Mechanisms involved in the interaction of dopamine with angiotensin II on aldosterone secretion in isolated and cultured rat adrenal glomerulosa cells

Primary Aldosteronism: Changing Definitions and New Concepts of Physiology and Pathophysiology Both Inside and Outside the Kidney

In the 60 years that have passed since the discovery of the mineralocorticoid hormone aldosterone, much has been learned about its synthesis (both adrenal and extra-adrenal), regulation (by renin-angiotensin II, potassium, adrenocorticotrophin, and other factors), and effects (on both epithelial and nonepithelial tissues). Once thought to be rare, primary aldosteronism (PA, in which aldosterone secretion by the adrenal is excessive and autonomous of its principal regulator, angiotensin II) is now known to be the most common specifically treatable and potentially curable form of hypertension, with most patients lacking the clinical feature of hypokalemia, the presence of which was previously considered to be necessary to warrant further efforts towards confirming a diagnosis of PA. This, and the appreciation that aldosterone excess leads to adverse cardiovascular, renal, central nervous, and psychological effects, that are at least partly independent of its effects on blood pressure, have had a profound influence on raising clinical and research interest in PA. Such research on patients with PA has, in turn, furthered knowledge regarding aldosterone synthesis, regulation, and effects. This review summarizes current progress in our understanding of the physiology of aldosterone, and towards defining the causes (including genetic bases), epidemiology, outcomes, and clinical approaches to diagnostic workup (including screening, diagnostic confirmation, and subtype differentiation) and treatment of PA.

Steroidogenesis-adrenal cell signal transduction

The purpose of this article is to review fundamentals in adrenal gland histophysiology. Key findings regarding the important signaling pathways involved in the regulation of steroidogenesis and adrenal growth are summarized. We illustrate how adrenal gland morphology and function are deeply interconnected in which novel signaling pathways (Wnt, Sonic hedgehog, Notch, β-catenin) or ionic channels are required for their integrity. Emphasis is given to exploring the mechanisms and challenges underlying the regulation of proliferation, growth, and functionality. Also addressed is the fact that while it is now well-accepted that steroidogenesis results from an enzymatic shuttle between mitochondria and endoplasmic reticulum, key questions still remain on the various aspects related to cellular uptake and delivery of free cholesterol. The significant progress achieved over the past decade regarding the precise molecular mechanisms by which the two main regulators of adrenal cortex, adrenocorticotropin hormone (ACTH) and angiotensin II act on their receptors is reviewed, including structure-activity relationships and their potential applications. Particular attention has been given to crucial second messengers and how various kinases, phosphatases, and cytoskeleton-associated proteins interact to ensure homeostasis and/or meet physiological demands. References to animal studies are also made in an attempt to unravel associated clinical conditions. Many of the aspects addressed in this article still represent a challenge for future studies, their outcome aimed at providing evidence that the adrenal gland, through its steroid hormones, occupies a central position in many situations where homeostasis is disrupted, thus highlighting the relevance of exploring and understanding how this key organ is regulated. © 2014 American Physiological Society. Compr Physiol 4:889-964, 2014.

Angiotensin II negatively modulates L-type calcium channels through a pertussis toxin-sensitive G protein in adrenal glomerulosa cells

In bovine adrenal glomerulosa cells, angiotensin II and extracellular K+ stimulate aldosterone secretion in a calcium-dependent manner. In these cells, physiological concentrations of extracellular potassium activate both T-type (low threshold) and L-type (high threshold) voltage-operated calcium channels. Paradoxically, the cytosolic calcium response to 9 mM K+ is inhibited by angiotensin II. Because K+-induced calcium changes observed in the cytosol are almost exclusively due to L-type channel activity, we therefore studied the mechanisms of L-type channel regulation by angiotensin II. Using the patch-clamp method in its perforated patch configuration, we observed a marked inhibition (by 63%) of L-type barium currents in response to angiotensin II. This effect of the hormone was completely prevented by losartan, a specific antagonist of the AT1 receptor subtype. Moreover, this inhibition was strongly reduced when the cells were previously treated for 1 night with pertussis toxin. An effect of pertussis toxin was also observed on the modulation by angiotensin II of the K+ (9 mM)-induced cytosolic calcium response in fura-2-loaded cells, as well as on the angiotensin II-induced aldosterone secretion, at both low (3 mM) and high (9 mM) K+ concentrations. Finally, the expression of both Go and Gi proteins in bovine glomerulosa cells was detected by immunoblotting. Altogether, these results strongly suggest that in bovine glomerulosa cells, a pertussis toxin-sensitive G protein is involved in the inhibition of L-type channel activity induced by angiotensin II.

