Dopamine decreases mesenteric blood flow in the anaesthetised dog through the stimulation of postsynaptic alpha 2-adrenoceptors

Role of dopamine and selective dopamine receptor agonists on mouse ductus arteriosus tone and responsiveness

BACKGROUND

Indomethacin treatment for patent ductus arteriosus (PDA) is associated with acute kidney injury (AKI). Fenoldopam, a dopamine (DA) DA₁-like receptor agonist dilates the renal vasculature and may preserve renal function during indomethacin treatment. However, limited information exists on DA receptor-mediated signaling in the ductus and fenoldopam may prevent ductus closure given its vasodilatory nature.

METHODS

DA receptor expression in CD-1 mouse vessels was analyzed by qPCR and immunohistochemistry. Concentration-response curves were established using pressure myography. Pretreatment with SCH23390 (DA₁-like receptor antagonist), phentolamine (α -adrenergic receptor antagonist) or indomethacin addressed mechanisms for DA-induced changes. Fenoldopam's effects on postnatal ductus closure were evaluated in vivo.

RESULTS

DA₁ receptors were expressed equally in ductus and aorta. High-dose DA induced modest vasoconstriction under newborn O₂ conditions. Phentolamine inhibited DA-induced constriction, while SCH23390 augmented constriction, consistent with a vasodilatory role for DA₁ receptors. Despite this, fenoldopam had little effect on ductus tone nor indomethacin- or O₂-induced constriction and did not impair postnatal closure in vivo.

CONCLUSION(S)

DA receptors are present in the ductus but have limited physiologic effects. DA-induced ductus vasoconstriction is mediated via α-adrenergic pathways. The absence of DA₁-mediated impairment of ductus closure supports the study of potential role for fenoldopam during PDA treatment.

Intrinsic vascular dopamine - a key modulator of hypoxia-induced vasodilatation in splanchnic vessels

Dopamine not only is a precursor of the catecholamines noradrenaline and adrenaline but also serves as an independent neurotransmitter and paracrine hormone. It plays an important role in the pathogenesis of hypertension and is a potent vasodilator in many mammalian systemic arteries, strongly suggesting an endogenous source of dopamine in the vascular wall. Here we demonstrated dopamine, noradrenaline and adrenaline in rat aorta and superior mesenteric arteries (SMA) by radioimmunoassay. Chemical sympathectomy with 6-hydroxydopamine showed a significant reduction of noradrenaline and adrenaline, while dopamine levels remained unaffected. Isolated endothelial cells were able to synthesize and release dopamine upon cAMP stimulation. Consistent with these data, mRNAs coding for catecholamine synthesizing enzymes, i.e. tyrosine hydroxylase (TH), aromatic l-amino acid decarboxylase, and dopamine-β-hydroxylase were detected by RT-PCR in cultured endothelial cells from SMA. TH protein was detected by immunohistochemisty and Western blot. Exposure of endothelial cells to hypoxia (1% O2) increased TH mRNA. Vascular smooth muscle cells partially expressed catecholaminergic traits. A physiological role of endogenous vascular dopamine was shown in SMA, where D1 dopamine receptor blockade abrogated hypoxic vasodilatation. Experiments on SMA with endothelial denudation revealed a significant contribution of the endothelium, although subendothelial dopamine release dominated. From these results we conclude that endothelial cells and cells of the underlying vascular wall synthesize and release dopamine in an oxygen-regulated manner. In the splanchnic vasculature, this intrinsic non-neuronal dopamine is the dominating vasodilator released upon lowering of oxygen tension.

The Variable Effects of Dopamine Among Human Isolated Arteries Commonly Used for Coronary Bypass Grafts

The direct actions of dopamine on human arterial coronary bypass grafts are not well known. We investigated its effects on isolated rings cut from radial arteries (RA), gastroepiploic arteries (GEA), and internal mammary arteries (IMA) harvested from patients undergoing coronary artery bypass surgery. Dopamine produced dose-dependent contractile responses in RA, an effect independent of the presence of a functional endothelium. The contractions were enhanced by the dopamine A(1) (DA(1))-receptor antagonist SCH23390, whereas they were blocked by an alpha(1)-adrenergic antagonist, prazosin. Results qualitatively similar to these were obtained in both GEA and IMA, although the contractile responses were far smaller. In RA, DA enhanced the norepinephrine (NE)-induced contraction, and this action of dopamine was enhanced by SCH23390. In GEA, small concentrations (<10(-7) mol/L) of DA attenuated the NE-induced contraction but larger concentrations did not. In IMA, DA induced a vasorelaxation on the NE-contraction only at higher concentrations (10(-6)-10(-5) mol/L). In both GEA and IMA, the dopamine-induced vasorelaxations on the NE contraction were completely inhibited by SCH23390. These results suggest that the affinities of DA for DA(1)- and alpha(1)-adrenergic receptors may explain its variable contractile and vasorelaxant effects among these arteries.

