Reappearance of beta-adrenergic receptors after isoproterenol treatment in intact C6-cells

Postendocytic Sorting of Adrenergic and Opioid Receptors: New Mechanisms and Functions

The endocytic pathway tightly regulates the activity of G protein-coupled receptors (GPCRs). Much of our understanding of this relationship between GPCR endocytic trafficking and signaling comes from studies done on catecholamine and opioid receptors. After ligand-induced endocytosis, a key sorting step in the endosome determines whether receptors are recycled back to the cell surface, leading to recovery of signaling, or are degraded in the lysosome, leading to desensitization. Recycling of GPCRs, unlike that of many other proteins, is an active process driven by specific sequences on the receptor and proteins that interact with this sequence. Recent data suggest that sequence-dependent recycling plays complex roles in regulating both the timing and location of GPCR signaling. This chapter will describe our current understanding of the mechanisms regulating GPCR sorting in the endosome and discuss emerging ideas on their role in GPCR signaling, focusing on adrenergic and opioid receptors as prototypes.

Involvement of tumor acidification in brain cancer pathophysiology

Gliomas, primary brain cancers, are characterized by remarkable invasiveness and fast growth. While they share many qualities with other solid tumors, gliomas have developed special mechanisms to convert the cramped brain space and other limitations afforded by the privileged central nervous system into pathophysiological advantages. In this review we discuss gliomas and other primary brain cancers in the context of acid-base regulation and interstitial acidification; namely, how the altered proton (H⁺) content surrounding these brain tumors influences tumor development in both autocrine and paracrine manners. As proton movement is directly coupled to movement of other ions, pH serves as both a regulator of cell activity as well as an indirect readout of other cellular functions. In the case of brain tumors, these processes result in pathophysiology unique to the central nervous system. We will highlight what is known about pH-sensitive processes in brain tumors in addition to gleaning insight from other solid tumors.

Differential regulation of the cell-surface targeting and function of beta- and alpha1-adrenergic receptors by Rab1 GTPase in cardiac myocytes

The molecular mechanism underlying the export from the endoplasmic reticulum (ER) to the cell surface and its role in the regulation of signaling of adrenergic receptors (ARs) remain largely unknown. In this report, we determined the role of Rab1, a Ras-like GTPase that coordinates protein transport specifically from the ER to the Golgi, in the cell surface targeting and function of endogenous beta- and alpha1-ARs in neonatal rat ventricular myocytes. Adenovirus-driven expression of Rab1 into myocytes selectively increased the cell-surface number of alpha1-AR, but not beta-AR, whereas the dominant-negative mutant Rab1N124I significantly reduced the cell-surface expression of beta-AR and alpha1-AR. Brefeldin A inhibited beta-AR and alpha1-AR export and antagonized the Rab1 effect on alpha1-AR expression. Manipulation of Rab1 function similarly influenced the transport of alpha1A- and alpha1B-ARs as well as beta1- and beta2-ARs. Fluorescent microscopy analysis demonstrated that expression of Rab1N124I and Rab1 small interfering RNA induced a marked accumulation of GFP-tagged beta2-AR and alpha1B-AR in the ER. Consistent with the effects on receptor cell-surface targeting, Rab1 selectively enhanced ERK1/2 activation and hypertrophic growth in response to the alpha1-AR agonist phenylephrine but not to the beta-AR agonist isoproterenol. Rab1N124I inhibited both agonist-mediated ERK1/2 activation and hypertrophic growth in neonatal myocytes. These results demonstrate that the cell-surface targeting and signaling of beta- and alpha1-ARs require Rab1 and are differentially modulated by augmentation of Rab1 function. Our data provide strong evidence implicating the ER-to-Golgi traffic as a site for selective manipulation of distinct AR function in cardiac myocytes.

Enhancement of the recycling and activation of beta-adrenergic receptor by Rab4 GTPase in cardiac myocytes

We investigate the role of Rab4, a Ras-like small GTPase coordinating protein transport from the endosome to the plasma membrane, on the recycling and activation of endogenous beta-adrenergic receptor (beta-AR) in HL-1 cardiac myocytes in vitro and transgenic mouse hearts in vivo. Beta1-AR, the predominant subtype of beta-AR in HL-1 cardiac myocytes, was internalized after stimulation with isoproterenol (ISO) and fully recycled at 4 h upon ISO removal. Transient expression of Rab4 markedly facilitated recycling of internalized beta-AR to the cell surface and enhanced beta-AR signaling as measured by ISO-stimulated cAMP production. Transgenic overexpression of Rab4 in the mouse myocardium significantly increased the number of beta-AR in the plasma membrane and augmented cAMP production at the basal level and in response to ISO stimulation. Rab4 overexpression induced concentric cardiac hypertrophy with a moderate increase in ventricle/body weight ratio and posterior wall thickness and a selective up-regulation of the beta-myosin heavy chain gene. These data provide the first evidence indicating that Rab4 is a rate-limiting factor for the recycling of endogenous beta-AR and augmentation of Rab4-mediated traffic enhances beta-AR function in cardiac myocytes.

