Coronary effects of cyclovirobuxine D in anesthetized pigs and in isolated porcine coronary arteries.

The present study was undertaken in anesthetized pigs and in isolated porcine coronary arteries to determine the primary coronary effects of cyclovirobuxine D. In six pigs, the intravenous administration of 1.5 mg/kg of cyclovirobuxine D whilst preventing changes in heart rate and aortic blood pressure caused increases in left ventricular dP/dtmax and coronary blood flow which respectively averaged 10% and 23.9%. These responses were progressively augmented by graded increases in the dose of the drug (four pigs) and were not affected by blockade of cholinergic and adrenergic receptors (five pigs). Intravenous blockade of nitric oxide synthase (L-NAME, five pigs) abolished both responses, while intracoronary injection of L-NAME (five pigs) abolished only the coronary vasodilatation. In ten isolated coronary segments, cyclovirobuxine D significantly reduced the degree of potassium chloride-induced contraction. This reduction was not affected by inhibition of cyclooxygenase with indomethacin (five segments) or potassium channels blockade with glibenclamide (five segments), but it was abolished by L-NAME (five segments) or removal of endothelium (five segments). The present study showed that cyclovirobuxine D caused a primary effect of coronary vasodilatation, which involved mechanisms related to the endothelial release of nitric oxide.     

The role of beta 2-adrenergic vascular receptors in the peripheral vasodilation caused by 17 beta-estradiol in anesthetized pigs

It has been previously shown in anesthetized pigs that intravenous infusion of 2 microg/h of 17beta-estradiol primarily dilated renal, iliac and coronary circulations, while higher doses of the hormone were required to cause vasodilation also in the mesenteric vascular bed. In the same experimental model, a tonic beta2-adrenoceptor mediated vasodilation, which could be argued to attenuate the vasodilator effect of 17beta-estradiol, has been described. The present study was planned to investigate the role of beta2-adrenergic receptors in the hemodynamic responses of renal and mesenteric vascular beds to 17beta-estradiol. Changes in flow caused by intravenous infusion of 2 microg/h of the hormone at constant heart rate and aortic blood pressure in the left renal and superior mesenteric arteries were assessed using electromagnetic flowmeters. In six pigs, infusion of 17beta-estradiol caused an increase in renal blood flow, which averaged 12.1% of the control values, without affecting mesenteric blood flow. In the same pigs, after hemodynamic variables had returned to the baseline values, blockade of beta2-adrenergic receptors with butoxamine caused an increase in aortic blood pressure and an increase in renal and mesenteric resistance. The subsequent infusion of 17beta-estradiol elicited increases in renal and mesenteric blood flow which respectively averaged 19.6% and 12.8%. Therefore, the present study in anesthetized pigs have shown that the vasodilator responses of the renal and mesenteric circulations to 17beta-estradiol were attenuated and even masked by a tonic beta2-adrenoceptor mediated vasodilation. This indicates that some vasodilator effects elicited by normally used replacement doses of the hormone may not be apparent.     

Characterization of [3H]pentagastrin binding in guinea pig gastric glands--an alternative convenient ligand for receptor binding assay

The binding of [3H]pentagastrin to guinea pig gastric glands was specific, saturable and of high affinity (Kd = 5 nM). The relative order of potencies for gastrin and CCK analogs in displacing [3H]pentagastrin binding correlated well with those obtained using [125I]gastrin and their reported biological potencies for stimulating acid secretion. Nonselective CCK/gastrin antagonists including carbobenzoxy-CCK (26-32), proglumide and benzotript, but not the selective peripheral CCK antagonist, asperlicin, inhibited specific [3H]pentagastrin binding. The results indicate that [3H]pentagastrin labels physiologically relevant gastrin receptors in guinea pig gastric glands.     

Role of cholecystokinin in the intestinal phase of pancreatic circulation in dogs

The regulatory mechanisms of postprandial pancreatic hyperemia are not well characterized. The aim of this study is to clarify the role of cholecystokinin (CCK) in the intestinal phase of pancreatic circulation. Pancreatic, gastric, and intestinal blood flows were measured by ultrasound transit-time blood flowmeters in five conscious dogs. Pancreatic and gastric secretion and blood pressure were also monitored. Synthetic CCK octapeptide (CCK-8) or gastrin heptadecapeptide (gastrin-17) was infused intravenously, and milk was infused into the duodenum with or without loxiglumide, a specific CCK-A receptor antagonist. CCK-8 induced dose-related increases of pancreatic, but not gastric or intestinal, blood flow and protein secretion without affecting systemic blood pressure. Gastrin-17 did not affect pancreatic blood flow. An intraduodenal infusion of milk increased pancreatic and intestinal blood flows and pancreatic protein secretion. Loxiglumide completely inhibited pancreatic blood flow and protein responses to CCK-8 and milk but not the intestinal blood flow response. CCK is a potent and specific pancreatic vasodilator, with its effect mediated by CCK-A receptors. CCK plays an important role in the regulation of the intestinal phase of the pancreatic circulation in dogs.     

