Decline In Receptor Number Research Articles

Transforming growth factor beta 1 binding and receptor kinetics in fetal mouse lung fibroblasts.

TGFbeta1 inhibits fetal lung maturation in vitro. As TGFbeta1 is present in fetal lung, mechanisms must exist to overcome this inhibition and allow late gestation maturation to progress. We studied the ontogeny of TGFbeta1 binding, and TGFbeta receptor kinetics and subtypes in primary cultures of fetal mouse lung fibroblasts from Day 16 to Day 18 of gestation. TGFbeta1 specific binding in fetal lung fibroblasts declined with advancing gestation. The decrease occurred earlier, and was more pronounced in female fibroblasts (50% decrease) than in the male fibroblasts (29% decrease). Dihydrotestosterone treatment of Day 18 female fibroblasts resulted in a dose-dependent increase in TGFbeta1 binding. Scatchard analysis revealed a decline in receptor number with advancing gestation (Day 16 female Bmax: 7.3 x 10(-16); Day 18 female Bmax: 5.5 x 10[-16]) whereas binding affinities remained constant. Affinity labeling followed by chemical cross-linking and autoradiography showed the three known TGFbeta receptor subtypes at both Days 16 and 18 of gestation. The relative abundance of nonsignaling Type III receptors in comparison to signaling Type II and Type I receptors was increased at Day 18 versus Day 16. Incorporation of [3H]thymidine into DNA after treatment with TGFbeta1 changed from Day 16 to Day 18, consistent with changes previously reported between fetal and adult lung fibroblasts. We conclude that as fetal mouse lung maturation progresses, TGFbeta receptor number decreases in fibroblasts, the relative proportion of nonsignaling versus signaling receptor types increases, and the response to TGFbeta1 stimulation changes. These changes may contribute to overcoming TGFbeta1 inhibition of lung maturation.

Use-Dependent Regulation of GabaA Receptors

Prolonged occupancy of GABAA receptors by ligands, including GABA and benzodiazepine agonists, sets in motion a series of mechanisms that can be termed use-dependent regulation. These mechanisms can be subdivided into two distinct pathways, one for GABAA receptor downregulation and another for upregulation. Treatment of cortical neurons with GABA or benzodiazepines in cultures opens the pathway for GABAA receptor downregulation, which includes (in putative temporal order): (1) desensitization (tachyphylaxis), (2) sequestration (endocytosis) of subunit polypeptides and uncoupling of allosteric interactions between GABA and benzodiazepine binding sites, (3) subunit polypeptide degradation, and (4) repression of subunit gene expression. The end-point of GABAA receptor downregulation, a reduction in receptor number, is postulated to be established initially by degradation of the receptor protein and then maintained by a diminished level of de novo synthesis. Benzodiazepine treatment of many preparations, including cells expressing recombinant GABAA receptors, may elicit only desensitization, sequestration, or uncoupling, without a decline in receptor number. Components of the GABAA receptor downregulation pathway are also evoked by chronic administration of GABAmimetics, benzodiazepines, barbiturates, and neurosteroids in animals. This downregulation correlates with the establishment of tolerance to and physical dependence on the pharmacological effects of these drugs, suggesting a cellular model for this behavior. The upregulation of GABAA receptors is observed as one of the neurotrophic actions of GABA, primarily in cultured cerebellar granule cells. Upregulation in culture is caused by enhanced expression of genes for GABAA receptor subunits and correlates with the establishment of GABAergic circuitry in the developing cerebellum. Thus, both the upregulation and downregulation of GABAA receptors appear to represent use-dependent pathways for guiding synaptic plasticity in the vertebrate central nervous system.

Homologous down-regulation of gonadotropin-releasing hormone receptor sites and messenger ribonucleic acid transcripts in alpha T3-1 cells.

