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

The decrease in mesenteric blood flow produced by dopamine administered intra-arterially in the anaesthetised dog was investigated by means of drugs selective for alpha 1- and alpha 2-adrenoceptors. The selective alpha 1-adrenoceptor agonist phenylephrine (0.3-100 microgram) given by intra-arterial injection (i.a.) into the superior mesenteric artery of the anaesthetized dog produced a decrease in mesenteric blood flow which was preferentially blocked by the alpha 1-adrenoceptor antagonist prazosin (30-300 microgram/kg i.v.). On the other hand, i.a. injections of the selective alpha 2-adrenoceptor agonist M7 (1-100 microgram) or of dopamine (1-300 microgram) produced a decrease in mesenteric blood flow which was blocked by the alpha 2-adrenoceptor antagonist yohimbine (100-300 microgram/kg i.v.) but was not significantly reduced by prazosin (300 microgram/kg i.v.). These results demonstrate that the mesenteric vascular bed of the dog contains both alpha 1- and alpha 2-adrenoceptors located postsynaptically and mediating vasoconstriction. The decrease in mesenteric blood flow produced by i.a. injections of dopamine is mediated predominantly via postsynaptic alpha 2-adrenoceptors.     

Functional postsynaptic alpha 2- but not alpha 1-adrenoceptors in dog saphenous vein exposed to phenoxybenzamine

Phenoxybenzamine greatly attenuated phenylephrine-induced contractions of dog saphenous vein in vitro, but had less effect on contractions induced by clonidine. The phenoxybenzamine-resistant responses to clonidine were not affected by prazosin or by corynanthine but were competitively antagonized by yohimbine (pA2 8.2). It is concluded that exposure of saphenous vein to phenoxybenzamine resulted in blockade of alpha 1-adrenoceptors to the extent that there remained a virtually homogeneous population of postsynaptic alpha 2-adrenoceptors. The effects of agents at postsynaptic alpha 2-adrenoceptors can be studied on this preparation without the complications caused by the presence of functional alpha 1-adrenoceptors.     

Norepinephrine constricts the canine coronary bed via postsynaptic alpha 2-adrenoceptors

The effect of alpha 2-blockade (0.3 mg/kg i.v. rauwolscine) and alpha 1-blockade (1.2 mg/kg i.v. prazosin) on coronary constrictions induced by intracoronary injections of azepexole (B-HT 933, alpha 2-agonist, 0.1-10 microgram/kg), phenylephrine (0.3-3 microgram/kg) and norepinephrine (0.001-0.1 microgram/kg) were studied in dog hearts perfused in situ under beta-blockade. Constrictions by azepexole (antagonized by rauwolscine, yet resistant to prazosin and methysergide) demonstrated coronary alpha 2-adrenoceptors. Norepinephrine-induced constrictions were more attenuated (22-fold) by alpha 2-blockade than by alpha 1-blockade (2.6-fold) and thus were mediated mainly by activation of postsynaptic alpha 2-receptors.     

Evidence for postsynaptic alpha 1- and alpha 2-adrenoceptors mediating catecholamine-induced negative chronotropic ventricular responses in the conscious dog

1. Adrenaline (0.25-1 microgram/kg), noradrenaline (0.125-0.5 microgram/kg) and dopamine (25-100 micrograms/kg) given in the conscious dog with chronic atrio-ventricular block after beta-adrenoceptor blockade, increased ventricular cycle length (VCL) and mean blood pressure (MBP). 2. Atropine (muscarinic receptor blocker) reduced the catecholamine-induced effects on VCL without modifying their hypertensive effects. 3. Phenoxybenzamine or phentolamine (alpha-adrenoceptor blockers) only decreased the effects of adrenaline on VCL but suppressed those of noradrenaline and dopamine. They only reduced the effects of adrenaline and noradrenaline on MBP, but reversed that of dopamine. 4. Yohimbine (alpha-adrenoceptor blocker) suppressed the catecholamine-induced effects on VCL, and reduced strongly the hypertensive effects of adrenaline and noradrenaline and reversed that of dopamine. 5. Thus, these results show the existence of negative chronotropic postsynaptic alpha-adrenoceptors in the ventricles.     

Dopamine receptors in the femoral vascular bed of the dog as mediators of a vasodilator and sympathoinhibitory effect.

