Effects of atipamezole--a selective alpha-adrenoceptor antagonist--on cardiac parasympathetic regulation in human subjects.

1 This double-blind, cross-over, placebo-controlled study on six healthy male volunteers was designed to evaluate the effects of alpha2-adrenoceptor antagonism on cardiac parasympathetic regulation. 2 The subjects received atipamezole intravenously as a three-step infusion, which aimed at steady-state serum concentrations of 10, 30 and 90 ng ml(-1) at 50-min intervals. 3 Drug effects were assessed with repeated recordings of blood pressure and electrocardiogram, in which the high-frequency (0.15-0.40 Hz) R-R interval variation is supposed to reflect cardiac parasympathetic efferent neuronal activity. 4 At the end of the three steps of the infusion, the mean (+/-SD) concentrations of atipamezole were 10.5 (3.9), 26.8 (5.6) and 81.3 (21.1) ng ml(-1). 5 Within this concentration range, atipamezole appeared to reduce slightly the high-frequency R-R interval fluctuations, indicating a minor vagolytic effect in the heart. 6 Atipamezole increased systolic and diastolic arterial pressure, on average by 20 and 14 mmHg (maxima at the second step of the infusion), which evidently reflects an overall sympathetic augmentation.

Reversal of the sedative and sympatholytic effects of dexmedetomidine with a specific alpha2-adrenoceptor antagonist atipamezole: a pharmacodynamic and kinetic study in healthy volunteers.

BACKGROUND: Specific and selective alpha2-adrenergic drugs are widely exploited in veterinary anesthesiology. Because alpha2-agonists are also being introduced to human practice, the authors studied reversal of a clinically relevant dexmedetomidine dose with atipamezole, an alpha2-antagonist, in healthy persons. METHODS: The study consisted of two parts. In an open dose-finding study (part 1), the intravenous dose of atipamezole to reverse the sedative effects of 2.5 microg/kg of dexmedetomidine given intramuscularly was determined (n = 6). Part 2 was a placebo-controlled, double-blinded, randomized cross-over study in which three doses of atipamezole (15, 50, and 150 microg/kg given intravenously in 2 min) or saline were administered 1 h after dexmedetomidine at 1-week intervals (n = 8). Subjective vigilance and anxiety, psychomotor performance, hemodynamics, and saliva secretion were determined, and plasma catecholamines and serum drug concentrations were measured for 7 h. RESULTS: The mean +/- SD atipamezole dose needed in part 1 was 104+/-44 microg/kg. In part 2, dexmedetomidine induced clear impairments of vigilance and psychomotor performance that were dose dependently reversed by atipamezole (P < 0.001). Complete resolution of sedation was evident after the highest (150 microg/kg) dose, and the degree of vigilance remained high for 7 h. Atipamezole dose dependently reversed the reductions in blood pressure (P < 0.001) and heart rate (P = 0.009). Changes in saliva secretion and plasma catecholamines were similarly biphasic (i.e., they decreased after dexmedetomidine followed by dose-dependent restoration after atipamezole). Plasma norepinephrine levels were, however, increased considerably after the 150 microg/kg dose of atipamezole. The pharmacokinetics of atipamezole were linear, and elimination half-lives for both drugs were approximately 2 h. Atipamezole did not affect the disposition of dexmedetomidine. One person had symptomatic sinus arrest, and another had transient bradycardia approximately 3 h after receiving dexmedetomidine. CONCLUSIONS: The sedative and sympatholytic effects of intramuscular dexmedetomidine were dose dependently antagonized by intravenous atipamezole. The applied infusion rate (75 microg x kg(-1) x min(-1)) for the highest atipamezole dose was, however, too fast, as evident by transient sympathoactivation. Similar elimination half-lives of these two drugs are a clear advantage considering the possible clinical applications.

Buccal delivery of an alpha 2-adrenergic receptor antagonist, atipamezole, in humans.

