Direct demonstration of adrenocorticotropin-induced changes in cytoplasmic free calcium with aequorin in adrenal glomerulosa cell

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.

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.

Biochemical and Ionic signaling mechanisms for ACTH-stimulated cortisol production

Adrenocorticotropic hormone (ACTH)-stimulated cortisol production by adrenal zona fasciculata cells requires coordinated biochemical and ionic signaling mechanisms that employ adenosine 3', 5'-cyclic monophosphate (cAMP) and Ca(2+) as intracellular messengers. As the primary messenger generated in response to ACTH receptor activation, cAMP acts at multiple sites to produce the full steroidogenic response that includes both rapid and delayed components. Biochemically, cAMP activates and induces the expression of multiple proteins that function in converting cholesterol to cortisol. These include the steroid acute regulatory (StAR) protein as well as steroidogenic enzymes. cAMP also inhibits a background K(+) channel (bTREK-1), which sets the resting potential of adrenal zona fasciculata (AZF) cells, thereby triggering membrane depolarization and Ca(2+) entry through voltage-gated Ca(2+) channels. Ca(2+) also accelerates the production of cortisol from cholesterol by activating or inducing the synthesis of steroidogenic proteins. In this scheme, background K(+) channels act pivotally by transducing a hormonal signal at the cell membrane to an ionic signal, leading to depolarization-dependent Ca(2+) entry. In this way, ACTH receptor activation increases cAMP and Ca(2+) in the AZF cell, yielding the full steroidogenic response. In addition to acutely regulating the activity of AZF cell ion channels, ACTH and cAMP also regulate the expression of genes coding for these ion channels. The tonic control of the expression of AZF cell ion channels through the hypothalamic-pituitary-adrenal axis suggests that prolonged stimulation of the AZF cell by ACTH may alter the electrical properties of these cells in a manner which matches the organism's requirement for cortisol.

Mechanism of action of ACTH: beyond cAMP

ACTH is the major regulator of adrenal cortex function, having acute and chronic effects on steroid synthesis and secretion. The precise molecular mechanisms by which ACTH stimulates steroid synthesis and secretion, as well as cell hypertrophy, survival, and migration are still poorly understood. Several studies have shown that ACTH action is mediated not only by cyclic adenosine monophosphate (cAMP), but also by calcium (Ca(2+)), both interacting closely through positive feedback loops to enhance steroid secretion. However, in spite of the evidence that ACTH could stimulate other signaling pathways, such as inositol phosphates and diacylglycerol or mitogenic-activated protein kinase pathway (MAPK), none is as potent as cAMP. Recent data indicate that duration and potency of the cAMP production could be modulated by several isoforms of adenylyl cyclases and phosphodiesterases. In addition, calcium is probably not a first second messenger per se; rather, there are several arguments indicating that its increase occurs following cAMP production. Finally, in addition to steroid secretion, ACTH, through cAMP, is a survival factor, protecting cells against apoptosis. All of the effects of ACTH are dependent on cytoskeleton integrity. In summary, after 30 years of intensive research in this field, cAMP remains the first obligatory second messenger of ACTH action. However, recent work emphasizes that cell environment (matrix and cytoskeleton) probably interacts with cAMP to coordinate functions other than steroid secretion.

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.

A bovine adrenocortical Kv1.4 K(+) channel whose expression is potently inhibited by ACTH

We have cloned a bovine adrenal cortical (bKv1.4) K(+) channel cDNA whose expression is rapidly inhibited by adrenocorticotropic hormone (ACTH). The 4386-nucleotide cDNA is homologous to other voltage-gated, rapidly inactivating Kv1.4 channels, and includes a 1986-nucleotide coding region and large 5'- and 3'-untranslated regions. Bovine Kv1.4-specific mRNA from adrenal zona fasciculata (AZF) cells was rapidly and potently reduced by ACTH, with a t(12) of approximately 1 h and an IC(50) of 1.2 pm. The membrane-permeable cAMP analog 8-pcpt-cAMP also reduced bKv1.4 mRNA expression with kinetics similar to that observed with ACTH. Reduction of bKv1.4 mRNA expression by ACTH and 8-pcpt-cAMP was only partially inhibited by the selective protein kinase A antagonist H-89. Consistent with their effect on bKv1.4 mRNA, ACTH and 8-pcpt-cAMP both dramatically reduced the expression of bKv1.4-associated A-type current measured over 72 h. These results demonstrate that bovine AZF cells synthesize a Kv1.4-type channel whose expression is inhibited at the pretranslational level by ACTH and 8-pcpt-cAMP by a mechanism that is partially dependent on the activation of protein kinase A. The rapid, potent reduction of bKv1.4 mRNA produced by ACTH and 8-pcpt-cAMP indicates that the expression of this K(+) channel is under tonic inhibitory control of the hypothalamic-pituitary-adrenal axis. The basic electrical properties of AZF cells might be tightly regulated at the transcriptional level by the normal diurnal pattern of ACTH secretion, and altered during bouts of stress by the enhanced release of this pituitary peptide. Under conditions of prolonged stress or adrenal insufficiency, persistent ACTH-induced changes in the electrical properties of AZF cells could be coupled to parallel changes in cortisol secretion.

