Histamine H3 receptors modulate reactive hyperemia in rat gut.

Reactive hyperemia (RH) is an abrupt blood flow increase following release from mechanical occlusion of an artery, with restoration of intra-arterial pressure. The mechanism of this postocclusion increase in blood flow in the gut is multifactorial. Relaxation of intestinal resistance vessels, observed during RH, may involve myogenic, metabolic, hormonal and neurogenic factors. Evidence exists that histamine is an important endogenous mediator of various functions of the gut, including blood flow. The vascular effects of histamine in the intestinal circulation are due its agonistic action on histamine H1, H2 and H3 receptors. In the present study the hypothesis was tested that peripheral histamine H3 receptors are involved in the mediation of RH in the intestinal circulation. In anesthetized rats, anterior mesenteric artery blood flow (MBF) was determined with ultrasonic Doppler flowmeter, and arterial pressure (AP) was determined with a transducer. The increase in the volume of blood accumulating during RH (RH-volume), the peak increase of arterial blood flow (RH-peak response) and the duration of the hyperemia (RH-duration) were used to quantify RH after occluding the anterior mesenteric artery for 30, 60 and 120 s. Hyperemia parameters were determined before and after administration of the selective histamine H3 receptor antagonist clobenpropit. Pretreatment with clobenpropit was without any effect on control MBF and AP but significantly reduced most of RH responses. These findings support the hypothesis that histamine H3 receptors do not play any role in the control of intestinal vasculature at basal conditions but these receptors participate in the intestinal hyperemic reaction in response to complete temporal intestinal ischemia.     

Neural and humoral control of regional vascular beds via A1 adenosine receptors located in the nucleus tractus solitarii

Our previous studies showed that stimulation of adenosine A(1) receptors located in the nucleus of the solitary tract (NTS) exerts counteracting effects on the iliac vascular bed: activation of the adrenal medulla and β-adrenergic vasodilation vs. sympathetic and vasopressinergic vasoconstriction. Because NTS A(1) adenosine receptors inhibit baroreflex transmission in the NTS and contribute to the pressor component of the HDR, we hypothesized that these receptors also contribute to the redistribution of blood from the visceral to the muscle vasculature via prevailing sympathetic and vasopressinergic vasoconstriction in the visceral (renal and mesenteric) vascular beds and prevailing β-adrenergic vasodilation in the somatic (iliac) vasculature. To test this hypothesis, we compared the A(1) adenosine-receptor-mediated effects of each vasoactive factor triggered by NTS A(1) adenosine receptor stimulation [N(6)-cyclopentyladenosine (CPA), 330 pmol in 50 nl] on the regional vascular responses in urethane/chloralose-anesthetized rats. The single-factor effects were separated using adrenalectomy, β-adrenergic blockade, V(1) vasopressin receptor blockade, and sinoaortic denervation. In intact animals, initial vasodilation was followed by large, sustained vasoconstriction with smaller responses observed in renal vs. mesenteric and iliac vascular beds. The initial β-adrenergic vasodilation prevailed in the iliac vs. mesenteric and renal vasculature. The large and sustained vasopressinergic vasoconstriction was similar in all vascular beds. Small sympathetic vasoconstriction was observed only in the iliac vasculature in this setting. We conclude that, although A(1) adenosine-receptor-mediated β-adrenergic vasodilation may contribute to the redistribution of blood from the visceral to the muscle vasculature, this effect is overridden by sympathetic and vasopressinergic vasoconstriction.     

Capsaicin-sensitive nerves modulate reactive hyperemia in rat gut.