ET 495 and apomorphine, injected in small doses (0.1--1 microng/kg) into the femoral artery, induced a dose-dependent increase in femoral blood flow. This dilator effect was abolished by section of the ipsilateral femoral nerve and sciatic nerve, transection of the spinal cord, alpha-adrenoceptor blockade, ganglionic blockade and guanethidine. In addition, the increase in blood flow was inhibited by intravenous administration of haloperiol (2 mg/kg i.v.) or pimozide (2 mg/kg i.v.) and by injection of small doses (10--50 microng/kg) of these drugs into the femoral artery. It was concluded that a dopaminergic component located in the femoral vascular bed of the dog may be involved in the local vasodilator and sympathoinhibitory effect of apomorphine and ET 495.     

Blockade of postsynaptic alpha 2-adrenoceptors enhances responses to mixed alpha 1/alpha 2-agonists in rat submaxillary gland

The effects of selective blockade of alpha 2-adrenoceptors with idazoxan on salivary secretion elicited by several alpha-adrenoceptor agonists known to differ in their alpha 1/alpha 2 potency ratios were studied in the submaxillary gland. Doses of idazoxan ranging between 0.01 to 10 000 micrograms/kg did not produce secretion in anaesthetized rats. Idazoxan (3 micrograms/kg) effectively blocked the inhibitory action of clonidine, 10 micrograms/kg, on the responses to methacholine but did not change the sialagogic responses to either phenylephrine or isoprenaline. On the other hand, idazoxan enhanced the secretion elicited by other agonists known to have mixed alpha 1/alpha 2-agonistic properties. The degree of augmentation of the responses observed after alpha 2 blockade was: clonidine much greater than alpha-methyl-norepinephrine greater than norepinephrine. Guanabenz did not elicit secretory responses in either the presence or absence of idazoxan. These results argue that two components determine the magnitude of the sialagogic response induced by agonists with mixed alpha 1/alpha 2 activity: (a) an alpha 1-mediated secretory effect and (2) an alpha 2-mediated inhibitory effect, both being localized at postsynaptic level.     

Characterization of postsynaptic alpha1-adrenoceptors in the rabbit iris dilator smooth muscle

Summary Interactions of several alpha-adrenoceptor agonists and antagonists with their receptors were studied in rabbit and guinea pig iris dilator smooth muscle and rabbit aortic strips using pharmacological procedures.In rabbit iris dilators and aortic strips, noradrenaline acted as a full agonist, while oxymetazoline, clonidine and tizanidine acted as partial agonists. The dissociation constants of full and partial agonists in the dilators, calculated after irreversible blockade of a proportion of the active receptors with phenoxybenzamine, were similar to those in the aortic strips. Furthermore, the relative intrinsic efficacies of partial agonists were practically equal in the two tissues, suggesting that these drugs act on the same alpha-adrenoceptors. Since the alpha₂-agonists clonidine and tizanidine had low affinity in the rabbit dilators, the alpha-adrenoceptors in this tissue appear to be of alpha₁-type. These results were further supported by the fact that the pA₂-value of prazosin, an alpha₁-antagonist, was approximately 2 log units higher than that of yohimbine, an alpha₂-antagonist.However, pA₂-values of four quinazolines (prazosin, bunazosin, SM911 and SM2470) and two yohimbine alkaloids (yohimbine and corynanthine) were significantly lower in the rabbit dilator muscle than in rabbit aortic strips. Two imidazoline antagonists (phentolamine and tolazoline) and a phenethanolamine (labetalol) acted on the alpha₁-adrenoceptors in the two tissues nonselectively. These results suggest that alpha₁-adrenoceptors in the rabbit dilator muscle and aortic strips may not be identical and that both selective and nonselective antagonists which act on these receptor sites exist. The pA₂-values of prazosin, SM911 and SM2470 against noradrenaline in guinea pig iris dilator smooth muscle were also significantly lower than those in the rabbit aortic strips, but practically equal to those in the rabbit iris dilators, suggesting that the alpha₁-adrenoceptors in the dilators are the same in these two species.     

