Quantitative in vitro autoradiographic characterization of [125I]angiotensin III binding sites in rat adrenal gland

Angiotensin II AT2 Receptors Contribute to Regulate the Sympathoadrenal and Hormonal Reaction to Stress Stimuli

Angiotensin II, through AT1 receptor stimulation, mediates multiple cardiovascular, metabolic, and behavioral functions including the response to stressors. Conversely, the function of Angiotensin II AT2 receptors has not been totally clarified. In adult rodents, AT2 receptor distribution is very limited but it is particularly high in the adrenal medulla. Recent results strongly indicate that AT2 receptors contribute to the regulation of the response to stress stimuli. This occurs in association with AT1 receptors, both receptor types reciprocally influencing their expression and therefore their function. AT2 receptors appear to influence the response to many types of stressors and in all components of the hypothalamic-pituitary-adrenal axis. The molecular mechanisms involved in AT2 receptor activation, the complex interactions with AT1 receptors, and additional factors participating in the control of AT2 receptor regulation and activity in response to stressors are only partially understood. Further research is necessary to close this knowledge gap and to clarify whether AT2 receptor activation may carry the potential of a major translational advance.

Distribution and functional significance of angiotensin-II AT1- and AT2-receptor subtypes in the rat adrenal gland

The distribution and the functional significance of angiotensin-II (ANG-II) receptor subtypes, AT1 and AT2, in the rat adrenal gland has been investigated in vitro. Autoradiographic assessment of the selective displacement of [125I]ANG-II binding by selective ligands of the two receptor subtypes indicated that zona glomerulosa (ZG) was provided with both AT1 and AT2, and adrenal medulla (AM) almost exclusively with AT2 receptors. ANG-II (10(-9) M) evoked a marked rise in the secretion of aldosterone by dispersed ZG cells and catecholamines by AM fragments. The selective AT1-receptor antagonist DuP753 blocked aldosterone response to ANG-II, while the selective AT2-receptor antagonist PD123319 was ineffective. Catecholamine response to ANG-II was inhibited by PD123319 and only moderately affected by high concentrations of DuP753. The selective AT2-receptor agonist CGP42112 did not change basal aldosterone release of ZG cells, but concentration-dependently enhanced basal catecholamine release by AM fragments. In light of these findings the conclusion is drawn that in the rat the aldosterone secretagogue effect of ANG-II is exclusively mediated by the AT1 receptors present in the ZG, while the catecholamine secretagogue action preminently involves the activation of AT2 receptor located on medullary chromaffin cells.

Influence of dietary sodium restriction on angiotensin II receptors in rat adrenals

We studied the distribution of angiotensin II (AII) receptors type 1 (AT1) and type 2 (AT2) and the effects of a low sodium intake on these two subtypes of receptors in male rat adrenals. Binding studies on adrenal slices, on cell membranes and on cell suspensions were performed using [125I]AII and specific analogs for AT1 (Losartan) and AT2 (PD 123319) receptors. The distribution of AT1 was also studied by immunofluorescence. Complementary approaches were necessary to reach our goal. Indeed, by autoradiography on adrenal slices, [125I]AII was shown to bind to the zona glomerulosa (ZG) and to the medulla (M). When coincubated with [125I]AII, PD 123319 displaced [125I]AII from the medulla and from the ZG, indicating the presence of AT2 receptors in both zones. Losartan partially displaced [125I]AII from the ZG, indicating the presence of AT1 receptors in that zone. Furthermore, the labeling intensity of the medulla (AT2 receptors) was much stronger in adrenal sections from rats kept on a low sodium regimen than from controls. Immunofluorescence microscopy revealed that AT1 receptors were located mainly in the ZG of control rats. After sodium restriction, AT1 receptors appeared to be uniformly distributed within an enlarged ZG; furthermore AT1 receptor-positive cells were found to a limited degree in the zona fasciculata and possibly in the zona reticularis, and a greater number of these positive cells appeared in these zones under sodium restriction. Cell suspensions from rats fed a low sodium diet showed a 2.7- and 2.1-fold increase in total AII receptors in adrenal ZG and ZFR + M cells when compared with controls. Based on Losartan displacement, we calculated that [125I]AII bound to AT1 and to AT2 receptors was increased in both ZG and ZFR + M cell preparations under sodium restriction. Results of binding studies on cell membranes were also indicative of an increasing effect of sodium restriction on AT1 and AT2 receptors binding capacity. Furthermore, Northern blotting analysis revealed 3.0- and 2.5-fold increases in the level of AT1 receptor mRNA in the ZG and the ZFR + M of rats fed a low sodium diet as compared with those fed a normal diet. The low sodium intake resulted in a weaker increase (1.5-fold) in the level of AT2 receptor messenger RNA in the ZG, with no changes in the ZFR + M preparations. In conclusion, in this study complementary approaches were needed to determine the localization of AT1 and AT2 receptors in the rat adrenal, and to show the increasing effects of a low sodium regimen on the adrenal level of these receptors. Immunofluorescence studies revealed AT1 receptors mainly in the ZG and also in some cells of the inner adrenal cortex zones; in adrenals of rats kept on a low sodium diet the ZG was markedly enlarged, and an increased number of immunoreactive cells with AT1 receptors were observed throughout that zone; also more immunoreactive cells were present in the inner zones of the adrenal cortex. Furthermore in the adrenals of rats kept on a low sodium diet, we observed: 1) an increased number of AT1 and AT2 receptors in cell suspensions from the ZG, and in cell suspensions of the ZFR + M; 2) an increased level of AT1 and AT2 receptor mRNAs in the ZG; 3) an increased level of AT1 receptor mRNA, with no changes in the AT2 mRNA level in the ZFR + M. These results suggest a role for AT1 as well as for AT2 receptors in controlling adrenal function and differentiation under normal as well as under physiological stimulation of AII production following sodium restriction.