Importance of sulfation of gastrin or cholecystokinin (CCK) on affinity for gastrin and CCK receptors

We investigated the importance of sulfation of gastrin or cholecystokinin (CCK) on influencing their affinity for gastrin or CCK receptors by comparing the abilities of sulfated gastrin-17 (gastrin-17-II), desulfated gastrin-17 (gastrin-17-I), CCK-8 and desulfated CCK-8 [des(SO3)CCK-8] to interact with CCK or gastrin receptors on guinea pig pancreatic acini. For inhibiting binding of 125I-gastrin to gastrin receptors, gastrin-17-II (Kd 0.08 nM) greater than CCK-8 (Kd 0.4 nM) greater than gastrin-17-I (Kd 1.5 nM) greater than des(SO3)CCK-8 (Kd 28 nM). For inhibiting binding of 125I-Bolton Hunter-labeled CCK-8 to CCK receptors the relative potencies were: CCK-8 much greater than des(SO3)CCK-8 = gastrin-17-II greater than gastrin-17-I. Each peptide interacted with both high and low affinity CCK binding sites. The relative abilities of each peptide to interact with high affinity CCK receptors showed a close correlation with their abilities to cause half-maximal stimulation of enzyme secretion. These results demonstrate that, in contrast to older studies, sulfation of both CCK and gastrin increase their affinities for both gastrin and CCK receptors. Moreover, the gastrin receptor is relatively insensitive to the position of the sulfate moiety, whereas the CCK receptor is extremely sensitive to both the presence and exact position of the sulfate moiety.     

Cholecystokinin- and gastrin-induced protein and amylase secretion from the parotid gland of the anaesthetized rat

I.V. infusion of pentagastrin (20 microg/kg/h) or cholecystokinin (CCK)-8 (1 microg/kg/h) for 10 min caused secretion of salivary proteins from the parotid gland in the anaesthetized rat without any accompanying overt fluid secretion. This "occult" response was revealed by a subsequent wash-out injection of methacholine (5 microg/kg, I.V.) 10 min after the end of the infusion period (aiming at avoiding synergistic interactions). While the fluid response to methacholine was unaffected by the preceding infusion of pentagastrin and CCK-8, the output of protein increased by 147% (pentagastrin) and 74% (CCK-8) and that of amylase by 45% (CCK-8) compared to the responses to methacholine upon saline infusion. Those increases were abolished by the CCK-A receptor blocker (lorglumide), but not by the CCK-B receptor blocker (itriglumide). Evisceration, combined sympathetic and parasympathetic denervation of the glands and assay under adrenoceptor blockade excluded contribution from the gastro-intestinal tract, central or ganglionic mechanisms and circulating catecholamines to the increase in protein/amylase. Furthermore, Western blot demonstrated CCK receptors for both A and B subtypes in normal and chronically denervated glands. In the submandibular gland, both pentagastrin and CCK-8 evoked a trace secretion of saliva but, under the present experimental set-up, no statistically significant increase in protein output. Thus, in addition to the autonomic nervous system, gastrointestinal hormones may, in some types of glands, be involved in the secretion of salivary gland proteins.     

