Bovine Adrenal Zona Glomerulosa Research Articles

Adrenic Acid Metabolites as Endogenous Endothelium-Derived and Zona Glomerulosa-Derived Hyperpolarizing Factors

Adrenic acid (docosatetraenoic acid), an abundant fatty acid in the adrenal gland, is identical to arachidonic acid except for 2 additional carbons on the carboxyl end. Adrenic acid is metabolized by cyclooxygenases, cytochrome P450s, and lipoxygenases; however, little is known regarding the role of adrenic acid and its metabolites in vascular tone. Because of its abundance in the adrenal gland, we investigated the role of adrenic acid in vascular tone of bovine adrenal cortical arteries and its metabolism by bovine adrenal zona glomerulosa cells. In adrenal cortical arteries, adrenic acid caused concentration-dependent relaxations, which were inhibited by the epoxyeicosatrienoic acid antagonist 14,15-epoxyeicosa-5(Z)-enoic acid and the cytochrome P450 inhibitor SKF-525A. The large-conductance calcium-activated potassium channel blocker iberiotoxin or removal of the endothelium abolished these relaxations. Reverse-phase high-pressure liquid chromatography and liquid chromatography/mass spectrometry isolated and identified numerous adrenic acid metabolites from zona glomerulosa cells, including dihomo-epoxyeicosatrienoic acids and dihomo-prostaglandins. In denuded adrenal cortical arteries, adrenic acid caused concentration-dependent relaxations in the presence of zona glomerulosa cells but not in their absence. These relaxations were inhibited by SKF-525A, 14,15-epoxyeicosa-5(Z)-enoic acid, and iberiotoxin. Dihomo-16,17-epoxyeicosatrienoic acid caused concentration-dependent relaxations of adrenal cortical arteries, which were inhibited by 14,15-epoxyeicosa-5(Z)-enoic acid and high potassium. Our results suggest that adrenic acid relaxations of bovine adrenal cortical arteries are mediated by endothelial and zona glomerulosa cell cytochrome P450 metabolites. Thus, adrenic acid metabolites could function as endogenous endothelium-derived and zona glomerulosa-derived hyperpolarizing factors in the adrenal cortex and contribute to the regulation of adrenal blood flow.

Endothelium-Derived Steroidogenic Factor Enhances Angiotensin II-Stimulated Aldosterone Release by Bovine Zona Glomerulosa Cells

Endothelium-derived steroidogenic factor (EDSF) is an endothelial peptide that stimulates aldosterone release from bovine adrenal zona glomerulosa (ZG) cells. The regulation of aldosterone release by combinations of EDSF and angiotensin II (AII) or EDSF and ACTH was investigated. Endothelial cells (ECs) and EC-conditioned media (ECCM) increased aldosterone release from ZG cells, an activity attributed to EDSF. AII (10−12 to 10−8m) and ACTH (10−12 to 10−9m) also stimulated the release of aldosterone from ZG cells. The stimulation by AII, but not ACTH, was greatly enhanced when ZG cells were coincubated with ECs. AII was metabolized by ECs to peptides identified by mass spectrometry as angiotensin (1-7) and angiotensin IV. There was very little metabolism of AII by ZG cells. Neither of these two AII metabolites altered aldosterone release from ZG cells, so they could not account for the enhanced response with ECs. AII-induced aldosterone release from ZG cells was enhanced by ECCM but not cell-free conditioned medium. This enhanced response was not due to increased EDSF release from ECs by AII. The synergistic effect of EDSF and AII was apparent when AII was added during or after the generation of ECCM and not observed when the AII component of the enhancement was blocked by the AII antagonist, losartan. These studies indicate that EDSF enhances the steroidogenic effect of AII. In the adrenal gland, ECs are in close anatomical relationship with ZG cells and may sensitize ZG cells to the steroidogenic action of AII by releasing EDSF.

Probenecid inhibits adrenal zona glomerulosa cell aldosterone release

This study examines the effect of the organic anion transporter inhibitor, probenecid, on aldosterone release and Fluo-3 acetoxymethyl ester dye sequestration from cultured bovine adrenal zona glomerulosa (ZG) cells. ZG cells were isolated and maintained in primary culture in Hams F12 medium supplemented with antioxidants, fetal bovine serum and antibiotics prior to their use in aldosterone or Fluo-3 experiments. Aldosterone release was quantified by enzyme-linked immunosorbant assay. Probenecid inhibited angiotensin II-stimulated aldosterone release in a concentration dependent manner and aldosterone release was completely inhibited with 10 mM probenecid. Potassium, adrenocorticotropic hormone, and 8-bromoadenosine 3′,5′-cyclic monophosphate-stimulated aldosterone release was similarly inhibited by probenecid treatment. The inhibitory effects of probenecid were also observed in ZG cells stimulated with exogenous 25-hydroxycholesterol and progesterone. Fluo-3 fluorescence was increased by probenecid treatment. Treatment of ZG cells with 10 mM probenecid during the Fluo-3 dye loading process caused a dramatic increase in visible fluorescence and more than doubled the average pixel intensity in fluorescing angiotensin II-stimulated ZG cells as compared to control. We conclude that probenecid inhibits transport of Fluo-3 dye by organic anion transporters in the cell membrane of the cultured ZG cell. We further conclude that probenecid interferes with either the enzymes involved in the process of aldosterone synthesis or a transporter required for aldosterone synthesis or release from the ZG cell. This work was supported by grants from the University of Wisconsin-Green Bay.

