Characterisation of postjunctional alpha-adrenoceptors in isolated human femoral veins and arteries

Trace amine-induced vasoconstriction of human mammary artery and saphenous vein

Sympathomimetic amines, including β-phenylethylamine (PEA), constrict animal blood vessels but their mechanism of action is not now thought to be through α-adrenoceptors and release of noradrenaline but via trace amine-associated receptors (TAARs). This information is not available for human blood vessels. Functional studies were therefore performed on human arteries and veins to establish whether they constrict to PEA and whether any constrictions are adrenoceptor-mediated. Isolated internal mammary artery or saphenous vein rings were set up in Kreb's-bicarbonate solution at 37 ± 0.5 °C gassed with O2:CO2 (95:5) under class 2 containment. Isometric contractions were measured and cumulative concentration-response curves for PEA or the α-adrenoceptor agonist, phenylephrine were established. PEA showed concentration-related contractions. The maximum was significantly greater in arteries (1.53 ± 0.31 g, n = 9) than veins (0.55 ± 0.18 g, n = 10), but not when plotted as % of KCl contractions. PEA showed slowly developing contractions plateauing at 17,3 ± 3.7 min in mammary artery. The reference α-adrenoceptor agonist, phenylephrine, exhibited more rapid onset (peak 5.0 ± 1.2 min) but non-sustained contractions. In saphenous veins, PEA (62.8 ± 10.7%) and phenylephrine (61.4 ± 9.7%, n = 4) displayed the same maximum, but phenylephrine was more potent. The α1-adrenoceptor antagonist, prazosin (1 μM), blocked phenylephrine contractions of mammary arteries but not PEA contractions in either vessel. PEA causes substantial vasoconstriction of human saphenous vein and mammary artery, which explains its vasopressor actions. This response, however, was not mediated via α1-adrenoceptors, but likely due to TAARs. The classification of PEA as a sympathomimetic amine on human blood vessels is therefore no longer valid and requires revision.

Dexmedetomidine inhibits vasoconstriction via activation of endothelial nitric oxide synthase

Despite the complex vascular effects of dexmedetomidine (DEX), its actions on human pulmonary resistance arteries remain unknown. The present study tested the hypothesis that DEX inhibits vascular tension in human pulmonary arteries through the endothelial nitric oxide synthase (eNOS) mediated production of nitric oxide (NO). Pulmonary artery segments were obtained from 62 patients who underwent lung resection. The direct effects of DEX on human pulmonary artery tension and changes in vascular tension were determined by isometric force measurements recorded on a myograph. Arterial contractions caused by increasing concentrations of serotonin with DEX in the presence or absence of L-NAME (endothelial nitric oxide synthase inhibitor), yohimbine (α₂-adrenoceptor antagonist) and indomethacin (cyclooxygenase inhibitor) as antagonists were also measured. DEX had no effect on endothelium-intact pulmonary arteries, whereas at concentrations of 10^–8~10^–6 mol/L, it elicited contractions in endothelium-denuded pulmonary arteries. DEX (0.3, 1, or 3×10^–9 mmol/L) inhibited serotonin-induced contraction in arteries with intact endothelium in a dose-dependent manner. L-NAME and yohimbine abolished DEX-induced inhibition, whereas indomethacin had no effect. No inhibitory effect was observed in endothelium-denuded pulmonary arteries. DEX-induced inhibition of vasoconstriction in human pulmonary arteries is mediated by NO production induced by the activation of endothelial α₂-adrenoceptor and nitric oxide synthase.

Left ventricular epicardial admittance measurement for detection of acute LV dilation

There are two implanted heart failure warning systems incorporated into biventricular pacemakers/automatic implantable cardiac defibrillators and tested in clinical trials: right heart pressures, and lung conductance measurements. However, both warning systems postdate measures of the earliest indicator of impending heart failure: left ventricular (LV) volume. There are currently no proposed implanted technologies that can perform LV blood volume measurements in humans. We propose to solve this problem by incorporating an admittance measurement system onto currently deployed biventricular and automatic implantable cardiac defibrillator leads. This study will demonstrate that an admittance measurement system can detect LV blood conductance from the epicardial position, despite the current generating and sensing electrodes being in constant motion with the heart, and with dynamic removal of the myocardial component of the returning voltage signal. Specifically, in 11 pigs, it will be demonstrated that 1) a physiological LV blood conductance signal can be derived; 2) LV dilation in response to dose-response intravenous neosynephrine can be detected by blood conductance in a similar fashion to the standard of endocardial crystals when admittance is used, but not when only traditional conductance is used; 3) the physiological impact of acute left anterior descending coronary artery occlusion and resultant LV dilation can be detected by blood conductance, before the anticipated secondary rise in right ventricular systolic pressure; and 4) a pleural effusion simulated by placing saline outside the pericardium does not serve as a source of artifact for blood conductance measurements.

Orthostatic hypotension occurs following α2-adrenoceptor blockade in chronic prazosin-pretreated conscious spontaneously hypertensive rats

1. Studies were performed to evaluate whether chronic prazosin treatment alters the alpha 2-adrenoceptor function for orthostatic control of arterial blood pressure in conscious spontaneously hypertensive rats (SHR). 2. Conscious SHR (male 300-350 g) were subjected to 90 degrees head-up tilts for 60 s following acute administration of prazosin (0.1 mg kg-1 i.p.) or rauwolscine (3 mg kg-1 i.v.). Orthostatic hypotension was determined by the average decrease (%) in mean arterial pressure (MAP femoral) over the 60-s tilt period. The basal MAP of conscious SHR was reduced to a similar extent by prazosin (-23%(-)-26% MAP) and rauwolscine (-16%(-)-33% MAP). However, the head-up tilt induced orthostatic hypotension in the SHR treated with prazosin (-16% MAP, n = 6), but not in the SHR treated with rauwolscine (less than +2% MAP, n = 6). 3. Conscious SHR were treated for 4 days with prazosin at 2 mg kg-1 day-1 i.p. for chronic alpha 1-adrenoceptor blockade. MAP in conscious SHR after chronic prazosin treatment was 14% lower than in the untreated SHR (n = 8). Head-up tilts in these rats did not produce orthostatic hypotension when performed either prior to or after acute dosing of prazosin (0.1 mg kg-1 i.p.). Conversely, administration of rauwolscine (3 mg kg-1 i.v.) in chronic prazosin treated SHR decreased the basal MAP by 12-31% (n = 4), and subsequent tilts induced further drops of MAP by 19-23% in these rats. 4. The pressor responses and bradycardia to the alpha 1-agonist cirazoline (0.6 and 2 micrograms kg-1 i.v.), the alpha 2-agonist Abbott-53693 (1 and 3 micrograms kg-1 i.v.), and noradrenaline (0.1 and 1.0 micrograms kg-1 i.v.) were determined in conscious SHR with and without chronic prazosin pretreatment. Both the pressor and bradycardia effects of cirazoline were abolished in chronic prazosin treated SHR (n = 4) as compared to the untreated SHR (n = 4). On the other hand, the pressor effects of Abbott-53693 were similar in both groups of SHR, but the accompanying bradycardia was greater in SHR with chronic prazosin treatment than without such treatment. Furthermore, the bradycardia that accompanied the noradrenaline-induced pressor effect in SHR was similar with and without chronic prazosin treatment despite a 47-71% reduction of the pressor effect in chronic alpha 1-receptor blocked SHR.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo studies on alpha-adrenergic receptor subtypes in human veins