OBJECTIVE: To evaluate the pharmacokinetics, systemic effects and clinical applicability of buccally administered atipamezole in healthy volunteers. METHODS: The study was carried out in two parts. In the first part, spray preparations of atipamezole hydrochloride in water/alcohol (50/50) solution were applied on buccal mucosa of six volunteers. Single doses of 5, 10, 20, and 40 mg atipamezole hydrochloride were administered in ascending order during separate sessions. In the second part, nine subjects received single 20 mg doses as buccal spray, intravenous infusion, or oral solution in randomized order. RESULTS: Values for area under the concentration-time curve for atipamezole (mean +/- SD) ranged from 26 +/- 4 ng x hr/ml after 5 mg to 112 +/- 21 ng x hr/ml after 40 mg and peak concentrations ranged from 11 +/- 3 ng/ml after 5 mg to 38 +/- 9 ng/ml after 40 mg. Individual peak concentrations were mainly measured at 30 and 60 minutes after administration. Mean elimination half-lives were approximately 1 1/2 hours after every treatment. In part two, a mean bioavailability of 33% was calculated for buccal administration (compared with intravenous), whereas systemic availability after an oral dose was < 2%. After intravenous administration the mean total clearance, apparent volume of distribution, and elimination half-life were 1.2 L/hr/kg, 2.9 L/kg, and 1.8 hours, respectively. The intravenous administration of 20 mg atipamezole hydrochloride produced a fivefold elevation in mean plasma norepinephrine concentration, a slight and short-lasting elevation in blood pressure and, in most subjects, increased tension, alertness and restlessness, and sweating. After buccal administration, some subjects reported short-lasting restlessness or tension after the 20 and 40 mg doses. No significant changes in heart rate, blood pressure, or plasma catecholamines were observed. No effects were observed after swallowing of 20 mg atipamezole hydrochloride. The spray caused local reactions at buccal mucosa. Superficial white spots or areas were observed for several hours; these disappeared gradually. Subjects also reported transient numbness at the application site. CONCLUSION: Atipamezole hydrochloride is well absorbed systemically through oral mucosa. The oral bio-availability of atipamezole is negligible, probably because of extensive first-pass metabolism.

The analysis of plasma kinetics and beta-receptor binding and -blocking activity of timolol following its small intravenous dose.

Plasma kinetics and beta-receptor blocking and -binding activity of timolol was studied in six healthy volunteers following its intravenous 0.25 mg dose. Timolol concentrations were measured using radioreceptor assay (RRA), blocking activity by comparing the dose ratios (DRs) of the infusion rates of isoprenaline required to increase heart rate by 25 bpm (I25) and binding activity by determining the extent to which timolol occupied beta 1-receptors of rabbit lung and beta 2-receptors of rat reticulocytes in undiluted plasma samples. Timolol was eliminated from plasma with a mean half-life for the elimination phase of 2.6 hours. The dose antagonized potently isoprenaline-induced tachycardia at least for four hours. The effect was excellently correlated with the estimated beta 2-receptor binding activity of timolol in the circulating plasma. In conclusion, the small intravenous timolol dose was eliminated from plasma by a fashion, which was very similar to its eighty-fold higher oral doses reported earlier in the literature. The 0.25 mg dose was of considerable systemic beta-receptor blocking and -binding activity, that may help to explain its reported side-effects following ocular drug administration. The extent to which beta-blocking agents occupy rabbit lung beta 1- and rat reticulocyte beta 2-receptors in the circulation appears to predict the intensity and selectivity of their beta-blocking effects in healthy volunteers.

Pharmacodynamics and pharmacokinetics of intramuscular dexmedetomidine.