Measurement of steroid synthesis in zona glomerulosa cells by liquid chromatography–electrospray ionization–mass spectrometry: inhibition by nitric oxide

A liquid chromatography–electrospray ionization–mass spectrometry method was developed to simultaneously determine the concentrations of aldosterone, corticosterone, cortisol, deoxycorticosterone, pregnenolone, and progesterone in bovine adrenal zona glomerulosa (ZG) cells. Steroids were extracted by liquid–liquid extraction, separated on a reverse-phase C 18 column, ionized by electrospray, and detected by single-quadrupole mass spectrometry in a positive ion mode. All steroids formed sodium adducts at high abundance. Factors affecting the formation and signal of sodium adducts were investigated. The limits of detection ( S/ N = 3) using selected ion monitoring are 2 pg for these steroids and 10 pg for pregnenolone. DETA NONOate, a nitric oxide donor, inhibited the basal, angiotensin-II-stimulated, and 25-hydroxycholesterol-stimulated syntheses of these steroids in ZG cells in a concentration-dependent manner. The technique demonstrates the ability to determine the individual steroid in each enzymatic step of aldosterone synthesis and the activity of steroidogenic enzymes in adrenal ZG cells.

Alpha1H T-type Ca2+ channel is the predominant subtype expressed in bovine and rat zona glomerulosa.

The low voltage-activated (T-type) Ca2+ channel has been implicated in the regulation of aldosterone secretion from the adrenal zona glomerulosa by extracellular K+ levels, angiotensin II, and ACTH. However, the identity of the specific subtype mediating this regulation has not been determined. We utilized in situ hybridization to examine the distribution of three newly cloned members of the T-type Ca2+ channel family, alpha1G, alpha1H, and alpha1I, in the rat and bovine adrenal gland. Substantial expression of only the mRNA transcript for the alpha1H-subunit was detected in the zona glomerulosa of both rat and bovine. A much weaker expression signal was detected for the alpha1H transcript in the zona fasciculata of bovine. Whole cell recordings of isolated bovine adrenal zona glomerulosa cells showed the native low voltage-activated current to be inhibited by NiCl2 with an IC50 of 6.4 +/- 0.2 microM. Because the alpha1H subtype exhibits similar NiCl2 sensitivity, we propose that the alpha1H subtype is the predominant T-type Ca2+ channel present in the adrenal zona glomerulosa.

Effects of angiotensin II on sodium potassium pumps, endogenous ouabain, and aldosterone in bovine zona glomerulosa cells.

Angiotensin (Ang) II stimulates secretions of aldosterone and an endogenous ouabain-like steroid (EO) from bovine adrenal zona glomerulosa (BAG) cells. The BAG cell sodium pump, a possible target of EO, affects aldosterone secretion although little is known about this pump. Here, we describe the effects of Ang II on the characteristics of this transporter and steroid secretions. Under serum-free conditions, 3H-ouabain bound to a single class of sites on BAG cells. Binding of label was time and concentration dependent, was sensitive to extracellular potassium ions, and was displaced by ouabain and digoxin with EC50 of approximately 218 and approximately 232 nmol/L, respectively. Sodium pump-mediated 86Rb uptake was inhibited by ouabain (EC50 approximately 301 nmol/L). Ang II dose dependently augmented secretions of EO and aldosterone, increased ouabain-sensitive 86Rb uptake and 3H-ouabain binding, and increased the affinity for 3H-ouabain binding (Kd, from 205 to 80 nmol/L) with no change in the maximal number of sodium pumps (5.45x10(6)) per cell. Losartan blocked all effects of Ang II except EO secretion, which was inhibited by PD123319. We conclude that BAG cells express sodium pumps in high density and bind ouabain to a single class of low-affinity sites. The characteristics of the sodium pumps protect BAG cells from EO autotoxicity but may exclude them from mediating feedback inhibition of EO secretion. The effects of Ang II on sodium pump activity, ouabain binding affinity, and aldosterone secretion are mediated via Ang II type 1 receptors, whereas Ang II type 2 receptors augment EO secretion. The role of the Ang II-mediated increase in the ouabain sensitivity of BAG cell sodium pumps in the secretions of aldosterone and EO remains to be elucidated.