Mechanisms of the renal vasodilation caused by insulin in anesthetized pigs

The present study was planned to determine the mechanisms involved in the renal vasodilation caused by insulin. Changes in flow caused by the intravenous infusion of 0.004 IU/kg/min of insulin at constant heart rate, aortic blood pressure, left ventricular contractility and blood levels of glucose and potassium in the left renal artery were assessed using an electromagnetic flowmeter. In ten pigs, infusion of insulin caused an increase in renal blood flow which averaged 12.8% of the control values. After hemodynamic variables had returned to control values, insulin infusion was repeated in five pigs following blockade of alpha-adrenergic receptors with injection of phentolamine into the renal artery and in the other five pigs following blockade of nitric oxide formation with injection in the same artery of Nomega-nitro-L-arginine methyl ester (L-NAME). After blockade of alpha-adrenergic receptors, insulin infusion caused an increase in renal blood flow which averaged 18.1% of the control values, being significantly enhanced with respect to the increase previously obtained in the same pigs. On the contrary, after blockade of nitric oxide formation insulin infusion caused a decrease in renal blood flow which averaged 6.5% of the control values. These responses were respectively abolished by the subsequent injection into the renal artery of L-NAME and phentolamine. The present study showed that the renal vasodilation caused by insulin in the anesthetized pig was the result of two opposite effects which involved a predominant vasodilation mediated by the release of nitric oxide from the endothelium and a sympathetic vasoconstrictor mechanism mediated by alpha-adrenergic receptors.     

促胰液素和胆囊收缩素族激素对豚鼠肝胆汁分泌的影响

用具备胃瘘和胆瘘的豚鼠于人工维持胆汁酸池恒定的条件下,观察促胰液素(SEC)和胆囊收缩素(CCK)族激素[包括雨蛙肽(CAE)、五肽胃泌素(G5)和内源性CCK]对肝胆汁分泌的影响及其相互作用。结果表明:静脉灌注SEC、CAE或肠内灌注左旋苯丙氨酸(L-PHE,促内源性CCK释放剂)后,胆汁流量、胆汁HCO~-_3和Cl~-排出量均显著增多,并呈剂量-效应关系,但静脉注射G5则无利胆效应。在恒速灌注SEC的背景下,CAE或CCK对胆汁HCO~-_3排出的效应分别大于它们单独给予时的效应(P<0.05或P<0.01)。这些激素对胆汁酸的排出量均无影响。上述结果表明,SEC,CAE和内源性CCK均有利胆作用,所刺激的肝胆汁属于不依赖胆汁酸部分。G5则无利胆效应。对胆汁中HCO~-_3的排出,SEC与CAE或内源性CCK间有相互加强作用。     

Role of NO in coronary and systemic vasodilation following cardiogenic reflexes

In acute experiments on anesthetized dogs under closed-chest conditions, we used the technique of double lumen catheterization of coronary vessels and peripheral vessel bed. We studied the role of endothelium-dependent relaxing factor/nitric oxide (EDRF/NO) in the development of parasympathetic coronary vasodilation after excitation of cardiac receptors. Under conditions of pharmacological stimulation of cardiac receptors of the left ventricle and short-lasting episodes of local myocardial ischemia, we also examined the effects of inhibition of NO synthesis on the development of cardiogenic depressor reflexes (hypotension and peripheral vasodilation). It was found that the reflex coronary dilatation following excitation of the cardiac (left ventricular) receptors significantly decreased after systemic NO synthase inhibition. Thus, NO production is one of the effector mechanisms of the development of coronary vessel dilatation; this conclusion is confirmed by changes in the dilatation level after blockade of this process with L-NNA (nitro-ω-L-arginine). We pioneered in demonstrating that after the blockade of NO synthesis peripheral vessel vasodilation decreases or disappeas altogether when cardiogenic reflexes are realized following pharmacological excitation of cardiac receptors with veratrine or catecholamine injections, and vasoconstrictor responses evoked by myocardial ischemia are significantly intensified. It is suggested that the influences of NO-dependent mechanisms exert a dual effect on sympathic control-mediated peripheral vasodilation during cardiogenic reflexes. Such mechanisms reduce central sympathetic tone and/or concurrently provide peripheral inhibition of neural sympathetic influences; in the latter case, NO-dependent cardiogenic reflexes play a crucial role in compensatory reactions after an injury to the heart.     

Redistribution of blood within the body is important for thermoregulation in an ectothermic vertebrate (<Emphasis Type="Italic">Crocodylus porosus</Emphasis>)

