Effects of drugs on pigeon erythrocyte membrane and asymmetric control or adenylate cyclase by the lipid bilayer

In pigeon erythrocyte membrane, the beta-adrenergic receptor and the enzyme adenylate cyclase can be uncoupled in two different ways depending on the type of drug used. Cationic drugs: chlorpromazine, methochlorpromazine, tetracaine, n-octylamine and a neutral alcohol, octanol, abolished alprenolol receptor binding ability and in the same range of concentration of the drug, sensitized adenylate cyclase to fluoride or Gpp(NH)p stimulation. Anionic drugs: di- and trinitro-phenols, indomethacin and octanoic acid did not affect the total number of beta-adrenergic receptor sites and, with the exception of trinitrophenol, did not change the association constant for alprenolol but they abolished the stimulation of adenylate cyclase by isoproterenol, fluoride or Gpp(NH)p. These modifications of the adenylate cyclase system occurred in a range of drug concentration where cell shape and protection against hemolysis were also affected. As chemical composition varies widely from one drug to another, it is suggested that these effects are largely nonspecific and mediated by the lipid bilayer. They are probably related to a preferential sidedness of action of the drugs in the lipid bilayer, displaying the role of an asymmetric control of the adenylate cyclase system in the membrane by the two halves of this bilayer.

Cardioselectivity of beta-adrenoceptor blocking agents 1. 1-[(4-Hydroxyphenethyl)amino]-3-(aryloxy)propan-2-ols

A series of 1-[(4-hydroxyphenethyl)amino]-3-(aryloxy)propan-2-ols was synthesized together with several 1-[(3,4-dimethoxyphenethyl)amino]-3-(aryloxy)propan-2-ols. Their affinity to beta 1- and beta-2-adrenoceptors was determined and compared with the affinity of known beta-blockers. We were able to confirm the substantial cardioselectivity of 1-(3,4-dimethoxyphenethyl)-3-[(4-substituted aryl)oxy]propan-2-ols when compared to those with a 1-(4-hydroxyphenethyl) group. An increase in the size of the 4 substitutent of the 3-(aryloxy) moiety to caproamido leads to a substantially higher affinity for the beta 1--adrenoceptor of rat ventricular muscle in the presence of the 3,4-dimethoxyphenethyl than in the presence of the 4-hydroxyphenethyl or isopropyl group; this combination also gave the highest cardioselectivity.

Evidence for internalization of the recognition site of beta-adrenergic receptors during receptor subsensitivity induced by (-)-isoproterenol

In the supernatant (30,000 x g) of frog erythrocyte homogenates, by using gel filtration we detected a protein that could bind [(3)H]dihydroalprenolol ([(3)H]DHA) with high affinity. This binding was greatly enhanced when the erythrocytes were preincubated with (-)-isoproterenol. After various periods of incubation with (-)-isoproterenol, the extent of the increase in the density of [(3)H]DHA binding sites in the cytosol was paralleled by a proportional decrease in the number of [(3)H]DHA binding sites in the corresponding pellet; both events peaked after 2-3 hr of incubation with (-)-isoproterenol. The K(a) of the (-)-isoproterenol-induced increase in [(3)H]DHA binding in cytosol and the decrease in this binding in the membrane ranged between 60 and 90 nM. The changes in the cytosol and particulate [(3)H]DHA binding sites were independent of RNA and protein synthesis. The increase in cytosol binding elicited by (-)-isoproterenol was blocked by exposure of the cells to (-)-alprenolol which per se failed to change the cytosol binding of [(3)H]DHA. Scatchard analysis revealed that the enhanced [(3)H]DHA binding to cytosol material was due to a 4-fold increase in the B(max) with little or no change in K(d) ( approximately 9 nM). Binding displacement data show that these soluble [(3)H]DHA binding sites resemble the surface membrane recognition sites. Moreover, the ability of various beta-adrenergic agents to increase [(3)H]DHA binding to cytosol after they were incubated with frog erythrocytes paralleled their affinity for membrane-bound beta receptors. These findings support the view that the beta-adrenergic receptor desensitization caused by prolonged exposure to (-)-isoproterenol is due, at least in part, to an internalization of the recognition site of beta-adrenergic receptors.