Real-time fluorescence analysis on molecular mechanisms for regulation of cytochrome P450scc activity upon steroidogenic stimulation in adrenocortical cells

Real-time fluorescence analysis revealed that the activity of cytochrome P450scc was related to Ca2+ signals arising from extracellular NADPH, ACTH and ATP stimulation in adrenocortical fasciculata cells. The side-chain cleavage reaction by cytochrome P450scc was measured with 3beta-hydroxy-22,23-bisnor-5-cholenyl ether (cholesterol-resorufin) by observing the distinct increase in fluorescence upon conversion of cholesterol-resorufin to resorufin and pregnenolone. Adrenocorticotropic hormone (ACTH) induced a relatively small stimulation of the P450scc activity. A significant production of resorufin was revealed after stimulation of cell cultures with 100 pM, 1 nM of ACTH for 3 h. On the other hand, extracellular NADPH was found to rapidly and greatly stimulate the resorufin production in intact cells immediately after the addition of 50-500 microM NADPH. The extracellular NADPH stimulation was prevented by the addition of thapsigargin and EGTA which abolished Ca2+ oscillations induced by NADPH. Suramin, a specific antagonist of the P2y type ATP receptor, also completely abolished the NADPH-induced cholesterol-resorufin conversion. These results imply that extracellular NADPH (membrane impermeable) produced Ca2+ oscillations through its binding to ATP receptor thereby stimulating the activity of P450scc. The application of 45-500 microM extracellular ATP to cells did not, however, significantly increase the resorufin production. These three stimulators produced very different types of Ca2+ signals. ACTH induced mainly a series of Ca2+ spikes superimposed on a long-lasting basal Ca2+ elevation. The Ca2+ signals induced by NADPH showed predominantly a series of Ca2+ spikes without elevation of the basal Ca2+ concentration. Only long-lasting Ca2+ elevation was induced by extracellular ATP. The stimulation of cytochrome P450scc may thus be correlated with the different patterns of Ca2+ signals.

Activation of separate calcium and A-kinase-dependent pathways by ACTH

Although cAMP has long been regarded as the primary intracellular messenger for ACTH-stimulated cortisol secretion, a requirement for Ca2+ is well established. However, a specific mechanism which couples ACTH receptor activation to increased intracellular calcium concentration in the adrenal cortical cell has not been elucidated. Here, we present evidence for a specific model in which ACTH at picomolar concentrations induces cAMP which acts through kinase-dependent and independent pathways to stimulate cortisol secretion. Along one of these pathways, cAMP acts directly to depolarize cells by inhibition of a specific non-inactivating K+ channel (I(AC)). This model provides a specific mechanism whereby cAMP-mediated inhibition of I(AC) is tightly coupled to depolarization-dependent Ca2+ entry and cortisol secretion. Ca2+ and cAMP are dual second messengers in the ACTH signalling pathway that are linked through I(AC) K+ channels.

Recent progress in understanding aldosterone secretion

1. The synthesis and secretion of aldosterone in the adrenal zona glomerulosa in physiologic conditions is controlled by adrenocorticotropin (ACTH), angiotensin II (AII), and extracellular (K+). 2. ACTH effects on aldosterone output are explained by cyclic AMP-(cAMP)- and Ca(2+)-dependent mechanisms. 3. All effects on aldosterone secretion are initiated by an increase in Ca2+ influx through hormone-operated Ca2+ channels and G-protein- and phospholipase C-(PLC) dependent hydrolysis of phosphoinositides leading to the generation of inositol 1,4,5 trisphosphate (IP3) and DAG that induce intracellular Ca2+ release and PKC activation, respectively. 4. ACTH increases DAG formation with marginal or undetectable IP3 generation. The effect of ACTH on DAG levels is discussed. 5. The requirement of external Ca2+ in PLC activation and aldosterone secretion also is discussed.