Potassium negatively regulates angiotensin II type 1 receptor expression in human adrenocortical H295R cells

We have previously shown that the human adrenocortical H295R cell line expresses the type 1 angiotensin II receptor (AT1-R) and that expression of this receptor is downregulated at the level of mRNA by forskolin or dibutyryl-cAMP as well as by angiotensin II (Ang II). In this study we examine the effects of K+ on both AT1-R mRNA and receptors, as monitored through 125I-Ang II binding in the presence of PD 123319. After treatment with a maximal stimulatory steroidogenic dose of K+ (14 mmol/L), H295R cells showed an increase in cytosolic free Ca2+ from 113 to 212 nmol/L. Unlike the effects of Ang II, this increase could be abolished by pretreatment with the Ca2+ channel antagonist nifedipine (1 mumol/L). AT1-R mRNA levels also fell in response to elevated extracellular K+ in a dose-dependent (Kd, 9 mmol/L; maximal fall in message at 12 mmol/L) and time-dependent (maximum 50% at 12 hours) manner. The change in AT1-R mRNA level was less rapid than that in response to activation of phosphoinositidase C by Ang II or adenylyl cyclase by forskolin or by dibutyryl-cAMP. Unlike the action of Ang II but similar to the action of forskolin or dibutyryl-cAMP, the action of K+ was sustained. Changes in mRNA level in response to treatment with K+, Ang II, or dibutyryl-cAMP were also paralleled by changes in 125I-Ang II binding in each case.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative autoradiography of angiotensin II AT2 receptors with [125I]CGP 42112

Most radiolabeled ligands for angiotensin II (Ang II) receptors do not discriminate between the AT1 and AT2 receptor subtypes, which must be distinguished by displacement with selective AT1 or AT2 ligands. We compared [125I]CGP 42112 with the non-selective agonist [125I]Sar1 Angiotensin II. We studied the inferior olive, medial geniculate nucleus and the adrenal medulla, areas rich in AT2 receptors, using both ligands with quantitative autoradiography and membrane binding techniques. [125I]CGP 42112 bound with high affinity (Kd = 0.07-0.3 nM, depending on the area studied). [125I]CGP 42112 binding was selective for AT2 receptors, as determined by lack of competition with the AT1 ligand losartan, and competition by the AT2 ligands PD 123177 and unlabeled CGP 42112 and the non-selective peptides Ang II and angiotensin III (Ang III). Using [125I]CGP 42112 binding, we found the same order of potency: CGP 42112 > Ang II = Ang III > PD 123177 using both quantitative autoradiography or membrane binding methods. Our results demonstrate that [125I]CGP 42112 is the most selective, highest affinity ligand available for AT2 receptors. Because of these characteristics, and low non-specific binding, quantitative autoradiography with [125I]CGP 42112 is the method of choice to selectively characterize AT2 receptors, especially in tissues like the brain, with a highly heterogeneous distribution of receptor subtypes.