We studied in vivo responsiveness of venous alpha 1- and alpha 2-adrenoceptors, measuring the diameter changes in superficial veins in response to alpha-adrenergic agonists and antagonists in healthy human volunteers. The dorsal hand vein technique was used because it permits complete dose-response studies of venous constriction without confounding reflex alterations. Local infusions of all agonists studied induced dose-dependent contraction of the hand vein; the maximal effects (Emax) were: norepinephrine (88% +/- 10%), methoxamine (97% +/- 5%), phenylephrine (95% +/- 6%), clonidine 54% +/- 12%), and azepexole (68% +/- 26%). Clonidine reduced the norepinephrine-induced venoconstriction by 11% +/- 10%. Oral doses of 1 mg prazosin antagonized the venoconstriction induced by norepinephrine, methoxamine, and clonidine, but not by azepexole. Yohimbine-antagonism was observed against all agonists studied. Inhibition by yohimbine of clonidine-induced venoconstriction was irreversible over 60-180 min. Results show that the in vivo effects on veins of alpha-adrenergic agonists are in good agreement with results from in vitro experiments. Agonists with alpha 1- and alpha 2-adrenoceptor subtype selectivity cause venoconstriction in vivo, but alpha 2-receptor mediated constriction is intrinsically weaker. Clonidine acts as a partial antagonist against norepinephrine, presumably on postsynaptic alpha 2-receptors. At high doses, alpha 2-adrenoceptor subtype selectivity of clonidine and yohimbine appear to be partially lost in vivo.

A comparison of the effects of angiotensin II and Bay K 8644 on responses to noradrenaline mediated via postjunctional alpha 1-and alpha 2-adrenoceptors in rabbit isolated blood vessels

1. The effects of angiotensin II (AII) and Bay K 8644 on responses to noradrenaline (NA) mediated via postjunctional alpha 1- and/or alpha 2-adrenoceptors have been compared in three isolated venous preparations from the rabbit, the lateral saphenous vein, the left renal vein and the ear vein. 2. A similar action of AII and Bay K 8644 was observed only in the lateral saphenous vein; each potentiated responses to NA after isolation of a homogeneous population of postjunctional alpha 2- adrenoceptors. However, even in this preparation the mechanism of action for these agents was not identical. The sensitivity of KCl-induced contraction to changes in extracellular calcium ions (reflecting activation of voltage-dependent Ca2+ channels) was enhanced by Bay K 8644 but reduced by AII. 3. All produced a selective facilitation of responses mediated via postjunctional alpha 2-adrenoceptors. In the lateral saphenous vein it reduced the effectiveness of prazosin and facilitated responses after isolation of alpha 2-adrenoceptors with phenoxybenzamine and rauwolscine. It directly enhanced responses to NA in the ear vein, where only alpha 2-adrenoceptors are involved. In contrast, AII did not influence responses mediated via postjunctional alpha 1-adrenoceptors in the left renal vein (even after the receptor reserve had been removed with phenoxybenzamine) nor the 'rauwolscine-resistant' component of responses to NA in the saphenous vein. 4. Bay K 8644 enhanced contractile responses to NA mediated both via alpha 2-adrenoceptors, in the lateral saphenous vein, and via alpha 1-adrenoceptors in the left renal vein. Thus, unlike angiotensin II, no preferential effect was apparent. 5. Bay K 8644 was inactive against responses to NA in the rabbit isolated ear vein. Since the sustained component of responses to NA in this preparation is dependent upon the influx of extracellular Ca2 , these observations suggest that the influx of Ca2+ stimulated by NA is mediated via receptor-operated (1,4-dihydropyridine-resistant) Ca2 + channels.

Postjunctional alpha-adrenoceptors in the rabbit isolated distal saphenous artery: indirect sensitivity to prazosin of responses to noradrenaline mediated via postjunctional alpha 2-adrenoceptors

1. Under normal experimental conditions, the rabbit isolated distal saphenous artery appears to contain a homogeneous population of postjunctional alpha 1-adrenoceptors. Prazosin competitively antagonized responses to noradrenaline (NA) with a pA2 value of 8.6, while a relatively high concentration of rauwolscine (1 microM), produced only a 2 fold rightward displacement of the NA cumulative concentration-response curve (CCRC). 2. Despite the fact that angiotensin II (AII) was without effect on responses to NA or phenylephrine in this preparation, this peptide made responses to NA less susceptible to the antagonistic action of prazosin. This was particularly evident on the lower portion of the CCRC for NA. These results suggest that in the presence of AII, NA produces contractile responses by an action mediated through a prazosin-resistant adrenoceptor. 3. An attempt was made to isolate a homogeneous population of postjunctional alpha 2-adrenoceptors by use of a receptor protection procedure involving the combination of rauwolscine and phenoxybenzamine. After the protection protocol no responses were observed to the alpha-adrenoceptor agonists NA, phenylephrine or UK-14304. In the presence of angiotensin II however, concentration-dependent contractions were observed to each of these agonists. Under these conditions the rank order of potency, UK-14304 greater than NA greater than phenylephrine, is consistent with that of an effect mediated through postjunctional alpha 2-adrenoceptors. 4. The responses to NA, after the protection protocol, in the presence of AII, were susceptible to the selective alpha 2-adrenoceptor antagonist, rauwolscine (1 microM), but resistant to the selective alpha 2-adrenoceptor antagonist prazosin (0.1 microM). Furthermore, the combination of rauwolscine (1 microM) and prazosin (0.1 I microM) was no more effective in blocking responses to NA than was rauwolscine (1 microM) alone. These results are consistent with the presence of a homogeneous population of postjunctional alpha 2-adrenoceptors. 5. Inducing a small degree of tone with a low concentration of the selective alpha 1-adrenoceptor agonist, phenylephrine, markedly increased the threshold sensitivity to the selective alpha 2-adrenoceptor agonist UK- 14304, in a manner analogous to that seen with All. 6. The results in the present study indicate that responses mediated via postjunctional alpha 2-adrenoceptors in the rabbit isolated distal saphenous artery are dependent upon a degree of vascular smooth muscle stimulation by some other receptor system. It is hypothesized that under normal experimental conditions, this function is fulfilled by stimulation of alpha l-adrenoceptors, while after alpha 1-adrenoceptor blockade the necessary positive influence can be provided by stimulation of All receptors. The implications for such an interaction between postjunctional alpha-adrenoceptor subtypes in demonstrating prazosin-resistant, rauwolscine- or yohimbine-sensitive responses in isolated blood vessels is discussed.

