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

Gαq protein-biased ligand of angiotensin II type 1 receptor mediates myofibroblast differentiation through TGF-β1/ERK axis in human cardiac fibroblasts

Angiotensin II receptors are members of G protein-coupled receptor superfamily that manifest biased signals toward G protein- and β-arrestin-dependent pathways. However, the role of angiotensin II receptor-biased ligands and the mechanisms underlying myofibroblast differentiation in human cardiac fibroblasts have not been fully elucidated. Our results demonstrated that antagonism of angiotensin II type 1 receptor (AT₁ receptor) and blockade of G_αq protein suppressed angiotensin II (Ang II)-induced fibroblast proliferation, overexpression of collagen I and α-smooth muscle actin (α-SMA), and stress fibre formation, indicating the AT₁ receptor/G_αq axis is necessary for fibrogenic effects of Ang II. Stimulation of AT₁ receptors by their G_αq-biased ligand (TRV120055), but not β-arrestin-biased ligand (TRV120027), substantially exerted fibrogenic effects at a level similar to that of Ang II, suggesting that AT₁ receptor induced cardiac fibrosis in a G_αq-dependent and β-arrestin-independent manner. Valsartan prevents TRV120055-mediated fibroblast activation. TRV120055 mediated the upregulation of transforming growth factor-beta1 (TGF-β1) through the AT₁ receptor/G_αq cascade. In addition, G_αq protein and TGF-β1 were necessary for ERK1/2 activation induced by Ang II and TRV120055. Collectively, TGF-β1 and ERK1/2 are downstream effectors of the G_αq-biased ligand of AT₁ receptor for the induction of cardiac fibrosis.

Ion Channel Function and Electrical Excitability in the Zona Glomerulosa: A Network Perspective on Aldosterone Regulation

Aldosterone excess is a pathogenic factor in many hypertensive disorders. The discovery of numerous somatic and germline mutations in ion channels in primary hyperaldosteronism underscores the importance of plasma membrane conductances in determining the activation state of zona glomerulosa (zG) cells. Electrophysiological recordings describe an electrically quiescent behavior for dispersed zG cells. Yet, emerging data indicate that in native rosette structures in situ, zG cells are electrically excitable, generating slow periodic voltage spikes and coordinated bursts of Ca²⁺ oscillations. We revisit data to understand how a multitude of conductances may underlie voltage/Ca²⁺ oscillations, recognizing that zG layer self-renewal and cell heterogeneity may complicate this task. We review recent data to understand rosette architecture and apply maxims derived from computational network modeling to understand rosette function. The challenge going forward is to uncover how the rosette orchestrates the behavior of a functional network of conditional oscillators to control zG layer performance and aldosterone secretion.

Stretch-Induced Biased Signaling in Angiotensin II Type 1 and Apelin Receptors for the Mediation of Cardiac Contractility and Hypertrophy

The myocardium has an intrinsic ability to sense and respond to mechanical load in order to adapt to physiological demands. Primary examples are the augmentation of myocardial contractility in response to increased ventricular filling caused by either increased venous return (Frank-Starling law) or aortic resistance to ejection (the Anrep effect). Sustained mechanical overload, however, can induce pathological hypertrophy and dysfunction, resulting in heart failure and arrhythmias. It has been proposed that angiotensin II type 1 receptor (AT1R) and apelin receptor (APJ) are primary upstream actors in this acute myocardial autoregulation as well as the chronic maladaptive signaling program. These receptors are thought to have mechanosensing capacity through activation of intracellular signaling via G proteins and/or the multifunctional transducer protein, β-arrestin. Importantly, ligand and mechanical stimuli can selectively activate different downstream signaling pathways to promote inotropic, cardioprotective or cardiotoxic signaling. Studies to understand how AT1R and APJ integrate ligand and mechanical stimuli to bias downstream signaling are an important and novel area for the discovery of new therapeutics for heart failure. In this review, we provide an up-to-date understanding of AT1R and APJ signaling pathways activated by ligand versus mechanical stimuli, and their effects on inotropy and adaptive/maladaptive hypertrophy. We also discuss the possibility of targeting these signaling pathways for the development of novel heart failure therapeutics.

Mechanoactivation of the angiotensin II type 1 receptor induces β-arrestin-biased signaling through Gαi coupling

Ligand activation of the angiotensin II type 1 receptor (AT1R), a member of the G protein-coupled receptor (GPCR) family, stimulates intracellular signaling to mediate a variety of physiological responses. The AT1R is also known to be a mechanical sensor. When activated by mechanical stretch, the AT1R can signal via the multifunctional adaptor protein β-arrestin, rather than through classical heterotrimeric G protein pathways. To date, the AT1R conformation induced by membrane stretch in the absence of ligand was thought to be the same as that induced by β-arrestin-biased agonists, which selectively engage β-arrestin thereby preventing G protein coupling. Here, we show that in contrast to the β-arrestin-biased agonists TRV120023 and TRV120026, membrane stretch uniquely promotes the coupling of the inhibitory G protein (Gα_i ) to the AT1R to transduce signaling. Stretch-triggered AT1R-Gα_i coupling is required for the recruitment of β-arrestin2 and activation of downstream signaling pathways, such as EGFR transactivation and ERK phosphorylation. Our findings demonstrate additional complexity in the mechanism of receptor bias in which the recruitment of Gα_i is required for allosteric mechanoactivation of the AT1R-induced β-arrestin-biased signaling.

