Hemodynamic and neurohormonal effects of clonidine in patients with preganglionic and postganglionic sympathetic lesions. Evidence for a central sympatholytic action

Rocuronium-induced neuromuscular block after long pretreatment of clonidine in rabbits

Background

Clonidine, an α-2 adrenergic agonist, is used in the perioperative period and in intensive care for the management of hypertension. The in vivo and in vitro effects of clonidine on the actions of nondepolarizing neuromuscular blocking drugs are conflicting. We evaluated the potency and time course of rocuronium-induced neuromuscular block after prolonged pretreatment with clonidine in rabbits.

Methods

Sixty rabbits were randomly assigned to three groups; control (C) group: normal saline 0.1 ml/kg daily subcutaneous for 6 weeks; S3 group: clonidine 4 µg/kg daily subcutaneous for 3 weeks; S6 group: clonidine 4 µg/kg daily subcutaneous for 6 weeks. The dose-response relations of rocuronium were tested in 30 rabbits (10 from each of the three groups) during ketamine-thiopental anesthesia, while the time course of rocuronium 0.6 mg/kg was examined in 10 rabbits each from the three groups.

Results

There was no difference in mean arterial pressure and pulse rate among the experimental groups. The calculated ED₅₀ for rocuronium decreased significantly from 64.1 µg/kg (C group) to 50.3 µg/kg (S3 group) and 47.8µg/kg (S6 group) (P < 0.001). There was no difference in the onset and the recovery times after rocuronium.

Conclusions

Rocuronium after pretreatment with clonidine for three or six weeks may have an increased effect, but no difference in the duration of action compared with control group.

Role of sympathetic responses on the hemodynamic consequences of rapid changes in posture in humans

Tolerance to +G(z) gravitational stress is reduced when +G(z) stress is preceded by exposure to hypogravity (fractional, 0, or negative G(z)). For example, there is an exaggerated fall in eye-level arterial pressure (ELAP) early on during +G(z) stress (head-up tilt; HUT) when this stress is immediately preceded by -G(z) stress (head-down tilt; HDT), termed the "push-pull effect." The aim of the present study was to test the hypothesis that sympathetic responses contribute to the push-pull effect. Young, healthy subjects (n = 7 males and 3 females) were subjected to 30 s of 30 degrees HUT from a horizontal position and to 30 s of 30 degrees HUT when HUT was immediately preceded by 20 s of -15 degrees HDT. Four bouts of HDT-HUT were alternated between five bouts of HUT in a counterbalanced design, and 1 min was allowed for recovery between tilts. This protocol was repeated during clonidine administration (2.5 microg/kg bolus over 30 min and then continuously at 0.36 microg x kg(-1) x h(-1)). Clonidine blunted the vasomotor responses to tilting, and this led to exaggerated changes in arterial pressure. Clonidine exerted little specific influence on the push-pull effect. Thus sympathetic responses appear neither to contribute to, nor protect against, the push-pull effect for the rate and duration of tilting imposed in the present study.

Hemodynamic effects of clonidine in two contrasting models of autonomic failure: Multiple system atrophy and pure autonomic failure