Changes in blood flow are a principal mechanism of thermoregulation in vertebrates. Changes in heart rate will alter blood flow, although multiple demands for limited cardiac output may compromise effective thermoregulation. We tested the hypothesis that regional differences in blood flow during heating and cooling can occur independently from changes in heart rate. We measured heart rate and blood pressure concurrently with blood flow in the crocodile, Crocodylus porosus. We measured changes in blood flow by laser Doppler flowmetry, and by injecting coloured microspheres. All measurements were made under different heat loads, with and without blocking cholinergic and β-adrenergic receptors (autonomic blockade). Heart rates were significantly faster during heating than cooling in the control animals, but not when autonomic receptors were blocked. There were no significant differences in blood flow distribution between the control and autonomic blockade treatments. In both treatments, blood flow was directed to the dorsal skin and muscle and away from the tail and duodenum during heating. When the heat source was switched off, there was a redistribution of blood from the dorsal surface to the duodenum. Blood flow to the leg skin and muscle, and to the liver did not change significantly with thermal state. Blood pressure was significantly higher during the autonomic blockade than during the control. Thermal time constants of heating and cooling were unaffected by the blockade of autonomic receptors. We concluded that animals partially compensated for a lack of differential heart rates during heating and cooling by redistributing blood within the body, and by increasing blood pressure to increase flow. Hence, measures of heart rate alone are insufficient to assess physiological thermoregulation in reptiles.     

Acceleration of ulcer healing by cholecystokinin (CCK): role of CCK-A receptors, somatostatin, nitric oxide and sensory nerves.

CCK exhibits a potent cytoprotective activity against acute gastric lesions, but its role in ulcer healing has been little examined. In this study we determined whether exogenous CCK or endogenously released CCK by camostate, an inhibitor of luminal proteases, or by the diversion of pancreatico-biliary secretion from the duodenum, could affect ulcer healing. In addition, the effects of antagonism of CCK-A receptors (by loxiglumide, LOX) or CCK-B receptors (by L-365,260), an inhibition of NO-synthase by N(G)-nitro-L-arginine (L-NNA), or sensory denervation by large neurotoxic dose of capsaicin on CCK-induced ulcer healing were examined. Gastric ulcers were produced by serosal application of acetic acid and animals were sacrificed 9 days after ulcer induction. The area of ulcers and blood flow at the ulcer area were determined. Plasma levels of gastrin and CCK and luminal somatostatin were measured by RIA and mucosal biopsy samples were taken for histological evaluation and measurement of DNA synthesis. CCK given s.c. reduced dose dependently the ulcer area; the threshold dose of CCK being 1 nmol/kg and the dose inhibiting this area by 50% being 5 nmol/kg. This healing effect of CCK was accompanied by a significant increase in the GBF at ulcer margin and the rise in luminal NO production, plasma gastrin level and DNA synthesis. Concurrent treatment with LOX, completely abolished the CCK-8-induced acceleration of the ulcer healing and the rise in the GBF at the ulcer margin, whereas L-365,260 remained without any influence. Treatment with camostate or diversion of pancreatic juice that raised plasma CCK level to that observed with administration of CCK-8, also accelerated ulcer healing and this effect was also attenuated by LOX but not by L-365,260. Inhibition of NO-synthase by L-NNA significantly delayed ulcer healing and reversed the CCK-8 induced acceleration of ulcer healing, hyperemia at the ulcer margin and luminal NO release, and these effects were restored by the addition to L-NNA of L-arginine but not D-arginine. Capsaicin denervation attenuated CCK-induced ulcer healing, and the accompanying rise in the GBF at the ulcer margin and decreased plasma gastrin and luminal release of somatostatin when compared to those in rats with intact sensory nerves. Detectable signals for CCK-A and B receptor mRNAs as well as for cNOS mRNA expression were recorded by RT-PCR in the vehicle control gastric mucosa. The expression of CCK-A receptor mRNA and cNOS mRNA was significantly increased in rats treated with CCK-8 and camostate, whereas CCK-B receptor mRNA remained unaffected. We conclude that CCK accelerates ulcer healing by the mechanism involving upregulation of specific CCK-A receptors, enhancement of somatostatin release, stimulation of sensory nerves and hyperemia in the ulcer area, possibly mediated by NO.     

Nitroglycerin reduces myocardial oxygen consumption during exercise despite vascular tolerance