Prolonged AT1R activation induces CaV1.2 channel internalization in rat cardiomyocytes

The cardiac L-type calcium channel is a multi-subunit complex that requires co-assembling of the pore-forming subunit Ca_V1.2 with auxiliary subunits Ca_Vα₂δ and Ca_Vβ. Its traffic has been shown to be controlled by these subunits and by the activation of various G-protein coupled receptors (GPCR). Here, we explore the consequences of the prolonged activation of angiotensin receptor type 1 (AT₁R) over Ca_V1.2 channel trafficking. Bioluminescence Resonance Energy Transfer (BRET) assay between β-arrestin and L-type channels in angiotensin II-stimulated cells was used to assess the functional consequence of AT₁R activation, while immunofluorescence of adult rat cardiomyocytes revealed the effects of GPCR activation on Ca_V1.2 trafficking. Angiotensin II exposure results in β-arrestin₁ recruitment to the channel complex and an apparent loss of Ca_V1.2 immunostaining at the T-tubules. Accordingly, angiotensin II stimulation causes a decrease in L-type current, Ca²⁺ transients and myocyte contractility, together with a faster repolarization phase of action potentials. Our results demonstrate that prolonged AT₁R activation induces β-arrestin₁ recruitment and the subsequent internalization of Ca_V1.2 channels with a half-dose of AngII on the order of 100 nM, suggesting that this effect depends on local renin-angiotensin system. This novel AT₁R-dependent Ca_V1.2-trafficking modulation likely contributes to angiotensin II-mediated cardiac remodeling.

G protein-coupled receptors as promising cancer targets

G protein-coupled receptors (GPCRs) regulate an array of fundamental biological processes, such as growth, metabolism and homeostasis. Specifically, GPCRs are involved in cancer initiation and progression. However, compared with the involvement of the epidermal growth factor receptor in cancer, that of GPCRs have been largely ignored. Recent findings have implicated many GPCRs in tumorigenesis, tumor progression, invasion and metastasis. Moreover, GPCRs contribute to the establishment and maintenance of a microenvironment which is permissive for tumor formation and growth, including effects upon surrounding blood vessels, signaling molecules and the extracellular matrix. Thus, GPCRs are considered to be among the most useful drug targets against many solid cancers. Development of selective ligands targeting GPCRs may provide novel and effective treatment strategies against cancer and some anticancer compounds are now in clinical trials. Here, we focus on tumor related GPCRs, such as G protein-coupled receptor 30, the lysophosphatidic acid receptor, angiotensin receptors 1 and 2, the sphingosine 1-phosphate receptors and gastrin releasing peptide receptor. We also summarize their tissue distributions, activation and roles in tumorigenesis and discuss the potential use of GPCR agonists and antagonists in cancer therapy.

Signaling Interactions in the Adrenal Cortex

The major physiological stimuli of aldosterone secretion are angiotensin II (AII) and extracellular K(+), whereas cortisol production is primarily regulated by corticotropin (ACTH) in fasciculata cells. AII triggers Ca(2+) release from internal stores that is followed by store-operated and voltage-dependent Ca(2+) entry, whereas K(+)-evoked depolarization activates voltage-dependent Ca(2+) channels. ACTH acts primarily through the formation of cAMP and subsequent protein phosphorylation by protein kinase A. Both Ca(2+) and cAMP facilitate the transfer of cholesterol to mitochondrial inner membrane. The cytosolic Ca(2+) signal is transferred into the mitochondrial matrix and enhances pyridine nucleotide reduction. Increased formation of NADH results in increased ATP production, whereas that of NADPH supports steroid production. In reality, the control of adrenocortical function is a lot more sophisticated with second messengers crosstalking and mutually modifying each other's pathways. Cytosolic Ca(2+) and cGMP are both capable of modifying cAMP metabolism, while cAMP may enhance Ca(2+) release and voltage-activated Ca(2+) channel activity. Besides, mitochondrial Ca(2+) signal brings about cAMP formation within the organelle and this further enhances aldosterone production. Maintained aldosterone and cortisol secretion are optimized by the concurrent actions of Ca(2+) and cAMP, as exemplified by the apparent synergism of Ca(2+) influx (inducing cAMP formation) and Ca(2+) release during response to AII. Thus, cross-actions of parallel signal transducing pathways are not mere intracellular curiosities but rather substantial phenomena, which fine-tune the biological response. Our review focuses on these functionally relevant interactions between the Ca(2+) and the cyclic nucleotide signal transducing pathways hitherto described in the adrenal cortex.