Nitric oxide inhibits aldosterone synthesis by a guanylyl cyclase-independent effect.

To investigate the mechanism of nitric oxide (NO) inhibition of aldosterone release, this study compared the effects of type A natriuretic peptide and heat-stable enterotoxin to a nitric oxide donor, deta nonoate, on cGMP production and angiotensin II-stimulated aldosterone synthesis ill primary cultures of bovine adrenal zona glomerulosa cells. Type A natriuretic peptide (10(-10)-10(-6) M) and deta nonoate (10(-6)-10(-3) M) stimulated concentration-related increases in cGMP production. Heat-stable enterotoxin (10(-6) M) failed to stimulate cGMP synthesis in zona glomerulosa cells. Type A natriuretic peptide and deta nonoate attenuated angiotensin II-stimulated aldosterone production over the same concentration range that stimulated cGMP production. Heat-stable enterotoxin (10(-6) M) was without effect on aldosterone release. To further test the hypothesis that cGMP mediated the inhibition of aldosterone synthesis, the selective inhibitor of soluble guanylyl cyclase, 1H-(1,2,4)oxadiazolo [4,3-a]quinoxalin-1-one (ODQ) was used. ODQ pretreatment (10(-5) M) completely prevented deta nonoate-stimulated cGMP production without altering the inhibitory effect of deta nonoate on angiotensin II-stimulated steroidogenesis. Consistent with its selectivity for inhibiting soluble guanylyl cyclase, ODQ did not block type A natriuretic peptide-stimulated cGMP synthesis or type A natriuretic peptide inhibition of steroidogenesis. Deta nonoate completely blocked 25-hydroxycholesterol- and progesterone-stimulated aldosterone synthesis in zona glomerulosa cells and inhibited the conversion of 25-hydroxycholesterol to pregnenolone in mitochondrial fractions from bovine adrenal cortex. Deta nonoate-derived NO gave an absorbance maximum of the mitochondrial cytochrome P450 of 453 nm and inhibited the absorbance at 450 nm caused by carbon monoxide binding to the enzyme. These results suggest that deta nonoate reduces steroidogenesis independent of guanylyl cyclase activation and that NO has a direct effect to inhibit the activity of cytochrome P450, probably by binding to the heme groups of the cytochrome.

Atrial natriuretic peptide inhibits calcium-induced steroidogenic acute regulatory protein gene transcription in adrenal glomerulosa cells.

Atrial natriuretic peptide (ANP) is a potent inhibitor of mineralocorticoid synthesis induced in adrenal glomerulosa cells by physiological agonists activating the calcium messenger system, such as angiotensin II (Ang II) and potassium ion (K+). While the role of calcium in mediating Ang II- and K(+)-induced aldosterone production is clearly established, the mechanisms leading to blockade of this steroidogenic response by ANP remain obscure. We have used bovine adrenal zona glomerulosa cells in primary culture, in which an activation of the calcium messenger system was mimicked by a 2-h exposure to an intracellular high-calcium clamp. The effect of ANP was studied on the following parameters of the steroidogenic pathway: 1) pregnenolone and aldosterone production; 2) changes in cytosolic ([Ca2+]c) and mitochondrial ([Ca2+]m) Ca2+ concentrations, as assessed with targeted recombinant aequorin; 3) cholesterol content in outer mitochondrial membranes (OM), contact sites (CS), and inner membranes (IM); 4) steroidogenic acute regulatory (StAR) protein import into mitochondria by Western blot analysis; 5) StAR protein synthesis, as determined by [35S]methionine incorporation, immunoprecipitation, and SDS-PAGE; 6) StAR mRNA levels by Northern blot analysis with a StAR cDNA; 7) StAR gene transcription by nuclear run-on analysis. While clamping Ca2+ at 950 nM raised pregnenolone output 3.5-fold and aldosterone output 3-fold, ANP prevented these responses with an IC50 of 1 nM and a maximal effect of 90% inhibition at 10 nM. In contrast, ANP did not affect the [Ca2+]c or [Ca2+]m changes occurring under Ca2+ clamp or Ang II stimulation in glomerulosa cells. The accumulation of cholesterol content in CS (139.7 +/- 10.7% of control) observed under high-Ca2+ clamp was prevented by 10 nM ANP (92.4 +/- 4% of control). Similarly, while Ca2+ induced a marked accumulation of StAR protein in mitochondria of glomerulosa cells to 218 +/- 44% (n = 3) of controls, the presence of ANP led to a blockade of StAR protein mitochondrial import (113.3 +/- 15.0%). This effect was due to a complete suppression of the increased [35S]methionine incorporation into StAR protein that occurred under Ca2+ clamp (94.5 +/- 12.8% vs. 167.5 +/- 17.3%, n = 3). Furthermore, while the high-Ca2+ clamp significantly increased StAR mRNA levels to 188.5 +/- 8.4 of controls (n = 4), ANP completely prevented this response. Nuclear run-on analysis showed that increases in intracellular Ca2+ resulted in transcriptional induction of the StAR gene and that ANP inhibited this process. These results demonstrate that Ca2+ exerts a transcriptional control on StAR protein expression and that ANP appears to elicit its inhibitory effect on aldosterone biosynthesis by acting as a negative physiological regulator of StAR gene expression.