Characterization of an ACTH-induced chloride current in rat adrenal zona glomerulosa cells

Adrenocorticotropic hormone (ACTH) is one of the principal activator of aldosterone secretion in rat zona glomerulosa cells, but its action on chloride currents is not well established. Here, we demonstrate that the hormone provoked a transient increase in a chloride current with a small unitary conductance estimated at 3.35 pS. Amplitude, as well as time-dependent increase of the ACTH-induced chloride current was independent of the intracellular cAMP concentration. In contrary, its decrease was sensitive to alkaline phosphatase and PKA-inhibitor H-89, indicating that protein phosphorylation, at least in part via PKA, is involved in the decline of the current.

Aldosterone release during the sleep-wake cycle in humans

The aim of this study was to assess the relative influence on the 24-h aldosterone profile of the adrenocorticotropic system, primarily modulated by a circadian rhythmicity, and the renin-angiotensin system, which is influenced by sleep. Cortisol, plasma renin activity (PRA), and aldosterone were measured for 24 h in healthy subjects under basal conditions, once with nocturnal sleep and once with a night of sleep deprivation followed by 8 h of daytime sleep. The sleep period displayed high mean aldosterone levels, pulse amplitude, and frequency that were reduced during waking periods. During sleep, aldosterone pulses were mainly related to PRA oscillations, whereas they were mainly associated with cortisol pulses during waking periods. Cross-correlation analysis between sleep electroencephalographic activity in the delta band and aldosterone levels yielded significant results, aldosterone following delta waves by approximately 30 min. This study demonstrates that the 24-h aldosterone profile is strongly influenced by sleep processes. A dual influence, by the renin-angiotensin system during sleep and by the adrenocorticotropic system during wakefulness, is exerted on aldosterone pulses throughout the 24-h period.

Dopamine receptors: from structure to function

The diverse physiological actions of dopamine are mediated by at least five distinct G protein-coupled receptor subtypes. Two D1-like receptor subtypes (D1 and D5) couple to the G protein Gs and activate adenylyl cyclase. The other receptor subtypes belong to the D2-like subfamily (D2, D3, and D4) and are prototypic of G protein-coupled receptors that inhibit adenylyl cyclase and activate K+ channels. The genes for the D1 and D5 receptors are intronless, but pseudogenes of the D5 exist. The D2 and D3 receptors vary in certain tissues and species as a result of alternative splicing, and the human D4 receptor gene exhibits extensive polymorphic variation. In the central nervous system, dopamine receptors are widely expressed because they are involved in the control of locomotion, cognition, emotion, and affect as well as neuroendocrine secretion. In the periphery, dopamine receptors are present more prominently in kidney, vasculature, and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. Numerous genetic linkage analysis studies have failed so far to reveal unequivocal evidence for the involvement of one of these receptors in the etiology of various central nervous system disorders. However, targeted deletion of several of these dopamine receptor genes in mice should provide valuable information about their physiological functions.

Inhibition of the T-type Ca2+ current by the dopamine D1 receptor in rat adrenal glomerulosa cells: requirement of the combined action of the G betagamma protein subunit and cyclic adenosine 3',5'-monophosphate