We assessed the effects of clonidine on blood pressure (BP) and heart rate (HR) in multiple system atrophy (MSA), where the autonomic nervous system lesion site is preganglionic, and in pure autonomic failure (PAF), where it is postganglionic. In normal subjects, intravenous infusion of the selective alpha2-adrenoceptor agonist clonidine reduces BP and plasma noradrenaline (NA) levels by means of central alpha2-adrenoceptor action, as well as inducing growth hormone (GH) release. Clonidine-induced GH release is impaired in MSA but spared in PAF. However, the hemodynamic effects of clonidine have not been studied extensively in these disorders. We examined intravenous clonidine test results (performed in our autonomic laboratories using the London Autonomic Units protocol) in 58 patients: 39 with probable MSA and 19 with PAF. Systolic BP (SBP), diastolic BP (DBP), HR, and NA levels were measured supine at baseline and for up to 60 minutes after clonidine. Clonidine resulted in a significant BP fall in MSA patients, which occurred earlier (within 15 minutes of clonidine) and to a greater extent than seen in PAF patients. MSA and PAF patients showed reduction in HR after clonidine administration, although this finding was significantly greater in MSA than in PAF patients. NA levels decreased significantly after clonidine administration in both groups. Although basal NA levels were lower in PAF than in MSA patients, there was no difference in NA reduction relative to baseline between groups. MSA patients showed significant negative correlation between basal NA levels and BP response to clonidine. Clonidine infusion reduces BP and HR in both MSA and PAF groups but to a greater extent in MSA patients. The greater vasodepressor action of clonidine in MSA patients suggests that there is partial preservation of brainstem sympathetic outflow pathways in MSA and may reflect its action at sites in the brainstem and spinal cord that were in part functionally preserved in MSA. Despite similar degrees of NA reduction after clonidine administration, the vasodepressor effect of clonidine was attenuated in PAF compared with MSA patients. This attenuation in PAF patients may reflect greater peripheral alpha2-adrenoceptor denervation supersensitivity due to the postganglionic lesion site. These BP differences, thus, may reflect the underlying lesion site in MSA and PAF, and the hemodynamic data after clonidine infusion may help differentiate these conditions.

Withdrawal syndrome following cessation of antihypertensive drug therapy

In this study, a review of the available information concerning abrupt withdrawal of antihypertensive drug therapy is presented. Abrupt withdrawal of these drugs can produce a syndrome of sympathetic overactivity that includes nervousness, tachycardia, headache, agitation and nausea 36-72 h after cessation of the drug. A withdrawal syndrome may occur after discontinuation of almost all types of antihypertensive drugs, but mostly occurs with clonidine, beta-blockers, methyldopa and guanabenz. Less commonly can produce a rapid increase of the blood pressure to pre-treatment levels or above, or both and/or myocardial ischaemia. Although the exact incidence of the syndrome is not known, it appears to be rare, at least in patients receiving standard doses of the above antihypertensive drugs. The best treatment is prevention. In this study regarding the withdrawal syndrome that follows cessation of antihypertensive drugs therapy, a reference to the abrupt discontinuation of the main categories of antihypertensive drugs is also attempted.

Clonidine Lowers Blood Pressure by Reducing Vascular Resistance and Cardiac Output in Young, Healthy Males

PURPOSE

Clonidine is a classical sympatholytic drug that is widely used for the treatment of hypertension. Experimental and clinical studies suggest that Clonidine may activate baroreflex. The aim of this study was to determine the hemodynamic response to Clonidine under physiological conditions and to test the hypothesis that Clonidine would reduce cardiac output and blood pressure resulting in an increase in total peripheral resistance.

METHODS

Clonidine's hemodynamic effect was evaluated in 28 young, healthy subjects after a single i.v. dose of 1 microg x kg(- 1). Impedance cardiography, systolic time intervals and pulse wave analysis were used to characterize myocardial and vascular function.

RESULTS

Clonidine lowered blood pressure, heart rate, left ventricular ejection time, and pulse wave velocity and increased pre-ejection period. Stroke volume and cardiac output decreased gradually over the investigation time of 240 min. Central systolic blood pressure (SBP) was lowered to a larger extent than peripheral SBP. Total peripheral resistance was characterized by an immediate fall of short duration followed by a continuous rise above baseline after 120 min. Placebo did not have any significant effect on hemodynamic parameters.

CONCLUSIONS

Clonidine's blood pressure lowering effect is mediated by both an immediate decrease in vascular resistance and a prolonged decrease in cardiac output, and Clonidine lowers central SBP more than peripheral SBP.