IMPLICATIONS

Differing affinities of dopamine for dopamine A(1)- and alpha(1)-adrenergic receptors may lead to it having variable contractile and vasorelaxant effects among the arteries supplying grafts for coronary bypass surgery.

Effects of dopamine infusion on cardiac and renal blood flows in dogs

In veterinary medicine, dopamine is currently being administered clinically by infusion for treatment of kidney disorders at low doses (< or = 3 microg/kg/min) and for assessment of hemodynamics at high doses (> or = 5 microg/kg/min). However, since high doses of dopamine cause peripheral vasoconstriction due to its effect on alpha adrenoceptors, high doses have no longer been recommended. The present study was conducted to explore possible regimens for the use of dopamine infusion in dogs. The regional (renal and cardiac) blood flow for 60 min was measured by using colored microspheres at three doses (3, 10 and 20 microg/kg/min) of dopamine infusion in healthy anesthetized mongrel dogs. The effects on kidney and peripheral hemodynamics at each dose and the resultant cardiac output, mean arterial blood pressure and total peripheral resistance were determined. Renal blood flow increased markedly at 3 microg/kg/min dopamine. Improvement in hemodynamics indicated by marked increase in cardiac blood flow, cardiac output and mean arterial blood pressure and decreased total peripheral resistance was observed at higher doses (10 and 20 microg/kg/min). At 10 microg/kg/min, in addition to the satisfactory increase in cardiac blood flow, there was also a stable satisfactory increase in renal blood flow. However, at 20 microg/kg/min, increased myocardial oxygen consumption (manifested by marked increased in cardiac output), arrythmia and irregular increase in renal blood flow were detected. This study suggests that the clinical use of dopamine infusion in dogs could be safely expanded to moderately higher doses.

Inotropic treatment and intestinal mucosal tissue oxygenation in a model of porcine endotoxemia

OBJECTIVE

To evaluate the dose-related effects of dopamine, dopexamine, and dobutamine on intestinal mucosal tissue oxygenation following short-time infusion of Escherichia coli lipopolysaccharide, which has previously been shown to decrease mucosal tissue oxygenation by 60% of control values.

DESIGN

Prospective, randomized, unblinded study.

SETTING

Animal research laboratory.

SUBJECTS

Anesthetized, mechanically ventilated domestic pigs.

INTERVENTIONS

Pigs were infused with 2 microg/kg of E. coli lipopolysaccharide over 20 mins via the superior mesenteric artery. Pulmonary artery occlusion pressure was maintained near 15 mm Hg, using a mixed infusion regimen of Ringer's lactate solution and hydroxyethyl starch. Following endotoxemia, a small segment of the jejunal mucosa was exposed by midline laparotomy and antimesenteric incision. The control group (n = 7) received no further interventions. Pigs in the dopamine (n = 7), dopexamine (n = 7), and dobutamine (n = 7) groups were infused with 2.5, 5, 10, and 20 microg/kg/min of the respective drug via a central venous catheter.

MEASUREMENTS AND MAIN RESULTS

Systemic hemodynamics as well as systemic, mesenteric, and femoral blood gas variables were measured using an arterial, a thermodilution pulmonary artery, a superior mesenteric venous, and a femoral venous catheter. Jejunal mucosal tissue PO2 was measured by means of two Clark-type surface oxygen electrodes. Oxygen saturation of jejunal mucosal microvascular hemoglobin was determined by tissue reflectance spectrophotometry. Infusion of endotoxin resulted in pulmonary hypertension. Systemic hemodynamics remained unchanged except for brief decreases in cardiac output and arterial blood pressure. Dopamine, dopexamine, and dobutamine increased systemic oxygen delivery in a dose-related manner by 80% (p < .01), 96% (p = .00), and 129% (p = .00) of values before inotropic treatment. Dopamine increased mucosal tissue PO2 by 109% (10-microg dose, p < .01) and 164% (20-microg dose, p = .00), and mucosal hemoglobin oxygen saturation by 61% (5-microg dose, p < .05), 102% (10-microg dose, p < 01) and 121% (20-microg dose, p = .00). Dopexamine increased mucosal tissue PO2 by 89% (20-microg dose, p < .01) and mucosal hemoglobin oxygen saturation by 26% (2.5-microg dose, p < .05) and 35% (5-, 10-, and 20-microg dose, p < .05). In the dobutamine and control groups, no significant effect on either mucosal tissue PO2 or hemoglobin oxygen saturation was observed.