NCI-H295R cell line as in vitro model of hyperaldosteronism lacks functional KCNJ5 (GIRK4; Kir3.4) channels

As a major cause of aldosterone producing adenomas, numerous gain-of-function mutations in the KCNJ5 gene (encoding the K(+) channel subunit GIRK4) have been identified. The human adrenocortical carcinoma cell line NCI-H295R is the most frequently used cellular model for in vitro studies related to regulation of aldosterone-synthesis. Because of the undefined role of KCNJ5 (GIRK4) in regulating synthesis of aldosterone, we aimed at identifying basal and G protein-activated GIRK4 currents in this paradigmatic cell line. The GIRK-specific blocker Tertiapin-Q did not affect basal current. Neither loading of the cells with GTP-γ-S via the patch-clamp pipette nor agonist stimulation of an infected A1-adenosine receptor resulted in activation of GIRK current. In cells co-infected with KCNJ5, robust activation of basal and adenosine-activated inward-rectifying current was observed. Although GIRK4 protein can be detected in Western blots of H295R homogenates, we suggest that GIRK4 in aldosterone-producing cells does not form functional G(βγ)-activated channels.

Calcium-dependent mitochondrial cAMP production enhances aldosterone secretion

Glomerulosa cells secrete aldosterone in response to agonists coupled to Ca(2+) increases such as angiotensin II and corticotrophin, coupled to a cAMP dependent pathway. A recently recognized interaction between Ca(2+) and cAMP is the Ca(2+)-induced cAMP formation in the mitochondrial matrix. Here we describe that soluble adenylyl cyclase (sAC) is expressed in H295R adrenocortical cells. Mitochondrial cAMP formation, monitored with a mitochondria-targeted fluorescent sensor (4mtH30), is enhanced by HCO3(-) and the Ca(2+) mobilizing agonist angiotensin II. The effect of angiotensin II is inhibited by 2-OHE, an inhibitor of sAC, and by RNA interference of sAC, but enhanced by an inhibitor of phosphodiesterase PDE2A. Heterologous expression of the Ca(2+) binding protein S100G within the mitochondrial matrix attenuates angiotensin II-induced mitochondrial cAMP formation. Inhibition and knockdown of sAC significantly reduce angiotensin II-induced aldosterone production. These data provide the first evidence for a cell-specific functional role of mitochondrial cAMP.

Regulation of aldosterone synthesis and secretion

Aldosterone is a steroid hormone synthesized in and secreted from the outer layer of the adrenal cortex, the zona glomerulosa. Aldosterone is responsible for regulating sodium homeostasis, thereby helping to control blood volume and blood pressure. Insufficient aldosterone secretion can lead to hypotension and circulatory shock, particularly in infancy. On the other hand, excessive aldosterone levels, or those too high for sodium status, can cause hypertension and exacerbate the effects of high blood pressure on multiple organs, contributing to renal disease, stroke, visual loss, and congestive heart failure. Aldosterone is also thought to directly induce end-organ damage, including in the kidneys and heart. Because of the significance of aldosterone to the physiology and pathophysiology of the cardiovascular system, it is important to understand the regulation of its biosynthesis and secretion from the adrenal cortex. Herein, the mechanisms regulating aldosterone production in zona glomerulosa cells are discussed, with a particular emphasis on signaling pathways involved in the secretory response to the main controllers of aldosterone production, the renin-angiotensin II system, serum potassium levels and adrenocorticotrophic hormone. The signaling pathways involved include phospholipase C-mediated phosphoinositide hydrolysis, inositol 1,4,5-trisphosphate, cytosolic calcium levels, calcium influx pathways, calcium/calmodulin-dependent protein kinases, diacylglycerol, protein kinases C and D, 12-hydroxyeicostetraenoic acid, phospholipase D, mitogen-activated protein kinase pathways, tyrosine kinases, adenylate cyclase, and cAMP-dependent protein kinase. A complete understanding of the signaling events regulating aldosterone biosynthesis may allow the identification of novel targets for therapeutic interventions in hypertension, primary aldosteronism, congestive heart failure, renal disease, and other cardiovascular disorders.

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.

Extramitochondrial OPA1 and adrenocortical function

We have previously described that silencing of the mitochondrial protein OPA1 enhances mitochondrial Ca(2+) signaling and aldosterone production in H295R adrenocortical cells. Since extramitochondrial OPA1 (emOPA1) was reported to facilitate cAMP-induced lipolysis, we hypothesized that emOPA1, via the enhanced hydrolysis of cholesterol esters, augments aldosterone production in H295R cells. A few OPA1 immunopositive spots were detected in ∼40% of the cells. In cell fractionation studies OPA1/COX IV (mitochondrial marker) ratio in the post-mitochondrial fractions was an order of magnitude higher than that in the mitochondrial fraction. The ratio of long to short OPA1 isoforms was lower in post-mitochondrial than in mitochondrial fractions. Knockdown of OPA1 failed to reduce db-cAMP-induced phosphorylation of hormone-sensitive lipase (HSL), Ca(2+) signaling and aldosterone secretion. In conclusion, OPA1 could be detected in the post-mitochondrial fractions, nevertheless, OPA1 did not interfere with the cAMP - PKA - HSL mediated activation of aldosterone secretion.

