Simultaneous perfusion of rat isolated superior mesenteric arterial and venous beds: comparison of their vasoconstrictor and vasodilator responses to agonists

1. A new isolated perfused preparation is described that allows a direct comparison to be made of the responses of the perfused arterial and retrogradely perfused venous circulations of the rat superior mesenteric vascular bed. 2. In experiments comparing the responses of the intact arterially perfused mesentery and small intestine to those of the same preparation following removal of the intestine and division of the circulations, the increases in perfusion pressure produced by arginine-vasopressin (30 pmol) and noradrenaline (1 nmol) were retained by the arterial circulation and those induced by angiotensin II (30 pmol) by the venous circulation. Endothelin-1 (30 pmol) constricted both portions of the vasculature but the prolonged nature of its response was associated with only the venous vessels. 3. In the simultaneously perfused arterial and venous preparation arginine vasopressin (3-100 pmol) was a selective constrictor of the arterial circulation and angiotensin II (3-100 pmol) of the venous circulation. In addition, noradrenaline (0.3-10 nmol), 5-hydroxytryptamine (0.3-10 nmol) and KCl (1-60 micromol) were more active as constrictors of the arterial than the venous vessels, and U46619 (10-300 pmol) a more active constrictor of the venous than the arterial vessels. Endothelin-1 (3-100 pmol) constricted both the arterial and venous portions of the vasculature but was significantly longer acting as a venoconstrictor than an arterioconstrictor. 4. Angiotensin I (300 pmol) caused constrictions of the venous circulation which were dependent upon the presence of angiotensin converting enzyme for captopril (10 microM) abolished constrictions caused by angiotensin I but not by angiotensin II.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of vasoconstrictor mechanisms by dazoxiben in the rat mesenteric vasculature

Sympathetic neurotransmission can be modulated by prostaglandins in a number of tissues, but it is not known whether thromboxane A₂ also influences neurotransmission. In this study, vasoconstrictor responses to electrical stimulation of the sympathetic nerves and to injection of noradrenaline were examined in the blood perfused mesentry of the rat in situ. The thromboxane synthetase inhibitor dazoxiben, infused into the perfusion circuit at 10–100 μmol/l, significantly inhibited constrictor responses to nerve stimulation and to injected noradrenaline and vasopressin. The cyclo-oxygenase inhibitor indomethacin (28 μmol/kg intravenously) had no effect on responses to nerve stimulation or noradrenaline, but pretreatment with indomethacin abolished the inhibitory effect of dazoxiben vasoconstrictor responses. The thromboxane-mimetic (U46619, 10 nmol/l) slightly reduced responses to nerve timulation (but not to noradrenaline), whereas prostacyclin (3–10 nmol/l) and PGE₂ (3 nmol/l) markedly reduced responses both to nerve stimulation and to injections of noradrenaline. These prostanoids did not alter perfusion pressure at these concentrations. The data suggest that the inhibitory effect of dazoxiben on sympathetic neurotransmission is unlikely to be due directly to inhibition of thromboxane synthesis. Inhibition might result from diversion of endoperoxide metabolism to endogenous prostanoids that, in turn, inhibit activation of vasoconstrictor mechanisms.     

Restoration of vasoconstrictor responses to noradrenaline by prostaglandin E2 after alpha-adrenoceptor blockade in rat isolated mesenteric artery

1 The effects of prostaglandin E2 (PGE2) on responses to noradrenaline (NA) after alpha-adrenoceptor blockade were studied in the isolated mesenteric artery of the rat. 2 Phentolamine (32 nM) tolazoline (41 microM) and yohimbine (1.28 microM) blocked NA-induced vasoconstriction competitively with dose-ratios of 13.9 +/- 1,22.01 +/- 1 and 26.6 +/- 0.9 respectively. 3 PGE2 (28 nM) restored responses to NA during alpha-adrenoceptor blockade and reduced NA dose-ratios to 2.8 +/- 0.1 (phentolamine), 5.9 +/- 0.4 (tolazoline) and 1.7 +/- 0.1 (yohimbine). 4 At low concentrations (0.29 nM), phenoxybenzamine blockade of NA-induced vasoconstriction was also antagonized by PGE2. 5 PGE2 did not reduce the pA2 of the competitive antagonists; therefore the antagonism of alpha-adrenoceptor block by PGE2 was not due to a reduction in the affinity of the antagonist for the receptor. 6 The calcium ionophore, A23187, also antagonized competitive alpha-adrenoceptor blockade but was less potent than PGE2. 7 Evidence is provided to suggest that although both PGE2 and A23187 can potentiate the action of NA in this preparation, the two compounds probably reverse alpha-adrenoceptor blockade by different mechanisms. 8 Inhibition of NA-induced vasoconstriction caused by the calcium antagonists cinnarizine, verapamil and high concentrations of phenoxybenzamine (greater than 2 nM) were not affected by PGE2. 9 It is proposed that PGE2 restores responses to NA after alpha-adrenoceptor blockade by increasing intracellular Ca2+ ion concentration or by activating alpha-adrenoceptor-associated Ca2+ channels.     