Binding of the beta2 adrenergic receptor to N-ethylmaleimide-sensitive factor regulates receptor recycling

Following agonist stimulation, most G protein-coupled receptors become desensitized and are internalized, either to be degraded or recycled back to the cell surface. What determines the fate of a specific receptor type after it is internalized is poorly understood. Here we show that the rapidly recycling beta2 adrenergic receptor (beta2AR) binds via a determinant including the last three amino acids in its carboxyl-terminal tail to the membrane fusion regulatory protein, N-ethylmaleimide-sensitive factor (NSF). This is documented by in vitro overlay assays and by cellular coimmunoprecipitations. Receptors bearing mutations in any of the last three residues fail to interact with NSF. After stimulation with the agonist isoproterenol, a green fluorescent protein fusion of NSF colocalizes with the wild type beta2AR but not with a tail-mutated beta2AR. The beta2AR-NSF interaction is required for efficient internalization of the receptors and for their recycling to the cell surface. Mutations in the beta2AR tail that ablate NSF binding reduce the efficiency of receptor internalization upon agonist stimulation. Upon subsequent treatment of cells with the antagonist propranolol, wild type receptors return to the cell surface, while tail-mutated receptors remain sequestered. Thus, the direct binding of the beta2AR to NSF demonstrates how, after internalization, the fate of a receptor is reliant on a specific interaction with a component of the cellular membrane-trafficking machinery.

Type-specific sorting of G protein-coupled receptors after endocytosis

The beta(2)-adrenergic receptor (B2AR) and delta-opioid receptor (DOR) are structurally distinct G protein-coupled receptors (GPCRs) that undergo rapid, agonist-induced internalization by clathrin-coated pits. We have observed that these receptors differ substantially in their membrane trafficking after endocytosis. B2AR expressed in stably transfected HEK293 cells exhibits negligible (<10%) down-regulation after continuous incubation of cells with agonist for 3 h, as assessed both by radioligand binding (to detect functional receptors) and immunoblotting (to detect total receptor protein). In contrast, DOR exhibits substantial (>/=50%) agonist-induced down-regulation when examined by similar means. Degradation of internalized DOR is sensitive to inhibitors of lysosomal proteolysis. Flow cytometric and surface biotinylation assays indicate that differential sorting of B2AR and DOR between distinct recycling and non-recycling pathways (respectively) can be detected within approximately 10 min after endocytosis, significantly before the onset of detectable proteolytic degradation of receptors ( approximately 60 min after endocytosis). Studies using pulsatile application of agonist suggest that after this sorting event occurs, later steps of membrane transport leading to lysosomal degradation of receptors do not require the continued presence of agonist in the culture medium. These observations establish that distinct GPCRs differ significantly in endocytic membrane trafficking after internalization by the same membrane mechanism, and they suggest a mechanism by which brief application of agonist can induce substantial down-regulation of receptors.

Specific changes in beta2-adrenoceptor trafficking kinetics and intracellular sorting during downregulation

Agonist-activated beta2-adrenoceptors rapidly internalize and then recycle to the cell surface, however chronic agonist eventually causes receptor downregulation. To characterize beta2-adrenoceptor trafficking kinetics and intracellular sorting during downregulation, human embryonic kidney cells expressing epitope-tagged receptors were examined by radioligand binding with (+/-)-[3H]4-(3-tertiarybutylamino-2-hydroxypropoxy)-benzimidazole- 2-on hydrochloride ([3H]CGP12177) and immunofluorescence microscopy. The first-order receptor recycling rate constant declined after 18 h of agonist compared with 15 min (0.05 min(-1) vs. 0.12 min(-1)), thus increasing the intracellular transit time (20.0 min vs. 8.3 min). There was also a reduction in the rate of receptor endocytosis and a decline in the total number of receptors. Although the intracellular receptor fraction did not increase between 15 min and 18 h of agonist, some receptors moved irreversibly into a protease-containing compartment while retaining radioligand binding activity. Our results indicate that beta2-adrenoceptor downregulation is associated principally with an increased intracellular transit time during recycling. This could promote the diversion of receptors into protease-containing compartments, where there is an irreversible commitment to downregulation prior to loss of radioligand binding activity.