Altered effect of insulin and catecholamines in brown adipose tissue of cold-acclimated rats

Data are presented indicating that in brown adipose tissue (BAT) of cold-acclimated (CA), but not cold-exposed (CE) rats, there was an alteration in the relative response to catecholamines and insulin as evidenced by increased binding of alprenolol and decreased binding of insulin to plasma membrane enriched fractions. In addition, the stimulatory effect of insulin on glucose incorporation into glycogen and its inhibitory action on adenylate cyclase activity were both blunted in the CA tissues. It is proposed that shifts in the capacity of BAT to respond to catecholamines and insulin may be involved in the mechanism of cold acclimation.

beta-Adrenergic and muscarinic cholinergic receptors in rat submaxillary glands. Effects of thyroidectomy

Administration of triiodothyronine to thyroidectomized rats increased the density of beta-adrenergic receptors in rat submaxillary gland without significantly changing the density of muscarinic cholinergic receptors. Thus, thyroid hormone appears to regulate beta-adrenergic sensitivity in the rat salivary gland.

Decreased number of beta-adrenergic receptors in hypertensive vessels

Responsiveness to inotropic agents is altered in hypertension and may contribute to its initiation and maintenance. A biochemical basis for this change was provided by the observation that the number of beta-adrenergic receptors, as reflected in specific [3H]dihydroalprenolol binding, was diminished in both arteries and veins of spontaneously hypertensive rats. There was no change in the affinity of dihydroalprenolol for the binding sites or in the capacity of isoproterenol to displace dihydroalprenolol. The decline in beta-adrenergic receptor numbers is not secondary to blood pressure elevation but may, instead, contribute to the pathogenesis of hypertension.

Biotransformation of alprenolol in dog, guinea-pig and rat liver microsomes

1. Metabolites of alprenolol were isolated and identified in dog, guinea-pig and rat liver microsomes by means of g.l.c.-mass spectrometry and comparison with synthetic reference compounds. 2. The compounds were chromatographed as n-butylboronate derivatives, giving a series of diagnostic ions in the mass spectral fragmentation, which was elucidated by using stable isotopes. 3. Alprenolol was metabolized by aromatic ring hydroxylation, oxidation of the allylic function, and degradation of the isopropylaminopropanol side-chain. Alprenolol and four metabolites were quantified by h.p.l.c. and batch extraction techniques based on radioactivity measurements. 4. Five metabolites were detected in rat and guinea-pig liver microsomes and four in the dog. A species variation in the biotransformation of the allyl function in alprenolol was observed. The metabolite formed by oxidation of the allyl double bond was detected in significant amounts in the guinea-pig, and was also formed in the rat but could not be detected in dog liver microsomes.

Identification of urinary and biliary metabolites of alprenolol in the rat

1. After oral administration of alprenolol to rat, 12 metabolites were isolated and characterized as trifluoroacetyl, trimethylsilyl and n-butylboronate derivatives, using a g.l.c.-mass spectrometry-computer system. Fragmentation pathways of derivatives in the mass-spectrometric analysis are discussed. 2. Metabolic reactions involved are oxidative degradation of the propanolisopropylamine side-chain, aromatic hydroxylation, oxidation of the allyl group, and conjugation. A method for direct analysis of epoxide functions in the allyl group is described. 3. In comparison with metabolism of alprenolol in vitro, more polar metabolites are formed in vivo but the same principal metabolic pathways are valid. Structural features for biliary excretion are discussed.

Kinetic studies of dose-dependent metabolism of alprenolol: in vitro and in vivo studies in different species

Kinetic studies of the metabolism of alprenolol were performed with isolated microsomes from the rat, guinea-pig, dog and man at an initial substrate concentration of 0.17--150 micrometer. In all species the rate of aromatic hydroxylation reached a plateu above 50 micrometer of alprenolol in contrast to the rate of desisopropylation, where consistent saturation level was not obtained. The Km-values for the aromatic hydroxylation in the guinea-pig and man, 2,7 micrometer and 1.3 micrometer respectively, showed no concentration dependency in contrast to the rat (Km1 = 0.20 micrometer, Km2 = 26 micrometer) and the dog (Km1 = 0.78 micrometer, Km2 = 66 micrometer). The apparent Km-value of 0.20 micrometer for aromatic hydroxylation in the rat seemed to be of the same order of magnitude as reported spectral dissociation constant (Ks = 0.34 micrometer). In vivo experiments in the rat by oral administration of 7--700 mu mol/kg demonstrated a dose-dependent presystemic elimination of alprenolol. The urinary excretion of hydroxy-alprenolol was significantly lower after the highest dose. It is proposed, that the saturation of the aromatic hydroxylation, catalyzed by a high affinity site or subspecies of cytochrome P-450 with a low capacity, contributes to the dose-dependent kinetics in vivo.