Functional capacity in healthy volunteers before and following β-blockade with controlled-release metoprolol

The effects of the ₁-selective -adrenergic blocker metoprolol on physiological responses, exercise capacity and gas exchange parameters were measured in healthy men using different graded bicycle exercise protocols on separate days before and following administration of 200 mg controlled-release metoprolol. Eleven men performed in randomised order maximal cardiopulmonary exercise testing on 50-W/6-min stage, 50-W/3-min stage and ramp (15-W/min^–1) protocols. Peak heart rate and peak heart rate-blood pressure products were similar on all exercise protocols, and were significantly reduced by metoprolol. Submaximal and peak oxygen consumption were similar before and following -adrenoceptor blockade. Depending on the exercise protocol applied, an insignificant decrease of 4–10% in maximal cumulated exercise capacity (work-rate × time integral) was observed following administration of metoprolol. It is concluded that in healthy men evaluated with different exercise protocols the ₁-selective controlled-release -adrenoceptor blocker metoprolol does not influence exercise capacity despite a marked reduction of heart rate and rate-pressure product.     

β-Adrenoceptor blocking effects and plasma levels of bornaprolol and propranolol in man

Summary The -adrenoceptor blocking effects and pharmacokinetics of bornaprolol (FM 24), a new -adrenoceptor blocking agent, have been compared with those of propranolol and a placebo in a double-blind trial in 6 healthy volunteers. Heart rate, systolic and diastolic blood pressures and peak expiratory flow rate were measured at rest and at the end of 3 min vigorous exercise on a bicycle ergometer, before and 2, 24 and 48 h after single oral doses of bornaprolol (120, 240 and 480 mg) and propranolol (40, 80 and 160 mg). Plasma renin activity at rest and the plasma concentrations of the two drugs were determined. Bornaprolol significantly reduced resting heart rate, dose-dependently lowered exercise-induced tachycardia and decreased peak expiratory flow rate and plasma renin activity. In addition, exercise-induced tachycardia was significantly reduced by bornaprolol up to 48 hours after drug intake (pharmacodynamic half-life approximately 63–86 h) and there was a correlation between this reduction and the log plasma bornaprolol concentration over the 48-h period. Thus, bornaprolol behaved in man as a non-cardioselective and long-lasting -adrenoceptor blocking drug, probably devoid of intrinsic sympathomimetic activity.     

Effect of pretreatment with the selective β1-adrenoceptor antagonist bisoprolol on the subsequent cardiovascular actions and β-adrenoceptor subtype specific occupancy of celiprolol in healthy man

Summary The cardiovascular effects at rest and during exercise and ₁- and ₂-adrenoceptor occupancy following a single dose of 1200 mg celiprolol p. o. were investigated in 8 healthy subjects with or without pretreatment with a single dose of 20 mg bisoprolol p. o., using a place-bo-controlled, 2-way cross-over design.The ergometric responses of heart rate (HR) and systolic blood pressure (SBP) after celiprolol were reduced to a similar extent as after bisoprolol, but the cardiovascular function at rest was affected in a different way: there was a rise in HR, clear enhancement of cardiac systolic performance, and a considerable drop in the estimated total peripheral vascular resistance, associated with median ₁-RRA and ₂-RRA occupancies of 88 and 34%, respectively. The cardiovascular effects of celiprolol were not affected by pretreatment with bisoprolol. Celiprolol thus binds extensively to ₁-adrenoceptors, moderately to ₂-adrenoceptors, acts as ₁-adrenergic antagonist (exemplified by the ergometric effects) but has vasodilator, positive chronotropic and cardiac systolic performance enhancing properties, which do not involve either direct or indirect ₁-adrenergic agonism, but which might reflect ₂-adrenergic agonism.     