CONCLUSIONS

In this model of porcine endotoxemia, dopamine and, to a lesser extent, dopexamine increase intestinal mucosal tissue oxygenation. Of all three inotropes used, dobutamine has the most pronounced effect on systemic oxygen delivery, but it does not improve mucosal tissue oxygenation. Selective vasodilation within the intestinal mucosa, mediated mainly by dopamine-1 receptors, seems to explain the observed intestinal mucosal effect of dopamine and dopexamine.

Dopamine-1-receptor stimulation and mucosal tissue oxygenation in the porcine jejunum

OBJECTIVE

To evaluate the effects of dopamine-1-receptor stimulation on intestinal mucosal tissue oxygenation.

DESIGN

Prospective, experimental, controlled trial.

SETTING

Animal research laboratory.

SUBJECTS

Anesthetized domestic pigs (30 to 45 kg).

INTERVENTIONS

A small segment of the jejunal mucosa and serosa was exposed by midline laparotomy and antimesenteric incision. Fenoldopam, a selective dopamine-1-receptor agonist, was infused in steps, exponentially increasing from 0.6 to 9.6 micrograms/kg/min via a central venous catheter (n = 8, fenoldopam group), whereas a second group (n = 6, saline group) was only given the solvent.

MEASUREMENTS AND MAIN RESULTS

Systemic hemodynamics as well as systemic and jejunal acid base and blood gas variables were measured using an arterial catheter, a thermodilution pulmonary artery catheter, and a jejunal venous catheter. Jejunal mucosal and serosal tissue PO2 were measured by means of Clark-type surface oxygen electrodes. Oxygen saturation and relative concentration of mucosal microvascular hemoglobin were measured by means of tissue reflectance spectrophotometry. In the fenoldopam group, systemic oxygen delivery (12.5 +/- 0.8 mL/kg/min at baseline) increased by 56% (p < .001) above baseline values. Mean arterial pressure remained unchanged. Fenoldopam produced a 51% increase in mucosal PO2 (23.8 +/- 2.8 torr [3.2 +/- 0.4 kPa] at baseline; p < .001) and a 31% increase in mucosal hemoglobin oxygen saturation (55.4 +/- 8.3% at baseline; p < .001), but not change in serosal PO2 (58 +/- 4.8 torr [7.7 +/- 0.6 kPa] at baseline).

CONCLUSIONS

Fenoldopam improves tissue oxygenation of the porcine jejunum in a dose-related manner. This effect is limited to the inner mucosal layer. Dopamine-1-receptor agonists should be evaluated in patients presenting with signs of intestinal mucosal ischemia.

Vascular responsiveness of rabbit common carotid, renal and femoral arteries to alpha-adrenoceptor agonists

The stainless steel cannula inserting method was used to observe vascular effects of alpha-adrenoceptor agonists, phenylephrine (PE), methoxamine (ME), clonidine (CL) and xylazine (XY), on the isolated, perfused rabbit common carotid, renal and femoral arteries. PE and ME induced a dose-dependent vasoconstriction that was readily suppressed by treatment with bunazosin, an alpha 1-adrenoceptor blocker. CL induced a constriction only in common carotid arteries, and this was readily suppressed by bunazosin. In preparations preconstricted by phenylephrine, CL and XY dose-dependently induced vasodilations in the 3 types of arteries, and these vasodilations were not modified by a potent alpha 2-adrenoceptor antagonist, midaglizole. In preparations preconstricted by prostaglandin F2 alpha, CL and and XY did not produce any significant vasodilation, but CL induced a vasoconstriction in common carotid arteries that was completely blocked by bunazosin. Thus, it is concluded that: 1) alpha 1-adrenoceptors are functionally predominant in rabbit peripheral arteries, 2) the alpha 2-adrenoceptor agonist-induced vasodilation may be due to an antagonistic action towards alpha 1-mediated constrictions, and 3) clonidine has alpha 1-adrenoceptor stimulating properties in rabbit common carotid artery but not in renal and femoral arteries.

Evaluation of a rat model for the study of local regulation of intestinal blood flow: ex vivo asanguineous perfusion of the ileal vascular bed

A new model of ex vivo vascularly perfused, isolated rat ileum was developed and evaluated. Segments of distal ileum (approximately 5 cm) from male Wistar rats were isolated on their vascular pedicles. Perfusion through an aortic cannula with oxygenated (95% O2, 5% CO2) Krebs solution containing 5% bovine albumin, 5.6 mM glucose, and 25 mM mannitol at 37 degrees C was initiated immediately after interruption of blood flow. The bowel preparations, including the abdominal aorta, were then transferred to a perfusion chamber. Perfusion pressure was maintained by gravity at 40 mm Hg. Flow was measured with an electromagnetic flow probe. The portal vein, together with the lymphatics, drained freely into collection tubes. The bowel lumen was perfused at 0.85 ml/min with isotonic modified Krebs solution containing [14C]polyethylene glycol, and the luminal perfusion pressure was monitored. Luminal effluents were collected through a large-bore outlet tubing. As determined by histology, O2 consumption, vascular reactivity, and mucosal permeability, the preparations were viable for at least 60 min of perfusion. With this model, a vasoconstrictor effect of the alpha 2-adrenoceptor agonist clonidine was documented for the first time in isolated rat bowel.

