Prazosin and presynaptic alpha-receptors in the cardioaccelerator nerve of the dog

Antihypertensive and Hypolipidemic Effects of DC-015, a Novel, Potent and Specific alpha(1)-Adrenoceptor Antagonist: Comparison with Prazosin in Spontaneously Hypertensive Rats

The hypotensive effect of DC-015, a newly synthesized quinazoline derivative, was investigated and compared with prazosin in spontaneously hypertensive rats (SHR). Intravenous administration of DC-015 and prazosin (both at 0.01, 0.05 and 0.1 mg/kg) induced a dose-dependent reduction of mean arterial pressure (MAP) which reached a maximal effect at 5 min after injection and persisted over 2 h in SHR. Furthermore, at higher doses DC-015 (0.1 mg/kg i.v. and 2.0 mg/kg orally, respectively) did not cause any significant changes in heart rate (HR); whereas the same doses of prazosin (0.1 mg/kg i.v. and 2.0 mg/kg orally, respectively) produced a decrease in HR which seems to parallel the time course of the hypotensive response in SHR. DC-015 and prazosin attenuated pressor responses to phenylephrine (10 &mgr;g/kg) but failed to inhibit the pressor effects of angiotensin II (0.5 &mgr;g/kg) even at the maximal hypotensive dose (0.1 mg/kg). This observation indicates that DC-015 appears to exert its hypotensive effect through alpha(1)-adrenoceptor blockade. On the other hand, in SHR fed a high-fat-high-cholesterol (HF-HC) diet, oral administration of DC-015 and prazosin (both at 1.0 mg/kg, twice a day) for 4 weeks caused significant reductions in total plasma cholesterol (CE), low-density lipoprotein (LDL)-cholesterol and total plasma triglyceride (TG). DC-015 therapy also increased high-density lipoprotein (HLD)-cholesterol levels, thus the ratio of total plasma cholesterol to HDL-CE was improved. In contrast, prazosin did not significantly increase the HDL-CE level in this study. It is concluded that DC-015 decreased MAP, plasma CE, LDL-CE, plasma TG and increased HDL-CE levels. DC-015 may have therapeutic potential as a potent antihypertensive drug via the alpha(1)-adrenoceptor antagonist. Concurrently, DC-015 may thus hold some advantage for the reduction of two of the major risk factors, hypertension and hyperlipidemia, for cardiovascular diseases. Copyright 1996 S. Karger AG, Basel

Pharmacological activity of DC-015, a novel potent and selective alpha 1-adrenoceptor antagonist

The pharmacological activity of 3-((4-(2-methoxyphenyl)piperazin-1-yl)methyl)-2,3-dihydroimidaz o(1,2 -c)quinazolin-5(6H)-one (DC-015), a newly synthesized quinazoline derivative, was determined in rat isolated thoracic aorta and pressor responses were determined in spontaneously hypertensive rats (SHR). Experimental results indicated that DC-015 is an alpha 1-adrenoceptor-blocking agent in rat thoracic aorta as revealed by its competitive antagonism of phenylephrine-induced vasocontraction (pA2 = 10.54 +/- 0.55). These effects still persisted in denuded aorta. It was as potent as prazosin (pA2 = 10.04 +/- 0.63). At higher concentration (1.0 microM), DC-015 also expressed 5-hydroxytryptamine (5-HT) receptor competitive antagonism, but this 5-HT blocking effect was not found in the prazosin-administration group. [3H]Inositol monophosphate formation stimulated by phenylephrine (30 microM) in rat thoracic aorta was diminished by DC-015 (3 and 10 nM) and prazosin (10nM); whereas the cAMP content of rat thoracic aorta was not altered by DC-015 and prazosin. Furthermore, intravenous administration of DC-015 and prazosin (both at 0.01, 0.05 and 0.1 mg/kg-1) induced a dose-dependent reduction of mean arterial pressure which reached a maximal effect at 5 mm after injection and persisted over 2 h in SHR. A higher dose of DC-015 (0.1 mg/kg-1, i.v.) did not cause any significant changes in heart rate, whereas, the same dose of prazosin (0.1 mg/kg-1, i.v.) produced a decrease which seems to parallel the time course of the hypotensive response. We can conclude that the DC-015 is a potent, highly selective alpha 1-adrenoceptor antagonist in vascular smooth muscle.

Haemodynamic effects of dicentrine, a novel alpha 1-adrenoceptor antagonist: comparison with prazosin in spontaneously hypertensive and normotensive Wistar-Kyoto rats