Influence of intravenous β-adrenergic blockade with or without partial agonist activity upon plasma cyclic AMP and catecholamines in healthy subjects

Summary In a randomised within-subject double-blind study, 7 healthy male volunteers, aged 32 to 40 years, received at rest intravenous infusions of 2 mg propranolol (devoid of partial agonist activity), 2 mg oxprenolol (with partial agonist activity) and placebo. Cuff blood pressure did not vary after any of the 3 treatments. The heart rate did not change after placebo, but fell in the first 5 min both after propranolol and oxprenolol (p<0.01); the rate was slightly lower after propranolol than oxprenolol (p<0.05). The heart rate remained lower after both -blockers than placebo from 5 to 60 min after the infusion (both p<0.01), but the difference between the two -blockers was no longer significant. Plasma cyclic AMP showed a peak rise at 2 and 3 min after oxprenolol, and remained unchanged at those times after propranolol and placebo. From the 5th to the 60th min after infusion, the cyclic AMP concentration was lower after both -blockers than placebo, and with a slightly but not significantly higher level on oxprenolol than propranolol. Plasma noradrenaline and adrenaline were higher after the -blockers compared to placebo. Oxprenolol evoked a smaller and non-significant rise in both catecholamines. That oxprenolol, unlike propranolol, causes a sudden rise in plasma cyclic AMP soon after an i.v. infusion may be due to its partial agonist activity.     

Effects of intravenous propranolol and metoprolol and their interaction with isoprenaline on pulmonary function,heart rate and blood pressure in asthmatics

Summary The effects of propranolol (0.06 mg/kg i.v.), the selective ₁-receptor antagonist metoprolol (0.12 mg/kg i.v.) and a placebo on pulmonary function, heart rate and blood pressure have been compared in asthmatics. The interaction of these drugs with increasing doses of isoprenaline on the same variables was also studied. The two-blockers reduced resting heart rate to the same extent, indicating the same degree of blockade of cardiac-receptors. Both-blockers reduced the basal forced expiratory volume in one second (FEV₁), and the effect tended to be more pronounced after propranolol. Isoprenaline caused a dose-dependent increase in FEV₁ and vital capacity (VC). These effects were almost completely blocked by propranolol, whereas after metoprolol the changes approached that of the placebo. The isoprenaline-induced increase in heart rate and fall in diastolic blood pressure was also inhibited to a considerably greater extent by propranolol than by metoprolol. The results show a selectivity of metoprolol for so-called ₁-receptors and indicate that metoprolol may be used in asthmatics provided that it is combined with ₂-receptor-stimulating drugs.     

Vasopressin stimulates release of β-lipotropin and β-endorphin in conscious rats as measured by radioimmunoassay of unextracted plasma

Summary Using a newly developed radioimmunoassay to determine the -endorphin-like immunoreactivity (-EI) in unextracted plasma, the effect of vasopressin injections on plasma -EI was investigated in conscious rats. Arginine vasopressin caused a dose-dependent increase of plasma -EI from 34.5±7.8 fmol ml^–1 (n=6) in vehicle-treated animals to 205.0±36.1 fmol ml^–1 (n=7) after injection of the highest vasopressin dose employed (486 ng/100 g b.w.). In view of the appreciable cross-reactivity of -lipotropin (-LPH) in the radioimmunoassay used, plasma was extracted and subjected to gel chromatography on a Sephadex G-50 column. On average, about 70% of the -EI co-eluted with human -LPH and about 30% with human -endorphin in plasma extracts obtained from both control and vasopressin-treated rats. No peripheral conversion of human -LPH occurred under the experimental conditions, since after i.v. bolus injection of human -LPH 97% of the -EI comigrated with human -LPH during gel filtration. A similar blood pressure increase to that induced by the vasopressin injections, when elicited by noradrenaline or angiotensin II i.v., was not followed by an elevation of plasma -EI.These data indicate that vasopressin stimulates -lipotropin and -endorphin release into the systemic circulation in vivo.     

Comparison of the effects of xamoterol,atenolol and propranolol on breathlessness,fatigue and plasma electrolytes during exercise in healthy volunteers

Summary The influence of clinical doses of drugs that affect -adrenoceptors has been examined on heart rate, blood pressure, duration of exercise, and on electrolyte concentrations (Na, K, Ca and Mg) during recovery from exercise in healthy volunteers. The drugs used were a ₁-adrenoceptor antagonist atenolol, a nonselective -adrenoceptor antagonist propranolol, and a cardioselective, partial ₁-adrenoceptor agonist with 43% ISA activity, xamoterol.The duration of exercise was smaller on propranolol. Maximum exercise heart rate and blood pressure were reduced significantly by propranolol and atenolol. Xamoterol reduced maximum exercise heart rate and had no effect on blood pressure. The degree of breathlessness and fatigue revealed no differences between treatments.Recent evidence has suggested an association between hyperkalaemia and hypomagnesaemia with an increase in the occurrence of arrythmias following acute myocardial infarction. Exercise-induced hyperkalaemia has been suggested as a factor in sudden death. The results confirmed a rise in serum potassium during exercise and attenuation of the fall during recovery under -adrenoceptor blockade. Xamoterol was no different from placebo in these respects. Exercise also produced a rise in magnesium levels and during recovery the level fell below baseline. Both these effects were attenuated by propranolol. Calcium levels were not affected by any of the treatments.     