Plasma catecholamines and alpha 1-adrenoceptor function in hemodialysis-associated hypotension

alpha 1-Adrenoceptor function and plasma catecholamine levels were investigated in chronically hemodialyzed patients with and without hemodialysis-induced hypotension. In the interdialytic period blood pressure responses as well as the mydriasis after topical application of the alpha 1-selective agonist phenylephrine were not significantly different in patients with or without hypotension during dialysis, although patients with hypotension were more susceptible to miosis induced by the muscarinergic agonist carbachol. In the normotensive group the blood pressure increasing effect of phenylephrine was attentuated after 120 min of hemodialysis therapy. Plasma noradrenaline levels were not significantly different in both groups and did not change significantly during hemodialysis, while plasma dopamine was significantly increased in the hypotensives. Thus, evidence for a pathophysiological role of a postsynaptic alpha-adrenoceptor dysfunction in hemodialysis-induced hypotension was lacking.

Postjunctional alpha 2-adrenoceptors mediate vasoconstriction in human subcutaneous resistance vessels

1. In vitro studies have been performed on human medium-sized muscular arteries (internal diameter 1-4 mm) in a classical organ bath and with human subcutaneous resistance arteries (internal diameter 103-626 microns) in a microvascular myograph. 2. Although the medium-sized muscular arteries showed no response to either of the alpha 2-agonists B-HT 933 or UK 14304 in concentrations up to 10 microM, the subcutaneous resistance arteries from all regions examined showed well-pronounced and concentration-dependent responses to B-HT 933, the pD2 (-log EC50) being 5.11 +/- 0.09. 3. In the resistance arteries the alpha 2-antagonist yohimbine caused a parallel shift to the right of the B-HT 933 concentration-response curve; the yohimbine pA2 for the B-HT 933 receptor was 7.86 +/- 0.12. 4. There was an inverse relationship between the maximum response to B-HT 933 and the calibre of the resistance vessels. 5. These results indicate the presence of a postjunctional alpha 2-adrenoceptor in human subcutaneous resistance arteries and not in medium sized muscular arteries.

Different effects of furosemide on alpha-adrenoceptors and on platelet aggregation in man

The effect of a long-term administration of furosemide (2 x 30 mg/day for 3 weeks) on platelet alpha 2-adrenoceptor density and the fraction of high-affinity binding sites, as well as on platelet aggregation induced by adrenaline and ADP, was studied ex vivo in 8 normotensive volunteers. For comparison the in vitro effect of furosemide on platelet aggregation was also evaluated. Furosemide decreased alpha 2-adrenoceptor-density (P less than 0.01) and the fraction of high-affinity binding sites (P less than 0.05). Adrenaline-induced platelet aggregation was not altered ex vivo and in vitro. Furosemide inhibited ADP-induced platelet aggregation ex vivo (P less than 0.05) and in parallel in vitro (P less than 0.01) in a dose-dependent manner. The reduction of the density of alpha 2-adrenoceptors in the high affinity state may be of functional importance for the hemodynamic effects of furosemide. The inhibitory effect of furosemide on ADP-induced platelet aggregation ex vivo and in vitro, which is not related to the effects on adrenoceptors, seems to involve direct effects of furosemide on platelet function. It remains to be seen whether the latter effect is of clinical importance.

Effect of chlorpromazine on sympathetic neuroeffector transmission in the rabbit isolated pulmonary artery and aorta

1. The effects of chlorpromazine on sympathetic neuroeffector transmission have been studied in the rabbit isolated pulmonary artery and aorta. 2. Chlorpromazine (10(-8)-10(-5) M), prazosin (10(-9)-10(-7) M) and phentolamine (3 x 10(-8)-3 x 10(-5) M) decreased the contractions of pulmonary artery evoked by electrical field stimulation (150 pulses; 3 Hz). The rank order of inhibitory potency (ID50) was prazosin greater than chlorpromazine greater than phentolamine. 3. Rauwolscine (3 x 10(-9) M-4 x 10(-6) M) enhanced the neurogenic response by up to 201%. However, higher concentrations (6 x 10(-6)-3 x 10(-5) M) reduced the contractions evoked by transmural stimulation. 4. The inhibitory effect of prazosin (10(-6) M) was reversible, while that of chlorpromazine (10(-8) M) was not. 5. Chlorpromazine (10(-8)-10(-4) M), desmethylimipramine (3 x 10(-9)-10(-5) M), cocaine (10(-7)-3 x 10(-4) M) and phentolamine (10(-5)-3 x 10(-4) M) reduced the accumulation of [3H]-noradrenaline ([3H]-NA, 10(-8) M) by aorta. The rank order of inhibitory potency (ID50) was: desmethylimipramine greater than chlorpromazine greater than cocaine greater than phentolamine. Prazosin (10(-7)-10(-5) M) and rauwolscine (10(-8)-10(-4) M) did not reduce [3H]-NA accumulation. 6. Chlorpromazine (10(-8)-10(-6) M) and prazosin (3 x 10(-9)-10(-7) M) antagonized the contractions of aorta evoked by exogenous noradrenaline (10(-9)-3 x 10(-4) M) and phenylephrine (10(-9)-3 x 10(-3) M). The pA2 values for chlorpromazine on the alpha 1-adrenoceptors were 8.24 (noradrenaline) and 8.27 (phenylephrine). The corresponding values for prazosin were 8.64 and 8.57, respectively. 7. It is concluded that chlorpromazine and prazosin are potent inhibitors of postsynaptic alpha 1-adrenoceptors. Chlorpromazine and phentolamine, unlike prazosin and rauwolscine, are also inhibitors of Uptake.