A novel beta-adrenoceptor ligand for positron emission tomography: evaluation in experimental animals

Myocardial and pulmonary beta-adrenoceptors can be imaged and quantified with the antagonist (S)-4-[3[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-b enzimidazol-2-[11C]-one (S-[11C]CGP-12177). The synthesis of this ligand (based on the reaction of a precursor with [11C]phosgene) is laborious and in many centers the final product has a low and variable specific activity. This prevents widespread use of S-[11C]CGP-12177 for studies in patients. We prepared S-[11C]CGP-12388, the isopropyl analogue of CGP-12177, by a reliable one-pot procedure and evaluated the radiopharmaceutical for beta-adrenoceptor imaging. Blocking experiments with subtype-selective beta-adrenergic drugs showed that myocardial and pulmonary uptake of S-[11C]CGP-12388 in anesthetized rats reflects ligand binding to beta1- and beta2-adrenoceptors. In this animal model, clearance, metabolism and tissue/plasma ratios of S-[11C]CGP-12388 were similar to those of S-[11C]CGP-12177. A [18F]fluoroisopropyl analogue of CGP-12177 showed less favorable characteristics. S-[11C]CGP-12388 was therefore selected for evaluation in humans and it may become the tracer of choice for clinical studies since it is easily prepared.

Effect of Ca2+-channel blockers on isoprenaline-induced desensitization in rat trachea

Isoprenaline-induced desensitization in vitro in rat trachea was studied in the presence of the Ca2+-channel blockers (Ca2+-CBs) verapamil and nitrendipine. The concentration-response curves for isoprenaline were determined in a noncumulative manner using carbachol as contracting agent, and then desensitization was achieved by 40-min incubation of the tracheal preparations with isoprenaline (1 microM). The effect of verapamil and nitrendipine was studied by the addition of each Ca2+-CB to the desensitizing solution. Both verapamil and nitrendipine reduced the isoprenaline-induced desensitization in the rat trachea.

Protection by dexamethasone of the functional desensitization to beta 2-adrenoceptor-mediated responses in human lung mast cells

1. The beta-adrenoceptor agonist, isoprenaline, inhibited the IgE-mediated release of histamine from human lung mast cells (HLMC) in a dose-dependent manner. Maximal inhibitory effects were obtained with 0.1 microM isoprenaline. However, the inhibition of histamine release from HLMC by isoprenaline (0.1 microM) was highly variable ranging from 33 to 97% inhibition (mean, 59 +/- 3%, n = 27). 2. Long-term (24 h) incubation of HLMC with isoprenaline led to a subsequent reduction in the ability of a second exposure of isoprenaline to inhibit IgE-mediated histamine release from HLMC. The impairment in the ability of isoprenaline (0.1 microM) to inhibit histamine release following desensitizing conditions (1 microM isoprenaline for 24 h) was highly variable amongst HLMC preparations ranging from essentially negligible levels of desensitization in some preparations to complete abrogation of the inhibitory response in others (mean, 65 +/- 6% desensitization, n = 27). 3. The ability of HLMC to recover from desensitization was investigated. Following desensitizing conditions (1 microM isoprenaline for 24 h), HLMC were washed and incubated for 24 h in buffer and the effectiveness of isoprenaline (0.1 microM) to inhibit IgE-mediated histamine release from HLMC was assessed. The extent of recovery was highly variable with some HLMC preparations failing to recover and others displaying a complete restoration of responsiveness to isoprenaline (mean, 40 +/- 6% recovery, n = 23). 4. The effects of the glucocorticoid, dexamethasone, were also investigated. Long-term (24-72 h) treatments with dexamethasone (0.1 microM) had no effect on IgE-mediated histamine release from HLMC. Additionally, long-term (24-72 h) treatments with dexamethasone (0.1 microM) had no effect on the effectiveness of isoprenaline to inhibit histamine release. However, long-term (24-72 h) treatments with dexamethasone (0.1 microM) protected against the functional desensitization induced by incubation (24 h) of HLMC with isoprenaline (1 microM). The protective effect was time-dependent and pretreatment of HLMC with dexamethasone for either 24, 48 or 72 h prevented desensitization by either 15 +/- 7, 19 +/- 5 or 51 +/- 10%, respectively (n = 5-7). 5. HLMC preparations which were relatively refractory to isoprenaline even after withdrawal (24 h) from desensitizing conditions responded more effectively to isoprenaline (0.1 microM) if dexamethasone (0.1 microM) was also included during the recovery period (19 +/- 9% recovery after 24 h in buffer; 50 +/- 8% recovery after 24 h with dexamethasone, n = 5). 6. These data indicate that the responses of different HLMC preparations to isoprenaline, the susceptibility of HLMC to desensitization and the ability of HLMC to recover from desensitizing conditions varies markedly. Dexamethasone, which itself has no direct effects on IgE-mediated histamine release from HLMC, protected HLMC from the functional desensitization to beta-adrenoceptor agonists. Because beta 2-adrenoceptor agonists and glucocorticoids are important in the therapeutic management of asthma and as the HLMC is probably important in certain types of asthma, these findings may have wider clinical implications.

