-pig portal vein.2 Intramural nerve stimulation of the rabbit portal vein produced a 13 fold increase in release of (3)H-adenyl compounds after preloading with [(3)H]-adenosine. About 50% of this release was abolished by guanethidine. All release was abolished by tetrodotoxin. No significant release of radioactive compounds was observed during intramural nerve stimulation of the guinea-pig portal vein in the presence of guanethidine, although there was a 6 fold increase in release of radioactivity in the absence of drugs.3 Histochemical studies using quinacrine, which binds ATP showed a fine fluorescent nerve plexus, nerve bundles, and ganglion cells in the rabbit portal vein, but not in the guinea-pig portal vein. This plexus was still present after chemical sympathectomy with 6-hydroxydopamine.4 Adenosine 5'-triphosphate (ATP) relaxed the rabbit portal vein, but usually produced a biphasic response, consisting of a contraction followed by a relaxation, of the guinea-pig portal vein.5 Prostaglandins E(1) and E(2) caused contraction of the rabbit portal vein. Indomethacin, a prostaglandin synthesis inhibitor, potentiated the relaxations of the rabbit portal vein produced by both non-adrenergic, non-cholinergic nerve stimulation and ATP.6 High concentrations of antazoline and phentolamine, which antagonize purinergic responses in the guinea-pig taenia coli, caused a loss of basal tone so that it was not possible to assess their effects on the responses of the portal vein to either non-adrenergic, non-cholinergic nerve stimulation, or ATP.7 Comparison of the results on the portal vein of the rabbit and guinea-pig provides support for the view that: (i) quinacrine fluorescence can be used to localize purinergic nerves and that the rabbit portal vein is supplied by these nerves; (ii) ATP is released from adrenergic nerve fibres, although, based on histochemical analysis, about 3 to 7 times less than is released from purinergic nerve fibres.

An examination of the pre- and postsynaptic alpha-adrenoceptors involved in neuroeffector transmission in rabbit aorta and portal vein.

1 The alpha-adrenoceptor agonists, clonidine and xylazine, reduced and the alpha-antagonists, yohimbine an rauwolscine, increased the stimulation-evoked tritium overflow from rabbit aorta and portal vein pre-incubated with [3H]-noradrenaline. 2 Based on an order of agonist potency of clonidine greater than xylazine greater than phenylephrine and antagonist potency of rauwolscine = yohimbine greater than prazosin, the presynaptic receptor mediating these effects is of the alpha 2 type. 3 In the aorta, stimulation-evoked contractions were abolished by prazosin (0.1 micrometers) and potentiated by rauwolscine and yohimbine in concentrations that increased the stimulation-evoked overflow tritium. 4 In the portal vein, prazosin was less potent in reducing, and rauwolscine and yohimbine failed to potentiate, the stimulation-evoked contraction. 5 In experiments in which tissues were pre-exposed to phenoxybenzamine (30 nM) to block some of the postsynaptic alpha-receptors, rauwolscine in concentrations that increased stimulation-evoked tritium overflow, reduced the evoked contraction in the portal vein but not in the aorta. 6 It is concluded that presynaptic alpha 2-autoreceptors are present in both tissues and that the postsynaptic alpha-receptors which mediate nerve stimulation-evoked contractions are alpha 1 in the aorta but a mixture of alpha 1 and alpha 2 in the portal vein.

Indirect evidence that purinergic modulation of perivascular adrenergic neurotransmission in the portal vein is a physiological process.

The effects of adenine nucleotides and nucleosides on the contractile response to perivascular nerve stimulation were compared in the isolated portal vein of rabbit, rat and guinea-pig. 2-Chloroadenosine was more potent than adenosine and ATP, which were equipotent in producing inhibition of neurogenic contractions in the rabbit and rat via prejunctional P1-purinoceptors. In contrast, neurogenic contractions of the guinea-pig portal vein were not inhibited by adenosine and were potentiated by 2-chloroadenosine and, to a lesser extent, by ATP. Fluorescence histochemical localization of quinacrine, which binds to high levels of ATP, revealed a dense perivascular nerve plexus in the portal vein of rabbit and rat but not of guinea-pig. After chemical sympathectomy, quinacrine-positive nerves persisted in the rabbit (supporting other evidence for the presence of purinergic nerves) but not in the rat (supporting other evidence for ATP as a cotransmitter in adrenergic nerves). It is concluded that a prejunctional purinergic modulatory mechanism operates in adrenergic neurotransmission in the portal vein of rabbit and rat but not guinea-pig, and it is suggested that this indicates a physiological mechanism.

Sustained prejunctional facilitation of noradrenergic neurotransmission by adrenaline as a co-transmitter in the portal vein of freely moving rats.

1. The duration of the facilitatory effect of adrenaline on the electrically evoked overflow of noradrenaline was studied in the portal vein of permanently adreno-demedullated freely moving rats. 2. Rats were infused with adrenaline (20 or 100 ng min-1) for 2 h. After an interval of 1 h, when plasma adrenaline had returned to undetectable levels, electrical stimulation resulted in an enhanced catecholamine overflow amounting to 219% (noradrenaline) and 241% (noradrenaline plus adrenaline) of control (saline infusion), respectively. 3. When stimulation was applied again, in the same animal, at 24, 48 and 72 h after the first stimulation episode, the evoked noradrenaline overflow was 150, 111 and 102% (after 20 ng ml-1 adrenaline) and 158, 134 and 105% (after 100 ng min-1 adrenaline) of control. 4. The beta 2-adrenoceptor antagonist, ICI 118,551 (0.3 mg kg-1), blocked the facilitatory effect obtained after the 100 ng min-1 adrenaline infusion on all days. 5. The results show that adrenaline, after being taken up by and released from sympathetic nerve terminals, is able to facilitate the evoked noradrenaline overflow through activation of prejunctional beta 2-adrenoceptors for at least 48 h after administration.