Imidazoline antihypertensive drugs: a critical review on their mechanism of action

It was long thought that the prototypical centrally acting antihypertensive drug clonidine lowers sympathetic tone by activating alpha(2)-adrenoceptors in the brain stem. Supported by the development of two new centrally acting drugs, rilmenidine and moxonidine, the imidazoline hypothesis evolved recently. It assumes the existence of a new group of receptors, the imidazoline receptors, and attributes the sympathoinhibition to activation of I(1) imidazoline receptors in the medulla oblongata. This review analyzes the mechanism of action of clonidine-like drugs, with special attention given to the imidazoline hypothesis. Two conclusions are drawn. The first is that the arguments against the imidazoline hypothesis outweigh the observations that support it and that the sympathoinhibitory effects of clonidine-like drugs are best explained by activation of alpha(2)-adrenoceptors. The second conclusion is that this class of drugs lowers sympathetic tone not only by a primary action in cardiovascular regulatory centres in the medulla oblongata. Peripheral presynaptic inhibition of transmitter release from postganglionic sympathetic neurons contributes to the overall sympathoinhibition.

Age- and gender-related growth hormone responses to intravenous clonidine in healthy adults

The alpha(2)-adrenoceptor agonist clonidine stimulates growth hormone (GH) release in both animals and humans. It has been used to test for GH deficiency in children, to assess central alpha(2)-adrenoceptor function in adults and to determine the pathophysiological basis and to confirm diagnosis in neurological diseases with autonomic failure. The dose and mode of administration, however, may be important, as in some studies in adults oral clonidine has minimal effects on GH. We report our experience following intravenous (i.v.) clonidine (2 microg/kg) in 98 normal adults on the neuroendocrine (GH, insulin, glucose and catecholamine) and cardiovascular (blood pressure) responses. In males between 25 and 89 years and females between 25 and 64 years there was a significant rise in GH secretion (P < 0.05) after clonidine. Males showed an age-sensitive secretory pattern, with the greatest response between 25 and 35 years (P < 0.02). Younger males (< 45 years) had significantly higher peak GH levels post-clonidine than younger females < 45 years (P < 0.03). No sex-related change was observed in older subjects (< 45 years). Clonidine caused a significant fall in plasma noradrenalin and adrenalin in all age-sex groups (P< 0.001). There were no significant changes in glucose or insulin. There were no effects of age on the fall in blood pressure induced by clonidine. In conclusion, i.v. clonidine stimulated GH in all age groups and there was a marked sexually dimorphic pattern in adults < 45 years. The results overall suggest that i.v. clonidine-GH testing provides a reliable method for investigation of central alpha(2)-adrenergic function in adult humans.

Sympathetic inhibition with clonidine prolongs survival in experimental chronic heart failure

Activation of the sympathetic nervous system is associated with increased mortality in congestive heart failure (CHF), and inhibition of the sympathetic nervous system by centrally acting sympatholytic agents has been shown to have beneficial effects on hemodynamics in these patients. However, the effect of sympathetic inhibition on survival in CHF is not clear. In the present study, the effect of sympathetic inhibition with clonidine on survival was examined in a rat model of heart failure. Myocardial infarction and heart failure was induced in rats by ligation of the left coronary artery and sham-operated rats served as the control. Two weeks after surgery, the ligated rats were randomly assigned to the clonidine (100 microg kg(-1) d(-1), n=30) group or the placebo (vehicle, n=31) group. All rats were followed daily for a 1-year period or until spontaneous death. Compared with placebo therapy, clonidine treatment reduced systolic blood pressure and heart rate throughout the experimental period. The plasma norepinephrine level determined at the end of the experiment was also reduced. Long-term sympathetic inhibition with clonidine treatment improved 1-year survival (50% vs. 22.6%, P<0.05) after surgery in this rat model of CHF.

