Intravenous infusion of iloprost in arterial occlusive disease: dose-dependent effects on skin microcirculation

Summary Transcutaneous oxygen pressure (tcPo₂), laser Doppler flux and capillary microscopy have been used to examine the forefoot skin in 5 healthy men and 8 patients with severe peripheral arterial occlusive disease in order to evaluate the dose dependent effects of iloprost on skin microcirculation. Iloprost was infused IV starting at 0.0625 ng·kg^–1·min^–1 and doubling the dose every 15 min up to 2 ng·kg^–1·min^–1.While tcPo₂ at an electrode core temperature of 44°C decreased in both patients and controls, there was a significant dose dependent increase in tcPo₂ (37°C) in the controls from 0.25 ng·kg^–1·min^–1. In the patients the reaction was variable: it was decreased in two and increased in 6, with a maximum either at 0.25–0.5 ng·kg^–1·min^–1 (n=3) or at the highest dose (1.0 or 2.0 ng·kg^–1·min^–1; n=3). Mean laser Doppler flux in both groups was increased, although the reaction was not consistent in the patients. Density of forefoot skin capillaries was reduced in 3 patients, and in the others the flow velocity was very low. During infusion of iloprost, both an increase in capillary density and blood cell velocity were observed. The effects were of variable intensity and occurred at varying doses, some appeared early and diminished as the dose was increased, and others were found only at 2 ng·kg^–1·min^–1.Adverse effects were numerous, extending from harmless skin flushing to mental changes and a quickly reversible attack of angina pectoris. It may be possible to divide patients into those with early effects on the microcirculation, at doses of 0.25–0.5 ng·kg^–1·min^–1, and those in whom the microcirculatory response is preceded and counteracted by the adverse effects.     

Effects of taprostene,a stable prostacyclin analogue,on haemodynamics,platelet function and arachidonate metabolism in healthy volunteers

Summary In order to assess the effect of taprostene on haemodynamics, platelet function and arachidonate metabolism in 4 healthy volunteers an intravenous infusion of 25 ng · kg^–1 · min^–1 was given for 6 h.During the infusion period systolic blood pressure dropped from 130 to 111 mm Hg and diastolic blood pressure from 77 to 69 mm Hg. The heart rate rose from 77 to 84 beats/min.During the taprostene infusion the slope and height of the ADP and collagen induced platelet aggregation curves were significantly inhibited and the sensitivity of platelets to PGI₂ and PGE₁ was increased.Plasma and serum thromboxane B₂, conversion of exogenous radiolabelled arachidonic acid, WU-test, circulating endothelial cell count, concentration of platelet factor 4, -thromboglobulin, malondialdehyde and the PGI₂-synthesis stimulating plasma factor did not show any clear drug-related alteration.It is concluded that infusion of taprostene 25 ng · kg^–1 · min^–1 caused measurable inhibition of platelet function ex vivo.     

Prostaglandin E1 increases binding of 123I-low-density lipoprotein to the human liver in vivo

Previous in vitro radioligand binding data have shown that prostaglandin E₁ (PGE₁) increases the number and the binding affinity of low-density lipoprotein (LDL) receptors of the human liver. Experimental data in normo- and hypercholesterolaemic rabbits have confirmed these findings, showing a significant increase in LDL-binding to the liver in vivo after prolonged PGE₁ therapy. Methods: This study aimed to confirm the experimental and animal data in human in vivo. ¹²³I-LDL binding to the liver was quantified in vivo in patients suffering from peripheral vascular disease, seven of them with heterozygous familial hypercholesterolaemia (HC) and five with normal total plasma cholesterol, after PGE₁ administration (5 ng·kg^–1·min^–1; 6 h daily for 5 days/week for 5 weeks). LDL uptake by the liver was quantified by single photon emission computer tomography (SPECT). Results: The amount of LDL trapped by the liver in normocholesterolaemics (45.6%) was significantly higher than in hypercholesterolaemics (22.0%). PGE₁ induced an increase in liver LDL binding, which was more pronounced in HC (+38.2%) than in normocholesterolaemic patients (+8.11%).     

