Characteristics of vasodilatation induced by acetylcholine and platelet-activating factor in the rat mesenteric arterial bed

Vascular Responses to Extractable Fractions of Ilex paraguariensis in Rats Fed Standard and High-Cholesterol Diets

The authors investigated the vasorelaxant properties of the aqueous (Aq-EF) and acid n-butanolic (acn-BuOH) extractable fractions from Ilex paraguariensis leaves. Perfusion pressure was evaluated using isolated and perfused mesenteric arterial beds (MABs) from rats fed hypercholesterolemic and standard diets. Extract-induced vasorelaxation in the presence and absence of various inhibitors was examined following precontraction of the MABs with methoxamine (30 μM) solution. In hypercholesterolemic-diet rats, relaxation in intact MABs was significantly decreased with ac-n-BuOH-EF bolus (300, 600, 900 μg) in comparison to those in standard-diet rats. After the endothelium was stripped from the MABs, the vascular responses to ac-n-BuOH-EF and 900 μg bolus of Aq-EF were significantly changed. Treatment of the MABs with an inhibitor of nitric oxide synthase, NG -nitro-L-arginine methylester hydrochloride (L-NAME, 10 mM), did not change either ac-n-BuOH-EF- or Aq-EF-induced vasodilation except for the 900 g bolus of Aq-EF. The guanilate cyclase inhibitor methylene blue (100 μM) did not affect vasodilation for either fraction in the MABs from the hypercholesterolemic-diet rats. The chronic oral administration of I. paraguariensis extract in hypercholesterolemic-diet rats resulted in a significant reduction in serum levels of cholesterol and triglycerides. These results suggest that I. paraguariensis ac-n-BuOH-EF and Aq-EF induce vasodilation in standard-diet rats in a dose-dependent manner and that the hypercholesterolemic diet substantially reduced the effect of ac-n-BuOH-EF on precontracted MABs.

Fish Balistes capriscus skin extract-induced relaxation in mesenteric arterial bed of rat

The vasorelaxing activity of the aqueous extract of fish Balistes capriscus skin (AEBc) on mesenteric arterial bed (MAB) of rats was studied. The bolus injections of AEBc (bolus of 5.1, 10.2, 20.5, and 41.1mg) significantly inhibited, in a concentration-dependent manner, the maximal contractile response induced by methoxamine (30 microM) in MAB. The vasodilatation action of AEBc is not mediated through beta-adrenoceptors or cyclo-oxigenase, since it was not affected by propranolol (20 microM) or diclofenac sodium (3 microM). The vasodilator response induced by subsequent addition of AEBc Balistes capriscus in bolus was significantly reduced in water infusion for endothelium removal. Treatment with an inhibitor of NO synthase (L-NAME, 10 microM) decreased AEBc effect. The guanylate cyclase inhibitor methylene blue (MB, 100 microM) had no significant effect on AEBc-induced vasodilatation. These results suggest that the vasorelaxing effect of AEBc is mediated by endothelium-dependent (NO/EDRF) and endothelium-independent neurally induced vasorelaxation from nonadrenergic and noncholinergic nerves (NO).

Platelet-activating factor enhanced the pressor response of endothelin-1

Both endothelin-1 (ET-1) and platelet-activating factor (PAF) have been suggested to play a role in the regulation of the cardiovascular system. In view of the limited data regarding the interaction between ET-1 and PAF, the hemodynamic effects of ET-1 and PAF, either alone or in combination, were investigated in the current study. Anesthetized male Sprague Dawley rats received bolus intravenous injections of ET-1 (1 and 2 nmol/kg) and/or PAF (0.075, 0.15 and 0.3 nmol/kg). In some experiments, the ET receptor antagonist, FR-139317 (2.5 or 5 mg/kg), were injected 5 min before the administration of ET-1 or PAF. ET-1 caused a biphasic response consisting of an initial depressor followed by a delayed but sustained pressor response. Injection of PAF to anesthetized rats resulted only in a decrease in arterial blood pressure. Interestingly, the pressor effect of ET-1 was significantly enhanced in the concomitant presence of PAF. Pretreatment with FR-139317 inhibited the magnitude of ET-1-induced hypertension and increased the duration of the depressor action of ET-1. The time-course of PAF-induced decrease of arterial blood pressure was also prolonged in rats pretreated with FR-139317. These data therefore suggested that ET receptors were activated, either directly or indirectly, by PAF, possibly to facilitate the return of blood pressure to resting level following a depressor response. Thus the activation of ET receptors by PAF might result in the enhancement of the pressor response of ET-1 observed in the current study.