Demonstration of an angiotensin II-induced negative feedback effect on aldosterone synthesis in isolated rat adrenal zona glomerulosa cells

Although both angiotensin II (Ang II) and potassium ion (K+) induce marked elevations of cytosolic free calcium concentration, [Ca2+]c, in adrenal zona glomerulosa cells-an effect which is thought to trigger aldosterone synthesis-Ang II is also known to reduce the sustained [Ca2+]c rise induced by K+. We have examined whether this effect of Ang II on the calcium messenger system is reflected at the level of the final biological response, aldosterone synthesis. In superfused isolated rat glomerulosa cells, K+ (8 mM) induced a sustained, 60-fold increase in aldosterone production. In contrast, the maximal response to Ang II (10 nM) amounted to only 10 times the basal production. When added subsequent to K+ stimulation, Ang II provoked an immediate and dramatic drop in aldosterone synthesis, to levels obtained with Ang II alone. Under conditions of maximal K+ stimulation, this effect depended upon Ang II concentration, while the well-known synergistic effect was observed with submaximal concentrations of both agonists. The inhibitory effect of Ang II could be reproduced with dioctanoylglycerol, a selective activator of protein kinase C. By contrast, the aldosterone response to adrenocorticotropic hormone (ACTH) was not affected by Ang II. At submaximal concentrations of ACTH, the steroidogenic effect of Ang II was even additive to that of ACTH. Thus, we have shown that, under conditions of maximal stimulation, Ang II exerts a profound inhibition of steroidogenesis in K(+)-stimulated rat adrenal glomerulosa cells. This counter-regulatory mechanism may ensure adequate levels of aldosterone production in vivo.

Adrenocorticotropic hormone activation of adenylate cyclase in raphe neurons: multiple regulatory pathways control serotonergic neuronal differentiation

The RN46A cell line was derived from embryonic day 13 rat medullary raphe cells by infection with a retrovirus encoding the temperature-sensitive mutant of SV40 large T antigen (tsT-ag). The RN46A cell line is neuronally restricted and constitutively differentiates following a shift to nonpermissive temperature. Differentiated RN46A cells express low levels of tryptophan hydroxylase (TPH) but no detectable levels of serotonin (5-HT). Treatment of cultures with the adrenocorticotropic hormone peptide ACTH4-10 up-regulates the expression of TPH immunoreactivity in differentiated RN46A cells, but 5-HT synthesis requires initial treatment with ACTH4-10, followed by partial membrane depolarizing conditions. Up-regulation of TPH by ACTH4-10 is apparently due to activation of adenylate cyclase, whereas the increased 5-HT synthesis with membrane depolarization can be blocked with the voltage-sensitive Ca(2+)-channel blockers nifedipine and omega-conotoxin. ACTH4-10 treatment also markedly up-regulates the expression of the 5-HT reuptake transporter, as do dibutyryl cyclic AMP and forskolin; chronic membrane depolarization has no effect on 5-HT reuptake. The expression of the high-affinity 5-HT1A receptor is increased threefold by ACTH4-10 treatment during differentiation and fivefold by differentiation under partial membrane depolarizing conditions. Combining ACTH4-10 treatment and membrane depolarization does not increase expression of the 5-HT1A receptor further. 5-HT release is constitutive in ACTH-treated RN46A cells and linked to spontaneous synaptic vesicle fusion in RN46A cells. Considered with previous results, these data indicate that multiple effectors, ACTH, brain-derived neurotrophic factor, and membrane depolarization, have both distinct and overlapping effects that regulate specific elements of the serotonergic neuronal phenotype during differentiation and maturation.

