Comparison of the Effects on Renin Release of Beta Adrenergic Antagonists with Differing Properties

Continuous 6-h infusions of the beta adrenergic blockers d,l-propranolol or oxprenolol significantly reduced plasma renin activity (PRA) and mean blood pressure in the resting rabbit and prevented the stimulatory effects of isoproterenol on renin release and heart rate. These actions were due to blockade of beta receptors, for the inactive isomer, d-propranolol, had no effect. Despite sustained high plasma concentrations of d,l-propranolol (0.2 mug/ml) in the unstimulated animal, PRA did not fall below 36% of control values, suggesting that basal renin secretion is maintained partly by factors other than beta adrenergic mechanisms.Prindolol, another beta blocker, also abolished the effects of isoproterenol on renin and on the heart, and reduced blood pressure in the resting animal. However, prindolol increased resting PRA and heart rate, and in animals already receiving d,l-propranolol, it raised PRA and heart rate without further altering blood pressure. This suggests that the effect on PRA of prindolol was due to its intrinsic sympathomimetic activity and not hypotension-mediated mechanisms. The observation that the blood pressure-lowering effect of prindolol was associated with a rise in PRA, while another beta antagonist, H 35/25, lowered PRA but had no effect on blood pressure, indicates that the hypotensive action of beta blockers is unrelated to their effects on renin release.In both unstimulated and isoproterenol-challenged animals, only blockers possessing beta-1 receptor affinity (d,l-propranolol, oxprenolol, prindolol, practolol, and metoprolol) affected heart rate, while effects on PRA were more prominent with agents possessing beta-2 activity (d,l-propranolol, oxprenolol, prindolol, and H 35/25). Thus, the changes in PRA caused by the beta adrenergic blockers appear to be dependent upon the summation of their direct effects, antagonistic or sympathomimetic, on beta-2 adrenergic receptors regulating renin release.     

Pafenolol, a highly selective beta 1-adrenoceptor-antagonist, in asthmatic patients: interaction with terbutaline

Pafenolol, a new beta 1-adrenoceptor antagonist, has been shown in animals to be more selective for beta 1-adrenoceptors than metoprolol. It was studied in asthmatic patients to evaluate whether it was more selective with respect to circulatory effects and especially whether this selectivity influenced bronchial muscle tone less than metoprolol, which has been shown to have beta 1-adrenoceptor selectivity similar to that of atenolol. Intravenous pafenolol (5 mg and 7.5 mg), metoprolol (15 mg), and saline were given double-blind at random and thereafter four increasing doses of terbutaline were given intravenously to seven asthmatic patients with reproducible reversibility of airways obstruction. After the terbutaline dose-response curve was determined, terbutaline was inhaled three times in increasing doses. A separately reported exercise study in the same patients showed that 5 mg pafenolol and 15 mg metoprolol were equipotent with respect to beta 1-adrenoceptor blockade, whereas 7.5 mg pafenolol tended to increase the blockade. The reflex tachycardia on terbutaline stimulation after pafenolol was greater than after metoprolol due to less blockade of beta 2-adrenoceptors in peripheral blood vessels. The bronchial effect of pafenolol was equal to that of saline, but there was a difference between the terbutaline dose-response curves after pafenolol and metoprolol that caused a rightward shift of the dose-response curve. Thus, pafenolol was shown to be more beta 1-selective than metoprolol.     

Isolated parietal cells: adrenergic response and pharmacology

Isoproterenol (Iso), epinephrine and norepinephrine each stimulated isolated gastric mucosal parietal cells as shown by an increased accumulation of [14C]aminopyrine (AP), an indirect measure of acid secretion. The beta receptor selective agonists metaproterenol, terbutaline and zinterol stimulated AP accumulation to the same extent as Iso, whereas the beta-1 receptor selective agonist dobutamine was only 20% as effective. The general beta receptor antagonists oxprenolol and dl-propranolol and the beta-2 receptor antagonist H35/25 inhibited Iso-stimulated AP accumulation. Receptor stereoselectivity was shown by the approximately 100-fold difference in potency of the I- and d-isomers of propranolol. The alpha receptor antagonists phentolamine and phenoxybenzamine, the beta-1 receptor antagonists metoprolol and practolol and the muscarinic receptor antagonist atropine were without effect. The histamine H2-receptor antagonist cimetidine inhibited Iso-stimulated AP accumulation an average of 40% at a concentration which inhibits completely histamine-stimulated AP accumulation. The data demonstrate that cells of the rat gastric mucosa have adrenergic beta-2 receptors which when stimulated result in an increase in acid secretion. The results also show that the response is in part mediated indirectly by catecholamine-stimulated release of histamine.     