Effect of Cyclodextrins on Protein Binding of Drugs: The Diflunisal/Hydroxypropyl-β-Cyclodextrin Model Case

The binding of diflunisal to hydroxypropyl--cyclodextrin (HPCD), bovine serum albumin (BSA), human serum albumin (HSA), normal human plasma, and mixed solutions of HPCD/ protein was studied at 25°C, pH 7.4, by potentiometry using an electrode selective to diflunisal. The experimental data for diflunisal/ HPCD fit well to the 1:1 binding model. The binding of diflunisal with each of the studied proteins was compatible with a model having two independent classes of binding sites. The binding of diflunisal in mixed solutions HPCD/BSA, HPCD/HSA, and HPCD/plasma increased considerably when the HPCD concentration was increased. The binding behavior of the two biomolecules in the mixed solutions of HPCD/BSA or HPCD/ HSA was described with an additive model formulated on the basis of the estimates of the binding parameters of diflunisal derived from the separate experiments with each one of the binders tested. The lower than theoretical binding observed in HPCD/plasma solutions was ascribed to the competitive displacement of diflunisal from the HPCD cavity by plasma cholesterol.     

Beta-blockade and lipolysis during endurance exercise

Summary Inhibition of adipose tissue lipolysis may be involved in the impairment of endurance capacity after administration of a -adrenoceptor blocker. During endurance exercise, no significant decrease in plasma glycerol and free fatty acid (NEFA) concentrations after -adrenoceptor blockade is found. However, the levels during recovery from exhaustion are lower after -adrenoceptor blockade. This study was designed to investigate whether the lower levels after exercise are due to -adrenoceptor blockade or to the shorter time to exhaustion after administration of a -adrenoceptor blocker.In a single-blind study, 11 well-trained male subjects (age 23 (0.9) y) performed a cycle ergometer test at 70% W_max until exhaustion 2 h after intake of 80 mg propranolol. One week later, the test was repeated after intake of placebo and was stopped at the time of exhaustion in the previous test. Average exercise time was 24 min. During exercise plasma glucose was lower, whereas plasma lactate and the respiratory exchange ratio were significantly higher when the subjects were on propranolol. Glycerol and NEFA concentrations during exercise were not significantly different between the two conditions. Despite an identical exercise time, glycerol and NEFA concentrations during recovery were significantly lower after propranol treatment.In conclusion, lipolysis is inhibited during exercise after propranolol, probably causing a shift from fat to carbohydrate combustion.     

Differential cardiovascular effects of propranolol,atenolol, and pindolol measured by impedance cardiography

Summary We have evaluated Sramek's method of impedance cardiography as a non-invasive way of detecting the cardiovascular effects of drugs. We made cardiovascular measurements using the method during passive tilting and exercise 2 h after the oral administration of atenolol (50 and 100 mg), propranolol (40 and 80 mg), pindolol (5 and 10 mg), and placebo in seven separate studies involving eight healthy male volunteers.Equivalent doses of the pure antagonists atenolol (₁) and propranolol (₁, ₂) produced similar reductions in heart rate, systolic blood pressure, and cardiac index, and increases in stroke volume and total peripheral resistance, particularly during exercise. In contrast the partial agonist pindolol produced increases in heart rate and cardiac index, and reductions in peripheral resistance at rest. During passive tilting and exercise pindolol reduced heart rate, but cardiac output and total peripheral resistance were unchanged except at the highest levels of exercise.The similar cardiovascular effects of atenolol and propranolol, but differing effects of pindolol, are consistent with reports using other methods of measurement. This suggests that impedance cardiography may have a place in the non-invasive assessment of the cardiovascular effects of drugs.     