Role of the sympathetic control of vascular resistance in ethanol-clonidine hemodynamic interaction in SHRs

Our previous studies showed that ethanol selectively counteracts centrally mediated hypotensive responses. In this study, we investigated the role of sympathetic nerve activity, cardiac output (CO), and total peripheral resistance (TPR) in this antagonistic hemodynamic interaction between ethanol and clonidine. Changes in blood pressure (BP), heart rate (HR), CO, stroke volume (SV), and TPR elicited by intracisternal (i.c.) clonidine and subsequent ethanol or saline were evaluated in conscious freely moving spontaneously hypertensive rats (SHRs). Clonidine (0.5 microg, i.c.) evoked hypotension was due to a significant reduction in TPR (from 3.6+/-0.21 to 2.8+/-0.17 mm Hg/ml/min/100 g), which was associated with a significant (p < 0.05) reduction in plasma norepinephrine (NE, from 660+/-115 to 310+/-50 pg/ml), measured as index of sympathetic activity. Ethanol (1 g/kg, i.v.) counteracted the hypotensive effect of clonidine and produced significant (p < 0.05) increases in plasma NE and TPR. Further support for the hypothesis that ethanol selectively counteracts centrally mediated hypotension was sought by investigating the effect of ethanol on peripherally mediated hemodynamic responses to hydralazine. Hydralazine (0.4 mg/kg, i.v.) produced a hypotension similar in magnitude to that produced by clonidine, which was also due to a significant reduction in TPR. However, unlike the case with clonidine, reflex increases in HR, SV, and hence CO were evident. Ethanol given after hydralazine produced a short-lived pressor effect (<10 min vs. 60 min in case of clonidine) in spite of a sustained increase in TPR. The latter was offset by the simultaneous decreases in CO, SV, and HR. A 30% increase in plasma NE caused by hydralazine returned to baseline level after ethanol or saline. Blood ethanol concentrations were similar in all treatment groups. These findings suggest that ethanol selectively counteracts centrally evoked hypotensive responses by counteracting the sympathoinhibition-mediated decreases in TPR elicited by centrally administered clonidine in conscious SHRs.

Abnormal suppression of arginine-vasopressin by clonidine in multiple system atrophy

In normal man, the centrally active alpha2-adrenoceptor agonist clonidine reduces arginine-vasopressin (AVP) secretion, probably by presynaptic inhibition of noradrenergic neuron terminals in the supraoptic nucleus. A lesion of noradrenergic pathways in animals abolishes this response to clonidine. At postmortem in multiple system atrophy (MSA) there is marked loss of hypothalamic noradrenergic innervation. We hypothesized that the AVP response to clonidine in MSA may be abnormal and therefore studied the AVP response to clonidine (2 microg/ kg iv) in 10 subjects with MSA and compared them to six healthy age-matched control subjects. Basal levels of AVP were similar in controls and MSA. Following clonidine there was a significantly greater fall in controls than MSA (-47 +/- 4% vs -25 +/- 6%; p < 0.05). There was a similar fall in mean arterial pressure (MAP) and plasma catecholamines in both groups, with no change in plasma osmolarity, excluding these as a contributary factor. In conclusion, there is an abnormal AVP response to clonidine in MSA, which probably represents loss of functional noradrenergic innervation of the supraoptic nucleus.

Cardiac and systemic sympathetic activity in response to clonidine in human heart failure

OBJECTIVES

We studied the effects of clonidine on cardiac sympathetic activity and left ventricular function in patients with congestive heart failure (CHF).

BACKGROUND

Sympathetic activation has major prognostic implications in patients with heart failure. Clonidine, an imidazoline and alpha2-receptor agonist, has been shown to cause a reduction in generalized sympathetic activity.

METHODS

Nine patients with CHF (left ventricular ejection fraction 22+/-4% [mean+/-SEM]) received a 50 microg and 100 microg bolus of clonidine intravenously. Study measurements included right and left heart hemodynamics, cardiac output, rate of rise in left ventricular peak positive pressure (LV + dP/dt) and tau, along with cardiac and total body norepinephrine spillover. The radiotracer method was used for calculation of norepinephrine spillover.

RESULTS

Right and left heart filling pressures did not change in response to either dose of clonidine. Mean arterial pressure fell after the second dose of clonidine, from 94+/-8 to 82+/-6 mm Hg (p < 0.05). The LV + dP/dt was reduced from 737+/-53 to 629+/-43 mm Hg/s (p < 0.05). Clonidine also caused a significant increase in tau, as measured by the method of Weiss (65+/-3 vs. 74+/-4 ms, p < 0.01) and the direct pressure half-time technique (48+/-2 vs. 54+/-3 ms, p < 0.01). Cardiac norepinephrine spillover fell from 121+/-29 to 52+/-20 pmol/min in response to 100 microg of clonidine (p < 0.01 vs. control).