1. The haemodynamic effects of dicentrine, an aporphine derivative isolated from the plant Lindera megaphylla, were investigated and compared with prazosin in rats. 2. In anaesthetized normotensive Wistar-Kyoto (WKY) rats, i.v. administration of dicentrine (0.1, 0.5, 1.0 mg kg-1) and prazosin (0.01, 0.05, 0.1 mg kg-1) induced a dose-related reduction of mean arterial pressure (MAP) which reached a maximal effect 5-10 min after injection and persisted for 2 h. 3. In anaesthetized WKY rats, a higher dose of dicentrine (1.0 mg kg-1, i.v.) did not cause any significant changes in heart rate (HR), cardiac output (CO) and stroke volume (SV) but markedly increased tail blood flow. In contrast, a higher dose of prazosin (0.1 mg kg-1, i.v.) produced a decrease in HR which paralleled the time course of the hypotensive response. 4. The hypotensive activity of dicentrine was completely abolished by alpha-adrenoceptor blockade. Both dicentrine and prazosin significantly attenuated pressor responses to noradrenaline but failed, even at maximal hypotensive doses, to impair the pressor effects of angiotensin II or vasopressin. These observations suggest that dicentrine appears to exert its hypotensive action through alpha 1-adrenoceptor blockade. 5. In conscious normotensive and spontaneously hypertensive (SH) rats, dicentrine (0.5-2.0 mg kg-1, i.v.) and prazosin (0.05-0.2 mg kg-1, i.v.) also evoked dose-related decreases in MAP which were of greater magnitude in SH rats. Oral administration of dicentrine (5 and 8 mg kg-1) to conscious SH rats caused a hypotensive effect which persisted for over 15 h.6. These results suggest that dicentrine may have therapeutic potential as an oral antihypertensive drug via alpha-adrenoceptor blockade.

Myocardial effects of selective alpha-adrenoceptor blockade during exercise in dogs

To study the effect of selective alpha 1- or alpha 2-adrenergic blockade on the myocardial contractile and chronotropic response to exercise, 29 dogs were chronically instrumented with a sonomicrometer for measuring myocardial wall thickness and a micromanometer for measuring left ventricular pressure. During treadmill exercise, either the selective alpha 1-blocker prazosin (80 micrograms/kg, n = 12) or the alpha 2-blocker idazoxan (80 micrograms/kg, n = 8) was infused into the left atrium beginning 2-3 minutes after the onset of exercise. alpha 1-Adrenoceptor blockade, like alpha 2-adrenoceptor blockade, was found to cause significant increases in systolic wall thickening, thickening velocity, heart rate, and left ventricular contractility, indicating an increase in inotropic state that was comparable to that with alpha 2-adrenoceptor blockade. Preventing the decrease in aortic blood pressure after selective alpha 1-blockade by using either systemic angiotensin II infusion (n = 6) or inflation of an intra-aortic balloon (n = 6) did not prevent the observed increases in wall thickening, heart rate, and left ventricular contractility. In four of the dogs treated with prazosin, the norepinephrine concentration in the coronary sinus was found to more than double after alpha 1-blockade. beta-Adrenergic blockade (propranolol, 1.0 mg/kg) prevented the increased contractile and chronotropic state caused by alpha 1- or alpha 2-blockade. Selective alpha-adrenergic blockade during adrenergic activation by intravenous norepinephrine infusion, in contrast to exercise, had no effect on wall thickening, heart rate, or left ventricular contractility. These data indicate that selective alpha 1-adrenergic blockade, like selective alpha 2-adrenergic blockade, causes a significant augmentation of heart rate and left ventricular contractility in the dog during dynamic exercise. These data are consistent with the hypothesis that this occurs through a presynaptic disinhibition of neural norepinephrine release mediated by a prejunctional alpha 1-adrenoceptor.

Modulation of noradrenaline release in slices of rat kidney cortex through alpha 1- and alpha 2-adrenoceptors

Slices of rat kidney cortex were incubated in [3H]noradrenaline, then placed in a flow cell and subjected to electrical field stimulation. At a stimulation frequency of 5 Hz, both the alpha 2-adrenoceptor antagonist idazoxan (0.1 microM) and the alpha 1-adrenoceptor antagonist prazosin (0.1 microM) significantly enhanced the stimulation-induced (S-I) outflow of radioactivity from the slice. However, neither clonidine (0.1 microM) nor methoxamine (10 microM), alpha 2- and alpha 1-agonists respectively, affected the S-I outflow of radioactivity at this stimulation frequency. At a lower stimulation frequency (1 Hz), the S-I outflow of radioactivity was not affected by idazoxan or prazosin, but was inhibited by both clonidine and methoxamine. The effect of clonidine was prevented by idazoxan (0.1 microM), but not by prazosin (0.1 microM). The effect of methoxamine was abolished by prazosin (0.1 microM), but not by idazoxan (0.1 microM). The inhibitory effect of methoxamine was not prevented by the prostaglandin synthesis inhibitor indomethacin (10 microM) or the adenosine receptor antagonist 8-phenyltheophylline (1 microM) and thus was not mediated by either prostaglandins or adenosine. The results suggest that both prejunctional alpha 1- and alpha 2-adrenoceptors are directly involved in modulation of noradrenaline release from the renal sympathetic nerves of the rat.

Pharmacological characterization of neuropeptide Y and noradrenaline mechanisms in sympathetic control of pig spleen

The mechanisms underlying the functional effects of neuropeptide Y (NPY)-like immunoreactivity (LI) and noradrenaline (NA) and their release evoked by nerve stimulation were studied with the blood-perfused pig spleen in vivo. Infusion of selective agonists and antagonists suggested the presence of alpha 1- and beta 2-adrenoceptors mediating vasoconstriction and vasodilatation, respectively. NPY caused a slight inhibition of stimulation-evoked [3H]NA release and a clearcut non-adrenergic vasoconstriction. Local pretreatment with phentolamine and prazosin as well as with clonidine and UK 14304 reduced the perfusion pressure response to nerve stimulation. Phentolamine, yohimbine and idazoxan enhanced while clonidine and UK 14304 decreased the output of [3H]NA or NA and NPY-LI. The subsequent addition of propranolol to the alpha-adrenoceptor antagonists was followed by reappearance at a considerable portion of the perfusion pressure response while the output of [3H]NA or NA and NPY-LI was slightly reduced. It is concluded that NPY exerts pre- and post-junctional actions in pig spleen that regulate both NA release and vascular tone. alpha 1-Adrenoceptors are mainly involved in vasoconstriction, and prejunctional alpha 2 mechanisms inhibit both NA and NPY release at a low frequency of stimulation. beta 2-Adrenoceptors mediate vasodilatation when NA release is enhanced with a minor effect on mediator secretion.

