Beta-adrenergic receptor: stereospecific interaction of iodinated beta-blocking agent with high affinity site

In vitro studies of skeletal muscle membranes. Adenylate cyclase of fast and slow twitch muscle and the effects of denervation

Sarcolemmal membranes were prepared from slow-twitch (red) and fast-twitch (white) skeletal muscle of the rat. A sensitive adenylate cyclase assay was used and basal, fluoride- and catecholamine-stimulated activities measured. The greater in vivo sensitivity of red muscle to the effects of catecholamines correlates, in the present study, with approximately a twofold stimulation of its sarcolemmal adenylate cyclase with isoproterenol (10 micronm). The white muscle enzyme, on the other hand, is only minimally stimulated (20%) at the same concentration of beta-adrenergic agonist. Fast-twitch muscle is known to be physiologically insensitive to catecholamine in vivo. A course of sciatic nerve denervation was followed to further distinguish these two metabolic types of skeletal muscle and their respective adenylate cyclases. The slow-twitch muscle enzyme activities were completely and permanently lost on denervation. The white muscle enzyme, however, recovered almost completely after an initial reduction in specific activity the first week. Interestingly, the NaF-stimulated activity lagged behind both the basal and hormone-stimulated activities of the white muscle enzyme, in returning to control levels. The activities of cyclic nucleotide phosphodiesterase were evaluated in homogenates of the two muscle types in innervated rats and following denervation, in order to further define the neural influence on skeletal muscle cyclic nucleotide metabolism. The results suggest that the motor nerve may regulate some of the metabolic properties of slow-twitch muscle (which may involve cyclic AMP) by controlling the responsiveness of its sarcolemmal-bound adenylate cyclase system.

Ontogeny of beta-adrenergic receptors in rat cerebral cortex

The ontogeny of beta-adrenergic receptors in rat cerebral cortex has been studied using [125I]iodohydroxybenzylpindolol as a ligand in an in vitro binding assay. The concentration of beta-adrenergic receptors was very low during the first week after birth. Between days 7 and 14 there was a rapid increase in the density of receptors. Adult levels were reached by the end of the second week. The affinities of 1-isoproterenol and iodohydroxybenzylpindolol for beta-adrenergic receptors did not vary with the age of the animal. Fluoride stimulated adenylate cyclase activity in the cerebral cortex was 40% of the adult level at birth and gradually increased to maximal levels over the next two weeks. On the other hand, catecholamine stimulated cyclic-3',5'-adenosine monophosphate accumulation was barely detectable during the first week after birth, but it increased rapidly to adult levels between days 7 and 14. The results suggest that it is the development of beta-adrenergic receptors that permits the expression of catecholamine sensitive adenylate cyclase activity. Norepinephrine stores in the cerebral cortex developed slowly reaching adult levels approximately two months after birth. There is therefore little correlation between the ontogeny of presynaptic adrenergic nerve terminals and the postsynaptic development of beta-adrenergic receptors.

Beta-adrenergic receptors in guinea-pig myocardial tissue

Stereospecific binding sites for (-) [3H]-alprenolol, a beta-adrenergic antagonist, have been identified in guinea-pig myocardial broken cell preparations. The concentration of the sites was 0.3 pmoles per mg of protein and the dissociation constant (at 37 degrees C) 10(-8) M. A close correlation between the ability of various beta-adrenergic antagonists to compete with tracer alprenolol binding and to block the response of isoprenaline-stimulated myocardial adenylate cyclase has been found. Low affinity sites for the labelled beta-adrenergic antagonist in contrast to stereospecific sites are heat stable and do not discriminate between the (-) and the (+) forms of the beta-adrenergic antagonists. Adenylate cyclase in guinea-pig myocardial tissue is poorly stimulated by isoprenaline or 5'-guanylylimidodiphosphate. This is attributed to a high basal activity which could be lowered by a preincubation at 37 degrees C.

