Effect of 5-hydroxytryptamine on neurogenic vasoconstriction in the isolated, autoperfused hindquarters of the rat

1. In the present study, we analysed the effect of different doses of 5-hydroxytryptamine (5-HT; intravenous infusions of 0.001-40 microg/kg per min) in the autoperfused hindquarters of the rat subjected to electrical stimulation (frequencies of 0.5-20 Hz) of the lumbar chains, investigating the relationship between the adrenergic and serotonergic systems in this vascular bed. 2. Because we observed that 5-HT inhibited the increases in perfusion pressure induced by electrical stimulation of the lumbar chains, we used different agonists and antagonists to analyse the mechanism of action of 5-HT. 3. The effect of 5-HT was inhibited by methiothepin (a non-specific 5-HT receptor antagonist), but not by ritanserin (a selective 5-HT2 receptor antagonist). The effects of 5-HT were mimicked by 5-carboxamidotryptamine (a 5-HT1 receptor agonist) and L-694 247 (a selective 5-HT1D receptor agonist), but not by 8-hydroxy-2-dipropylaminotetralin (a 5-HT1A receptor agonist), CGS-12066B (a 5-HT1B receptor agonist), alpha-methyl-5-HT (a 5-HT2 receptor agonist), 1-(3-chlorophenyl) piperazine (a 5-HT2C receptor agonist) or 1-phenylbiguanide (a 5-HT3 receptor agonist). The selective 5-HT1D/1B receptor antagonist BRL 15572 inhibited the effect of the agonist L-694 247. 4. Our data suggest that 5-HT inhibits the increases in perfusion pressure induced by the electrical stimulation of the lumbar chains, acting on presynaptic 5-HT1D receptors and decreasing the release of noradrenaline from the sympathetic nerves in the hindquarter vascular bed of the rat.     

Effects of clonidine and guanethidine on peripheral sympathetic nerve function in the pithed rat

Clonidine, in low intravenous doses, inhibited the increased heart rate of pithed rats caused by peripheral sympathetic nerve stimulation. The magnitude of this effect was greatest at low frequencies of nerve stimulation, responses to high frequencies being little affected by the drug. In contrast, guanethidine reduced cardiac responses to both low and high rates of nerve stimulation. The difference between the depressant effects of the two drugs on responses to various frequencies of sympathetic nerve traffic may contribute to the differences known to occur between their properties as hypotensive agents.     

The factors influencing the time course of drug action at alpha-adrenoceptors: an investigation of the effects of clonidine in the pithed rat.

1 The time courses of the pre- and post-junctional effects of clonidine were examined on heart rate and blood pressure and on the isometric tension responses of the vas deferens and anococcygeus in situ in the pithed rat. Plasma levels of clonidine were also monitored. 2 The time courses of the pre-junctional inhibition of sympathetic nerve mediated responses in all tissues monitored were related to the plasma levels of clonidine, but the degree of inhibition which was observed varied with the tissue and with the test stimulus employed. 3 The post-junctional agonist effects of clonidine, expecially at low doses, exhibited an initial 'peak' followed by a decline to a lower plateau. This decline of the response was not due to receptor desensitization but was related to a decline in the plasma clonidine level from an initially high value caused by injection of a bolus. 4 The factors determining the time course of drugs' effects in the preparation are discussed and it is concluded that pre- and post-junctional responses should be compared at a point in time after injection of the-drug, at which equilibration has occurred.     

Characterization of alpha-adrenoceptors mediating sympathetic vasoconstriction in rat autoperfused hindlimb: effects of SK&F 104078