CONCLUSIONS

Despite a significant fall in arterial pressure, clonidine caused a marked reduction in sympathetic activity directed at the heart. The negative inotropic and lusitropic effects appear to be secondary to this reduction in sympathetic drive. Because increased cardiac and generalized sympathetic activity are strong predictors of an adverse outcome in patients with CHF, the role of centrally active sympathoinhibitory agents in the therapy of CHF deserves further exploration.

Free plasma catecholamines in spinal cord injured persons with different injury levels at rest and during exercise

Spinal cord lesion leads to an interruption of pathways from brain to the peripheral sympathetic nervous system, which results in pathological changes in sympathetic innervation. Free epinephrine (E), norepinephrine (NE) and dopamine (DA) were measured in 30 tetraplegics (TETRA), 15 high-lesion paraplegics (T1 and T4, HPARA), 15 paraplegics with injuries between T5 and T10 (MPARA), 15 low-lesion paraplegics (below T10, LPARA) and 16 non-handicapped control persons (C) at rest, at 60 and 100% of maximal oxygen consumption during graded wheelchair ergometry (WCE). The TETRA showed significant lower E and NE levels at rest and only slight increases during physical exercise. The E and NE concentrations of the paraplegics with a lesion below T5 were significantly higher than those of the high-lesion paraplegics, as well as those of the control persons at every point in the study. All paraplegics and the control persons showed, at submaximal and maximal exercise, a significant increase in NE. Only a slight increase in E in HAPRA was shown. No differences were found at rest and during exercise in E and NE levels in the MPARA and LPARA. No significant differences were found in the dopamine concentration at rest or during exercise for any of the groups. In summary, different levels of lesion and the resulting interruption to sympathetic pathways in the spinal cord are decisive factors in the degree of impairment of sympathetic innervation in SCI persons. Tetraplegics show less preganglionic resting activity because of interruption of impulses from central centers and no considerable stimulation of the sympathetic nervous system during maximal exercise. Individuals with high paraplegia have a partial impairment of catecholamine release, especially of epinephrine, at rest and during exercise. Paraplegics with a lesion level below T5 showed an augmented basal and exercise-induced upper spinal thoracic sympathetic activity in comparison to control persons.

Separation of peripheral and central cardiovascular actions of angiotensin II

The pressor and vasoconstrictor action of angiotensin II (ANG II) is considered to be caused by a combination of its direct and indirect vascular effects, the latter mediated by the sympathetic nervous system. The purpose of this study was to determine the extent to which the direct and indirect actions of ANG II contribute to its pressor and vascular effects. Blood pressure, cutaneous vascular, and plasma norepinephrine responses to intravenous ANG II were measured in conscious rabbits before and after inhibition of central sympathetic outflow with intravenous and intracisternal clonidine and after ganglionic blockade with intravenous pentolinium. Intravenous ANG II caused a similar dose-related rise in blood pressure before and after sympathetic blockade with intravenous clonidine, intracisternal clonidine, and intravenous pentolinium. In contrast, the dose-related fall in cutaneous ear blood flow and cutaneous ear temperature and rise in cutaneous ear vascular resistance induced by intravenous ANG II were abolished after intravenous clonidine, intracisternal clonidine, and intravenous pentolinium. Heart rate was unchanged after ANG II. There were no changes in back skin or rectal temperature. There was a nonsignificant fall in plasma norepinephrine and no change in epinephrine after ANG II. These results demonstrate that the acute pressor response to intravenous ANG II is mediated by its direct vascular effects and is not dependent on central or ganglionic stimulation of the sympathetic nervous system, in contrast to the effect of ANG II on cutaneous ear vasoconstriction, which is predominantly caused by a centrally mediated increase in sympathetic nervous activity. Our results separate, in conscious rabbits, the direct vascular effects of ANG II from its indirect vascular actions, which are mediated by central sympathetic stimulation in the brain.

