Selective alpha2-adrenergic properties of dexmedetomidine over clonidine in the human forearm.

We tested the hypothesis that dexmedetomidine (Dex) has greater alpha(2)- vs. alpha(1) selectivity than clonidine and causes more alpha(2)-selective vasoconstriction in the human forearm. After local beta-adrenergic blockade with propranolol, forearm blood flow (plethysmography) responses to brachial artery administration of Dex, clonidine, and phenylephrine (alpha(1)-agonist) were determined in healthy young adults before and after alpha(2)-blockade with yohimbine (n = 10) or alpha(1)-blockade with prazosin (n = 9). Yohimbine had no effect on phenylephrine-mediated vasoconstriction but blunted Dex-mediated vasoconstriction (mean +/- SE: -41 +/- 5 vs. -11 +/- 2%; before vs. after yohimbine) more than clonidine-mediated vasoconstriction (-39 +/- 5 vs. -28 +/- 4%; before vs. after yohimbine) (P < 0.02). Prazosin blunted phenylephrine-mediated vasoconstriction (-39 +/- 4 vs. -8 +/- 2%; before vs. after prazosin) but had similar effects on both Dex- (-30 +/- 4 vs. -39 +/- 6%; before vs. after prazosin) and clonidine-mediated vasoconstriction (-29 +/- 3 vs. -41 +/- 7%; before vs. after prazosin) (P > 0.7). Both Dex and clonidine reduced deep forearm venous norepinephrine concentrations to a similar extent (-59 +/- 12 vs. -55 +/- 10 pg/ml; Dex vs. clonidine, P > 0.6); this effect was abolished by yohimbine and blunted by prazosin. These results suggest that Dex causes more alpha(2)-selective vasoconstriction in the forearm than clonidine. The similar vasoconstrictor responses to both drugs after prazosin might be explained by the presynaptic effects on norepinephrine release.     

Postsynaptic alpha-adrenoceptor characterization and Ca2+ channel antagonist and activator actions in rat tail arteries from normotensive and hypertensive animals

Postsynaptic alpha-adrenoceptors in the rat tail artery have been examined by determining the pA2 values for antagonists against several alpha-adrenoceptor agonists. In this tissue the alpha-adrenoceptor agonists all produce concentration-dependent mechanical responses with the following rank order of potency: clonidine greater than norepinephrine greater than phenylephrine greater than UK 14304 greater than B-HT 920. Antagonism by prazosin and yohimbine of phenylephrine, norepinephrine, and clonidine responses does not reveal the anticipated discrimination between alpha 1- and alpha 2-adrenoceptors. Thus, pA2 values for prazosin (9.1-9.5), yohimbine (7.2-7.4), and corynanthine (7.0-7.1) and idazoxan (7.6) do not show large differences between these receptor agonists and suggests the predominance of alpha 1-adrenoceptor mediated contractile responses in this preparation. Significant differences between antagonist activities (pA2 values) in Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) artery preparations have not been observed. The sensitivity sequence of alpha-adrenoceptor agonist-induced responses to nifedipine and D 600 is B-HT 920 greater than clonidine greater than phenylephrine greater than norepinephrine. Dependence of agonist response upon extracellular Ca2+ parallels the sensitivity to Ca2+ channel antagonists. Sensitivity to D 600 of phenylephrine responses increased with decreasing concentration of phenylephrine or with receptor blockade by phenoxybenzamine: sensitivity of responses to B-HT 920 was not affected by these procedures. Tail artery strips from WKY and SHR do not exhibit major differences in sensitivity to D 600 or to Ca2+ depletion. Bay k 8644, a Ca2+ channel activator, produces concentration-dependent mechanical responses in the tail artery in the presence of modestly elevated K+ concentrations (10-15 mM): these actions of elevated K+ can be mimicked by both alpha 1- and alpha 2-adrenoceptor agonists including methoxamine, St 587, UK 14304, and clonidine. These studies do not provide clear evidence for the existence of discrete postsynaptic alpha 1- and alpha 2-adrenoceptor populations in rat tail artery as indicated by pA2 values or Ca2+ dependence of response.     