In the autoperfused hindlimb of pithed rats, vasoconstrictor responses to intra-arterial infusions of the selective alpha 2-adrenoceptor agonist, B-HT 933, were antagonized by the alpha 2-adrenoceptor antagonist, rauwolscine (1 mg/kg i.v.), and by the selective postjunctional alpha 2-adrenoceptor antagonist, SK&F 104078 (1 mg/kg), but not by the selective alpha 1-adrenoceptor antagonist, prazosin (0.1 mg/kg). In contrast, responses to the selective alpha 1-adrenoceptor agonist, methoxamine, were antagonized by prazosin, but not by rauwolscine or SK&F 104078. Vasopressor responses to stimulation of sympathetic nerves were inhibited by prazosin, increased by rauwolscine, and not affected by SK&F 104078. The results indicate that vascular neuroeffector transmission in rat hindlimb is mediated by postjunctional alpha 1-adrenoceptors, and that SK&F 104078 is a selective antagonist of postjunctional alpha 2-adrenoceptor, and lacks the prejunctional alpha 2-adrenoceptor antagonist action of rauwolscine.     

Vasoconstrictor responses to 5-hydroxytryptamine in the autoperfused hindquarters of spontaneously hypertensive rats

In this work we studied the responses and receptors involved in the effects of intra-arterial 5-hydroxytryptamine (5-HT) in the in situ autoperfused hindquarters of spontaneously hypertensive rats (SHR). Intra-arterial administration of the highest doses (50-1,000 ng/kg) produced a vasoconstrictor effect that was inhibited by ritanserin (a selective 5-HT2 receptor antagonist), SB 206553 (a selective 5-HT(2B/2C) receptor antagonist) and spiperone (a nonspecific 5-HT(1/2A) receptor antagonist), and was mimicked by alpha-methyl-5-HT (a selective 5-HT2 receptor agonist) and m-CPP (a selective 5-HT2C receptor agonist), but not by the intra-arterial administration of BW 723C86, a selective 5HT2B receptor agonist. SB 206553 and spiperone inhibited alpha-methyl-5HT-induced vasoconstriction in the hindquarters of SHR. Our data suggest that the vasoconstrictor response induced by 5-HT in the autoperfused hindquarters of SHR is mainly mediated by the activation of 5-HT2A and 5-HT2C receptors.     

Vasodilator and vasoconstrictor responses induced by 5-hydroxytryptamine in the in situ blood autoperfused hindquarters of the anaesthetized rat

In the present study we attempted to characterise the responses and receptors involved in the effects of 5-hydroxytryptamine (5-HT, serotonin) in in situ autoperfused rat hindquarters. Intra-arterial administration of the lowest doses of 5-HT used (0.12-12.5 ng/kg) induced vasodilator responses, whereas the highest doses (25-1000 ng/kg) produced vasoconstriction. The vasodilator effect was inhibited by methiothepin (a non-specific 5-HT(1,2,5,6,7) receptor antagonist) and by a 5-HT(1D/1B) receptor antagonist, i.e., 3-[4-(4-chlorophenyl)piperazin-1-yl]-1,1-diphenyl-2-propanolol (BRL 15572), but not by ritanserin (a selective 5-HT(2) receptor antagonist), 5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2,3-f] indole (SB 206553, a selective 5-HT(2B/2C) receptor antagonist) or mesulergine (a non-specific serotonergic antagonist that shows affinity to the 5-HT(7) receptor). This vasodilator effect was mimicked by administration of a selective 5-HT(1) receptor agonist - 5-carboxamidotryptamine (5-CT) - and by 2-[5-[3-(4-methylsulphonylamino)benzyl-1,2,4-1 H-indol-3-yl]ethanamine (L-694,247, a selective 5-HT(1D/1B) receptor agonist). Methiothepin, but not mesulergine, inhibited 5-CT-induced vasodilatation and the selective 5-HT(1D/1B) receptor antagonist (BRL 15572) inhibited the vasodilator action induced by L-694,247.The vasoconstrictor effect of 5-HT was significantly decreased by methiothepin, ritanserin and SB 206553, and was mimicked by alpha-methyl-5-HT (a selective 5-HT(2) receptor agonist) but not by administration of BW 723C86, a selective 5HT(2B) receptor agonist. Ritanserin, SB 206553 and spiperone (a non-specific 5-HT(1/2A) receptor antagonist) inhibited the alpha-methyl-5HT-induced vasoconstriction.Our data suggest that the vasodilator response induced by 5-HT in autoperfused rat hindquarters is mainly mediated by 5-HT(1D/1B) receptors, whereas the vasoconstrictor effect is mainly due to the activation of 5-HT(2A) receptors.     