GnRH is known to down-regulate its pituitary receptors by mechanisms that include endocytosis of the agonist-receptor complex. To evaluate the extent to which changes in receptor synthesis contribute to this process, the effects of GnRH and its analogs on GnRH receptor number and messenger RNA (mRNA) levels were analyzed in the alpha T3-1 gonadotroph cell line. Treatment with GnRH or its potent agonist analog, des-Gly10-[D-Ala6]GnRH N-ethylamide, reduced GnRH receptor number in a time- and dose-dependent manner, with a half-maximal decrease in response to 10(-6) M GnRH or agonist analog by 75 min. The maximum decrease in receptor number (to 31% of the control value) was sustained for up to 72 h. In alpha T3-1 cells incubated with 10(-8) M GnRH or agonist analog, the GnRH receptors fell by 28% and 46% after 2 h, respectively; no change in receptors occurred after treatment with 10(-8) M GnRH antagonist ([D-pGlu1,D-Phe2,D-Trp3,6]GnRH). Time- and dose-dependent reductions in the level of receptor mRNA were also observed after treatment of alpha T3-1 cells with GnRH and the agonist analog. However, the maximal reduction in mRNA levels (to 60-70% of the control value) was consistently less than the decline in receptor number. These results indicate that the mechanism of GnRH receptor down-regulation in alpha T3-1 gonadotrophs includes reduction of receptor synthesis secondary to decreases in receptor mRNA levels. The finding that reductions in mRNA levels were relatively less than the decreases in receptor number is consistent with the involvement of additional mechanisms, including endocytosis and degradation, in down-regulation of the GnRH receptor.

Muscarinic Receptor Characteristics and Regulation in Rat Cerebral Cortex: Changes during Development, Aging and the Oestrous Cycle

The effects of postnatal development, aging and the oestrous cycle on muscarinic acetylcholine receptor (mAChR) properties were examined in in vitro living slices of rat neocortex. Using the hydrophilic antagonist ([3H]NMS) to label cell surface mAChRs, an increase in both Bmax and Kd was found during the first postnatal weeks. These values peaked at between 20-40 days postnatally and then declined to adult levels. After 3 months of age, a steady decline in receptor number started: it was 10.1% lower at 10 months and 38.7% lower at 17 months of age. In contrast, Kd values increased, being 31.7 and 20% higher respectively at these ages. Carbachol-induced (4 h at 37 degrees C) down-regulation of receptor number was approximately 22.2% in newborn and 26.1% in adult (3-month-old) rats, but only 16.3% at 20-40 days of age. The degree of carbachol-induced down-regulation of mAChR was not affected in the older animals. Veratridine, which increases neural activity, also induced a significant reduction in [3H]NMS binding sites of 11.4% in rats aged 0-20 days and 22.4% in 3-month-old rats, but at 20-40 and 40-60 days of age no significant down-regulation of receptor number was observed. Furthermore, down-regulation was absent in the 10-month-old rats as well. Since a great variation in Bmax and Kd values was seen in 3-month-old females but not in male rats, we investigated mAChR characteristics during the oestrous cycle of female rats. In pro-oestrus, mACh receptor number was increased and affinity decreased in comparison with di-oestrus.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging: changes in cardiac α1-adrenoceptor responsiveness and

Several investigators have reported a diminished responsiveness of senescent cardiac muscle to norepinephrine and β-adrenoceptor agonists. In contrast, relatively little is known regarding the effects of aging on myocardial actions mediated specifically by α-adrenoceptor stimulation. Thus, the current study examined aging-dependent changes in: (a) the inotropic response to methoxamine, and α-adrenoceptor agonist; (b) characteristics of myocardial α 1-adrenoceptors as monitored by specific [ 3H]prazosin binding; and (c) steady state levels of α 1-adrenoceptor mRNA as determined by Northern blot analysis. Cardiac preparations were isolated from 4-, 14-, and 25-month-old F344 rats. An aging-associated decline was observed in the maximum positive inotropic effect elicited by methoxamine in right ventricular strips (160 ± 23, 134 ± 13 and 79 ± 26% increase above control developed tension in 4, 14 and 25 months, respectively) with no change in ED 50 values. [ 3H]Prazosin binding to ventricular sarcolemmal membranes revealed a reduction in receptor number (82 ± 7, 69 ± 6 and 59 ± 5 fmol/mg protein in 4, 14 and 25 months, respectively); the apparent dissociation constant was not affected. Steady state levels of α 1-adrenoceptor mRNA decreased progressively between 4 and 25 months of age (14- and 25-month levels were approximately 71 and 38% of 4 months, respectively), while steady state levels of β-actin mRNA did not change with age. These results suggest that the aging-related decline in α 1-adrenergic responsiveness in rat ventricular muscle is mediated, at least in part, by a decrease in cardiac α 1-adrenoceptor density. In turn, the decline in receptor number may be a direct result of diminished levels of α 1-adrenoceptor gene transcripts in aging myocardium.