Activation of β2-adrenoceptors by isoprenaline and adrenaline enhances noradrenaline release in cortical kidney slices of young spontaneously hypertensive rats

Summary The aim of the present study was to investigate -adrenoceptor modulation of noradrenaline release from sympathetic nerves in superfused cortical kidney slices of 4-week-old spontaneously hypertensive rats (SHR) and age-matched controls (WKY). After preincubation with ³H-noradrenaline the kidney slices were electrically stimulated in superfusion chambers. The stimulation induced (S-I) outflow of radioactivity was mainly composed of unmetabolized 3H-noradrenaline in both strains and thus taken as an index of noradrenaline release. There was a frequency-dependent (1.25–20 Hz) increase in the S-1 outflow of radioactivity. At all stimulation frequencies tested S-I outflow of radioactivity was similar or even slightly lower in SHR than in WKY kidney slices in either the absence or presence of cocaine (10 mol/l). The non-selective -adrenoceptor agonists isoprenaline (0.l gmol/1) and adrenaline (0.01 and 0.1 mol/l) enhanced S-I outflow of radioactivity. The facilitatory effects of isoprenaline (0.1 mol/l) and adrenaline (0.1 mol/l) were blocked by the selective ₂-adrenoceptor antagonist ICI 118551 (0.1 mol/l) but not by the selective ₁-adrenoceptor antagonist atenolol (0.3 mol/l). The cell-permeable CAMP analogue 8-bromo-cAMP (300 mol/l) enhanced S-1 outflow of radioactivity to a similar extent in both SHR and WKY kidney slices. A combination of 8-bromo-cAMP (300 mol/l) and adrenaline (0.1 mol/l) did not enhance S-1 outflow of radioactivity to a greater extent than 8-bromo cAMP (300 mol/l) alone in both strains. However, the facilitatory effects of isoprenaline (0.1 mol/l) and adrenaline (0.1 mol/l) but not that of adrenaline (0.01 mol/l) were significantly greater in SHR than in WKY. The results suggest that stimulation of prejunctional ₂-adrenoceptors by adrenaline even in the absence of a-adrenoceptor blockade enhances noradrenaline release in kidney cortex of young SHR and WKY. This ₂-adrenoceptor mediated effect may possibly be dependent on cAMP formation. The greater facilitatory effects of isoprenaline (0.1 mol/l) and adrenaline (0.1 mol/l) in SHR as compared to WKY are in accord with receptor binding studies which show a higher density of ₂-adrenoceptors in SHR than in WKY kidney cortex.Abbreviations SHR Spontaneously hypertensive rats - WKY WistarKyoto rats - cAMP 3-5-cyclic adenosine monophosphate - S-I stimulation induced Send offprint requests to: L. C. Rump     

Plasma concentration and vascular effect of β-endorphin in spontaneously hypertensive and Wistar Kyoto rats

Summary In order to find out whether -endorphin (-E) is involved in the development of hypertension, we performed two series of experiments. Firstly, spontaneously hypertensive rats (SHR) and their normotensive Wistar Kyoto controls (WKY) were submitted to ether stress. Plasma concentrations of -endorphin-like immunoreactivity (-EI), adrenocorticotropin (ACTH) and -melanotropin (-MSH) were measured by radioimmunoassay. The basal concentration of -EI was similar in WKY and SHR, whereas WKY had higher levels of ACTH and lower levels of -MSH than SHR. In both strains acute stress enhanced the plasma concentration of -EI to the same extent and with a similar time-course. The increase of plasma -El coincided with a rise in ACTH but not -MSH. Gel chromatography of -EI revealed that plasma extracts contain similar amounts of -lipotropin- (-LPH) and -E-sized immunoreactive components, and that acute stress elevated both forms of -El. Secondly, isolated tail arteries of SHR and WKY were perfused and field stimulated with two pulses at 1 Hz. -E depressed stimulation-evoked vasconstriction with the same potency in both strains. Thus, basal and stress-induced levels of -EI did not differ in SHR and WKY. Moreover, in the tail artery of both strains the sensitivity of presynaptic opioid receptors towards -E was almost identical. If the -E sensitivity of these receptors in other arteries of WKY and SHR is also similar, a major role of the circulating peptide in the development of hypertension is rather unlikely.This work was partly supported by the Deutsche Forschungsgemeinschaft (SFB 325) Send offprint requests to B. Bucher at the above address     

Lisinopril reduces postexercise albuminuria more effectively than atenolol in primary hypertension