The alpha adrenoceptors on endothelial cells

Endothelial cells release a powerful factor (endothelium-derived relaxing factor [EDRF]) that relaxes smooth muscle cells in response to some vasodilating agents such as acetylcholine. Contraction curves to norepinephrine (NE) in greyhound, mongrel dog, and pig coronary artery rings were studied in vitro in the presence of propranolol. Removal of endothelium increased the sensitivity and maximum contraction in response to NE. In other experiments pig coronary rings were precontracted with a thromboxane mimetic U 46619 in the presence of propranolol. NE relaxed these arteries only if endothelium was present. Methoxamine was without effect but the relaxation response to NE was antagonized by phentolamine, idazoxan, and yohimbine, which suggests that there are alpha 2 adrenoceptors on endothelial cells that mediate the release of EDRF. Greyhound and mongrel dog large coronary arteries relaxed to NE only if prazosin was present, which suggests that alpha 1-adrenoceptor stimulation on the vascular smooth muscle can override the relaxation response to EDRF. Comparison of NE responses in carotid, mesenteric, renal, and femoral large arteries of the pig, greyhound, and mongrel dog indicate the nonuniformity of distribution of alpha 2 adrenoceptors on endothelium and alpha 1 and alpha 2 adrenoceptors on vascular smooth muscle. The integrity of the endothelium must now be considered in interpreting the vascular responses to alpha-adrenoceptor agonists.     

Attenuating effects of intrathecal clonidine on the exercise pressor reflex

We tested the hypothesis that intrathecal injection of clonidine, an alpha 2-adrenergic agonist, attenuated the reflex cardiovascular and ventilatory responses to static muscular contraction in cats. Before clonidine (1 microgram in 0.2 ml), contraction-induced reflex increases (n = 10) in mean arterial pressure and ventilation averaged 25 +/- 3 mmHg and 359 +/- 105 ml/min, respectively, whereas after clonidine these increases averaged 8 +/- 4 mmHg and 200 +/- 114 ml/min, respectively (P less than 0.05). Clonidine had no effect on the heart rate response to contraction. Intrathecal injection of yohimbine (10 micrograms; n = 5), an alpha 2-adrenergic antagonist, but not prazosin (10 micrograms; n = 3), an alpha 1-adrenergic antagonist, prevented the attenuating effects of clonidine on the reflex pressor and ventilatory responses to contraction. Our findings were not due to the spread of clonidine to the medulla, because the reflex pressor and ventilatory responses to contraction were not attenuated by injection of clonidine (1 microgram) onto the medulla (n = 3). In addition, our findings were not due to a clonidine-induced withdrawal of sympathetic outflow, because intrathecal injection of clonidine (1 microgram) did not attenuate increases in arterial pressure and ventilation evoked by high-intensity electrical stimulation of the cut central end of the sciatic nerve (n = 5). Furthermore, our findings were not due to a local anesthetic action of clonidine, because application of this agent to the dorsal roots had no effect on the discharge of group IV muscle afferents. We conclude that stimulation of alpha 2-adrenergic receptors in the spinal cord attenuates the reflex pressor and ventilatory responses to static contraction.     

Catecholamine stimulation of ion transport in the toad urinary bladder

We have observed that serosal catecholamines increase the amplitude of the short-circuit current (Isc) in the toad urinary bladder by as much as 450%. Chemical sympathectomy with 10(-6) M 6-hydroxydopamine and the sympathomimetic effects of 10(-5) M tyramine indicate a reservoir of amines in the serosal stroma of the tissue. The urinary epithelium from the toad responds to six adrenoceptor agonists: (-)-epinephrine, (-)-norepinephrine, (-)-phenylephrine, clonidine, methoxamine and oxymetazoline. The alpha 2-adrenoceptor agonist clonidine is most potent for stimulating Isc. Some agonists were found to diminish Isc. Apparently this is related to a simultaneous increase in the transepithelial flux of both chloride and sodium. The Isc response to the catecholamines is also inhibited by several adrenoceptor antagonists. The alpha 2-adrenoceptor antagonist yohimbine is more effective than the alpha 1-antagonist prazosin for blocking the stimulation of epithelial transport. As a result of these studies, we have tentatively classified the serosal adrenoceptor of the toad urinary bladder as alpha 2.     