An examination of pre- and postsynaptic alpha-adrenoceptors in the autoperfused rabbit hindlimb

Pre- and postsynaptic alpha-adrenoceptors in autoperfused rabbit hindlimbs were examined employing the yohimbine diastereomers rauwolscine and corynanthine, as well as other drugs with varying selectivities for alpha 1-and alpha 2-adrenoceptors. Vasoconstrictor responses were elicited by sympathetic nerve stimulation and by intraarterial injection of agonists. The alpha 2-selective antagonist rauwolscine (10(-7) and 10(-6) M) reduced responses to injected noradrenaline (mixed alpha 1- and alpha 2-agonist) but not to injected phenylephrine (alpha 1-selective agonist); rauwolscine (10(-6) M) actually enhanced nerve-mediated responses. Corynanthine (alpha 1-selective antagonist) was about equipotent against agonist- and nerve-mediated responses. In another series of experiments, rauwolscine (10(-6) M) strongly antagonized the response to xylazine (alpha 2-selective agonist), antagonized the responses to alpha-methylnoradrenaline and noradrenaline (mixed agonists) less strongly, and did not affect the response to phenylephrine, Conversely, prazosin (alpha 1-selective antagonist, 10(-8) M) strongly antagonized phenylephrine, antagonized noradrenaline and alpha-methylnoradrenaline less strongly, and did not antagonize xylazine. Potentiation of neuroeffector responses by rauwolscine demonstrates the operation of an endogenous autoinhibition of noradrenaline release mediated by presynaptic alpha 2-adrenoceptors. However, in comparison with previous in vitro results on rabbit pulmonary artery, the potentiation was small due to the presence of postsynaptic alpha 2-adrenoceptors.     

可乐定对自家血灌流毁脑脊髓大鼠后肢血管平滑肌的作用

本实验观察了可乐定对自家血恒速灌流的毁脑脊髓大鼠(pithed rat)后肢血管平滑肌的作用和该处突融后膜两种α受体亚型的存在。可乐定引起灌流压(PP)升高,呈剂量依赖关系,但效能远比选择性α_1受体激动剂苯福林低。对预先用利血平处理的大鼠,可乐定的作用被显著增强。高度选择性的α_1受体阻断剂哌唑嗪0.3mg/kg iv可部分阻断可乐定100μg ia的作用,增加哌唑嗪的剂量并不能进一步增加对可乐定的阻断作用。对于哌唑嗪不能阻断的部分,α_1受体阻断剂育亨宾却能表现出十分明显的抑制作用。这些结果提示在大鼠后肢血管平滑肌突触后膜上存在有α_1和α_2肾上腺素受体,但α_1受体似乎占优势。     

Interaction of calmodulin antagonists with alpha-adrenergic responses in pithed rats and in the perfused hindquarters of the rat

The effect of calmodulin antagonists was studied on the alpha 1- and alpha 2-adrenoceptor-mediated increase in diastolic blood pressure in pithed rats and on the alpha 1-adrenoceptor-mediated reduction of flow in the perfused hindquarters of the rat. B-HT 920 was used as a selective alpha 2-adrenoceptor agonist in the pithed rat experiments, whereas cirazoline was used as a selective agonist for alpha 1-adrenoceptors. The latter was used after pretreatment with nifedipine (1 mg/kg) or phenoxybenzamine (30 micrograms/kg), revealing calcium influx-insensitive and -sensitive mechanisms of vasoconstriction, respectively. Papaverine, calmidazolium and W-7 did not influence the dose-response curves for the agonists in the pithed rat experiments. The modest effects of high doses of flunarizine and bepridil on the dose-response curve for B-HT 920 and of trifluoperazine on the dose-response curve for cirazoline can be explained by the well-known calcium entry (flunarizine) and alpha 1-adrenoceptor-blocking (bepridil) effects of these drugs. Bepridil and calmidazolium caused an elevation of the cirazoline dose-response curves in the perfused rat hindquarters; flunarizine and trifluoperazine showed a parallel and dose-dependent displacement of the cirazoline dose-response curve to the right, whereas W-7 was inactive. Our results do not implicate calmodulin-associated effects in the alpha-adrenoceptor-mediated vasoconstriction in pithed rats and in the perfused rat hindquarters.     