Prostaglandin E 2 –Induced Aldosterone Release Is Mediated by an EP 2 Receptor

Prostaglandin E2 (PGE2) is an endogenous hormone of adrenal zona glomerulosa cells and is released in response to stimulation by agonists such as angiotensin II (Ang II). It stimulates the release of aldosterone from cultured bovine adrenal zona glomerulosa cells. These studies were designed to determine whether this steroidogenic effect of PGE2 was mediated by an EP1, EP2, or EP3 receptor. Prostaglandin E2 and 11-deoxy PGE1, an EP2-selective agonist, stimulated aldosterone release in a concentration-related manner with an ED50 of 300 nmol/L for PGE2 and 2 micromol/L for 11-deoxy PGE1. The maximal effect of PGE2 was less than that of angiotensin II. 17-Phenyl trinor PGE2, an EP1-selective agonist, required concentrations of 100 micromol/L to stimulate aldosterone release and sulprostone, an EP3/EP1-selective agonist, failed to alter aldosterone release. The EP1-selective antagonist SC19220 failed to alter basal or PGE2-stimulated aldosterone release over a range of concentrations. PGE2 and 11-deoxy PGE1 also stimulated an increase in both intracellular and extracellular cAMP. This increase was time- and concentration-related. The ED50 for PGE2 was 9.8 micromol/L. 17-Phenyl trinor PGE2 and sulprostone were without effect. Using fura-2 loaded cells, PGE2 (2 micromol/L), dibutyryl cAMP (2 mmol/L), and Ang 11 (2 micromol/L) increased intracellular calcium over basal concentrations by 5.5-fold, 3-fold, and 6.2-fold, respectively. Like PGE2, dibutyryl cAMP also stimulated aldosterone release. PGE2- and dibutyryl cAMP-induced aldosterone release were blocked by the calcium channel inhibitor diltiazem. These studies indicate that PGE2 is a potent stimulus for aldosterone release and that the effect is mediated by EP2 receptors. Both cAMP and calcium appear to mediate the steroidogenic effect of PGE2 and calcium seems to be distal to cAMP.

Submitochondrial Distribution of Three Key Steroidogenic Proteins (Steroidogenic Acute Regulatory Protein and Cytochrome P450scc and 3β-Hydroxysteroid Dehydrogenase Isomerase Enzymes) upon Stimulation by Intracellular Calcium in Adrenal Glomerulosa Cells

In adrenal glomerulosa cells, angiotensin II (Ang II) and potassium stimulate aldosterone synthesis through activation of the calcium messenger system. The rate-limiting step in steroidogenesis is the transfer of cholesterol to the inner mitochondrial membrane. This transfer is believed to depend upon the presence of the steroidogenic acute regulatory (StAR) protein. The aim of this study was 1) to examine the effect of changes in cytosolic free calcium concentration and of Ang II on intramitochondrial cholesterol and 2) to study the distribution of StAR protein in submitochondrial fractions during activation by Ca2+ and Ang II. To this end, freshly prepared bovine zona glomerulosa cells were submitted to a high cytosolic Ca2+ clamp (600 nM) or stimulated with Ang II (10 nM) for 2 h. Mitochondria were isolated and subfractionated into outer membranes, inner membranes (IM), and contact sites (CS). Stimulation of intact cells with Ca2+ or Ang II led to a marked, cycloheximide-sensitive increase in cholesterol in CS (to 143 +/- 3. 2 and 151.1 +/- 18.1% of controls, respectively) and in IM (to 119 +/- 5.1 and 124.5 +/- 6.5% of controls, respectively). Western blot analysis revealed a cycloheximide-sensitive increase in StAR protein in mitochondrial extracts of Ca2+-clamped glomerulosa cells (to 159 +/- 23% of controls). In submitochondrial fractions, there was a selective accumulation of StAR protein in IM following stimulation with Ca2+ (228 +/- 50%). Similarly, Ang II increased StAR protein in IM, and this effect was prevented by cycloheximide. In contrast, neither Ca2+ nor Ang II had any effect on the submitochondrial distribution of cytochrome P450scc and 3beta-hydroxysteroid dehydrogenase isomerase. The intramitochondrial presence of the latter enzyme was further confirmed by immunogold staining in rat adrenal fasciculata cells and by immunoblot analysis in MA-10 mouse testicular Leydig cells. These findings demonstrate that under acute stimulation with Ca2+-mobilizing agents, newly synthesized StAR protein accumulates in IM after transiting through CS. Moreover, our results suggest that the import of StAR protein into IM may be associated with cholesterol transfer, thus promoting precursor supply to the two first enzymes of the steroidogenic cascade within the mitochondria and thereby activating mineralocorticoid synthesis.

