Receptor-mediated internalization of angiotensin II in bovine adrenal medullary chromaffin cells in primary culture

Targeted Delivery of Antisense Oligonucleotides Through Angiotensin Type 1 Receptor

We evaluated the potential of AGTR1, the principal receptor for angiotensin II (Ang II) and a member of the G protein-coupled receptor family, for targeted delivery of antisense oligonucleotides (ASOs) in cells and tissues with abundant AGTR1 expression. Ang II peptide ASO conjugates maintained robust AGTR1 signaling and receptor internalization when ASO was placed at the N-terminus of the peptide, but not at C-terminus. Conjugation of Ang II peptide improved ASO potency up to 12- to 17-fold in AGTR1-expressing cells. Additionally, evaluation of Ang II conjugates in cells lacking AGTR1 revealed no enhancement of ASO potency. Ang II peptide conjugation improves potency of ASO in mouse heart, adrenal, and adipose tissues. The data presented in this report add to a growing list of approaches for improving ASO potency in extrahepatic tissues.

Effect of angiotensin II on voltage-gated sodium currents in aortic baroreceptor neurons and arterial baroreflex sensitivity in heart failure rats

BACKGROUND

Impairment of arterial baroreflex sensitivity is associated with mortality in patients with chronic heart failure (CHF). Elevation of plasma angiotension II (Ang II) contributes to arterial baroreflex dysfunction in CHF. A reduced number of voltage-gated sodium (Nav) channels in aortic baroreceptor neurons are involved in CHF-blunted arterial baroreflex.

METHOD

In this study, we investigated acute effect of Ang II on Nav currents in the aortic baroreceptor neuron and on arterial baroreflex in sham and coronary artery ligation-induced CHF rats.

RESULTS

Using Ang II I radioimmunoassay, real-time reverse transcription-PCR and western blot, we found that Ang II levels, and mRNA and protein expression of angiotension II type 1 receptor in nodose ganglia from CHF rats were higher than that from sham rats. Local microinjection of Ang II (0.2  nmol) into the nodose ganglia decreased the arterial baroreflex sensitivity in sham rats, whereas losartan (1  nmol, an angiotension II type 1 receptor antagonist) improved the arterial baroreflex sensitivity in CHF rats. Data from patch-clamp recording showed that Ang II (100  nmol/l) acutely inhibited Nav currents in the aortic baroreceptor neurons from sham and CHF rats. In particular, inhibitory effect of Ang II on Nav currents in the aortic baroreceptor neurons was larger in CHF rats than that in sham rats. Losartan (1  μmol/l) totally abolished the inhibitory effect of Ang II on Nav currents in sham and CHF aortic baroreceptor neurons.

CONCLUSION

These results suggest that elevation of endogenous Ang II in the nodose ganglia contributes to impairment of the arterial baroreflex function in CHF rats through inhibiting Nav channels.

Participation of D 1-4 dopamine receptors in the pro-cognitive effects of angiotensin IV and des-Phe 6 angiotensin IV

Angiotensin IV (Ang IV) and des-Phe(6)Ang IV are naturally occurring neuroactive peptides of the renin-angiotensin system (RAS) involved in memory processing. However, the relevant mechanisms are poorly understood. In this review it is proposed that the pro-cognitive effects of these peptides are, at least partly, mediated by dopamine (DA). Recent studies demonstrated that the improvement of several memory aspects; recall of appetitively and aversively motivated behaviors and learning of spatial tasks by Ang IV and des-Phe(6)Ang IV was abolished, or significantly diminished by behaviorally inactive per se doses of the D(1) and D(2) receptor blockers SCH 23390 (R-[+]-7-chloro-8-hydroxy-3 methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) and remoxipride, respectively. The D(3) receptor inhibition with nafadotride was almost ineffective but again, the D(4) receptor blockade by L745,870 hydrochloride (3-{[4-(4-chlorophenyl)piperazin-1-yl]methyl}-1H-pyrrolo[2,3-b]pyridine hydrochloride) diminished all, except for spatial memory, improving actions of the peptides. These results suggest that Ang IV and des-Phe(6)Ang IV enhance memory in a brain region-specific manner, dependent on local DA receptor subpopulations and the memory aspects controlled by them. The data reviewed here, demonstrating DA-Ang IV and des-Phe(6)Ang IV interactions in brain, strongly suggest probability of clinically relevant effects of concomitant use of antipsychotic and RAS affecting drugs.