Analysis of responses to sympathetic nerve stimulation in the feline pulmonary vascular bed

The adrenergic receptor subtypes mediating the response to sympathetic nerve stimulation in the pulmonary vascular bed of the cat were investigated under conditions of controlled blood flow and constant left atrial pressure. The increase in lobar vascular resistance in response to sympathetic nerve stimulation was reduced by prazosin and to a lesser extent by yohimbine, the respective alpha 1- and alpha 2-adrenoceptor antagonists. Moreover, in animals pretreated with a beta-adrenoceptor antagonist to prevent an interaction between alpha- and beta 2-adrenoceptors, responses to nerve stimulation were reduced by prazosin, but yohimbine had no significant effect. On the other hand, in animals pretreated with a beta-adrenoceptor antagonist, yohimbine had an inhibitory effect on responses to tyramine and to norepinephrine. Propranolol had no significant effect on the response to nerve stimulation, whereas ICI 118551, a selective beta 2-adrenoceptor antagonist, enhanced responses to nerve stimulation and injected norepinephrine. The present data suggest that neuronally released norepinephrine increases pulmonary vascular resistance in the cat by acting mainly on alpha 1-adrenoceptors and to a lesser extent on postjunctional alpha 2-adrenoceptors but that this effect is counteracted by an action on presynaptic alpha 2-receptors. The present studies also suggest that neuronally released norepinephrine acts on beta 2-adrenoceptors and that the response to sympathetic nerve stimulation represents the net effect of the adrenergic transmitter on alpha 1-, alpha 2-, and beta 2-adrenoceptors in the pulmonary vascular bed.     

Adrenergic response of splanchnic arteries from cirrhotic patients: role of nitric oxide, prostanoids, and reactive oxygen species

Peripheral and splanchnic vasodilatation in cirrhotic patients has been related to hyporesponsiveness to vasoconstrictors, but studies to examine the vascular adrenergic response provide contradictory results. Hepatic arteries from cirrhotic patients undergoing liver transplantation and mesenteric arteries from liver donors were obtained. Segments 3 mm long from these arteries were mounted in organ baths for testing isometric adrenergic response. The concentration-dependent contraction to noradrenaline (10(-8) to 10(-4) M) was similar in hepatic and mesenteric arteries, and prazosin (alpha 1-adrenergic antagonist, 10(-6) M), but not yohimbine (alpha 2-adrenergic antagonist, 10(-6) M), produced a rightward parallel displacement of this contraction in both types of arteries. Phenylephrine (alpha 1-adrenergic agonist, 10(-8) to 10(-4) M) and clonidine (alpha 2-adrenergic agonist, 10(-8) to 10(-4) M) also produced concentration-dependent contractions that were comparable in hepatic and mesenteric arteries. The inhibitor of cyclooxygenase meclofenamate (10(-5) M), but not the inhibitor of nitric oxide synthesis N(w)-nitro-l-arginine methyl ester (l-NAME, 10(-4) M), potentiated the response to noradrenaline in hepatic arteries; neither inhibitor affected the response to noradrenaline in mesenteric arteries. Diphenyleneiodonium (DPI; 5 x 10(-6) M), but neither catalase (1000 U/ml) nor tiron (10(-4) M), decreased the maximal contraction for noradrenaline similarly in hepatic and mesenteric arteries. Therefore, it is suggested that, in splanchnic arteries from cirrhotic patients, the adrenergic response and the relative contribution of alpha 1- and alpha 2-adrenoceptors in this response is preserved, and prostanoids, but not nitric oxide, may blunt that response. Products dependent on NAD(P)H oxidase might contribute to the adrenergic response in splanchnic arteries from control and cirrhotic patients.     