The I1-imidazoline-binding site is a functional receptor mediating vasodepression via the ventral medulla

I1-imidazoline-binding sites fulfill all essential criteria for identification as receptors, including specificity of binding, association with physiological functions, appropriate anatomic and cellular and subcellular localization, and specific cell signaling pathways. Moreover, binding affinities correlate with functional drug responses. The evidence linking I1 receptors to vasodepression includes expression in RVLM and consistent correlations between vasodepressor potency in humans and animals and I1 binding affinity. Some I1 agonists are antagonists at alpha 2-adrenergic receptors (alpha 2AR), and these elicit vasodepression in RVLM. Potent alpha 2-agonists with phenylethylamine or guanidine structures are inactive in RVLM, yet highly effective in nucleus of the solitary tract, a region with well-defined alpha 2-mediated vasodepressor responses. Selective I1 agonists are used clinically to lower blood pressure with minimal alpha 2-mediated sedation. Moreover, when microinjected into the RVLM only antagonists active at I1 receptors can block the vasodepressor action of either local or systemic imidazolines. RVLM alpha 2-blockade has no effect. Some reports appear to conflict with the I1 receptor hypothesis; but these reports often make incorrect assumptions regarding drug specificity, overlook systemic effects of alpha 2-antagonists, or inappropriately analyze data. Blockade of gamma-aminobutyric acid (GABA) receptors blocks the vasodepressor action of imidazolines, implying a multisynaptic pathway. Thus imidazolines act via I1 receptors in RVLM to lower blood pressure, although alpha 2AR are also important, especially in NTS.

Oral clonidine premedication decreases energy expenditure in human volunteers

PURPOSE

Clonidine not only stops postoperative shivering and decreases oxygen consumption, but also decreases energy expenditure with or without a reduction in shivering during recovery from anaesthesia. It is important to see if clonidine decreases energy expenditure at rest since this may contribute to a postoperative decrease in energy expenditure. The authors tested the hypothesis that oral clonidine decreases energy expenditure at rest.

METHODS

Twenty healthy male volunteers were randomly assigned to one of two groups. Ten volunteers received oral clonidine approximately 5 micrograms.kg-1 (clonidine group), while the remaining 10 volunteers received placebo (control group). Blood pressure, heart rate, body temperature at the tympanic membrane, sedation score graded from 1 (alert) to 5 (sleeping and difficult to be aroused by tactile stimulation) were measured before and at 30-min intervals for three hours after administration of clonidine or placebo. Measurements of energy expenditure and respiratory quotient were made with a head canopy system at one-minute intervals and averaged over 15 min before, and at 30, 60, 90, 120, and 180 min after administration of clonidine or placebo.

RESULTS

Sedation score increased from 1 to 3 (median) after clonidine administration. Energy expenditure decreased from 1452 +/- 225 kcal.24hr-1 (mean +/- SD) at baseline to 1258 +/- 175 kcal.24hr-1 at 180 min after clonidine administration (P < 0.05).

CONCLUSION

This study suggests that oral clonidine at a dose of 5 micrograms.kg-1 decreases energy expenditure at rest.

Pharmacological manipulation of human gastrointestinal blood flow

The splanchnic circulation is one of the largest vascular regions in man. In the past, this has been difficult to study because of methodological problems. The adapting of noninvasive Doppler techniques has made it possible to develop reproducible measurements of coeliac and superior mesenteric artery blood flow, which are the main contributors to the gastrointestinal vasculature. This has resulted in the further understanding of neurogenic and humoral control of this region in a number of physiological and pathophysiological states, and has contributed towards the knowledge of its pharmacological control. These studies are of relevance to cardiovascular homeostasis and, in particular, systemic blood pressure control which depends upon various factors including responses in different vascular regions. In this review the key physiological factors which influence pharmacological studies on this circulation will be discussed. Examples will be provided, in subjects with cardiovascular and neurological disorders, of how administration of endogenous and exogenous substances, including drugs with specific pharmacological effects, alter human gastrointestinal blood flow. These will include insulin, alcohol, the somatostatin analogue octreotide, the central acting sympatholytic clonidine and the angiotensin II-converting inhibitor captopril. The relevance of these studies to subjects with postural hypotension due to sympathetic denervation and to primary hypertension, in particular, will be discussed.