The pharmacodynamics and pharmacokinetics of intramuscular dexmedetomidine--a novel alpha 2-adrenergic receptor agonist under development for preanesthetic use--were studied in healthy male volunteers. Single intramuscular doses of dexmedetomidine (0.5, 1.0, and 1.5 microgram/kg) and placebo were administered to six subjects in a single-blind, multiple crossover study. Dexmedetomidine induced dose-related impairment of vigilance assessed both objectively and subjectively. The drug also caused moderate decreases in blood pressure and heart rate. Plasma norepinephrine was dose-dependently (maximum 89%) decreased. The intramuscular doses resulted in linearly dose-related plasma concentrations of dexmedetomidine. Pharmacokinetic calculations revealed a time to maximum concentration from 1.6 to 1.7 hours, an elimination half-life of 1.6 to 2.4 hours, an apparent total plasma clearance of 0.7 to 0.9 L/hr/kg, and apparent volume of distribution of 2.1 to 2.6 L/kg. The sedative effect of dexmedetomidine dissipated during the 6-hour observation time, but all other effects were still evident 6 hours after administration of the higher doses, paralleling the plasma concentration curves. The relationship of plasma concentrations of dexmedetomidine to pharmacodynamic variables was consistent with a linear pharmacodynamic model. The pharmacodynamic-pharmacokinetic profile of intramuscular dexmedetomidine may be suited to the proposed preanesthetic clinical use of this alpha 2-agonist.

Rapid reversal of alpha 2-adrenoceptor agonist effects by atipamezole in human volunteers.

1. The ability of atipamezole, a specific and selective alpha 2-adrenoceptor antagonist, to reverse the pharmacological effects induced by the alpha 2-adrenoceptor agonist dexmedetomidine was studied in six healthy male volunteers. Each volunteer received in four sessions in a randomized and single-blind manner three different doses (6.7 micrograms kg-1, 27 micrograms kg-1 and 67 micrograms kg-1) of atipamezole or saline placebo as 5 min i.v. infusions preceded by a fixed i.v. dose of dexmedetomidine (0.67 micrograms kg-1). 2. Dexmedetomidine caused profound sedation, with the subjects actually falling asleep. This was effectively reversed by the two highest doses of antipamezole. 3. Dexmedetomidine reduced salivary flow on average by 70%. A rapid and full reversal of this effect was seen after the highest dose of antipamezole. 4. Hypotension induced by dexmedetomidine was also effectively antagonized by atipamezole. Bradycardia was very modest after dexmedetomidine in this study, and thus no reversal of alpha 2-adrenoceptor agonist-induced bradycardia could be demonstrated. 5. Plasma noradrenaline concentrations were reduced by 80% by dexmedetomidine. This was effectively antagonized by atipamezole, and the highest dose caused a 50% overshoot in plasma noradrenaline concentrations over the basal levels. 6. It is concluded that the effects of dexmedetomidine are effectively reversible by atipamezole. A dose ratio of 10:1 for atipamezole:dexmedetomidine was clearly insufficient for this purpose, but ratios in the range of 40:1 to 100:1 were found to be effective in the current experimental situation.

Plasma 3,4-dihydroxyphenylglycol (DHPG) and 3-methoxy-4-hydroxyphenylglycol (MHPG) are insensitive indicators of alpha 2-adrenoceptor mediated regulation of norepinephrine release in healthy human volunteers.

The usefulness of the plasma concentrations of two major metabolites of norepinephrine (NE), 3,4-dihydroxyphenylglycol (DHPG) and 3-methoxy-4-hydroxyphenylglycol (MHPG), as indicators of neuronal NE release was investigated. The potent alpha 2-adrenoceptor agonist, dexmedetomidine, induced only about 15% maximal reductions in the metabolite concentrations, in spite of almost total inhibition of neuronal NE release, as evidenced by 90% reductions in plasma NE concentrations. Similarly, administration of the alpha 2-adrenoceptor antagonist atipamezole was followed by only small increases in plasma DHPG and no change in MHPG levels, in spite of almost six-fold, albeit short-lasting, increases in plasma NE. In contrast, a single dose of the reversible monoamine oxidase type A (MAO-A) inhibitor moclobemide reduced plasma DHPG levels by 78% and MHPG levels by 51%. It is concluded that the plasma concentrations of DHPG and MHPG are largely determined by intraneuronal, MAO-A-dependent metabolism of NE, and do not accurately reflect acute alterations in neuronal NE release. The concentration of NE in venous plasma is clearly a more sensitive indicator of alpha 2-adrenoceptor-mediated regulation of NE release.