Effects of Tunicamycin on Growth Hormone Binding in Rat Adipocytes*

Digestion of covalently linked [125I]human (h) GH-receptor complexes with neuraminidase or endoglycosidase F reduced the mass of the principal hormone receptor complex from about 130 kilodaltons (kDa) to 120 and 110 kDa, respectively, suggesting that about 20% of the mass of the GH receptor of rat adipocytes consists of N-linked sialocarbohydrates. Incubation of adipocytes with tunicamycin, an inhibitor of N-linked glycosylation, decreased the incorporation of [35S]methionine into membrane glycoproteins by more than 50% in 4 h and decreased specific binding of [125I]hGH by about 70% after 8 h. Decreased binding and incorporation of [35S]methionine were seen only after a lag time of about 2 h. Cross-linking of [125I] hGH to cells that had been treated with tunicamycin resulted in the appearance of a new labeled species of hormone-receptor complex with an apparent mass of about 110 kDa. This band appeared after a delay of about 3 h and reached approximately equal prominence with the 130 kDa band at 5 h. By 8 h, the 110 kDa complex was the predominant band in radioautograms, but some of the 130 kDa species remained. Scatchard analysis of binding data in tunicamycin-treated adipocytes indicated that decreased binding of [125I]hGH resulted from a 3- to 4-fold decrease in affinity accompanied by only a small (30%) decline in receptor number. Tunicamycin did not affect the rate of receptor turnover in cells that were also treated with cycloheximide to block protein synthesis, but receptor turnover decelerated with increasing time of incubation. Treatment with tunicamycin for 8 h markedly slowed the rate at which specifically bound [125I]hGH disappeared from adipocytes, suggesting that N-linked carbohydrates may play some role in internalization and processing of labeled hormone. We conclude that 1) N-linked carbohydrates contribute about 20 kDa to the apparent mass of the GH receptor of rat adipocytes; 2) N-linked glycosylation is not required for GH receptors to be inserted into the adipocyte membrane in the proper orientation and to retain their ability to recognize and bind GH; 3) N-linked sugar chains are required for maintenance of a normal high affinity of receptors for GH; 4) N-linked carbohydrates are necessary for normal rates of internalization and processing of bound hGH.

Demonstration and partial characterization of the interferon‐gamma receptor on human B lymphocytes

The expression of interferon-gamma (IFN-gamma) receptors on normal human B cells and four B cell lines was studied. Recombinant human IFN-gamma was labeled with [gamma-32P]ATP using the catalytic subunit of a cAMP-dependent protein kinase. All four B cell lines, although differing in their responsiveness to IFN-gamma, were found to express high-affinity receptors (1,000-11,000 receptors/cell). Normal unactivated B lymphocytes were also found to express constitutively high-affinity receptors, approximately 1,400 receptors per cell with an estimated affinity of 295 pM. Activation of the normal B cells in vitro with the polyclonal B cell activator, Staphylococcus aureus Cowan strain I (SAC), resulted in a slight decline in receptor number and a more pronounced fall in receptor density. One of the B cell lines and unactivated normal B cells were shown to internalize labeled IFN-gamma rapidly. Chemical cross-linking of 32P-IFN-gamma to the CB B cell line and to freshly isolated B lymphocytes revealed one major cross-linked receptor-ligand complex which had an estimated molecular weight of approximately 110 kilodaltons. This complex corresponded to a 93 kD receptor cross-linked to recombinant IFN-gamma. Our data indicate that normal B lymphocytes constitutively express an approximately 93 kD IFN-gamma receptor which is similar to the receptor present on Epstein-Barr virus-transformed B cell lines.

Down-regulation of the glucocorticoid receptor by dexamethasone in cultured human skin fibroblasts: implications for the regulation of aromatase activity.