Effects of short- and long-term sympathectomy on vasoconstrictor responses of the rat mesenteric arterial bed.

1. The effects of short- and long-term sympathectomy were evaluated on vasoconstrictor function of constantly perfused mesenteric arterial beds isolated from rats: the effects of short-term sympathectomy were assessed at 3 and 8 days after 6-hydroxydopamine (6-OHDA) treatment of adult rats; the effects of long-term sympathectomy were assessed in adult rats treated at youth with guanethidine. 2. The relative degree of residual sympathetic innervation of the mesenteric arterial preparations was assessed by responses to electrical field stimulation (EFS; 16 Hz, 1 ms, 90 V, 30 s). Control responses were 95.6 +/- 3.9 mmHg (n = 35). Responses after sympathectomy were: 3 days after 6-OHDA, 2.9 +/- 0.9 mmHg (n = 15) < 8 days after 6-OHDA, 14.1 +/- 2.1 mmHg (n = 14) < guanethidine, 21.1 +/- 4.1 mmHg (n = 16). 3. Three days after 6-OHDA treatment there was an increase in the sensitivities of response to vasopressin and endothelin, producing leftward shifts of the dose-response curves of 0.66 +/- 0.11 and 0.88 +/- 0.13 log units respectively (n = 7-11), and a small increase in sensitivity of responses to noradrenaline (NA) and ATP. The maximal response to 5-hydroxytryptamine (5-HT) was increased. In contrast, there was a decrease in maximal constriction to NA and to the alpha 1-adrenoceptor agonist methoxamine. The alpha 2-adrenoceptor agonist clonidine did not elicit vasoconstriction at basal tone. There was no difference in vasodilator responses to the beta-adrenoceptor agonist isoprenaline in preparations with tone raised with prostaglandin F2 alpha (PGF2 alpha; 0.1-0.3 microM). 4. Eight days after 6-OHDA sympathectomy there was no significant difference in sensitivities or maximal responses to ATP, vasopressin and endothelin, but a small increase in the sensitivity of responses to 5-HT. Maximal responses to NA and methoxamine were significantly lower than the controls, but sensitivities were similar. There was no significant difference in vasodilator responses to isoprenaline in PGF2 alpha-raised tone preparations. 5. After long-term guanethidine sympathectomy maximal responses to 5-HT and NA were significantly reduced. Responses to ATP, vasopressin and endothelin were unchanged. 6. In mesenteric arterial preparations from untreated rats, ouabain (0.1 mM), a blocker of the Na+/K+ pump, significantly augmented the sensitivity and maximal responses to EFS, NA, methoxamine and 5-HT. Responses to ATP, vasopressin and endothelin were unaffected. 7. It is concluded that in the rat mesenteric arterial bed, short-term sympathectomy, where only 3% of the sympathetic nerve-mediated response remained, results in non-uniform changes in sensitivity and maximal responses to different vasoconstrictors, which cannot be entirely explained by changes in the Na+/K+ pump. Most of these changes disappeared at 8 days after 6-OHDA treatment, when nerve-mediated responses had partially returned. After long-term guanethidine sympathectomy, there was little change in responses to vasoconstrictors, and nerve-mediated responses were reduced to 22%. Although the variable factors are complex, it appears that in general, changes in responses of smooth muscle to vasoconstrictor substances after sympathetic denervation only occur if there is near-complete loss of nerve-mediated responses.     

Bay K 8644, a dihydropyridine calcium agonist, augments vasoconstrictor responses to endogenous and exogenous noradrenaline in the peripheral vasculature of the dog.