Comparative Effect of Pituitary Adenylate Cyclase-Activating Polypeptide on Aldosterone Secretion in Normal Bovine and Human Tumorous Adrenal Cells*

The purpose of this study was to investigate the mechanisms of action of pituitary adenylate cyclase-activating polypeptide (PACAP) in stimulating aldosterone production in two different models: bovine adrenal zona glomerulosa (ZG) cells in primary culture and the human adrenocortical carcinoma cell line H295R. PACAP binds to two major groups of receptors: type I, which prefers PACAP38 and PACAP27 over vasoactive intestinal peptide (VIP); and type II, which has approximately equal affinity for PACAP38, PACAP27, and VIP. The type I subclass comprises multiple splice variants that can be distinguished by their specificity to PACAP38 and PACAP27 in their activation of adenylate cyclase and phospholipase C. Type II PACAP/ VIP receptors couple only to AC. In bovine ZG cells, PACAP38 and PACAP27 stimulated aldosterone production in a dose-dependent manner, whereas VIP was ineffective. In H295R cells, PACAP38, PACAP27, and VIP dose-dependently stimulated aldosterone production with roughly the same ED50. In bovine ZG cells, PACAP38 and PACAP27 stimulated cAMP production with similar efficacy, whereas VIP had no effect. In H295R cells, all three peptides stimulated cAMP accumulation. PACAP38 and PACAP27 also activated PLC in bovine ZG cells as they induced an increase in Ins(1,4,5)Ps production. In H295R cells, neither of these peptides was able to stimulate IP turnover. These results indicate that PACAP stimulation of aldosterone production is mediated by the PVR1s or the PVR1hop splice variants of the type I PACAP-specific receptor subtype in bovine ZG cells, whereas only type II PACAP/VIP receptors seemed to occur in the human H295R cell line. In addition, PACAP-stimulated aldosterone production was inhibited by atrial natriuretic peptide in bovine and human adrenocortical cells, however not by the same mechanism. This further supports species-specific and/or cell type-specific signaling pathways for PACAP in the regulation of aldosterone production.

Possible role for mitochondrial calcium in angiotensin II- and potassium-stimulated steroidogenesis in bovine adrenal glomerulosa cells.

In adrenal zona glomerulosa cells, the action of angiotensin II (Ang II) and of potassium (K+) on aldosterone synthesis is mediated by the Ca2+ messenger system. The major part of the steroidogenic pathway takes place inside the mitochondria, and Ca2+ must enter the mitochondrial matrix to stimulate the steroidogenic cascade. To examine how changes in the cytosolic free calcium concentration ([Ca2+]c) induced by Ang II and K+ are relayed into the mitochondrial matrix, we transfected bovine adrenal zona glomerulosa cells in primary culture with a chimeric complementary DNA encoding for the signal presequence targeting human cytochrome c oxidase subunit VIII to the matrix, linked to a complementary DNA coding for the Ca2+-sensitive photoprotein aequorin. Resting mitochondrial free calcium concentration ([Ca2+]m) amounted to 0.41 +/- 0.18 microM (n = 40). Ang II induced a concentration-dependent (EC50 = 11.3 +/- 6.0 nM), biphasic rise of [Ca2+]m. After a large transient initial peak (5.13 +/- 0.89 microM, n = 28), [Ca2+]m decreased to a plateau that remained higher than basal [Ca2+]m for several minutes in the presence of the hormone. By contrast, studies in cells transfected with cytosolic aequorin indicated that the rise of [Ca2+]c triggered by Ang II was confined to 1.34 +/- 0.26 microM (n = 17). In Ca2+-free medium, a reduced peak [Ca2+]m response to Ang II occurred without a secondary plateau. On readdition of extracellular Ca2+, in the presence of the hormone, the resulting Ca2+ influx was accompanied by small rise of [Ca2+]m. The mitochondrial uncoupler, carbonyl cyanide p-(trifluoro-methoxy)phenyl-hydrazone, prevented the Ang II-induced [Ca2+]m rise but not the [Ca2+]c response, thus demonstrating the mitochondrial location of transfected aequorin. In contrast to Ang II, K+ (13 mM) induced a sustained [Ca2+]c response, which was relayed without amplification into the mitochondrial matrix as a plateau of[Ca2+]m. This plateau of[Ca2+]m was suppressed by the addition of the dihydropyridine, nifedipine (200 nM). The inhibitor of the mitochondrial Na+/Ca2+ exchanger, CGP37157, reduced significantly the rate of decrease of [Ca2+]m following the peak induced by Ang II. In cells whose [Ca2+]c was clamped at various levels (0.05-0.860 microM) with ionomycin, a concentration-dependent stimulation of pregnenolone output was induced by Ca2+. Under these conditions, the output of pregnenolone--the early product of steroidogenesis--was markedly potentiated by CGP37157. These results suggest the existence of microdomains of high [Ca2+]c elicited by Ang II in the proximity of mitochondria. Moreover, our observations are consistent with a mitochondrial site of action for calcium in the activation of the steroidogenic cascade.