Alpha1 and alpha2 adrenoceptor mediated melanosome aggregatory responses in vitro in Oreochromis mossambica (Peters) melanophores

Effects of specific and non-specific adrenoceptor agonists and antagonists were examined on the isolated scale melanophores of O. mossambica in physiological Ringer solution. The responses were recorded as melanophore size index. It was observed that adrenaline, nor-adrenaline, phenylpropanolamine, clonidine and phenylepherine induced melanosome aggregation in a dose-dependent manner. Denervation of the fish melanophores increased the sensitivity of the melanophores to adrenaline but not to nor-adrenaline. Phentolamine (3.55 x 10(-5) M), prazosin (2.38 x 10(-5) M) and yohimbine (2.821 x 10(-5) M) significantly inhibited the aggregatory responses of the fish melanophores to adrenaline, nor-adrenaline, clonidine and phenylepherine. The blocking effect of yohimbine was significantly higher than that of prazosin. It is concluded that the effect of adrenaline is directly mediated through the receptors and alpha2 adrenoceptors are predominantly involved in the aggregatory responses of this fish melanophores, while alpha1 adrenoceptors presence has been indicated.     

Time-dependent potentiation of contractile response to norepinephrine in canine isolated cerebral arteries.

The time course of contractile responses to alpha-adrenoceptor agonists was investigated using various arteries isolated from dogs and monkeys. The contractile response to norepinephrine was increased during the time course of the experiment in canine basilar and internal carotid arteries, whereas the response of isolated canine external carotid arteries and monkey internal carotid arteries did not change significantly. Treatment with 10(-7) M propranolol, 5 x 10(-6) M cocaine plus 10(-5) M hydrocortisone, or 5 x 10(-5) M acetylsalicylic acid did not significantly affect the time-dependent potentiation of the norepinephrine-induced contraction in canine internal carotid arteries. The time-dependent enhancement in the response to norepinephrine was also observed in the arterial preparations from which the endothelial cells were removed. The contractile response of canine internal carotid arteries to phenylephrine did not alter significantly throughout the experiments. On the other hand, the responses to clonidine and xylazine were markedly enhanced with time. Significant potentiation of the norepinephrine-induced contraction was observed in canine internal carotid arteries treated with 10(-8) M prazosin, whereas 10(-8) M yohimbine attenuated the time-dependent potentiation. These results suggest that the contractile responses of isolated canine basilar and internal carotid arteries to norepinephrine are potentiated during the course of the experiment, which is likely to be related, in part, to an enhancement in alpha 2-adrenoceptor mediated contraction.     

Enhanced neurally evoked responses and inhibition of norepinephrine reuptake in rat mesenteric arteries after spinal transection

In patients with high thoracic spinal lesions that remove most of the central drive to splanchnic preganglionic neurons, visceral or nociceptive stimuli below the lesion can provoke large increases in blood pressure (autonomic dysreflexia). We have examined the effects of T4 spinal transection on isometric contractions of mesenteric arteries isolated from spinalized rats. Nerve-evoked contractions involved synergistic roles for norepinephrine and ATP. At 7 wk after spinal transection, responses to perivascular stimulation at 1-5 Hz were enhanced fivefold, whereas the alpha1-adrenoceptor antagonist prazosin (10 nM) produced a twofold larger reduction in contraction (to 20 pulses at 10 Hz) than in unoperated controls. In contrast, the reduction in nerve-evoked contractions by the P2-purinoceptor antagonist suramin (0.1 mM) and the responses to the P2-purinoceptor agonist alpha,beta-methylene ATP or to high K+ concentration did not greatly differ between groups, indicating that arteries from spinalized rats were not generally hyperreactive. Sensitivity to the alpha1-adrenoceptor agonist phenylephrine was enhanced in arteries from spinalized rats, and the difference from controls was abolished by the norepinephrine uptake blocker desmethylimipramine. Sensitivity to the alpha1-adrenoceptor agonist methoxamine, which is not a substrate for the neuronal norepinephrine transporter, was similar among the groups. Thus the increased neurally evoked response after spinal transection appeared to be due to a reduction in neuronal uptake of released norepinephrine, a mechanism that did not explain the enhanced response of tail arteries after spinal transection that we previously reported. The findings provide further support for potentiated neurovascular responses contributing to the genesis of autonomic dysreflexia.     