Comparative assessment of the anaesthetic and analgesic effects of intramuscular and epidural clonidine in humans

PURPOSE

The aim of the study was to assess and compare in analogous controlled experimental conditions, the anaesthetic sparing and analgesic effects of the same dose of clonidine administered by the intramuscular (im) and epidural (ep) routes.

METHODS

We used a randomized, double blind and placebo controlled protocol. Sixty patients undergoing abdominal hysterectomy were distributed into three groups who, 30 min before surgical incision, received: 300 micrograms ep clonidine plus im saline; ep saline plus 300 micrograms im clonidine; or ep and im saline (ss). General anaesthesia was maintained with 60% N2O in O2, and isoflurane administered at concentrations to maintain mean arterial pressure (MAP) and heart rate (HR) within 20% of basal values. Isoflurane requirements (mass spectrometry), cardiovascular variables (MAP, HR), and plasma concentrations of glucose, cortisol and prolactin were evaluated at critical time points. In the recovery room (RR), sedation (Ramsay) and pain intensity (VAS) were estimated at the time of analgesia request (TAR).

RESULTS

Intramuscular and ep clonidine decreased isoflurane requirements similarly by about 85% (P < 0.001). Patients in the ep group had lower MAP (P < 0.03) and HR (P < 0.001) than in the im group, but im and ep clonidine similarly blunted the plasma prolactin increase induced by intubation. In RR, ep but not im clonidine (P < 0.01) induced postoperative analgesia demonstrated by a prolonged TAR 80.8 +/- 7.3 (ep) 35.9 +/- 3.2 (im) and 44.5 +/- 5.1 (ss) min and a lower VAS (P < 0.05).

CONCLUSIONS

Epidural and intramuscular clonidine decreased isoflurane requirements similarly, but only the epidural route provided postoperative analgesia, suggesting a spinal site for the analgesic action.

Oral clonidine does not alter vecuronium neuromuscular blockade in anaesthetized patients

Since clonidine, an alpha 2-agonist, inhibits the release of norepinephrine or acetylcholine which can decrease nondepolarizing muscle relaxant-induced neuromuscular blockade, the authors examined whether clonidine given as an oral preanaesthetic medication would alter the onset, duration or recovery of a vecuronium neuromuscular blockade in lightly anaesthetized patients. Thirty-eight patients (aged 20-73 yr) randomly received oral clonidine either approximately 5 micrograms.kg-1 (n = 21) or none (n = 17), 90 min before arrival in the operating room. We measured acceleration of thumb contraction with ulnar nerve stimulation at the wrist to assess neuromuscular blockade. The onset time (the time from injection to decrease to 5% of baseline twitch height), duration (the time interval between injection and return of the first twitch to 25% of the baseline value), and recovery index (the time interval of the first twitch from 25% to 75% of the baseline value) of neuromuscular blockade from a single bolus of vecuronium 0.1 mg.kg-1 i.v. were determined and compared between the clonidine-treated and control patients during lower abdominal or extremity surgery under epidural plus general anaesthesia with fentanyl and nitrous oxide in oxygen. No differences were noted between the control and clonidine groups in onset time (100 +/- 6 sec (mean +/- SE) vs 101 +/- 6 sec), duration (44.5 +/- 2.7 min vs 42.9 +/- 2.7 min), or recovery index (21.6 +/- 2.8 min vs 19.1 +/- 1.9 min) of neuromuscular blockade from vecuronium, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Dual effect of clonidine on isolated rabbit pulmonary arteries