The H295R human adrenocortical cell line contains functional atrial natriuretic peptide receptors that inhibit aldosterone biosynthesis

The inhibitory effect of atrial natriuretic peptide (ANP) on angiotensin II (AII)-stimulated aldosterone secretion has been previously studied in rat and bovine adrenal zona glomerulosa cells in primary culture. However the understanding of the mode of action of ANP at the molecular level has been hampered by limitations of those primary cell culture systems and by the lack of cell lines from human adrenal cortex. Here we demonstrate the presence of fully functional ANP receptors in the recently characterized AII-responsive adrenocortical carcinoma cell line H295R. Specific saturable binding of 125I-rANP to H295R cell membrane preparations revealed a single class of high affinity binding sites with a density of 20 fmol/mg of protein. The pharmacological profile of this ANP receptor was documented by competitive binding of 125I-rANP with naturally occurring natriuretic peptides. rANP was the most potent with a K d of 42 pM. pBNP32 was less potent with a K d of 174 pM. 125I-rANP binding was not competed by pCNP (NPRB-specific ligand) nor by C-ANF (NPRC-specific ligand). Photoaffinity labeling of membrane preparations with 125I-BPA-ANP revealed a single specific protein of molecular weight around 130 kDa. This protein was further identified by immunodetection with a specific antibody directed to the human ANP-specific receptor NPRA. Natriuretic peptides stimulated cGMP production by the receptor-coupled guanylate cyclase with the same specificity. Aldosterone production by AII-stimulated H295R cells was dose-dependently inhibited by rANP with an ED 50 of 1.5 nM. In addition, we used this model to test two chimeric analogs of ANP and BNP. pBNP1 and pBNP3 were, respectively, 4- and 2-fold more potent than rANP in competing for 125I-rANP binding with K d of 10 and 20 pM. pBNP1 was 24-fold more potent in inhibiting AII-stimulated aldosterone production with ED 50 of 63 pM. pBNP1 is therefore the most potent natriuretic peptide analog tested. In summary, the human H295R cell line contains NPRA receptors positively coupled to the particulate guanylate cyclase and that antagonize angiotensin II stimulation of aldosterone secretion.

C-type natriuretic peptide as a possible local modulator of aldosterone secretion in bovine adrenal zona glomerulosa

Although atrial and brain natriuretic peptides are well known to be involved in the regulation of cardiovascular and endocrine functions as circulating hormones, the roles of the C-type natriuretic peptide (CNP) remain unknown. We examined the effects of CNP on the secretion of aldosterone and cyclic nucleotides from bovine adrenal zona glomerulosa cells in culture. CNP produced a dose-dependent increase in the basal secretion of cGMP, with an EC50 of 3.8 x 10(-10)M. CNP significantly inhibited the ACTH-induced increase in aldosterone and cAMP in a dose-related manner, with an IC50 of 3.6 x 10(-10)M. Although ACTH itself did not increase cGMP secretion, the addition of CNP elicited a significant increase in cGMP secretion. The effects of CNP on the basal secretion of cGMP and the ACTH-induced secretion of aldosterone were significantly reversed by a nonpeptide natriuretic peptide receptor antagonist, HS-142-1. CNP immunoreactivity was localized in the zona glomerulosa by immunohistochemical staining. In addition, expression of CNP messenger RNA and natriuretic peptide B receptor messenger RNA was demonstrated by RT-PCR in the zona glomerulosa tissue and cells in culture. These findings suggest that CNP is a local factor regulating ACTH-induced aldosterone secretion through a guanylyl cyclase-cGMP pathway.

C-type natriuretic peptide as a possible local modulator of aldosterone secretion in bovine adrenal zona glomerulosa.

Although atrial and brain natriuretic peptides are well known to be involved in the regulation of cardiovascular and endocrine functions as circulating hormones, the roles of the C-type natriuretic peptide (CNP) remain unknown. We examined the effects of CNP on the secretion of aldosterone and cyclic nucleotides from bovine adrenal zona glomerulosa cells in culture. CNP produced a dose-dependent increase in the basal secretion of cGMP, with an EC50 of 3.8 x 10(-10)M. CNP significantly inhibited the ACTH-induced increase in aldosterone and cAMP in a dose-related manner, with an IC50 of 3.6 x 10(-10)M. Although ACTH itself did not increase cGMP secretion, the addition of CNP elicited a significant increase in cGMP secretion. The effects of CNP on the basal secretion of cGMP and the ACTH-induced secretion of aldosterone were significantly reversed by a nonpeptide natriuretic peptide receptor antagonist, HS-142-1. CNP immunoreactivity was localized in the zona glomerulosa by immunohistochemical staining. In addition, expression of CNP messenger RNA and natriuretic peptide B receptor messenger RNA was demonstrated by RT-PCR in the zona glomerulosa tissue and cells in culture. These findings suggest that CNP is a local factor regulating ACTH-induced aldosterone secretion through a guanylyl cyclase-cGMP pathway.