Enigma homolog 1 scaffolds protein kinase D1 to regulate the activity of the cardiac L-type voltage-gated calcium channel

AIMS

In cardiomyocytes, protein kinase D1 (PKD1) plays a central role in the response to stress signals. From a yeast two-hybrid assay, we have identified Enigma Homolog 1 (ENH1) as a new binding partner of PKD1. Since in neurons, ENH1, associated with protein kinase Cepsilon, was shown to modulate the activity of N-type calcium channels, and the pore-forming subunit of the cardiac L-type voltage-gated calcium channel, alpha1C, possesses a potential phosphorylation site for PKD1, we studied here a possible role of ENH1 and PKD1 in the regulation of the cardiac L-type voltage-gated calcium channel.

METHODS AND RESULTS

PKD1-interacting proteins were searched by yeast two-hybrid screening. In vivo protein interactions in cardiomyocytes isolated from heart ventricles of newborn rats were tested by co-immunoprecipitation. Small interfering RNA and a dominant negative mutant of PKD1 were delivered into cardiomyocytes by use of an adenovirus. Calcium currents were measured by the patch-clamp technique. Both ENH1 and PKD1 interact with alpha1C in cardiomyocytes. This interaction is increased upon stimulation. Silencing of ENH1 prevented the binding of PKD1 to alpha1C. Moreover, a dominant negative mutant of PKD1 or the silencing of ENH1 inhibited the alpha-adrenergic-induced increase of L-type calcium currents.

CONCLUSION

We found a new binding partner, ENH1, and a new target, alpha1C, for PKD1 in neonatal rat cardiomyocytes. We propose a model where ENH1 scaffolds PKD1 to alpha1C in order to form a signalling complex that regulates the activity of cardiac L-type voltage-gated Ca(2+) channels.

Regulation of aldosterone production from zona glomerulosa cells by ANG II and cAMP: evidence for PKA-independent activation of CaMK by cAMP

Aldosterone production in zona glomerulosa (ZG) cells of adrenal glands is regulated by various extracellular stimuli (K(+), ANG II, ACTH) that all converge on two major intracellular signaling pathways: an increase in cAMP production and calcium (Ca(2+)) mobilization. However, molecular events downstream of the increase in intracellular cAMP and Ca(2+) content are controversial and far from being completely resolved. Here, we found that Ca(2+)/calmodulin-dependent protein kinases (CaMKs) play a predominant role in the regulation of aldosterone production stimulated by ANG II, ACTH, and cAMP. The specific CaMK inhibitor KN93 strongly reduced ANG II-, ACTH-, and cAMP-stimulated aldosterone production. In in vitro kinase assays and intact cells, we could show that cAMP-induced activation of CaMK, using the adenylate cyclase activator forskolin or the cAMP-analog Sp-5,6-DCI-cBIMPS (cBIMPS), was not mediated by PKA. Activation of the recently identified cAMP target protein Epac (exchange protein directly activated by cAMP) by 8-pCPT-2'-O-Me-cAMP had no effect on CaMK activity and aldosterone production. Furthermore, we provide evidence that cAMP effects in ZG cells do not involve Ca(2+) or MAPK signaling. Our results suggest that ZG cells, in addition to PKA and Epac/Rap proteins, contain other as yet unidentified cAMP mediator(s) involved in regulating CaMK activity and aldosterone secretion.

Pleiotropic AT1 receptor signaling pathways mediating physiological and pathogenic actions of angiotensin II

Angiotensin II (Ang II) activates a wide spectrum of signaling responses via the AT1 receptor (AT1R) that mediate its physiological control of blood pressure, thirst, and sodium balance and its diverse pathological actions in cardiovascular, renal, and other cell types. Ang II-induced AT1R activation via Gq/11 stimulates phospholipases A2, C, and D, and activates inositol trisphosphate/Ca2+ signaling, protein kinase C isoforms, and MAPKs, as well as several tyrosine kinases (Pyk2, Src, Tyk2, FAK), scaffold proteins (G protein-coupled receptor kinase-interacting protein 1, p130Cas, paxillin, vinculin), receptor tyrosine kinases, and the nuclear factor-kappaB pathway. The AT1R also signals via Gi/o and G11/12 and stimulates G protein-independent signaling pathways, such as beta-arrestin-mediated MAPK activation and the Jak/STAT. Alterations in homo- or heterodimerization of the AT1R may also contribute to its pathophysiological roles. Many of the deleterious actions of AT1R activation are initiated by locally generated, rather than circulating, Ang II and are concomitant with the harmful effects of aldosterone in the cardiovascular system. AT1R-mediated overproduction of reactive oxygen species has potent growth-promoting, proinflammatory, and profibrotic actions by exerting positive feedback effects that amplify its signaling in cardiovascular cells, leukocytes, and monocytes. In addition to its roles in cardiovascular and renal disease, agonist-induced activation of the AT1R also participates in the development of metabolic diseases and promotes tumor progression and metastasis through its growth-promoting and proangiogenic activities. The recognition of Ang II's pathogenic actions is leading to novel clinical applications of angiotensin-converting enzyme inhibitors and AT1R antagonists, in addition to their established therapeutic actions in essential hypertension.