Role of vascular alpha 2-adrenoceptors as targets for circulating catecholamines in the maintenance of blood pressure in anaesthetised spontaneously hypertensive rats

We examined the blood pressure-lowering effects of the alpha 2-adrenoceptor antagonist rauwolscine in anaesthetised spontaneously hypertensive rats (SHR). In the absence of prior drug, rauwolscine (1-10 mg kg-1) produced dose-dependent falls in diastolic blood pressure (DBP). In the presence of prazosin (1 mg kg-1) to eliminate alpha 1-adrenoceptor-mediated responses, rauwolscine (0.1-1 mg kg-1) lowered DBP in unoperated and sham-operated animals but not in adrenal demedullated animals. Hence, we were able to demonstrate that alpha 2-adrenoceptors are involved in the control of blood pressure in intact anaesthetised SHR but not following adrenal demedullation, suggesting that circulating catecholamines are responsible for these alpha 2-adrenoceptor-mediated pressor effects.     

Location of vascular alpha 2-adrenoceptors in man.

To determine whether vascular postsynaptic alpha 2-adrenoceptors are innervated by sympathetic nerves in man, normal volunteers were infused for 90 min with angiotensin II (A II) and the alpha 2-adrenoceptor agonist alpha-methylnoradrenaline (MNA) in a double-blind cross-over study. Whilst systolic blood pressure returned to baseline within 5 min of terminating the A II infusion it remained elevated 40 min after stopping MNA. The prolongation of the pressor response to MNA, a substrate for neuronal uptake, was probably due to activation of alpha 2-adrenoceptors by MNA re-released from contiguous sympathetic nerve endings. The proximity of alpha 2-adrenoceptors to sympathetic nerve terminals suggests that they could contribute to blood pressure regulation in man.     

Postsynaptic alpha 1- and alpha 2-adrenoceptors in the vascular system of the herring gull, Larus argentatus.

The nature of the vascular alpha-adrenoceptors has been studied in the herring gull, Larus argentatus. In the anaesthetized herring gull, both phenylephrine and clonidine elicited dose-dependent increases in arterial blood pressure. The alpha 1-adrenoceptor antagonist prazosin was a better antagonist of phenylephrine than were the alpha 2-adrenoceptor antagonists yohimbine and rauwolscine. Yohimbine and rauwolscine were better antagonists of clonidine than was prazosin. The maximum response to phenylephrine, but not clonidine, was lower in reserpine-treated birds, indicating that phenylephrine in high doses liberates endogenous catecholamines, which contribute to the effect. It is concluded that the herring gull possesses postsynaptic, vascular alpha-adrenoceptors, of both the alpha 1- and alpha 2-subtypes, similar to those found in mammals.     

Further investigation of the sites of vascular alpha 1 and alpha 2 adrenoceptors in the anaesthetised spontaneously hypertensive rat

We have looked at the role played by alpha 1 and alpha 2 adrenoceptors in the control of blood pressure in the anaesthetised spontaneously hypertensive rat (SHR) by examining the blood-pressure-lowering effects of the alpha 1-adrenoceptor antagonist prazosin and the alpha 2-adrenoceptor antagonist rauwolscine in 5 groups of animals: unoperated, adrenal demedullated, shamoperated, sympathectomised with 6-hydroxydopamine, and vehicle treated. Prazosin (1 mg/kg) produced significant falls in diastolic blood pressure (DBP) in all groups of rat but the fall was significantly less in sympathectomised animals. In the presence of prazosin (1 mg/kg), rauwolscine (0.1-1 mg/kg) lowered DBP in all groups of rat except the adrenal demedullated animals. In the absence of prazosin, rauwolscine (0.1 mg/kg) produced a significant fall in DBP only in sympathectomised animals. These results demonstrate that alpha 2 adrenoceptors are involved in blood pressure control in the intact or sympathectomised SHR but not following adrenal demedullation, suggesting that circulating catecholamines rather than neurally released noradrenaline (NA) are responsible for these alpha 2-adrenoceptor-mediated pressor effects. In contrast, only alpha 1-adrenoceptor-mediated pressor responses are reduced by sympathectomy, suggesting that neurally released NA acts mainly on alpha 1-adrenoceptors.     

