Endorphinergic mechanism in the central cardiovascular and analgesic effects of clonidine

In urethane-anesthetized male rats, injection of 5 nmol clonidine into the nucleus of the solitary tract (NTS) causes hypotension and bradycardia. These effects are greater in spontaneously hypertensive rats (SHR) and normotensive Sprague-Dawley (SD) rats than in normotensive Wistar-Kyoto (WKY) rats. The effects of clonidine are stereoselectively inhibited by 100 ng intra-NTS naloxone in SHR and SD but not in WKY rats. In SHR, the effects of clonidine are also inhibited by intra-NTS administration of ICI 174864 (a delta-receptor antagonist) but not by beta-funaltrexamine (a mu-receptor antagonist), while in SD rats only the mu- and not the delta-antagonist was effective. Neonatal treatment of SHR with monosodium glutamate (MSG) reduced the beta-endorphin content of the arcuate nucleus and the NTS, reduced the cardiovascular effects of clonidine, and abolished their naloxone sensitivity. MSG treatment of newborn WKY reduced the beta-endorphin content of the arcuate nucleus but not the NTS and did not affect the responses to clonidine. Measurement of pain sensitivity by the formalin test indicated that clonidine was more potent as an analgesic in SHR and SD than in WKY rats, and its effect was inhibited by naloxone (2 mg/kg i.p.) in the former two strains but not in WKY. It is proposed that a naloxone-sensitive component of the cardiovascular effects of clonidine is due to release of a beta-endorphin-like opioid from the NTS, and that this mechanism is present in SHR and SD but not in WKY rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Enhanced depressor effect of muscimol in the DOCA/NaCl hypertensive rat: evidence for altered GABAergic activity in brain

To elucidate the role of the central gamma-aminobutyric acid (GABA) system in the maintenance of deoxycorticosterone (DOCA)NaCl hypertension, the responses of mean arterial pressure (MAP), plasma norepinephrine (NE), and epinephrine (EP) to intracerebroventricular (ICV) administration of muscimol, a GABA agonist, and the responses of MAP to bicuculline, a GABA antagonist, and to clonidine, an alpha 2-adrenoceptor agonist known to lower blood pressure by inhibiting sympathetic tone, were examined in conscious, unrestrained 4 week DOCA/NaCl hypertensive rats and age-matched uninephrectomized control rats. Muscimol (50-1000 ng/300 g, ICV) caused dose-dependent decreases in MAP which were greater in DOCA/NaCl rats than in controls. Basal plasma NE and EP were significantly higher in DOCA/NaCl rats than in controls. Muscimol (1000 ng/300 g, ICV) induced decreases in plasma EP which were greater in DOCA/NaCl rats than in controls without changing NE levels in either group. Bicuculline (3 micrograms/300 g, ICV) caused increases in MAP which were the same in both groups. The depressor response to clonidine (5 micrograms/300 g) was greater in DOCA/NaCl rats than in controls. These results suggest that the activity of the central GABAergic system is altered in the rat with established DOCA/NaCl hypertension and that the alteration in central GABAergic function may be related to the increased sympathoadrenal activity and the maintenance of hypertension in this model.     

Clonidine and morphine increase atrial natriuretic peptide secretion in anesthetized rats

In order to determine whether the activity of central alpha 2-adrenergic and opioid receptors influence plasma atrial natriuretic peptide (ANP) levels, clonidine and morphine were infused into the lateral cerebral ventricle for 45 min in anesthetized Sprague-Dawley rats. The central administration of a low dose of clonidine (10 ng/min) caused a significant increase in plasma ANP without changing arterial blood pressure or central venous pressure. Pretreatment with yohimbine (5 micrograms/min) completely blocked the effect of clonidine. Central infusion of morphine (100 ng/min) also elevated plasma ANP levels and naloxone (5 micrograms/min) blunted this effect. Intravenous infusion of the same dose of clonidine or morphine did not affect plasma ANP levels. Moreover, the effect of clonidine on plasma ANP was partially blocked by pretreatment with naloxone (5 micrograms/min). These results suggest that central alpha 2-adrenergic and opioid receptors may be involved in ANP secretion.     