Pharmacokinetics and pharmacodynamics of thiazinamium in asthmatic patients

Summary The pharmacokinetics and pharmacodynamics of thiazinamium (Multergan) were studied after intravenous and intramuscular administration to 7 males with chronic reversible airways obstruction.Disposition after i.v. administration was described by a clearance of 0.54 l·min^–1, central compartment volume of 14.8 l, distribution rate constant 0.092 min^–1, and an elimination rate constant of 0.0044 min^–1. The corresponding estimates after i.m. administration were 0.324 l·min^–1, 34.1 l, 0.035 min^–1, and 0.0018 min^–1. The bronchodilator response (expressed as % predicted FEV1) after i.v. administration was characterized by maximum increase in FEV1 of 33.9%, with an EC₅₀ of 12.8 ng·ml^–1 and an equilibration half-time of 11 min. Corresponding parameter estimates after i.m. administration were 32.2%, 18.8 ng·ml^–1, and 9 min.Anticholinergic activity, measured by the change in heart rate after i.v. administration, showed maximum increase of 76 beats·min^–1, with an EC₅₀ of 176 ng·ml^–1 and an equilibration half-time of 1.3 min. After i.m. administration the corresponding values were 120 beats·min^–1, 250 ng·ml^–1, and 3 min.The optimal plasma concentration of thiazinamium was about 100 ng·ml^–1, which should give a near maximal bronchodilator response (over 80% of predicted normal) and a heart rate of about 100 beats·min^–1.     

Comparison of verapamil and bepridil,two slow channel inhibitors,in protection against calcium-induced arrhythmias

Summary Verapamil and bepridil share the common property of antagonizing the slow inward calcium-mediated current, but bepridil has some additional antiarrhythmic properties. The efficacy of these two compounds against CaCl₂-induced arrhythmias has been compared in rats. CaCl₂ was administered i.v. by continuous infusion until death (25 mg·kg^–1·min^–1 or 40 mg·kg^–1·min^–1) or by bolus injection (160 mg·kg^–1). Bepridil (5, 10 mg·kg^–1) or verapamil (2.5,5 mg·kg^–1) were injected 10 min before CaCl₂. Bepridil (10 mg·kg^–1) or verapamil (5 mg·kg^–1) prolong the survival time during CaCl₂ infusion. After pretreatment, the injection of 160 mg·kg^–1 CaCl₂ is less toxic: 25% of animals are protected by bepridil (5 mg·kg^–1), 41% by bepridil (10 mg·kg^–1) or verapamil (5 mg·kg^–1).At death the myocardial Ca²⁺ level is not different in controls and pretreated animals, thus, the ratio myocardial Ca²⁺/total injected Ca²⁺ is significantly lowered by bepridil (10 mg·kg^–1) or verapamil (5 mg·kg^–1). The efficacy of the two drugs on this model appears related solely to inhibition of slow inward current despite the additional antiarrhythmic profile of bepridil.     

Plasma insulin during physiological and pathophysiological changes in atrial natriuretic factor

Summary Changes in plasma insulin in response to a physiological or pathophysiological elevation in circulating atrial natriuretic factor (ANF) have been investigated.Plasma insulin, glucose, immunoreactive (ir) ANF, effective renal plasma flow (ERPF), glomerular filtration rate (GFR), absolute and fractional excretion of sodium (FENa), have been measured in 14 volunteers before and during infusion of low doses of ANF or vehicle (V). Each subject received single-blind in a randomized sequence at 2 week-intervals: V alone, or ANF 4, 8 and 16 ng·kg^–1·min^–1, indused over 90 min.Plasma irANF was increased 2.5- to 11-fold during the ANF infusion as compared to the test with the vehicle. Plasma insulin did not change during V administration (baseline vs V: 22 vs 21 U·ml^–1) and was unchanged during ANF at 4, 8 and 16 ng·kg^–1·min^–1 (19, 19, 21 U·ml^–1, respectively). Blood pressure, ERPF and GFR were not affected, and diuresis, FENa and urinary Na excretion were increased significantly and dose-dependently during ANF, but not V infusion. Compared to baseline, ANF 4, 8 and 16 ng·kg^–1·min^–1 increased urinary Na excretion by 147, 241 and 446 mol·min^–1, respectively.The findings indicate that, in normal humans, an acute increase in irANF within or slightly above the physiological range, which modified natriuresis and diuresis, did not alter circulating plasma insulin.The work was supported in part by the Swiss National Science Foundation     

