Interaction of a radiolabeled agonist with cardiac muscarinic cholinergic receptors

The interaction of a radiolabeled muscarinic cholinergic receptor agonist, [methyl-3H]oxotremorine acetate [( 3H]OXO), with a washed membrane preparation derived from rat heart, has been studied. In binding assays at 4 degrees C, the rate constants for association and dissociation of [3H]OXO were 2 X 10(7) M-1 min-1 and 5 X 10(-3) min-1, respectively, Saturation binding isotherms indicated that binding was to a single population of sites with a Kd of approximately 300 pM. The density of [3H]OXO binding sites (90-100 fmol/mg of protein) was approximately 75% of that determined for the radiolabeled receptor antagonist [3H]quinuclidinyl benzilate. Both muscarinic receptor agonists and antagonists inhibited the binding of [3H]OXO with high affinity and Hill slopes of approximately one. Guanine nucleotides completely inhibited the binding of [3H]OXO. This effect was on the maximum binding (Bmax) of [3H]OXO with no change occurring in the Kd; the order of potency for five nucleotides was guanosine 5'-O-(3-thio-triphosphate) greater than 5'-guanylylimidodiphosphate greater than GTP greater than or equal to guanosine/diphosphate greater than GMP. The [3H]OXO-induced interaction of muscarinic receptors with a guanine nucleotide binding protein was stable to solubilization. That is, membrane receptors that were prelabeled with [3H]OXO could be solubilized with digitonin, and the addition of guanine nucleotides to the soluble, [3H]OXO-labeled complex resulted in dissociation of [3H]OXO from the receptor. Pretreatment of membranes with relatively low concentrations of N-ethylmaleimide inhibited [3H]OXO binding by 85% with no change in the Kd of [3H]OXO, and with no effect on [3H]quinuclidinyl benzilate binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of an altered membrane form of the beta-adrenergic receptor produced during agonist-induced desensitization

Incubation of 1321N1 human astrocytoma cells with 1 microM isoproterenol rapidly results in the conversion of a portion of the beta-adrenergic receptors to a membrane form that can be separated from markers for the plasma membrane by sucrose density gradient or differential centrifugation. This "light peak" form of the receptor reaches a maximal level within 10 min of incubation of cells with catecholamine. Two types of experiments suggest that the early phase of catecholamine-induced desensitization of the beta-adrenergic receptor-linked adenylate cyclase can be separated into at least two reactions. First, the agonist-induced loss of catecholamine-stimulated adenylate cyclase activity precedes the appearance of beta-adrenergic receptors in the light peak fraction by 1-2 min. Second, pretreatment of cells with concanavalin A prior to induction of desensitization blocks the formation of the light peak form of beta-adrenergic receptors without blocking the "uncoupling" reaction as measured by catecholamine-stimulated adenylate cyclase activity. Specificity for the reaction that converts beta-adrenergic receptors to the light peak form is indicated by the lack of a catecholamine-induced alteration in the sucrose density gradient distribution of muscarinic cholinergic receptors, adenylate cyclase or the guanine nucleotide-binding proteins, Ns and Ni. The light peak of beta-adrenergic receptors migrates at a density similar to that of at least a portion of the activity of galactosyltransferase, a marker for Golgi. Enzyme marker activities for lysosomes and endoplasmic reticulum are not associated with this population of beta-adrenergic receptors. Taken together, these and other data suggest that incubation of 1321N1 cells with isoproterenol results in a rapid uncoupling of beta-adrenergic receptors from adenylate cyclase which is followed by a change in the membrane form of the receptor. This latter step most likely represents internalization of receptors into a vesicular form which may then serve as the precursor state from which receptors are eventually lost from the cell.