Possible involvement of presynaptic alpha 1-adrenoceptors in the effects of idazoxan and prazosin on 3H-noradrenaline release from tail arteries of SHR

The effects of several alpha-adrenoceptor antagonists have been examined on tritium release elicited by electrical stimulation from isolated perfused SHR tail artery preparations prelabelled with 3H-noradrenaline (3H-NA). Phentolamine and yohimbine potently facilitated the stimulation evoked release of tritium at low frequencies of stimulation, but the alpha 2-adrenoceptor antagonist idazoxan was only weakly active at 1 mumol/l, despite antagonising the clonidine-evoked inhibition of 3H-release at a lower concentration of 0.1 mumol/l. The alpha 1-adrenoceptor antagonists prazosin and corynanthine also increased stimulation evoked tritium release in this preparation, suggesting the presence of prejunctional alpha 1-adrenoceptors. Furthermore, the alpha 1-adrenoceptor agonist methoxamine (3 mumol/l) caused a significant inhibition of tritium-evoked release, an effect which was blocked by prazosin (10 nmol/l). When alpha 1-adrenoceptors were blocked in the presence of prazosin, idazoxan (0.1 mumol/l) produced a significant facilitatory effect on the electrically-evoked release of 3H-transmitter. On the other hand, when alpha 2-adrenoceptors were blocked in the presence of yohimbine, exposure to idazoxan (0.1 mumol/l) reduced significantly the stimulation-evoked release of tritium elicited by electrical stimulation. The results indicate that in the SHR tail arteries, idazoxan has a partial agonist inhibitory activity on transmitter release, which can mask the facilitatory effects due to blockade of presynaptic alpha 2-adrenoceptors. The inhibitory effects of idazoxan appear to involve presynaptic alpha 1-adrenoceptors, which when stimulated, reduce 3H-NA release in SHR tail arteries.

Influence of urapidil on alpha- and beta-adrenoreceptors in pithed rats

The interaction of urapidil with pre- and postsynaptic alpha-adrenoreceptors and with postsynaptic beta-adrenoreceptors was studied in pithed normotensive rats and compared to the effects of clonidine, prazosin, and atenolol. I.v. injection of urapidil did not substantially change blood pressure, while clonidine raised blood pressure. Urapidil dose-dependently antagonized the pressor effects of the alpha 1-agonist L-phenylephrine (pDR2 6.8) and of the alpha 2-agonist azepexole (pDR2 5.2). Compared to urapidil, prazosin was a more potent antagonist of phenylephrine at postsynaptic vascular alpha 1-adrenoreceptors (pDR2 8.7) and of azepexole at alpha 2-adrenoreceptors (pDR2 5.6). Urapidil inhibited the tachycardia produced by discontinuous or continuous electrical stimulation of the thoracic spinal outflows (ID50 4.8 and 27.2 mumol/kg, respectively). In contrast to the inhibitory action of clonidine (ID50 0.039 and 0.023 mumol/kg, respectively), the inhibition by urapidil was not reversed by the selective alpha 2-antagonist rauwolscine (10 mumol/kg). Prazosin did not change stimulation-evoked tachycardia but atenolol caused pronounced inhibition (ID50 0.158 mumol/kg, discontinuous stimulation). Urapidil dose-dependently antagonized the tachycardic effect of isoprenaline at beta 1-adrenoreceptors (pDR2 5.1) but also exhibited intrinsic activity by increasing basal heart rate (maximum effect of urapidil was 30% of that of isoprenaline). Urapidil did not change the vasodilatory beta 2-adrenoreceptor-mediated effect of isoprenaline. The results suggest that urapidil is an antagonist at postsynaptic vascular alpha 1- and alpha 2-adrenoreceptors, with a greater potency against alpha 1-adrenoreceptors. An agonistic interaction of urapidil with presynaptic alpha 2-adrenoreceptors could not be demonstrated in pithed rats. Instead, the inhibition by urapidil of stimulation-evoked tachycardia could be accounted for by its beta 1-adrenoreceptor antagonistic effect.

The influence of repeated administration of prazosin on its hypotensive effect and on renin release in conscious dogs. A comparison with urapidil

The effects of prazosin (0.1, 0.5 and 2.5 mg/kg, orally) on blood pressure, heart rate and PRA were investigated after acute and chronic administration in conscious, normotensive dogs. The acute administration of prazosin caused a reduction in both systolic and diastolic blood pressure and an increase in heart rate and PRA. After chronic administration, twice daily for four days, tolerance developed, which was not due to persistent counterregulation. On comparison with urapidil, another alpha-adrenoceptor antagonist, the hypotensive and counterregulatory response to acute administration were similar with both substances; the development of tolerance to prazosin after chronic administration contrasted with the results with urapidil.