A comparison of the inhibitory effects of clonidine and guanfacine on the behavioral and autonomic components of morphine withdrawal in rats

Intravenous administration of naloxone (0.5 mg/kg) to morphine dependent rats elicited classical autonomic and behavioral symptoms of narcotic abstinence including hypertension, tachycardia, withdrawal body shakes, escape attempts, diarrhea, etc. Pretreatment of dependent rats with either clonidine (3–90 μg/kg) or guanfacine (3–900 μg/kg) produced a dose-dependent reduction in the hypertensive response to subsequent injection of naloxone. Clonidine was about 12 times more potent than guanfacine in inhibiting this autonamic symptom of withdrawal. Both drugs were less effective at blocking body shakes and escapes, however, when all symptoms were combined in a ranked score, guanfacine was less effective than clonidine at reducing the ranked abstinence intensity score. Since clonidine blocked the autonomic component of withdrawal at doses more consistent with its clinical anti-withdrawal actions, it is possible that 1) measurement of behavioral signs of withdrawal in rats is a less sensitive index than is measurement of autonomic changes associated with withdrawal, or, 2) a reduction in autonomic outflow in general is most relevant to suppressing the apparent intensity of the abstinence syndrome.     

Dorsal noradrenergic bundle lesions fail to alter opiate withdrawal or suppression of opiate withdrawal by clonidine

Previous work has shown that clonidine effectively supresses many of the signs of opiate withdrawal. The present study was designed to test the hypothesis that the supression of opiate withdrawal by clonidine is mediated by forebrain noradrenergic projections of the locus coeruleus. Two groups of 24 rats each were subjected to either a 6-hydroxydopamine lesion of the dorsal noradrenergic bundle (Lesion group) or a sham, vehicle injection (Sham group). All rats were made dependent on morphine by subcutaneous implantation of one 75 mg silastic morphine pellet for three days followed by 3 more days with two additional 75 mg pellets. Following removal of the morphine pellet, withdrawal was precipitated in all rats by subcutaneous injection of 4 mg/kg of naloxone. Pretreatment 10 min. before withdrawal with clonidine (0.1 or 0.2 mg/kg) produced a significant attenuation of withdrawal signs as compared to saline injected rats; this effect was equally significant in both sham and lesion groups. Lesions of the locus coeruleus had no effect on withdrawal, nor did they affect the ameliorating action of clonidine. These results substantiate the observation that clonidine can effectively attenuate signs of opiate withdrawal in the rat, but fail to support the hypothesis that these effects are mediated by the forebrain projections of the locus coeruleus.     

Neuropeptide Y depresses reflex urinary bladder contractions in rats and modifies central activity of opioid agonists

The 36 amino acid peptide neuropeptide Y (NPY) has been found distributed in central structures associated with nociception and the actions of opioid analgesics. We therefore studied its central actions on reflex bladder contractions which we have shown to be inhibited by supraspinal and spinal opioid administrations in urethane anesthetized rats. Neuropeptide Y produced a dose related (0.5-2 micrograms per rat) inhibition of bladder contractions following intracerebroventricular (ICV) and spinal intrathecal (IT) administrations. These effects could not be antagonized by naloxone (2 micrograms, ICV or IT) or by ICI 174,864 [N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH: Aib = alpha-aminoisobutyric acid] (3 micrograms, ICV or IT). NPY (0.5-1 micrograms) reduced the ICV and IT effects of morphine but potentiated the action of the selective delta-receptor ligand [2-D-penicillamine, 5-L-penicillamine] enkephalin (DPLPE). The effect of the mu-selective opioid ligand [D-Ala2, Me-Phe4, Gly(ol)5] enkephalin (DAGO) were unaffected as were the submaximal ICV and IT actions of noradrenaline. It was concluded that NPY-induced inhibition of bladder activity was not due to a direct opioid receptor interaction. However since NPY consistently changed the activity of opioids (morphine and DPLPE), this suggested a possible physiological role in the regulation of opioid receptors, central neural excitability and thereby visceral activity.     