Positive interaction between alpha-1 adrenergic and dopamine-2 receptors in locomotor activity of normo and supersensitive mice

In normosensitive mice either the D1 antagonist SCH 23390 or the D2 antagonist sulpiride inhibited the reversion of reserpine-induced akinesia elicited by the mixed D1/D2 agonist pergolide. In mice rendered supersensitive by a five days' reserpine treatment, sulpiride did not prevent the pergolide-induced reversal of akinesia while SCH 23390 disclosed two subpopulations of mice. One population responded to pergolide with marked locomotor activity whereas in the other subpopulation this response was absent. However, all mice challenged with pergolide failed to reverse reserpine-akinesia after alpha-methyl-p-tyrosine (AMPT) pretreatment. The alpha 1/alpha 2 agonist clonidine restored the ability of pergolide to overcome reserpine akinesia in supersensitive mice pretreated with SCH 23390. Clonidine reversed the akinesia in supersensitive mice but in normal animals it did not. However, in these last conditions, the combined use of clonidine plus the D2 agonist LY 171555 was effective to induce locomotion. Neither AMPT nor SCH 23390 inhibited this response whereas the alpha-adrenergic antagonists prazosin and yohimbine did prevent it. The alpha 2 agonist B-HT 920 failed to induce locomotor responses when given together with LY 171555. The same occurred with the D1 agonist SKF 38393 when given together with clonidine. The combined use of SCH 23390 plus prazosin in chronic reserpinized mice prevented pergolide-induced locomotion. Adrenergic stimulation, acting on alpha 1 receptors, could be an alternative to D1 stimulation as a necessary factor to obtain D2-induced motor responses under normo and supersensitive conditions.     

Multiple Adrenergic Receptor Subtypes Controlling Cyclic AMP Formation: Comparison of Brain Slices and Primary Neuronal and Glial Cultures

The adrenergic receptor subtypes involved in cyclic AMP responses to norepinephrine (NE) were compared between slices of rat cerebral cortex and primary neuronal and glial cultures from rat brain. In neuronal cultures, NE and the beta-adrenergic receptor agonist isoproterenol (ISO) caused similar increases in cyclic AMP, which were not altered by the alpha-adrenergic receptor antagonist phentolamine. In glial cultures, NE caused a much smaller cyclic AMP response than did ISO, and this difference was reversed by alpha-adrenergic receptor antagonists (phentolamine greater than yohimbine greater than prazosin). alpha 2-Adrenergic receptor agonists partially inhibited the ISO response in glial cultures to a level similar to that observed with NE alone (clonidine = UK 14,304 greater than NE greater than 6-fluoro-NE greater than epinephrine). In slices from cerebral cortex, NE caused a much larger increase in cyclic AMP than did ISO, and this difference was reversed by alpha-adrenergic receptor antagonists with a different order of potency (prazosin greater than phentolamine greater than yohimbine). alpha 1-Adrenergic receptor agonists potentiated the response to ISO to a level similar to that observed with NE alone (epinephrine = NE greater than phenylephrine greater than 6-fluoro-NE greater than methoxamine). In all three tissue preparations, large responses to both alpha 1-receptor activation (increases in inositol phosphate accumulation) and alpha 2-receptor activation (decreases in forskolin-stimulated cyclic AMP accumulation) were observed. These data indicate that all of the major adrenergic receptor subtypes (beta, alpha 1, alpha 2) are present in each tissue preparation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Higher sensitivity of cerebral arteries isolated from premature and newborn baboons to adrenergic and cholinergic stimulation