The disposition of intravenous doxapram in man

Summary The pharmacokinetics of intravenous doxapram in healthy individuals is consistent with a three-compartment open model. Doxapram was administered by bolus injection (1.5 mg · kg^–1) and by intravenous infusion (6.5 mg · kg^–1 for 2 h) to 5 subjects on separate occasions. There was no significant difference in mean terminal plasma half-lives (355 and 448 min) or in mean total body clearances (5.9 and 5.6 ml · min^–1 · kg^–1) following i. v. bolus injection or infusion respectively. In 3 subjects plasma doxapram concentrations during and after i. v. infusion agreed with those predicted from pharmacokinetic values obtained from the bolus injection study. Since mean steady-state concentrations (9.9 µg · ml^–1) would be reached only after an extended interval (mean 15.2 h), a variable-rate infusion regimen was calculated to produce and maintain a concentration of 2 µg · ml^–1 from 15–25 min onwards. A regimen in which the infusion rate is reduced step-wise is recommended to achieve early near-constant plasma doxapram concentrations.     

Raised plasma levels of atrial natriuretic factor in cardiac allograft recipients: evidence of increased cardiac secretion and decreased renal clearance

The mechanism(s) causing high levels of plasma atrial natriuretic factor (ANF) in cardiac allograft recipients is(are) unclear. The kidney is important for the clearance of ANF and renal function may decline with cyclosporin A therapy in these patients. The relationship between plasma ANF level and renal function and also the pharmacokinetics of a continuous infusion of ANF (15.5 ng·kg^–1·min^–1 for 60 min) was examined in 6 cardiac allograft recipients on cyclosporin A therapy.Resting plasma ANF levels were significantly higher in these patients than in 8 healthy subjects (71 vs. 21 ng·l^–1). Both effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) were significantly lower in these patients than in healthy subjects (215 vs. 617 ml·min^–1 and 55 vs. 102 ml·min^–1 respectively). There was a significant inverse correlation between plasma ANF and ERPF (r=-0.86) and between plasma ANF and GFR (r=-0.81). During the period of ANF infusion, steady state plasma ANF levels were significantly higher in cardiac allograft recipients. Total body clearance of ANF was marginally lower in these patients than in healthy subjects (60 vs. 10.0 l·min^–1) although this difference did not reach statistical significance. Derived endogenous secretion rate of ANF was threefold higher in patients when compared to healthy subjects (633 vs. 208 ng·min^–1).We have therefore shown that cardiac allograft recipients on cyclosporin A have elevated plasma ANF levels and also decreased renal function. Pharmacokinetic analysis have shown that this increase in plasma ANF levels is due more to increased ANF secretion than to decreased ANF clearance in these patients.     