3H-D-aspartate release from cerebellar granule neurons is differentially regulated by glutamate- and K+-stimulation

Neurotransmitter release in response to either 55 mM K+ or 25 microM glutamate as well as its dependency on Ca2+ from different sources was compared in cultured glutamatergic cerebellar granule cells from rat brain. The intracellular Ca2+ concentration was monitored at the single cell level in neurites as well as cell bodies employing the fluorescent Ca2+ indicator fura-2. Transmitter release was assayed using 3H-D-aspartate to label the exogenously accessible glutamate pools, which in these neurons is believed to also include the transmitter pool. In an attempt to distinguish whether transmitter release was dependent on an intact cytoskeleton or not, the colchicine-like drug Nocodazole, which also blocks transport of vesicles, was used. K(+)-stimulated transmitter release consisted for the major part (around 70%) of a Ca(2+)-dependent, Nocodazole sensitive release component and this K(+)-induced release appeared to be almost exclusively dependent on N-type Ca2+ channels. In contrast, 50% of the glutamate-induced Ca(2+)-dependent release was triggered by Ca2+ from a Dantrolene sensitive intracellular Ca2+ pool. Since these neurons undergo a pronounced maturational change in which neurotransmitter vesicles become increasingly prominent, the Ca2+ responses and transmitter release evoked by the two different stimuli were investigated as a function of the culture period. K+ and glutamate were found to increase intracellular [Ca2+] differentially. In 1-day-old cultures K+ elicited a small albeit significant increase in [Ca2+]i while glutamate was completely without effect. In 7-day-old neurons both agents induced a large increase in [Ca2+].(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channels and control of cytosolic calcium in rat and bovine zona glomerulosa cells

Rat and bovine adrenal zona glomerulosa (ZG) cells possess a low-threshold, voltage-dependent Ca2+ current that was characterized using whole cell voltage clamp techniques. Activation of this current is observed at membrane potentials above -80 mV with maximal peak Ca2+ current elicited near -30 mV. Inactivation of the Ca2+ current was half-maximal between -74 and -58 mV, depending on the external Ca2+ concentration and was nearly complete at -40 mV. The voltage dependency of the current indicates that a calcium current could be sustained at membrane potentials between -80 and -40 mV and thereby elevates cytosolic calcium (Cai) levels. Under basal conditions, Cai is stable in single rat ZG cells, whereas more than half of the bovine ZG cells produce repeated Cai transients. These Cai transients, which are blocked by removal of external Ca2+ or addition of Ni2+, are likely due to repetitive electrical activity in bovine ZG cells. Cai responses can be elicited by small increases in external K+ concentration (5-10 mM) in both rat and bovine ZG cells, indicating the opening of low-threshold Ca2+ channels. However, these Cai changes remain robust at high external K+ concentrations (20-40 mM). In experiments combining Cai measurements and whole cell voltage clamp, a steep dependence of Cai on membrane potential was revealed beginning at depolarizing voltages near a holding membrane potential of -80 mV. A maximal increase in Cai occurred near -30 mV (equivalent to an external K+ concentration of 40 mM), a membrane voltage at which sustained current through low-threshold Ca2+ channels should be negligible. These data raise the possibility of additional voltage-dependent pathways for Ca2+ influx.

Effects of ACTH and angiotensin II on cytosolic calcium in cultured adrenal glomerulosa cells. Role of cAMP production in the ACTH effect