Coupling of catecholamine receptor from one cell with adenylate cyclase from another cell by cell fusion

The experiments test the hypothesis that beta-adrenergic receptor is an independent unit that can be transferred from one adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4-6-1-1[ system to another. Turkey erythrocytes in which the catalytic activity of adenylate cyclase had been inactivated by N-ethylmaleimide or by heat contributed the beta-adrenergic receptor. Friend erythroleukemia cells (F cells) that possessed no measurable beta-adrenergic receptor contributed the adenylate cyclase. The erythrocytes in which the enzyme had been inactivated were fused with the F cells by Sendai virus. The cell ghosts of the fused preparation demonstrated adenylate cyclase activity which was strikingly enhanced by isoproterenol. Controls of fusion of F cells with each other or with human erythrocytes failed to show a response to isoproterenol. It was therefore concluded that the beta-adrenergic receptor of the turkey erythrocytes must have become functionally coupled to the adenylate cyclase of the mouse F cells. Activation by isoproterenol was demonstrable within a few minutes after fusion, and inhibitors of protein synthesis had no effect. Thus, coupling must have occurred between the preexisting components. The findings suggest that it may be possible in the future to confer on cells that possess an adenylate cyclase system new hormonal responses by inserting a receptor into their cell membrane. It is proposed that the procedure of massive heterologous cell fusion, as used in the present study, can be used to analyze the function of other cell membrane components.

beta-Adrenergic receptor involvement in 6-hydroxydopamine-induced supersensitivity in rat cerebral cortex

The intraventricular administration of 6-hydroxydopamine, a procedure which destroys noradrenergic nerve terminals in the central nervous system, caused an increase in the density of beta-adrenergic receptors in rat cerebral cortex, without affecting their affinity for isoproterenol. The results suggest that changes in the density of adrenergic receptors are involved in 6-hydroxydopamine-induced supersensitivity at central noradrenergic synapses.

Mechanism of inhibition of organic acid transport in rabbit renal cortex by cyclic AMP

The mechanism by which isoproterenol and cyclic AMP inhibit organic acid transport was examined. Accumulation of 131I-Hippuran (S/M) was used as an index of organic acid transport. Propranolol, an antagonist of isoproterenol's beta-adrenergic response, not only inhibited accumulation of Hippuran but, when combined with isoproterenol, caused further inhibition. PTH elicited larger increases than isoproterenol in cortical-slice cyclic AMP content but did not inhibit accumulation of Hippuran. This lack of correlation between a positive effect on the adenylate-cyclase-cyclic AMP system and inhibition of transport was also seen when other agents were tested. Cyclic GMP, which has been postulated to work in opposition to cyclic AMP, was as potent as cyclic AMP in decreasing S/M. The decreasing of S/M by adenine and uric acid could not be related to any known effect of these agents on tissue cyclic AMP content. Similarly, although 0.1 mM theophylline significantly decreased S/M, cortical cyclic-AMP content was not increased until a 100-fold greater concentration of theophylline was employed. The data suggest that the inhibitory effect of these agents was attributable to their molecular configurations, which interact directly with the organic acid transport system rather than indirectly via the adenylate-cyclase-cyclic AMP system.

The insulin receptor of the turkey erythrocyte. Characterization of the membrane-bound receptor

The insulin receptor of the turkey erythrocyte has previously been shown to be very similar to that of the mammalian insulin receptors. As a first step in the isolation of this receptor a highly purified plasma membrane fraction has been prepared. The binding characteristics of the purified membrane-bound receptor were identical to those found with intact erythrocytes, but the membrane preparation had very little insulin-degrading activity. Isolation of the membrane by the methods described gave a 100-fold purification of the insulin receptor with 67% yield.