In vivo regulation of AT1a receptor-mediated intracellular uptake of [125I]Val5-ANG II in the kidneys and adrenals of AT1a receptor-deficient mice

Using type 1a angiotensin receptor (AT1a) receptor-deficient (Agtr1a-/-) mice and in vivo autoradiography, we tested the hypothesis that intracellular uptake of ANG II in the kidney and adrenal glands is primarily mediated by AT1a receptors and that the response is regulated by prevailing endogenous ANG II. After pretreatment of wild-type (Agtr1a+/+) and Agtr1a-/- mice (n = 6-9 each group) with or without captopril (25 mg.kg(-1).day(-1)) or losartan (10 mg.kg(-1).day(-1)) for 2 wk, [125I]Val5-ANG II was infused for 60 min. Intracellular uptake of [125I]Val5-ANG II was determined by quantitative in vivo autoradiography after washout of circulating [125I]Val5-ANG II. Basal intracellular ANG II levels were 65% lower in the kidney (P < 0.001), but plasma ANG II levels were threefold higher, in Agtr1a-/- than wild-type mice (P < 0.01). Although plasma [125I]Val5-ANG II levels were similar, urinary excretion of [125I]Val5-ANG II was fourfold higher in Agtr1a-/- mice (P < 0.001). By contrast, intracellular [125I]Val5-ANG II levels were approximately 80% lower in the kidney and adrenal glands of Agtr1a-/- mice (P < 0.01). Captopril decreased endogenous plasma and renal ANG II levels (P < 0.01) but increased intracellular uptake of [125I]Val5-ANG II in the kidney and adrenal glands of wild-type and Agtr1a-/- mice (P < 0.01). Losartan largely blocked renal and adrenal uptake of [125I]Val5-ANG II in wild-type and Agtr1a-/- mice. Thus 80% of intracellular ANG II uptake in the kidney and adrenal glands is mediated by AT1a receptors, whereas AT1b receptor- and other non-receptor-mediated mechanisms account for 20% of the response. Our results suggest that AT1a receptor-mediated uptake of extracellular ANG II may play a physiological role in the kidney and adrenal glands.

Nano-LC-MS/MS for the monitoring of angiotensin IV in rat brain microdialysates: Limitations and possibilities

To broaden our knowledge about the central role of the angiotensin IV (Ang IV) peptide, we aimed to monitor its extracellular concentration in the brain using in vivo microdialysis. Ang IV was measured in the dialysates using a previously developed nano-LC-MS/MS assay with an LOD of 50 pM. Using this assay, baseline levels of Ang IV in dialysates from different brain structures were undetectable. However, immediately after microdialysis probe insertion, Ang IV could be detected in a concentration that varied between 120 and 187 pM. Using the zero-net-flux method, the extracellular levels of Ang IV in the striatum were estimated at 46 pM. These data may indicate that Ang IV is mainly present intracellularly. In addition, Ang IV was clearly measurable after striatal perfusion of Ang II. On the other hand, our nano-LC-MS/MS method was successful for the detection of Met-enkephalin and neurotensin in dialysates from the rat. In conclusion, the nano-LC-MS/MS method coupled with microdialysis is well suited to monitor the biologically significant conversion between Ang II and Ang IV in vivo, but physiological extracellular levels of Ang IV appear too low to be detected.

Cognitive effects attributed to angiotensin II may result from its conversion to angiotensin IV

This study tests the hypothesis that the facilitation of learning and improvement of memory observed after an intracerebroventricular (i.c.v.) injection of angiotensin II (Ang II) is, in fact, caused by its derivative angiotensin IV (Ang IV). We ran two memory tests as well as an auxiliary test assessing motor performance in rats injected (i.c.v., 1 nmol in 2 microl saline) with Ang II or Ang IV. There were separate groups receiving peptide or saline five, 10 and 15 minutes before testing. Ang IV significantly increased step-through latencies in a passive avoidance paradigm as well as improved discrimination between familiar and unfamiliar objects in an object recognition test in all groups showing better retrieval of memory of aversive as well as appetitive stimuli in the peptide-treated groups regardless of the time of its injection. In contrast, rats treated with Ang II demonstrated significant improvement of memory of aversive and appetitive stimuli in the same tests only 15 minutes after its i.c.v. injection, with no effect in the groups injected five minutes before testing and slight efficacy in those injected 10 minutes before the test. Numbers of crossings, rearings and bar approaches in an open field were similar both in the peptide-treated and control groups making it unlikely that changes in motor performance affected the memory tests. In line with the present views on the intracellular metabolism of Ang II, these results suggest degradation to Ang IV by aminopeptidases A and N is necessary before the cognitive effects can occur.