High-affinity binding of agonists to beta-adrenergic receptors on intact cells

The interactions of agonists and antagonists with beta-adrenergic receptors on intact 1321N1 human astrocytoma and C62B rat glioma cells were studied by using the radioligand (-)-[125I]iodopindolol. Competition binding assays were performed at 37 degrees C under equilibrium conditions and in short-time nonequilibrium assays that approximated initial velocity conditions for binding of the radioligand. The theoretical basis and experimental validation of the initial velocity approach for determining binding affinities of rapidly equilibrating ligands are presented. For the agonists isoproterenol and epinephrine, high binding affinities that approximated their apparent affinities for binding in membranes and for increase of cyclic AMP concentrations in intact cells could be demonstrated only in short-time assays; in contrast, much lower affinities were observed with equilibrium (60-min) assays as reported previously for various cell lines. High-affinity binding of isoproterenol to 1321N1 cells also was observed in equilibrium (6-hr) binding assays carried out on ice. These results indicate that in the native state the intact cell beta-adrenergic receptor has a high binding affinity for agonists and suggest that incubation at 37 degrees C in the presence of an agonist converts the receptors to a form with low affinity for agonists.

Muscarinic cholinergic receptor-mediated control of cyclic AMP metabolism. Agonist-induced changes in nucleotide synthesis and degradation

Activation of muscarinic cholinergic receptors on 1321N1 human astrocytoma cells results in a 40-70% inhibition of isoproterenol- or prostaglandin E1 (PGE1)-stimulated accumulation of cyclic AMP. Previous investigations have demonstrated that this effect is due to a Ca2+-dependent activation of phosphodiesterase in the presence of muscarinic receptor agonists. However, during prolonged exposure of 1321N1 cells to a cholinergic agonist, a series of adaptive changes occurs which culminates in a complete loss of the muscarinic receptor-mediated inhibition of cyclic AMP accumulation. These alterations include: (a) A 50-100% increase in the capacity of isoproterenol and PGE1 to stimulate cyclic AMP accumulation. This phenomenon was rapid in onset, reached a maximum in 15-20 min, and disappeared over the next 2 hr even in the continued presence of carbachol. (b) A loss of the effects of muscarinic receptor stimulation on cyclic AMP accumulation. This phenomenon was apparent within 15 min after addition of carbachol, and complete desensitization was observed after 75 min. The loss of muscarinic receptor-mediated effects on cyclic AMP levels was due to a loss of the Ca2+-dependent stimulation of phosphodiesterase activity by muscarinic receptor agonists. (c) A loss of muscarinic receptors as assessed by [3H]quinuclidinyl benzilate binding. This effect was apparent after 90 min in the presence of carbachol. More than 80% of the receptors were lost after 24 hr, with no change occurring in the KD of [3H]quinuclidinyl benzilate. The concentration-effect curve for carbachol-induced changes in agonist responsiveness of the cyclic AMP system was similar to that for carbachol-induced reductions in cyclic AMP levels. Coincubation of carbachol with a saturating concentration of atropine prevented these adaptive changes from occurring. Although incubation of cells in Ca2+-free buffer or in the presence of 20 mM Co2+ prevented the inhibitory effects of muscarinic receptor stimulation on cyclic AMP accumulation, carbachol preincubations under these conditions still produced the adaptive changes in agonist responsiveness. The divalent cation ionophore, A23187, mimics the effects of muscarinic receptor stimulation on cyclic AMP levels by activating phosphodiesterase. Following complete carbachol-induced loss of responsiveness to muscarinic receptor agonists, A23187 was still capable of inhibiting cyclic AMP accumulation.

Recovery of beta-adrenergic receptors following long term exposure of astrocytoma cells to catecholamine. Role of protein synthesis