The effect of Bay K 8644 (a substance known to increase calcium influx through the voltage-dependent calcium channel) on vasoconstrictor responses of resistance vessels to endogenous and exogenous noradrenaline (NA) was investigated in pentobarbitone-anaesthetized dogs which had also undergone spinal anaesthesia and bilateral vagotomy and received atropine. In these dogs the saphenous arterial bed was perfused at fixed flow rates with autologous blood to give perfusion pressure close to the systemic blood pressure. Electrical stimulation (3-30 Hz) of the saphenous nerve and single intra-arterial (i.a.) injections of noradrenaline (NA, 0.03-3 micrograms) produced an increase in perfusion pressure (vasoconstriction) in a frequency- and a dose-dependent manner, respectively. Intra-arterial infusions of Bay K 8644 (3 and 10 micrograms min-1) per se produced no significant change in perfusion pressure. However, these infusions augmented vasoconstrictor responses to both saphenous nerve stimulation (endogenous NA) and i.a. NA (exogenous NA). These results suggest that augmentation by Bay K 8644 of vasoconstrictor responses of resistance vessels to endogenous and exogenous NA is probably due to promotion of the calcium influx through calcium channels closely associated with alpha-adrenoceptors in smooth muscle cells there.     

Influence of the endothelium on the amplification by serotonin of vasoconstrictor responses to noradrenaline and sympathetic nerve stimulation in rabbit isolated ear artery

The influence of the vascular endothelium on the amplifying effect of serotonin on steady-state responses to noradrenaline (NA) and sympathetic nerve stimulation (SNS) has been studied in the rabbit isolated ear artery. Removal of the endothelium abolished the dilator effect of acetylcholine (ACh, 1 microM) obtained in the presence of NA (0.3 microM). Responses to NA were increased after endothelium removal; however, responses to SNS were unaffected. Serotonin in a concentration which did not itself produce vasoconstriction (100 nM) increased responses of endothelium-intact preparations to NA and SNS and of endothelium-denuded preparations to SNS but had no effect on NA responses of endothelium-denuded preparations. If neuronal uptake was inhibited with cocaine (1 microM), serotonin markedly amplified responses to NA and SNS in both endothelium-intact and endothelium-denuded preparations. These results suggest that the endothelium influences the amplifying effect of serotonin on responses of rabbit ear artery to NA but not to SNS. Inhibition of removal of serotonin and NA by neuronal uptake may extend the facilitatory interaction between NA and serotonin to smooth muscle cells which are not subject to the inhibitory influence of an endothelium derived relaxing factor.     

Potentiation of responses to sympathetic nerve stimulation and vasoconstrictor agents by SK&F 103829 in the feline mesenteric circulation.

1. The amplification of vasoconstrictor effects of several agonists and sympathetic nerve stimulation, caused by 5-HT2 receptor activation, was studied in the autoperfused mesenteric circulation of anaesthetized cats. To produce long lasting and selective 5-HT2 receptor stimulation we used SK&F 103829 (2,3,4,5 tetrahydro-8[methyl-sulphonyl]-1H3-benzazepin-7-ol methensulphonate). We assessed that SK&F 103829 was a strong contractile partial agonist in isolated preparations of rat tail artery and calf pulmonary artery. 2. The intrinsic activity of SK&F 103829 with respect to 5-hydroxytryptamine (5-HT) was 0.8 in rat tail artery and 0.6 in calf pulmonary artery. SK&F 103829-induced contractile responses were surmountably antagonized by ketanserin with a potency expected from its affinity for 5-HT2 receptors. SK&F 103829 surmountably antagonized the effects of 5-HT in rat tail artery with a pKp of 5.8. 3. Concentrations of SK&F 103829 causing greater than threshold constrictions enhanced vasoconstrictor responses of sympathetic nerve stimulation, noradrenaline, angiotensin II, methoxamine and alpha, beta-methylene ATP in the mesenteric arterial bed. Increases in mesenteric arterial pressure by noradrenaline, observed in the presence of prazosin, were also potentiated by SK&F 103829. 4. Ketanserin prevented both the constrictor effect of SK&F 103829 and the SK&F 103829-evoked potentiation of the responses to noradrenaline and angiotensin II in the mesenteric arterial bed. Ketanserin, however, failed to abolish (once established) the SK&F 103829-evoked potentiation of the constrictor effects caused by both noradrenaline and angiotensin II.(ABSTRACT TRUNCATED AT 250 WORDS)     

Blocking effects of nipradilol on vasoconstrictor responses to periarterial nerve stimulation and alpha-adrenoceptor agonists in isolated and perfused canine mesenteric arteries

The stainless steel cannula inserting method was used to observe effects of nipradilol and prazosin on responses to periarterial electrical nerve stimulation and intraluminal administration of noradrenaline or phenylephrine in isolated and perfused canine mesenteric arteries. With small doses, nipradilol slightly potentiated vasoconstrictor responses to noradrenaline, but not periarterial stimulation. With a relatively large dose, nipradilol almost uniformly suppressed both periarterial stimulation-induced and noradrenaline- or phenylephrine-induced vasoconstriction. On the other hand, prazosin inhibited noradrenaline-induced vasoconstriction at small doses but not periarterial nerve stimulation-induced vasoconstrictions. At any doses, prazosin strongly inhibited noradrenaline-induced constrictions more markedly than periarterial stimulation-induced constrictions. It is concluded that nipradilol has a dominant inhibitory property on periarterial nerve stimulation-induced constriction in isolated canine mesenteric arteries.     