Theoretical and functional studies on alpha 1-and alpha 2-adrenoreceptors: an examination using the Schild plot

1 The influence on the shape of the Schild plot by a two-receptor system was studied in both functional and theoretical studies. In the functional studies, the alpha-adrenoreceptors in cat lingual arteries were studied since both alpha 1- and alpha 2-adrenoreceptors have been suggested to contribute to the noradrenaline-induced contractile response in this tissue. 2 The Schild plots constructed using noradrenaline (NA) as agonist and prazosin as antagonist gave a straight line with a slope close to unity. In contrast, the corresponding Schild plot constructed for rauwolscine appeared to be biphasic. 3 The results obtained in the functional study were discussed in view of Schild plots obtained from a theoretical model which was designed to take into consideration the presence of two distinct types of receptor, the percentile proportions of which could be altered. 4 The theoretical model indicates that the shape of the Schild plot can vary considerably depending on the relative contribution of each receptor subtype to the contractile response, the selectivity of the antagonist, and the range and number of antagonist concentrations used. 5 It is suggested that a response is predominantly mediated by alpha 1-adrenoreceptors and less by alpha 2-adrenoreceptors when the Schild plot for prazosin gives a slope close to unity and has a pA2-value representative for alpha 1-adrenoreceptors, and rauwolscine gives a slope less than unity and a significant shift of the NA concentration-response curve in concentrations around 10(-8) M. When the reverse is true (but with a significant shift caused by prazosin already at a concentration around 10(-9) M) the contraction elicited by NA is proposed to be predominantly mediated by alpha 2-adrenoreceptors and less by alpha 1-adrenoreceptors.

Postjunctional alpha 2-adrenoceptors mediate venoconstriction in the hindquarters circulation of anaesthetized cats

1 A study was made of the subtypes of postjunctional alpha-adrenoceptors which mediate arterial and venous constriction in the hindquarters circulation of anaesthetized cats, as measured by changes in perfusion pressure and vena cava blood flow, respectively. 2 It was found that, while noradrenaline caused constriction in both the arterial and venous compartments, methoxamine caused only arterial constriction. Clonidine and B-HT 920 also caused arterial and venous constriction although autodesensitization to both drugs occurred. 3 The ability of either prazosin or yohimbine to antagonize the constrictor effects of noradrenaline was also examined. It was found that the combination of both antagonist drugs abolished both the arterial and venous constrictor effects of noradrenaline. However, there was a greater prazosin-resistant response to noradrenaline in the venous compartment as compared with the arterial effects of noradrenaline. Yohimbine caused approximately equal reductions in the effect of noradrenaline in both arteries and veins, which was greater than that observed with prazosin. 4 These results suggest that, in the cat hindquarters, both alpha 1- and alpha 2-adrenoceptors are present in the arterial circulation, whereas there are mainly alpha 2-adrenoceptors in the venous circulation.

Comparative effects of some calcium-channel blockers on human peripheral arteries and veins

We investigated the effects of five different calcium-channel blockers (CCBs), verapamil, nifedipine, diltiazem, flunarizine and lidoflazine, on contractions evoked in vitro by noradrenaline (NA) in small human arteries and veins from the epigastric region. Vessels were obtained from patients without obvious vascular diseases undergoing surgery because of inguinal hernias. The human superficial epigastric artery has previously been shown to contain mainly alpha 1-adrenoceptors, whereas in the vein alpha 2-adrenoceptors predominate. In experiments where NA (10(-5) M) was added non-cumulatively, it was found that nifedipine was the most potent relaxant agent in both arteries and veins, but that this drug showed no preference for any type of vessel. In contrast verapamil (10(-6) M) and (10(-5) M) diltiazem, flunarizine and lidoflazine inhibited the NA-induced contractions to a significantly greater extent in the arteries than in the veins. Comparison between diltiazem and nifedipine on contractions induced by cumulative addition to NA showed that both drugs had significantly more depressive effects on arteries than on veins if the vessels were contracted by relatively high concentrations of NA (10(-6) and 10(-5) M). The results thus confirm the clinical finding that CCBs have more pronounced effects on the arterial than on the venous side of the circulation. They do not support the view that CCBs are more effective inhibitors of alpha 2- than alpha 1-adrenoceptor mediated contraction in isolated human blood vessels.

Characterization of post-junctional alpha-adrenoceptors in the rat isolated perfused femoral artery

A comparison was made of contractile responses to alpha-adrenoceptor agonists in the rat aorta and in the rat isolated perfused femoral artery. Dose-response curves were constructed to noradrenaline (alpha 1/alpha 2), methoxamine (alpha 1-selective) and B-HT 920 (alpha 2-selective). Methoxamine behaved as a full agonist in both tissues as compared with noradrenaline, while B-HT 920 was only a partial agonist in the aorta and produced small responses in the femoral artery preparation which were not dose-dependent. pA2 or -log KB values were calculated for prazosin and idazoxan against noradrenaline and methoxamine. Similar -log KB values for prazosin against both agonists were obtained in both tissues, while idazoxan was approximately ten times more potent in the femoral artery preparation than in the aorta. These results suggest that the aorta contains a single population of alpha 1-adrenoceptors, while the perfused femoral artery preparation contains predominantly alpha 1-adrenoceptors but also a small population of alpha 2-adrenoceptors.

Nifedipine vasodilates human forearm arteries and dorsal hand veins constricted by specific alpha-adrenoceptor stimulation

1. The local dilative effect of the calcium entry blocker nifedipine on forearm arteries and dorsal hand veins has been studied in 27 healthy male volunteers. 2. Nifedipine induced an increase of blood flow by 1190% (P less than 0.001) in the forearm. 3. The construction of the hand veins induced by stimulation of either postsynaptic alpha 1- or alpha 2-adrenoceptors was reduced (P less than 0.001) by nifedipine. 4. The calcium entry blocker nifedipine is a potent dilator of human forearm arteries as well as of dorsal hand veins.