The long-term benefits of nitroglycerin (NTG) therapy are limited by the development of vascular tolerance and endothelial dysfunction in conductance coronary arteries. We have determined whether nitrate tolerance extends to NTG effects on myocardial O2 consumption (MV(O2)) and the ability of endogenous nitric oxide (NO) to modulate MV(O2) during exercise. In chronically instrumented dogs (n = 8), hemodynamic and MV(O2) responses to treadmill exercise were measured before, during tolerance (3 and 7 days of NTG delivery), and 7 days after NTG withdrawal. Acute NTG delivery caused a parallel downward shift of the MV(O2)-triple product (TP) relations and reversed the disproportionate increases in MV(O2) caused by the blockade of NO formation. After 7 days of continuous transdermal NTG delivery, vascular tolerance was displayed as a >75% reduction of coronary blood flow (CBF) responses to NTG boluses. Despite vascular nitrate tolerance, MV(O2)-TP relations were shifted downward compared with pre-NTG exercise. Seven days after NTG withdrawal, vascular responses to boluses of NTG had recovered from tolerance, and MV(O2)-TP relations during exercise were back to pre-NTG level. At that time, blockade of NO formation failed to alter MV(O2)-TP relations. Thus NTG caused a sustained reduction of cardiac MV(O2), independent of metabolic demand during exercise, despite tolerance of the coronary microcirculation. NTG-induced vascular tolerance and MV(O2) reductions were reversible by NTG withdrawal, but endogenous NO-dependent modulation of O2 consumption was severely impaired.     

ECG-synchronized thoracic vest inflation during autonomic blockade, myocardial ischemia, or cardiac arrest.

To evaluate, in the absence of lung inflation, the cardiovascular effects of single and repetitive pleural pressure increments induced by thoracic vest inflations and timed to occur during specific portions of the cardiac cycle, seven chronically instrumented dogs were studied. Reflexes and left ventricular (LV) performance were varied by autonomic blockade, circumflex coronary occlusion (with and without beta-blockade), or cardiac arrest. Single late systolic, but not early systolic, vest inflations significantly increased LV stroke volume both before (+12.4%) and after myocardial depression by coronary occlusion+beta-blockade (+18.5%) when performed after a period of apnea to control preload and rate. During vest inflations, LV and aortic pressures increased to a greater degree than esophageal pressure (by 51 vs. 39 mmHg, P = 0.0001). Lung inflations (26 trials in 3 dogs) during early or late systole failed to increase stroke volume, despite peak esophageal pressures of 11-26 mmHg. With autonomic reflexes intact, repetitive vest inflations coupled to early systole, late systole, or diastole induced a large (40%) but unspecific systemic flow increase. In contrast, during autonomic blockade, flow increased slightly (7.5%, P < 0.05) with late systolic compared with diastolic inflations but not relative to baseline. During coronary occlusion (with or without beta-blockade), no cycle-specific differences were seen, whereas matched vest inflations during cardiac arrest generated 20-30% of normal systemic flow. Thus only single late systolic thoracic vest inflations associated with large increments in pleural pressure increased LV emptying, presumably by decreasing LV afterload and/or focal cardiac compression. However, during myocardial ischemia and depression, coupling of vest inflation to specific parts of the cardiac cycle revealed no hemodynamic improvement, suggesting that benefits of this circulatory assist method, if any, are minor and may be restricted to conditions of cardiac arrest.     

Gastrin and pentagastrin enhance the tumour proliferation of human stable cultured gastric adenocarcinoma cells.

The effect of gastrin on stimulating tumour proliferation has been evaluated on human pancreas cancer cells in culture and in tumours transplanted to nude mice. The presence of CCK-B/gastrin-like receptor responsible for that effect of gastrin has been proved in colonic (WiDr, HT-29, YAMC) and pancreatic (PANC-1, BON) cell lines. The aim of our study was to examine the stimulating effect of gastrin and pentagastrin on the growth of human gastric adenocarcinoma cell line. The human gastric adenocarcinoma cell line (AGS, CRL-1739) was purchased from ATCC (Rockville, MA, USA). Gastrin-17 was purchased from Sigma-Aldrich (Budapest, Hungary), pentagastrin was from Zeneca Limited (Macclasfield, UK). The cells were incubated in DMEM containing 10% FCS on 96-well culturing plate with 10(4) cells/well starting cell number at 37 degrees C with 5% CO2. The proliferation rates were detected: by the measurements of the metabolically active cells with Owen's reagent and the determination of protein content, and by cell counting in a haemocytometer at several incubation times. As a result, we detected similar proliferation rates using gastrin-17 or pentagastrin in the incubation medium. The stimulating effect of gastrin/pentagastrin on cell line proliferation was in correlation with its concentration. Our results proved that pentagastrin is a 10 times less effective stimulator of proliferation of gastric cancer than gastrin-17, and that AGS human adenocarcinoma cell line might be CCK receptor positive.     