Physical exercise causes transient albuminuria. The mechanisms of postexercise albuminuria are not fully clarified but stimulation of the reninangiotensin system (RAS) probably plays a major role through intrarenal haemodynamic changes causing an elevated filtration pressure. In a randomised, double-blind, crossover study we compared the effects on urinary albumin excretion (UAE) of lisinopril (L) and atenolol (A) therapy, i.e. we aimed to investigate whether inhibition of the RAS or inhibition of ₁-adrenoceptor-mediated effects of the sympathetic nervous system differed with regard to changes in UAE. Sixteen patients with uncomplicated primary hypertension were studied. Four standardised bicycle ergometer exercise tests were performed, before and after each active treatment period. UAE 30 min postexercise, determined by radioimmunoassay, was significantly lowered by both treatments: -278 g·min^-1 (L) and-199 g·min^-1 (A). The reduction of postexercise UAE achieved by treatment with the angiotensin-converting enzyme (ACE) inhibitor (L) was significantly greater than that achieved by the ₁-selective adrenoceptor blocker treatment. Blood pressure (BP) at rest and during exercise were equally reduced by both drugs. In conclusion, this study showed that antihypertensive treatment with an ACE inhibitor was more effective in reducing exercise-induced albuminuria than a ₁-selective adrenoceptor-blocking agent with a similar degree of BP reduction in patients with uncomplicated primary hypertension. This suggests that the RAS plays a major role in postexercise albuminuria in hypertensive subjects. The clinical significance of this finding, however, remains to be clarified.     

In Vitro Evaluation of Dexamethasone-β-D-Glucuronide for Colon-Specific Drug Delivery

Dexamethasone--D-glucuronide is a potential prodrug for colonic delivery of the antiinflammatory corticosteroid dexamethasone. Previous studies [T. R. Tozer et al., Pharm. Res. 8:445–454 (1991)] indicated that a glucoside prodrug of dexamethasone was susceptible to hydrolysis in the upper gastrointestinal tract. Resistance of dexamethasone--D-glucuronide to hydrolysis in the upper gastrointestinal tract was therefore assessed. Conventional, germfree, and colitic rats were used to examine enzyme levels along the gastrointestinal tract to compare the stability of two model substrates (p-nitrophenyl--D-glucoside and --D-glucuronide) and to evaluate the prodrug dexamethasone--D-glucuronide. Hydrolytic activity was examined in the luminal contents, mucosa, and underlying muscle/connective tissues in all three types of rats. Enzymatic activity (-D-glucosidase and -D-glucuronidase) was greatest in the lumen of cecum and colon of conventional rats. In contrast, germ-free rats exhibited relatively high levels of -D-glucosidase activity (about 80% of total activity in the conventional rats) in the proximal small intestine (PSI) and the distal small intestine (DSI). Rats with induced colitis (acetic acid) showed reduced levels of luminal -D-glucuronidase activity in the large intestine; however, -D-glucosidase activity was relatively unchanged relative to that of the conventional rat. Mucosal -D-glucuronidase activity was significantly lower in the colitic rats compared with that in the conventional animals. Despite reduced luminal levels of -D-glucuronidase activity in the colitic rats, there was still a sharp gradient of activity between the small and the large intestines. Permeability of the glucoside and glucuronide prodrugs of dexamethasone through a monolayer of Caco-2 cells was relatively low compared to that of dexamethasone. The results indicate that dexamethasone--D-glucuronide should be relatively stable and poorly absorbed in the upper gastrointestinal tract. Once the compound reaches the large intestine, it should be hydrolyzed to dexamethasone and glucuronic acid. Specificity of colonic delivery in humans should be even greater due to lower levels of -D-glucuronidase activity in the small intestine compared with that in the laboratory rat.     