Inhibitory effects of apomorphine and pergolide on neurogenic vasoconstriction in the hindquarters of the rat

Summary The effects of locally administered apomorphine and pergolide were studied in the isolated autoperfused hindquarters of the rat, in an attempt to assess the possible role of presynaptic dopamine receptors at the level in the hypotensive effect of these dopamine agonists.Local infusion of apomorphine (1g·kg^–1·min^–1 for 5 min) or pergolide (1g·kg^–1·min^–1 for 5 min) [into the hindquarters] did not alter perfusion pressure per se, but reduced the pressor response to electrical stimulation of the lumbar sympathetic chains for the whole frequency range used during a cumulative frequency-response curve (0.25–16 Hz, 1 ms, supramaximal voltage). Apomorphine and pergolide reduced the pressor response elicited by 4 Hz electrical stimulation (applied until maximum response was reached) to 54.8±7.1% and 53.9±1.7% respectively, but they did not modify similar increases of perfusion pressure produced by locally administered noradrenaline.The inhibition by apomorphine and pergolide of the 4 Hz stimulation-evoked pressor response was completely antagonized by local administration of the dopamine antagonist haloperidol (1g·kg^–1), but was not influenced by the ₂-antagonist rauwolscine (100g·kg^–1). This dose of rauwolscine antagonized the inhibitory effect of the ₂-agonist UK-14,304, which was not influenced by haloperidol.Local administration of rauwolscine increased the pressor response to stimulation at 4 Hz by 37.4–46.2%. In contrast, local administration of haloperidol did not influence the 4 Hz stimulation-evoked pressor response.These results indicate that dopamine receptors are pressent on the sympathetic innervation of the vascular bed in the rat hindquarters but do not provide evidence for a physiological role of these receptors in modulating peripheral sympathetic neurotransmission. Stimulation of these receptors, leading to a decrease of noradrenaline release and thus of vasomotor tone, might—at least in part—explain the blood pressure lowering effects of intravenous apomorphine and pergolide in the rat.     

The effect of prostaglandin E1 during cardiopulmonary bypass on renal function after cardiac surgery

Summary We have evaluated the effect of prostaglandin E₁(PGE₁) on renal function after cardiac surgery with cardiopulmonary bypass in 20 patients, ten of whom received 0.02 g·kg^–1·min^–1 of PGE₁ by infusion into the oxygenator during bypass; ten patients served as controls.Serum ₂-microglobulin fell significantly and urine ₂-microglobulin increased significantly after surgery in both groups. Urine N-acetyl--D-glucosaminidase was high after surgery in both groups, but it was significantly lower in the PGE₁ group. Free water clearance fell significantly on the 1st, 3rd, and 5th postoperative days compared with preoperative values in the control but not in the PGE₁ group.These results suggest that PGE₁ may prevent renal dysfunction after cardiopulmonary bypass.     

The pharmacokinetics of amiloride-hydrochlorothiazide combination in the young and elderly

Summary The pharmacokinetics of amiloride and hydrochlorothiazide were studied in 12 healthy young volunteers following a single dose of a fixed combination of amiloride and hydrochlorothiazide and in 11 elderly hypertensive patients at steady-state. Following modelling of the single dose data, simulated steady-state plasma concentrations for the 2 drugs were generated to examine the effect of age and/or hypertension on pharmacokinetics.The apparent systemic plasma clearance for both amiloride and hydrochlorothiazide was significantly reduced in the elderly when compared to the young (from 753 to 325 ml·min^–1, amiloride; and from 418 to 157 ml·min^–1, hydrochlorothiazide). The plasma concentrations at steady state for both drugs were greatly increased in the elderly patients (Amiloride: from 7 to 25 ng·ml^–1, C_ss,max; from 2 to 8 ng·ml^–1, C_ss,min; and from 4 to 14 ng·ml^–1, C_av; Hydrochlorothiazide: from 184 to 651 ng·ml^–1, C_ss,max; from 31 to 121 ng·ml^–1, C_ss,min; and from 89 to 273 ng·ml^–1, C_av).The decreased clearance of the diuretics in the elderly was believed due to deterioration of renal function, and there was a significant correlation between the plasma clearance of hydrochlorothiazide and creatinine clearance in both age groups (r=0.62, young;r=0.72, elderly).As a result of the pharmacokinetic findings caution may be indicated in the clinical dosage of the diuretics particularly when in fixed dose combination.     