Regulation of receptor-coupling to (multiple) G proteins. A challenge for basic research and drug discovery

G protein-coupled receptors induce intracellular signals via interaction of with cytosolic/peripheral membrane proteins, mainly G proteins. There has been much debate about the mode of interaction between the receptors, G proteins and effectors, their mobility and the ways of determining the specificity of interaction. Additional complexity has been added to system upon the discovery of i) coupling of single receptors to several G proteins and ii) active direction of this by different ligands (stimulus trafficking). These data suggest that the most primary unit in the signal transduction is the receptor complexed with a specific G protein, making the investigation of the mechanism of receptor-G protein selection and interaction even more important. In this review, I will summarize the general knowledge of receptor interaction with G proteins and effectors and the ways of investigating this.

Unconventional homologous internalization of the angiotensin II type-1 receptor induced by G-protein-independent signals

Internalization of a G-protein-coupled receptor (GPCR) is essential to the desensitization, endocytosis, and signal transduction of the receptor. It has been the general view that conventional homologous internalization of a GPCR requires activation of the G-protein(s) coupled to the receptor. However, whether and how GPCR-mediated G-protein-independent signals trigger receptor internalization remains unknown, although G-protein-independent internalization has been reported. Here we show that an angiotensin II (Ang II) type-1 (AT1) receptor mutant incapable of activating any G-protein still undergoes normal internalization. Substitution of Asp125 with Ala and Arg126 with Leu at the highly conserved DRY motif of the AT1 receptor disabled the ability of the receptor to activate G-proteins, as shown by various Ang II binding studies, GDP-GTP exchange, and inositol phosphate production assays. Surprisingly, the mutant internalized normally in the presence of Ang II and transactivated the epidermal growth factor receptor (EGFR). Similar to the wild-type receptor, overexpression of a dominant-negative K220R mutant GRK2 diminished the internalization of D125A-R126L but not the transactivation of EGFR. These data indicate that G-protein-independent specific signals may also trigger homologous internalizations of the AT1 receptor through beta-arrestin-dependent and -independent pathways, suggesting a possible mechanism for G-protein-independent activation of G-protein-coupled receptor kinases (GRKs). This may represent a general mechanism for triggering GPCR internalization.

TREK-1 K+ channels couple angiotensin II receptors to membrane depolarization and aldosterone secretion in bovine adrenal glomerulosa cells

Bovine adrenal glomerulosa (AZG) cells were shown to express bTREK-1 background K(+) channels that set the resting membrane potential and couple angiotensin II (ANG II) receptor activation to membrane depolarization and aldosterone secretion. Northern blot and in situ hybridization studies demonstrated that bTREK-1 mRNA is uniformly distributed in the bovine adrenal cortex, including zona fasciculata and zona glomerulosa, but is absent from the medulla. TASK-3 mRNA, which codes for the predominant background K(+) channel in rat AZG cells, is undetectable in the bovine adrenal cortex. In whole cell voltage clamp recordings, bovine AZG cells express a rapidly inactivating voltage-gated K(+) current and a noninactivating background K(+) current with properties that collectively identify it as bTREK-1. The outwardly rectifying K(+) current was activated by intracellular acidification, ATP, and superfusion of bTREK-1 openers, including arachidonic acid (AA) and cinnamyl 1-3,4-dihydroxy-alpha-cyanocinnamate (CDC). Bovine chromaffin cells did not express this current. In voltage and current clamp recordings, ANG II (10 nM) selectively inhibited the noninactivating K(+) current by 82.1 +/- 6.1% and depolarized AZG cells by 31.6 +/- 2.3 mV. CDC and AA overwhelmed ANG II-mediated inhibition of bTREK-1 and restored the resting membrane potential to its control value even in the continued presence of ANG II. Vasopressin (50 nM), which also physiologically stimulates aldosterone secretion, inhibited the background K(+) current by 73.8 +/- 9.4%. In contrast to its potent inhibition of bTREK-1, ANG II failed to alter the T-type Ca(2+) current measured over a wide range of test potentials by using pipette solutions of identical nucleotide and Ca(2+)-buffering compositions. ANG II also failed to alter the voltage dependence of T channel activation under these same conditions. Overall, these results identify bTREK-1 K(+) channels as a pivotal control point where ANG II receptor activation is transduced to depolarization-dependent Ca(2+) entry and aldosterone secretion.