Pharmacological characterization of KUR-1246, a selective uterine relaxant

The aim of the present study was to evaluate the efficacy and beta 2-adrenoceptor (AR) selectivity of KUR-1246, a new uterine relaxant. Inhibition of spontaneous or drug-induced uterine contractions by KUR-1246 was evaluated in pregnant rats and rabbits by an organ bath method or by a balloon method. The selectivity of KUR-1246 was assessed simultaneously in organs isolated from late-pregnant rats. The affinity of KUR-1246 for human beta 1-, beta 2-, and beta 3-ARs was determined using two radioligands. KUR-1246 suppressed both spontaneous and drug-induced contractions in isolated uteri, the rank order of potency being isoproterenol > KUR-1246 > terbutaline > ritodrine. ICI-118551 (selective beta 2-AR antagonist) competitively antagonized the KUR-1246-induced inhibition of spontaneous uterine contractions, but CGP-20712A (selective beta 1-AR antagonist) and SR-58894A (selective beta 3-AR antagonist) did not. All beta-AR agonists tested produced significant inhibition of spontaneous uterine contractions in vivo: ED(30) value for KUR-1246 was 0.13 microg/kg/min, a potency about 6 times and 400 times greater than that of terbutaline and ritodrine, respectively. In contrast, the positive chronotropic effect was minimal in KUR-1246-treated rats. KUR-1246 displaced radioligand binding to beta 1-, beta 2-, and beta 3-ARs, the pK(i) values being 5.75 +/- 0.03, 7.59 +/- 0.08, and 4.75 +/- 0.03 for beta 1-, beta 2-, and beta 3-ARs, respectively. For the selectivity of KUR-1246 for human beta 2-AR, we obtained values of 39.2 ([IC(50) for beta 1-AR]/[IC(50) for beta 2-AR]) and 198.2 ([IC(50) for beta 3-AR]/[IC(50) for beta 2-AR]), indicating an apparently higher affinity for human beta 2-AR than for other beta-AR subtypes. The present study clearly demonstrated that KUR-1246 is a more selective beta 2-AR agonist than the drugs presently used for relaxing uterine muscle.     

Racial differences in drug response: isoproterenol effects on heart rate following intravenous metoprolol

Healthy young black men and white men received single intravenous doses of metoprolol (0.07 mg/kg) or participated in an isoproterenol sensitivity study before and after metoprolol (0.07 mg/kg followed by 50 micrograms/min) in a randomized, crossed-over fashion. Noncompartmental pharmacokinetic parameters were calculated. The dose of isoproterenol versus change in heart rate response curves were constructed, and comparisons of dose ratio, ED50, Emax, and Ka, with the apparent association constant for metoprolol binding to beta 1-receptors, were made. There were no pharmacokinetic differences observed between the groups. The predicted Emax for the black group was 52.7 +/- 8.7 beats/min at a metoprolol concentration of 29.8 +/- 6.1 ng/ml, which was higher (p less than 0.05) than that in the white group, i.e., 43.7 +/- 7.3 beats/min at a concentration of 27.6 +/- 9.1 ng/ml. There were no differences in dose ratio, ED50, or Ka. The racial differences in beta 1-receptor responses to exogenous isoproterenol following metoprolol can simply be explained by an increase in beta 1-receptor activity in the black subjects, assuming homogeneity in cardiac beta 2-receptor responses.     