D2 dopamine receptor blockade prevents cognitive effects of Ang IV and des-Phe6 Ang IV

Angiotensins, especially angiotensin IV (Ang IV), have recently been found to be potent cognitive enhancers in rodents. However, the precise mechanisms of their memory improving effects remain unknown. In this study we tested the hypothesis that D2 dopamine receptors at least partially mediate cognitive effects of Ang IV and its derivative des-Phe6 Ang IV. Namely, the well known cognitive effects of both peptides [facilitation of a conditioned avoidance responses (CARs) acquisition, increase of a passive avoidance behavior (PAB) retrieval, and improvement of object recognition] were evaluated in rats either pretreated or not with a selective D2 dopamine receptor antagonist remoxipride {(S)-(-)-3-Bromo-N-[(1-ethyl-2-pyrrolidinylOmethyl]2,6-dimethoxybenzamide hydrochloride}. To control for the unspecific motor and emotional effects of our treatments that could confound results of the memory tests we used respectively, 'open' field and elevated 'plus' maze tests. Ang IV as well as des-Phe6 Ang IV remarkably improved learning of CARs, recall of PAB and recognition of the previously seen objects. D2 receptors blockade by remoxipride abolished all these effects of both peptides. In the elevated 'plus' maze remoxipride abolished anxiogenic effects of both Ang IV and des-Phe6 Ang IV. Also, the drug followed by Ang IV decreased number of crossings and by des-Phe6 Ang IV number of crossings and rearings. The results point to importance of the functional D2 dopamine receptors in cognitive effects of Ang IV and its naturally occurring product devoid of C-terminal Phe6.

Effects of high salt intake on brain AT1 receptor densities in Dahl rats

To assess effects of dietary salt on brain AT1 receptor densities, 4-wk-old Dahl salt-sensitive (Dahl S) and salt-resistant (Dahl R) rats were fed a regular (101 mumol Na/g) or high (1,370 mumol Na/g)-salt diet for 1, 2, or 4 wk. AT1 receptors were assessed by quantitative in vitro autoradiography. AT1 receptor densities did not differ significantly between strains on the regular salt diet. The high-salt diet for 1 or 2 wk increased AT1 receptor binding by 21-64% in the Dahl S rats in the subfornical organ, median preoptic nucleus, paraventricular nucleus, and suprachiasmatic nucleus. No changes were noted in the Dahl R rats. After 4 wk on a high-salt diet, increases in AT1 receptor binding persisted in Dahl S rats but were now also noted in the paraventricular nucleus, median preoptic nucleus, and suprachiasmatic nucleus of Dahl R rats. At 4 wk on the diet, intracerebroventricular captopril caused clear decreases in blood pressure only in the Dahl S on the high-salt diet but caused largely similar relative increases in brain AT1 receptor densities in Dahl S and R on the high-salt diet versus regular salt diet. These data demonstrate that high salt intake rapidly (within 1 wk) increases AT1 receptor densities in specific brain nuclei in Dahl S and later (by 4 wk) also in Dahl R rats. Because the brain renin-angiotensin system only contributes to salt-induced hypertension in Dahl S rats, further studies are needed to determine which of the salt-induced increases in brain AT1 receptor densities contribute to the hypertension and which to other aspects of body homeostasis.

Presence of cellular renin-angiotensin system in chromaffin cells of bovine adrenal medulla

The presence of a local renin-angiotensin system has been established in organs that serve as angiotensin targets. In this study, the expression of angiotensinogen mRNA and subcellular localization of renin, angiotensin-converting enzyme, and angiotensin II were investigated in bovine adrenal medullary cells in primary culture. By light microscopy, expression of angiotensinogen mRNA, immunoreactive renin, angiotensin-converting enzyme, and angiotensin II were readily detectable only in the chromaffin cells. The density distribution of renin and angiotensin II in sucrose gradients suggested a concentration in chromaffin granules, a localization directly confirmed by immunoelectron microscopy. Reverse transcriptase-polymerase chain reaction and sequencing confirmed the expression of angiotensinogen in bovine chromaffin cells and the adrenal medulla. In addition, in vitro autoradiography indicated that both angiotensin-converting enzyme and angiotensin type 1 receptors were present in the adrenal medulla. These results provide the first direct evidence that chromaffin cells in the adrenal medulla are not only the target for angiotensin but should also be considered as potential local angiotensin-generating and -storing cells.