Presence of presynaptic inhibitory alpha 1-adrenoceptors in the cardiac sympathetic nerves of the dog: effects of prazosin and yohimbine on sympathetic neurotransmission to the heart

The effects of prazosin and yohimbine on sympathetic neurotransmission to the heart were investigated in perfused dog hearts in situ in an attempt to determine whether alpha 1-adrenoceptors are located presynaptically in the cardiac sympathetic nerves. Intra-arterial injections of prazosin (1-30 micrograms) and yohimbine (0.3-10 micrograms) into the right coronary artery during cardiac sympathetic nerve stimulation further increased the tachycardia resulting from the stimulation. Continuous infusions of methoxamine (20-40 micrograms/min) and of clonidine (2-4 micrograms/min) into the right coronary artery during cardiac sympathetic nerve stimulation caused sustained reduction of the tachycardia. Prazosin under methoxamine infusion enhanced the tachycardia to a greater extent than in the absence of methoxamine. Prazosin under clonidine infusion enhanced the tachycardia to the same extent as it did in the absence of clonidine. These results suggest that prazosin antagonizes the effect of methoxamine but does not antagonize that of clonidine. The results obtained with yohimbine were in contrast to the effects of prazosin, showing the antagonism of clonidine by yohimbine. Prazosin and yohimbine both had little effect on the heart rate during either the resting state or the infusion of norepinephrine. These results suggest that the prazosin- and yohimbine-induced enhancement of the tachycardia resulting from cardiac sympathetic nerve stimulation is due to a presynaptic effect. However, the presynaptic effect of prazosin appears to differ from that of yohimbine. The presence of presynaptic alpha 1-adrenoceptors regulating norepinephrine release, as well as of alpha 2-adrenoceptors, is suggested in the cardiac sympathetic nerves of the dog.

In vitro and in vivo antiarrhythmic effects of prazosin in the rat

The possible antiarrhythmic action of prazosin was studied on two experimental models: a focus of ectopic automaticity in the rat isolated right ventricle and arrhythmias induced by i.v. CaCl2 in anaesthetized animals. In vitro, prazosin at 10(-5)M abolished the ectopic automaticity focus in 83% of cases (10/12). The effects of prazosin were not modified by reserpine pretreatment. In vivo, prazosin modified the range of CaCl2 arrhythmias, reduced the mean control sinus rate (by 21.7% at 1 mg/kg), reduced incidence and mortality by ventricular fibrillation and lengthened survival time of animals that eventually died. The results indicate that antagonism of postsynaptic alpha 1-adrenoreceptors may partly explain the efficacy of prazosin against some experimental cardiac arrhythmias.

Immediate hemodynamic changes produced by urapidil in normotensive and spontaneously hypertensive rats

The immediate effects on regional and systemic hemodynamics of urapidil (1 mg/kg IV), a recently synthesized vasodilator with a possible combined central and peripheral action, were studied in spontaneously hypertensive (SHR) and normotensive (WKY) rats. Maximal decrease in mean arterial pressure was achieved within the first minute after injection (154 +/- 4 vs 113 +/- 6 mm Hg in SHR and 111 +/- 4 vs 82 +/- 4 mm Hg in WKY, p less than 0.01). This effect was accompanied by a transient (10 min) significant increase in heart rate in both strains. There was a significant fall in total peripheral resistance (0.43 +/- 0.02 vs 0.30 +/- 0.02 U/kg in WKY and 0.62 +/- 0.03 vs 0.43 +/- 0.03 U/kg in SHR, p less than 0.01) and rise in cardiac index 15 min after drug injection (371 +/- 9 vs 425 +/- 12 ml/min/kg in WKY and 395 +/- 8 vs 432 +/- 112 ml/min/kg in SHR, p less than 0.01). Organ vascular resistance decreased significantly in all the organs of /KY and most of the organs of SHR rats. However, a significant increase in blood flow was observed only in skeletal muscle. The data indicate that urapidil is a potent hypotensive agent. The pressure fall is mediated through a decreased total peripheral resistance that is distributed through all circulations. The increased cardiac output and heart rate are most likely reflexly induced.

Ergotamine-induced constriction of cranial arteriovenous anastomoses in dogs pretreated with phentolamine and pizotifen

Previous investigations from our laboratory have shown that ergotamine causes a selective vasoconstriction in the carotid vascular bed of the dog and that the drug constricts arteriovenous anastomoses (AVAs) in cats and pigs. Since ergotamine can act via alpha-adrenergic or D-serotonergic receptors in certain vascular and non-vascular tissues, we have attempted to ascertain here if these receptors mediate the constriction of AVAs. Using radioactive microspheres of 15 microns diameter we found in the dog that about 40% of the carotid arterial blood is shunted to the venous side via AVAs. Ergotamine (2-20 micrograms X kg-1, i.v.) reduced total carotid blood flow to a larger extent in the AVA part than in the extracerebral part (muscles, ears, skin and fat). The cerebral component of carotid blood did not change. These results, confirming that ergotamine decreases arteriovenous shunting, show that the drug has a more selective action on the AVAs than on the arterioles. Pretreatment with phentolamine (0.5 mg X kg-1), pizotifen (0.5 mg X kg-1) or their combination did not effectively modify the responses to ergotamine. It is concluded that the vasoconstriction of cranial AVAs (and arterioles) by ergotamine does not appear to be primarily mediated by either alpha-adrenergic or D-serotonergic receptors. However, the role of atypical serotonin receptors has yet to be determined.