Beta adrenoceptor subtype binding activity in plasma and beta blockade by propranolol and beta-1 selective bisoprolol in humans. Evaluation with Schild-plots

In the present study we investigated whether the beta adrenoceptor subtype binding activity in plasma samples can predict selective and nonselective beta blockade in humans. From the right shifts of isoprenaline dose-response curves 0 to 84 hr after administration of propranolol and the beta-1 selective bisoprolol, in vivo beta blockade was assessed. In an in vitro radioreceptor assay with membrane preparations of beta-1 or beta-2 adrenoceptors, plasma samples were assayed for subtype selective blocking activity. After propranolol administration, in vitro beta-1 and beta-2 adrenoceptor occupancy declined from initially 97% to less than 10% within 48 hr. An isoprenaline dose ratio (DR)-1 of 1 coincided with a 50% occupancy of the beta-1 or the beta-2 subtype in vitro. In Schild-plots using plasma concentrations (radioreceptor assay) and the isoprenaline DR-1 for heart rate, diastolic blood pressure and inotropy (QS2C), slopes of unity were observed. After bisoprolol administration, in vitro beta-1 occupancy shifted from initially 95% to less than 10% within 72 hr. For the beta-2 subtype, an occupancy of greater than 10% was detectable only within the first 12 hr. An isoprenaline DR-1 of 1 coincided with a 50% occupancy of beta-1 adrenoceptors. The bisoprolol Schild-plots yielded a slope of unity for inotropy, but less than unity for the heart rate and diastolic blood pressure. From an extended analysis of subtype selective antagonism in Schild-plots, the fractions of the beta-2 adrenoceptor subtype participating in the isoprenaline response were calculated: heart rate 0.45 +/- 0.12 and diastolic blood pressure 0.23 +/- 0.13. It is concluded that in vitro receptor occupancy can predict beta blockade in humans for propranolol. Beta adrenoceptor subtype-mediated effects in humans can be evaluated with a selective antagonist and a refined analysis of Schild-plot data.     

Modeling of relationships between pharmacokinetics and blockade of agonist-induced elevation of intraurethral pressure and mean arterial pressure in conscious dogs treated with alpha(1)-adrenoceptor antagonists.

Fiduxosin is a new alpha(1)-adrenoceptor antagonist targeted for the treatment of symptomatic benign prostatic hyperplasia. The purpose of this study was to determine and compare the potencies of the alpha(1)-adrenoceptor antagonists terazosin, doxazosin, tamsulosin, and fiduxosin, based on relationships between plasma drug concentrations and blockade of phenylephrine (PE)-induced intraurethral (IUP) and mean arterial pressure (MAP) responses after single oral dosing in conscious male beagle dogs. Magnitude of blockade and plasma concentrations were evaluated at selected time points over 24 h. All drugs produced dose-dependent antagonism of PE-induced IUP and MAP responses. When IUP and MAP blockade effects were plotted against drug plasma concentrations, direct relationships were observed that were well described by the sigmoidal maximal effect model. IUP IC(50) values for terazosin, doxazosin, tamsulosin, and fiduxosin were 48.6, 48.7, 0.42, and 261 ng/ml, respectively. MAP IC(50) values were 12.2, 13.8, 1.07, and 1904 ng/ml, respectively. Uroselectivity index values, defined as MAP IC(50)/IUP IC(50), were 0.25, 0.28, 2.6, and 7.3, respectively. These results extend previous observations with terazosin in this model, showing that doxazosin exhibits a uroselectivity index comparable to terazosin, consistent with the lack of alpha(1)-adrenoceptor subtype selectivity or uroselectivity of these drugs. Tamsulosin, an alpha(1a)-/alpha(1d)-subtype selective agent, had an index value approximately 10-fold greater than the nonselective drugs. Based on its pharmacokinetic profile and a relative uroselectivity 29-fold greater than the nonselective drugs, fiduxosin is expected to exhibit greater selectivity for urethral compared with vascular alpha(1)-adrenoceptors in human and should be a novel, long-acting, uroselective alpha(1)-adrenoceptor antagonist.     