Vascular responses to sodium arachidonate in experimental hypertension

This study characterizes vascular responsiveness to sodium arachidonate (C 20:4) in four models of hypertension [deoxycorticosterone acetate (DOCA) hypertensive rats, two kidney-one clip (2K-1C) renal hypertensive rats, spontaneously hypertensive rats (SHR), and psychosocial hypertensive mice]. Isolated arterial strips (aorta, mesenteric artery, tail artery) were equilibrated under optimal resting tension in physiological salt solution for measurement of isometric force generation. Dose-response curves to arachidonate (10(-10) to 10(-4) g/ml) in arteries from DOCA and 2K-1C hypertensive rats were shifted to the left compared to those in arteries from control rats. In arteries from SHR and psychosocial hypertensive mice, the dose-response relationships were unchanged compared to normotensive values. Arteries from DOCA hypertensive and 2K-1C hypertensive rats developed greater maximal contractile responses to arachidonate than controls; maximal responses in arteries from SHR and psychosocial hypertensive mice were unchanged compared to normotensive values. Contractions to arachidonate were inhibited by indomethacin (0.5 and 5 micrograms/ml) and by aspirin (5 and 50 micrograms/ml). The fatty acid, oleate (C 18:1), had no effect on the contractile state of the arteries, whereas prostaglandin F2 alpha caused contraction. These results indicate altered responsiveness to exogenous arachidonate in arteries from DOCA and 2K-1C hypertensive rats, but not in arteries from SHR and psychosocial hypertensive mice.     

Clonidine fails to reduce pressor responsiveness of conscious spontaneously hypertensive rats to vasopressin

Pressor responses and heart rate responses to intravenous injections (3.5-50.0 pmol/kg) of arginine vasopressin (AVP) were recorded in saline- and clonidine-treated spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats. Clonidine (20 micrograms/kg, i.v.) caused a marked fall of arterial pressure in SHR but not in WKY rats so that, 20 min after the injection of the alpha 2-adrenoceptor agonist, arterial pressure was similar in the two strains of rats. The curve expressing the relationship between the dose of AVP and the increase of arterial pressure for saline-treated SHR was positioned to the left of that for saline-treated WKY rats. This enhanced pressor responsiveness of SHR to AVP may have been related to impaired reflex activity since heart rate fell much less in SHR than in WKY rats for a given elevation in pressure. Pressure responses to AVP were augmented by clonidine in both SHR and WKY rats so that, similar to saline-treated rats, pressor responsiveness to the peptide was still greater in SHR. Heart rate responses to AVP were not altered significantly by clonidine. The results indicate that clonidine fails to enhance reflex activity and reduce pressor responsiveness of SHR to AVP. The increased pressor responsiveness of both SHR and WKY rats to AVP following clonidine was an unexpected finding and may be related to a peripheral interaction between alpha-adrenergic agonists and AVP.     

Neuropeptides and thermoregulation: the interactions of bombesin, neurotensin, TRH, somatostatin, naloxone and prostaglandins

In this study we have examined the interactions of bombesin (1 microgram ICV), neurotensin (1 microgram ICV), TRH (10 micrograms ICV), somatostatin (10 micrograms ICV), PGE2 (10 micrograms ICV) and naloxone (10 mg/kg SC) on thermoregulation in the rat at room temperature (20 +/- 1 degree C). Given alone, bombesin, neurotensin, somatostatin and naloxone all produced hypothermia (bombesin greater than neurotensin greater than somatostatin congruent to naloxone). PGE2 was hyperthermic, and TRH had no effect. Bombesin and PGE2 neutralized one another's effects. Neurotensin had no effect on PGE2-induced hyperthermia. Naloxone enhanced the hypothermic effect of bombesin and somatostatin enhanced the rate of onset of hypothermia after bombesin. TRH had no effect on bombesin-induced hypothermia. TRH, somatostatin and naloxone had no effect on neurotensin-induced hypothermia. TRH antagonized the hypothermia due to naloxone and somatostatin.     

Cardiovascular effects of dynorphin A (1–13) in conscious rats and its modulation of morphine bradycardia over time

The short-term cardiovascular effects of dynorphin A (1–13), as well as its effects upon morphine bradycardia were investigated. In unanesthetized, unrestrained rats, intracerebroventricular (ICV) dynorphin A (1–13) injections (10–20 μg) produced a dose-related pressor effect, whereas intravenous (IV) dynorphin A (1–13) (1.0 mg/kg) produced a depressor effect; these responses persisted less than five min. Heart rate was not significantly altered by these doses or routes of administration. Dynorphin A (1–13) also produced behavioral effects in the unanesthetized animals, such as wet dog shakes in response to IV administration and wet dog shakes accompanied by barrel rolling in response to ICV administration. To evaluate the effects of dynorphin A (1–13) pretreatment on the bradycardic response to IV morphine, rats were pretreated with 10 μg dynorphin A (1–13) ICV four, six or eight hours prior to challenge with morphine sulfate (0.1 mg/kg IV). Four hour pretreatment with dynorphin A (1–13) (tested at 14:00 hr) resulted in a potention of morphine bradycardia, with six hours pretreatment (tested at 16:00 hr) no effect was observed, and eight hours following dynorphin A (1–13) pretreatment (tested at 18:00 hr) morphine bradycardia was attenuated. Additionally, the bradycardic response to IV morphine alone became more exaggerated as rats approached their nocturnal activity cycle. These data further establish that dynorphin A (1–13) exerts a potent, long lasting modulatory effect on morphine bradycardia and emphasize the importance of circadian variables in altering the magnitude of cardiovascular responses to opioid agonists.     