Synthesis and properties of fluorescent beta-adrenoceptor ligands

We describe the synthesis of bordifluoropyrromethene (BODIPY), fluorescein, and related fluorescent derivatives of the beta-adrenergic ligand CGP 12177. With these probes we screened insect (Sf9) cells stably transformed with the human beta 2-adrenoceptor gene and expressing (2-3.5) x 10(5) human beta 2-adrenoceptors per cell. Among these derivatives only BODIPY-CGP gave a receptor-specific signal sufficiently strong for measuring the on- and off-rate constants and the equilibrium dissociation constant of beta-adrenoceptor-specific binding by spectrofluorometry or photon counting. Similar KD values for BODIPY-CGP binding were obtained by kinetic measurements (approx. 250 pM) and under equilibrium conditions (400 +/- 180 pM), and these were in the same range as those obtained with [3H]CGP 12177 (200 +/- 32 pM). The cell-bound fluorescence could be quenched specifically with nonfluorescent CGP 12177 to near background levels. The disposition of the beta 2-adrenoceptors in BODIPY-CGP-stained Sf9 cells was mainly restricted to the cell surface at 4 and 30 degrees C. Hence, beta-adrenoceptor-expressing cells can be stained specifically with BODIPY-CGP, and beta-adrenoceptors on a single cell can be assessed by photon counting under the fluorescence microscope. Cells can also be scanned by fluorescence-activated flow cytometry.

Regulation of beta-adrenoceptors in the guinea-pig sinoatrial node

This study examined the changes of beta-adrenoceptors in the guinea-pig sinoatrial nodal region following 7 day (-)-isoprenaline (400 micrograms/kg/h s.c.) infusion and the relationship between beta-adrenoceptor desensitization and receptor down-regulation. Changes in beta 1- and beta 2-adrenoceptor density were measured using quantitative autoradiography and function in organ bath studies. (-)-Isoprenaline treatment produced a marked decrease in total (from 57.5 to 33.9 fmol/mg protein), beta 1- (from 49.4 to 32.8 fmol/mg protein), and beta 2-adrenoceptor density (from 8.1 to 1.05 fmol/mg protein) in the sinoatrial node. In adjacent right atrium, treatment produced no change in total (39.5 and 36.7 fmol/mg protein) or beta 1-adrenoceptors (35.9 and 36.4 fmol/mg protein) but did decrease beta 2-adrenoceptors (from 3.7 to 0.3 fmol/mg protein). Chronotropic effects were measured in spontaneously beating right atrium. Procaterol, a selective beta 2-adrenoceptor agonist, caused a biphasic chronotropic response in control right atria, the first part of which was abolished in the tissue from treated animals. The maximum increase in right atrial rate to RO363, a beta 1-adrenoceptor selective partial agonist, was reduced from 114 bpm in control to 43 bpm in treated animals. In electrically driven right atrium with the sinoatrial node removed procaterol failed to produce a positive inotropic response via beta 2-adrenoceptors, but the maximum response to RO363 was reduced from 0.75 g in the control tissue to 0.12 g in the treated tissue. This study showed that changes in beta 2-adrenoceptor density following 7 day (-)-isoprenaline infusion are compatible with reduced functional responsiveness in the SA node. The reduction of beta 1-adrenoceptor number in the SA node was also compatible with the reduced chronotropic response in this tissue. However the lack of effect on beta 1-adrenoceptor density in the right atrium was not consistent with the decrease in beta 1-adrenoceptor mediated inotropic response in this tissue. This suggests that beta-adrenoceptor desensitization is not always associated with receptor down-regulation but depends also on the changes in the cell signalling system beyond the level of the receptor which differ according to the cardiac location.

Recovery of beta-receptors and adenylate cyclase from desensitization induced by short term heat exposure in rat parotid glands

1. The recovery of rat parotid beta-adrenergic receptors (beta-AR) and adenylate cyclase (AC) from heat (33 degrees C)-induced desensitization was studied. 2. Down-regulated cell surface beta-AR and AC activity in response to isoprenaline (IPR) returned to the control level 120 hr after the termination of heat exposure. 3. However, beta-AR in parotid crude membranes increased over the control level for 48-120 hr. 4. Coupling between beta-AR and G protein(s) was attenuated at 120 hr. 5. These data suggest that beta-AR on the cell surface, but not those internalized, can transduce biological responses.