Imipramine and desipramine disposition in the elderly

Forty-six healthy male and female volunteers aged 21 to 88 received single 12.5-mg i.v. doses of imipramine, and 35 of these people received single 50-mg p.o. imipramine doses on a different occasion. Thirty-five similar volunteers received single 50-mg p.o. desipramine doses. Among these subjects 25 participated in studies of both imipramine and desipramine. Kinetic variables for the respective drugs were determined from multiple plasma drug concentrations from samples obtained during 96 hr after the dosage. Imipramine half-life was markedly prolonged in elderly vs. young males (28.6 vs. 16.5 hr; P less than .001) and females (30.2 vs. 17.8 hr; P less than .01) due to decreased clearance (males: 567 vs. 945 ml/min, P less than .01; females: 599 vs. 975 ml/min, P less than .005) with no change in volume of distribution. After p.o. imipramine doses time to peak imipramine concentration was shorter in elderly females (2.1 vs. 4.8 hr; P less than .005) but no different in males. Peak concentration achieved was greater in the elderly of both sexes (males: 40.2 vs. 19.5 ng/ml, P less than .005; females: 44.7 vs. 10.4 ng/ml, P less than .01). Comparison of p.o. and i.v. imipramine doses indicated no difference in absolute bioavailability between the elderly and young of either sex. In contrast, after p.o. desipramine more limited age-related changes were noted. Desipramine half-life was slightly prolonged in elderly males (30.8 vs. 21.2 hr; P less than .05) apparently related to a nonsignificant decrease in p.o. clearance.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of the prejunctional beta adrenoceptors in canine bronchial smooth muscle

Prejunctional beta adrenoceptors in canine bronchi (3rd to 6th order) were characterized by observing the effects of beta receptor agonists and antagonists on field stimulation-induced contractions and excitatory junction potentials (EJPs). Contractions were antagonized by norepinephrine (IC50 = 9.4 X 10(-7) M), isoproterenol (IC50 = 1.9 X 10(-8) M) or salbutamol (IC50 = 4.0 X 10(-8) M). EJPs were also decreased by all three agonists, with little or no effect on resting membrane potential or on carbachol-induced depolarization when used at concentrations sufficient to eliminate EJPs. These inhibitory effects were blocked by propranolol or timolol, as well as by the selective antagonists ICI 89,406 (beta-1-selective) and ICI 118,551 (beta-2-selective); pA2 values for the selective antagonists were 8.4 and 7.2 (norepinephrine as agonist) or 6.5 and 9.0 (salbutamol as agonist), respectively. Control responses were also sometimes potentiated by the nonselective antagonists. Schild plot analysis of the data indicated clearly that both beta-1 and beta-2 receptors are involved in the inhibitory effect. Electron microscopic studies showed this tissue to be densely innervated by adrenergic and cholinergic nerves with close apposition of adrenergic and cholinergic nerve varicosities, providing a structural basis for prejunctional interactions between them. From the data presented, we conclude that catecholamines act on prejunctional beta-1 and beta-2 receptors leading to inhibition of cholinergic neurotransmission in canine bronchi.     

Lidocaine elimination: effects of metoprolol and of propranolol

The effects of administration of metoprolol and propranolol on lidocaine elimination were studied in six healthy young men who did not smoke. Each received three single intravenous doses of lidocaine (2.5 to 3.0 mg/kg injected over 10 min): one alone, one after 1 day pretreatment with propranolol (40 mg orally every 6 hr), and one after 1 day pretreatment with metoprolol (50 mg orally every 6 hr). Lidocaine clearance was 0.88 +/- 0.28 l X hr-1 X kg-1 before beta blockade, 0.61 +/- 0.20 l X hr-1 X kg-1 during metoprolol dosing, and 0.47 +/- 0.16 l X hr-1 X kg-1 during propranolol dosing. There was no correlation between the change in lidocaine elimination and the steady-state concentrations of metoprolol or propranolol, nor between the change in lidocaine clearance and the change in resting heart rate produced by either beta blocker. Metoprolol and propranolol reduce lidocaine elimination significantly.     

Characterization of functional beta-adrenoceptor subtypes in rabbit urinary bladder smooth muscle

Spontaneous contractile force of muscle strips isolated from male rabbit urinary bladder dome (detrusor) and base (trigonal muscle) was significantly inhibited by isoproterenol (10(-7)-10(-5) M), a non-specific beta-adrenoceptor agonist or by terbutaline (10(-8)-10(-5) M), a selective beta 2-adrenoceptor agonist. The EC50 values for isoproterenol and terbutaline in detrusor were the same as those in trigonal muscle but the maximum relaxant response to isoproterenol or terbutaline was significantly greater in detrusor than in trigonal muscle. Dobutamine (10(-5)-10(-4) M), a relatively specific beta 1-adrenoceptor agonist caused a small but significant relaxant response in trigonal muscle but no change in detrusor. In trigonal muscle the relaxant response to dobutamine was less than that to terbutaline. The relaxant response to 10(-6) M isoproterenol in detrusor was completely blocked by butoxamine (10(-4) M), a selective beta 2-antagonist or by propranolol (10(-6) M), a non-specific beta-antagonist but not by metoprolol (10(-6)-10(-4) M), a selective beta 1-antagonist. Relaxation of trigonal muscle induced by 10(-6) M isoproterenol was inhibited by 10(-5) M metoprolol by 30%, by 10(-4) M butoxamine by 70%, or completely by 10(-6) M propranolol. These findings are consistent with the view that the density of beta-adrenoceptors is higher in the detrusor than in trigonal muscle, and that the relaxant response to beta-adrenoceptor stimulation is mediated by beta 2-subtype in the detrusor and by both of beta 1- and beta 2-subtypes in trigonal muscle of the male rabbit.     