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

Adenosine and its receptors in the heart: regulation, retaliation and adaptation

The purine nucleoside adenosine is an important regulator within the cardiovascular system, and throughout the body. Released in response to perturbations in energy state, among other stimuli, local adenosine interacts with 4 adenosine receptor sub-types on constituent cardiac and vascular cells: A(1), A(2A), A(2B), and A(3)ARs. These G-protein coupled receptors mediate varied responses, from modulation of coronary flow, heart rate and contraction, to cardioprotection, inflammatory regulation, and control of cell growth and tissue remodeling. Research also unveils an increasingly complex interplay between members of the adenosine receptor family, and with other receptor groups. Given generally favorable effects of adenosine receptor activity (e.g. improving the balance between myocardial energy utilization and supply, limiting injury and adverse remodeling, suppressing inflammation), the adenosine receptor system is an attractive target for therapeutic manipulation. Cardiovascular adenosine receptor-based therapies are already in place, and trials of new treatments underway. Although the complex interplay between adenosine receptors and other receptors, and their wide distribution and functions, pose challenges to implementation of site/target specific cardiovascular therapy, the potential of adenosinergic pharmacotherapy can be more fully realized with greater understanding of the roles of adenosine receptors under physiological and pathological conditions. This review addresses some of the major known and proposed actions of adenosine and adenosine receptors in the heart and vessels, focusing on the ability of the adenosine receptor system to regulate cell function, retaliate against injurious stressors, and mediate longer-term adaptive responses.     

Adenosine modulation of vasoconstrictor responses to stimulation of sympathetic nerves and norepinephrine infusion in the superior mesenteric artery of the cat

Vasoconstriction induced by sympathetic nerve stimulation and by norepinephrine infusion in the superior mesenteric artery of cats anesthetized with pentobarbital was inhibited by adenosine infusions in a dose-related way. The responses to nerve stimulation were not inhibited to a greater extent than the responses to norepinephrine, thus suggesting no presynaptic modulation of sympathetic nerves supplying the resistance vessels of the feline intestinal vascular bed. Blockade of adenosine receptors using 8-phenyltheophylline did not alter the degree of constriction induced by nerve stimulation or norepinephrine infusion, indicating that in the fasted cat, endogenous adenosine co-released or released subsequent to constriction does not affect the peak vasoconstriction reached. Isoproterenol caused similar degrees of vasodilation as adenosine but did not show significant antagonism of the pooled responses to nerve stimulation or norepinephrine infusion; there was no tendency for the degree of dilation induced by isoproterenol to correlate with the inhibition of constrictor responses. Thus, the effect of adenosine on nerve- and norepinephrine-induced constriction is not secondary to nonspecific vasodilation.     

Control of the Splanchnic Circulation

The major regulatory factors in the mesenteric circulation include general hemodynamic forces, the autonomic nervous system, circulating vasoactive substances, tissue metabolites and intrinsic characteristics of vascular smooth muscle. During mesenteric ischemic states smooth muscle spasm elevates resistance to blood flow and aggravates intestinal tissue hypoxia leading to mucosal necrosis. Close intraarterial infusion of potent vasodilator drugs holds the promise of reversing intestinal ischemia and preserving viability of the gut.     

Vasoactive receptors in abdominal blood vessels of the dogfish shark, Squalus acanthias

Previous studies have demonstrated that the ventral aorta of the dogfish shark, Squalus acanthias, responds to a variety of cell-signaling agents. To investigate the generality of vasoactive receptors in the shark vasculature, in particular a conductance artery (anterior mesenteric) and vein (posterior intestinal), I measured the effect of acetylcholine, endothelin, nitric oxide, natriuretic peptides, and prostaglandins on tension in isolated rings from these vessels. Both vessels responded to these agents, and responses to receptor-specific ligands for endothelin and natriuretic peptide receptors suggest that B-type endothelin receptors are expressed in both vessels and that the artery expresses both A- and B-type natriuretic peptide receptors; however, the vein (like the ventral aorta) expresses only the B-type natriuretic peptide receptor. My data suggest that a suite of signaling systems is ubiquitous in both arteries and veins in at least this elasmobranch species. Their role in hemodynamics and osmoregulation (perfusion of gill and rectal gland) remains to be determined.     

