Christof H. Wachter

Nitric oxide-dependent and -independent hyperaemia due to calcitonin gene-related peptide in the rat stomach.

1 Calcitonin gene‐related peptide (CGRP) potently enhances mucosal blood flow in the rat stomach. The aim of this study was to examine whether CGRP also dilates extramural arteries supplying the stomach and whether the vasodilator action of CGRP involves nitric oxide (NO). 2 Rat CGRP‐α (0.03–1 nmol kg−1, i.v.) produced a dose‐dependent increase in blood flow through the left gastric artery (LGA) as determined by an ultrasonic transit time technique in urethane‐anaesthetized rats. Blockade of NO synthesis by NG‐nitro‐l‐arginine methyl ester (l‐NAME, 20 and 60 μmol kg−1, i.v.) significantly reduced basal blood flow (BF) in the LGA and attenuated the hyperaemic activity of CGRP by a factor of 2.8–4. d‐NAME tended to enhance basal BF in the LGA but had no influence on the dilator activity of CGRP. The ability of vasoactive intestinal polypeptide to increase left gastric arterial blood flow remained unaltered by l‐NAME. 3 l‐NAME (20 and 60 μmol kg−1, i.v.) evoked a prompt and sustained rise of mean arterial blood pressure (MAP) and caused a slight decrease in the hypotensive activity of CGRP. In contrast, d‐NAME induced a delayed and moderate increase in MAP and did not influence the hypotensive activity of CGRP. 4 Rat CGRP‐α dilated the isolated perfused bed of the rat LGA precontracted with methoxamine and was 3 times more potent in this respect than rat CGRP‐β. The dilator action of rat CGRP‐α in this preparation was not affected by l‐NAME or d‐NAME (40 μm). 5 L‐NAME (60 μmol kg−1, i.v.) reduced gastric mucosal blood flow as assessed by laser Doppler flowmetry and diminished the hyperaemic activity of rat CGRP‐α in the gastric mucosa by a factor of 4.5, whereas d‐NAME was without effect. 6 These data show that CGRP is a potent dilator of mucosal and extramural resistance vessels in the rat stomach. Its dilator action involves both NO‐dependent and NO‐independent mechanisms.

Nitric oxide‐dependent and ‐independent vascular hyporeactivity in mesenteric arteries of portal hypertensive rats

Increased production of nitric oxide (NO) has been suggested to underlie both the vascular hyporeactivity to vasoconstrictors and the splanchnic vasodilatation seen in portal hypertension. This study assessed the role of NO in the vasoconstrictor hyporeactivity of portal vein‐ligated (PVL) rats in isolated and in situ perfused mesenteric arterial beds. Isolated perfused mesenteric arteries of PVL rats were significantly less reactive to noradrenaline (NA), methoxamine (METH), arginine vasopressin (AVP) and endothelin‐1 (ET‐1) than those from sham‐operated (Sham) rats. Blockade of NO synthesis with NG‐nitro‐L‐arginine methyl ester (L‐NAME, 100 μM) in isolated perfused mesenteric arteries from PVL rats restored the reactivity to bolus injections of AVP and ET‐1, but had little effect on the hyporeactivity to NA or METH. Cyclo‐oxygenase inhibition with indomethacin (5 μM) likewise did not restore reactivity to METH of isolated perfused mesenteric arteries of PVL rats. The hyporeactivity to METH seen in isolated perfused mesenteric arteries from PVL rats was reduced by low concentrations of AVP (20 nM) or ET‐1 (1 nM) which per se caused only a slight increase in perfusion pressure. When L‐NAME (100 μM) was combined with AVP (20 nM) or ET‐1 (1 nM), respectively, reactivity to METH of isolated perfused mesenteric arteries of PVL rats was restored to the level seen in Sham rats. These effects of AVP and ET‐1 were not mimicked by precontracting the vessels with 5‐hydroxytryptamine (5 μM). The differential effects of L‐NAME and AVP on the hyporesponsiveness to methoxamine and AVP were corroborated by experiments performed with the in situ perfused mesenteric vascular bed preparation. These data indicate that both NO‐dependent and NO‐independent mechanisms are involved in the vasoconstrictor hyporesponsiveness of mesenteric arteries from portal hypertensive rats. The hyporeactivity to AVP and ET‐1 is mediated by NO whereas the reduced responsiveness to adrenoceptor agonists appears to be predominantly NO‐independent. AVP and ET‐1, in addition, seem to inhibit the NO‐independent mechanism of vascular hyporeactivity, since the hyporesponsiveness to METH was reduced in the presence of AVP or ET‐1 and abolished by the combination of these peptides with L‐NAME.

Visceral vasodilatation and somatic vasoconstriction evoked by acid challenge of the rat gastric mucosa: diversity of mechanisms.