Direct and remote modulation of L-channels in chromaffin cells: distinct actions on alpha1C and alpha1D subunits?

Understanding precisely the functioning of voltage-gated Ca2+ channels and their modulation by signaling molecules will help clarifying the Ca(2+)-dependent mechanisms controlling exocytosis in chromaffin cells. In recent years, we have learned more about the various pathways through which Ca2+ channels can be up- or down-modulated by hormones and neurotransmitters and how these changes may condition chromaffin cell activity and catecolamine release. Recently, the attention has been focused on the modulation of L-channels (CaV 1), which represent the major Ca2+ current component in rat and human chromaffin cells. L-channels are effectively inhibited by the released content of secretory granules or by applying mixtures of exogenous ATP, opioids, and adrenaline through the activation of receptor-coupled G proteins. This unusual inhibition persists in a wide range of potentials and results from a direct (membrane-delimited) interaction of G protein subunits with the L-channels co-localized in membrane microareas. Inhibition of L-channels can be reversed when the cAMP/PKA pathway is activated by membrane permeable cAMP analog or when cells are exposed to isoprenaline (remote action), suggesting the existence of parallel and opposite effects on L-channel gating by distinctly activated membrane autoreceptors. Here, the authors review the molecular components underlying these two opposing signaling pathways and present new evidence supporting the presence of two L-channel types in rat chromaffin cells (alpha1C and alpha1D), which open new interesting issues concerning Ca(2+)-channel modulation. In light of recent findings on the regulation of exocytosis by Ca(2+)-channel modulation, the authors explore the possible role of L-channels in the autocontrol of catecholamine release.

Control of aldosterone secretion: a model for convergence in cellular signaling pathways

Aldosterone secretion by glomerulosa cells is stimulated by angiotensin II (ANG II), extracellular K(+), corticotrophin, and several paracrine factors. Electrophysiological, fluorimetric, and molecular biological techniques have significantly clarified the molecular action of these stimuli. The steroidogenic effect of corticotrophin is mediated by adenylyl cyclase, whereas potassium activates voltage-operated Ca(2+) channels. ANG II, bound to AT(1) receptors, acts through the inositol 1,4,5-trisphosphate (IP(3))-Ca(2+)/calmodulin system. All three types of IP(3) receptors are coexpressed, rendering a complex control of Ca(2+) release possible. Ca(2+) release is followed by both capacitative and voltage-activated Ca(2+) influx. ANG II inhibits the background K(+) channel TASK and Na(+)-K(+)-ATPase, and the ensuing depolarization activates T-type (Ca(v)3.2) Ca(2+) channels. Activation of protein kinase C by diacylglycerol (DAG) inhibits aldosterone production, whereas the arachidonate released from DAG in ANG II-stimulated cells is converted by lipoxygenase to 12-hydroxyeicosatetraenoic acid, which may also induce Ca(2+) signaling. Feedback effects and cross-talk of signal-transducing pathways sensitize glomerulosa cells to low-intensity stimuli, such as physiological elevations of [K(+)] (< or =1 mM), ANG II, and ACTH. Ca(2+) signaling is also modified by cell swelling, as well as receptor desensitization, resensitization, and downregulation. Long-term regulation of glomerulosa cells involves cell growth and proliferation and induction of steroidogenic enzymes. Ca(2+), receptor, and nonreceptor tyrosine kinases and mitogen-activated kinases participate in these processes. Ca(2+)- and cAMP-dependent phosphorylation induce the transfer of the steroid precursor cholesterol from the cytoplasm to the inner mitochondrial membrane. Ca(2+) signaling, transferred into the mitochondria, stimulates the reduction of pyridine nucleotides.

Angiotensin II-induced ionic currents and signalling pathways in submandibular ganglion neurons

Angiotensin II (Ang II) is one of the most important vasoconstrictive hormones but is also known to act as a neuromodulator and a neurotransmitter in the central and peripheral nervous system. The submandibular ganglion (SMG) neuron is a parasympathetic ganglion which receives inputs from preganglionic cholinergic neurons, and innervates the submandibular salivary gland to control saliva secretion. In this study, the effects of Ang II on SMG neurons were investigated using the whole-cell patch clamp technique. Membrane currents evoked by a ramp pulse from +50 to -100 mV (-150 mV/500 ms) were compared in both the absence and presence of Ang II. In eight neurons tested, 1 microM Ang II increased inward currents by 42.0+/-8.2%. The reversal potentials of the Ang II-induced current were 0.2+/-0.6 mV. These increase of inward currents by Ang II were antagonized by losartan, a selective antagonist of AT(1) receptors. Intracellular dialysis with 0.1mM guanosin 5'-O-(2-thiodiphosphate) (GDP-beta-S), a G-proteins blocker, and anti-G(q/11) antibody attenuated Ang II-induced ionic current. In addition, pretreatment of neurons with 10 microM staurosporine (stauro), a protein kinase C (PKC) inhibitor, 0.5 microM PMA, a PKC activator, and 10 microM KN-93, a Ca2+/calmodulin-dependent protein kinase II (CaM K II) inhibitor, attenuated Ang II-induced ionic current in SMG neurons. The data presented here demonstrated that Ang II-induced ionic current via G(q/11)-proteins involving both PKC and CaM K II pathways in SMG neurons.