Blood vascular system of the sea cucumber,Stichopus moebii

Stichopus moebii, a sea cucumber, has a closed circulatory system which is unique in its degree of development for the phylum Echinodermata. The gross anatomy, histology and fine structure of the system were studied. Blood vessels consist of a coelomic surface of ciliated epithelium, a layer of muscle and nerve cells, followed by connective tissue and luminal lining of endothelium. Basically the blood vascular system consists of two major vessels running parallel to the gut: the dorsal vessel pumps colorless blood via the vessels within the walls of the intestine into the ventral vessel. There are two specialized areas of the circulation: (1) At the upper small intestine 120 to 150 muscular single-chambered hearts pump blood from the dorsal vessel into a series of intestinal plates. (2) At the lower region of the small intestine the vasculature is associated with the left respiratory tree. Blood passing from the dorsal pulmonary vessel can take two routes to the gut, it either passes through myriads of minute respiratory shunt vessels entangled with the respiratory tree or it passes through a unique follicle network consisting of tiny channels periodically dilated into chambers filled with iron deposits, necrotic cells and developing coelomocytes.     

Bile acids and their receptors in regulation of gut health and diseases

It is well established that bile acids play important roles in lipid metabolism. In recent decades, bile acids have also been shown to function as signaling molecules via interacting with various receptors. Bile acids circulate continuously through the enterohepatic circulation and go through microbial transformation by gut microbes, and thus bile acids metabolism has profound effects on the liver and intestinal tissues as well as the gut microbiota. Farnesoid X receptor and G protein-coupled bile acid receptor 1 are two pivotal bile acid receptors that highly expressed in the intestinal tissues, and they have emerged as pivotal regulators in bile acids metabolism, innate immunity and inflammatory responses. There is considerable interest in manipulating the metabolism of bile acids and the expression of bile acid receptors as this may be a promising strategy to regulate intestinal health and disease. This review aims to summarize the roles of bile acids and their receptors in regulation of gut health and diseases.     

Autoregulation of superior mesenteric artery is blocked by adenosine antagonism

Pressure-flow autoregulation of the intact superior mesenteric artery (SMA) was demonstrated in the fasted, pentobarbital-anesthetized cat by use of a micrometer-controlled screw clamp to produce progressive decreases in vascular pressure. Administration (ia) of bolus doses of 8-phenyltheophylline (8-PT) were followed by infusion of adenosine to verify adenosine antagonism. 8-PT doses were progressively doubled until adenosine responses were blocked. If higher doses of 8-PT were used, SMA flow declined to very low levels and autoregulatory curves could not be obtained. Comparison of vasodilator responses to isoproterenol and adenosine before and after adenosine receptor blockade verified that, whereas adenosine responses were blocked, isoproterenol effects were not altered. The autoregulation was quantitated using three methods (the autoregulatory index, the percent decrease in vascular resistance, and the slope index) as blood pressure was reduced from a standardized control pressure of 110 mmHg (1 mmHg = 133.3 Pa). Maximal vasodilation appeared at a blood pressure of 56 +/- 5 mmHg (range 34-70). 8-PT resulted in dose-related antagonism of the dilator response to exogenous adenosine and autoregulation. All indices of autoregulation were significantly reduced by 8-PT. The data are compatible with the hypothesis that pressure-flow autoregulation in the SMA is not myogenic (responding to altered transmural pressure) but is dependent upon local concentrations of adenosine.     

Differential inhibition of noradrenaline release mediated by inhibitory A1-adenosine receptors in the mesenteric vein and artery from normotensive and hypertensive rats