Rewarding properties of β-endorphin as measured by conditioned place preference

The role of -endorphin as a possible mediator in the reinforcing properties of opiates was investigated using a conditioned place preference paradigm. Heroin, a synthetic opiate known to have reinforcing properties, produced a strong preference for an environment previously paired with heroin injection at all doses tested (0.25, 0.5, 1.0, 2.0 mg/kg SC). No such place preference was observed following saline injections. Rats also showed dose-dependent place preference for the environment paired with -endorphin when injected intracerebroventricularly (significant dose was 2.5 g). At higher doses (5.0 and 10.0 g) rats showed no preference for the paired environment, but were catatonic. Pretreatment with naloxone (0.04, 0.2, 1.0 mg/kg SC) attenuated the rewarding effect of -endorphin (2.5 g) at all doses tested. The lowest dose of naloxone which had no aversive effect when tested alone could also significantly block the positive effect of -endorphin. The reinforcing dose of -endorphin (2.5 g) also produced an increase in locomotor activity, when tested in photocell cages. This suggests that the hyperactivity induced by -endorphin may contribute to the preference for an environment previously paired with the same drug. The reinforcing effect of -endorphin is most probably mediated by the mu and/or delta opioid subtype receptor, since -endorphin has a high affinity for these receptors. These results demonstrate positive reinforcing properties of -endorphin in the central nervous system.     

Comparative pharmacology of human β-adrenergic receptor subtypes—characterization of stably transfected receptors in CHO cells

Although many ₁-receptor antagonists and ₂-receptor agonists have been used in pharmacotherapy for many years their pharmacological properties at all three known subtypes of -adrenergic receptors are not always well characterized. The aim of this study was, therefore, to provide comparative binding characteristics of agonists (epinephrine, norepinephrine, isoproterenol, fenoterol, salbutamol, salmeterol, terbutalin, formoterol, broxaterol) and antagonists (propranolol, alprenolol, atenolol, metoprolol, bisoprolol, carvedilol, pindolol, BRL 37344, CGP 20712, SR 59230A, CGP 12177, ICI 118551) at all three subtypes of human -adrenergic receptors in an identical cellular background. We generated Chinese hamster ovary (CHO) cells stably expressing the three -adrenergic receptor subtypes at comparable levels. We characterized these receptor subtypes and analyzed the affinity of routinely used drugs as well as experimental compounds in competition binding studies, using the non-selective antagonist ¹²⁵I-cyanopindolol as a radioligand. Furthermore, we analyzed the -receptor-mediated adenylyl cyclase activity in isolated membranes from these cell lines. The results from our experiments show that all compounds exhibit distinct patterns of selectivity and activity at the three -receptor subtypes. In particular, a number of ₂- or ₃-receptor agonists that are inverse agonists at the other subtypes were identified. In addition, ₁-receptor antagonists with agonistic activity at ₂- and ₃-receptors were found. These specific mixtures of agonism, antagonism, and inverse agonism at different subtypes may have important implications for the therapeutic use of the respective compounds.     

Interactions of three inotropic agents,ASL-7022, dobutamine and dopamine,with α- and β-adrenoceptors in vitro

Summary Three inotropic agents, ASL-7022, dobutamine and dopamine, were evaluated for their -and -adrenoceptor mediated effects in vitro in a variety of isolated organs and in radioligand binding studies. All compounds were ₁-adrenoreceptor agonists in rat and guinea pig aortae, but the rank orders of potency were exactly opposite in these two tissues. Only the rank potency order of dobutamine>ASL-7022>dopamine obtained in rat aorta was consistent with the results obtained in radioligand binding studies to ₁-adrenoreceptors in rat cerebral cortex and to previous results obtained in vivo in the pithed rat. The results obtained in guinea pig aorta did not parallel the radioligand binding studies in rat brain or our previous results in pithed rat, and suggests that species differences exist between postsynaptic vascular ₁-adrenoreceptors in rat and guinea pig aorta, consistent with previous conclusions. ASL-7022 was found to be a potent ₂-adrenoreceptor agonist in field-stimulated guinea pig ileum, and was approximately 10-fold more potent than dobutamine in this respects, which was also confirmed by radioligand binding studies to ₂-adrenoreceptors in rat cerebral cortex. The ₁-adrenoreceptor mediated effects of these compounds were evaluated in guinea pig atria, where the rank order of potency was dobutamine>ASL-7022>dopamine. An identical rank order of affinity was established for these compounds by displacement of ³H-dihydroalprenolol from ₁-adrenoreceptors in rat cerebral cortex. The ₁-adrenoreceptor mediated effects of dobutamine and ASL-7022 in guinea pig atria were completely direct in nature and not secondary to the release of endogenous catecholamines. In contrast, a major component of the ₁-adrenoreceptor mediated tachycardia produced by dopamine in guinea pig atria was indirect in nature as evidenced by the marked attenuation in potency that occurred following catecholamine depletion with reserpine. All three compounds elicited ₂-adrenoreceptor mediated inhibition of tone in rat uterus, with the rank order of potency being ASL-7022>dobutamine>dopamine. Again, this rank order of ₂-adrenoreceptor potency was also reflected in ₂-adrenoreceptor affinity as assessed by displacement of ³H-dihydroalprenolol from ₂-adrenoreceptors in rat cerebellum. Based on these results, it may be concluded that for -adrenoceptors, dobutamine is a selective ₂-adrenoreceptor agonist, ASL-7022 is a selective ₂-adrenoreceptor agonist, and dopamine is a nonselective -adrenoceptor agonist. For -adrenoceptor mediated effect, ASL-7022 is a selective ₂ agonist, while dobutamine and dopamine are nonselective -adrenoceptor agonists. It is likely that the complex inotropic and hemodynamic activities of ASL-7022, dobutamine and dopamine result from the sum of their individual effects at the -and -adrenoceptor subtypes.     