Reserpine and sympathetic denervation cause an increase of postsynaptic alpha 2-adrenoceptors

The changes in alpha-adrenoceptors in rat vasa deferentia after injections of reserpine and 6-hydroxydopamine (6-OHDA) were examined in binding studies with [3H]WB4101 and [3H]clonidine. On intraperitoneal injection of reserpine (0.5 mg/kg body weight per day, 2 days), [3H]clonidine binding sites increased by about 2.0 pmol/g wet weight; the binding sites were not detectable in vasa from control rats. [3H]WB4101 binding sites showed no apparent change in amount. After a single injection of 6-OHDA (30 mg/kg body weight), [3H]clonidine binding sites amounted to about 1.1 pmol/g wet weight. The data suggest that both reserpine treatment and incomplete chemical denervation with 6-OHDA cause an increase of [3H]clonidine binding sites in the postsynaptic region of rat vasa deferentia. The possibility was tested by completely denervating rat vasa deferentia by chemical treatment and operation. The treatments increased the binding sites to about 0.9 and 1.0 pmol/g wet weight, respectively over the control level. The results imply that the binding sites indeed also increased in the postsynaptic area.     

Pharmacological subclassification of alpha 1-adrenoceptors in vascular smooth muscle

1. We examined whether alpha 1-adrenoceptors in various blood vessels can be divided into subtypes by antagonist affinity or by susceptibility to chloroethylclonidine or nifedipine. 2. Noradrenaline or phenylephrine produced concentration-dependent contractions in all the tissues tested, which were competitively inhibited by phentolamine, yohimbine, prazosin, WB4101 and HV723. However, there were large differences between the tissues in the pA2 values for all the antagonists except phentolamine. 3. The blood vessels could be classified into three groups (I, II and III) on the basis of their affinity variation. In group I (dog mesenteric artery and vein, saphenous vein), the pA2 values for HV723 were greater than 9, and those for HV723 and WB4101 were approximately 1 log unit higher than for prazosin. This rank order of affinity reversed in group II (dog carotid artery and rat thoracic aorta), where prazosin was more potent (pA2 values greater than 9.5) than HV723 or WB4101. In group III (rabbit mesenteric artery, thoracic aorta and carotid artery and guinea-pig thoracic aorta), on the other hand, prazosin, HV723 and WB4101 inhibited the noradrenaline response with a similar affinity (pA2 values ranging from 8 to 9). 4. Yohimbine inhibited the responses to noradrenaline and phenylephrine with a lower affinity than prazosin, HV723 or WB4101. The pA2 values for yohimbine were similar in groups I and II (the values greater than 6.5), which were greater than those in group III (values less than 6.4).(ABSTRACT TRUNCATED AT 250 WORDS)     

Chloroethylclonidine irreversibly activates postjunctional alpha2-adrenoceptors in the dog saphenous vein

Summary This study was aimed at analysing the contractile response of the dog saphenous vein to chloroethylclonidine. At 37°C, chloroethylclonidine (0.1–100 mol·1^–1) caused along-lasting contraction in both proximal and distal segments of the dog saphenous vein, reaching 77.6 and 52.6% of the maximal response to phenylephrine, respectively. At 18°C, and in both segments, the maximal response to chloroethylclonidine was markedly reduced, whereas that to phenylephrine was not changed and that to UK-14,304 was enhanced. The response to chloroethylclonidine was unaffected by pretreatment with cocaine. Warming to 37°C caused contraction of strips which at 18°C had remained unresponsive to chloroethylclonidine, even if these strips were repeatedly washed before warming. At 18°C, chloroethylclonidine (100 mol·1^–1) did not alter the responses to UK-14,304 and phenylephrine.At 37°C, the contractile response to chloroethylclonidine was antagonized by yohimbine, rauwolscine and prazosin, with the potency rank yohimbine = rauwolscine > prazosin. Phenoxybenzamine (30 nmol · 1^–1) displaced the concentration-response curve to chloroethylclonidine to the right and depressed its maximum. After phenoxybenzamine, yohimbine continued to be more effective than prazosin, which remained very potent.We concluded that: 1) the contractile response of the canine saphenous vein to chloroethylclonidine (both in the absence and in the presence of phenoxybenzamine) is predominantly alpha2-adrenoceptor-mediated since it is larger at the proximal than at the distal level of the vein and since it is more sensitive to yohimbine and rauwolscine than to prazosin; 2) the response to chloroethylclonidine and UK-14,304 are apparently due to activation of different alpha2-adrenoceptor subtypes, since prazosin was much more effective against chloroethylclonidine than against UK-14,304, and since at 18°C chloroethylclonidine occupies receptors without changing the response to UK-14,304; 3) there is a component of alphal-adrenoceptor stimulation in the response to chloroethylclonidine, since 30 nmol·1^–1 phenoxybenzamine partly antagonized the effect of chloroethylclonidine; 4) since the responses to UK-14,304 and chloroethylclonidine are differently affected by cooling, there is some step (or steps) in the chain of events between the receptor and the final response, which is different in the two pathways. Correspondence to S. Guimarães at the above address     