As a part of the process of agonist-induced desensitization, 1321N1 human astrocytoma cells lose up to 95% of their beta-adrenergic receptors, as detected by 125I-hydroxybenzylpindolol (125IHYP) binding, after 12-24 h of exposure to isoproterenol. In preconfluent cultures the loss of beta-receptors is completely reversible upon removal of isoproterenol, with receptor levels reaching 100% of control levels within 48-72 h. Addition of cycloheximide (5 micrograms/ml) upon removal of agonist does not prevent the recovery of receptors. After an initial 4-h lag, receptors accumulate in the presence of cycloheximide until the same receptor level is reached that was present at the onset of desensitization. Confluent cultures, which have a reduced number of receptors per cell, recover beta-receptors to only 60 to 70% of control levels following removal of isoproterenol. In addition, cycloheximide blocks the recovery of receptors in these cultures. The effects of cycloheximide on the accumulation of receptors during cell growth suggest that receptors are stable in preconfluent cultures and that turnover only occurs later when cultures are confluent. The data also indicate that long term exposure of cells to catecholamine results in a form of the beta-adrenergic receptor that is undetectable by 125IHYP binding but, nonetheless, retains its primary amino acid structure. The undetectable receptors appear to be retained until agonist is removed, whereupon they become detectable by 125IHYP binding with a t1/2 of about 36 h in the presence of cycloheximide.

Catecholamine-induced alteration in sedimentation behavior of membrane bound beta-adrenergic receptors

Incubation of astrocytoma cells with catecholamines results in a decrease in catecholamine-stimulated adenylate cyclase activity and a concomitant alteration in the sedimentation properties of particulate beta-adrenergic receptors. The altered receptors exhibit agonist binding properties similar to those of receptors that are "uncoupled" from adenylate cyclase.

Cyclic AMP and beta-adrenergic receptors during the development of physical dependence on ethanol in the rat

1. The half-time for maximal withdrawal scores in the rat is 30 to 36 hours following around the clock intubation with ethanol. 2. Cyclic AMP levels in all brain areas decline with ethanol treatment and rise again to control levels during withdrawal. 3. Norepinephrine-stimulated adenylate cyclase activity is increased by chronic treatment with ethanol. However, it is unlikely that this change is related to physical dependence, as it occurs 50 to 60 hours following the development of half-maximal withdrawal scores. 4. Ethanol had no effect on the concentration of beta-receptors in the cerebral cortex.

Adaptive changes in the responsiveness of adenylate cyclase to catecholamines

Exposure of the intact astrocytoma cell to isoproterenol not only causes the activation of adenylate cyclase and the accumulation of cyclic AMP but sets in motion a complicated series of events designed to down-regulate the system if exposure to the agonist is extended in time. We have identified at least three of these processes: (1) a rapid uncoupling of the beta-receptor--adenylate cyclase system with subsequent loss of beta-receptors; (2) a slower, nonspecific desensitization of adenylate cyclase to the effects of all classes of receptor agonists by a process that may be mediated by cyclic AMP; and (3) a slow induction of phosphodiesterase activity that is probably mediated by cyclic AMP.

Isoproterenol-induced desensitization of adenylate cyclase in human astrocytoma cells. Relation of loss of hormonal responsiveness and decrement in beta-adrenergic receptors

Incubation of human astrocytoma cells (1321N1) with low concentrations of isoproterenol results in a specific loss of responsiveness to catecholamines as evidenced by a decreased accumulation of cAMP in intact cells, a reduction in isoproterenol-stimulated adenylate cyclase activity, and a decrease in beta-adrenergic receptor density, as measured by the specific binding of 125I-hydroxybenzylpindolol. The kinetics of desensitization suggest the involvement of two different reactions. The initial reaction involves a rapid loss of adenylate cyclase activity with little loss of beta-adrenergic receptors. Subsequently, a slower reaction results in the loss of measurable beta-adrenergic receptors. The degree of loss of both parameters was similar after 24 h of desensitization. It is concluded that the loss of beta-adrenergic receptors is an event that occurs as a result of the initial uncoupling of the beta-receptor-linked adenylate cyclase.