To find whether effects of adrenergic and cholinergic agents on cerebral artery were dependent on maturity, we examined responses of isolated cerebral artery strips harvested from premature, term newborn and adult baboons. Although cerebral arteries from many species are only mildly sensitive to norepinephrine, we found the perinatal cerebral arteries to be quite responsive to the amine. Cerebral arteries from premature and newborn baboons were significantly (P less than 0.001) more sensitive to norepinephrine than were arteries from adults; medium effective concentration (EC50) for norepinephrine were 3 X 10(-8), 6 X 10(-8) and 32 X 10(-8)M for prematures, newborns and adults, respectively. Arteries showed a similar age-dependence in the sensitivity of the response to phenylephrine, an alpha 1-adrenoceptor agonist. EC50 values for KC1 did not differ among groups, nor did the maximum response to norepinephrine. Arteries from premature and newborn baboons showed marked contractile response to acetylcholine (maximum tensions 5.9 +/- 0.6 and 6.4 +/- 0.8 g/mm2, respectively), whereas arteries from adult baboons showed little response (0.6 +/- 0.1 g/mm2). Arteries from premature and newborn animals showed a more marked relaxation response to isoproterenol than did arteries from adult animals; the degree of relaxation from an induced contraction was 63% (premature), 72% (newborn) and 10% (adult). There was no age-dependence in the relaxation response to sodium nitrite. We conclude that the events coupling alpha 1, beta or muscarinic receptor activation with cerebral arterial contraction or relaxation are more effective in perinatal than in adult baboons.     

The bindings of 3H-prazosin and 3H-yohimbine to alpha adrenoceptors in the guinea-pig stomach

Alpha adrenoceptor subtypes have been investigated by radioligand binding study in guinea-pig stomach using 3H-prazosin and 3H-yohimbine. The specific 3H-prazosin binding to guinea-pig stomach was saturable and of high affinity (KD = 1.4 nM) with a Bmax of 33 fmol/mg protein. Specific 3H-yohimbine binding to the tissue was also saturable and of high affinity (KD = 25.5 nM) with a Bmax of 150 fmol/mg protein. Adrenergic drugs competed for 3H-prazosin binding in order of prazosin greater than phentolamine greater than methoxamine greater than norepinephrine greater than clonidine greater than epinephrine greater than yohimbine. These drugs competed for 3H-yohimbine binding in order of yohimbine greater than phentolamine greater than clonidine greater than epinephrine greater than norepinephrine greater than prazosin greater than greater than prazosin greater than methoxamine. We also examined whether dopamine receptors exist in guinea-pig stomach, using radioligand binding study. Specific binding of 3H-spiperone, 3H-apomorphine, 3H-dopamine and 3H-domperidone was not detectable in the stomach. Dopaminergic drugs such as dopamine, haloperidol, domperidone and sulpiride competed for 3H-prazosin binding in order of haloperidol greater than domperidone greater than dopamine greater than sulpiride. Metoclopramide, sulpiride and dopamine competed for 3H-yohimbine binding in order of metoclopramide greater than sulpiride greater than dopamine. These results suggest that guinea-pig stomach has alpha 1 and alpha 2 adrenoceptors and has no specific dopamine receptors. It is also suggested that some dopamine receptor antagonists such as domperidone, haloperidol, sulpiride and metoclopramide have antagonistic actions on alpha adrenoceptors.     

Adrenoceptors of the human internal thoracic artery.

Adrenoceptor function in the human internal thoracic artery (ITA) was characterized in vitro using segments of the artery obtained during coronary bypass operations. Specimens were prepared as isolated arterial rings mounted in a tissue bath, and mechanical activity (isometric tension) was measured in response to drugs. The ITA responded to phenylephrine (PE), epinephrine, and norepinephrine with concentration-dependent contractions. The PE-induced contractions were antagonized by phenoxybenzamine, prazosin, and high concentrations of yohimbine. The ITA was not effectively contracted by clonidine in the concentration range normally associated with alpha 2-adrenoceptor stimulation. The beta-adrenoceptor agonist, isoproterenol, had a weak and variable effect on the ITA; samples from 9 out of 12 subjects did not respond to isoproterenol, whereas samples from 3 subjects responded with relaxations of between 33 and 42%. These in vitro studies indicate that the most important adrenoceptors of the human ITA are alpha-adrenoceptors; this may be relevant for the pharmacologic management of patients undergoing coronary bypass surgery using the ITA.     