Effects of calcium antagonists on beta-receptors of cultured cardiac myocytes isolated from neonatal rat ventricle

The effects of calcium antagonists (verapamil, diltiazem, and nicardipine) on beta-adrenergic receptors of cultured cardiac myocytes isolated from neonatal rat ventricle were studied with the hydrophilic ligand [3H]CGP-12177, which identifies cell surface-bound beta-receptors. The three calcium antagonists suppressed spontaneous beating of the myocytes, increased the number of beta-receptors, but did not alter the affinity (Kd). These effects were dose and time dependent. Verapamil (10(-6) M) increased the beta-receptor density by about 13% after 6 hours of incubation, and this increase in density reached a plateau of about 45% after 24 hours of incubation. beta-Receptor density increased by 15% with 5 x 10(-7) M and by 37% with 10(-6) M verapamil. The increased beta-receptors appeared to retain their normal function, as assessed by the increased spontaneous beating of the myocytes in response to applied isoproterenol. The increase in beta-receptors was abolished by colchicine but not by cycloheximide. When the calcium ion concentration of the medium was lowered to 0.1 mM, no significant change occurred in the density of beta-receptors compared with that in 1.8-mM Ca2+ medium. The results suggest that calcium antagonists increase beta-receptors by accelerating recycling by microtubules but not by decreasing the inward calcium current. Such effects of calcium antagonists may be clinically important and promise insight into the mechanism of the withdrawal phenomenon of calcium antagonists.

Localization of beta-adrenergic receptors in A431 cells in situ. Effect of chronic exposure to agonist

The status of beta-adrenergic receptors was investigated in A431 cells exposed to chronic stimulation by the beta-adrenergic agonist, (-)-isoproterenol. Specific binding of beta-adrenergic antagonist (-)-[125I]iodocyanopindolol declined to 60-80% below control values within 12 h of agonist treatment. This decline in ligand binding was also observed in high-speed membrane fractions prepared from agonist-treated cells. Immunoblots probed with anti-receptor antibodies revealed both that beta-adrenergic receptors from untreated and treated cells migrated as 65,000-Mr peptides and that the cellular complement of receptor was unchanged. Indirect immunofluorescence localization of beta-adrenergic receptors was comparable in control (untreated) cells and cells challenged with (-)-isoproterenol for 1, 12, or 24 h. Thus receptor complement, migration on SDS/polyacrylamide-gel electrophoresis, and localization in situ are largely unaffected by agonist stimulation. Receptor binding of antagonist radioligands, in contrast, is markedly down-regulated in cells stimulated chronically with beta-adrenergic agonists. These data argue in favour of agonist-induced alteration(s) in the conformation of the receptor that preclude radioligand binding rather than agonist-induced receptor sequestration and/or degradation.

Role of receptor cycling in the regulation of angiotensin II surface receptor number and angiotensin II uptake in rat vascular smooth muscle cells

In vivo data on the factors controlling angiotensin II (AII) cell surface binding are conflicting. We studied the specific effects of AII on AII binding in rat mesenteric artery vascular smooth muscle cells in culture. Incubation with unlabeled AII at 21 degrees C resulted in time- and concentration-dependent decreases in AII surface binding at 4 degrees C, with a 30% reduction after exposure to 300 nM AII for 15 min. Reductions in cell surface binding were due to decrements in receptor number rather than changes in binding affinity. Loss of surface receptors was mediated by receptor internalization as maneuvers that blocked ligand internalization (cold temperature and phenylarsine oxide [PAO]) attenuated AII-induced loss of surface receptors. After removal of AII, recovery of surface binding was rapid (t1/2 = 15 min) and was mediated by reinsertion of a preexisting pool of receptors into the surface membrane rather than by new receptor synthesis. To determine the role of receptor cycling on AII-induced surface receptor loss, cells were incubated with the endosomal inhibitor chloroquine during exposure to AII at 21 degrees C. Incubation with AII plus chloroquine resulted in a 70% greater loss of surface binding than after incubation with AII alone. To determine the role of receptor cycling on uptake of ligand, cells were incubated with PAO or endosomal inhibitors during exposure to AII at 4 and 21 degrees C. Compared with buffer these agents did not alter AII uptake at 4 degrees C, but decreased uptake by 12-50% at 21 degrees C. These results indicate that after binding AII receptors cycle and that receptor cycling attenuates AII-induced losses of surface receptors and enhances ligand uptake by providing a continuous source of receptors to the cell surface.