Stomach-brain communication by vagal afferents in response to luminal acid backdiffusion, gastrin, and gastric acid secretion

Vagal afferents play a role in gut-brain signaling of physiological and pathological stimuli. Here, we investigated how backdiffusion of luminal HCl or NH(4)OH and pentagastrin-stimulated acid secretion interact in the communication between rat stomach and brain stem. Rats were pretreated intraperitoneally with vehicle or appropriate doses of cimetidine, omeprazole, pentagastrin, dexloxiglumide (CCK(1) receptor antagonist), and itriglumide (CCK(2) receptor antagonist) before intragastric administration of saline or backdiffusing concentrations of HCl or NH(4)OH. Two hours later, neuronal activation in the nucleus of the solitary tract (NTS) and area postrema was visualized by c-Fos immunohistochemistry. Exposure of the rat gastric mucosa to HCl (0.15-0.5 M) or NH(4)OH (0.1-0.3 M) led to a concentration-dependent expression of c-Fos in the NTS, which was not related to gender, gastric mucosal injury, or gastropyloric motor alterations. The c-Fos response to HCl was diminished by cimetidine and omeprazole, enhanced by pentagastrin, and left unchanged by dexloxiglumide and itriglumide. Pentagastrin alone caused an omeprazole-resistant expression of c-fos, which in the NTS was attenuated by itriglumide and prevented by dexloxiglumide but in the area postrema was reduced by dexloxiglumide and abolished by itriglumide. We conclude that vagal afferents transmit physiological stimuli (gastrin) and pathological events (backdiffusion of luminal HCl or NH(4)OH) from the stomach to the brain stem. These communication modalities interact because, firstly, acid secretion enhances afferent signaling of gastric acid backdiffusion and, secondly, gastrin activates NTS neurons through stimulation of CCK(1) receptors on vagal afferents and of CCK(2) receptors on area postrema neurons projecting to the NTS.     

Receptor type for cholecystokinin on isolated intestinal muscle cells of the guinea pig

Smooth muscle cells isolated from the longitudinal muscle layer of guinea pig ileum were used to determine the presence and type of cholecystokinin/gastrin receptor mediating contraction. This was accomplished with a series of cholecystokinin and gastrin agonists (CCK-8, des(SO3)CCK-8, gastrin-17, des(SO3)gastrin-17 and pentagastrin) and antagonists (glutaramic acid derivatives CR 1392, CR 1409, CR 1505 and proglumide). The order of potency of agonists based on EC50 values derived from concentration-response curves was: CCK-8 greater than des(SO3)CCK-8 greater than gastrin-17 greater than des(SO3)gastrin-17. The inhibitory dissociation constant (Ki) for the antagonist CR 1505 derived from Schild plots was the same whether sulfated CCK-8 or desulfated gastrin-17 was used as agonist (4.47 +/- 0.76 versus 4.68 +/- 0.78 nM). Pentagastrin acted as a partial agonist and inhibited partially the response to CCK-8. The Ki values determined for all antagonists with pentagastrin as agonist were similar to those with CCK-8 as agonist. The order of potency of agonists and the independence of Ki values from the type of agonist used implied that CCK and gastrin interact with one receptor type; the receptor is more sensitive to CCK-8 but is minimally influenced by sulfation of the tyrosine residue. In this respect, the receptor appears to be distinct from the CCK receptor on gallbladder muscle cells and pancreatic acinar cells.     

Hindlimb unloading alters nitric oxide and autonomic control of resting arterial pressure in conscious rats

After periods of microgravity or bed rest, individuals often exhibit reduced Vo(2 max), hypovolemia, cardiac and vascular effects, and autonomic dysfunction. Recently, alterations in expression of vascular and central nervous system NO synthase (NOS) have been observed in hindlimb-unloaded (HU) rats, a model used to simulate physiological effects of microgravity or bed rest. We examined the effects of 14 days of hindlimb unloading on hemodynamic responses to systemic NOS inhibition in conscious control and HU rats. Because differences in NO and autonomic regulation might occur after hindlimb unloading, we also evaluated potential differences in resting autonomic tone and effects of NOS inhibition after autonomic blockade. Administration of nitro-L-arginine methyl ester (L-NAME; 20 mg/kg iv) increased mean arterial pressure (MAP) to similar levels in control and HU rats. However, the change in MAP in response to L-NAME was less in HU rats, that had an elevated baseline MAP. In separate experiments, atropine (1 mg/kg iv) increased heart rate (HR) in control but not HU rats. Subsequent administration of the ganglionic blocker hexamethonium (30 mg/kg iv) decreased MAP and HR to a greater extent in HU rats. Administration of L-NAME after autonomic blockade increased MAP in both groups to a greater extent compared with intact conditions. However, the pressor response to L-NAME was still reduced in HU rats. These data suggest that hindlimb unloading in rats reduces peripheral NO as well as cardiac parasympathetic tone. Along with elevations in sympathetic tone, these effects likely contribute to alterations in vascular control and changes in autonomic reflex function following spaceflight or bed rest.     