延髓弧束核微量注射可乐宁对自发性高血压大鼠...

In order to study whether atrial natriuretic factor (ANF) is involved in the depressor effect of clonidine, microinjection of the latter into nucleus tractus solitarii (NTS) was carried out in anesthetized stroke-prone spontaneously hypertensive rats (SHRsp) and normotensive Wistar-Kyoto (WKY) rats. Each strain was randomly divided into three groups by injecting: (1) clonidine (1.0 microgram/0.2 microliter); (2) yohimbine (3.3 micrograms/0.2 microliter) followed by (1); (3) artificial cerebral spinal fluid (ACSF, 0.2 microliter) as control. A decrease of blood pressure and heart rate and a suppression of ANF release elicited by clonidine were significantly greater in SHRsp than in WKY rats. After blockade of alpha 2-receptor with yohimbine, the hypotensive effect of clonidine was blocked completely in WKY rats, but only partially in SHRsp, while the suppression effect on ANF release was eliminated in both strains. In addition, the decrease of plasma catecholamine produced by clonidine could also be blocked after yohimbine. The results suggest that ANF probably does not contribute to the depressor effect of centrally administered clonidine, while in SHRsp the decrease of plasma ANF might be a blood pressure-dependent compensatory response.     

Different response of ANP secretion to adrenoceptor stimulation in renal hypertensive rat atria

Sympathetic nervous system and atrial natriuretic peptide (ANP) system play fundamental roles in the regulation of cardiovascular functions. Overactivity of sympathetic nervous system can lead into cardiovascular diseases such as heart failure and hypertension. The present study aimed to define which adrenergic receptors (ARs) affect atrial contractility and ANP release and to determine their modification in renal hypertensive rat atria. An alpha(1)-AR agonist, cirazoline increased ANP release with positive inotropism. These alpha(1)-AR agonist-mediated responses were attenuated by the alpha(1A)-AR antagonist, but not by the alpha(1B)- or alpha(1D)-AR antagonist. An alpha(2)-AR agonist, guanabenz and clonidine increased ANP release with negative inotropism and decreased cAMP level. The order of potency for the increased ANP release was cirazoline>phenylephrine=guanabenz>clonidine. In contrast, a beta-AR agonist, isoproterenol decreased ANP release with positive inotropism and these responses were blocked by the beta(1)-AR antagonist but not by the beta(2)-AR antagonist. The increased cAMP level by isoproterenol was suppressed by pretreatment with both beta(1)- and beta(2)-AR antagonists. In renal hypertensive rat atria, the effects of isoproterenol on atrial contractility, ANP release, and cAMP level were attenuated whereas the effect of cirazoline on ANP release was unaltered. Atrial beta(1)-AR mRNA level but not alpha(1A)-AR mRNA level was decreased in renal hypertensive rats. These findings suggest that alpha(1A)- and beta(1)-AR oppositely regulate atrial ANP release and that atrial beta(1)-AR expression/function is impaired in renal hypertensive rats.     

Stimulus properties of clonidine in chronically morphine treated rats trained to a morphine-saline discrimination

In an operant behavior procedure of lever pressing on an FR 10 schedule of food reinforcement, morphine dependent and nondependent rats were trained to respond on a lever on one side of the food tray after a morphine (10 mg/kg i.p.) injection and to respond on a lever on the alternate side after a saline injection. Following discrimination training, in both dependent and nondependent rats saline was generalized to various doses of clonidine (10, 30 and 50 micrograms/kg i.p.). A response inhibition of about 65% was obtained with the highest dose. It was concluded that, even if clonidine can suppress signs of narcotic withdrawal, the internal state induced by morphine in an abstinent rat does differ from the one induced by clonidine in the same animal.     