Presynaptic modulation of beta adrenergic receptors in rat cerebral cortex after treatment with antidepressants

Treatment with desmethylimipramine (DMI), a tricyclic antidepressant, for 7 to 21 days resulted in a 35 to 45% decrease in the accumulation of adenosine cyclic 3':5'-monophosphate (cAMP) in response to a maximally effective concentration of (-)-isoproterenol (ISO) in rat cerebral cortical slices. The EC50 for ISO-stimulated cAMP accumulation was not affected by DMI administration. The diminution in responsiveness to catecholamines was accompanied by a 35 to 40% decrease in the density of beta adrenergic receptors as measured by the binding of [125I]iodohydroxybenzylpindolol. Decreases in ISO-sensitive cAMP accumulation and in beta adrenergic receptor density were temporally correlated, maximal decreases being observed within 5 to 7 days. Within 7 days after cessation of chronic DMI treatment ISO-stimulated cAMP accumulation and beta adrenergic receptor density returned to normal. The role of presynaptic nerve terminals in mediating these phenomena was also investigated. Treatment of newborn rats with 6--hydroxydopamine inhibited the development of noradrenergic nerve terminals in the cerebral cortex and blocked the effects of DMI on cortical cAMP accumulation and on beta adrenergic receptor density. The administration of the beta adrenergic receptor antagonist propranolol led to increases in maximal ISO-stimulated cAMP accumulations and beta adrenergic receptor density in the rat cerebral cortex. This increase was not affected by the simultaneous administration of propranolol and DMI. Thus, the effect of DMI appears to be mediated through an action of norepinephrine at beta adrenergic receptors. Chronic treatment with two other clinically effective antidepressants, pargyline and iprindole, led to effects similar to those observed with DMI administration. Pretreatment of neonates with 6-hydroxydopamine blocked the effect of iprindole on beta adrenergic receptors. Preincubation of cortical membranes with guanosinetriphosphate before determination of the density of beta adrenergic receptors had no effect on the decreased number of receptors had no effect on the decreased number of receptors seen in DMI-treated animals. These experiments suggest that antidepressants, acting presynaptically, increase the concentration of transmitter at noradrenergic synapses and induce a compensatory decrease in the density of beta adrenergic receptors.

Studies of cAMP metabolism in cultured hepatoma cells: presence of functional adenylate cyclase despite low cAMP content and lack of hormonal responsiveness

The ability of isoproterenol, glucagon, PGE1 and cholera toxin to stimulate the synthesis of cAMP and protein kinase activity in line of liver cells (BRL) and a line of rat hepatoma cells (H35) has been determined. The concentration of cAMP in BRL cells (approximately 10 pmoles/mg protein) is in the range reported for other cultured cell lines but H35 cells contain extraordinarily low amounts of this cyclic nucleotide (approximately 0.05 pmoles/mg protein). Isoproterenol and PGE1 caused an increase in cAMP content, and protein kinase activation in BRL cells, although glucagon was ineffective. H35 cells, in contrast, were completely insensitive to all hormonal agonists. Despite this fact, cholera toxin was able to produce a marked increase in cAMP content, adenylate cyclase activity and protein kinase activation in H35 cells. binding studies with [125 I]-iodohydroxybenzylpindolol, a specific beta-adrenergic receptor antagonist, revealed that each H35 cell possesses fewer than 10 beta-adrenergic receptors whereas BRL cells contain 2-5,000 receptors per cell. The low level of cAMP in H35 cells appears to result from a combination of totally unstimulated adenylate cyclase and apparently elevated phosphodiesterase activities.

Adrenergic storage vesicles in rat heart: quantitation of membrane-bound and membrane-enclosed dopamine beta-hydroxylase

Norepinephrine-containing storage particles in rat heart have been disrupted to determine their stability and to quantitate the amount of dopamine beta-hydroxylase (DBH) that exists in a soluble, membrane enclosed pool. Vesicles were relatively resistant to lysis by hypotonic shock or by up to five cycles of freezing and thawing. These procedures have been shown to release catecholamines from adrenal chromaffin granules readily. However, rat heart vesicles were sensitive to temperature and they could be lysed by mechanical disruption using a sonicator or a Polytron homogenizer. Increasing the force or duration of the disruption resulted in an increased release of norepinephrine and DBH into a supernatant fraction after high-speed centrifugation. Extrapolation of the data from such experiments gave an estimate for membrane-enclosed DBH of 37% of the total amount of enzyme activity. This estimate of the proportion of membrane-enclosed enzyme is considerably higher than those previously reported for membrane-enclosed DBH in sympathetic neurons.