Ang II accumulation in rat renal endosomes during Ang II-induced hypertension: role of AT(1) receptor

Hypertension induced by long-term infusion of angiotensin II (Ang II) is associated with augmented intrarenal Ang II levels to a greater extent than can be explained on the basis of the circulating Ang II levels. Although part of this augmentation is due to AT(1) receptor-dependent internalization, the intracellular compartments involved in this Ang II accumulation remain unknown. In the present study, we sought to determine whether Ang II trafficking into renal cortical endosomes is increased during Ang II hypertension, and if so, whether the AT(1) receptor antagonist, candesartan, prevents this accumulation. Compared with controls (n=12; 114+/-2 mm Hg), Ang II-infused rats (n=12; 80 ng/kg/min, SC, for 13 days) developed hypertension with systolic blood pressure rising to 185+/-4 mm Hg by Day 12. In Ang II hypertensive rats, plasma renin activity was suppressed, whereas plasma and kidney Ang II levels were increased by 3-fold (348+/-58 versus 119+/-16 fmol/mL) and 2-fold (399+/-39 versus 186+/-26 fmol/g). Intracellular endosomal Ang II levels were increased by more than 10-fold (1100+/-283 versus 71+/-12 fmol/mg protein), whereas intermicrovillar cleft Ang II levels were increased by more than 2-fold (88+/-22 versus 37+/-7 fmol/mg protein). Flow cytometric analysis detected significant increases in AT(1A) receptor antibody binding in endosomal and intermicrovillar clefts of Ang II-infused rats. The hypertension induced by Ang II was prevented in rats treated concurrently with candesartan (2 mg/kg/d, 119+/-3 mm Hg). Candesartan treatment (n=8) also prevented increases in kidney (215+/-19 fmol/g), endosomal (96+/-29 fmol/mg protein), and intermicrovillar cleft Ang II levels (11+/-2 fmol/mg protein). These results indicate that there is substantial intracellular accumulation of angiotensin peptides in renal cortical endosomes during Ang II-dependent hypertension via an AT(1) receptor-mediated process.

Angiotensin IV enhances LTP in rat dentate gyrus in vivo

Angiotensins have been shown to play a significant role in a variety of physiological functions including learning and memory processes. Relatively recent evidence supports the increasing importance of angiotensin IV (Ang IV), in many of these functions previously associated only with Ang II, including learning and memory. An interesting hypothesis generated by these results has been that Ang II is a precursor for the production of a more active peptide fragment, Ang IV. Since Ang II impairs learning and memory, when administered directly or released into the hippocampal dentate gyrus, and inhibits long term potentiation (LTP) in medial perforant path-dentate granule cell synapses, as well; it remained to be seen what effects Ang IV had on LTP in these same synapses. Results of this study show clearly that Ang IV significantly enhances LTP, and the enhancement is both dose and time dependent. The following solutions of Ang IV were administered over a five min period, at the end of baseline and before the first tetanus was applied: 2.39, 4.78, and 9.56 nM. An inverted U-type dose related effect was observed. A complex time related effect was observed with a maximum at 5 min, a return to normal LTP at 30 min and a minimum below normal at 90 min, and a return to normal LTP at 120 min. The effects of the 4.78 nM solution were determined at the following intervals between administration and the first tetanus: 5, 15, 30, 60, 90, and 120 min. The enhancement of LTP can be prevented by pretreatment with Divalinal, an Ang IV antagonist, without any effect on normal LTP. Two solutions of Divalinal were used; 5 nM and 5 microM, and the 5 microM was more effective and completely blocked the enhancement of normal LTP. Results were also obtained with 4.78 nM Nle1-Ang IV (Norleucine), an Ang IV agonist. Norleucine was less effective than Ang IV in the enhancement of normal LTP and displayed a similar time course of activity. Both Ang IV and Norleucine produced a significant suppression of normal LTP at 90 min; that remains to be explained. However, the inhibition by Ang IV was dose dependent and was blocked by Divalinal. The fact that the Ang IV enhancement of normal LTP was blocked by losartan, an Ang II AT1 receptor antagonist, is puzzling since Divalinal had no effect on the inhibition of LTP by Ang II.