Plasma levels and effects of metoprolol on blood pressure and heart rate in hypertensive patients after an acute dose and between two doses during long-term treatment.

Plasma levels and the effect of orally administered metoprolol on the resting arterial blood pressure and heart rate have been studied during acute and steady-state conditions in patients with mild hypertension. The patients receiving an 80-mg dose had a mean maximum plasma level of about 100 ng/ml plasma in single-dose studies and about 140 ng/ml plasma during steady-state conditions. The corresponding values for the patients on the 50-mg dose were about 60 and 100 ng/ml plasma, respectively. The maximum concentrations were reached 1 hr after administration. After the single dose the elimination half-life of metoprolol in plasma was 4.3 plus or minus 0.7 hr in the patients receiving the 80-mg dose and 3.8 plus or minus 0.3 hr in the other group. The difference was not statistically significant. The elimination half-life in the plasma was about the same in the single-dose study and during steady state in both groups. The morning dose induced a decrease of the systolic blood pressure whereas the diastolic blood pressure was not significantly different from that recorded immediately before administration of metoprolol. For the 80-mg dose the systolic pressure dropped from 167 plus or minus 4 to 146 plus or minus 4 mm Hg in the single dose study and from 160 plus or minus 8 to 140 plus or minus 4 mm Hg at steady state. The corresponding values for the 50-mg dose were 150 plus or minus 3 to 135 plus or minus 3 mm Hg and 144 plus or minus 3 to 138 plus or minus 3 mm Hg, respectively. In experiments with placebo the systolic blood pressure was not significantly changed. There was no correlation between the plasma levels and the effect on the systolic blood pressure. Both doses of metoprolol markedly reduced the heart rate after the single dose as well as at steady state. The effect was linearly related to the logarithm of the plasma concentration, and the relationship was virtually the same as obtained previously for the effect on exercise heart rate in healthy volunteers.     

In vitro effects of beta adrenoceptor agonists and antagonists on the rat ovarian suspensory ligament

The mesovarian suspensory ligament of the rat was used to compare the activities of beta adrenoceptor agonists and antagonists. The following beta adrenoceptor agonists, in descending order of potency, inhibited spontaneous activity in a dose-related manner: zinterol greater than isoproterenol much greater than dobutamine. Several noncardioselective, beta-2 adrenoceptor antagonists with intrinsic sympathomimetic activity (ISA) also inhibited the activity of the ligament: pindolol greater than alprenolol = bucindolol = oxprenolol greater than labetalol. Maximal relaxation induced by the antagonists was equivalent to that caused by the beta receptor agonists. Two cardioselective, beta adrenoceptor antagonists with ISA, acebutolol and practolol, did not inhibit the activity of the suspensory ligament but did increase the rate of the isolated right atrium of the rat. The maximal increases in atrial rate evoked by the antagonists were significantly less than those induced by the beta adrenoceptor agonists. Studies with ICI 118,551 or atenolol as beta-2 or beta-1 selective adrenoceptor blockers, respectively, suggest that the beta adrenoceptors of the suspensory ligament are predominantly of the beta-2 subtype. The possible relevance of these results to the induction of mesovarian leiomyomas in rats by noncardioselective beta adrenoceptor agonists and antagonists with ISA is discussed.     

Beta-1 and beta-2 adrenoceptors mediate smooth muscle relaxation in bovine isolated mesenteric lymphatics.