Human platelet beta 2-adrenoceptors: agonist-induced internalisation and down-regulation in intact cells

1. The effect of isoprenaline (10 microM at 37 degrees C for 30 min) pretreatment on [125I]-(-)-pindolol ([125I]-(-)-Pin) binding to beta 2-adrenoceptors on intact human platelets has been examined. 2. By use of saturation analysis, maximal binding capacity (Bmax) of [125I]-(-)-Pin binding in control and treated cells was assessed in the presence of 1 microM (-)-propranolol or 1 microM (+/-)-CGP 12177 which were taken to represent total or cell surface beta-adrenoceptors respectively. Assay incubations were performed at 37 degrees C and 4 degrees C, the latter to prevent recycling of internalised receptors. 3. Isoprenaline treatment resulted in an identical, highly significant, loss of binding sites (approximately equal to 25%) defined by (-)-propranolol at both assay temperatures as compared to control cells. Binding sites identified in the presence of (+/-)-CGP 12177 were reduced to a much greater extent (approximately equal to 70%), but this was only seen when assays were performed at 4 degrees C. 4. Agonist-induced changes in receptor numbers were concentration-dependent with half maximal receptor loss occurring at an isoprenaline concentration of approximately 2 x 10(-8) M. These effects were inhibited by the presence of a beta-adrenoceptor antagonist and absent if agonist pretreatment was performed at 4 degrees C. 5. Recovery experiments showed that the isoprenaline-induced reduction in total receptor number defined by (-)-propranolol was irreversible whereas the reduction in cell surface receptors defined by (+/-)-CGP 12177 was rapidly reversible (less than 40 min). 6. These data suggest that isoprenaline treatment of intact human platelets causes redistribution of beta 2-adrenoceptors. A proportion are sequestered away from the cell surface (internalised), becoming inaccessible to the hydrophilic ligand (+/-)-CGP 12177. A smaller proportion defined by (-)-propranolol are apparently totally lost from the cell (down regulated).

Effect of thyroid hormone on slow calcium channel function in cultured chick ventricular cells

The hyperthyroid state is associated with increased myocardial contractility. To clarify responsible mechanisms, we examined the effects of thyroid hormone on slow Ca channels, beta-adrenergic receptors, transsarcolemmal 45Ca flux and cytosolic free calcium in cultured chick ventricular cells. Compared with cells grown without triiodothyronine (T3), cells grown in 10 nM T3 possessed 67% (P less than 0.05) more dihydropyridine 3H-PN200-110 binding sites, 24% (P less than 0.05) more beta-adrenergic antagonist 3H-CGP12177 binding sites, a 57% (P less than 0.05) greater nifedipine-sensitive initial 45Ca uptake rate, and a 31% (P less than 0.05) greater nifedipine-sensitive 45Ca uptake rate in response to BAY k 8644. Time-averaged mean intracellular free Ca concentration ([Ca]i) measured with fura-2, total protein content, and dissociation constant values for 3H-PN200-110 or 3H-CGP12177 binding was not significantly different in the two groups of cells. BAY k 8644 (1 microM) increased mean [Ca]i 2.85- or 2.16-fold in cells grown with or without 10 nM T3, respectively. l-Isoproterenol (1 microM) increased [Ca]i 1.53- or 1.28-fold in cells grown with or without 10 nM T3, respectively. We conclude that thyroid hormone augments transsarcolemmal Ca influx, at least in part via slow Ca channels associated with increased numbers of these channels. T3-treated cells appear to be more responsive to the effects of BAY k 8644 or isoproterenol on [Ca]i.

Brain opioid receptors in the hibernating bat, Myotis lucifugus: modification by low temperature and comparison with rat, mouse and hamster

Several studies have provided evidence that brain opioid peptides may be involved in the control of the hibernation cycle. We have now examined the influence of hibernation on central opioid receptors. We have characterized the receptor binding properties of [3H]-naloxone in slices of the cerebral cortex and hypothalamus of the bat Myotis lucifugus. These receptors possess those characteristics expected of an opioid site namely high affinity, stereospecificity and saturability. Under normal incubation conditions (30 degrees C) we observed no effect of hibernation on [3H]-naloxone binding. However, when binding assays were performed at temperatures corresponding to the appropriate body temperature (e.g., 4 degrees C for hibernation) we detected a significant low temperature-induced increase in hypothalamic binding. Similar experiments in rat, mouse and hamster revealed that [3H]-naloxone binding was also increased at 4 degrees C when compared to 30 degrees C. This was true in hypothalamus and cortex. Additional studies in the rat demonstrated that the opioid receptor is of higher affinity at low temperatures. The behavioural and neurochemical consequences of this change in the opioid receptor, and whether this might be involved in the regulation of hibernation, remain to be studied.