Bidirectional modulation of exocytosis by angiotensin II involves multiple G-protein-regulated transduction pathways in chromaffin cells

Angiotensin II (AngII) receptors couple to a multitude of different types of G-proteins resulting in activation of numerous signaling pathways. In this study we examined the consequences of this promiscuous G-protein coupling on secretion. Chromaffin cells were voltage-clamped at -80 mV in perforated-patch configuration, and Ca(2+)-dependent exocytosis was evoked with brief voltage steps to +20 mV. Vesicle fusion was monitored by changes in membrane capacitance (DeltaC(m)), and released catecholamine was detected with single-cell amperometry. Ca(2+) signaling was studied by recording voltage-dependent Ca(2+) currents (I(Ca)) and by measuring intracellular Ca(2+) ([Ca(2+)](i)) with fura-2 AM. AngII inhibited I(Ca) (IC(50) = 0.3 nm) in a voltage-dependent, pertussis toxin (PTX)-sensitive manner consistent with G(i/o)-protein coupling to Ca(2+) channels. DeltaC(m) was modulated bi-directionally; subnanomolar AngII inhibited depolarization-evoked exocytosis, whereas higher concentrations, in spite of I(Ca) inhibition, potentiated DeltaC(m) fivefold (EC(50) = 3.4 nm). Potentiation of exocytosis by AngII involved activation of phospholipase C (PLC) and Ca(2+) mobilization from internal stores. PTX treatment did not affect AngII-dependent Ca(2+) mobilization or facilitation of exocytosis. However, protein kinase C (PKC) inhibitors decreased the facilitatory effects but not the inhibitory effects of AngII on stimulus-secretion coupling. The AngII type 1 receptor (AT1R) antagonist losartan blocked both inhibition and facilitation of secretion by AngII. The results of this study show that activation of multiple types of G-proteins and transduction pathways by a single neuromodulator acting through one receptor type can produce concentration-dependent, bi-directional regulation of exocytosis.

Angiotensin II receptor internalization and signaling in isolated rat hepatocytes

Since angiotensin II (Ang II)-induced receptor internalization is required to maintain the production of certain intracellular signals in some target cells, we investigated the relationships between Ang II receptor endocytosis and the generation of second messengers in rat hepatocytes. The results of the present study demonstrate that in response to exposure of hepatocytes to Ang II, a decrease in surface Ang II receptors occurred, consistent with a rapid endocytosis of the receptor-bound hormone complex. Pretreatment of cells with okadaic acid (OA) did not have any effect on receptor-mediated internalization. In contrast, a marked reduction of the Ang II receptor endocytosis process occurred after treatment of hepatocytes with phenylarsine oxide (PAO), indicating that cysteine residues could be involved in receptor-mediated endocytosis. Stimulation of cells with Ang II blocked the generation of cyclic adenosine monophosphate (cAMP), which follows the stimulation of hepatocytes with forskolin. Moreover, Ang II increased both inositol 4,5-bisphosphate (IP2) and inositol 1,4,5-trisphosphate (IP3) generation, and enhanced intracellular calcium concentration ([Ca2+]i). Exposure of cells to PAO did not alter the effect of Ang II on the accumulation of cAMP after forskolin stimulation, indicating that endocytosis of the agonist-receptor complex is not involved in adenylate cyclase inhibition. Conversely, PAO and OA markedly reduced IP2 and IP3 synthesis, and the plateau phase of Ang II-induced Ca2+ mobilization. The relationship between Ang II-induced endocytosis and the generation of phosphoinositols and increment in [Ca2+]i indicates that sequestration of the Ang II receptor is necessary to maintain the production of these intracellular signals in rat hepatocytes.

Receptor-mediated angiotensin II transcytosis by brain microvessel endothelial cells

Angiotensin II (Ang II) uptake and transport across monolayers of bovine brain microvessel endothelial cells (BMECs) was demonstrated. Ang II transport was linear up to 2 h, saturable with a K(m) of 1.7 nM, and tended to be polarized with the apical-to-basolateral transport being greater. [3H]Ang II transport was found to be inhibited by excess unlabeled Ang II, by the Ang II analog sarathrin, and by the endocytic inhibitor phenylarsine oxide. Ang II-(2-8) and-(3-8) were shown to significantly increase the transport of Ang II. These results demonstrate for the first time the receptor-mediated transcytosis of Ang II across brain microvessel endothelium.