The relaxant effects of beta adrenoceptor agonists were investigated in isolated bovine mesenteric lymphatics which had been contracted by 5-hydroxytryptamine. Addition of isoproterenol (a nonselective beta agonist), denopamine (a selective beta-1 agonist) and procaterol (a selective beta-2 agonist) caused concentration-dependent relaxations in the lymphatic preparations. There was no significant difference in the relaxant responses to the beta adrenoceptor agonists between the preparations with and without endothelium. Treatment with 10(-7) to 3 x 10(-6) M metoprolol (a selective beta-1 antagonist) shifted the concentration-response curve for denopamine to the right, whereas 10(-9) to 3 x 10(-8) M ICI 118,551 (a selective beta-2 antagonist) did not affect the relaxant response to denopamine. The relaxations of bovine mesenteric lymphatics induced by isoproterenol were suppressed by both metoprolol and ICI 118,551. The procaterol-induced relaxations were inhibited by 10(-9) to 3 x 10(-8) M ICI 118,551 but not by 10(-7) to 3 x 10(-6) M metoprolol. Schild plot analyses showed that the slope and pA2 values for metoprolol against denopamine were 1.10 and 7.59, respectively, and that those for ICI 118,551 against procaterol were 0.91 and 9.96. These results suggest that both beta-1 and beta-2 adrenoceptors are located on the smooth muscle cells in bovine mesenteric lymphatics and that stimulation of either receptor produces a marked relaxation.     

Beta adrenoceptors in the canine large coronary arteries: beta-1 adrenoceptors predominate in vasodilation

Beta adrenoceptors of the canine large coronary artery were characterized by observing the effects of the subtype selective antagonists, metoprolol (beta-1) and ICI 118,551 (beta-2), on the vasodilator responses of isolated and perfused preparations to beta adrenoceptor agonists and in the radioligand binding assay. The integrity of the endothelium was checked by acetylcholine-induced vasodilations. Without any precontraction, isoproterenol, norepinephrine, epinephrine and procaterol (selective beta-2 agonist) dilated the canine large coronary artery pretreated with phentolamine (10(-5) M). The rank order of agonist potency was isoproterenol greater than norepinephrine greater than epinephrine greater than procaterol. The pA2 values for metoprolol and ICI 118,551 were determined by the antagonisms of the vasodilator responses to isoproterenol and procaterol. The slopes of Schild plots for metoprolol and ICI 118,551 against isoproterenol and the value for ICI 118,551 against procaterol were not significantly different from unity, but the value for metoprolol against procaterol was significantly less than unity. The pA2 value for metoprolol against isoproterenol was 7.48 and those values for ICI 118,551 against isoproterenol and procaterol was 7.19 and 7.25, respectively. These pA2 values are typical for beta-1 adrenoceptors. The beta adrenoceptors of the canine large coronary artery were examined further using an antagonist [125I]iodocyanopindolol as a ligand for the binding of beta adrenoceptors. The [125I]iodocyanopindolol binding to the canine coronary artery smooth muscle membrane was saturable with a KD of 63.7 pM and a total number of radioligand binding sites of 44 fmol/mg of protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

ASSESSMENT OF β-1 SELECTIVITY OF BISOPROLOL AND ATENOLOL BY MEANS OF THEIR INFLUENCE ON THE LIPOLYTIC RESPONSE TO AN INFUSION OF TERBUTALINE

We have investigated the beta-1 selectivity of a new beta-blocker, Bisoprolol, by comparing its effect on lipolysis induced by intravenous terbutaline infusion with that of Atenolol. At a dose of 5 mg, Bisoprolol had virtually no beta-2 blocking activity as measured by free fatty acid (FFA) release during terbutaline infusion. At a dose of 10 mg, Bisoprolol had a small but statistically insignificant effect on FFA release similar to 50 mg Atenolol. At a dose of 20 mg, Bisoprolol had significant beta-2 blocking activity. At lower doses, therefore, Bisoprolol is a very selective beta-blocker.     