Adverse effect of therapeutic vasoconstrictors in experimental acute pancreatitis

Alpha-adrenergic drugs commonly are used to treat hypotension resulting from severe acute pancreatitis. It was shown previously that although systemic arterial pressure is increased by phenylephrine, pancreatic microcirculatory perfusion is decreased. Because inadequate tissue perfusion may be critical in the progression of edematous pancreatitis to parenchymal necrosis, it was hypothesized that vasoconstrictors might be harmful in pancreatitis. Therefore the effect of phenylephrine on cerulein-induced mild pancreatitis were studied. Sprague-Dawley rats (n = 54) were randomly allocated to 6 experimental groups and subjected to the following infusion regimens: (1) cerulein (cae) + phenylephrine (phe), (2) cae + saline (NS), (3) NS + phe, (4) cae + phenoxybenzamine (pbz) + phe, (5) NS + pbz + phe, and (6) NS. Initial and terminal hematocrit, serum amylase activity, and blood ionized calcium concentration were determined. The animals were killed 9 hours after starting the infusion. Macroscopic and histologic changes were scored by a 'blinded' pathologist. Phenylephrine increased the severity of cerulein-induced pancreatitis as manifested by statistically significant adverse changes in serum amylase, hematocrit, ionized calcium, peripancreatitic soap formation, and acinar cell vacuolization. These changes were antagonized by alpha-adrenergic receptor blockade with phenoxybenzamine. It is concluded that phenylephrine is deleterious in acute experimental pancreatitis, the first demonstration of such an effect by a pharmacologic vasoconstrictor, and suggested that microcirculatory changes may be important in the transition of mild to severe pancreatitis. Caution in the use of vasoconstrictor drugs in patients with acute pancreatitis is recommended.

Effects of thoracic epidural anesthesia and adrenoceptor blockade on the cardiovascular response to dopamine in the dog

The cardiovascular effects of dopamine are different before and during thoracic epidural anesthesia (TEA). To evaluate underlying adrenoceptor-mediated mechanisms, dopamine effects were investigated in nine chloralose-anesthetized dogs. The circulatory response to dopamine (0-40 micrograms.kg-1.min-1) was studied before and during TEA, and during TEA after introducing the alpha 1-antagonist prazosin (0.3 mg.kg-1), the alpha 2-antagonist rauwolscine (0.3 mg.kg-1), and the beta 1-antagonist metoprolol (0.5 mg.kg-1). TEA decreased mean arterial pressure (MAP) by 29%, cardiac output (CO) by 36%, heart rate (HR) by 27%, and the maximum rate of change of left ventricular pressure (LVdP/dt) by 52%. Systemic vascular resistance, pulmonary vascular resistance and mean pulmonary artery pressure (MPAP) remained unaltered by TEA. Dopamine-induced increases in MAP and HR were augmented by TEA. Both MAP and LVdP/dt increased above pre-TEA levels at 10 micrograms.kg-1.min-1. Prazosin attenuated the increases in MAP and MPAP by dopamine. Adding rauwolscine almost abolished the dopamine response in MAP and MPAP. Metoprolol almost eliminated the dopamine effects on CO and LVdP/dt. Only minor alterations in cardiac filling pressures were observed during the study. Plasma norepinephrine (NE) concentration was lower during than before TEA at corresponding dopamine infusion rates. NE was reduced by the beta 1-blockade. During TEA, the plasma dopamine levels were generally higher, and they were further increased by adding beta 1-blockade. In conclusion, myocardial contractility and arterial pressure were restored to pre-TEA values by dopamine at 5-10 micrograms.kg-1.min-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Presynaptic dopamine DA2-receptors in rabbit jejunal arteries. An electrophysiological study