We have used microspectrofluorometry and video imaging techniques in order to study and compare the changes in intracellular calcium concentrations [( Ca2+]i) of individual Fura-2 loaded glomerulosa cells cultured for three days and stimulated either with angiotensin II (AT), K+, or adrenocorticotropin (ACTH). As previously demonstrated for freshly isolated cells, K+ ion induces an immediate increase in [Ca2+]i, although AT induces a biphasic response, characterized by an initial transient spike, followed by a sustained plateau. In this study, we demonstrate, for the first time, that ACTH is able to induce a [Ca2+]i increase in cultured glomerulosa cells from rat and bovine sources. Moreover, it is clear that the pattern of [Ca2+]i increase elicited by ACTH is different from that observed with AT. In most cases, addition of ACTH leads to a slow increase in [Ca2+]i after a long latency period ranging from 10-15 min, which could be correlated to cAMP time-production. The present results show that: (a) in the absence of extracellular Ca2+, ACTH does not increase [Ca2+]i; (b) the response develops slowly and cases immediately after [Ca2+]e depletion or addition of calcium channel blockers, such as nifedipine or omega-conotoxin; (c) the addition of the calcium channel agonist Bay K 8644 enhances the ACTH response; (d) the cAMP analog, 8-Br-cAMP, induces an increase in [Ca2+]i similar to that observed with ACTH, which is also dependent of the presence of calcium in the extracellular medium; (e) time-production of ACTH-induced cAMP follows quite well the increase in [Ca2+]i; (f) Bay K 8644 also enhances the 8-Br-cAMP induced increase in [Ca2+]i; and (g) ACTH-induced Cai response is inhibited by the specific protein kinase A blocker, HA1004. These observations, combined with previous results obtained on the effects of ACTH on calcium currents and action potentials, suggest that the [Ca2+]i increase induced by ACTH results from a calcium influx through dihydropyridine and omega-conotoxin sensitive calcium channels, which need to be phosphorylated by cAMP for full activation. The use of video-imaging techniques has allowed us to examine the spatial distribution of changes in [Ca2+]i in single cells. The ability to simultaneously record images of a number of cells confirm the heterogeneity of cellular responses, and corroborate results obtained through photocounting only. Our results indicate that ACTH initially increases [Ca2+]i locally beneath the cell membrane and throughout the cell thereafter, whereas angiotensin II elicits a more prominent effect in certain regions of the cell and eventually extends to the entire cell surface.

N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) does not modify the angiotensin II-stimulated calcium signal in cultured bovine glomerulosa cells

Studies were performed to determine if the sustained elevation in [Ca2+]c noted previously in glomerulosa cells in response to Ang II resulted from the presence of HEPES in the experimental medium. At confluence, primary cultures of bovine glomerulosa cells were maintained for 24-30 h in the presence of either 14 mM NaHCO3/5% CO2 or 25 mM HEPES/4 mM NaHCO3/air. During subsequent experimental periods, cells were incubated in the presence of the corresponding or reciprocal buffer, and the effects of Ang II on [Ca2+]c were monitored by fura 2 fluorescence. Increases in [Ca2+]c produced by Ang II in cells continuously maintained in either HCO3(-) - or HEPES-buffered media were similar, and with the same monolayer the nature of the Ang II-stimulated Ca2+ signal was independent of the buffer employed. Moreover, the Ang II-stimulated Ca2+ signal was not significantly affected by the removal of HCO3- from the superfusate. These results indicate that the sustained increase in [Ca2+]c is not an artifact introduced by the use of HEPES as an experimental buffer, but rather a normal component of the Ang II-stimulated Ca2+ signal.

Evidence that angiotensin II decreases mitochondrial calcium in the glomerulosa cell

The present studies were performed using primary monolayer cultures of bovine glomerulosa cells to determine whether the elevation in cytosolic calcium concentration produced by angiotensin II was accompanied by an elevation in mitochondrial calcium. Exchangeable mitochondria calcium content was assessed indirectly by measuring the changes in cytosolic calcium concentration and calcium efflux produced by the mitochondrial uncoupler, carbonyl cyanide m-chlorophenylhydrazone (CCCP). Total mitochondrial calcium content was also assessed directly by atomic absorption spectroscopy. CCCP had a direct effect to promote calcium release from an oligomycin/antimycin-sensitive (mitochondrial) calcium pool in permeabilized cells. In intact cells, CCCP caused rapid reductions in cellular ATP content and the ratio of ATP to ADP. Still, its effects on calcium dynamics were exerted primarily at the mitochondrial level as evidenced by inhibition with ruthenium red, but not dantrolene. As expected, angiotensin II produced a rapid increase in calcium efflux and an equally rapid and sustained increase in cytosolic calcium concentration. Nonetheless, CCCP-stimulated elevations in cytosolic calcium concentration and calcium efflux were reduced by angiotensin II in a concentration-dependent manner. Total mitochondrial calcium content was also lower in angiotensin-treated than in control cells. These results indicate that angiotensin II causes a net decrease in mitochondrial calcium stores. On the basis of these data, it is proposed that alterations in calcium metabolism initiated by angiotensin II are exerted not only at the membrane and cytosolic levels but also at the level of the mitochondria. Changes in mitochondrial calcium dynamics may directly contribute to the regulation of mitochondrial steroidogenic enzymes by angiotensin II.