Postjunctional alpha-adrenoceptors in human superficial epigastric arteries and veins

A pharmacological characterization of the postjunctional alpha-adrenoceptors in human superficial epigastric artery and vein was performed, using several alpha-adrenoceptor subtype selective agonists, and the antagonists prazosin (alpha 1) and rauwolscine (alpha 2). In the arteries prazosin fulfilled the criteria for a competitive antagonism in concentrations 10(-9)-10(-7) M, giving a pA2-value of 9.17 in the Schild plot. Rauwolscine in concentrations 10(-8)-10(-6) M caused less pronounced but significant dextral shifts of the noradrenaline (NA) concentration-response curves. In the veins rauwolscine behaved like a competitive antagonist (10(-8)-10(-7) M). The pA2-value was 9.16. Prazosin 10(-9) M displaced the NA concentration-response curve, but higher concentrations (10(-8) and 10(-7) M) caused no further displacement. Prazosin reduced the Emax-values in the veins. In the arteries the rank order of potency for the agonists was: cirazoline (alpha 1) greater than NA greater than naphazoline (alpha 2) greater than guanfacine (alpha 2) greater than phenylephrine (alpha 1). The intrinsic activities of clonidine (alpha 2), ST 587 (alpha 1), B-HT 920 (alpha 2) and B-HT 933 (alpha 2) were too low to allow meaningful comparisons to be made. The rank order of potency in the veins was: NA greater than clonidine (alpha 2) greater than naphazoline (alpha 2) greater than guanfacine (alpha 2) greater than phenylephrine (alpha 1) greater than B-HT 920 (alpha 2) greater than cirazoline (alpha 1) greater than B-HT 933 (alpha 2). The intrinsic activity of ST 587 was low.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of adrenoceptor mechanisms in isolated guinea-pig uterine arteries

The adrenoceptors of the guinea-pig uterine artery were characterized pharmacologically. Circular segments of the artery, approximately 2 mm long, and with an external diameter of 250 micron, were mounted in miniaturized tissue baths. Noradrenaline, methoxamine and phenylephrine (concentrations ranging from 10 nM to 1 mM), in the presence of propranolol (0.1 microM) and cocaine (1 microM), induced concentration-dependent contractions of the arterial segments. Clonidine (10 nM to 0.1 mM) was less effective in producing contraction of the vessel. Prazosin (10 nM to 1 microM) antagonized noradrenaline-induced contractions; its pA2 was 7.68. Rauwolscine (10 nM to 1 microM) had no effect on noradrenaline-induced contractions. Isoprenaline (10 nM to 0.1 mM) in the presence of prazosin (1 microM) and cocaine (1 microM) had no relaxant effect on arteries contracted submaximally by prostaglandin F2 alpha (5 microM). Cocaine or normetanephrine treatment did not influence the noradrenaline-induced contractions. It is concluded that in guinea-pig uterine arteries, amine-induced contractions are mediated predominantly by alpha 1-adrenoceptors and that in this arterial preparation, relaxant beta-adrenoceptor effects and neuronal or extraneuronal uptake are of minor if any importance.

Postsynaptic alpha 1- and alpha 2-adrenergic receptors in adrenergic control of capacitance vessel tone in vivo

The involvement of postsynaptic alpha 2-adrenergic receptors in the adrenergic constriction of the capacitance vessels was studied in anesthetized, spontaneously breathing dogs under ganglionic blockade (hexamethonium, 10 mg/kg + 10 mg/kg/hr; methylatropine, 0.5 mg/kg). Effective vascular compliance was measured as an indicator of venous tone (blood volume was varied by +/- 4 ml/kg in an 11-minute cycle of infusion, withdrawal, withdrawal, and reinfusion) and was calculated from the correlation between the observed changes in central venous pressure and the changes in blood volume. Sympathetic activity and central venous pressure were lower and effective vascular compliance was higher than values in untreated conscious dogs. The alpha 2-agonist UK 14,304 (5-bromo-6-[imidazolin-2-ylamino]-quinoxaline; 0.04 and 0.12 micrograms/kg/min; n = 6) dose-dependently lowered compliance and increased central venous pressure to levels found in conscious dogs, as did the alpha 1-agonist methoxamine (10 and 30 micrograms/kg; n = 6). Rauwolscine (alpha 2-antagonist), 0.3 mg/kg, significantly attenuated the effects of UK 14,304, but not those of methoxamine, while prazosin (alpha 1-antagonist), 0.12 mg/kg, attenuated the effects of methoxamine, but not those of UK 14,304 (n = 6 each). Under beta-blockade (nadolol, 2 mg/kg; n = 12) venous tone was increased to about physiological levels by norepinephrine, 0.15 micrograms/kg/min i.v., or by neuronal norepinephrine release induced by tyramine, 10 micrograms/kg/min i.v. These increases were significantly attenuated by prazosin as well as by rauwolscine and were abolished by a combination of both. These results indicate that postsynaptic alpha 2-adrenergic receptors (in addition to alpha 1-adrenergic receptors) are functional in the venous system in vivo and contribute substantially to adrenergic sympathetic and humoral regulation of venous tone.

Contractile effects of noradrenaline and 5-hydroxytryptamine in human hand veins: a pharmacological receptor characterization

The postjunctional receptors mediating contractile responses to noradrenaline (NA) and 5-hydroxytryptamine (5-HT) were characterized in ring segments of human hand veins by using subtype selective agonists and antagonists. The mechanical characteristics of the preparations were also examined by length-tension measurements. The length-active wall tension curve was bell-shaped and reached a maximum at a length corresponding to a passive distending pressure of approximately 14 mmHg. (-)-Phenylephrine consistently contracted the veins and was 24 times less potent than (+/-)-NA whereas clonidine produced a contraction in only two out of 11 vessel segments. Neither prazosin nor rauwolscine competitively inhibited the contractile response to NA, and large inter-individual differences were found in the degree of inhibition produced by the antagonists. However, application of both prazosin and rauwolscine almost abolished the NA-induced contraction. Ketanserin and methergoline inhibited the contractile response to 5-HT; the former in an apparently competitive manner with a pA2 value of 8.94, whereas the latter substantially reduced the maximum 5-HT response. It is suggested that NA elicits contraction in human hand veins by acting at a mixed population of alpha 1- and alpha 2-adrenoreceptors. The contractile response to 5-HT, on the other hand, appears to be mediated predominantly by 5-HT2 receptors.