Excitatory junction potentials (e.j.ps) evoked by nerve stimulation with 15 pulses at 1 Hz were recorded from muscle cells of rabbit isolated jejunal arteries. LY 171555 1 mumol/l, SKF 38393 10 mumol/l, dopamine 10 mumol/l and clonidine 0.1 mumol/l depressed all e.j.ps in the train. The percentage inhibition was inversely related to the number of pulses. S- and R-sulpiride, 10 mumol/l, domperidone 1 mumol/l, SCH 23390 1 mumol/l and rauwolscine 1 mumol/l did not change, or even depressed the first e.j.ps. Of these compounds only S- and R-sulpiride, 10 mumol/l and rauwolscine 1 mumol/l facilitated the late e.j.ps. The percentage facilitation increased with the number of pulses until a maximum was reached; rauwolscine 1 mumol/l had the largest effect. S- and R-sulpiride, 10 mumol/l, as well as domperidone 1 mumol/l antagonized the action of LY 171555 1 mumol/l. S-Sulpiride was more potent than its R-isomer. SCH 23390 1 mumol/l and rauwolscine 1 mumol/l blunted the effect of SKF 38393 10 mumol/l. Rauwolscine 1 mumol/l slightly reduced the inhibition by dopamine 10 mumol/l; S-sulpiride 10 mumol/l was antagonistic only in the presence of rauwolscine 1 mumol/l. When rauwolscine 1 mumol/l, prazosin 0.1 mumol/l, propranolol 1 mumol/l and cocaine 10 mumol/l was added to the medium, dopamine 10 mumol/l continued to produce the same depression of e.j.ps, as in the absence of these compounds. Under such conditions S-sulpiride 10 mumol/l also counteracted dopamine 10 mumol/l. Rauwolscine 1 mumol/l prevented the effect of clonidine 0.1 mumol/l. The antagonists were not absolutely selective against only one type of agonist. We suggest that both presynaptic DA2- and postsynaptic DA1-receptors are present in rabbit jejunal arteries. The activation of either receptor-type may depress the e.j.ps. Dopamine interferes with neuroeffector transmission due to alpha 2-adrenoceptor agonist properties; its DA2-effect is unmasked only after alpha 2-adrenoceptor blockade. There was no evidence for a co-transmitter function of dopamine.

An introduction to the pharmacologic properties of Dopacard (dopexamine hydrochloride)

Dopexamine hydrochloride (Dopacard) has been developed as a peripherally acting dopamine receptor agonist with afterload reducing properties for use in the acute management of low cardiac output states. Dopexamine hydrochloride is one-third as potent as dopamine in stimulating DA1 receptors but 60 times as potent as a beta 2-adrenoceptor agonist. Unlike dopamine, it is a weak beta 1-adrenoceptor agonist and does not stimulate vascular alpha adrenoceptors. Its stimulant properties at vascular DA1 receptors and at vascular beta 2 adrenoceptors endow it with the ability to improve renal blood flow and to increase cardiac output secondary to afterload reduction. In addition, mild positive inotropic activity arises from stimulation of cardiac beta 2 adrenoceptors, potentiation of endogenous norepinephrine due to uptake-1 blockade, and activation of the baroreceptor reflex. Other features of dopexamine hydrochloride that should enhance its clinical use are lack of arrhythmogenicity and rapid responsiveness to alterations in infusion rate.

Renal and iliac vascular effects of dopamine in the anaesthetized rat

The renal and iliac vascular effects of dopamine were compared in pentobarbital anaesthetized rats. Local vascular resistances were calculated from simultaneous measurement of blood pressure, renal and iliac blood flow. Without pretreatment, dopamine increased renal and iliac vascular resistance. After pretreatment with prazosin, dopamine decreased the renal vascular resistance while the iliac vascular resistance was still increased. After a combination of yohimbine and prazosin pretreatment, dopamine lowered both the renal and iliac vascular resistance by 30%. These responses were not modified by the beta-adrenoceptor antagonist, sotalol, or by pretreating the rats with reserpine. The renal but not the iliac vascular response to dopamine was abolished by (+)-butaclamol, a stereoselective dopamine receptor antagonist, and by SCH 23390, the DA1-selective dopamine receptor antagonist. The decrease in iliac vascular resistance was not modified by indomethacin or the non-selective 5-HT receptor antagonist, metitepin. These results show that after blockade of alpha 1 and alpha 2-adrenoceptors, dopamine induces iliac vasodilation by a postsynaptic mechanism independent of an interaction with beta-adrenoceptors, dopamine or serotonin receptors. They also confirm in the rat in vivo the existence of renal vasodilation mediated by DA1 dopamine receptors.