Chronopharmacokinetics of beta-receptor blocking drugs of different lipophilicity (propranolol,metoprolol, sotalol,atenolol) in plasma and tissues after single and multiple dosing in the rat

Summary Comparative pharmacokinetic studies with the -receptor blocking drugs propranolol, metoprolol, sotalol and atenolol, differing greatly in lipophilicity, and their main route of elimination were performed in light-dark-synchronized rats after equimolar single (6 moles/kg) or multiple (6x6 moles/kg) drug application. Drug concentrations were determined in plasma and various target organs of the drugs, e.g. heart, muscle, lung and brain, after drug application in the light period (L) and dark period (D), respectively. After single drug administration pharmacokinetic parameters of all drugs depended on the L and D conditions. Elimination half-lives in plasma and organs were shorter during D than during L. No L-D-differences were found in initial drug concentrations of the hydrophilic drugs sotalol and atenolol. In contrast, C₀-values of the lipophilic propranolol in highly perfused organs (muscle, lung, brain) and of metoprolol in muscle tissue were significantly higher in D than in L. No obvious temporal dependency was found in other pharmacokinetic parameters (AUC, plasma clearance,V _d) with the exception inV _d of propranolol. Due to the different physico-chemical properties of the compounds inter-drug-differences in pharmacokinetic parameters including drug accumulation into lung and brain tissue were observed. Multiple drug dosing abolished the circadian-phase-dependency in the elimination half-lives of the drugs due to an increase in D. Only for the highly lipophilic propranolol half-lives in highly perfused organs were still shorter in D than in L. It is concluded that L-D-differences in drug half-lives after single dose application are mainly due to circadian variations in drug elimination with a higher hepatic (propranolol, metoprolol) or renal (sotalol, atenolol) elimination in the activity period of rats during D. Additional studies with propranolol on heart rate of conscious rats revealed that a maximum in -receptor blockade was achieved at 10 moles/kg in L but not in D. Thus, it is assumed that abolition of circadian-phase-dependency in half-lives after 6x6 moles/kg of the drugs may be due to the longer lasting and more pronounced -receptor blockade after multiple drug dosing over a period of several hours in D. Thereby, liver-flow-dependent elimination of propranolol and metoprolol and renal elimination of sotalol and atenolol is reduced to base-line levels found in L.Parts of this work were presented at the 22nd Spring Meeting (Lemmer 1981) and at the Joint Meeting (Lemmer et al. 1983a) of the German Pharmacological Society     

Penbutolol: β-Adrenoceptor interaction and the time course of plasma concentrations explain its prolonged duration of action in man

Summary -adrenoceptor binding of (—) penbutolol and its active metabolite 4-hydroxy-penbutolol to rat reticulocyte membranes was shown in the presence of native human plasma. Due to the high plasma protein binding (90%) the apparent K_i-values of penbutolol were shifted 100-fold to the right after inclusion of plasma in the assay; the K_i was 40–70 ng/ml. That value is comparable to the IC₅₀-values calculated from clinical studies. The interaction of 4-hydroxy-penbutolol with -adrenoceptors was not affected to the same extent by inclusion of plasma protein binding 80%, apparent K_i-value 7 ng/ml. Thus, the active metabolite of penbutolol displays higher potency at -adrenoceptors in vitro due to its lesser degree of plasma protein binding. A prediction procedure for antagonist activity after penbutolol administration using -adrenoceptor interaction and plasma concentration kinetics suggests that, in addition to a rapid elimination process from human plasma, a slow elimination phase of penbutolol (or an active metabolite) is necessary to explain the long duration of action observed in clinical studies after a single oral dose. Inhibition in vitro of -adrenoceptor binding by plasma samples obtained after oral administration of 40 mg penbutolol to 3 healthy volunteers indicated a biphasic concentration-time profile of the antagonist in plasma and was in accordance with the time course of the reported reduction in exercise tachycardia. Finally, plasma concentrations of penbutolol equivalents derived from the receptor assay were in the range of penbutolol concentrations detected by physico-chemical methods.It is concluded that the time course of antagonism against -adrenoceptor-mediated effects after a single oral dose of penbutolol in man can readily be explained by its biphasic elimination kinetics from human plasma together with the properties of the -adrenoceptor interaction detectable in vitro.This work is dedicated to Professor Dr. K. Greeff on occasion of his 65th birthday     