A role for protein tyrosine kinase in the steroidogenic pathway of angiotensin II in bovine zona glomerulosa cells.

Stimulation of aldosterone synthesis in bovine adrenal zona glomerulosa (ZGB) cells by angiotensin II (AngII) is believed to be mediated by the phospholipase C (PLC) pathway that results in the increase of cytosolic free calcium concentration and in the activation of protein kinase C (PKC). However, the cell proliferation and contraction associated with AngII action are known to be mediated in part by protein tyrosine kinases (PTK). To assess the potential role of PTK in the stimulatory effect of AngII on adrenal steroidogenesis, the actions of a series of PTK inhibitors on this metabolic pathway were examined in isolated ZGB cells. Tyrphostin 23 (TP23) caused a dose-dependent inhibition of AngII-stimulated aldosterone production with an IC50 of 15 microM and reached complete inhibition at 100 microM. Genistein (GS) was more potent with an IC50 of 35 nM and complete inhibition at 10 microM. The stimulation of aldosterone production by the calcium-mobilizing agent thapsigargin (Thaps) was also dose-dependently inhibited by TP and GS with the same potency. A specific PKC inhibitor, calphostin C (0.1 microM) caused only a 51.7% inhibition of AngII-stimulated aldosterone production. In the same way, a specific Ca2+/calmodulin-dependent protein kinase inhibitor, KN-62 (1 microM), reduced aldosterone production stimulated by AngII by 64%. As expected, thapsigargin-stimulated aldosterone biosynthesis was not affected by calphostin C, but was completely inhibited by KN-62. These results demonstrate for the first time that protein tyrosine kinase activity is part of the angiotensin II signalling pathway in bovine zona glomerulosa cells. The activation of this PTK occurs subsequently to the mobilization of intracellular calcium. This calcium-dependent protein tyrosine kinase pathway is essential for the steroidogenic response to AngII in bovine zona glomerulosa cells.

The site of action of Ca2+ in the activation of steroidogenesis: studies in Ca(2+)-clamped bovine adrenal zona-glomerulosa cells.

The Ca(2+)-messenger system plays a crucial role in the regulation of steroid production in adrenal zona-glomerulosa cells, as it is known to mediate the action of both angiotensin II and K+. In the present study we used intact isolated glomerulosa cells in which the cytosolic free Ca2+ concentration ([Ca2+]c) was clamped at various levels with the Ca2+ ionophore ionomycin in order to locate the site(s) of action of Ca2+. By measuring in parallel steroid synthesis and [Ca2+]c, we show that Ca2+ levels (50-860 nM) regulate the production of both pregnenolone (up to 669 +/- 71.1% of the basal production) and aldosterone (up to 301 +/- 42.2%; EC50 = 303 nM). By contrast, Ca2+ did not stimulate the conversion of 11-deoxycorticosterone into aldosterone. Ca2+ modulation did not affect the formation of pregnenolone from freely diffusible analogues of cholesterol, indicating that Ca2+ acts at a step upstream of cholesterol side-chain cleavage. Moreover cycloheximide, an inhibitor of protein translation and of adrenocorticotropin-induced facilitation of intramitochondrial cholesterol transport, the rate-limiting step in steroidogenesis, also blocked Ca(2+)-triggered pregnenolone formation. This is consistent with a model in which Ca2+ promotes cholesterol transfer between mitochondrial membranes. In addition, agents using the cyclic AMP pathway as well as angiotensin II potentiated the steroidogenic response to increases in [Ca2+]c by augmenting both the efficacy and the potency of Ca2+. This effect of angiotensin II did not involve protein kinase C. These results establish a direct link between agonist-induced [Ca2+]c rises and a specific step of the steroidogenic pathway.