Pharmacological characterization of angiotensin II AT(2) receptor subtype heterogeneity in the rat adrenal cortex and medulla

Adrenal angiotensin II (AII) receptors have been pharmacologically and structurally divided into two main subtypes, AT(1) and AT(2). Radioligand receptor binding assays with(125)I-sarcosine(1), isoleucine(8) angiotensin II ((125)I-SI AII) in the presence of losartan, an AT(1) selective ligand, and PD123177 an AT(2) selective ligand, indicated that the AT(1) subtype was predominant in membrane homogenates of the rat adrenal cortex (AT(1) Bmax=649 ± 62 fmol/mg protein; AT(2) Bmax=237 ± 29 fmol/mg protein). In membrane homogenates of the adrenal medulla, the AT(2) subtype was predominant (AT(1) Bmax=55 ± 5 fmol/mg protein; AT(2) Bmax=109 ± 29 fmol/mg protein). Overall 58% of the(125)I-SI AII binding in the rat adrenal was to the AT(1) subtypes, and 42% was to the AT(2) subtypes. The outer cortex contained 59% of the AH receptor binding sites in the adrenal, while the medulla accounted for the remaining 41%. The affinity of the AT(1) binding sites in membrane homogenates of the cortex and medulla (K( D )=672 ± 123 pM and 573 ± 85 pM, respectively) was not significantly different. The affinity for(125)I-SH AII of AT(2) binding sites in membrane homogenates was higher than that of AT, binding sites. The affinity for(125)I-SI All of AT(2) binding sites in membrane homogenates of the outer cortex (K( D )=265 ± 35 pM) was significantly less than that in the medulla (K( D )=133 ± 11 pM).In vitro receptor autoradiography also demonstrated that the AT(2) subtype in frozen sections of the cortex had a lower affinity (K( D )=1512 ± 191 pM) than that in the medulla (K( D )=867 ± 72 pM). The heterogeneous affinity of adrenal AT(2) binding sites may indicate existence of multiple AT(2) receptor subtypes in the rat adrenal.

Effects of atrial natriuretic factor on norepinephrine release evoked by angiotensins II and III in the rat adrenal medulla

Atrial natriuretic factor (ANF) effects on neuronal norepinephrine (NE) release evoked by angiotensin II (ANG II) or angiotensin III (ANG III) were studied in the rat adrenal medulla. ANF 10 nM diminished the increase of NE release induced by ANG II (1 microM), ANG III (1 microM) or 100 mM KCl. When 10 nM ANF was added to the medium containing KCl plus ANG II or KCl plus ANG III, the reduction of 3H-NE output by ANF was greater than when the atrial factor was added to the medium containing only ANG II or ANG III. Since both ANG II and ANG III have a physiological role on catecholamine metabolism, these peptides could modulate the adrenal medulla functions. ANG II and ANG III enhance NE release and decrease NE uptake in the rat adrenal medulla. Present results show that ANF is a physiological antagonist of both ANG II and ANG III, in the process of NE secretion. The interaction between ANF and the renin-angiotensin system could contribute to the regulation of the adrenal medulla catecholamines pathway and sympathetic activity.

Effects of [Sar1]angiotensin II on proenkephalin gene expression and secretion of [Met5]enkephalin in bovine adrenal medullary chromaffin cells

We have studied the effect of [Sar1]angiotensin II [S1-AII; a degradation-resistant analogue of angiotensin II (AII) on the release of [Met5]enkephalin (ME) and proenkephalin A (proENK) gene expression. Short-term (15-min to 1-h) stimulation of bovine adrenal medullary chromaffin (BAMC) cells with S1-AII at concentrations from 0.1 to 100 nM had no significant effect on secretion of ME, whereas high concentrations of S1-AII (3 to 100 microM) produced a concentration-dependent increase in the concentration of ME in the incubation media. In contrast, long-term (3- to 24-h) stimulation with low concentrations (0.1 nM-1 microM) of S1-AII increased the secretion of ME in a concentration-dependent manner (EC50 = 1 nM). The intracellular level of ME was not changed by long-term treatment with S1-AII (100 nM). In addition to increased ME secretion, long-term (24-h) stimulation with S1-AII increased the expression of proENK mRNA in a concentration-dependent manner (EC50 = 4 nM). Losartan (2-n-butyl-4 chloro-5-hydroxymethyl-1-[(2'-(1 H-tetrazol-5-yl)biphenyl-4-yl)- methyl]imidazole potassium salt, a type 1 AII receptor antagonist) inhibited these effects, whereas PD123319 (50 microM, a type 2 AII receptor antagonist) was inactive. Our results suggest that AII in BAMC cells exerts a major effect on the long-term regulation of expression of proENK mRNA and secretion of ME. These effects appear to be mediated by type 1-like AII receptors.