Postsynaptic alpha 1-and alpha 2-adrenoceptor involvement in the vascular responses to neuronally released and exogenous noradrenaline in the hindlimb of the dog and cat

The contribution of postsynaptic alpha 1-and alpha 2-adrenoceptors to vascular responses was investigated in the blood perfused hindlimbs of pentobarbitone-anaesthetised dogs and chloralose-anaesthetised cats. Responses were obtained to sympathetic nerve stimulation, noradrenaline, phenylephrine and UK-14,304. In the dog and cat hindlimbs, the response to phenylephrine was reduced by the selective alpha 1-adrenoceptor antagonist, prazosin, while that to UK-14,304, in the dog, was reduced by the alpha 2-adrenoceptor antagonist, rauwolscine. Responses to noradrenaline and sympathetic nerve stimulation were only partially blocked by prazosin in both the dog and cat, but were further reduced by the addition of rauwolscine. These results suggest that the hindlimb vasculature of the dog and cat responds to neuronally released, as well as exogenous, noradrenaline by activation of both postsynaptic alpha 1- and alpha 2-adrenoceptors.

Evidence in favour of a selective alpha 1-adrenoceptor blocking action of WB 4101 in vivo

The alpha-adrenoceptor blocking potency of WB 4101 at alpha 1- and alpha 2-adrenoceptors has been investigated in pithed rats. WB 4101 was approximately 97 times more potent at antagonizing the vasopressor responses produced by the selective alpha 1-adrenoceptor agonist phenylephrine, than those produced by the selective alpha 2-adrenoceptor agonist M-7. A dose of WB 4101 (3 mg/kg) that caused extensive blockade of vascular alpha 1-adrenoceptors, but little or no blockade of vascular alpha 2-adrenoceptors, exerted no significant blockade of the presynaptic alpha 2-adrenoceptors in the rat heart. The results support the view that WB 4101 is a highly selective antagonist at alpha 1-adrenoceptors in vivo.

The effects of prazosin, phentolamine and phenoxybenzamine on inhibitory alpha-adrenoceptors in the guniea-pig isolated ileum

1 The relaxant effect of noradrenaline on strips of guinea-pig isolated terminal ileum was blocked by pretreatment with prazosin, phentolamine, yohimbine and phenoxybenzamine. 2 The presence of a very high concentration of noradrenaline (50 micrometers) during exposure to the blocking agent protected against the blocking effect of the drugs. 3 Yohimbine, prazosin and phentolamine partially protected against irreversible blockade by phenoxybenzamine. 4 Spontaneous release of acetylcholine in the unstimulated ileum was blocked by noradrenaline (0.05-5.9 micrometers) this effect of noradrenaline was antagonized by phentolamine (0.13-26 micrometers) and yohimbine (0.051-0.51 micrometers) but not prazosin (0.53-5.3 micrometers) or phenoxybenzamine (4.2-42 nm). All four antagonists reversed the noradrenaline-induced relaxation of the ileum. 5 Acetylcholine output in the transmurally stimulated ileum was inhibited by noradrenaline. This effect of noradrenaline was antagonized by phentolamine and yohimbine but not by prazosin or phenoxybenzamine. The first two antagonists blocked the noradrenaline-induced inhibition of evoked twitches of the ileum while the last two had no effect. 6 The results show (a) that prazosin has no effect on presynaptic alpha-adrenoceptor located on cholinergic nerve endings in the guinea-pig ileum and (b) that prazosin, phentolamine and phenoxybenzamine act on the same subgroup of postsynaptic alpha-adrenoceptors on the smooth muscle of the guniea-pig ileum.

Interaction between mianserin and clonidine at alpha 2-adrenoceptors

The purpose of the present study was to characterize the effects of mianserin at alpha 2-adrenoceptors. Firstly, the action of mianserin on postganglionic sympathetic fibres has been studied using the tachycardia induced by stimulation of the cardiac nerve in dogs. Mianserin increased this tachycardia, but could not prevent the inhibitory effect of clonidine in this model. However, an antagonistic effect of mianserin against clonidine was observed when animals were pretreated with desipramine. Secondly, mianserin antagonized the inhibitory effect of clonidine on the electrically stimulated guinea-pig ileum. In high concentrations, mianserin reduced both electrically and acetylcholine induced contractions. Thirdly, mianserin antagonised the sleep induced by clonidine in chickens. These results are consistent with alpha 2-adrenoceptor blocking properties of mianserin in peripheral noradrenergic fibres in dogs, in cholinergic fibres in guinea-pig ileum and in the central nervous system in chickens.