Neuronally released (−)-noradrenaline relaxes smooth muscle of calf trachea mainly through β1-adrenoceptors : comparison with (−)-adrenaline and relation to adenylate cyclase stimulation

Summary The nature of the receptors that mediate the relaxation of smooth muscle by field stimulation, (–)-noradrenaline and (–)-adrenaline was investigated in calf tracheal smooth muscle. The relation between relaxation, stimulation of the adenylate cyclase and density of -adrenoceptor subtypes was studied with the help of antagonists of ₁- and ₂-adrenoceptors. The question of the existence of catecholamine-containing nerves was also investigated. (1) Nerves with varicosities exhibiting catecholaminergic fluorescence were observed between bundles of smooth muscle cells. (2) Consistent with the existence of adrenergic nerves (–)-noradrenaline was also found. The content of (–)-noradrenaline (1 g · g^–1 w.w.) was the same in smooth muscle strips from the sublaryngeal region and from the region close to the bifurcation of the calf trachea. (–)-Adrenaline was not detected. (3) Smooth muscle relaxation by low (–)-noradrenaline concentration (0.6–2 nmol/l) was mediated through ₁-adrenoceptors. Low concentrations of (–)-adrenaline (0.06–1 nmol/l) relaxed through ₂-adrenoceptors. High concentrations of (–)-noradrenaline and (–)adrenaline also caused relaxation through ₂- and ₁-adrenoceptors respectively. (4) Field stimulation caused relaxation which was half maximal at 0.2–0.8 Hz. Blockade of ₁-adrenoceptors strongly attenuated the relaxant response to field stimulation and shifted the frequency-relaxation curves to 4 times higher frequencies. These results are consistent with a ₁-adrenoceptor-mediated relaxation caused by (–)-noradrenaline released from sympathetic nerve endings at low stimulation frequencies. (5) Blockade of ₂-adrenoceptors failed to reduce smooth muscle relaxation caused by field stimulation at low stimulation frequencies (0.1–1 Hz). However, after ₁-adrenoceptor blockade, additional blockade of ₂-adrenoceptors reduced the relaxant effects observed at high frequencies (2–400 Hz). The results suggest that high concentrations of endogenous (–)-noradrenaline cause relaxation through ₂-adrenoceptors. (6) Binding experiments with ³H-(–)-bupranolol and ³H-ICI 118,551 revealed between 10,000 and 20,000 -adrenoceptors per smooth muscle cell of which 3/4 were ₂ and 1/4 ₁. The equilibrium dissociation constant of (–)-adrenaline for both ₁- and ₂-adrenoceptors and of (–)-noradrenaline for ₁-adrenoceptors was 1 mol/l. The affinity of (–)noradrenaline for ₂-adrenoceptors was 10 to 20 times lower than for ₁-adrenoceptors. (7) The adenylate cyclase of smooth muscle cells was stimulated more through ₂-adrenoceptors by both (–)-noradrenaline (77%) and adrenaline (83%) than through ₁-adrenoceptors. Half maximum stimulation of the cyclase was observed at 2 to 4 times lower catecholamine concentrations than the concentrations that half saturate ₁- or ₂-adrenoceptors. (8) Both the low affinity and low cyclase stimulating potency of the catecholamines suggest considerable amplification of the receptor signals. Half maximum relaxation of tracheal muscle by (–)-noradrenaline is associated with the activation of less than 50 ₁-adrenoceptors per cell producing less than 250 molecules cyclic AMP per second.