Stress induced desensitization of lymphocyte beta-adrenoceptors in young and aged rats

The effects of different times of immobilization stress on intact lymphocyte beta-adrenoceptors and plasma corticosterone were compared in 3-month and 24-month-old rats. In young animals after 30 min restraint 3H-dihydroalprenolol specific binding was significantly reduced (61% of control value) and plasma corticosterone significantly raised (186% of control). The effect on beta-adrenoceptors was due changes in receptor number (Bmax) without any effect on affinity (KD). In aged rats both effects were only seen after 180 min restraint and were less pronounced. Isoproterenol treatment in vitro reduced beta-adrenoceptors on lymphocytes. This effect was less pronounced in lymphocytes from aged rats. Corticosterone in vitro increased 3H-dihydroalprenolol specific binding. We therefore suggest that the decrease of beta-adrenoceptors reflects an adaptive response to the stress-induced catecholamine release and that corticosterone could play a role in reversing this effect. This adaptive response to stress seems to be impaired in aged animals.

Evidence for the rapid internalization and recycling of lutropin receptors in rat testis Leydig cells

A study into the binding of 125I-human chorionic gonadotropin (hCG) to the lutropin (LH) receptor in rat testis Leydig cells, and subsequent internalization of the hormone-receptor complex, has been carried out. The results show that there is rapid internalization of the hormone-receptor complex; 240 receptors/cell (from a total of approx. 4000 receptors/cell) were internalized each minute in the first hour after exposure to hCG. Radioactivity was released from the cell 1 h after internalization and was found to be associated with highly degraded hCG. The endocytic process was found to have two temperature-sensitive steps. At 4 degrees C, movement of the hormone-receptor complex inside the cell did not occur, and at 21 degrees C hormone accumulated within the cytoplasm but was not degraded or released from the cell. At 34 degrees C, internalization, degradation and loss of the degraded hormone from the cell occurred. These processes appeared to reach a steady state after 2 h. Even though there is rapid internalization of the hormone-receptor complex following exposure to hCG, the binding sites on the cell surface were maintained for at least 4 h. The number of binding sites on the cell surface was not decreased by a protein synthesis inhibitor but was reduced to undetectable levels by monensin. This compound inhibits acidification of endocytic vesicles, which is known to be an important prerequisite to receptor cycling. It is concluded that, in the rat testis Leydig cells, following binding of hCG to the LH receptor there is rapid internalization of the complex and that recycling of the receptor occurs to the cell surface. This process may be essential in maintaining the capacity of the Leydig cell to bind fresh hormone.

Evidence for the appearance of an uncoupled form of the beta-adrenergic receptor distinct from the internalized receptor

Agonist treatment of C6-glioma cells induces two altered states in beta-adrenergic receptors, a low affinity for the hydrophilic antagonist CGP-12177 and a low affinity for agonists like isoproterenol. We present evidence that, in cells not treated to inhibit receptor internalization, the two properties occur with a different time course, the low affinity for isoproterenol preceding that for CGP-12177. In that the low affinity for CGP-12177 is due to the internalization of the receptor, the results indicate that uncoupling of the receptor, indicated by the low affinity for isoproterenol, occurs while the receptor is still located on the cell surface. Removal of the agonist leads to reappearance of the receptor to the plasma membrane followed by loss of the uncoupled state.

Beta-adrenergic [( 3H]CGP-12177) binding to brain slices and single intact pineal glands

We have characterized and quantified the binding of [3H]CGP-12177 to beta-adrenergic receptor sites in slices (300 microns) of rat cerebral cortex. The receptors are stereospecific, saturable and of high affinity. Binding of [3H]CGP is readily reversible and demonstrates appropriate drug specificity. This assay method allows the demonstration of isoproterenol-induced down-regulation (internalization) of beta-adrenoreceptors. Receptor recycling is observed at 37 degrees C in the absence of beta-agonist but can be blocked by low temperature (0 degree C) or by monensin. beta-Adrenoreceptors can also be labeled and quantified in intact, single pineal glands of rat, mouse and hamster. Rat pineals contain approximately 10 times more binding sites than do hamster or mouse pineals and up to 8 times more sites than found in rat cerebral cortex. Rat pineal [3H]CGP binding can be up- and down-regulated but not to the same degree as seen in brain slices. This assay method is simple, rapid and provides new opportunities for the study of other receptor types in intact tissue.