Characterisation of the alpha-adrenoceptors of the rat renal vascular bed

Postjunctional renal alpha-adrenoceptors were studied (1) in vivo, on the renal vasculature of the anaesthesized rat and compared with those in the femoral vasculature, and (2) in vitro, on the renal vascular bed of isolated perfused rat kidney. In vivo, renal and iliac blood flows were measured with an electromagnetic flow meter. The i.v. injection of (-)-phenylephrine (1-16 micrograms/kg) and B-HT 920 (0.6-600 micrograms/kg) induced an increase in both renal and iliac vascular resistance, inhibited respectively with prazosin (300 micrograms/kg) or yohimbine (300 micrograms/kg). In the kidney, maximum response to B-HT 920 was equivalent to 64% of that to (-)-phenylephrine; on the iliac vasculature, vasoconstrictor responses to both drugs were identical, but only corresponded to 50% of the maximum renal response to (-)-phenylephrine. This indicates the predominance of alpha 1- over alpha 2-adrenoceptors in the renal vascular bed. In vitro, on the isolated perfused rat kidney, vasoconstriction was induced by the preferential alpha 1-adrenoceptor agonists [(-)-phenylephrine, cirazoline and methoxamine] and the preferential alpha 2-adrenoceptor agonists (alpha-methylnoradrenaline, dopamine and clonidine) at concentrations at which they lose their selectivity for the alpha 2-adrenoceptors; all responses were antagonised by prazosin but not by yohimbine. B-HT 920, the selective alpha 2-adrenoceptor agonist, only induced renal vasoconstriction in vitro under concomitant infusion of rabbit plasma.

Presynaptic dopamine receptors in the cardiovascular system

Stimulation by agonists of presynaptic dopamine receptors on nerve terminals of peripheral sympathetic neurons results in inhibition of norepinephrine release and a concomitant reduction of end organ responses to sympathetic nerve stimulation. These presynaptic dopamine receptors are of the DA-2 subtype and can be blocked selectively by the antagonist S-sulpiride. Presynaptic DA-2 receptors are the target of action of agonists with potential antihypertensive and bradycardic effects. Under control conditions exposure to S-sulpiride on its own does not enhance norepinephrine release. Following chronic treatment of cats with pargyline, S-sulpiride produced a small but significant increase in the electrically-evoked release of 3H-norepinephrine from perfused atrial slices. The possible involvement of peripheral presynaptic DA-2 receptors in the antihypertensive effects of monoamine oxidase inhibitors is discussed.

Characterization of alpha-adrenoceptors in the vasculature of the canine nasal mucosa

alpha-Adrenoceptors present in the vasculature of the nasal mucosa in beta-adrenoceptor blocked dogs have been characterized pharmacologically using selective alpha 1- and alpha 2-adrenoceptor agonists and antagonists. In pentobarbitone-anaesthetized dogs, intra-arterial (i.a.) administration of the selective alpha 1-agonists cirazoline and phenylephrine, the selective alpha 2-agonist UK-14,304 and the mixed alpha 1/alpha 2-agonists adrenaline, noradrenaline and oxymetazoline produced dose-related nasal vasoconstrictor responses (as measured by decreases in nasal cavity pressure). The rank order of agonist potency was adrenaline greater than oxymetazoline = UK-14,304 greater than noradrenaline greater than cirazoline greater than phenylephrine. The nasal response to cirazoline was inhibited by the selective alpha 1-adrenoceptor antagonist prazosin but not by the new, potent selective alpha 2-adrenoceptor antagonist RX811059. In contrast, UK-14,304 was inhibited only by RX811059. Either prazosin or RX811059 reduced the effect of the mixed agonist adrenaline. In spinal dogs, the noradrenaline-evoked fall in nasal cavity pressure was reduced by either prazosin or RX811059. Prazosin attenuated markedly the nasal vasoconstrictor response to electrical stimulation of postganglionic fibres emerging from the superior cervical ganglion (SNS) whereas RX811059 was ineffective. Administration of the neuronal re-uptake inhibitor cocaine potentiated the effect of i.a. noradrenaline but reduced marginally the maximal response to SNS. After cocaine, RX811059 enhanced the effect of SNS and attenuated the response to noradrenaline. Prazosin reduced effectively the responses to both SNS and noradrenaline after cocaine. Pretreatment with the alpha 2-agonist UK-14,304 did not affect the response to noradrenaline in the nasal cavity but evoked a persistent (up to 2 h) reduction in the response to SNS. RX811059 antagonized the inhibitory effect of UK-14,304. These results demonstrate that both postjunctional alpha 1- and alpha 2-adrenoceptors mediating vasoconstriction are present in the canine nasal mucosa. In addition, sympathetic neurones innervating the nasal mucosa are characterized by a very efficient re-uptake process and contain prejunctional alpha 2-adrenoceptors.

The DA1 receptor agonist fenoldopam (SK & F 82526) is also an alpha 2-adrenoceptor antagonist

The alpha-adrenoceptor activity of fenoldopam was evaluated in three different in vitro tissue preparations. In rabbit isolated aortic rings fenoldopam displayed weak alpha 1-adrenoceptor antagonist activity (-log KB = 5.41) and displayed no alpha 1-adrenoceptor agonist activity. In contrast, fenoldopam demonstrated moderately potent antagonist properties at the alpha 2-adrenoceptor in two other tissue preparations. In in vitro model systems used for the characterization of alpha 2-adrenoceptors, fenoldopam competitively antagonized the effects of the alpha 2-adrenoceptor agonist B-HT 920. In the dog isolated saphenous vein and in the isolated field-stimulated guinea-pig ileum, fenoldopam antagonized the effects of B-HT 920 with -log KB values of 7.78 and 7.60, respectively. These data indicate that in addition to being an agonist at DA1 receptors, fenoldopam is also a relatively selective antagonist at alpha 2-adrenoceptors.