Relationship between endogenous prostaglandin E and the vasoconstrictor response to noradrenaline in a perfused arterial segment

The close interrelation between endogenous prostaglandin E (PGE) and the vasoconstrictor response to noradrenaline (NA) was evaluated using a perfused central arterial segment of a rabbit ear. The endogenous PGE level, measured radioimmunologically, was estimated from that in the perfusate. Pretreatment with arachidonic acid (AA) caused a decrease in the response to NA which was accompanied by a rise in the PGE level. The response to NA (% control) correlated significantly and negatively with the PGE level (ng/ml) in the perfusate. In addition, repetitively applied NA elicited gradual augmentation in the response to NA, which was accompanied by a reduction in the PGE level in the perfusate. These results seem to suggest that the endogenous PGE in the perfused central arterial segment of the rabbit ear is related to the inhibitory action on the response to NA.     

In vitro studies on responses to noradrenaline, serotonin, and potassium of intramyocardial and mesenteric resistance vessels from Wistar rats

We compared the responses to noradrenaline, serotonin, and potassium of isolated intramyocardial (flow-regulating) coronary and mesenteric resistance vessels of similar internal diameter (approximately 200 microns) from Wistar rats. The coronary but not the mesenteric resistance vessels had spontaneous basal tone, which was reduced by elimination of extracellular calcium, by increasing the extracellular potassium concentration up to 22 mM, and by stimulation of coronary vessels with noradrenaline. Noradrenaline always caused contraction of mesenteric resistance vessels and in coronary vessels after incubation with propranolol (3 X 10(-6) M). The noradrenaline and potassium concentration-response characteristics of the mesenteric resistance vessels were modulated by perivascular adrenergic nerve terminals, whereas no influence of nerve terminals could be shown in the coronary resistance vessels. The sensitivity of mesenteric vascular smooth muscle to noradrenaline [concentration required to give half maximal response (EC50) approximately 0.4 X 10(-6) M] and serotonin (EC50 approximately 0.3 X 10(-6) M) was higher (p less than 0.001) than the sensitivity of coronary smooth muscle (EC50 values 1.6 X 10(-6) and 1.3 X 10(-6) M, respectively). The potassium sensitivity of the coronary smooth muscle (EC50 approximately 35 mM) was higher than that of the mesenteric smooth muscle (EC50 approximately 41 mM; p less than 0.01). The respective maximal noradrenaline and serotonin responses relative to the maximal potassium response were 35 and 55% in the coronary and 127 and 120% in the mesenteric resistance vessels (p less than 0.001).     

A comparison of the responses of the isolated vas deferens of the rat to field stimulation and to noradrenaline added cumulatively and non-cumulatively

The maxima, slopes and positions of cumulative and non-cumulative dose-response curves for noradrenaline were determined on the isolated vas deferens of the rat. Cumulative log dose-response curves were flatter and reached a lower maximum than the non-cumulative curves. However transformation of the responses to fractions of the maximal responses produced curves with identical slopes and only slight differences in positions. Field stimulation led to larger and more rapidly attained maximal responses than did the addition of exogenous noradrenaline. Responses to field stimulation applied at the plateau of the cumulative dose-response curves were similar to those occurring in the control situation. It is suggested that the effectiveness of noradrenaline in producing contraction of the vas deferens may be related to the rate at which its concentration rises at the effector cells.     

Pharmacological analysis of vasoconstrictor responses to periarterial purinergic nerve stimulation