Effect of cyclosporin A on the membrane potential and Ca2+ level of human lymphoid cell lines and mouse thymocytes

The effect of the immunosuppressive cyclosporin A (CsA) on the cytosolic free Ca2+ concentration ([Ca2+]i) and membrane potential of human B and T lymphoblastoid cells and mouse thymocytes was studied in order to reveal some features of the early stage of drug-cell interaction. Cytosolic free Ca2+ concentration of the cells was measured by spectrofluorimetry using indo-1 and quin2 fluorescent calcium indicators. Membrane potential was monitored in a flow cytometer with oxonol dye. CsA applied at 2-20 micrograms/ml final concentrations caused a dose-dependent, rapid, transient rise of [Ca2+]i in all cell types. This effect could be blocked by chelating the extracellular Ca2+ with EGTA but was not sensitive to Ca2+ channel blockers verapamil and nifedipine or K+ channel blocker 4-aminopyridine. A possible explanation for the calcium mobilizing effect of CsA is an ionophore-like mode of action at the cell membrane level. Besides directly interfering with mitogenic signals, the elevation of [Ca2+]i could be responsible for an initial hyperpolarization observed in CsA-treated T lymphocytes. This hyperpolarization, however, was not detectable in B lymphoblastoid cells. A further difference between B and T cells was the diverse pattern of depolarization following CsA treatment. This variance in the behaviour of T and B lymphocytes and the diversity of membrane transport systems in its background could account for the different final outcome of the drug-cell interaction.

Receptor-effector coupling by G proteins

The primary structure of G proteins as deduced from purified proteins and cloned subunits is presented. When known, their functions are discussed, as are recent data on direct regulation of ionic channels by G proteins. Experiments on expression of alpha subunits, either in bacteria or by in vitro translation of mRNA synthesized from cDNA are presented as tools for definitive assignment of function to a given G protein. The dynamics of G protein-mediated signal transduction are discussed. Key points include the existence of two superimposed regulatory cycles in which upon activation by GTP, G proteins dissociate into alpha and beta gamma and their dissociated alpha subunits hydrolyze GTP. The action of receptors to catalyze rather than regulate by allostery the activation of G proteins by GTP is emphasized, as is the role of subunit dissociation, without which receptors could not act as catalysts. To facilitate the reading of this review, we have presented the various subtopics of this rapidly expanding field in sections 1-1X, each of which is organized as a self-contained sub-chapter that can be read independently of the others.

Roles of G proteins in coupling of receptors to ionic channels and other effector systems

Guanine nucleotide binding (G) proteins are heterotrimers that couple a wide range of receptors to ionic channels. The coupling may be indirect, via cytoplasmic agents, or direct, as has been shown for two K+ channels and two Ca2+ channels. One example of direct G protein gating is the atrial muscarinic K+ channel K+[ACh], an inwardly rectifying K+ channel with a slope conductance of 40 pS in symmetrical isotonic K+ solutions and a mean open lifetime of 1.4 ms at potentials between -40 and -100 mV. Another is the clonal GH3 muscarinic or somatostatin K+ channel, also inwardly rectifying but with a slope conductance of 55 pS. A G protein, Gk, purified from human red blood cells (hRBC) activates K+ [ACh] channels at subpicomolar concentrations; its alpha subunit is equipotent. Except for being irreversible, their effects on gating precisely mimic physiological gating produced by muscarinic agonists. The alpha k effects are general and are similar in atria from adult guinea pig, neonatal rat, and chick embryo. The hydrophilic beta gamma from transducin has no effect while hydrophobic beta gamma from brain, hRBCs, or retina has effects at nanomolar concentrations which in our hands cannot be dissociated from detergent effects. An anti-alpha k monoclonal antibody blocks muscarinic activation, supporting the concept that the physiological mediator is the alpha subunit not the beta gamma dimer. The techniques of molecular biology are now being used to specify G protein gating. A "bacterial" alpha i-3 expressed in Escherichia coli using a pT7 expression system mimics the gating produced by hRBC alpha k.