Relative selectivity of different beta-adrenoceptor antagonists for human heart beta 1- and beta 2-receptor subtypes assayed by a radioligand binding technique

The affinity constants of inhibition (Ki values) for both beta 1- and beta 2-receptor subtypes were determined for four different beta-adrenoceptor antagonists by a radioligand binding technique in a human myocardial membrane preparation. The radioligand was the high affinity antagonist [125I]-(-)-iodocyanopindolol (ICYP), and the drugs tested were atenolol, metoprolol, ICI 141,292 and ICI 118,551. Different concentrations of the drugs at test were allowed to compete with a constant concentration of ICYP for the specific binding sites (beta-receptors). Ki values for beta 1- and beta 2-receptors for each beta-adrenoceptor antagonist were developed from these data by computer calculations. Atenolol and metoprolol were found to differ slightly regarding potency (absolute Ki values) and to be practically equal regarding relative selectivity (approx. 40; i.e. ratio between high and low Ki values), while ICI 141,292 was found to have slightly higher relative selectivity (approx. 60) and much higher potency. All these drugs exhibited highest affinity for the beta 1-receptor population. In contrast, ICI 118,551 exhibited a very high relative selectivity (approx. 300) with highest affinity for the beta 2-receptor subtype. The method represents a good supplement to physiological and clinical examinations of selectivity of beta-blockers, and offers several advantages regarding simplicity, specificity and accuracy.     

Uncoupling of the beta-adrenergic receptor as a mechanism of in vitro neutrophil desensitization

Human leukocytes have been useful in studying desensitization phenomena to beta-adrenergic agonists in a number of clinical conditions. For example, we have previously shown that oral terbutaline causes a time-dependent decrease in neutrophil (PMN) beta receptor number, using the beta antagonist ligand [3H]dihydroalprenolol (DHA), in conjunction with a significant loss of isoproterenol-induced adenylate cyclase activity. In the present in vitro study we have explored the mechanism for beta-adrenergic desensitization and have compared conditions for homologous and heterologous desensitization, using the intact PMN model. PMN preincubated with isoproterenol (10(-4)M), washed thoroughly, then restimulated, desensitize rapidly so that within 10 min 80% of control isoproterenol-induced cyclic AMP stimulation is lost. Cells washed free of isoproterenol recover full responsiveness in 1 to 2 hr. The estimated isoproterenol desensitization EC50 in cells washed and then restimulated is 1 X 10(-5)M, and the EC50 in unwashed cells that are restimulated is 9 X 10(-8)M. Rank-order potency studies of catecholamine desensitization show isoproterenol greater than epinephrine greater than norepinephrine, a beta-2 pattern. Isoproterenol-induced desensitization results in a small reduction in [3H]DHA binding sites, which becomes statistically significant (p less than 0.05) from control values at 1 hr (67% of control) and 3 hr (64%). Since the change in number of beta receptors did not explain the profound, rapid loss of beta agonist-induced cyclic AMP responsiveness, we explored the possibility of an uncoupling phenomenon. In the absence of GTP, isoproterenol binding is characterized by an EC50 of 6.6 +/- 2.6 X 10(-7)M, which is significantly different (p less than 0.05) from the EC50 of 38.1 +/- 9.1 X 10(-7)M found when cells are previously desensitized with isoproterenol for 10 min. GTP does not affect the EC50 of desensitized cells. These findings are consistent with the uncoupled receptor state fitting the model described by Su et al. Finally, prolonged (3 hr) isoproterenol preincubation results in a small but significant (p less than 0.05) loss of cyclic AMP responsiveness to histamine (67.7% +/- 11.7 of control) and PGE1 (59.3% +/- 7.4), suggesting heterologous desensitization. These studies suggest that the human PMN is a suitable model to study both homologous and heterologous desensitization in vitro.     

Duration of action of beta blockers

Three randomized, placebo-controlled, crossover experimental designs were used to define a suitable interdose interval and to study the adequacy of once-daily administration for applications in preventive trials on manifest or latent ischemic patients. Suppression of exercise tachycardia was used as the major effect variable. All measurements were made at different intervals after the last dose when the healthy subjects had been treated for at least 1 wk. Reductions of exercise tachycardia were found 24 hr after the last dose for atenolol, metoprolol, penbutolol, pindolol, propranolol, sotalol, and timolol. Penbutolol and propranolol induced equal reduction of exercise tachycardia at the end of the dose interval regardless of whether the total daily dose was given once daily or divided in 2 daily doses. Atenolol and sotalol, both with long half-lifes (t1/2s), were not superior to other beta blockers. Neither were slow-release preparations of metoprolol and propranolol markedly more effective 24 hr after the preparation than after ordinary tablets. Plasma concentration-time patterns after slow-release preparations may be important in patients with adverse experiences during peak plasma levels after conventional tablets.