Inhibitory effects of clonidine on responses to sympathetic nerve stimulation in the pithed rat.

1. The spinal sympathetic outflow to the eyelid, heart, splanchnic blood vessels, vas deferens and anococcygeus muscle was stimulated in pithed rats. 2. Clonidine inhibited sympathetic outflow to all of the tissues studied. The inhibitory effects of clonidine on cardiac nerves and hypogastric nerves were antagonized by phentolamine. 3. Clonidine produced a postsynaptic alpha-adrenoceptor agonist action on the eyelid, splanchnic blood vessels and the anococcygeus muscle. These effects were also antagonized by phentolamine. 4. The effects of clonidine, naphazoline and oxymetazoline on pre- and postsynaptic alpha-adrenoceptors were determined. 5. The presynaptic alpha-adrenoceptors employed were situated in either the sympathetic cardiac or hypogastric nerve terminals. Increases in diastolic blood pressure were used to assess concurrent postsynaptic alpha-adrenoceptor agonist activity. 6. The presynaptic alpha-adrenoceptor agonist potencies of clonidine, naphazoline and oxymetazoline were very similar on cardiac nerve terminals whereas on the hypogastric nerve terminals oxymetazoline was about 6 times more potent than either naphazoline or clonidine. 7. The results support the view that presynaptic alpha-adrenoceptors regulate transmitter release in sympathetic nerves. There appear to be subtle differences between the presynaptic alpha-adrenoceptors of different sympathetic nerve endings.     

Effects of guanfacine on pre- and postsynaptic alpha-adrenoceptors studied in comparison with those of clonidine

Intravenous injections of guanfacine (1 microgram-3 mg/kg) elevated dose-dependently blood pressure in adrenalectomized, pitched rats. The vasopressor activity of guanfacine was weaker than those of clonidine, an alpha 2-agonist and 1-phenylephrine, an alpha 1-agonist. However, the maximal response produced by guanfacine was the same as that produced by clonidine, while it was much smaller than that produced by 1-phenylephrine. The dose-response curve to guanfacine for increase in blood pressure was shifted in a parallel fashion to the right by 1 mg/kg of yohimbine, an alpha 2-antagonist and by 1 mg/kg of phentolamine, a nonselective alpha 1- and alpha 2-antagonist. However, 0.1 mg/kg of prazosin, a selective alpha 1-antagonist, produced an inhibition of the blood pressure rise induced by higher doses of guanfacine, but not that induced by lower doses. Guanfacine (1 microgram-1 mg/kg) inhibited dose-dependently the tachycardia induced by electrical stimulation of spinal nerves at C7-Th1 in adrenalectomized, pithed rats. The maximal inhibition by guanfacine of heart rate increase induced by electrical stimulation was about 60%. These inhibitory effects were antagonized by yohimbine and phentolamine, but not by prazosin. These results indicate that guanfacine is an alpha 2-agonist with approximately equal potency towards the pre- and postsynaptic alpha 2-adrenoceptors, just as clonidine is, but is weaker as an alpha 2-agonist than clonidine.     