Receptor-mediated endocytosis of angiotensin II in rat myometrial cells

The events involved in the processing of the angiotensin II (Ang II)-receptor complex were studied in primary cultures of rat myometrial cells. Ang II bound to rat myometrial cells in a specific, time- and temperature-dependent fashion. Pretreatment with cycloheximide did not interfere with binding up to 3 hr, but inhibited increases in binding observed over longer periods. The [3H]Ang II binding to intact cells was inhibited by dithiothreitol (DTT), and the rank order of potency of Ang II and nonpeptide antagonists to inhibit the [3H]Ang II binding was Ang II > Losartan >> PD 123319 or CGP 42112B, indicating the presence of the AT1 receptor type. Whereas most of the [3H]Ang II binding at 4 degrees was susceptible to acid or pronase treatment, binding at 35 degrees was resistant to both treatments, suggesting an internalization of the Ang II-receptor complex. Phenylarsine oxide (PAO) and N-ethylmaleimide (NEM) caused a concentration-dependent inhibition when the binding assay was performed at 35 degrees, but no effect was observed at 4 degrees, indicating that these agents did not alter cell-surface binding but actually prevented the internalization process. Simultaneous treatment with 1 mM DTT or beta-mercaptoethanol prevented the inhibitory effect of NEM, but only DTT could prevent the inhibition caused by PAO, indicating that two closely located sulfhydryl groups must be involved in the internalization process. Chloroquine (100 microM) inhibited the [3H]Ang II dissociation from cells, and monensin (25 microM) induced a 30% inhibition of [3H]Ang II binding (35 degrees, 3 hr), suggesting endosomal processing of the Ang II-receptor complex with receptor recycling to the cell surface. These results indicate that Ang II binding to AT1 receptors in rat myometrial cells is followed by internalization of the Ang II-receptor complex and recycling of the receptor to the cell surface.

Angiotensin II type 1 (AT1) receptor-mediated accumulation of angiotensin II in tissues and its intracellular half-life in vivo

Angiotensin II (Ang II) is internalized by various cell types via receptor-mediated endocytosis. Little is known about the kinetics of this process in the whole animal and about the half-life of intact Ang II after its internalization. We measured the levels of 125I-Ang II and 125I-Ang I that were reached in various tissues and blood plasma during infusions of these peptides into the left cardiac ventricle of pigs. Steady-state concentrations of 125I-Ang II in skeletal muscle, heart, kidney, and adrenal were 8% to 41%, 64% to 150%, 340% to 550%, and 680% to 2100%, respectively, of the 125I-Ang II concentration in arterial blood plasma (ranges of six experiments). The tissue concentrations of 125I-Ang I were less than 5% of the arterial plasma concentrations. 125I-Ang II accumulation seen in heart, kidney, and adrenal was almost completely blocked by a specific Ang II type 1 (AT1) receptor antagonist. Steady-state concentrations of 125I-Ang II were reached within 30 to 60 minutes in the tissues and within 5 minutes in blood plasma. The in vivo half-life of intact 125I-Ang II in heart, kidney, and adrenal was approximately 15 minutes, compared with 0.5 minute in the circulation. Thus, Ang II, but not Ang I, from the circulation is accumulated by some tissues, and this is mediated by AT1 receptors. The time course of this process and the long half-life of the accumulated Ang II support the contention that this Ang II has been internalized after its binding to the AT1 receptor, so that it is protected from rapid degradation by endothelial peptidases. The results of this study are in agreement with growing evidence of an important physiological role for internalized Ang II.

Presence and differential internalization of two distinct insulin-like growth factor receptors in rat hippocampal neurons