Nitric oxide (NO) modulation of PAF-induced cardiopulmonary action: interaction between NO synthase and cyclo-oxygenase-2 pathways

1. To further investigate into the mechanisms of PAF-induced cardiopulmonary actions, we examined the effects of the nitric oxide synthase (NOS) inhibitor L-N(omega)-nitro-L-arginine (L-NNA), of the specific cyclooxygenase-2 (COX-2) inhibitor NS 398, and of the combined presence of both COX and NOS inhibitors on the PAF responses in the heart lung preparation of guinea-pig (HLP). 2. In HLPs perfused with homologous blood, dose-response curves for the haemodynamic and bronchial effects of PAF (1 - 32 ng) were carried out in the absence or presence of L-NNA (200 microM). L-NNA caused an increase in the resting pulmonary arterial pressure (PAP) without affecting the other basal values, and strongly potentiated the bronchoconstriction and pulmonary hypertension elicited by PAF. An enhancement of the PAF-induced actions on right atrial pressure (RAP) and cardiac output (CO) was also observed. All the effects of L-NNA were antagonized by L-arginine (2 mM). 3. The presence of L-NNA in the perfusing blood of HLPs failed to affect the pulmonary hypertensive and bronchoconstrictor responses induced by the thromboxane A(2) mimetic U46619 (0.05 - 1.6 microg), 5-hydroxytryptamine (0.1 - 1.6 microg), and histamine (0.1 - 1.6 microg), thus suggesting that these PAF secondary mediators are not responsible for the hyper-responsiveness to PAF induced by L-NNA. 4. Blocking COX-2 pathway with NS 398 (15 - 30 microM) did not alter the cardiopulmonary resting variables. However, a reduction of the PAF-mediated pulmonary hypertension, but not of bronchoconstriction, was observed. When L-NNA was added to the perfusing medium of HLPs pre-treated with NS 398 or with indomethacin (15 microM), the basal PAP values were enhanced. However, in the combined presence of COX and NOS inhibitors, only a slight increase in the hypertensive responses to the highest doses of PAF was observed, whereas the PAF mediated actions at bronchial and cardiac level were unaffected. 5. This study indicates that (i) the cardiopulmonary actions induced by PAF are specifically modulated by endogenous NO through the NOS pathway, and (ii) COX-2 isoform is involved in the pulmonary hypertensive, but not bronchoconstrictor, effects of PAF. Furthermore, an interaction between PAF stimulated COX, particularly COX-2, and NOS pathways appears to take a functional role at both bronchial and cardiovascular level.

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.

Possible involvement of endothelium-derived hyperpolarizing factor (EDHF) in the depressor responses to platelet activating factor (PAF) in rats

1. In anaesthetized rats, platelet activating factor (PAF; 1 microg kg(-1)) decreased mean arterial blood pressure by around 60 mmHg (n=18). This depressor response was completely blocked by the PAF antagonist, CV-6209 (1 mg kg(-1)), indicating the role of PAF-specific receptor in the response. 2. N(G)-nitro-L-arginine methyl ester (L-NAME; 50 mg kg(-1)), an NO synthase inhibitor, profoundly elevated systemic blood pressure (n=19), indicating an important role of NO in the basal blood pressure regulation. The depressor response to PAF (1 microg kg(-1)) normalized against that to sodium nitroprusside (SNP) (10 microg kg(-1)) was not substantially different between rats treated without and with L-NAME (n=4). In contrast, the depressor effect of acetylcholine (0.03 - 1.0 microg kg(-1)) normalized against that of SNP (10 microg kg(-1)) was significantly attenuated by L-NAME (n=5). 3. Charybdotoxin (0.4 mg kg(-1)) plus apamin (0.2 mg kg(-1)) significantly attenuated the depressor response to PAF (1 microg kg(-1)) (n=5) without affecting the blood pressure change due to SNP (1 mg kg(-1)) (n=3). Charybdotoxin (0.4 mg kg(-1)) (n=4) or apamin (0.2 mg kg(-1)) (n=4) alone did not affect the PAF-induced depressor response. 4. These findings suggest that EDHF may make a significant contribution to the depressor response to PAF in rats. Although NO plays the determinant role in the basal blood pressure regulation, its contribution to PAF-produced depressor response seems to be less as compared with that to the depressor response to acetylcholine.

Platelet-activating factor (PAF) receptor and genetically engineered PAF receptor mutant mice

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active phospholipid mediator. Although PAF was initially recognized for its potential to induce platelet aggregation and secretion, intense investigations have elucidated potent biological actions of PAF in a broad range of cell types and tissues, many of which also produce the molecule. PAF acts by binding to a unique G-protein-coupled seven transmembrane receptor. PAF receptor is linked to intracellular signal transduction pathways, including turnover of phosphatidylinositol, elevation in intracellular calcium concentration, and activation of kinases, resulting in versatile bioactions. On the basis of numerous pharmacological reports, PAF is thought to have many pathophysiological and physiological functions. Recently advanced molecular technics enable us not only to clone PAF receptor cDNAs and genes, but also generate PAF receptor mutant animals, i.e., PAF receptor-overexpressing mouse and PAF receptor-deficient mouse. These mutant mice gave us a novel and specific approach for identifying the pathophysiological and physiological functions of PAF. This review also describes the phenotypes of these mutant mice and discusses them by referring to previously reported pharmacological and genetical data.