Thapsigargin inhibits voltage-activated calcium channels in adrenal glomerulosa cells

Thapsigargin, an inhibitor of the microsomal Ca2+ pumps, has been extensively used to study the intracellular Ca2+ pool participating in the generation of the agonist-induced Ca2+ signal in various cell types. A dual effect of this agent was observed in bovine adrenal zona glomerulosa cells. At nanomolar concentrations, thapsigargin stimulated a sustained Ca2+ influx, probably resulting from Ca(2+)-store depletion. In contrast, when added at micromolar concentrations, thapsigargin prevented the rise in cytosolic free Ca2+ concentration ([Ca2+]c) induced by K+. This inhibitory effect of thapsigargin on voltage-activated Ca2+ channels was confirmed by measuring Ba2+ currents by the patch-clamp technique. Both low-threshold (T-type) and high-threshold (L-type) Ca2+ channels were affected by micromolar concentrations of thapsigargin. Analysis of the current-voltage relationship for T-type channels revealed that thapsigargin did not modify the sensitivity of these channels to the voltage, but decreased the maximal current flowing through the channels. In conclusion, thapsigargin appears to exert a dual effect on adrenal glomerulosa cells. At lower concentrations, this agent induces a sustained Ca2+ entry, whereas at higher concentrations it decreases [Ca2+]c by blocking voltage-activated Ca2+ channels.

Effect of CO2/pH on the aldosterone response to hypoxia in bovine adrenal cells in vitro.

Acidosis increases and hypoxia decreases aldosterone production from the adrenal zona glomulerosa in vivo, in situ, and in vitro. These effects appear to be located at different steps in the steroidogenic process. Because respiratory acidosis and hypoxemia are common sequelae of chronic lung disease, the present experiments evaluated the interaction of hypoxia and CO2 (with uncompensated or compensated extracellular pH) on aldosteronogenesis in vitro. Bovine adrenal zona glomerulosa cells were stimulated with angiotensin II (ANG II) or adenosine 3',5'-cyclic monophosphate under room air control (21% O2-0% CO2), CO2 per se (21% O2-10% CO2), hypoxia per se (10% O2-0% CO2), and the combination of CO2 and hypoxia (10% O2-10% CO2). Furthermore, under CO2, pH was either allowed to decrease from 7.2 to 6.8 (uncompensated) or its decrease was minimized (> 7.05) with NaOH (compensated). CO2 without pH compensation led to a significant increase in ANG II-stimulated aldosterone release; when the decrease in pH was minimized, CO2 inhibited ANG II-stimulated aldosterone release. Hypoxia inhibited aldosterone release; the inhibitory effect of hypoxia predominated when combined with CO2. In the presence of cyanoketone, pregnenolone production from endogenous precursors (early pathway) was unaffected. However, the conversion of corticosterone to aldosterone (late pathway) was inhibited by low O2 but unaffected by CO2. It is concluded that the inhibitory effect of low O2 on the late pathway predominates over the effects of uncompensated or compensated simulated respiratory acidosis on aldosteronogenesis.

Early and late effects of angiotensin-II on Ca2+ fluxes in bovine adrenal zona glomerulosa cells.

Previous studies have yielded conflicting results concerning the effects of angiotensin-II (Ang II) on Ca2+ fluxes in adrenal zona glomerulosa (ZG) cells. The present study was designed to investigate the kinetics and dose-dependency of Ang II-mediated changes in Ca2+ influx and efflux in cultured bovine ZG cells. At a high (10 nM) Ang II concentration, cytosolic Ca2+ (Cai) shows a peak-plateau response for the first 15 min, with small Cai transients commonly observed with longer stimulations. At 50 pM Ang II, more sustained Cai changes were elicited, typically consisting of Cai oscillations. The underlying changes in Ca2+ influx and efflux were studied. The early modifications of Ca2+ influx after 2 min of agonist stimulation were biphasic, with uptake increased by 90% between 1-100 pM Ang II and inhibited by 30% at 10 nM Ang II. Furthermore, high (10 nM) Ang II doses inhibited extracellular K(+)-stimulated Ca2+ influx. After 30 min of Ang II stimulation, the later dose response of Ca2+ influx was of similar magnitude but shifted to the left, showing a maximal influx at 10 pM Ang II and a modest enhancement at 10 nM. Basal Ca2+ efflux followed a two-compartment exponential decay, reflecting rapid Ca2+ displacement from extracellular sites (k1) and active Ca2+ transport (k2). A high (10 nM) Ang II concentration induced a transient large increase (130%) in k2 during the initial phase of Ang II stimulation, which returned to basal values within 10 min. A low (50 pM) Ang II concentration induced a small sustained increase (30%) in k2. A 10-nM Ang II concentration markedly reduced the exchangeable Ca2+ pool, as Ca2+ mobilized from intracellular stores into the cytosol was rapidly extruded, while Ca2+ influx was inhibited. A more physiological (50 pM) concentration of Ang II did not significantly alter the total exchangeable Ca2+ pool due to modest stimulation of both Ca2+ efflux and influx. In summary, the initial transient Cai response to high Ang II results from a large Ca2+ mobilization combined with inhibition of Ca2+ influx, which does not allow for the refilling of Ca2+ stores. At later times, small increases in Ca2+ influx allow for the eventual recovery of exchangeable cell Ca2+ and an enhanced elevation in Cai. At low Ang II concentrations, stimulation of both Ca2+ influx and efflux are concurrent and maintained, allowing for a sustained increase in Cai with little change in exchangeable cell Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)