Modulation of ionic Ca2+ currents by dopamine (DA) could play a pivotal role in the control of steroid secretion by the rat adrenal glomerulosa cells. In the present study, we report that DA decreases the T-type Ca2+ current amplitude in these cells. The use of pharmacological agonists and antagonists reveals that this effect is mediated by activation of the D1-like receptors. Modulation by cAMP is complex inasmuch as preincubation of the cells with 8-Br-cAMP or the specific adenylyl cyclase inhibitor, 2',3'-dideoxyadenosine, have no effect per se, but prevent the DA-induced inhibition. The inhibitory effect of DA was abolished by addition of GDPbetaS to the pipette medium but not by pertussis toxin. If a cell is dialyzed with medium containing G alpha(s)-GDP, the inhibitory effect is reduced and cannot be recovered by the addition of GTPgammaS, indicating that the alpha(s) is not involved, but rather the betagamma-subunit. Indeed, DA-induced inhibition was mimicked by G betagamma in the pipette and 8-Br-cAMP in the bath. Similarly, G betagamma release from the activation of the AT1 receptor of angiotensin II did affect the current amplitude only in the presence of 8-Br-cAMP in the bath. The mitogen-activated protein kinase cascade, which can be activated by receptors coupled to Gs, was not involved as shown by the lack of activation of p42mapk by DA and the absence of effect of the mitogen-activated protein kinase inhibitor, PD 098059, on the DA-induced inhibition. Because the binding of G betagamma-subunits to various effectors involves the motif QXXER, we therefore tested the effect of the QEHA peptide on the inhibition of the T-type Ca2+ current induced by DA. The peptide, added to the medium pipette (200 microM), abolished the effect of DA. We conclude that the presence of the G betagamma and an increase in cAMP concentration are both required to inhibit the T-type Ca2+ current in rat adrenal glomerulosa cells.

Innervation of the adrenal cortex, its physiological relevance, with primary focus on the noradrenergic transmission

The current knowledge of the catecholaminergic innervation of the mammalian adrenal cortex is summarized, and macro- and microscopic neuromorphology, including the central nervous system connections of the adrenal cortex, is briefly discussed. Morphological and functional data on the catecholaminergic (i.e., noradrenergic) innervation of the adrenal cortex are reviewed. Experimental data suggest that in addition to the regulation of adrenal blood flow, the noradrenergic innervation has a primary influence on zona glomerulosa cells possibly via beta 1 adrenergic and dopaminergic receptors (DA2 subtype via inhibiting T-type Ca2+ channels) It is concluded that the local, modulatory effect of noradrenergic nerve fibres, terminating in the close vicinity of the zona glomerulosa cells, on the systemic renin-angiotensin-aldosterone and other peptide cascade may be influenced by neuropeptides, particularly neuropeptide Y and vasoactive intestinal peptide.

Localization of dopamine D1A receptor protein and messenger ribonucleic acid in rat adrenal cortex

Pharmacological, physiological, and autoradiographic studies have suggested the presence of dopamine receptors in the adrenal gland. Dopaminergic ligands have been shown to modulate adrenocortical aldosterone biosynthesis and secretion as well as adrenomedullary catecholamine production and release. Using a combination of light microscopic immunochemistry and in situ amplification and hybridization, the present study sought to determine the site-specific expression of the recently cloned D1A receptor subtype in rat adrenal gland. Light microscopic immunohistochemistry was conducted using polyclonal antisera raised to the putative rat D1A receptor. Immunoreactive product was detected using an avidin-biotin immunoperoxidase method. D1A receptor messenger RNA (mRNA) was detected using a transcription-based isothermal in situ amplification and hybridization approach using receptor-specific mRNA oligonucleotide probes. The amplified product was localized using an alkaline phosphatase 4-nitro blue tetrazolium chloride/5-bromo-4-chloro-3-indolyl-phosphate technique. This combined experimental approach, using both receptor subtype-selective antibodies and oligonucleotide probes, allows for the site-specific localization of the D1A receptor subtype, which would otherwise not be possible with the pharmacological methods currently available. The D1A receptor protein and mRNA were expressed solely in the zona glomerulosa of the rat adrenal gland, with no signal evident in any of the other cortical layers or in the medulla. Such a distribution raises the possibility that the D1A receptor subtype could modulate, at least in part, some of the known effects of dopamine on aldosterone secretion.

Dual effects of fluoroaluminate on activation of calcium influx and inhibition of agonist-induced calcium mobilization in rat glomerulosa cells