Interaction between nifedipine and postsynaptic alpha 1- and alpha 2-adrenoceptors following oral pretreatment of spontaneously hypertensive rats

In spontaneously hypertensive rats (SHR), oral antihypertensive doses of nifedipine exert potent inhibitory effects on pressor responses elicited by xylazine and angiotensin II, and by stimulation of the complete sympathetic outflow, suggesting that extracellular calcium is a prerequisite for responses to these stimuli. On the other hand, only those pressor responses to low doses of phenylephrine are affected by nifedipine, suggesting less dependence of phenylephrine on extracellular calcium. The results, therefore, indicate that postsynaptic alpha-adrenoceptors mediating pressor responses to neuronally released noradrenaline and phenylephrine may differ in their dependence on extracellular calcium, although they are both considered to be of the alpha 1-subtype. The evidence also suggests that this ability of nifedipine to inhibit pressor responses to neuronally released noradrenaline, as well as pressor responses to angiotensin II, contributes to the efficacy of this drug as an antihypertensive in SHR.     

Pharmacological characterization of alpha1-adrenoceptors in mouse isolated femoral small arteries

Arteries were isolated from male DBA/2 mice and mounted on a small vessel wire myograph for isometric recording. Responses to exogenous noradrenaline were inhibited with high affinity by prazosin (pKB, 9.3) and 5-methyl-urapidil (pKB, 9.2) and with low affinity by 8-[2-[4-(2-methoxyphenyl)-1 piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY 7378) (pA(2), 6.7). Chloroethylclonidine (10 microM) produced only a small reduction in the maximum response to noradrenaline. Responses to electrical field stimulation were also inhibited with high affinity by prazosin (pIC50, 9.3-9.5) and 5-methyl-urapidil (pIC50, 8.0-8.3). Responses were sensitive to BMY 7378 at low frequencies of stimulation (pIC50 at 2 Hz, 8.2) but not at high frequencies (pIC50 at 20 Hz, 6.5). In conclusion, contractions to exogenous and endogenous noradrenaline in mouse femoral small arteries are mediated mainly by alpha1A-adrenoceptors. alpha1D-adrenoceptors are not involved in responses to exogenous noradrenaline but appear to be activated by neurally released noradrenaline at a low frequency of stimulation.     

Influence of Bay K 8644 on vascular responses mediated by alpha 1- and alpha 2-adrenoceptors

1. The calcium channel activator Bay K 8644 increased the potency of noradrenaline in cat middle cerebral (alpha 2-adrenoceptors) and mesenteric (atypical or mixed alpha 1- and alpha 2-adrenoceptor population) arteries, but not in rat middle cerebral and mesenteric arteries (alpha 1-adrenoceptors). 2. In cat arteries, exposure to 15 mM K+ solution shifted the noradrenaline concentration-response curve to the left in an almost identical manner as did Bay K 8644. 3. Bay K 8644 completely reversed the relaxation produced by nifedipine in K+-contracted cat middle cerebral arteries, whereas the relaxation induced by verapamil, diltiazem or flunarizine was only partially reversed. This suggests a specific interaction between Bay K 8644 and the dihydropyridine receptors on the calcium channels. 4. It is concluded that the degree to which noradrenaline promotes calcium influx through membrane channels is at least partly related to the alpha-adrenoceptor subtype mediating the response.     

Molecular characterization of alpha 1- and alpha 2-adrenoceptors

Three 'alpha 1-adrenoceptors' and three 'alpha 2-adrenoceptors' have now been cloned. How closely do these receptors match the native receptors that have been identified pharmacologically? What are the properties of these receptors, and how do they relate to other members of the cationic amine receptor family? Kevin Lynch and his colleagues discuss these questions in this review.