Human genital skin fibroblasts grown in cell culture contain aromatase activity and glucocorticoid receptors, providing a model for studying the relationship between changes in levels of dexamethasone (DEX) receptor binding and changes in the response of aromatase to DEX. Incubation of cells with media containing DEX produced a time-dependent decline in receptor number, with a nadir at 18-24 h. When DEX was removed from the media, only 40-60% of the lost receptor content was recovered. Binding of DEX to its receptor declined in a dose-dependent fashion after 24-h exposure to this glucocorticoid; the concentration of DEX that produced a half-maximal decline (2.2 nmol/L) was in the same range as the Kd for the receptor-DEX complex (14 nmol/L). Incubation of cells with DEX for 20 h also reduced the level of DEX receptor binding in purified nuclei and nuclear matrix by 76% and 89%, respectively. When subcellular fractions were prepared after incubation of cells with DEX for 1-24 h, whole cell, cytosolic and nuclear DEX receptor binding declined in parallel with time. Incubation of skin fibroblasts with DEX resulted in progressive stimulation of aromatase activity, with a peak at 24 h followed by a return to baseline at 72 h. The initial stimulation of aromatase was mediated by DEX receptors. However, the decline in aromatase activity after prolonged DEX exposure did not appear to be due to a decline in the level of DEX receptor binding. The data supporting this last conclusion included the following. When cells were washed free of DEX after 48 h, DEX receptor binding recovered within 24 h, whereas aromatase activity could not be maximally restimulated until 36 h; when cells were incubated with media containing DEX and cycloheximide for 1-48 h, DEX receptor binding declined to a nadir by 24 h, whereas aromatase activity rose continuously up to 48 h. These findings are consistent with the concept that the aromatase gene in skin fibroblasts is subject to both positive and negative regulation.

N-ethylmaleimide blocks the modulatory effects of divalent cations and guanine nucleotides on the brain substance P receptor

The binding of [ 3H]physalaemin ([ 3H]PHY) to rat brain substance P receptors is modulated by cations and guanine nucleotides. [ 3H]PHY binding in the presence of either monovalent or divalent cations (125 mM Na 2SO 4 or 2.5 mM MnCl 2) shows a K D of 5.9 and 5.5 nM and a B max of 44.4 and 63.9 fmol/mg protein respectively. In the presence of both, there is a 2-fold increase in the affinity (K D 2.8 nM) and a 25–80% increase in the B max (81.6 fmol/mg protein). Addition of 100 μM GTP or Gpp(NH)p in either 125 mM Na 2SO 4 or 2.5 mM MnCl 2 or both decreases the B max by 25–55%. However, the receptor affinity for [ 3H]PHY is not significantly altered by guanine nucleotides. N-Ethylmaleimide (NEM) irreversibly inhibits the receptor binding with an IC 50 of 1.0 mM, demosntrating that SH groups play a critical role in the interaction of the ligand with the receptor. If the SP receptors are protected with 1 μM PHY, NEM irreversibly inhibits the effect of divalent cations and guanine nucleotides. Analysis of [ 3H]PHY binding in 125 mM Na 2SO 4, 2.5 mM MnCl 2 on membranes that were protected with 1 μM PHY and then preincubated with NEM demonstrates a variable decline in receptor number and a 2-fold decrease in the affinity (K D, from 2.8 to 6.9 nM). These observations indicate the existence of a second class of SH groups that are essential for the interaction of divalent cations and guanine nucleotides with the receptor. The blockade of the modulatory effects of divalent cations and guanine nucleotides by NEM treatment further suggests that brain SP receptors are coupled to a guanine nucleotide binding regulatory protein.

Variations in age-related decline in striatal D 2-dopamine receptors in a variety of mouse strains

To determine the importance of inheritance on the age-associated decline in D 2-dopamine receptor number, the binding [ 3H] spiperone to mouse striatal membranes was measured in animals ranging from 7 to 104 weeks of age from 5 murine strains (C57BL/6J, C3/HeJ, A/J, SJL/J and DBA/1J). In young mice, receptor number (B max) was influenced by genetic background such that C57BL/6J < SJL/J < A/J = DBA/1J = C3H/HeJ. A 50–60% decline in B max with age was found in all strains except for C57BL/6J. Bmax in the C57BL/6J mice were lower than in the other strains of young animals (7–15 weeks) but remained relatively constant throughout life (measured up to 104 weeks of age). Furthermore, the maximal decline in receptor number was observed relatively early in life (16–30 weeks) and remained constant thereafter. Neither age nor genetic background influenced ligand affinity ( K d. Thus the results of this study suggest that the maximal decline in B max for the dopamine receptor occurs before the second half of life and that the magnitude of this decline is polymorphic.

Stimulatory and inhibitory effects of protein kinase C activation and calcium ionophore on cultured pig Leydig cells.