Mesenteric arteries and veins are densely innervated by sympathetic nerves and are crucial in the regulation of peripheral resistance and capacitance, respectively, thus, in the control of blood pressure. Presynaptic adenosine receptors are involved in vascular tonus regulation, by modulating noradrenaline release from vascular postganglionic sympathetic nerve endings. Some studies also suggest that adenosine receptors (AR) may have a role in hypertension. We aim at investigating the role of presynaptic adenosine receptors in mesenteric vessels and establish a relationship between their effects (in mesenteric vessels) and hypertension, using the spontaneously hypertensive rats (SHR) as a model of hypertension. Adenosine receptor-mediated modulation of noradrenaline release was investigated through the effects of selective agonists and antagonists on electrically-evoked [³H]-noradrenaline overflow. CPA (A₁AR selective agonist: 1–100 nM) inhibited tritium overflow, but the inhibition was lower in SHR mesenteric vessels. IB-MECA (A₃AR selective agonist: 1–100 nM) also inhibited tritium overflow but only in WKY mesenteric veins. CGS 21680 (A_2AAR selective agonist: up to 100 nM) failed to facilitate noradrenaline release in mesenteric veins, from both strains, but induced a similar facilitation in the mesenteric arteries. NECA (non-selective AR agonist: 1, 3 and 10 μM), in the presence of A₁ (DPCPX, 20 nM) and A₃ (MRS 1523, 1 μM) AR selective antagonists, failed to change tritium overflow. In summary, the modulatory effects mediated by presynaptic adenosine receptors were characterized, for the first time, in mesenteric vessels: a major inhibition exerted by the A₁ subtype in both vessels; a slight inhibition mediated by A₃ receptors in mesenteric vein; a facilitation mediated by A_2A receptors only in mesenteric artery (from both strains). The less efficient prejunctional adenosine receptor mediated inhibitory effects can contribute to an increase of noradrenaline in the synaptic cleft (both in arteries and veins), which might conduce to increased vascular reactivity.     

Vasomotor control: functional hyperemia and beyond

Historically, functional hyperemia has been viewed largely as an interaction between a parenchymal cell and its associated microvasculature. Locally released metabolites have been thought to produce relaxation of the smooth muscle and a vasodilation that increases blood flow in proportion to metabolic need. This symposium report presents evidence from a variety of disciplines and a number of different types of biological preparations that demonstrates that functional hyperemia is a complex process involving several classes of microvessels including capillaries, arterioles, and small arteries. These vessels do not function independently but are coordinated by a complex set of interrelations involving at least three different modes of interaction between parenchymal cells and the various segments of the vascular bed. These are local metabolic effects, propagated effects extending over long segments of the vasculature, and flow-dependent vasodilation induced by local changes in blood flow. In addition to these acute responses to metabolic demand it appears that tissues may be capable of more long-term structural alterations of the arterial and arteriolar network in response to sustained changes in the relationship between supply and demand. The vascular bed appears to be able to adapt either by increasing the maximal anatomic diameter of the large arteries or by inserting new arterioles into the parenchyma. Thus, classical functional hyperemia appears to be but one manifestation of a multifaceted process leading to highly coordinated responses of many vascular elements, resulting finally in vascular patterns that are optimized to meet parenchymal cell demands.     

Lymphatic pump function in the inflamed gut

The role of the lymphatic circulation to actively remove fluid, cells, proteins, and other particles from the interstitium to prevent mounting edema is well appreciated, but whether and how this function is compromised during inflammation has been scarcely investigated. We discuss here the mechanisms of lymphatic pumping and their modulation in inflammatory conditions or by inflammatory mediators in the context of inflammatory bowel disease (IBD), an ensemble of disorders typically described with abnormal or dysfunctional intestinal or mesenteric lymphatic vessels. We report our findings showing impaired mesenteric lymphatic contractile activity in an animal model of intestinal inflammation that recapitulates some features of IBD and suggests a role for prostanoids in this dysfunction. With the knowledge that prostaglandin E(2) and prostacyclin are implicated in IBD pathogenesis and induce a potent inhibition of lymphatic pumping, we established the pharmacological profile for these prostaglandin receptors in mesenteric lymphatic vessels and their respective role in pumping inhibition. Inhibition of mesenteric lymphatic pumping during inflammation may be a cause of edema, compromised immune response, and granuloma associated with IBD.     