Variability of thiopental clearance in routine critical care patients

Thiopental was administered to neurosurgical patients as a drug for cerebral protection for up to 9 days either by continuous infusion (seven subjects) or repeated intravenous bolus (nine subjects). The efficient levels of thiopental measured at steady state ranged between 5 and 63 mg · l^–1. Steady-state clearance evidenced considerable interindividual variability. Overall mean plasma clearance was lower than 1 ml · min^–1 · kg^–1 for one subjects, ranged between 1 and 2 ml · min^–1 · kg^–1 for six subjects, between 2 and 3 ml · min^–1 · kg^–1 for seven subjects and was up to 5 ml · min^–1 · kg^–1 for one subject. Extreme values for apparent daily clearance were 0.268 ml · min^–1 · kg^–1 and 5.40 ml · min^–1 · kg^–1. Daily intersubject CV gradually increased from day to day, increasing from 37% on day 1 to 51% on day 7. During the treatment for seven subjects, variations were minimal and clearance was considered to be constant; the maximum variation of clearance expressed as a percentage of the initial value was lower than 21% over a period longer than 3 days. For the other subjects, clearance was not stable with time. A decrease in clearance was evidenced for four subjects over a period of 7 days and more; the maximum variation of clearance was greater than –35%. An increase in clearance was noted for three subjects after 2 days of treatment; the maximum variation of clearance was greater than +44%. Evaluation of clearance at steady state made it possible to analyse interindividual variability and its clinical implications.Professor Bernard Roquefeuil deceased on January 22nd, 1994     

Pharmacokinetics of ornidazole in neonates and infants after a single intravenous infusion

Summary The single dose pharmacokinetics of ornidazole has been evaluated in 12 neonates or infants (aged 1 to 42 weeks) after the infusion of 20 mg/kg over 20 min. Plasma disposition was described by a two-compartment open model. The distribution phase was short (T_1/2 (1)=0.31 h) and was followed by an elimination phase (t_1/2 (2)=14.67 h). The mean apparent volume of distribution was 0.96 l/kg^–1. These results did not differ from data previous by reported in adults. Total plasma clearance was between 0.4 and 1.4 ml·min^–1·kg^–1. The plasma concentration 24 h after the infusion was 7.32 mg·l^–1, which was above the minimum inhibitory concentration for clinically significant anaerobic bacteria. Based on the pharmacokinetic results and residual concentrations at 24 h, a single daily infusion of ornidazole 20 mg·kg^–1 appears adequate for therapy in neonates and infants.     

Renal effects of atrial natriuretic peptide during dopa-decarboxylase inhibition in patients with essential hypertension

Summary To assess whether intrarenal dopamine synthesis could contribute to the renal response to ANP in essential hypertension, the effects of -human ANP influsion (50 ng·min^–1·kg^–1 b.w. for 30 min) on the urinary excretion of dopamine and sodium, urine flow rate and arterial pressure were evaluated in 7 patients with mild-moderate essential hypertension before (control period) and during DOPA-decarboxylase inhibition with carbidopa (carbidopa period).In the control period, urinary dopamine excretion was 400 pg·min^–1 in baseline conditions and 340 pg·min^–1 during ANP infusion. Carbidopa significantly decreased urinary dopamine excretion both before (210 pg·min^–1) and during ANP (99 pg·min^–1). In contrast, carbidopa did not affect sodium excretion (control from 184 to 460 Eq·min^–1; carbidopa period from 140 to 390 Eq·min^–1) or urine flow rate (control from 5.35 to 11.21 ml·min^–1; carbidopa period from 4.29 to 11.54 ml·min^–1).Arterial pressure fell significantly during ANP infusion in both periods, and no significant difference was observed between the two study days, i.e. in the absence of and during carbidopa administration.We conclude that DOPA-decarboxylase inhibition does not influence the diuretic and natriuretic response to -human ANP infusion in patients with essential hypertension.     