The pharmacological characteristics, localization and process of internalization of the insulin-like growth factor I and II receptors were studied in rat primary hippocampal cultured neurons grown under serum-free conditions. [125I]insulin-like growth factor-I binding was specific with an apparent affinity (Kd) of 0.1 nM and IC50 values of 0.1, 2.9 and 99.7 nM for insulin-like growth factor-I, insulin-like growth factor-II and insulin, respectively. The competition by insulin suggests the presence of genuine insulin-like growth factor-I receptors and not insulin-like growth factor binding proteins. In contrast, [125I]insulin-like growth factor-II binding showed a Kd of 0.1 nM and IC50 values of 0.2 and 20.5 nM for insulin-like growth factor-II and insulin-like growth factor-I while insulin was inactive, a well established characteristic of the insulin-like growth factor-II receptor. Using emulsion autoradiography, specific binding sites for [125I]insulin-like growth factor-I and -II were over the whole cultured neurons. The use of selective insulin-like growth factor-I and -II receptor antibodies further confirmed the existence of these receptors in rat hippocampal cultured neurons. To investigate the respective internalization profile of [125I]insulin-like growth factor-I and [125I]insulin-like growth factor-II receptor-ligand complexes in neurons, a technique of acid stripping was used. The apparent rate of endocytosis was found to be greater for the insulin-like growth factor-II than for the insulin-like growth factor-I receptor complexes. The internalization of [125I]insulin-like growth factor-I and [125I]insulin-like growth factor-II ligand-receptor complexes was confirmed using phenylarsine oxide which significantly blocked both internalization processes. In order to eliminate possible receptor recycling, monensin was used and shown to have no effect on the internalization of either ligand. Since the insulin-like growth factor-I receptor is coupled to tyrosine kinase activity, tyrphostin 47, a specific tyrosine kinase inhibitor. was used and shown to decrease [125I]insulin-like growth factor-I but not the [125I]insulin-like growth factor-II receptor internalization profile. Accordingly, insulin-like growth factor-I is apparently internalized mostly via the insulin-like growth factor-I tyrosine kinase type receptor, while insulin-like growth factor-II is not. The insulin-like growth factor-II receptor ligand complex is likely internalized via a pathway possibly related to mannose-phosphorylated residues as the insulin-like growth factor-II/mannose-6-phosphate receptor has been implicated in the intracellular targeting of lysosomal proteins containing glycosylated residues. Taken together, our results indicate that primary hippocampal cultured neurons represent a unique model for investigating the differential role and intracellular trafficking of both insulin-like growth factor-I and insulin-like growth factor-II receptor ligand complexes and their relevance to the respective functional role of these two-related trophic factors in the central nervous system.

Renal accumulation of circulating angiotensin II in angiotensin II-infused rats

Previous studies have demonstrated that low-dose angiotensin II (Ang II) infusion for 14 days mimics two-kidney, one clip Goldblatt hypertension and increases intrarenal Ang II levels. The objective of the present study was to determine whether the augmented intrarenal Ang II is due to intrarenal accumulation of the infused Ang II and/or to an increase in intrarenal formation of endogenous Ang II. Male Sprague-Dawley rats were uninephrectomized and divided into three groups: control (N=6), those infused with [Ile5]Ang II (endogenous form) (N=6), and those infused with [Val5]Ang II (n=8). [Ile5]Ang II or [Val5]Ang II was infused at 40 ng/min via an osmotic minipump implanted subcutaneously. By day 12, systolic blood pressure increased significantly in both [Val5]Ang II-infused rats (197 +/- 7 mm Hg) and [Ile5]Ang II-infused rats (173 +/- 3 mm Hg). Blood and kidney samples were harvested, subjected to high-performance liquid chromatography to separate [Val5]Ang II from [Ile5]Ang II, and then measured by radioimmunoassay. Plasma renin activity was markedly suppressed in both [Ile5]Ang II- and [Val5]Ang II-infused rats. Plasma Ang II levels were elevated in rats infused with both [Ile5]Ang II (121 +/- 24 fmol/mL) and [Val5]Ang II (119 +/- 14 fmol/mL) compared with controls (69 +/- 15 fmol/mL). Both [Ile5]Ang II- and [Val5]Ang II-infused rats exhibited an enhancement of total intrarenal Ang II. Only [Ile5]Ang II (358 +/- 53 fmol/g) was detected in the kidneys of rats infused with -Ile5-Ang II. In [Val5]Ang II-infused rats, a significant portion of total renal Ang II (371 +/- 57 fmol/g) was in the form of [Val5]Ang II (256 +/- 44 fmol/g). Renal [Ile5]Ang II levels were maintained in the [Val5]Ang II-infused rats (116 +/- 15 fmol/g) compared with control rats (116 +/- 11 fmol/g) despite marked suppression of renin release. These results support the hypothesis that infused circulating ANG II is bound to receptor or taken up intrarenally in a manner that protects against degradation.