Adenosine-Mediated Enteric Neuromuscular Function Is Affected during Herpes Simplex Virus Type 1 Infection of Rat Enteric Nervous System

Adenosine plays an important role in regulating intestinal motility and inflammatory processes. Previous studies in rodent models have demonstrated that adenosine metabolism and signalling are altered during chronic intestinal inflammatory diseases. However, the involvement of the adenosinergic system in the pathophysiology of gut dysmotility associated to a primary neurodysfunction is still unclear. Recently, we showed that the neurotropic Herpes simplex virus type-1 (HSV-1), orally inoculated to rodents, infects the rat enteric nervous system (ENS) and affects gut motor function without signs of systemic infection. In this study we examined whether changes in purinergic metabolism and signaling occur during permanent HSV-1 infection of rat ENS. Using isolated organ bath assays, we found that contraction mediated by adenosine engagement of A₁ or A_2A receptors was impaired at 1 and 6 weeks post-viral administration. Immunofluorescence studies revealed that viral infection of ENS led to a marked redistribution of adenosine receptors: A₁ and A_2B receptors were confined to the muscle layers whereas A_2A and A₃ receptors were expressed mainly in the myenteric plexus. Viral-induced ENS neurodysfunction influenced adenosine metabolism by increasing adenosine deaminase and CD73 levels in longitudinal muscle-myenteric plexus with no sign of frank inflammation. This study provides the first evidence for involvement of the adenosinergic system during HSV-1 infection of the ENS. As such, this may represent a valid therapeutic target for modulating gut contractility associated to a primary neurodysfunction.     

Perfusion pressure manipulation in porcine sepsis: effects on intestinal hemodynamics

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

Effect of raised portal venous pressure and postocclusive hyperemia on superior mesenteric arterial resistance in control and adenosine receptor blocked state in cats

Superior mesenteric arterial (SMA) blood flow was measured in pentobarbital-anesthetized cats using a noncannulating electromagnetic flowprobe. The selective adenosine antagonist 8-phenyltheophylline (8-PT) antagonized the dilator effect of infused adenosine but not isoproterenol. The vasodilation in response to reduced arterial perfusion pressure (autoregulation) was blocked by the adenosine receptor blockade, which also reduced the degree of postocclusive (1 min) hyperemia by one-half to two-thirds. The remainder of the hyperemia may have been due partially to adenosine, since exogenous adenosine still produced a small vasodilation (26%), so effects of endogenous adenosine could also still be expected to exert a small effect. Myogenic effects appear unlikely to be the mechanism of the small remaining hyperemia, since venous pressure increments within physiologically relevant ranges did not cause altered SMA conductance, and arterial dilation in response to large decreases in arterial pressure could be blocked by adenosine antagonism. Portal pressure was increased using hepatic nerve stimulation (8 Hz) to raise pressure from 7.0 to 12.4 mmHg (1 mmHg = 133.3 Pa). The small vasoconstriction seen in the SMA was due to the rise in systemic blood pressure, since prevention of a rise in SMA pressure prevented the response and 8-PT blocked the response (previously shown to block arterial pressure-flow autoregulation). An equal rise in PVP imposed by partial occlusion of the portal vein did not lead to changes in SMA vascular conductance. Thus, we conclude that within physiologically relevant ranges of arterial and portal venous pressure, the SMA does not show myogenic responses of the resistance vessels.     

Nucleotide- and nucleoside-converting ectoenzymes: Important modulators of purinergic signalling cascade

The involvement of extracellular nucleotides and adenosine in an array of cell-specific responses has long been known and appreciated, but the integrative view of purinergic signalling as a multistep coordinated cascade has emerged recently. Current models of nucleotide turnover include: (i) transient release of nanomolar concentrations of ATP and ADP; (ii) triggering of signalling events via a series of ligand-gated (P2X) and metabotropic (P2Y) receptors; (iii) nucleotide breakdown by membrane-bound and soluble nucleotidases, including the enzymes of ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) family, ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) family, ecto-5'-nucleotidase/CD73, and alkaline phosphatases; (iv) interaction of the resulting adenosine with own nucleoside-selective receptors; and finally, (v) extracellular adenosine inactivation via adenosine deaminase and purine nucleoside phosphorylase reactions and/or nucleoside uptake by the cells. In contrast to traditional paradigms that focus on purine-inactivating mechanisms, it has now become clear that "classical" intracellular ATP-regenerating enzymes, adenylate kinase, nucleoside diphosphate (NDP) kinase and ATP synthase can also be co-expressed on the cell surface. Furthermore, data on the ability of various cells to retain micromolar ATP levels in their pericellular space, as well as to release other related compounds (adenosine, UTP, dinucleotide polyphosphates and nucleotide sugars) gain another important insight into our understanding of mechanisms regulating a signalling cascade. This review summarizes recent advances in this rapidly evolving field, with particular emphasis on the nucleotide-releasing and purine-converting pathways in the vasculature.     