The acute and the long-term (24 h) effects of protein kinase C activators, phorbol 12 myristate 13-acetate (PMA) and 1-oleoyl-2-acetyl-sn-glycerol, and the calcium ionophore A23187 on cultured pig Leydig cell functions were investigated. None of these drugs modified basal cAMP production, but they induced a small (3-4-fold) increase in testosterone secretion. The stimulatory effects of human choriogonadotropin (hCG; 1 nM) on both cAMP and testosterone productions were inhibited by short-term incubation with these drugs. In addition, they suppressed the stimulation of testosterone output by forskolin and 8-bromo-adenosine 3',5'-monophosphate, whereas the forskolin-dependent cAMP production was unaffected. The inhibitory effects of PMA on hCG stimulation of both cAMP and testosterone were due mainly to a decrease of the Vmax without modification of the ED50. Moreover, PMA did not modify the binding of 125I-hCG. Pretreatment of Leydig cells with the three drugs for 24 h induced more pronounced modifications, such as a reduction in the number of hCG binding sites and a decreased responsiveness to hCG and forskolin, the testosterone production being drastically reduced. The effects of PMA were dose- and time-dependent; however, the concentration of PMA required to induce half-maximal effects on hCG receptors (10 nM) was about one order of magnitude higher than those required to reduce cAMP and testosterone productions. Further, the inhibitory effects on cAMP and testosterone secretions appeared within the first 3 h, whereas the hCG receptor number remained constant for at least 8 h. It appears therefore, that the main alteration responsible for the steroidogenic refractoriness of PMA-treated Leydig cells is located beyond cAMP formation. Moreover, since conversion of exogenous pregnenolone to testosterone by control and PMA-treated cells was similar, the alteration was probably located before pregnenolone formation. Kinetic studies with 125I-hCG showed that the rate of internalization of the hormone-receptor complexes was similar in control cells and in PMA-treated cells, suggesting that the decline in receptor number observed in the latter group after an 8-h delay is not due to an increased rate of internalization nor to sequestration of the internalized receptors inside the cells. Since cycloheximide blocked the effects of PMA on hCG down-regulation, it is likely that the phorbol esters and 1-oleoyl-2-acetyl-sn-glycerol induce the synthesis of some proteins which blocked the recycling of internalized receptors. A similar hypothesis has been put forward recently to explain the hCG-induced down regulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Activators of protein kinase C and 5-azacytidine induce IL-2 receptor expression on human T lymphocytes.

Resting human T lymphocytes do not express receptors for interleukin-2, but expression is rapidly induced by exposure to PHA. After maximal expression 2-3 days after stimulation, a progressive decline in receptor number is observed. Receptor expression can be augmented by reexposure to PHA. In this study we show that activators of protein kinase C including phorbol diester, phospholipase C, and the diacylglycerol congener diC8 also increase IL-2 receptor expression. Moreover, 5-azacytidine, which inhibits cytosine methyltransferase, and hydroxyurea, which inhibits ribonucleotide reductase, also increased receptor number. These studies demonstrate that IL-2 receptor expression can be altered in vitro, and that IL-2 receptor number, in combination with IL-2 secretion, may contribute to the regulation of IL-2-dependent immune responses.

Regulation of interleukin 2 receptor expression: effects of phorbol diester, phospholipase C, and reexposure to lectin or antigen.

Anti-Tac monoclonal antibody identifies the receptor for interleukin 2 (IL 2, or T cell growth factor) present on activated human T lymphocytes. By using tritiated anti-Tac, we now report a sensitive and specific binding assay to evaluate cell surface IL 2 receptor expression. IL 2 receptors on human peripheral blood lymphocytes can be detected within 6 hr after PHA stimulation. PHA-induced receptor expression is inhibited by actinomycin D and cycloheximide, but not by mitomycin C, suggesting a requirement for de novo RNA and protein synthesis, but not DNA synthesis. Scatchard analysis of [3H]-anti-Tac binding to lymphocytes stimulated with PHA for 3 days revealed from 20,000 to 60,000 molecules of antibody bound per cell, and a Kd of 1 to 3 x 10(-10) mol/l. Sequential binding studies of activated human lymphocytes maintained in long-term culture with IL 2 demonstrated a progressive decline in receptor number correlating with diminished growth rate. Restimulation with lectin or antigen increased the number of IL 2 receptors, suggesting that IL 2 dependent immune responses may be regulated, at least in part, by IL 2 receptor expression. Receptor number was also increased by PMA. Moreover, similar effects were produced by incubation with phospholipase C but not interleukin 1. Because both PMA and phospholipase C result in activation of protein kinase C, these data suggest the possibility that activation of protein kinase C may induce IL 2 receptor expression.

In vitro desensitization of beta adrenergic receptors in human neutrophils. Attenuation by corticosteroids.