Results presented in this study demonstrate that, in rat glomerulosa cells, fluoroaluminate (AlF4-) alone stimulates both cAMP accumulation (maximal stimulation 10-fold, ED50, 24 mM) and total inositol phosphate accumulation (maximal stimulation 12-fold, ED50 14 mM). Despite a transient accumulation of Ins(1,4,5)P3 after AlF4- stimulation, no rapid and transient intracellular calcium mobilization was observed. In contrast to angiotensin II (Ang II) or vasopressin (AVP), AlF4- induces only a slow and sustained increase in intracellular Ca2+. We demonstrate that this increase results from a Ca2+ influx mediated by cAMP-protein kinase A (PKA) pathway since preincubation with H-89, a potent PKA inhibitor, inhibits this influx. Moreover, a short preincubation (15 min at 37 degrees C) of cells with AlF4- or ACTH prevents the initial release of Ca2+ from intracellular stores induced by Ang II, but does not affect the amount of InsPs accumulated under Ang II stimulation. This rapid inhibition of Ang II action is mediated by ACTH- or AlF4(-)-stimulated cAMP production since pretreatment with H-89 leads to a complete reversal. cAMP most likely acts at the level of Ins(1,4,5)P3 receptors since an increase in intracellular cAMP blunts the calcium response induced by addition of exogenous Ins(1,4,5)P3 to permeabilized cells. These results point out that, in rat glomerulosa cells, activation of the cAMP pathway can induce a rapid desensitization of the phospholipase C pathway by acting downstream of inositol phosphate accumulation.

Metoclopramide stimulates kaliuresis during felodipine without affecting its natriuresis

Calcium entry blockers such as felodipine induce natriuresis without a parallel rise of potassium excretion. Previous studies with exogenous aldosterone and felodipine have suggested that the absence of kaliuresis might be explained by a felodipine-induced inhibition of aldosterone release. The natriuresis with calcium entry blockers could not be attributed to a similar mechanism but might be due to the stimulation of intrarenal natriuretic systems such as the dopaminergic system. We studied whether the aselective dopamine antagonist metoclopramide prevents the natriuresis with low and therapeutic felodipine doses and whether metoclopramide-induced aldosterone release promotes kaliuresis with felodipine. Twelve healthy male volunteers participated in a randomized, placebo-controlled, crossover study comparing felodipine infusion during metoclopramide with felodipine alone. Metoclopramide had no significant influence on the pronounced and dose-dependent increases of renal plasma flow and urinary sodium excretion with felodipine. Metoclopramide increased plasma aldosterone concentration from 0.17 +/- 0.03 to 0.60 +/- 0.14 nmol/L, and subsequent felodipine infusion clearly increased urinary potassium excretion by 23 +/- 6 and 35 +/- 8 mumol/min (low and therapeutic doses, respectively). In contrast, potassium excretion remained stable with felodipine alone (+5 +/- 4 and +7 +/- 5 mumol/min, respectively). In conclusion, the natriuretic action of calcium entry blockers cannot be blocked by the aselective dopamine antagonist metoclopramide. This natriuresis is accompanied by kaliuresis only in the presence of elevated endogenous aldosterone concentrations. The ability of calcium entry blockers to prevent a rise of plasma aldosterone thus seems essential for the prevention of urinary potassium losses.

A model for studying regulation of aldosterone secretion: freshly isolated cells or cultured cells?

Practically all studies relating to zona glomerulosa function have been performed either with freshly isolated cells or with cells used after 2 or 3 days in culture. This study compares the step-by-step response (binding, second messenger production and aldosterone response) of isolated glomerulosa cells vs cells maintained in primary culture to the main stimuli of aldosterone secretion. One day in culture induces a decrease of 77 and 65% in the basal level of corticosterone and aldosterone secretions, compared to that observed in freshly isolated cells. In these conditions, the cells become more sensitive to most of their stimuli, but not all: e.g. important differences are noted in the dose-response of aldosterone secretion to adrenocorticotropin (ACTH), which is often shifted to a lower concentration sensitivity in cultured cells. For example, 0.1 nM ACTH stimulates steroid secretion by three-fold in isolated cells while 1 pM ACTH already induces a 25 and nine-fold increase, respectively, in corticosterone and aldosterone output in cultured cells. Moreover, some stimuli such as isoproterenol do not have any effect in isolated cells but do stimulate steroid secretion in cultured cells. In contrast, other stimuli, such as serotonin or DA (via DA2 receptors) act preferentially in freshly isolated cells. The main observation derived from this study is that glomerulosa cells, under appropriate conditions, are able to respond to their main secretagogues even after 4 days in culture. At this time, glomerulosa cells maintain their ultrastructural characteristics and functional properties and, aside from a few exceptions, demonstrate higher sensitivity to their known stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)