Different effects of chronic K+ depletion on forebrain and peripheral angiotensin II receptors in young rats

K+ depletion stimulates the circulating renin-angiotensin system and affects the regulation of peripheral angiotensin II receptors. The effects of K+ depletion on the regulation of central angiotensin II receptors are unknown. We studied the effects of selective K+ depletion (less than 0.05% in diet for 16 days) on angiotensin II receptor number in kidneys, adrenal glands, and selected brain areas of young rats. K+ depletion caused a significant increase in plasma renin activity and significantly decreased angiotensin II receptor number in the kidney glomeruli and medulla, and in the adrenal zona glomerulosa and adrenal medulla. In the brain, the angiotensin II receptor number was unchanged in the subfornical organ and the hypothalamic paraventricular nucleus after 16 days of K+ depletion. An additional NaCl supplementation (0.02% in the drinking water) to K(+)-depleted rats produced a decrease in plasma renin activity but failed to affect subfornical organ or paraventricular angiotensin II receptor number. Our results suggest that in young animals, K+ depletion has a significant impact on the peripheral renin-angiotensin system without affecting the density of forebrain angiotensin II receptors.

Mn2(+)-activated aspartate aminopeptidase activity, subcellular localization in young and adult rat brain

The levels of soluble and membrane-bound aminopeptidase activities were assayed in subcellular fractions from young (4 weeks old) and adult (20 weeks old) rat brains, using Asp-2-naphthylamide as substrate. The young rats showed the highest soluble and membrane-bound levels of activity in the microsomal fraction but no differences among fractions were found at 20 weeks of age. The membrane-bound activity was significantly higher than the soluble one in all subcellular fractions of young rats. Soluble activity of the homogenate and the mitochondrial fraction was significantly increased in adult animals when compared to that of younger ones.

Down-regulation of angiotensin II receptors in subfornical organ of young male rats by chronic dietary sodium depletion

The effect of a 4-week period of selective dietary sodium depletion on the regulation of peripheral and central angiotensin II receptors was studied in young rats, by quantitative autoradiography. Moderate sodium depletion (0.05% sodium in diet) significantly impaired growth rate and stimulated the renin-angiotensin system, but did not result in significant changes in peripheral or central angiotensin II receptors. In young rats, the impairment of the growth rate and the stimulation of the peripheral renin-angiotensin system were more notable after profound sodium depletion (0.005% sodium in diet). Such sodium depletion corresponded to a down-regulation of kidney angiotensin II receptors, and to an up-regulation of adrenal zona glomerulosa angiotensin II receptors. These effects are similar to those reported in adult rats. In the brain, profound sodium depletion down-regulated angiotensin II receptors in the subfornical organ. There were no changes in angiotensin II receptors in another brain structure, the paraventricular nucleus. Our results indicate a participation of selective central angiotensin II receptors in the regulation of sodium metabolism and suggest that factors other than circulating angiotensin II levels might contribute to regulate the number of angiotensin II receptors in the subfornical organ.

Calibration of 125I-polymer standards with 125I-brain paste standards for use in quantitative receptor autoradiography

125I-Polymer standards were calibrated by interpolation of their optical densities in [125I]-brain paste standard curves to obtain dpm/mg protein. There was a linear relationship between the calibrated polymer standards and the dpm/mg polymer, as provided by the manufacturer. One dpm/mg polymer was equivalent to 7.34 +/- 0.22 dpm/mg protein. Receptor quantification in selected rat brain areas with comparison to either brain paste or calibrated polymer standards yielded similar results.

Identification of angiotensin II receptor subtypes

We have demonstrated the existence of two distinct subtypes of the angiotensin II receptor in the rat adrenal gland using radioligand binding and tissue section autoradiography. The identification of the subtypes was made possible by the discovery of two structurally dissimilar, nonpeptide compounds, DuP 753 and EXP655, that show reciprocal selectivity for the two subtypes. In the rat adrenal cortex, DuP 753 inhibited 80% of the total AII binding with an IC50 value on the sensitive sites of 2 x 10(-8) M, while EXP655 displaced only 20%. In the rat adrenal medulla, EXP655 gave 90% inhibition of AII binding with an IC50 value of 3.0 x 10(-8) M, while DuP 753 was essentially inactive. The combination of the two compounds completely inhibited AII binding in both tissues.