The receptor alterations involved in catecholamine-induced desensitization of adenylate cyclase in human neutrophils have been investigated as has the ability of hydrocortisone to modify such alterations. Incubation of human neutrophils with isoproterenol for 3 h in vitro resulted in an 86% reduction in the ability of isoproterenol to stimulate cyclic AMP accumulation in the cells. Two types of receptor alterations were documented. There was a 40% reduction in the number of beta adrenergic receptors (42 vs. 25 fmol/mg protein, P < 0.005) present after desensitization as assessed by [(3)H]dihydroalprenolol ([(3)H]DHA) binding. In addition the receptors appeared to be relatively uncoupled from adenylate cyclase. This uncoupling was assessed by examining the ability of the agonist isoproterenol to stabilize a high-affinity form of the receptor, detected by computer modelling of competition curves for [(3)H]DHA binding. Desensitized receptors were characterized by rightward-shifted agonist competition curves. When hydrocortisone was added to the desensitizing incubations (combined treatment) there was a statistically significant attenuation in the desensitization process as assessed by the ability of isoproterenol to increase cyclic AMP levels in the cells. Although combined treatment did not prevent the decline in receptor number, it did attenuate the uncoupling of the receptors. Combined treatment resulted in competition curves intermediate between the control and the rightward-shifted desensitization curves. Prednisolone was similar to hydrocortisone in attenuating isoproterenol-induced uncoupling. Thus, steroids appeared to attenuate agonist-induced desensitization of the beta adrenergic receptor-adenylate cyclase system by dampening the ability of agonists to uncouple receptors without modifying their ability to promote down-regulation of beta adrenergic receptors.

Temperature shifts induce the selective loss of alveolar-macrophage plasma membrane components.

A shift in the incubation temperature of rabbit alveolar macrophages (0 degree C leads to 37 degrees C leads to 0 degree C) resulted in a 40-60% reduction in the ability of cells to bind alphamacroglobulin. 125I-trypsin complexes (alphaM. 125I-T). The reduction in binding activity did not reflect a disruption of cell integrity since the levels of intracellular components (lactate dehydrogenase, beta-N-acetyl-hexosaminidase) or other plasma membrane components (alkaline phosphodiesterase) were unaltered. Analysis of receptor-ligand interaction indicated that the temperature shift effected a decline in receptor number rather than an alteration in ligand-receptor affinity. Studies indicated that a temperature shift resulted in the loss of unoccupied receptors, and that ligand bound to receptors was not lost. However, after ligand internalization, receptors were removed by the temperature shift. The rate of receptor loss was maximal when cells were incubated at temperatures greater than 24 degrees C. Receptor loss was not prevented by treatment of cells with colchicine, cytochalasin B, or N-ethylamaleimide, but was prevented by treatment with the cross-linking agent paraformaldehyde. Data indicate that the reduction in alphaM. 125I-T binding activity resulted from shedding of receptors into the media since media obtained from temperature-shifted cells contained material that competed with cell-bound receptors for alphaM. 125I-T. Additionally, binding of alphaM. 125I-T was diminished on membrane fragments obtained from temperature-shifted cells. Incubation with Triton X-100, of cells whose receptors were occupied with alphaM. 125I-T, led to the extraction of 40% of cell-bound activity. However, no radioactivity was extracted from cells labeled with alphaM. 125I-T after a temperature shift. Measurement of ligand accumulation by control and temperature-shifted cells incubated at 20 degrees C indicated that control cells exhibited a subpopulation of receptors capable of binding ligand but only slowly internalizing it. This subpopulation was not present on temperature-shifted cells. These results indicate that surface receptors for alphamacroglobulin . protease complexes are heterogeneous and that the temperature shift resulted in the selective loss of membrane components.

Glucocorticoids down-regulate the number of 1,25-dihydroxyvitamin D 3 receptors in mouse intestine

This study examined the hypothesis that glucocorticoids might regulate intestinal receptors for 1,25(OH) 2vitamin D 3. Mice were treated with saline vehicle or dexamethasone in doses of 50–200 μg/100 gm body weight for 7 days. A dose-dependent fall in the number of receptors was found in dexamethasone treated animals (∼30%); there was no change in the receptor affinity for 1,25(OH) 2D 3 (0.2 nM). The decline in receptor number required 5–7 days of dexamethasone treatment depending on dose. Adrenalectomy led to a 30% rise in receptor number which could be prevented by the 50 μg dose of dexamethasone. Receptor changes of equal magnitude were exhibited in vitamin D replete or deplete mice.