Plasma concentrations and haemodynamic effects of nimodipine in patients resuscitated after cardiac arrest

Summary As the pharmacokinetics of a drug may be altered in haemodynamically compromised patients, the plasma concentrations and haemodynamic effects of the calcium entry blocker nimodipine have been examined in patients resuscitated after out-of-hospital cardiac arrest.In 7 patients nimodipine was infused at increasing rates up to 30 µg·kg^–1·h^–1. The plasma concentrations increased with increasing dose; at the highest dose a mean steady-state plasma concentration of 22.1 ng·ml^–1 was obtained, and the mean plasma clearance was 1.4 l·kg^–1·h^–1. There were no marked changes in mean arterial blood pressure or heart rate.In 9 other patients nimodipine was given as a bolus infusion of 10 µg·kg^–1 over 3 min, followed by a continuous infusion of 30 µg·kg^–1·h^–1. A mean steady-state plasma concentration of 17.6 ng·ml^–1 was obtained and the mean plasma clearance was 1.9 l·kg^–1·h^–1. Heart rate did not change significantly, but the mean arterial blood pressure fell.The data indicate that in patients resuscitated after cardiac arrest, the pharmacokinetics of nimodipine are not markedly different from patients with other conditions, e.g. subarachnoid haemorrhage. However, if a loading dose is given to obtain a steady-state concentration sooner, there will be a fall in arterial blood pressure.     

Pharmacokinetics of oral salbutamol controlled-release in Asian patients with asthma

Summary Using a double blind, double dummy crossover design, single dose and steady state pharmacokinetics of oral controlled release (SCR) salbutamol 4 mg and 8 mg tablets b. d. has been studied in 8 Asian patients.Plasma salbutamol was measured over 12 h. In 8 patients the single dose mean C_max was 4.2 ng·ml^–1 and 7.7 ng·ml^–1 after 4 and 8 mg, respectively. In 5 patients the steady state mean C_max, C_min and t_max were 8.1 ng·ml^–1 and 4.7 ng·ml^–1 and 6 h for the 4 mg tablets and 14.1 ng·ml^–1 and 7.1 ng·ml^–1 and 4 h for the 8 mg tablets.It is concluded that both doses of SCR show features of controlled release and that they produced a relatively constant plasma level of salbutamol in Asian patients.     

Yohimbine bioavailability in humans

Summary Pharmacokinetic profiles were determined in seven healthy young male subjects following single oral and intravenous doses of 10 mg of yohimbine hydrochloride.The drug was rapidly eliminated (t_1/2 0.58 h orally and t_1/2 0.68 h intravenously). Following intravenous administration the data fit a two-compartment pharmacokinetic model, with a very rapid distribution phase (t_1/2a was approximately 6 min). Both the oral and the intravenous yohimbine clearance values were high but oral clearance values were much higher (mean 9.77 ml·min^–1·kg^–1 intravenous versus 55.9 ml·min^–1·kg^–1 oral). The oral bioavailability showed great variability, ranging from 7% to 87% (mean value was 33%).The imcomplete oral bioavailability of yohimbine may reflect either incomplete absorption from the gastrointestinal tract or an hepatic first pass effect. Although yohimbine is rapidly absorbed when given orally, the bioavailability is quite variable and considerable individualization of dosing may be necessary when the drug is used orally for clinical indications.Supported in part by NIH grant # MOIRR 0042     

Low-dose Iloprost infusion improves insulin action and non-oxidative glucose metabolism in hypertensive patients