Effects of ergotamine on cardiovascular catecholamine receptors in the pithed rat

1. Ergotamine (3-10 micrograms/kg) inhibited the electrical stimulation-induced pressor and cardiac responses without modifying pressor responses of noradrenaline and tyramine in the pithed rat. 2. Yohimbine (0.3 mg/kg) partially prevented the ergotamine cardiac and vascular inhibitory effects but sulpiride (0.3 mg/kg) only prevented it at vascular level. Both antagonists together abolished the ergotamine inhibition of electrical stimulation-induced pressor responses. 3. The cumulative dose-response curve of ergotamine (1-100 micrograms/kg) vasoconstrictor effects was partially inhibited to the same extent by prazosin (1 mg/kg) and yohimbine (0.3 mg/kg). A greater inhibition was observed with both antagonists administered together. 4. Ergotamine (30 micrograms/kg), in presence of yohimbine, inhibited the pressor responses of methoxamine, without any effect on xylazine pressor responses. 5. These data indicate that ergotamine acts as an agonist of both the presynaptic dopamine receptors and alpha 2-adrenoceptors, of alpha 1 and alpha 2-postsynaptic adrenoceptors, and also as an antagonist of the postsynaptic alpha 1-adrenoceptors.     

Lack of involvement of alpha-adrenoceptors in sympathetic neural vasoconstriction in the hindquarters of the rabbit.

The hypothesis that sympathetic nerves in arterial blood vessels activate excitatory receptors distinct from alpha-adrenoceptors was investigated in vivo in the rabbit. In anaesthetized, ganglion-blocked rabbits, graded stimulation of the lumbar sympathetic nerve chains caused graded hind limb vasoconstriction. The responses to single pulses and short trains of stimuli were unaffected by benextramine (10 mg kg-1) and the longer trains were enhanced. Phenoxybenzamine (5 mg kg-1) slightly reduced the responses to short trains of stimuli and did not affect the responses to long trains. The dose-response curve to intra-arterial noradrenaline (after beta-adrenoceptor blockade) was shifted rightwards about ten fold by benextramine (10 mg kg-1) and by phenoxybenzamine (5 mg kg-1). In conscious rabbits the vasoconstriction caused by the nasopharyngeal reflex initiated by smoke inhalation was unaffected by benextramine (10 mg kg-1). Small mesenteric arteries (less than 250 microns) taken from untreated rabbits responded to noradrenaline with a threshold concentration of about 1 microM. Similar tissues from benextramine (10 mg kg-1)-treated rabbits were unresponsive to noradrenaline at concentrations up to 300 microM. However, these tissues were able to respond to potassium and angiotensin II. Aortic ring segments taken from the same rabbits were only about ten fold less sensitive to noradrenaline than segments from control rabbits. These results are in accord with the hypothesis that sympathetic nerves activate non-alpha-receptors in the vasculature of the rabbit.     

Effects of St-587 on the alpha-adrenoceptors in the bisected rat vas deferens

The effects of the alpha-adrenoceptor agonist St-587 have been studied on the twitch responses induced by field stimulation in the prostatic portion of rat vas deferens. Moreover the drug's influence on the unstimulated prostatic and epididymal halves of rat vas deferens has also been determined. Alone and after addition of yohimbine (0.3 microM) it enhanced in a concentration-dependent manner the twitch responses in the prostatic half. Prazosin competitively antagonized (pA2 = 8.41 +/- 0.03) this effect. The enhancing effect of St-587 was not reduced in reserpinized animals. These results suggest that post-synaptic alpha 1-adrenoceptors are involved in the potentiation of twitch responses induced by St-587. When alpha 1-adrenoceptors were blocked by prazosin (0.1 microM), St-587 partially inhibited the twitch responses of the prostatic portion of rat vas deferens (Emax = 49.5 +/- 3.5%). Yohimbine completely reversed the inhibitory effects of both St-587 and clonidine. Furthermore St-587 antagonized the inhibitory effects of clonidine on twitch responses. Thus it appears that St-587 also behaves as a partial agonist of presynaptic alpha 2-adrenoceptors in this portion of rat vas deferens, but it did not induce contractions in the unstimulated prostatic half of the vas deferens. However, it competitively antagonized the alpha 1-adrenoceptor agonist phenylephrine by acting as an antagonist of prostatic postsynaptic alpha 1 adrenoceptors. These alpha 1-adrenoceptors are probably different from those that mediate the twitch enhancing response to St-587 in that portion. On the other hand, St-587 was a partial agonist of alpha 1-adrenoceptors in the epididymal half.(ABSTRACT TRUNCATED AT 250 WORDS)