Effect of angiotensin II on calcium reabsorption by the luminal membranes of the nephron

In the rat and the rabbit, a number of studies have reported the effects of angiotensin II (ANG II) on Na(+) reabsorption by the proximal (PT) and distal (DT) convoluted tubules of the kidney. The aim of the present study was to examine the effect of ANG II on Ca(2+) uptake by the luminal membranes of the PT and DT of the rabbit. Incubation of PT and DT with 10(-12) M ANG II enhanced the initial Ca(2+) uptake in the two segments. Dose-response experiments revealed, for Ca(2+) as well as for Na(+) transport, a biphasic action with a maximal effect at 10(-12) M. Ca(2+) transport by the DT luminal membrane presents a dual kinetic. ANG II action influenced the high-affinity Ca(2+) channel, increasing maximal velocity from 0.72 +/- 0.03 to 0.90 +/- 0.05 pmol x microg(-1) x 10 s(-1) (P < 0.05, n = 3) and leaving the Michaelis-Menten constant unchanged. The effect of ANG II was abolished by losartan, suggesting that the hormone is acting through AT1 receptors. In the PT, calphostin C inhibited the effect of the hormone. It is therefore probable that protein kinase C is involved as a messenger. In the DT, however, neither Rp cAMP, calphostin C, nor econazole (a phospholipase A inhibitor) influenced the hormone action. Therefore, the mechanisms involved in the hormone action remain undetermined. Finally, we questioned whether ANG II acts in the same DT segment as does parathyroid hormone on Ca(2+) transport. The two hormones increased Ca(2+) transport, but their actions were not additive, suggesting that they both influence the same channels in the same segment of the distal nephron, i.e., the segment responsible for the high-affinity calcium channel.

Effect of insulin and angiotensin II on cell calcium in human skin fibroblasts

We have recently shown that insulin attenuates angiotensin II-induced intracellular Ca(2+) mobilization in human skin fibroblasts from normotensive subjects. This study was designed to investigate the effects of angiotensin II and the interactions between insulin and angiotensin II on intracellular Ca(2+) mobilization in skin fibroblasts from patients with essential hypertension. Fibroblasts were obtained from 9 normotensives and 18 hypertensives. Spectrofluorophotometric free Ca(2+) measurement was performed in monolayers of 24-hour serum-deprived cells. Resting intracellular Ca(2+) level and angiotensin II-stimulated intracellular Ca(2+) peak were higher in fibroblasts from hypertensives compared with those from normotensives. The effect of acute insulin exposure was evaluated in fibroblasts from hypertensives subdivided on the basis of insulin sensitivity. In insulin-sensitive hypertensives, insulin significantly blunted the effects of angiotensin II on intracellular Ca(2+) response, whereas in insulin-resistant patients, insulin did not modify intracellular Ca(2+) response to angiotensin II. Pertussis toxin, a G(ialpha)-inhibitor, reduced angiotensin II-stimulated Ca(2+) peak in insulin-sensitive but not in insulin-resistant hypertensives. In conclusion, the effects of angiotensin II on intracellular Ca(2+) mobilization are more pronounced in fibroblasts from hypertensives compared with those from normotensives, and the inhibitory effect of insulin is blunted in insulin-resistant hypertensives by a G(ialpha) pertussis toxin-sensitive abnormality.

Role of membrane depolarization and T-type Ca2+ channels in angiotensin II and K+ stimulated aldosterone secretion