1. Periarterial electrical nerve stimulation at a low frequency (1 Hz) readily induced a vasoconstrictor response of the canine splenic artery in a pulse number-related manner (1-30 pulses of trains). The vasoconstrictor response to trains of up to 10 pulses at 1 Hz of stimulation appeared to be monophasic, whereas it became clearly distinguished into two phases at a longer train of 30 pulses. 2. The monophasic vasoconstrictor responses to trains of 1, 3 or 10 pulses were not modified by an alpha1-adrenoceptor blocking agent, prazosin (0.1 microM), but were completely inhibited by the P2X receptor desensitization with alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-methylene ATP; 1 microM). The 1st phase of vasoconstriction induced by a train length of 30 pulses was not influenced by the treatment with prazosin, but was abolished by alpha,beta-methylene ATP. The 2nd phase response was markedly inhibited by prazosin, and the remaining response of this phase was blocked by alpha,beta-methylene ATP. 3. Rauwolscine (0.3 microM), an alpha2-adrenoceptor antagonist, enhanced the vasoconstrictor responses to trains of 1, 3 or 10 pulses. Particularly at 10 pulses of electrical stimulation, the vasoconstrictor responses were significantly potentiated. The blockade of neuronal uptake of noradrenaline with imipramine (1 microM) did not affect the vasoconstrictor responses to trains of 1, 3 or 10 pulses. 4. It is concluded that short pulse trains of stimulation at a low frequency may selectively activate a purinergic component of sympathetic cotransmission, and the prejunctional alpha2-adrenergic feedback mechanism may tonically participate into the modulation of ATP release. Imipramine-sensitive neuronal uptake mechanism may not play an important role in regulating vascular responses to periarterial purinergic nerve stimulation.     

Pharmacological analysis of the double peaked vasoconstrictor responses to periarterial electric stimulation

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Escape from tension induced by noradrenaline or electrical stimulation in isolated mesenteric arteries

Isolated mesenteric arteries, studied under 25-30 mg resting tension, responded to prolonged noradrenaline or electrical stimulation with a 50-500 mg increase in tension from which they subsequently escaped towards resting tension levels.     

Effects of reduced calcium ion concentration and of diltiazem on vasoconstrictor responses to noradrenaline and sympathetic nerve stimulation in rat isolated tail artery.

In isolated, perfused proximal segments of Sprague-Dawley rat tail artery, idazoxan (100 nmol l-1) displaced the concentration-response curve to noradrenaline (NA) to the right. The log shift of the NA concentration-response curve was greater at lower concentrations than at higher concentrations of NA. Idazoxan (100 nmol l-1) had no effect on responses to electrical stimulation. Prazosin (10 nmol l-1) displaced the concentration-response curve to NA to the right as well as markedly reducing responses to sympathetic nerve stimulation. The concentration-response curve to NA, obtained after reducing the concentration of calcium ions in the Krebs solution from 2.5 to 0.6 mmol l-1, was significantly displaced to the right. Responses to sympathetic nerve stimulation were not affected by this reduction in the concentration of calcium ions. Diltiazem (1 and 10 mumol l-1) significantly displaced the concentration-response curve to NA to the right but had no effect on sympathetic nerve stimulation. These in vitro results in peripheral arterial smooth muscle confirm the findings of previous in vivo studies which suggest that alpha2-adrenoceptors contribute to the vasoconstrictor responses elicited by alpha-adrenoceptor agonists and that these responses but not those mediated by alpha1-adrenoceptors are dependent on extracellular calcium.     

Effects of alpha-adrenoceptor antagonists on electrical and mechanical responses of the isolated dog mesenteric vein to perivascular nerve stimulation and exogenous noradrenaline

The effects of four different alpha-adrenoceptor antagonists (prazosin, phentolamine, yohimbine, and nipradilol) on the electrical and mechanical responses of smooth muscle cells of the dog isolated mesenteric vein to perivascular nerve stimulation and exogenous noradrenaline were investigated. Perivascular nerve stimulation generated an excitatory junction potential (e.j.p.), a spike potential and a slow depolarization. The latter component was blocked by yohimbine or phentolamine at doses over 10(-7) M, while the former two components were suppressed by 10(-6)-10(-5) M yohimbine, but not by prazosin, nipradilol or phentolamine (up to 10(-5) M). Nerve-mediated muscle contractions were suppressed by these alpha-adrenoceptor antagonists in a concentration-dependent manner, at doses over 10(-7) M. The order of potency was yohimbine greater than nipradilol = phentolamine greater than prazosin. Exogenously applied noradrenaline (10(-6) M) depolarized the smooth muscle membrane and generated slow waves. The slow waves were blocked by all of these alpha-adrenoceptor antagonists (10(-5) M), while the depolarizations were inhibited by yohimbine (greater than 10(-7) M) or phentolamine (10(-5) M), but not by nipradilol or prazosin (up to 10(-5) M). Contractions produced by exogenously applied noradrenaline (10(-6) M) were inhibited by the alpha-adrenoceptor antagonists; yohimbine or phentolamine (10(-6)-10(-5) M) showed complete inhibition and prazosin or nipradilol (up to 10(-5) M) partial inhibition. Contractions produced by high-potassium or current-stimulation were suppressed by high-concentrations (10(-6)-10(-5) M) of these alpha-adrenoceptor antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)