Fourteen hypertensive (174.3/98.3 mmHg) non-diabetic patients were given a euglyceamic glucose clamp along with infusion of 0.9% NaCl and the prostacyclin (PGI₂) analogue Iloporst (0.7 ng · kg · min^–1). Substrate oxidation was also determined by indirect calorimetry. Over the last 60 min of the clamp, Iloprost vs saline improved whole body glucose disposal (WBGD) (35 vs 28.3 mol · kg^–1 LBM) and non-oxidative glucose metabolism (24.7 vs 18.1 mol · kg^–1 LBM · min^–1). Iloprost delivery was associated with a significant decrease in membrane microviscosity (0.253 vs 0.205), but did not affect arterial blood pressure and heart rate. In nine patients, skeletal muscle blood flow (SMBF) and insulin-stimulated glucose uptake (GU) were also studied. At the end of the study, despite a similar SMBF (37 vs 38 ml · min^–1 · kg^–1), GU (0.55 vs 0.46 mmol · 1^–1) was significantly increased by Iloprost infusion. Percentage decrease in membrane microviscosity was correlated with percentage increase in WBGD (r=0.65) and non-oxidative glucose metabolism (r=0.68). In conclusion, low-dose Iloprost infusion improves insulin action and non-oxidative glucose metabolism in hypertensive patients.     

Bioavailability and pharmacokinetics of oxazepam

Summary Six healthy volunteers received oxazepam 15 mg i.v. and orally at an interval of at least one week. The kinetic variables of i.v. oxazepam were: elimination half-life (t_1/2) 6.7 h, total clearance (CL) 1.07 ml·min^–1·kg^–1, volume of distribution (V_c) 0.27 l·kg^–1 (0.21–0.49) and volume of distribution at steady-state (V_ss) 0.59 l·kg^–1. The intravenous disposition of unbound oxazepam was characterized by a clearance of 22.5ml·min^–1·kg^–1 and a distribution volume of 12.3 l·kg^–1. After oral oxazepam the peak plasma level was reached in 1.7 to 2.8 h. The plasma t_1/2 at 5.8 h was not significantly different from the i.v. value. Absorption was almost complete, with a bioavailability of 92.8%. Urinary recovery was 80.0 and 71.4% of the dose after intravenous and oral administration, respectively. Renal clearance (CL_R) of the glucuronide metabolite was 1.10 ml·min^–1·kg^–1 (0.98–1.52). Oxazepam was extensively bound to plasma protein with a free fraction of 4.5%.     

Esmolol,an ultrashort-acting,selective β1-adrenoceptor antagonist: pharmacodynamic and pharmacokinetic properties

The effects of esmolol at different rates of infusion (100, 250 and 500 g·kg^–1 BW·min^–1) were compared with -adrenoceptor occupancy (₁ and ₂, estimated by a subtype selective radioreceptor assay) and plasma concentrations of esmolol and its acid metabolite were measured by HPLC. Up to a rate of infusion of esmolol of 500 g·kg^–1 BW·min^–1 there was a maximal ₁-receptor occupancy of 84.7% while ₂-receptor occupancy was below the detection limit; confirming the ₁ selectivity of esmolol. Exercise-induced increases in heart rate and systolic blood pressure were reduced by esmolol in a dose-dependent manner. The estimated EC₅₀ values of rate of infusion for the reduction in heart rate and systolic blood pressure during exercise were 113 and 134 g·kg^–1 BW · min^–1, respectively. Additionally, heart rate and systolic blood pressure were reduced moderately at rest. Because of the short elimination half-life of esmolol caused by the rapid hydrolysis to its acid metabolite, 45 min after end of infusion high plasma concentrations of the metabolite (maximally 80 g·ml^–1) but no esmolol were detectable. Since no in vivo effects have been observed, despite the presence of high plasma concentrations of the metabolite, the metabolite did not participate in the observed effects up to an infusion rate of esmolol of 500 g·kg^–1 BW·min^–1. The plasma concentrations of antagonist detected by radioreceptor assay and plasma concentrations of esmolol detected by HPLC showed a good correlation (r=0.97). Since the cardiovascular effects, determined before and 45 min after termination of infusion of esmolol were similar, it can be concluded that the observed effects on heart rate and systolic blood pressure are exclusively mediated by esmolol.Dedicated to Dr. P.Rajagopal, Kuantan Specialist Hospital, Kuantan, Malaysia