1. Acid back‐diffusion through a disrupted gastric mucosal barrier increases blood flow to the stomach without any change in systemic blood pressure. This study was undertaken to examine the gastric acid‐evoked changes in blood flow in a number of visceral and somatic arterial beds and to elucidate the mechanisms which lead to the regionally diverse haemodynamic responses. 2. The gastric mucosa of urethane‐anaesthetized rats was challenged with acid by perfusing the stomach with ethanol (15%, to disrupt the gastric mucosal barrier) in 0.15 M HCl. Blood flow was estimated by laser Doppler flowmetry, the hydrogen clearance method or the ultrasonic transit time shift technique. 3. Gastric acid challenge increased blood flow in the gastric mucosa and left gastric artery while blood flow in the femoral artery and skin declined. 4. Afferent nerve stimulation by intragastric administration of capsaicin enhanced blood flow in the left gastric artery but did not diminish blood flow in the femoral artery when compared with the vehicle. 5. The gastric acid‐evoked dilatation of the left gastric artery was depressed by acute extrinsic denervation of the stomach, capsaicin‐induced ablation of afferent neurones or hexamethonium‐induced blockade of autonomic ganglionic transmission. 6. The gastric acid‐induced constriction of the femoral artery was attenuated by acute extrinsic denervation of the stomach but left unaltered by capsaicin, hexamethonium, guanethidine, indomethacin, telmisartan (an angiotensin II antagonist), [d(CH2)5(1), Tyr(Me)2, Arg8]‐vasopressin (a vasopressin antagonist), bosentan (an endothelin antagonist) and acute ligation of the blood vessels to the adrenal glands. 7. These data show that acid challenge of the gastric mucosa elicits visceral vasodilatation and somatic vasoconstriction via divergent mechanisms. The gastric hyperaemia is brought about by extrinsic vasodilator nerves, whereas the reduction of somatic blood flow seems to be mediated by non‐neural, probably humoral, vasoconstrictor messengers that remain to be identified.

DIFFERENTIAL EXPRESSION OF C-FOS MESSENGER RNA IN THE RAT SPINAL CORD AFTER MUCOSAL AND SEROSAL IRRITATION OF THE STOMACH

Expression of the immediate early gene c-fos is considered to be a marker for neuronal activation in the spinal cord in response to afferent input. Since the stomach is continually exposed to injurious chemicals, the present study examined whether application of acid (0.15 M HCl) and formalin (5%) to the gastric mucosa or serosal surface of the stomach stimulates c-fos transcription in the caudal thoracic spinal cord of anaesthetized rats. The spinal cord was removed 15, 45 or 120 min after exposure of the stomach to the noxious chemicals and processed for quantitative in situ hybridization autoradiography of c-fos messenger RNA. Exposure of the gastric mucosa to acid or formalin failed to increase the expression of c-fos messenger RNA in the thoracic spinal cord. Application of acid to the serosal surface of the stomach was also unable to stimulate c-fos transcription, whereas serosal application of formalin led to substantial expression of c-fos messenger RNA in the superficial but also deeper laminae of the spinal dorsal horn when examined 45 min, but not 15 or 120 min, post-stimulation. The highest expression of c-fos messenger RNA was seen when formalin was injected subcutaneously into one hindpaw and c-fos transcription was examined in the lumbar spinal cord. These data indicate that acute exposure of the gastric mucosa to chemical injury does not provide the afferent input which is necessary to cause appreciable c-fos transcription in second order neurons within the spinal cord. Stimulation of the gastric mucosa by acid and formalin was followed, however, by gastric hyperaemia in which spinal afferents releasing vasodilator peptides have been implicated. It is concluded, therefore, that acute stimulation of nociceptive afferents in the stomach causes local homoeostatic reactions but does not necessarily provide afferent input sufficient to recruit spinal nociceptive circuits.

Mediation by 5-hydroxytryptamine of the femoral vasoconstriction induced by acid challenge of the rat gastric mucosa

1 Gastric mucosal barrier disruption in the presence of luminal acid causes femoral vasoconstriction via a pathway that appears to be stimulated by messengers generated in the injured gastric mucosa. This study was undertaken to analyse the gastric factors that are responsible for the femoral vasoconstrictor response. 2 Gastric mucosal barrier disruption in the presence of luminal acid was induced by perfusing the stomach of urethane‐anaesthetized rats with ethanol (15 %) in 0.01‐0.15 M HCl. Blood flow in the left gastric and right femoral artery was estimated by the ultrasonic transit time shift technique. 3 Gastric perfusion of ethanol in HCl caused loss of H+ ions from the gastric lumen, decreased the HCO3− concentration in hepatic portal vein blood, induced macroscopic histological damage to the gastric mucosa, dilated the left gastric artery and constricted the femoral artery. These responses were related to the HCl concentration in the ethanol‐containing perfusion medium. 4 The femoral vasoconstriction was also seen when, instead of ethanol, taurocholate (20 mM) was used to disrupt the gastric mucosal barrier in the presence of 0.15 M HCl. 5 The femoral vasoconstriction evoked by gastric perfusion of ethanol in HCl was left unaltered by pharmacological blockade of gastrin and histamine receptors. In contrast, the 5‐hydroxytryptamine 5‐HT1/2 receptor antagonist methiothepin, but not the 5‐HT2A receptor antagonist ketanserin or the 5‐HT3 receptor antagonist granisetron, inhibited the ability of both 5‐hydroxytryptamine and gastric acid back‐diffusion to constrict the femoral artery. 6 Gastric acid back‐diffusion caused release of 5‐hydroxytryptamine into the gastric lumen, which was related to the HCl concentration in the ethanol‐containing perfusion medium. 7 These data show that femoral vasoconstriction evoked by gastric mucosal barrier disruption depends on back‐diffusion of acid into the mucosa. The acid‐induced damage results in release of 5‐hydroxytryptamine from the gastric mucosa, and the pathway leading to constriction of the femoral artery involves 5‐hydroxytryptamine acting via 5‐HT1/2 receptors as a messenger molecule.