The hypothesis that Ca2+ influx necessary for angiotensin II (AngII) and K+ stimulation of aldosterone secretion is primarily mediated by membrane depolarization and activation of T-type Ca2+ channels was examined in isolated rat adrenal glomerulosa cells. Perforated-patch clamp recordings of membrane potential (Vm) demonstrated that AngII and K+ induce concentration-dependent depolarizations capable of activating T channels and, at high K+ and AngII concentrations, activating L channels and inactivating T channels. K+-induced depolarizations were stable and readily reversible. Vm was proportional to K+ concentration, exhibiting a linear slope of 53.7 mV per 10-fold increase in K+. AngII-induced depolarizations were complex, consisting of a slow maintained component superimposed with small amplitude depolarizing fluctuations. Slow oscillations in Vm were occasionally observed in response to 10(-9) M AngII or greater. The slow, maintained component of depolarization coincided with inhibition of K+ conductance. Neither rapid fluctuations nor slow oscillations in Vm were blocked by mibefradil or other treatments that inhibit voltage-gated Ca2+ channels. Perforated-patch clamp experiments also demonstrated that AngII (10(-8) M) inhibited L channels by 45.6% without affecting T channels. Thus AngII activates T channels by depolarization rather than T channel modulation in rat cells. The concentration dependencies of mibefradil inhibition of T channels and AngII- and K+-induced aldosterone secretion were compared. Under whole-cell patch clamp mibefradil induced a concentration-dependent inhibition of T channels, exhibiting a K(app) of 0.62 microM. Mibefradil inhibition was use-dependent but mibefradil neither acted as an open channel blocker nor significantly affected T channel inactivation or activation. Mibefradil inhibited K+- and AngII-induced secretion at concentrations similar to that for T channel inhibition; at high concentrations (10 microM) mibefradil inhibited AngII-induced secretion by 88% and completely inhibited K+-induced secretion. The IC50 for K+-induced secretion was dependent on K+ concentration, increasing from 0.2 microM for 6 mM K+ to 2.5 microM for 10 mM K+ or greater. Mibefradil exhibited an IC50 of 1.1 microM for inhibition of secretion at all AngII concentrations examined (0.1, 1.0, and 10 nM). Mibefradil also exhibited multiple nonspecific effects, which complicated the assessment of T channel function, including; inhibition of leak and voltage-dependent K+ conductances, inhibition of Ca2+-independent aldosterone secretion, and inhibition of secretion under conditions expected to completely inactivate T channels (10 nM AngII or 20 mM K+). In summary, these results indicate that voltage-gated T channels represent the primary Ca2+ influx pathway activated by physiological concentrations of AngII and K+ but other Ca2+ influx pathways must mediate aldosterone secretion induced by high K+ or AngII concentrations.

Angiotensin II and calcium channels

Sixty years after its initial discovery, the octapeptide hormone angiotensin II (AngII) has proved to play numerous physiological roles that reach far beyond its initial description as a hypertensive factor. In spite of the host of target tissues that have been identified, only two major receptor subtypes, AT1 and AT2, are currently fully identified. The specificity of the effects of AngII relies upon numerous and complex intracellular signaling pathways that often mobilize calcium ions from intracellular stores or from the extracellular medium. Various types of calcium channels (store- or voltage-operated channels) endowed with distinct functional properties play a crucial role in these processes. The activity of these channels can be modulated by AngII in a positive and/or negative fashion, depending on the cell type under observation. This chapter reviews the main characteristics of AngII receptor subtypes and of the various calcium channels as well as the involvement of the multiple signal transduction mechanisms triggered by the hormone in the cell-specific modulation of the activity of these channels.

The versatility and universality of calcium signalling

The universality of calcium as an intracellular messenger depends on its enormous versatility. Cells have a calcium signalling toolkit with many components that can be mixed and matched to create a wide range of spatial and temporal signals. This versatility is exploited to control processes as diverse as fertilization, proliferation, development, learning and memory, contraction and secretion, and must be accomplished within the context of calcium being highly toxic. Exceeding its normal spatial and temporal boundaries can result in cell death through both necrosis and apoptosis.

Modulation of stimulus-secretion coupling in porcine adrenal chromaffin cells by receptor-mediated increases in protein kinase C activity

Catecholamine (CAT) secretion by adrenal chromaffin cells is primarily triggered by nicotinic receptor-dependent increases in cytosolic Ca(2+). The principal aim of the present study was to determine whether pituitary adenylate cyclase activating peptide (PACAP), which is coreleased with acetylcholine from the splanchnic nerve, can modulate nicotinic receptor-dependent Ca(2+) signaling and catecholamine secretion in porcine adrenal medullary chromaffin (PAMC) cells. Activation of protein kinase C (PKC) with phorbol myristate acetate (PMA) dose- and time-dependently inhibited nicotine (NIC)-induced Ca(2+) transients. At 100 nM PMA, peak Ca(2+) levels were reduced by 27% +/- 2% (P < 0.05) and 41% +/- 3% (P < 0. 05) after 10 and 20 min exposure, respectively. The inhibitory effects of PMA were significantly reduced by preincubation with the PKC inhibitor staurosporine. KCl-induced Ca(2+) transients were also reduced by 20 min PMA treatment (Delta -27% +/- 4%; P < 0.05), suggesting that PKC affects voltage-gated Ca(2+) channel activity. Pretreatment with PACAP also resulted in both time- and concentration-dependent suppression of Ca(2+) transients. After 20 min exposure to 1 microM PACAP, NIC- and KCl-induced transients were reduced by 36% +/- 5% (P < 0.05) and 51% +/- 6% (P < 0.05), respectively. These effects could also be prevented by staurosporine pretreatment. NIC-induced CAT secretion was significantly reduced by pretreatment with both PMA (Delta -56% +/- 2%; P < 0.05) and PACAP (Delta-53% +/- 7%; P < 0.05). This suppressive effect on secretion could be prevented by pretreatment with staurosporine. These data suggest that, in addition to having direct stimulatory effects on catecholamine synthesis and secretion, PACAP can also negatively modulate nicotinic receptor-dependent Ca(2+) signaling and secretion in PAMC cells.