Characterization of alpha-adrenoceptors participating in the central hypotensive and sedative effects of clonidine using yohimbine, rauwolscine and corynanthine

The central alpha-adrenoceptors responsible for mediating the clonidine-induced central hypotension in anaesthetized cats and sedation in mice have been characterized according to their sensitivities to the alpha-adrenoceptor antagonist yohimbine and its two diastereomeric congeners rauwolscine and corynanthine. Yohimbine and rauwolscine (1-10 microgram/kg) dose-dependently antagonized the central hypotensive response to clonidine (1 microgram/kg) applied 15 min later. Greater amounts of corynanthine (30-100 micrograms/kg) had to be administered to diminish the central depressor effect of clonidine. In these studies the drugs were infused via the left vertebral artery. The prolongation of the hexobarbitone-induced loss of the righting reflex in mice by clonidine (0.3 mg/kg, i.p.) was inhibited by previous treatment with yohimbine and rauwolscine (0.04-5 mg/kg, i.p.) in a dose-dependent manner, but not by corynanthine. Binding experiments with rat isolated cerebral membranes demonstrated the higher affinity of yohimbine and rauwolscine for the [3H] clonidine- than for the [3H]prazosin-specific binding sites. The reverse was found for corynanthine. The relative potencies of yohimbine, rauwolscine and corynanthine in inhibiting these central effects of clonidine are comparable to their order of efficacies in blocking peripheral alpha 2-adrenoceptors. Accordingly, clonidine-induced central hypotension and sedation are mediated by alpha 2-adrenoceptors.

Correlation between decreased heart rate and central inhibition of sympathetic discharge after prazosin administration in the spontaneously hypertensive rat

Mean arterial blood pressure, heart rate and splanchnic sympathetic discharge were recorded in conscious and anaesthetized spontaneously hypertensive rats (SHR). Administration of prazosin (1 mg/kg i.v.) reduced heart rate in most rats and this effect was closely parallelled by a decrease in sympathetic discharge. In contrast, administration of hydralazine (1 mg/kg i.v.) caused a sustained tachycardia and increased sympathetic nervous activity. The cardiovascular effects of prazosin (0.2-0.3, 1 and 3 mg/kg i.v.) were examined in different rat strains. Sprague-Dawley and Wistar-Kyoto rats responded with tachycardia whereas a significant bradycardia was observed in SHR following the higher doses of prazosin. It is suggested that the bradycardia obtained after prazosin administration is due to a central inhibition of sympathetic outflow, an effect possibly caused by blockade of alpha-adrenoreceptors. The different heart rate responses in different rat strains may be interpreted to reflect differences in sensitivity of central alpha-adrenoreceptors.

Effects of low doses of clonidine in the monkey cranial circulation

Migraine therapy using low doses of clonidine has been based on the proposal that clonidine directly inhibits vascular smooth muscle reactivity. In anaesthetized monkeys in which internal and external carotid vascular resistances were measured, the only significant effects of clonidine administered acutely (0.5 and 2 microgram x kg-1 i.v.) or chronically (2 microgram x kg-1 i.m. daily for 7 days) on cranial vascular responses to the constrictors noradrenaline and 5-hydroxytryptamine, and the dilators histamine, prostaglandin E1 and bradykinin, were small potentiations of some of the responses. Acute clonidine initially increased blood pressure and constricted the cranial vasculature, then induced hypotension without involvement of the cranial circulation. It also decreased the external carotid vasoconstrictor response to low frequency cervical sympathetic nerve stimulation. The low chronic dose of clonidine had no hypotensive effect. The pressor response to common carotid occlusion was inhibited by both acute and chronic clonidine. These experiments thus provide no evidence that clonidine inhibits cranial vascular reactivity at doses equivalent to those used in migraine.

Evaluation of the selectivity of alpha-adrenoreceptor blocking drugs for postsynaptic alpha 1- and alpha 2-adrenoreceptors in a simple animal model

1 The increase of diastolic pressure of pithed, normotensive rats was determined after i.v. administration of the alpha-adrenoreceptor agonists L-phenylephrine and B-HT 933. 2 alpha-Adrenoreceptor antagonists varied widely in their relative inhibitory effects towards either L-phenylephrine- or B-HT 933-induced vasoconstrictor responses. Prazosin displayed the highest affinity for the postsynaptic alpha-adrenoreceptor triggered by L-phenylephrine. The rank order of potency was further: phenotlamine greater than dihydroergotamine greater than clozapine greater than corynanthine greater than azapetine greater than yohimbine greater than piperoxan greater than tolazoline greater than mainserin greater than rauwolscine. On the other hand, the rank order of potency towards B-HT 933 was: dihydroergotamine greater than rauwolscine greater than yohimbine greater than phentolamine greater than piperoxan greater than prazosin greater than tolazoline greater than mainserin greater than corynanthine greater than azapetine greater than clozapine. These data are in general agreement with the classification of alpha 1-(triggered by L-phenylephrine) and alpha 2-(triggered by B-HT 933) adrenoreceptors. Both populations are present postsynaptically in vascular smooth muscle of the pithed rat and are involved in vasoconstriction. 3 The ratio of KB post alpha 2/KB post alpha 1 was calculated as a measure of selectivity for either alpha-adrenoreceptor site. The alpha-adrenoreceptor antagonists used cover a 20,000-fold range of activity ratios. The antagonists most selective for either type were prazosin (alpha 1) and rauwolscine (alpha 2). The selectivity of the alpha-adrenoreceptor antagonists for postsynaptic alpha 1- and alpha 2-adrenoreceptors in the intact circulatory system of the pithed rat is comparable with the reported selectivity of these blocking agents for alpha 1 (postsynaptic)- and alpha 2(presynaptic)-adrenoreceptors in the rabbit isolated pulmonary artery. 4 It is concluded that two distinct types of postsynaptic alpha-adrenoreceptors participate in vasoconstriction in the pithed rat. Apart from the classical alpha 1-adrenoreceptor, vascular smooth muscle of the pithed rat contains postsynaptic alpha 2-adrenoreceptors resembling those previously found mainly presynaptically. The presence of separate classes of postsynaptic alpha 1- and alpha 2-adrenoreceptors in the intact circulatory system of the pithed rat offers the possibility to use this relatively simple animal model as an in vivo test system for the pharmacological characterization of alpha-adrenoreceptors agonists and antagonists.