Beta-blocking effects of timolol at low plasma concentrations.

The concentration-effect relationship of 0.25 mg intravenous timolol with and without pretreatment with 100 mg quinidine was studied in six healthy young volunteers with a randomized, double-blind, crossover study design. Blockade of cardiac beta-adrenoceptors was assessed by determining the dose ratios (DR) of isoproterenol infusions required to increase heart rate by 25 beats/min before and after timolol infusion. The logarithm of timolol concentration in plasma was linearly related to the logarithm (DR-1) of isoproterenol infusion, with a mean Pearson correlation coefficient of 0.89 +/- 0.11 (+/- SD; n = 24) at timolol concentrations well below 1 ng/ml. The increases in cyclic adenosine monophosphate (cAMP) and norepinephrine plasma levels caused by isoproterenol infusions were attenuated after timolol. Quinidine administration increased timolol plasma levels and cardiac beta-blocking effects by 10% to 40%. It was concluded that timolol at concentrations below 1 ng/ml in plasma competitively antagonizes cardiac and noncardiac effects of isoproterenol infusions. Timolol effects are augmented after quinidine administration. The beta-blockade occurring at low plasma levels can explain side effects and actions of ocularly applied timolol.

Pharmacological effects and pharmacokinetics of atipamezole, a novel alpha 2-adrenoceptor antagonist--a randomized, double-blind cross-over study in healthy male volunteers.

1. Single doses (10, 30 and 100 mg) of atipamezole (MPV-1248), a new potent and selective imidazole-type alpha 2-adrenoceptor antagonist, and saline placebo were administered as 20 min intravenous infusions to six healthy male volunteers in a randomized double-blind, cross-over phase I study. Later, 100 mg atipamezole was given orally to the same subjects in an open fashion. 2. The i.v. doses resulted in linearly dose-related concentrations of atipamezole in plasma. Pharmacokinetic calculations revealed an elimination half-life of 1.7-2.0 h, an apparent volume of distribution of 3.0-3.5 l kg-1 and a total plasma clearance of 1.1-1.5 l h-1 kg-1. No atipamezole could be detected in plasma after oral dosing. 3. Subjective drug effects were seen mainly after the largest i.v. dose and included increased alertness and nervousness, coldness and sweating of hands and feet, tremor and shivering, motor restlessness, and increased salivation. Salivation was also quantitated using dental cotton rolls, with dose-related increases produced by the i.v. doses. 4. The 100 mg i.v. dose increased plasma noradrenaline concentrations on average by 484 +/- 269 (s.d.)%, and also elevated both systolic and diastolic blood pressure (mean increases 17 +/- 7/14 +/- 2 mm Hg). The 30 mg dose had minor and the 10 mg dose no effects on these variables. Adrenaline and cyclic AMP levels in plasma were increased only after the largest dose. No drug effects were observed after oral dosing. 4. Plasma C-peptide and blood glucose levels were not markedly influenced by the drug, and cortisol secretion was not stimulated. 5. The observed effects are compatible with the presumed alpha 2-adrenoceptor antagonistic action of atipamezole and are in general concordance with the reported results of other alpha 2-adrenoceptor antagonists (yohimbine and idazoxan). 6. Although not orally active, atipamezole may prove to be a useful agent in studies of alpha 2-adrenoceptor function in man.

Evidence that the reversible MAO-A inhibitor moclobemide increases prolactin secretion by a serotonergic mechanism in healthy male volunteers.