Vascular responses of isolated, perfused canine femoral arteries and veins to vasoactive substances

The stainless steel cannula inserting method was used to observe vascular effects of 9 vasoactive substances on the isolated, perfused canine femoral artery and vein. In the vein, the order of potency for inducing vasoconstriction was norepinephrine greater than histamine = phenylephrine = serotonin greater than prostaglandin F2 alpha greater than ATP greater than xylazine greater than tyramine much greater than KCl. It was demonstrated that the cannula inserting method is useful for observations of venous vascular reactivity.

A study on the postjunctional excitatory alpha-adrenoreceptor subtypes in the mesenteric arterial bed of the rat

The nature of the postsynaptic adrenoreceptor subtypes which mediate vasoconstriction in the mesenteric arterial bed of the rat was investigated using mixed and selective alpha 1, alpha 2 and beta-agonists and antagonists. Phenylephrine (PE) an alpha 1-selective agonist and noradrenaline (NA) a mixed alpha1 and alpha 2-agonist, produced a rise in perfusion pressure (vasoconstriction). The responses to NA remained stable with time whereas responses to PE considerably increased. UK14304 an alpha 2-selective agonist at low doses (10(-8)-10(-7) moles), caused small, slow contractions in most preparations. Repeated administration of these doses or slightly higher ones, densensitized the tissue to this compound but not to NA or PE. Finally, UK14304 given simultaneously with NA or PE, at doses higher than 5 X 10(-7) moles, reduced contractions to the latter compounds and this effect was not altered by 10(-7) M rauwolscine, an alpha 2-selective antagonist. Prazosin, an alpha1-selective antagonist, as expected, reduced contractions to NA considerably at 10(-10)-10(-8) M and abolished contractions to UK14304 at 2 X 10(-9) M. Rauwolscine, at 10(-8) M, potentiated contractions to NA and at 10(-6) M reduced contractions to both NA and PE (when compared to time controls). When propranolol (10(-6) M), a beta-antagonist was included in the perfusion fluid, rauwolscine no longer potentiated responses to NA but reduced them at all concentrations. Under the same conditions rauwolscine affected the responses to PE in a similar direction to that observed in the absence of propranolol. These results suggest that in the rat mesenteric arterial bed: rauwolscine exerts an effect additional to alpha2-adrenoreceptor antagonism.(ABSTRACT TRUNCATED AT 250 WORDS)

Animal pharmacology of guanfacine

The pharmacologic data obtained from animal experiments with guanfacine, a novel, centrally acting antihypertensive agent, are reviewed. When given orally, guanfacine lowers systemic blood pressure in conscious DOCA-NaCl-hypertensive rats, Grollman rats and spontaneously hypertensive rats in a dose-dependent manner. It is also effective in renal hypertensive cats. Guanfacine reduces blood pressure in cats, rabbits and rats after injection into the lateral cerebral ventricle and in dogs after infusion into the vertebral artery at intravenously ineffective doses. Vagally mediated reflex bradycardia in dogs is enhanced. The preganglionic splanchnic (sympathetic) nerve activity is reduced in cats. In rats, guanfacine reduces the noradrenaline turnover in the brain stem. All these findings indicate a central site of action. Peripheral alpha-adrenoceptor stimulant properties of guanfacine have been demonstrated in various studies. In addition to postsynaptic stimulant effects, presynaptic guanfacine-induced inhibition of sympathetic heart nerve stimulation is antagonized by rauwolscine but not by prazosin, indicating a highly preferential alpha 2-agonistic presynaptic action of the drug. In receptor binding studies using rat cortex membranes and human platelets, guanfacine exhibited a high selectivity for alpha 2 adrenoceptors. Guanfacine has the advantage over other centrally acting antihypertensives of being less sedative and causing no rebound hypertension after discontinuation of treatment. The latter is mainly due to its pharmaco-kinetic properties.

Management of chronic orthostatic hypotension

Chronic orthostatic hypotension is characterized by recurrent symptoms of cerebral hypoperfusion due to low upright blood pressure levels. The initial approach should be to identify and correct reversible causes. Persistence of orthostatic hypotension suggests autonomic failure. The goal of management is to minimize symptoms and maximize functional capacity; therefore the magnitude of blood pressure fall is not as important as the advent of symptoms. Therapy is based upon the underlying pathophysiology and the risk/benefit ratio of interventions. Patient education and nondrug measures form the cornerstone of management. Drug therapy is often limited by unacceptable supine hypertension. Rational drug use can be governed by individualized trials of therapy. Patients with moderate or severe orthostatic hypotension are difficult to treat, but can be helped toward resumption of a normal life.

Effects of alpha-adrenoceptor subtype-selective antagonists on the human saphenous vein in vivo

The effects of the alpha-adrenoceptor subtype-selective antagonists prazosin (alpha 1) and yohimbine (alpha 2) on the saphenous vein of six healthy male subjects were investigated in vivo. The drugs were infused locally into the congested (40 mmHg), long saphenous vein constricted by simultaneous local infusion of noradrenaline (NA). Prazosin 10(-9) M (concentration in the infusion solution, infusion rate 0.3 ml min-1) did not reduce the NA-induced venoconstriction, but at a concentration of 10(-8) M there was a significant reduction; in two subjects no response to NA could be elicited in the presence of 10(-8) M prazosin. Prazosin 10(-7) M caused no further reduction of the NA effect compared to that produced by 10(-8) M in three of the subjects, whereas in one, prazosin 10(-9), 10(-8) and 10(-7) M caused a dose-dependent blockade. Yohimbine, 10(-9), 10(-8) and 10(-7) M caused a dose-dependent reduction of the NA-induced venoconstriction in all subjects. The results suggest that the human saphenous vein is endowed with functionally important populations of both alpha 2- and alpha 1-adrenoceptors.