Growth hormone responses to clonidine and GRF in spontaneously hypertensive rats: neuroendocrine evidence for an enhanced responsiveness of brain alpha 2-adrenoceptors in genetical hypertension

Clonidine induces growth hormone (GH) release in rat. According to previous investigations this effect is mediated by postsynaptic alpha 2-adrenoceptors in the hypothalamus exerting a stimulatory influence on the recently discovered GH releasing factor (GRF). In the present study it is demonstrated that spontaneously hypertensive rats (SHR) of the Wistar-Kyoto strain display enhanced GH responses to clonidine as compared to normotensive Wistar-Kyoto control rats. In contrast, the GH responses to GRF are similar in hypertensive and normotensive animals. These findings indicate that brain alpha 2-adrenoceptors are more responsive in SHR than in normotensive controls. Since the enhanced GH responses to clonidine were observed also in young, prehypertensive SHR they are probably not secondary to the elevated blood pressure. The possible importance of an altered alpha 2-adrenergic neurotransmission for the development of elevated blood pressure in SHR is discussed.     

Central and peripheral components of dermorphin's effect on rat intestinal propulsion in comparison to morphine

Dermorphin, injected intracerebroventricularly (ICV) to rats, provokes, like to morphine, an inhibition of intestinal propulsion linearly related to the log of the administered doses (in the range from 0.06 to 0.56 μg/rat), but it is 143 times more active than morphine. Naloxone, ICV or IP, antagonizes dermorphin less effectively than morphine. Quaternary naloxone ICV administered antagonizes the intestinal effect of ICV dermorphin, while IP administered it is not effective until 8 mg/kg. The dose of dermorphin maximally active by the ICV route (0.56 μg/rat) is completely inactive when injected IP. Increasing doses of dermorphin IP (from 12 to 6400 μg/kg) inhibit intestinal propulsion to the same extent irrespectively of the doses employed, but never by more than 50%. Only a high dose of naloxone (30 mg/kg/IP) antagonizes this IP effect. The central and peripheral components of this intestinal effect of dermorphin are discussed.     

Reduced tolerance to morphine thermoregulatory effects in senescent rats

Age-related differences in the thermoregulatory response to morphine have been shown in rats. To determine if these age-related differences would be reflected in the acquisition of tolerance, we studied morphine tolerance induced by either a single morphine dose or implantation of a morphine pellet. precipitated withdrawal was also analyzed by inducing withdrawal with naloxone in morphine-pelleted rats. Senescent (26 or 27 month old), mature (10 or 11 month old) and young (3 or 4 month old) male Fischer 344 rats were restrained and changes in rectal temperature were monitored for six hours after morphine administration. Only mature and young rats exhibited increased hyperthermic responses to a second low dose of morphine (5 mg/kg s.c.). Only young rats became tolerant after a single higher morphine dose (25 mg/kg s.c.). All age groups showed tolerance three days after morphine pellet implantation. Hypothermia was equivalent in all age groups when withdrawal was induced by naloxone in morphine-pelleted rats. These results indicate that older rats were more resistant to the acquisition of tolerance to the thermic effects of morphine; however; with continued morphine treatment, rats became tolerant regardless of age.     

Treatment with pertussis toxin does not prevent central effects of eel calcitonin.

To determine whether or not the CNS inhibitory activity of eel calcitonin (eCT) on adenylyl cyclase is the endocellular mechanism underlying the antinociceptive effect of the peptide, as shown for morphine analgesia, we administered Bordetella pertussis toxin (PTX) by intracerebroventricular (ICV) injection (0.5 microgram/rat) to block the receptor-mediated inhibition of adenylyl cyclase. In PTX-treated rats there was no change in eCT (2.5 micrograms/rat, ICV)-induced antinociceptive activity (hot-plate test) nor in eCT (100 ng/rat, ICV) inhibition of gastric acid secretion (Shay test) whereas morphine (5 micrograms/rat, ICV) analgesia was significantly reduced. In vitro studies showed no reduction of eCT binding in the CNS of rats treated with PTX in vivo. Moreover, PTX treatment did not change the inhibitory effect of eCT on adenylyl cyclase in isolated membranes from rat striatum in contrast with opiates (DAME and morphine) whose effects were lost. As PTX is known to inactivate the guanidine binding inhibitory protein Gi, these data suggest that a G protein, distinct from the Gi protein involved in the coupling of opiate receptors into a functional response, could be responsible for regulating the intracellular pathways resulting in eCT-induced antinociceptive effect and inhibition of gastric acid secretion.     