The serotonin receptor antagonist methysergide was used to investigate the mechanism mediating stimulation of prolactin release after single doses of the reversible MAO-A inhibitor moclobemide. Eight healthy male volunteers participated in a placebo-controlled cross-over study, where pretreatment with methysergide almost totally prevented the moclobemide-induced increase in plasma prolactin levels. MAO-A inhibition, as evidenced by up to 80% decreases in the plasma concentration of 3,4-dihydroxyphenylglycol, a deaminated metabolite of norepinephrine, was similar after both pretreatments. This result suggests that moclobemide stimulates prolactin release through activation of serotonergic receptors, and provides evidence that the drug is capable of augmenting central serotonergic neurotransmission in humans.

No involvement of alpha 2-adrenoceptors in the regulation of basal prolactin secretion in healthy men.

The role of alpha 2-adrenoceptors in the regulation of basal prolactin secretion was investigated in healthy male volunteers. Two independent, placebo-controlled, double-blind experiments were performed. In the first experiment, 50 micrograms dexmedetomidine (MPV-1440), a selective alpha 2-adrenoceptor agonist, and saline placebo were infused (IV) in five subjects. In the second experiment, 100 mg atipamezole (MPV-1248), a selective alpha 2-adrenoceptor antagonist, and saline placebo were infused in six volunteers. The concentrations of prolactin in plasma were not affected by the drugs. These results suggest that alpha 2-adrenoceptors do not a play significant role in the control of basal prolactin secretion in healthy men.

Sympathetic activity and blood pressure increases with bladder distension in humans.

Microneurographic recordings of muscle nerve sympathetic activity, which is governed by baroreceptors and involved in blood pressure regulation, were made in the peroneal nerve in 16 healthy volunteers during physiological bladder distension. When the urge to urinate was pronounced, sympathetic outflow increased from a baseline level of 16.3 +/- 1.7 to 23.2 +/- 1.9 bursts/min (mean +/- SEM, p less than 0.01). There was a concomitant significant rise in both systolic and diastolic blood pressure, from 125 +/- 2/74 +/- 2 to 140 +/- 4/84 +/- 3 mm Hg. After micturition, sympathetic activity and blood pressure returned toward initial values. It is concluded that 1) increased sympathetic outflow contributed to the rise in blood pressure, 2) there is a vesicovascular response mediated by sympathetic vasoconstrictor neurons in humans corresponding to mechanisms observed in animals, and 3) the described functional relation between bladder distension and sympathetic vasoconstrictor activity probably plays a role in clinical conditions such as autonomic dysreflexia in humans with cervical spinal cord lesions and nocturnal micturition syncope.

The cold pressor test: effects on sympathetic nerve activity in human muscle and skin nerve fascicles.

Micro-electrode multi-unit recordings of muscle nerve sympathetic activity (MSA) involved in cardiovascular homeostasis or skin nerve sympathetic activity (SSA) involved in thermoregulation were made in the right peroneal nerve of 48 healthy volunteers during performance of the cold pressor test, i.e. immersion of one hand in ice water (2 +/- 0.5 degrees C) for 1 min. Eleven subjects underwent the same procedure on a second MSA recording occasion. As a rule, immersion evoked an increase in MSA, with a gradual decrease on emersion. The response showed a wide range of variation between and within subjects; the intra-individual difference between first and second immersion on the same recording occasion was up to sevenfold, and from first to second recording up to fivefold. The increase in MSA correlated with the degree of discomfort from the ice water. In nine subjects with a large increase in MSA on ice water immersion, intracutaneous painful electrical stimulation to a level equalling the discomfort from the ice water was added, but it was not accompanied by any change in MSA. The increase in MSA was accompanied by and correlated quite well with an increase in blood pressure. Intra-arterial blood pressure recordings showed that MSA occurred at pressure levels normally associated with total inhibition of MSA, and that an inverse linear relationship between diastolic blood pressure and MSA at rest was abolished during the ice water immersion. SSA showed no consistent change with ice water immersion. It is concluded that the cold pressor test is a powerful activator of MSA, i.e. baroreceptor-governed vasoconstrictor outflow; that MSA contributes to the blood pressure elevation with this manoeuvre; that MSA operates at another blood pressure level during the manoeuvre and that the baroreflex inhibitory level consequently is changed; and that the response is not a reaction to pain only.