Relative contribution of different vascular beds to the pressor effects of alpha-adrenoreceptor agonists and vasopressin in pithed rats: radioactive microsphere determination

The relative fractional distribution of 51Cr-labelled microspheres was evaluated in pithed rats during equieffective vascoconstrictor responses evoked by infusions of the alpha-adrenoreceptor agonists methoxamine (alpha 1-selective), UK-14,304 (alpha 2-selective) or vasopressin. The proportion of injected radioactive microspheres trapped in each tissue during a sustained pressor response relative to saline treated controls is considered a reflection of the degree of local vascoconstriction in the tissue analysed. All three agonists (methoxamine, UK-14,304 and vasopressin) decreased the number of microspheres trapped in the mesentery and tail. Only methoxamine reduced the blood flow to the kidney and spleen. UK-14,304 did not modify the number of microspheres in the sample of skeletal muscle, however, both vasopressin and methoxamine reduced the blood flow to this tissue. Vasopressin increased the counts in the lungs and particularly in the liver but decreased the number of spheres trapped in the stomach and skin. In contrast to the alpha-adrenoreceptor agonists, vasopressin did not increase the number of microspheres trapped in the heart. Since a reduction in the number of microspheres trapped in the tissue reflects a decrease in blood flow, to that organ it is reasonable to conclude that alpha 1-adrenoreceptor stimulation increases kidney, spleen, mesentery, caudal and skeletal muscle vascular resistance, whereas alpha 2-adrenoreceptors appear to preferentially vasoconstrict the mesenteric and the caudal vascular beds.

Dopexamine: a novel agonist at peripheral dopamine receptors and beta 2-adrenoceptors

Dopexamine is an agonist at peripheral dopamine receptors and at beta 2-adrenoceptors. Dopexamine has approximately one-third the potency of dopamine in stimulating the vascular DA1-receptor in the dog, resulting in a fall in renal vascular resistance of 20% at 2.3 X 10(-8) mol kg-1 (i.a.). Prejunctional DA2-receptors are also stimulated by dopexamine, resulting in a reduction of neurogenic vasoconstriction in the rabbit isolated ear artery (IC50 of 1.15 X 10(-6)M) and of neurogenic tachycardia in the cat (ID50 of 5.4 X 10(-8) mol kg-1, i.v.), with a potency six and four times less respectively than that of dopamine. By contrast, dopexamine is approximately 60 times more potent than dopamine as an agonist at the beta 2-adrenoceptor of the guinea-pig isolated tracheal chain, with an EC50 of 1.5 X 10(-6)M. Both dopexamine and dopamine are weak agonists at the guinea-pig atrial beta 1-adrenoceptor over the concentration range 10(-7) to 10(-4) M, but dopexamine has an intrinsic activity of only 0.16 relative to dopamine. Dopexamine does not stimulate postjunctional alpha 1- or alpha 2-adrenoceptors in the canine isolated saphenous vein, whereas dopamine is an agonist, approximately 120 times less potent than noradrenaline. Unlike dopamine and salbutamol, dopexamine does not cause arrhythmias in the guinea-pig isolated perfused heart at doses of up to 10(-5) mol, which is a thousand times the minimum cardiostimulant dose. The combination of agonist properties at peripheral dopamine receptors and at beta 2-adrenoceptors, with little or no activity at alpha- and beta 1-adrenoceptors gives dopexamine a novel pharmacological profile. This may confer advantages over dopamine in the treatment of acute heart failure.

Mechanism of action of cholecystokinin on intestinal blood flow; interactions with opioid peptides and vasoactive intestinal peptide

The effects of local administration of CCK-OP on mesenteric blood flow was studied in anaesthetized dogs using an electromagnetic flowmeter and interactions with the opioid peptides and VIP examined. CCK-OP was found to enhance enteral blood flow and tissue metabolism. Investigations with pharmacological antagonists showed that the vasoregulatory effect was exerted neurogenically, by influencing the activities of sympathetic alpha 2 adrenergic, dopaminergic, cholinergic, muscarinergic and nicotinic neurones. The flow-increasing action of CCK-OP was enhanced by D-Met2-NleS5-enkephalinamide; the effect was additive and partially blocked by naloxone. D-Met2-Pro5-enkephalinamide blocked the increases caused by CCK-OP in blood flow and tissue acetylcholine levels. The blockade was not competitive and could be totally eliminated by naloxone. VIP blocked the flow-increasing effect of CCK-OP by a non-competitive mechanism. The results provide data on the peptidergic regulatory mechanisms of the mesenteric circulation.