Chronic central ghrelin infusion reduces blood pressure and heart rate despite increasing appetite and promoting weight gain in normotensive and hypertensive rats

Acute studies showed that ghrelin acts on the central nervous system (CNS) to reduce blood pressure (BP), heart rate (HR) and sympathetic activity. However, the long-term CNS cardiovascular actions of ghrelin are still unclear. We tested whether chronic intracerebroventricular (ICV) infusion of ghrelin causes sustained reductions in BP, HR and whether it alters baroreceptor sensitivity (BRS) and autonomic input to the heart. A cannula was placed in the lateral ventricle of male Sprague–Dawley (SD) rats for ICV infusions via osmotic minipump (0.5 μl/h). BP and HR were measured 24-h/day by telemetry. After 5 days of control measurements, ghrelin (0.21 nmol/h) or saline vehicle were infused ICV for 10 days followed by a 5-day post-treatment period. Chronic ICV ghrelin infusion increased food intake (22 ± 3 to 26 ± 1 g/day) leading to ∼50 g body weight gain. BP fell slightly during ghrelin infusion while HR decreased by ∼26 bpm. In control animals BP and HR increased modestly. ICV Ghrelin infusion caused a 50% reduction in sympathetic tone to the heart but did not alter BRS. We also tested if the depressor responses to ICV ghrelin infusion were enhanced in spontaneously hypertensive rats (SHR) due to their high basal sympathetic tone. However, we observed similar BP and HR responses compared to normotensive rats. These results indicate that ghrelin, acting via direct actions on the CNS, has a sustained effect to lower HR and a modest impact to reduce BP in normotensive and hypertensive animals despite increasing appetite and body weight.     

Sensitivity and adrenoceptor affinity in the mesenteric artery of the deoxycorticosterone acetate hypertensive rat

This study examines vascular reactivity to alpha-adrenoceptor agonists in mineralocorticoid (deoxycorticosterone acetate (DOCA-salt) hypertensive and normotensive rats. The rats were anesthetized and the mesenteric artery was excised and cut helically into strips that were mounted in a muscle bath for the measurement of isometric force development. Addition of norepinephrine, epinephrine, phenylephrine, methoxamine, or clonidine to the bath caused contractions in all arteries. Arteries from hypertensive rats were more sensitive (lower ED50 values) to each of the agonists than arteries from normotensive rats. alpha-Adrenoceptor affinity for phentolamine (Schild analysis; norepinephrine as the agonist) in hypertensive arteries was not significantly different from that in normotensive arteries. Maximal force generation to clonidine was greater in hypertensive arteries than in normotensive arteries. These results demonstrate an augmented vascular sensitivity to several alpha-adrenoceptor agonists in DOCA hypertensive rats. This change in sensitivity is independent of a change in affinity for the adrenoceptor antagonist, phentolamine. It may be that a change in receptor number or an alteration in a post-receptor activation event accounts for this enhanced adrenoceptor responsiveness in mineralocorticoid hypertension.     

Evidence for antagonistic activity of endothelin for clonidine induced hypotension and bradycardia.

Effect of endothelin (ET) on clonidine induced cardiovascular effects was studied in male Sprague-Dawley rats. Clonidine (75 micrograms/kg, iv) produced significant decrease in blood pressure and heart rate. ET-1 (50 ng/kg, iv) pretreatment completely antagonized the hypotension and bradycardia induced by clonidine. ET-2 (50 ng/kg, iv) and ET-3 (50 ng/kg, iv) had similar antagonistic effect on clonidine induced hypotension and bradycardia. The antagonistic effect of ET lasted for several hours, however, 4 hours after ET pretreatment only partial blockade of clonidine induced hypotension and bradycardia was observed. This indicated that the antagonistic effect of ET was reversible. Initial hypertensive response induced by high dose of clonidine (750 micrograms/kg, iv) could not be antagonized by ET-1, ET-2 or ET-3, while phenoxybenzamine, an alpha adrenoceptor antagonist, blocked the hypertensive response of clonidine. Thus, ET has no antagonistic effect on the initial hypertensive response but antagonizes the hypotensive and bradycardic effect induced by clonidine. Clonidine induced hypotension and bradycardia are mediated through central alpha 2 adrenoceptors while hypertension is mediated through peripheral alpha 2 adrenoceptors. It is concluded that central alpha 2 adrenoceptors are different from peripheral alpha 2 adrenoceptors and ET antagonizes the effect of clonidine only on central alpha 2 adrenoceptors but has no antagonistic activity on peripheral alpha 2 adrenoceptors.