Beta-adrenoceptor antagonist activities and binding affinities of timolol enantiomers in rat atria.

S-Timolol is an effective anti-glaucoma drug, but has potentially hazardous side effects. Recently, R-timolol, also, has been reported to be effective in lowering elevated intraocular pressure. In the present study, the beta-adrenoceptor antagonist activities and binding of R- and S-enantiomers of timolol have been examined on rat atrial preparations. The beta-antagonistic activities were investigated using spontaneously beating rat heart atria. Both timolol enantiomers inhibited (-)-isoprenaline-induced chronotropic action competitively. S-Timolol was about 54 times more potent than R-timolol. The apparent binding affinities of timolol enantiomers to beta 1- and beta 2-adrenoceptors were determined by a radioligand binding assay using (-)-[125I]iodocyanopindolol (ICYP) as a marker and CGP 20712 A as a beta 1- and ICI 118,551 as a beta 2-adrenoceptor antagonist. Both enantiomers of timolol inhibited ICYP binding in nanomolar concentrations with Hill coefficients near unity. Neither enantiomer showed selectivity between beta 1- and beta 2-adrenoceptors, but R-timolol was approximately 30 times less active than S-timolol. It is concluded that R-timolol is a relatively potent non-selective beta-adrenoceptor blocking agent, but may possibly exert a more localized beta-adrenoceptor action in the eye than S-timolol, thus improving the safety of ocular timolol therapy.

Hormonal, haemodynamic, and subjective effects of intravenously infused indomethacin: no change in the physiological response to hypertonic saline challenge.

This double-blind, placebo-controlled human study was performed to determine the endocrine responses to intravenously administered indomethacin at two dose rates (0.36 or 0.72 mg/kg bolus followed by 0.071 or 0.143 mg/kg/hr for 150 min.). A 5% hypertonic saline infusion was used for further assess the hormonal systems regulating body fluid and electrolyte balance. Plasma renin activity (PRA) and concentrations of aldosterone and vasopressin (AVP) were unaffected by indomethacin. Hypertonic saline caused a 5% increase in plasma sodium and a 4.2% increase in serum osmolality, with a concomitant two-fold rise in plasma AVP levels and significant declines in PRA and aldosterone. Indomethacin had no effects on these responses, and did not affect plasma catecholamine concentrations, but the hypertonic saline infusion doubled the noradrenaline levels in plasma. Atrial natriuretic peptide (ANP)-like immunoreactivity in plasma was not affected by indomethacin nor by hypertonic saline. The higher dose rate of indomethacin resulted in significant stimulation of growth hormone release, but plasma prolactin levels were not influenced. Thus acute intravenous administration of indomethacin proved to be devoid of significant effects on the multihormonal system regulating fluid and electrolyte balance.

Effect of guar gum on glipizide absorption in man.

The effect of 4.75 g guar gum, a fibre preparation, on the absorption of 2.5 mg glipizide has been studied in 10 healthy volunteers given a standard breakfast. Three different experimental protocols were used: glipizide without guar gum (Treatment 1), glipizide with guar gum (Treatment 2) and guar gum 30 min after the drug together with breakfast (Treatment 3). The serum glipizide at 30 minutes was higher during Treatment 2 than Treatment 3 (p less than 0.01), but neither differed from the control treatment. The AUCs for glipizide were calculated up to 8 hours. They did not differ significantly between the treatment, although there was a non-significant trend to lower values during Treatment 3. Serum insulin and blood glucose levels were determined up to 3 h. Corresponding to differences in the glipizide concentration, serum insulin was highest and blood glucose lowest at 30 minutes during Treatment 2. According to this single dose study, guar gum does not have any substantial deleterious effect on the absorption of glipizide. The lack of effect may be due to the complete gastrointestinal absorption of glipizide.