The relaxation induced by nicotine in the rat isolated renal artery

The mechanism of nicotine-induced relaxation was investigated in the rat isolated renal artery. Nicotine (10(-3) M) produced a relaxation when preparations were precontracted by phenylephrine (3 x 10(-6) M). Nicotine-induced relaxation was 27.3 +/- 2.5% of phenylephrine contraction and was not affected by atropine (10(-5) M), guanethidine (10(-5) M), hexamethonium (10(-4) M), indomethacin (10(-5) M), N(G)-nitro-L-arginine (10(-4) M), methylene blue (10(-5) M), glibenclamide (10(-5) M), quinacrine (3 x 10(-6) M), tetrodotoxin (10(-6) M), capsaicin (10(-6) M), tetraethylammonium (10(-3) M), 4-aminopyridine (10(-3) M), and ouabain (10(-6) M) (n = 6, Mann-Whitney U-test). A calcium antagonizing effect of nicotine was not observed. Therefore, it appears that nicotine relaxes rat isolated renal artery by a nonspecific action on the vascular smooth muscle.

Endothelium-dependent vasorelaxing activity of aqueous extracts of Ilex paraguariensis on mesenteric arterial bed of rats

The effects of aqueous extracts of Ilex paraguariensis leaves (AEIp) were studied. Mesenteric arterial bed (MAB), precontracted by methoxamine with or without intact endothelium, was mounted on a tissue bath and exposed to plant extracts (bolus). The bolus injections of AEIp (300-1050 microg) significantly inhibited, in a concentration-dependent manner, the maximal contractile response induced by methoxamine (30 microm) in MAB. The endothelium-dependent relaxations were reversed by N(G)-nitro-L-arginine methyl ester (10 mM), whereas methylene blue (100 microM) was not capable of effectively inhibiting the AEIp-induced vasodilatation of MAB. The vasorelaxing effect of AEIp persisted in the presence of indomethacin (10 microM). These results suggest the involvement of NO of endothelial source (or others factors) in this vasodilatory effect.

A comparative study on the rat aorta and mesenteric arterial bed of the possible role of nitric oxide in the desensitization of the vasoconstrictor response to an alpha 1-adrenoceptor agonist

1. In thoracic aortic strips with intact endothelium, the first and second (1 h later) dose-response curves obtained with methoxamine were almost the same. 2. Methoxamine caused a dose-dependent increase in perfusion pressure in the rat isolated mesenteric arterial bed, but the second (1 h later) dose-response curve for methoxamine showed a significant attenuation of the response in comparison with the first. 3. The attenuation shown by the second dose-response curve for methoxamine was significantly reduced, but not abolished, in mesenteric arterial beds without endothelium. Incubating endothelium-intact mesenteric arterial beds with NG-nitro-L-arginine (L-NOARG) caused a significant, but not complete, reversal of the attenuation shown in the second dose-response curve. 4. Incubating the mesenteric arterial bed with capsaicin, tetrodotoxin, indomethacin or with isotonic high k+ (60 mM) plus nicardipine did not affect the above attenuation seen in the second dose-response curve. 5. The guanosine 3':5'-cyclic monophosphate (cyclic GMP) level in the effluent from the perfused mesenteric arterial bed was significantly increased after the second exposure to methoxamine. This effect was significantly smaller after removal of the endothelium or pretreatment with L-NOARG. 6. These results suggest that a desensitization to methoxamine develops rapidly in the mesenteric arterial bed, but not in the aorta, and that release of nitric oxide from the endothelium plays a major role in this desensitization.

Mechanisms of desensitization of vasodilatation induced by platelet-activating factor in hypertensive rats

We found that vasodilator effects of platelet-activating factor (PAF) on the mesenteric arterial bed of the rat were significantly attenuated in spontaneously hypertensive rats (SHR) and renal hypertensive rats (RHR). Perfusion of the mesentery with acetylcholine and PAF caused endothelium-dependent vasodilatation accompanied by an increase in cyclic GMP levels in the mesentery from normotensive Wistar Kyoto rats (WKY). Acetylcholine caused a significant increase in cyclic GMP levels in the effluent in both SHR and RHR, whereas PAF could not increase cyclic GMP levels in SHR and slightly increased cyclic GMP in RHR. Incubating the mesentery with PAF markedly inhibited the vasodilatation induced by PAF, but not acetylcholine or sodium nitroprusside. The cyclic GMP accumulation in the effluent was impaired in the mesenteric arterial bed pretreated with PAF and in that obtained from rats given islet-activating protein (IAP). The PAF-induced vasodilatation was completely reversed by the PAF receptor antagonist, CV-6209 (2-[N-acetyl-N-(2-methyl-3-octadecylcarbamoyl-oxypropoxycarbony l) aminomethyl]-1-ethylpyridinium chloride). These results suggest that (1) attenuated vasodilator effects of PAF and decreased cyclic GMP levels in the mesentery from SHR and RHR are due to desensitization but not to impairment of the endothelium; (2) GTP-binding protein, which is IAP-sensitive, may be involved in PAF-induced vasodilatation and cyclic GMP accumulation; (3) desensitization of the mesentery to PAF in SHR and RHR may be due to PAF receptor and GTP-binding protein uncoupling.