A kinetic analysis of a catechol-specific binding site in the microsomal fraction from the rabbit aorta

(-)-3/-Norepinephrine (3H-NE) binding to the microsomal fraction of the rabbit aorta has been studied. Binding appears to increase linearly with time up to at least 30 min, shows no evidence of stereoselectivity and may be inhibited only by compounds possessing the catechol or 3-methoxy-4hydroxyphenyl moieties, with the latter being 100-fold less effective. 3H-NE binding is saturable with a Km of 8.5 X 10(-8) M and V max of 28 pmoles/mg protein. A Hill plot indicates that binding is noncooperative whereas a Scatchard plot suggests that two sites may be present. Binding does not appear to require physiological concentrations of Ca2+ or Mg2+ and is inhibited significantly by EDTA and sodium metabisulfite. In addition, binding is markedly enhanced by low and high pH values. This binding is also inhibited by sodium metabisulfite which suggests that an oxidized form of the catecholamine is the active binding species. Experiments with several group specific reagents indicate that binding may require a free sulfhydryl group but not a carboxyl function. The binding process requires an energy of activation of 14.8 kcal/mole whose magnitude may be partly explained, with the aid of optical rotatory dispersion spectra, by a non-stereoslective conformational change in protein structure induced by the amine. The characteristics of the 3H-NE binding sites observed in the microsomal fractional of the rabbit aorta appear to be different from those expected if binding were to the adrenoreceptors. A possible mechanism for catecholamine binding to free sulfhydryl groups on protein is presented.

Hormone action at the membrane level. V. Binding of (+/-)-[3H]isoproterenol to intact chicken erythrocytes and erythrocyte ghosts

The binding of (+/-)-[7-3H]isoproterenol to intact chicken erythrocytes has been investigated by a rapid centrifugation technique. The binding is displaceable by a one thousand-fold excess of cold isoproterenol and consists of two fractions, only one of which is inhibitable by the beta antagonist (--)-propranolol. The total displaceable binding to intact cells amounts to 80 or 127 molecules per cell at a (+/-)-isoproterenol concentration of 0.4 muM depending on the method employed to analyze the binding. Under similar conditions, the total displaceable binding to isolated membrane ghosts is 12600 molecules per cell. The propranolol-inhibitable binding to intact cell reaches saturation within 5 min at 4 degrees C and gives by scatchard analysis a maximum binding of 108 molecules per cell and with a KD of 0.4 muM. 50% inhibition of binding is obtained with 0.3 muM unlabeled (--)-isoproterenol as compared to 20 muM unlabeled (+)-isoproterenol. The binding of isoproterenol thus shows a marked stereospecific preference for the (--)-isomer.

Alpha-adrenergic receptor identification by (3H)dihydroergocryptine binding

A radioactively labeled alpha-adrenergic antagonist, [3H]dihydroergocryptine, binds specifically to a site on rabbit uterine membranes. Binding is rapid, reaching equilibrium in less than 17 minutes at 25 degrees C. Adrenergic agonists compete for this binding site with an order of affinities identical to the pharmacological potency order of these agents as alpha-adrenergic agonists (epinephrine greater than norepinephrine greater than isoprotereonl). The (-) stereoisomers of epinephrine and norepinephrine are 30 times more potent in competing for the site than the corresponding (+) stereoisomers. alpha-Adrenergic antagonists, such as phentolamine and phenoxybenzamine, potently compete for the binding sites while the beta-adrenergic antagonist propranolol does not. Structural analogs of catecholamines that are devoid of alpha-adrenergic physiological activity do not compete for [3H]dihydroergocryptine binding sites. These data suggest that alpha-adrenergic receptors can be directly identified and studied by [3H]dihydroergocryptine binding.