Innervation of human omental arteries and veins and vasomotor response to noradrenaline, neuropeptide Y, substance P and vasoactive intestinal peptide

Human omental arteries and veins are supplied with nerve fibers containing noradrenaline (NA) and neuropeptide Y (NPY); these two agents probably co-exist in perivascular sympathetic nerve fibers. Substance P (SP)- or vasoactive intestinal peptide (VIP)-containing fibers could not be detected. In studies on isolated omental vessels NA produced constriction. The results of blockade experiments suggest that human omental arteries are equipped predominantly with alpha 1-adrenoceptors and omental veins with a mixture of alpha 1- and alpha 2-adrenoceptors. NPY at a concentration of 10(-7) M or higher had a weak contractile effect on veins and virtually no effect on arteries. NPY at a concentration of 3 X 10(-8) M shifted the NA concentration response curve to the left in arteries (pD2 = 5.8 for NA versus 6.6. for NA in the presence of NPY; P less than 0.001) but not in veins. Both SP and VIP relaxed arteries precontracted with NA or prostaglandin F2 alpha (PGF2 alpha). The potency of SP as a relaxant agent was similar in arteries and veins; the effect of VIP was elicited at lower concentrations in veins than in arteries.

Preferential antagonism by diltiazem of alpha 2-adrenoceptor mediated vasoconstrictor responses in perfused tail arteries of spontaneous hypertensive rats

Vasoconstrictor responses mediated by the alpha 2-adrenoceptor agonist TL99, were particularly sensitive to blockade by the calcium antagonist drug diltiazem in isolated perfused tail arteries of spontaneously hypertensive rats (SHR). In contrast, the vasoconstrictor responses induced by the alpha 1-adrenoceptor agonist methoxamine were significantly more resistant to antagonism by diltiazem. At higher concentrations (greater than 300 nmol/l) diltiazem became an effective antagonist of all alpha-adrenoceptor mediated responses. In normotensive Wistar Kyoto (WKY) or Sprague-Dawley (SD) rats diltiazem was significantly less potent against vasoconstrictor responses to TL99 than in SHR. The blockade of alpha 1-adrenoceptor mediated vasoconstriction by diltiazem was not significantly different when normotensive rats and SHR were compared. The vasoconstrictor responses evoked by 5HT in the perfused tail arteries were particularly resistant to blockade by diltiazem in SHR arteries. The responses to endogenously released noradrenaline, evoked by electrical field stimulation, were significantly antagonised by diltiazem (30 nmol/1-3 mumol/l) in SHR-tail arteries, while they were not modified in WKY-tail arteries. At the concentrations of diltiazem which blocked end organ responses to field stimulation, there was no modification of total tritium overflow from SHR-tail arteries after labelling the tissue with 3H-noradrenaline, indicating that diltiazem does not inhibit transmitter release at these concentrations. The tail artery preparation of SHR contains a population of postsynaptic alpha 2-adrenoceptors which mediate contraction in this blood vessel and the calcium entry blocker diltiazem is a potent antagonist of vasoconstrictor responses mediated by vascular alpha 2-adrenoceptors in hypertensive rats. These findings may be relevant to the antihypertensive action of diltiazem.

Evidence for neuro-effector transmission through postjunctional alpha 2-adrenoceptors in human saphenous vein

The effects of the alpha 1-adrenoceptor antagonist prazosin and the alpha 2-adrenoceptor antagonist yohimbine were examined against stimulation-evoked contractions in human isolated saphenous veins. The concentration of yohimbine producing 30% inhibition of stimulation-evoked contractions (IC30) was 13.2 nM, whereas the IC30 of prazosin was greater than 250 nM. The inhibition of stimulation-evoked contractions by yohimbine was not prejunctionally mediated since yohimbine (0.01-0.1 microM) significantly potentiated the stimulation-evoked overflow of tritium in tissues pre-incubated with [3H]-noradrenaline. The high potency of yohimbine and the low potency of prazosin indicate that neuro-effector transmission in human saphenous vein is mediated predominantly by postjunctional alpha 2-adrenoceptors.

Contraction-mediating alpha-adrenoreceptors in isolated human omental, temporal and pial arteries

The alpha-adrenoreceptors mediating contraction in human omental (OA), temporal (TA), and pial (PA) arteries obtained during surgery, were characterized by means of subtype selective agonists and antagonists. In the OAs and TAs, prazosin concentration-dependently shifted the noradrenaline (NA) concentration-response (cr) curve towards higher concentrations, without depression of maximum. The corresponding Schild plots had slopes close to unity. Also rauwolscine caused a rightward displacement of the NA cr-curve in both OAs and TAs, without affecting the maximum response. In the TAs, OAs, rauwolscine 3 x 10(-8) M shifted the curve and the Schild plot seemed to be biphasic. Oxymetazoline, but not clonidine, produced contractile responses in the TAs and OAs, and phenylephrine was a full agonist in both types of vessel. The PAs showed a pronounced inter- and intra-individual variation in the response to NA, and often exhibited spontaneous activity. Prazosin was considerably more effective than rauwolscine and yohimbine to inhibit NA-induced responses. Clonidine had no contractant effect, whereas, oxymetazoline was more, and phenylephrine less potent than NA. It is concluded that in human OAs, TAs and PAs, the alpha-adrenoreceptor mediating contraction is mainly of the alpha 1-type.

Postsynaptic alpha-adrenoreceptor populations in several vascular systems of the anaesthetized rat

The contribution of postsynaptic alpha 1- and alpha 2-adrenoreceptors to vasoconstrictor responses was investigated in several vascular systems of pentobarbital-anaesthetized rats pretreated with atropine and propranolol. In the intact circulatory system of the anaesthetized rat, pressor responses were obtained to noradrenaline and phenylephrine. The pressor responses to noradrenaline were only partially blocked by prazosin and the responses which remained after prazosin were significantly reduced further by the subsequent addition of yohimbine. However, the responses to phenylephrine were largely antagonized by prazosin alone. In the blood-perfused hindquarter of the anaesthetized rat, a differential blocking activity of prazosin against noradrenaline and phenylephrine was also demonstrated. Prazosin, as observed in the intact circulatory system of the anaesthetized rat, was a more potent antagonist against phenylephrine than against noradrenaline. In the blood-perfused mesentery of the anaesthetized rat, sympathetic nerve stimulation, noradrenaline and phenylephrine produced a marked vasoconstrictor response whilst B-HT 920 hardly induced a pressor response. The pressor responses to nerve stimulation, noradrenaline and phenylephrine were largely blocked by prazosin alone. However, only the responses to all frequencies of nerve stimulation were enhanced by yohimbine pretreatment. These results obtained from the intact animal and blood-perfused hindquarter indicate that the pressor responses to exogenous noradrenaline result from the activation of both postsynaptic alpha 1- and alpha 2- adrenoreceptors. However, the result obtained from the blood-perfused mesentery indicates that the vasoconstrictor responses to neuronally released noradrenaline are largely mediated by activation of postsynaptic alpha 1-adrenoreceptors. Consequently, these results suggest that, in rats, the postsynaptic alpha-adrenoreceptor population in the mesenteric resistance blood vessels differs from that in other tissues.