gamma-Aminobutyric acid and postganglionic sympathetic transmission in the pulmonary artery of the rabbit

1 The effect of gamma-aminobutyric acid (GABA) on postganglionic sympathetic neurotransmission was studied in strips of the rabbit pulmonary artery. The strips were preincubated with 3H-noradrenaline and then superfused with 3H-amine-free medium. They were stimulated either electrically at 2 Hz, or by 60 mM potassium, or by 1 microM tyramine. 2 GABA (1 - 1000 microM) did not change the basal outflow of tritium, but decreased the electrically evoked overflow as well as the contractile response. GABA 1 microM decreased the evoked overflow by 12%, and GABA 1000 microM, by 42%. The effect of GABA was not changed by yohimbine, propranolol, cocaine, corticosterone, or indomethacin. It was not antagonized by picrotoxin or bicuculline methiodide. GABA 100 microM also slightly reduced the potassium-evoked overflow of tritium but did not change the tyramine-evoked overflow. 3 The results show that, in the pulmonary artery of the rabbit, GABA inhibits the release of noradrenaline. Its effect is independent of alpha- and beta-adrenoreceptors and is not mediated by prostaglandins. The effect may be due to activation of presynaptic receptors which appear to differ from conventional GABA receptors inasmuch as they are insensitive to blockade by either picrotoxin or bicuculline.

alpha 1- and alpha 2-vascular adrenoceptors in the dog

Prazosin inhibited contractions of isolated electrically stimulated dog pulmonary artery preincubated with (-)-7-3H-norepinephrine, but had no effect on 3H-overflow, whereas yohimbine enhanced both responses. In pithed dogs the cumulative doses of yohimbine required for 50% inhibition of the pressor effects of clonidine, phenylephrine and norepinephrine were 23, 188 and 35 micrograms/kg i.v., and those for prazosin were 9, 1.5 and 5 micrograms/kg i.v., respectively. The agonists could be ranked in pairs depending on the extent to which the antagonists discriminated members of the pair: clonidine-phenylephrine > phenylephrine-norepinephrine > clonidine-norepinephrine. The results suggest that pressor effects in dogs are mediated by both alpha 1- and alpha 2-vascular (i.e. postsynaptic) adrenoceptors.

Evidence for a presynaptic inhibitory action of 5-hydroxytryptamine on sympathetic neurotransmission to the myocardium

A study was undertaken to determine the effect of 5-Hydroxytryptamine (5-HT) on sympathetic neurotransmission to the myocardium. Intravenous infusion of 5-HT (20 microgram/kg/min) produced a decrease in mean blood pressure and caused significantly impairment of cardioacceleration observed during the stimulation of right post-ganglionic cardiac sympathetic nerve. The pressor and tachycardic responses to norepinephrine as well as tyramine were potentiated during 5-HT infusion. The inhibitory effect of 5-HT on responses to cardiac nerve stimulation was not prevented by desipramine. Prior treatment with cyproheptadine prevented the depressor effect of 5-HT and significantly antagonized the inhibitory action of 5-HT on the positive chronotropic effect of cardiac nerve stimulation. Methysergide administration to a different group of dogs antagonized the effect of 5-HT on blood pressure, but did not alter the impairment of cardioacceleration caused by 5-ht during cardiac nerve stimulation. Neither cyproheptadine nor methysergide had any effect on the responses to cardiac nerve stimulation. These results suggest that 5-HT inhibits sympathetic neurotransmission of the myocardium via an action on 'trypaminergic' receptors which may be located on sympathetic nerve terminal and that these receptors can be selectively blocked by cyproheptadine but not by methysergide.

Lack of correlation between presynaptic inhibition of noradrenaline release and end organ responses during nerve stimulation