Dopamine preferentially stimulates postsynaptic alpha 2-adrenoceptors in the femoral vascular bed, but alpha 1-adrenoceptors in the renal vascular bed of the anaesthetised dog

The decrease in blood flow in response to dopamine (DA) injected intraarterially (i.a.) into the femoral or renal vascular beds was examined in the anaesthetised dog. DA or noradrenaline (NA) were 10 times more potent as vasoconstrictor agents in the femoral than in the renal vasculature. In the femoral bed, the DA induced vasoconstriction was completely resistant to antagonism by prazosin (30-300 micrograms/kg i.v.), but was dose-dependently blocked by the alpha 2-receptor antagonist idazoxan (30-300 micrograms/kg i.v.). In the renal bed the vasoconstrictor effects of DA were resistant to blockade by idazoxan, but were prazosin sensitive indicating that alpha 1-adrenoceptors were involved in this response. The alpha-receptor agonist profile for DA was not modified in the femoral bed after blockade of dilatory D1-receptors with SCH 23390 (0.5 mg/kg i.v. and 0.1 mg/kg per h i.v.). However, this antagonist significantly increased the vasoconstrictor potency for DA in the renal bed. The decrease in femoral blood flow induced by an injection of DA, appears to be mediated by alpha 2-adrenoceptors. In the renal vascular bed where the predominant alpha-adrenoceptor corresponds to the alpha 1-subtype and there are few postsynaptic alpha 2-receptors subserving vasoconstriction, DA can stimulate alpha 1-receptors but this action requires higher doses of agonist than those needed for alpha 2-adrenoceptor stimulation.

Identification of adrenoceptors and dopamine receptors mediating vascular responses in the superior mesenteric arterial bed of the rat

The nature of the adrenoceptors and dopamine receptors mediating vascular responses in the in-situ blood perfused superior mesenteric arterial bed of the rat have been studied. alpha 1-Adrenoceptor agonists produced vasoconstriction but alpha 2-agonists had no significant effect on vascular resistance. The vasoconstrictor effects of noradrenaline were antagonized by low doses of prazosin (26 nmol kg-1 i.v.). Isoprenaline and salbutamol produced vasodilation when the vasculature was preconstricted with arginine vasopressin. The responses to isoprenaline were potently antagonized by propranolol (1.69 mumol kg-1 i.v.) and weakly but significantly reduced by practolol (3.75 mumol kg-1 i.v.) whereas the responses to salbutamol were unaffected by the same dose of practolol. After preconstriction of the vasculature and alpha-adrenoceptor blockade, dopamine and apomorphine produced dilator responses with both compounds producing the same maximal response and apomorphine being 1.8 times more potent than dopamine. The dopamine responses were present after the animals had been pithed and were resistant to spiperone (506 nmol kg-1 i.v.) but were antagonized by cis-alpha-flupenthixol (460 nmol kg-1 i.v.). These results suggest that this vascular bed possesses vasoconstrictor alpha 1- but not alpha 2-adrenoceptors, vasodilator beta 1- and beta 2-adrenoceptors and vasodilator dopamine receptors which appear similar to the D1-type found centrally.

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.

Influence of sympathectomy on alpha 2 adrenoceptor binding sites in canine blood vessels

The effects of denervation of alpha 2 adrenoceptor binding sites were examined in canine arteries and veins. Denervation of the lower abdominal aorta, renal and femoral arteries and femoral veins marked reduced vessel norepinephrine concentrations. Denervation had little effect on the concentration of alpha 2 adrenoceptor binding sites or the affinity of (3H)yohimbine for these sites. The apparent lack of any significant reduction in receptor binding sites suggests that the majority of these sites are located on smooth muscle cells of blood vessels. The failure of any appreciable rise in receptor concentration following denervation is consistent with the hypothesis from functional studies that postsynaptic alpha 2 adrenoceptors on blood vessels are located extra-synaptically and hence not influenced by neurally released norepinephrine.

alpha 1/alpha 2 Selectivity ratio in a series of agonists and their relation to pre/postsynaptic activity ratios

beta-Blocked pithed rats were used to obtained dose-response curves (blood-pressure increase) for 13 various alpha-adrenoceptor agonists of clinical and theoretical interest, in the absence and in the presence of the selective antagonists rauwolscine (alpha 2) and prazosin (alpha 1). Antagonist doses which shifted the dose-response curves for an agonist 10-fold to the right along the abscissa were then calculated (D10rauwolscine = D10R; D10prazosin = D10P). The ratio D10R/D10P is considered to be a quantitative estimation of an agonist's alpha 1/alpha 2 selectivity ratio. A correlation with post/presynaptic activity ratios is presented.