Dilatation by angiotensin II of the rat femoral arterial bed in vivo via pressure/flow‐induced release of nitric oxide and prostaglandins

1 The haemodynamic effects of angiotensin II (AII) and, for comparison, arginine vasopressin (AVP) in the femoral and superior mesenteric artery of urethane‐anaesthetized rats were analysed with the ultrasonic transit time shift technique. 2 I.v. bolus injection of AII (0.1–3 nmol kg−1) and AVP (0.03–1 nmol kg−1) increased blood pressure which was accompanied by a decrease in blood flow through the superior mesenteric artery and an increase in femoral blood flow. The femoral hyperaemia was in part due to vasodilatation as indicated by a rise of femoral vascular conductance up to 200% relative to baseline. The femoral vasodilatation caused by AVP, but not AII, was followed by vasoconstriction. 3 Blockade of angiotensin AT1 receptors by telmisartan (0.2–20 μmol kg−1) prevented all haemodynamic responses to AII. 4 The femoral dilator responses to AII and AVP depended on the increase in vascular perfusion pressure since vasodilatation was reversed to vasoconstriction when blood pressure was maintained constant by means of a gravity reservoir. However, the AII‐evoked femoral vasodilatation was not due to an autonomic or neuroendocrine reflex because it was not depressed by hexamethonium (75 μmol kg−1), prazosin (0.25 μmol kg−1) or propranolol (3 μmol kg−1). 5 The AII‐induced femoral vasodilatation was suppressed by blockade of nitric oxide (NO) synthesis with NG‐nitro‐L‐arginine methyl ester (L‐NAME, 40 μmol kg−1) and reversed to vasoconstriction when L‐NAME was combined with indomethacin (30 μmol kg−1), but was left unaltered by antagonism of endothelin ETA/B receptors with bosentan (37 μmol kg−1). 6 These results demonstrate that the effect of AII to increase systemic blood pressure and the resulting rise of perfusion pressure in the femoral artery stimulates the formation of NO and prostaglandins and thereby dilates the femoral arterial bed. This local vasodilator mechanism is sufficient to mask the direct vasoconstrictor response to AII.

Gastric Mucosal Blood Flow Regulation in Response to Different Stimuli

We compared changes in gastric mucosal bloodflow (GMBF) and left gastric artery blood flow (LGABF)in response to pharmacological, physiological, andpathological stimuli. GMBF and LGABF were measured by the hydrogen gas clearance and perivascularultrasonic transit time techniques, respectively, underbaseline conditions and following intravenous infusionof vasopressin or pentagastrin, isovolemic hemodilution, or gastric perfusion with HCl-taurocholate.Blood flow changes following vasopressin or hemodilutionwere significantly larger in the left gastric arterythan in the gastric mucosa. In contrast, the increment in blood flow associated withpentagastrin-stimulated acid secretion was significantlygreater in the gastric mucosa than in the extramuralartery. Barrier disruption with acid-taurocholateinduced similar changes in both measurement sites. The gastrichyperemia induced by either mechanism was significantlyattenuated by blockade of NO synthesis. These datademonstrate that although functional changes in GMBF are primarily supported by changes inblood flow at the extramural gastric arteries, thegastric mucosal microvasculature is also under theinfluence of independent local controlmechanisms.

Dilator effect of the angiotensin II antagonist telmisartan on the gastric, but not femoral, arterial bed of the rat

In the present study the effects were examined of the non-peptide angiotensin 11 AT1 receptor antagonist, telmisartan, and of-angiotensin II on the vascular conductance in the left gastric and the femoral arterial bed of anaesthetized rats. Blood flow was recorded with the ultrasonic transit time shift technique and vascular conductance was calculated as blood flow divided by mean arterial blood pressure. I.v. injection of angiotensin II (1 nmol kg−1)led to a transient fall of gastric arterial conductance, while femoral arterial conductance was not altered. Telmisartan (1 mg kg−1, i.v.) increased vascular conductance in the left gastric, but not femoral, arterial bed and inhibited the constrictor response of the left gastric artery to angiotensin II. The ability of arginine-vasopressin (0.3 nmolkg-1, i.v.) to cause hypertension and to constrict the left gastric and the femoral artery was not influenced by telmisartan. These data indicate that the angiotensin II antagonist telmisartan dilates the gastric, but not femoral, arterial bed of the rat and point to a constrictor role of endogenous angiotensin II in the gastric vasculature.