Clonidine, a partially selective agonist for alpha 2-adrenoceptors, has been increasingly used in anesthesia. Its direct effect on pulmonary arteries has not yet been clearly characterized. This in vitro study was performed to determine the vasoactive effects of clonidine on isolated rabbit pulmonary arteries. Responses of pulmonary artery rings from New Zealand white rabbits were assessed in the presence and absence of intact endothelium and with or without precontraction by norepinephrine (NE, 3 x 10(-6) M) or potassium chloride (KCl, 3 x 10(-2) M). Using tissue bath preparation, cumulative concentration response curves of clonidine were obtained at different concentrations (10(-8), 10(-7), 10(-6), 10(-5), 10(-4) M) after a period of stabilization. Clonidine caused vasoconstriction of isolated pulmonary arteries without any pretreatment. The magnitude of the constriction was dose related at lower concentrations and reached maximum of 300 g/g wet tissue when above 10(-6) M. On KCl-precontracted pulmonary arteries, clonidine caused significant dose-related vasoconstriction. On the NE-precontracted vessel rings, it elicited significant dose-dependent vasodilation up to 80% relaxation at 10(-4) M. All the above effects were endothelium independent. In conclusion, clonidine has dual endothelium-independent vasoactive effects, causing vasoconstriction on isolated rabbit pulmonary arteries, either untreated or precontracted with KCl, and vasodilation on those precontracted with NE. Clonidine may act as a competitive alpha-adrenoceptor blocking agent.

Ethanol-clonidine hemodynamic interaction in normotensive rats is modified by anesthesia

Our previous findings have shown that ethanol attenuates the decreases in plasma norepinephrine (NE) levels, and blood pressure elicited by centrally acting antihypertensive drugs in spontaneously hypertensive rats (SHRs). The present study investigated whether this interaction can be influenced by baseline blood pressure (BP) and sought direct evidence to support the involvement of the SNS by recording the sympathetic neural activity (SNA) of the splanchnic nerve. The conscious aortic barodenvervated (ABD) rat model was utilized because it exhibits greater hypotensive responses to clonidine compared with sham-operated (SO) rats. Although ABD and SO rats (332 + 33 vs. 227 + 18 pg/ml). Clonidine (30 micrograms/kg, i.v.) elicited significantly greater decreases in mean arterial pressure (MAP; -20.0 + 2.1 vs. -10.4 + 0.8 mm Hg) and plasma NE (-194 + 26 vs. -50 + 11 pg/ml) in conscious ABD vs. SO rats. Ethanol (1 g/kg, i.v.) reversed clonidine-evoked decreases in BP and plasma NE levels, but the interaction was more prominent in ABD rats. To support the hypothesis that the interaction occurs within the CNS, the effect of ethanol was studied on clonidine-evoked decreases in preganglionic SNA and BP in anesthetized rats. In contrast to its effects in conscious rats, ethanol augmented both the hypotensive and sympathoinhibitory responses to clonidine in anesthetized rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone response to clonidine in central and peripheral primary autonomic failure

Patients with primary autonomic failure may have either pure autonomic failure (PAF) or multiple system atrophy (MSA) in which there is additional neurological involvement. Distinction between PAF and MSA at an early stage is important because a wide range of complications is associated with MSA, which has a poor response to drug therapy and a less favourable prognosis. We have investigated the growth hormone (GH) releasing effects of clonidine in patients with PAF and MSA to see whether this hormonal response could serve as a neuroendocrine marker to distinguish between the groups. Age-matched normal subjects were studied as controls. Both groups of patients had severe postural hypotension due to primary sympathetic failure of presumed central origin in MSA and peripheral origin in PAF. After clonidine, plasma GH concentrations increased in controls and PAF, with no change in MSA. Changes in plasma glucose and insulin concentrations were similar in all groups. Clonidine, therefore, stimulates growth hormone release in PAF but not MSA and may serve as a neuroendocrine marker in differentiating patients with MSA and a central autonomic defect from those with PAF with a peripheral defect.