Use of acute phenolic denervation to show the neuronal dependence of Ca2+-induced relaxation of isolated arteries

We recently showed that perivascular sensory nerves of mesenteric resistance arteries (MRA) express a receptor for extracellular Ca2+ (CaR) and proposed that activation of the CaR by Ca2+ causes nerve-dependent vascular relaxation. We now describe a novel procedure for acutely denervating isolated arteries and have used this method to test the hypothesis that Ca2+-induced relaxation of MRA is nerve dependent. MRA were studied using a wire myograph equipped with electrodes for electrical field stimulation (EFS) which caused sympathetic nerve-mediated contraction, and when applied in the presence of guanethidine, induced nerve-mediated relaxation. Ca2+-induced relaxation was produced by the cumulative addition of Ca2+ to MRA precontracted with norepinephrine. Exposure of MRA to 6.5% phenol in ethanol for 20 sec significantly attenuated EFS-induced contraction and relaxation, and Ca2+-induced relaxation. The magnitude of the relaxation response to EFS correlated significantly with the decrease in Ca2+-induced relaxation. In contrast, endothelium-dependent relaxation induced by acetylcholine was slightly, but nonsignificantly decreased by phenol treatment and did not correlate with Ca2+-induced relaxation. These data indicate that brief exposure of isolated MRA to phenol significantly impairs perivascular nerve function and support the hypothesis that Ca2+-induced relaxation is neurally mediated.     

Alpha 1 adrenergic receptor function in senescent Fischer 344 rat aorta

There have been numerous conflicting reports concerning alpha 1 adrenergic receptor-mediated blood vessel contraction during aging and possible changes in alpha 1 receptor transduction mechanisms have not been investigated. These studies assess capacity of the aging vascular alpha 1 receptor to stimulate production of inositol phosphates, which are its intracellular second messengers, and to elicit a contractile response via this pathway. Aortic ring segments from mature adult (6 month old) and senescent (24 month old) Fischer 344 rats were incubated with [3H]myo-inositol and then stimulated with the alpha 1 agonist norepinephrine (NE, 10(-7)M-3 x 10(-5)M) in the presence of LiCl (10mM), an inhibitor of inositol phosphate metabolism. There was a substantial increase in inositol phosphate accumulation throughout the dose range in aortic rings from 24 month old rats compared to 6 month old rats. This is an alpha 1 receptor response since it is blocked by the alpha 1 antagonist prazosin but not by the alpha 2 antagonist yohimbine. Aortic inositol phosphate accumulation in response to serotonin did not change with age. To assess second messenger stimulated contraction, aortic ring segments were placed in Ca++ free buffer and then stimulated with NE. Under these conditions Ca++ influx is eliminated and contraction depends on the actions of intracellular second messengers. There is an age-related reduction in aortic contraction in Ca++ free buffer. These results suggest that aortic alpha 1 receptor-mediated formation of inositol phosphate intracellular second messengers is enhanced during aging. Despite this, the capacity of senescent arteries to elicit contraction utilizing second messenger pathways seems to be deficient.     

Centrally Acting Imidazolines Stimulate Vascular Alpha 1A-Adrenergic Receptors in Rat-Tail Artery