Agonist-induced internalization of the beta adrenergic receptor in a smooth muscle cell line

The mechanism of agonist-induced desensitization of the beta adrenergic receptor coupled adenylate cyclase has been studied in a smooth muscle cell line, BC3H-1, which expresses both alpha and beta adrenergic receptors and nicotinic receptors. beta receptors have been investigated in intact cells using as radioligand 3HCGP-12177, an hydrophilic compound which labels only surface receptors. The treatment of BC3H-1 cells with the agonist Isoproterenol, at 37 degrees but not at 4 degrees, induced a dose dependent internalization of the beta adrenergic receptor. Agonist-induced internalization was very rapid, in the order of few minutes. beta adrenergic receptor internalization was very specific: the alpha adrenergic agonist Phenylefrine had almost no effect on beta receptor levels, while Isoproterenol treatment had no effect on the number of alpha adrenergic or nicotinic receptors expressed at the cell surface of these cells. beta adrenergic receptor internalization is probably the major mechanism responsible for catecholamine desensitization in smooth muscle cells.

Rapid recovery of cardiac beta-adrenergic receptors after isoproterenol-induced "down"-regulation

Desensitization of beta-receptor-linked adenylate cyclase occurs after prolonged occupancy of the beta-receptors by their agonists. We have followed the development and recovery from "down"-regulation of beta-receptors in enzymatically dissociated cardiac myocytes by using the hydrophilic antagonist [3H]-CGP-12177 to identify surface-bound beta-receptors. After in vitro incubation with (-)-isoproterenol, almost 50% of the beta-receptors are lost within 10 minutes. Isoproterenol-mediated cyclic adenosine monophosphate accumulation by isolated myocytes was also decreased after a 15-minute preincubation with isoproterenol. "Lost" beta-receptors can, however, be recovered when isoproterenol-pretreated, washed cardiac myocytes are incubated at 37 degrees C, 85 +/- 7% of the lost beta-receptors have returned to the cell surface after 20 minutes of incubation. The requirements for such recycling were investigated. Receptor recovery does not depend on de novo protein synthesis, since it is unaffected by prior exposure to cycloheximide. It is, however, dependent on cellular energy, because it is prevented by adenosine triphosphate depletion and involves a lysosomal step since it is inhibited by the lysomotropic agent, chloroquine. In addition, the Golgi apparatus and the microtubules are involved in the beta-receptor recycling to the cell surface, as evidenced by the inhibitory effects of monensin and colchicine, respectively. The mechanism of isoproterenol-induced down-regulation of cardiac beta-receptors involves a rapid, reversible cycling to and from the cytosol and the cell membrane. This intracellular receptor traffic is energy dependent, requires several structures, including lysosomes and microtubules, and may be modified by pathological processes involving the heart.

Receptor-specific mechanisms of desensitization of beta-adrenergic receptor function

beta-Adrenergic receptor (beta AR)-specific, agonist-induced desensitization of adenylate cyclase can be shown in most mammalian cells examined to involve at least three reactions. An initial 'uncoupling' reaction leads to a 40-60% loss of catecholamine-stimulated adenylate cyclase activity at a time when no detectable loss of beta AR has occurred. This process precedes by 45-90 sec the appearance of beta AR in cytoplasmic vesicles. Such beta AR exhibit ligand binding properties consistent with their existence on the inside of membrane vesicles; thus, they appear to be formed by a process of agonist-induced beta AR internalization (endocytosis). A third process results in the loss of beta AR, at least in some cases due to receptor degradation. In general, agonist-induced desensitization or down-regulation reactions do not require protein synthesis. Recovery from the desensitized states does not require protein synthesis, whereas recovery from beta AR down-regulation (degraded receptors) requires new receptor synthesis. Agonist-induced beta AR desensitization and down-regulation reactions appear to have much in common with the process of polypeptide hormone-induced receptor down-regulation. The availability of a large number of ligands (agonists, partial agonists and antagonists) for the beta AR should allow the use of this receptor system to gain unique insights into the general processes of ligand-induced, cell surface receptor endocytosis.

Brain slices in radioligand binding assays: quantification of opiate, benzodiazepines and beta-adrenergic ([3H] CGP-12177) receptors

We have characterized and quantified specific binding of [3H]-flunitrazepam (FNZ; (benzodiazepine), [3H]-naloxone (NAL; (opiate) and [3H] CGP-12177(CGP; (beta-adrenergic) to thick slices (230-400 micron) of mouse and rat brain. The binding sites are stereospecific, saturable and of high affinity. In all cases, the binding of the ligands is readily reversible and demonstrates the appropriate drug specificity. In mouse brain [3H]-NAL binding is elevated by chronic treatment with naloxone (via capsules). We have been unsuccessful in quantifying beta adrenoreceptors with the archetypal ligand [3H]-dihydroalprenolol (DHA). However, the use of [3H]-CGP 12177 enabled us to detect high-affinity beta adrenoreceptors in brain slices. [3H]-CGP also permits the demonstration of rapid and reversible agonist-induced down-regulation (internalization) of beta binding sites. We have been successful in quantifying beta adrenergic sites in single pineal glands of rat and hamster.