Identification of beta-adrenergic receptors in human lymphocytes by (-) (3H) alprenolol binding

Human lymphocytes are known to posessess a catecholamine-responsive adenylate cyclase which has typical beta-adrenergic specificity. To identify directly and to quantitate these beta-adenergic receptors in human lymphocytes, (-) [3H] alprenolol, a potent beta-adrenergic antagonist, was used to label binding sites in homogenates of human mononuclear leukocytes. Binding of (-) [3H] alprenolol to these sites demonstrated the kinetics, affinity, and stereospecificity expected of binding to adenylate cyclase-coupled beta-adrenergic receptors. Binding was rapid (t1/2 less than 30 s) and rapidly reversible (t1/2 less than 3 min) at 37 degrees C. Binding was a saturable process with 75 +/- 12 fmol (-) [3H] alprenolol bound/mg protein (mean +/- SEM) at saturation, corresponding to about 2,000 sites/cell. Half-maximal saturation occurred at 10 nM (-) [3H] alprenolol, which provides an estimate of the dissociation constant of (-) [3H] alprenolol for the beta-adrenergic receptor. The beta-adrenergic antagonist, (-) propranolol, potently competed for the binding sites, causing half-maximal inhibition of binding at 9 nM. beta-Adrenergic agonists also competed for the binding sites. The order of potency was (-) isoproterenol greater than (-) epinephrine greater than (-)-norepinephrine which agreed with the order of potency of these agents in stimulating leukocyte adenylate cyclase. Dissociation constants computed from binding experiments were virtually identical to those obtained from adenylate cyclase activation studies. Marked stereospecificity was observed for both binding and activation of adenylate cyclase. (-)Stereoisomers of beta-adrenergic agonists and antagonists were 9- to 300-fold more potent than their corresponding (+) stereoisomers. Structurally related compounds devoid of beta-adrenergic activity such as dopamine, dihydroxymandelic acid, normetanephrine, pyrocatechol, and phentolamine did not effectively compete for the binding sites. (-) [3H] alprenolol binding to human mononuclear leukocyte preparations was almost entirely accounted for by binding to small lymphocytes, the predominant cell type in the preparations. No binding was detectable to human erythrocytes. These results demonstrate the feasibility of using direct binding methods to study beta-adrenergic receptors in a human tissue. They also provide an experimental approach to the study of states of altered sensitivity to catecholamines at the receptor level in man.

Rapid changes in rat pineal beta-adrenergic receptor: alterations in l-(3H)alprenolol binding and adenylate cyclase

The properties of the beta-adrenergic receptor which regulates adenylate cyclase [ATP pyrophosphate-lyase (cyclizing)8 EC 4.6.1.1] in the pineal gland are similar to the properties of the sites which specifically bind l-[3H]alprenolol, a potent beta-adrenergic antagonist. Stimulation of the beta-adrenergic receptor results in a 30-fold increase in the activity of N-acetyltransferase (= arylamine acetyltransferase; acetyl CoA:arylamine N-acetyltransferase, EC 2.3.1.5), an enzyme involved in the synthesis of thepineal hormone melatonin. In the normal diurnal light-dark cycle there is greater physiological stimulation of the beta-adrenergic receptor in the pineal during the night than during the day. Pineals from rats kept in constant light for 24 hr possess more hormone-sensitive adenylate cyclase and specifically bind more l-[3H]alprenolol than do pineals from rats kept in the dark overnight. When rats, exposed to light for 24 hr, are treated with the beat-adrenergic agonist isoproterenol, there is a rapid loss of both hormone-sensitive adenylate cyclase activity and specific l-[3H]alprenolol binding sites. There is no change in the affinity of adenylate cyclase for isoproterenol or for its substrate, ATP. Similarly, although there are fewer binding sites, there is no change in the affinity of the remaining sites for either agonist or antagonist. Inhibition of protein synthesis with cycloheximide does not affect the loss of either adenylate cyclase activity or specific binding sites. The data suggest that stimulation of the beta-adrenergic receptor causes a rapid decrease in the number of available receptors and in hormone-sensitive adenylate cyclase activity; conversely, lack of stimulation causes an increase in these parameters. It is suggested that these changes contribute to the phenomena of super- and subsensitivity in the pineal gland by regulating the capacity of the pineal to synthesize cyclic AMP in response to beta-adrenergic stimulation.