Sensitization by chronic diazepam treatment of A2A adenosine receptor-mediated relaxation in rat pulmonary artery

The effects of a 10-day i.p. treatment of rats with diazepam on responses to subtype selective adenosine receptor agonists were studied 3 h, 2 and 8 days after termination of diazepam treatment in isolated cardiovascular tissues possessing distinct adenosine receptors. After long-lasting diazepam exposure, the relaxation elicited by the specific A2A receptor agonist CGS 21680 was enhanced in rat main pulmonary arteries (a tissue containing A2A adenosine receptors). The increased sensitivity of A2A receptors observed 3 h and 2 days after withdrawal of diazepam was completely restored by the 8th day of the wash-out period. N6-cyclopentyladenosine (CPA)-induced suppression in mechanical activity of electrically stimulated rat atrial myocardium (a tissue containing A1 adenosine receptors) was not altered following diazepam treatment. In order to reveal the possible role of inhibition of membrane adenosine transport in the effects of diazepam (a moderate inhibitor of membrane adenosine transport), the action of a 10-day treatment with dipyridamole or S-(p-nitrobenzyl)-6-thioinosine (NBTI; prototypic adenosine uptake inhibitors) was also studied. Dipyridamole or NBTI treatment, like diazepam, increased the responsiveness of rat pulmonary artery to CGS 21680, but did not influence the cardiodepressive effect of CPA in electrically driven left atrial myocardium. The CGS 21680-induced relaxations were significantly antagonized by 10 nM ZM 241385 (a selective A2A adenosine receptor antagonist) in vessels of diazepam-treated rats. The relaxation responses to verapamil were unaltered in pulmonary arteries obtained from animals chronically treated with diazepam, dipyridamole or NBTI. These results suggest that chronic diazepam treatment is able to enhance the A2A adenosine receptor-mediated vascular functions, but does not modify the responses mediated via A1 receptors of rat myocardium, where nucleoside transport inhibitory sites of membrane are of a very low density. It is possible that sensitization of A2A adenosine receptor-mediated vasorelaxation is due to a long-lasting inhibition of membrane adenosine transporter during diazepam treatment.     

Role of polyamines in regulation of uterine blood flow of ovariectomized rats.

Polyamines have been shown to relax several types of smooth muscle including vasculature. In the estrogen-treated uterus, uterine blood flow and polyamine levels in the uterus are increased. The relaxant effect of polyamines on blood vessels suggest that polyamines may act on uterine vasculature to cause uterine hyperemia. In this study, we examined the roles of polyamines in regulating uterine blood flow. Ovariectomized rats were administered with polyamines or estrogen. The uterine blood flow was then measured by employing radioactive microspheres. The direct injection of polyamines into systemic circulation caused a 3-fold increase in uterine blood flow within 30 seconds. The polyamine-induced uterine hyperemia was counteracted by increasing extracellular calcium concentration. When rats were treated with estradiol, uterine blood flow increased. However, the treatment with alpha-difluromethylornithine, a polyamine synthesis inhibitor, did not attenuate the estrogen-induced uterine blood flow. Our results suggest that polyamines may affect uterine blood flow via antagonizing the entry of extracellular calcium. However, the detailed mechanisms via which polyamines involved in estrogenic stimulation of uterine hyperemia may require further studies.