Postjunctional alpha 1- and alpha 2-adrenoceptors mediating contraction in isolated human groin arteries and veins

By means of selective agonists and antagonists for alpha 1- and alpha 2-receptors, the alpha-receptor subtypes in human groin arteries and veins were characterized and compared. In the arteries the alpha 1-receptor blocker prazosin caused a concentration-dependent parallel displacement of the noradrenaline (NA) concentration-response (cr) curve without reduction of maximum (pA2 = 9.86); the selective alpha 2-receptor antagonist rauwolscine in the concentration 10(-8) M caused a right-ward shift of the NA cr-curve without reduction of Emax, but 10(-7) M and 10(-6) M caused little or no further shift. In the veins, the two antagonists had the opposite effects. Rauwolscine caused a concentration-dependent right-ward shift of the NA cr-curve without depression of maximum (pA2 = 9.03); prazosin 10(-9) M significantly displaced the NA cr-curve, whereas 10(-8) M and 10(-7) M caused little or no further shift. The responses to the alpha 2-receptor agonist clonidine in the arteries were too small to allow calculations of pEC50 values; in the veins contractions were elicited in all vessel segments investigated (pEC50 = 6.24). Phenylephrine, selective for alpha 1-receptors, was significantly more potent in arteries than in veins. NA was significantly more potent in veins than in arteries. It is concluded that in human groin vessels, there is a functional predominance of alpha 1-receptors in the arteries and of alpha 2-receptors in the veins.

The postjunctional alpha-adrenoceptors of the human saphenous vein

The postjunctional alpha-adrenoceptors in human long saphenous vein were characterized using alpha-adrenoceptor subtype selective agonists and antagonists. The order of potency for the alpha-adrenoceptor agonists used was: Clonidine (alpha 2) greater than BHT-920 (alpha 2) greater than naphazoline (alpha 2) greater than guanfacine (alpha 2) greater than cirazoline (alpha 1) greater than phenylephrine (alpha 1) greater than ST 587 (alpha 1) greater than BHT-933 (alpha 2). Clonidine had the same potency as noradrenaline (NA), but was 52 times more potent than phenylephrine (pEC50 7.09 and 5.37, respectively). Phenylephrine and guanfacine had intrinsic activities that did not differ from that of NA, whereas the intrinsic activities of the other agonists were significantly lower. The highly selective alpha 1-adrenoceptor antagonist prazosin in concentrations 10(-9)-10(-7) M was unable to cause a significant shift of the NA concentration response (cr) curve to the right. However, the alpha 2-adrenoceptor antagonists yohimbine and rauwolscine in concentrations 10(-8)-10(-6) M shifted the NA cr-curve towards higher concentrations. No concentration of any antagonist used significantly attenuated the maximum contraction. The slope of the regression line in the Schild plot differed significantly from unity for rauwolscine but not for yohimbine, and the pA2-values were 9.00 and 8.27, respectively. These results suggest that the contraction mediating alpha-adrenoceptors in the human saphenous vein are mainly of the alpha 2-type. However, the low slope value of the Schild plot for rauwolscine may indicate the presence of a small population of alpha 1-adrenoceptors.

Alpha-adrenoceptors, 5-hydroxytryptamine receptors and the action of dihydroergotamine in human venous preparations obtained during saphenectomy procedures for varicose veins

Changes in tension were monitored isometrically on spiral strips from human saphenous veins obtained during surgical removal of varicose veins. Concentration-response curves for noradrenaline and 5-hydroxytryptamine (5-HT) were established by cumulative administrations, curves for dihydroergotamine were constructed from the mean responses to single concentrations. The use of the antagonists prazosin, yohimbine and pizotifen provided evidence for the existence of both postjunctional alpha 1- and alpha 2-adrenoceptors and for the existence of 5-HT receptors. The venoconstrictor effects of dihydroergotamine were unchanged by prazosin. Yohimbine antagonized both dihydroergotamine and 5-HT at about 60 times higher concentrations than required against noradrenaline whereas pizotifen inhibited responses to both dihydroergotamine and 5-HT at about 100 times lower concentrations than those to noradrenaline. These new results are in contrast to conclusions drawn from animal studies and do not support the suggestion that in man the venoconstrictor activity of dihydroergotamine is mediated through stimulation of alpha-adrenoceptors. The present results strongly suggest that in human saphenous veins the constrictor activity of dihydroergotamine is mediated at least in part through stimulation of 5-HT receptors.

Effects of peripheral alpha 1- and alpha 2-adrenoceptor agonists on baroreceptor responsiveness in conscious dogs

Baroreceptor responsiveness was investigated in conscious dogs following increasing doses (i.v.) of the selective alpha-adrenoceptor agonists methoxamine (alpha 1) and oxymetazoline (alpha 2), in the presence and absence of beta-adrenoceptor blockade. The study was repeated in another group of dogs with background afferent baroreceptor nerve activity reduced by continuous infusion of sodium nitroprusside. Both agonists dose dependently increased mean arterial pressure and reflexly decreased heart rate. In dogs pretreated with a beta-adrenoceptor antagonist a correlation between increase in mean arterial pressure (increase up to 70 mmHg) and increase in heart period (baroreceptor responsiveness) indicated no difference in the regression lines between methoxamine and oxymetazoline for both the normotensive and the sodium nitroprusside groups. However, in the dogs not pretreated with a beta-adrenoceptor antagonist the slope of the regression line for oxymetazoline was steeper than that for methoxamine (P less than 0.01) in the normotensive group. In the sodium nitroprusside group the regression line for oxymetazoline was situated significantly to the left of the methoxamine line (P less than 0.05). It is suggested that this greater bradycardic response to the alpha 2-adrenoceptor agonist oxymetazoline was caused by suppression of the cardiac sympathetic component (presynaptic modulation of noradrenaline release) in addition to the vagal activation and the sympathetic withdrawal component of the reflex. This indicates that drugs with alpha 2-adrenoceptor agonistic activity can influence a reflex physiological situation under conditions of low sympathetic nerve activity.