1 LD 3098 (cirazoline) is an imidazoline derivative, possessing agonist properties at alpha-adrenoceptor sites.2 When transmitter release was measured directly as tritium overflow from perfused cat spleen preparations, prelabelled with [(3)H]-noradrenaline, LD 3098 was found to be 10 times more selective for presynaptic than for postsynaptic alpha-adrenoceptors.3 In addition, in this preparation, LD 3098 appears to induce a postsynaptic sensitization to the transmitter released by nerve depolarization because under conditions in which [(3)H]-noradrenaline overflow decreased, there was a paradoxical potentiation in the response to nerve stimulation. This potentiation also occurred with a concentration of LD 3098 that did not per se affect stimulation-evoked [(3)H]-noradrenaline release or the basal perfusion pressure of the spleen.4 Both the reduction in (3)H-transmitter release induced through activation of alpha-presynaptic adrenoceptors and the potentiation of the responses to nerve stimulation were concentration-dependent phenomena.5 In pentobarbitone anaesthetized dogs, the heart rate response to low frequency ansa-subclavia stimulation was not affected by LD 3098. Whilst the alpha(1) mediated increase in blood pressure responses to injected noradrenaline and tyramine was significantly potentiated by LD 3098, the beta(1)-mediated heart rate responses to these injected amines were not modified in the presence of LD 3098.6 Thus it is possible that the failure to detect any presynaptic effects with LD 3098 when transmitter release is measured indirectly at the level of the postsynaptic responses is due to end organ sensitivity changes.7 These findings emphasize that caution is necessary when assessing presynaptic alpha-adrenoceptor effects through end organ responses to nerve stimulation both in vitro and in vivo and the need for measurements of transmitter overflow as well as adequate postsynaptic controls in such experiments.

Effects of clonidine, prazosin and phentolamine on heart rate and coronary sinus catecholamine concentration during cardioaccelerator nerve stimulation in spinal dogs

1 In spinal dogs, continuous electrical stimulation of the cardioaccelerator nerve produced a transient rise in aortic blood pressure and a sustained increase in both heart rate and coronary sinus blood flow. The latter effects were accompanied by a significant elevation in the coronary sinus plasma noradrenaline concentration without significant changes in the levels of dopamine and adrenaline. The concentrations of the three catecholamines in thoracic aorta plasma were not significantly changed by cardioaccelerator nerve stimulation.2 Clonidine (20 mug/kg, i.v.), given during cardioaccelerator nerve stimulation, increased both mean aortic blood pressure and coronary sinus blood flow and decreased heart rate and coronary sinus venous plasma noradrenaline overflow.3 Phentolamine (0.3 mg/kg, i.v.) completely antagonized these effects of clonidine. Prazosin (0.3 mg/kg, i.v.) inhibited by only 43 and 38% the respective reductions in heart rate and noradrenaline overflow elicited by clonidine.4 On termination of cardioaccelerator stimulation (about 10 min after either prazosin or phentolamine), heart rate and coronary sinus noradrenaline overflow returned to control prestimulation levels.5 Phentolamine or prazosin, administered alone during stimulation of the cardioaccelerator nerve, increased heart rate and noradrenaline overflow into the coronary sinus plasma. However, intravenous phentolamine and prazosin, in contrast to desipramine (0.3 mg/kg, i.v.) or tyramine (1.0 mg, i.a.), failed to change the tachycardia resulting from the local administration of noradrenaline into the sinus node artery (i.a.).6 These results show that in spinal dogs the clonidine-induced reduction in heart rate (elevated by electrical stimulation of the cardioaccelerator nerve) is accompanied by a fall in the quantity of noradrenaline overflowing into the coronary sinus plasma. The latter effect is presumably the result of an action of clonidine on cardiac presynaptic alpha-adrenoceptors, the activation of which is followed by a reduction in the release of noradrenaline per nerve impulse. Phentolamine and prazosin are both antagonists of cardiac presynaptic alpha-adrenoceptors in spinal dogs, as suggested by their action against clonidine and by their positive chronotropic effect when administered during stimulation of the cardioaccelerator nerve.

Presynaptic inhibition by ergotamine of the responses to cardioaccelerator nerve stimulations in the cat

The effects of ergotamine on the tachycardia elicited by postganglionic stimulation of the cardioaccelerator nerve have been investigated in both intact and spinal (desmethylimipramine-pretreated) cats. Ergotamine (2.5--20 microgram kg-1) selectively antagonized the response to low-frequency (0.3--3 Hz) stimulation. The responses elicited by stimulation of the nerve with higher frequencies (greater than 10 Hz) either remained unaltered or were facilitatated by the drug. Pretreatment of the cats with pimozide (0.2 mg kg-1) completely antagonized the inhibitory effects of ergotamine on the sympathetic neuron. Although phentolamine (0.5 mg kg-1) did appear to attenuate the effects of ergotamine, the changes were, however, not statistically significant. Since the drug did not reduce the cardiac effects of isoprenaline or tyramine, it is concluded that ergotamine interferes with the release of noradrenaline from the cardioaccelerator nerve, mainly by stimulating the presynaptic dopaminergic receptors. This effect may be an important mechanism by which the drug causes bradycardia in almost all species regardless of the blood pressure response.

The mechanism of the clonidine-induced reduction in peripheral parasympathetic submaxillary salivation

The mechanism of the clonidine induced reduction in submaxillary salivation evoked by electrical stimulation of the chorda tympani was investigated in anaesthetised cats. This effect of clonidine was found to be dose and frequency dependent. In addition to clonidine, tramazoline, also a preferential presynaptic alpha-adrenoceptor agonist, produced a reduction in electricallly evoked salivation. Methoxamine, noradrenaline and naphazoline, which are less potent presynaptic alpha-receptor agonists, caused increases in salivation. Phentolamine only partially antagonized the decrease in salivation produced by clonidine whereas it was virtually abolished by yohimbine. Clonidine increased salivation evoked by intra-arterial injections of carbachol. These findings suggest that clonidine reduces peripheral parasympathetically evoked submaxillary salivation by activation of presynaptic alpha-adrenoceptors which inhibit cholinergic transmission.