Role of ET-1 in the regulation of coronary circulation

Given that circulating ET levels in heart failure, in particular, may reach physiological threshold for coronary constrictor responses, the primary objective of the present review is to consider coronary vessels as an important target for circulating and locally produced endothelin(s). In healthy vessels, ET-1 causes biphasic coronary responses characterized by a transient dilation of large and small arteries followed by a sustained constriction. ETB receptors are pivotal in the early dilation of resistance vessels, whereas dilation of conductance vessels may be a secondary phenomenon triggered by flow increases. Exogenous ET-1 causes coronary constriction almost exclusively through ETA receptor activation. Human and canine large epicardial coronary vessels display significant baseline ET-1 dependent tone in vitro and in vivo, an ETA-dependent process. In contrast, ETB receptors located on smooth muscle cells are apparently less important for producing constrictor responses. NO production may serve as an important counter-regulatory mechanism to limit ET-dependent effects on coronary vessels. Conversely, in a dysfunctional endothelium, the loss of NO may augment ET-1 production and activity. By lifting the ET-dependent burden from coronary vessels, ET receptor blockade should help to ensure a closer match between cardiac metabolic demand and coronary perfusion.     

Endothelium-derived relaxing factor participates in the increased blood flow in response to pentagastrin in the rat stomach mucosa

The stimulation of gastric-acid secretion by pentagastrin, a synthetic analogue of the endogenous peptide gastrin, is associated with an increased blood flow to the stomach mucosa, commonly referred to as functional hyperaemia. There are at least two potent vasodilator substances, the local release of which from endothelial cells could contribute to this hyperaemia, endothelium-derived relaxing factor (EDRF) and prostacyclin. EDRF has been identified as nitric oxide, released enzymatically from the guanidino group of L-arginine. In the present studies, the involvement of prostacyclin in the pentagastrin-induced increase in stomach blood flow was eliminated by using the cyclo-oxygenase inhibitor indomethacin. Thus this work was designed to elucidate the participation of EDRF/NO in the pentagastrin-induced hyperaemia and not its relative importance to prostacyclin. The increase in blood flow to the gastric mucosa in response to pentagastrin was measured by laser Doppler flowmetry in situ. Inhibition of EDRF/NO biosynthesis with the L-arginine analogues NG-monomethyl-L-arginine (MeArg) or N omega-nitro-L-arginine (NO2Arg) significantly attenuated (by more than 80%) the increase in mucosal blood flow in response to pentagastrin. However, infusions of the natural substrate L-arginine reversed the inhibitor effect of MeArg on pentagastrin-induced increase in mucosal blood flow. Local intra-arterial injections of the endothelium-independent vasodilator glyceryl trinitrate produced a dose-related increase in blood flow to the rat stomach mucosa that was unaffected by infusion of MeArg. Thus, in the absence of prostacyclin, EDRF/NO participates in the pentagastrin-induced increase in blood flow to the rat stomach mucosa.     

The effect of dibutyril cyclic guanosine monophosphate,a competitive antagonist to cholecystokinin-pancreozymin,on gastric acid secretion in the isolated mouse stomach

Dibutyryl cyclic guanosine monophosphate (dbcGMP), a specific competitive inhibitor of the gastrin, cholecystokinin-pancreozymin (CCK-PZ) family of peptides in pancreas, gallbladder and ileum, had no effect on basal acid secretion in the isolated mouse stomach nor on secretion stimulated by bethanechol or histamine. Secretion evoked by low doses of pentagastrin were likewise unaffected by dbcGMP but responses to high doses of pentagastrin were augmented. CCK-PZ and glucagon each inhibited acid secretion evoked by pentagastrin. DbcGMP blocked CCK-PZ-mediated inhibition but was without effect on inhibition by glucagon. These observations suggest that in the gastric glands there exist two receptors with different affinities for gastrin and CCK-PZ which mediate excitation and inhibition respectively.