  1. Centrally acting imidazoline antihypertensive agents clonidine and moxonidine also act peripherally to contract blood vessels. While these agents act at both I₁-imidazoline and alpha 2 adrenergic receptors centrally, the receptor types by which they mediate contraction require further definition. We therefore characterized the receptor subtype by which these agents mediate contraction of proximal rat-tail artery.2. Dose–response curves were determined for phenylephrine and for several imidazoline ligands, using endothelium denuded, isolated ring segments, of tail arteries from adult male Sprague-Dawley rats. Ring segments were mounted on a force transducer with platinum wires and immersed in a tissue bath containing Krebs solution, to which drugs could be added. Signals were digitized and recorded by a computer.3. Tail artery contractions expressed as a percent of contraction to 106 mM potassium were phenylephrine (96%), moxonidine (88%), clonidine (52%), and UK14304 (30%). Neither rilmenidine nor harmane caused contraction. Contraction of tail artery to moxonidine or clonidine could be blocked by alpha 1 antagonist urapidil or prazosin, and also by alpha 1A subtype selective antagonist WB4101. Schild plots were generated and a calculated pA2 value of 9.2 for prazosin in the presence of clonidine confirms clonidine as an agonist at alpha 1A receptors in proximal segments of rat-tail artery.4. Our work suggests that clonidine and moxonidine are promiscuous compounds at micromolar concentrations and that harmane and rilmenidine are more selective compounds for in vivo imidazoline research.*This work represents a portion of a dissertation to be submitted to the School of Graduate Studies, Loma Linda University, for the degree of Doctor of Philosophy.     

Existence of two types of postjunctional alpha-adrenoceptors in the isolated canine internal carotid artery

The pharmacological characteristics of postjunctional alpha-adrenoceptors in isolated canine internal carotid arteries were investigated by the use of selective agonists and antagonists for alpha 1- and alpha 2-adrenoceptors. Norepinephrine, phenylephrine, and xylazine caused concentration-dependent contractions in the helical strips. The contraction induced by 10(-4)M xylazine was significantly smaller than that produced by 10(-4)M norepinephrine or 10(-4)M phenylephrine. The contraction induced by 10(-4)M phenylephrine was almost the same value as that induced by 10(-4)M norepinephrine. Phentolamine (10(-8) and 10(-7)M) caused a parallel shift to the right of the concentration-response curve to norepinephrine. The contractile responses to low concentrations of norepinephrine were significantly suppressed by pretreatment with an alpha 2-antagonist such as yohimbine (10(-9) and 10(-8)M) or DG5128 (10(-7) and 10(-6)M). On the other hand, the responses to higher concentrations of norepinephrine were mainly reduced by low concentrations of an alpha 1-antagonist, prazosin (3 x 10(-10) and 3 x 10(-9)M). These results suggest that both alpha 1- and alpha 2-adrenoceptors are located on the plasma membrane of smooth muscle cells in canine internal carotid arteries and that the norepinephrine-induced contractions at low and high concentrations are mainly mediated by activation of alpha 2- and alpha 1-adrenoceptors, respectively.     

Identification of alpha 1 adrenergic receptors in rabbit aorta with [125I] BE2254

Alpha-adrenergic receptors may play an important role in regulating vascular tone and reactivity. To study alpha-adrenergic receptors in blood vessels, we have developed a method to characterize and quantitate alpha-adrenergic receptors in a particulate fraction of individual rabbit aortas using the high specific activity alpha antagonist [125I] BE2254. [125I] BE2254 specifically labels a single class of binding sites with a dissociation constant of 286 pM and a maximal binding capacity of 16.7 fmoles/mg protein. Catecholamines compete for [125I] BE2254 binding stereospecifically and with the characteristic alpha-adrenergic potency series of (-)epinephrine greater than or equal to (-)norepinephrine much greater than (-)isoproterenol. The alpha 1-selective antagonist prazosin (KD = 0.7 nM) is much more potent in competing for [125I] BE2254 binding than is the alpha 2-selective antagonist yohimbine (KD = 1000 nM), which suggests that the alpha adrenergic receptor identified is predominantly of the alpha 1 subtype. Also, the dissociation constants from these binding studies were in good agreement with those reported in rabbit aorta from classical pharmacological experiments where contraction was found to be mediated via alpha 1 receptors. This extension of radioligand binding techniques to individual rabbit aortas should simplify the study of vascular alpha adrenergic receptor regulation, and provide a basis for broadening the understanding of vascular alpha adrenergic receptors.