Proopiomelanocortin-derived peptides, phosphoinositides, cAMP, and aldosterone secretion

Since the intracellular messengers of various proopiomelanocortin-derived peptides remain ambiguous at best, we have investigated the possible involvement of phosphoinositide metabolism in aldosterone secretion evoked by alpha-MSH, beta-LPH, as well as ACTH in rat and calf adrenal glomerulosa cells. We have also examined the cAMP responses in the adrenal glomerulosa cells to alpha-MSH comparing it with those of ACTH. Our results showed that neither alpha-MSH, beta-LPH, nor ACTH increased inositol triphosphate (IP3) or other inositol phosphates in adrenal glomerulosa cells while increasing aldosterone secretion from the same cells. Angiotensin II, known to cause hydrolysis of the phosphoinositides, increased IP3 in these adrenal cells in a dose-dependent manner. Both ACTH and alpha-MSH raised the cAMP levels in the calf adrenal glomerulosa cells, although the magnitude of the increase of cAMP in response to ACTH was greater. These findings suggest that IP3 as a mediator of alpha-MSH- and beta-LPH-induced aldosterone secretion is not likely and other mediator(s) may be involved.

Adrenocorticotropic hormone inhibits angiotensin II-stimulated inositol phosphate accumulation in rat adrenal glomerulosa cells

Rat adrenal glomerulosa cells labelled for 18 h with [3H]inositol responded to angiotensin II with a dose-dependent stimulation of the accumulation of inositol monophosphate, inositol bisphosphate and inositol trisphosphate. Addition of adrenocorticotropic hormone (ACTH) (10(-7)M) reduced the maximum responses without altering the EC50 values for angiotensin II. Thus, ACTH acted as a non-competitive inhibitor with respect to angiotensin II. No inhibition was observed in cells labelled for 2 h with [3H]inositol. Detailed examination of the inhibition showed that ACTH(1-24) was the most potent inhibitor, with ACTH(1-39) being 10-fold less potent. A mixture of alpha-melanocyte-stimulating hormone (alpha-MSH) (ACTH(1-13] and corticotropin-like intermediate lobe peptide (ACTH(18-39] was similarly inactive. ACTH(5-24) did not produce detectable inhibition. In terms of specificity, the receptor mediating ACTH inhibition of phosphatidylinositol turnover was similar to the receptor which mediated stimulation of aldosterone synthesis. Inhibition by ACTH was additive with inhibition produced by dibutyryl cAMP demonstrating that it was not mediated by rises in intracellular cAMP. ACTH inhibition also was additive with inhibition by the calcium channel blocker, nifedipine. These results demonstrate an interaction between ACTH receptors and angiotensin II receptors in adrenal glomerulosa cells at the level of their receptor-second messenger pathways.

Na-Ca exchanger as a calcium influx pathway in adrenal glomerulosa cells

When aequorin-loaded glomerulosa cells were incubated in isotonic Na2+-free medium containing N-methyl-D-glucamine instead of NaCl, there was an increase in cytoplasmic free calcium concentration, [Ca2+] c, which was not observed when extracellular calcium concentration was reduced to 1 microM. Upon removal of extracellular sodium, there was nearly five-fold increase in fractional efflux ratio of calcium. The reduction of extracellular sodium resulted in a stimulation of calcium influx rate, the magnitude of which was dependent on extracellular sodium concentration. Similar stimulation of calcium influx was observed when extracellular sodium was replaced with lithium. Nitrendipine did not affect the calcium influx induced by the reduction of extracellular sodium while a derivative of amiloride 3',4'-dichlorobenzamil, which inhibits Na-Ca exchange, attenuated calcium influx observed in sodium-free medium. These results indicate that removal of extracellular sodium leads to an increase in [Ca2+] c by stimulating calcium influx and that calcium enters the cell via Na-Ca exchanger.

Inhibitory actions of somatostatin on cyclic AMP and aldosterone production in agonist-stimulated adrenal glomerulosa cells

Somatostatin (SRIF) is a potent inhibitor of angiotensin II (AII)-stimulated aldosterone production in rat adrenal glomerulosa cells. This inhibition can be prevented by pretreatment of the cells with pertussis toxin, but little else is known about either the specificity or the biochemical bases of SRIF action in this tissue. We therefore conducted detailed studies of the influence of SRIF on steroidogenesis elicited by AII and the other two physiological stimuli of aldosterone production, K+ and adrenocorticotropic hormone (ACTH), in rat adrenal glomerulosa cells. We also determined the effects of SRIF on cytosolic calcium concentration ([Ca2+]i) and cellular cAMP levels. In these studies, SRIF was found to inhibit the aldosterone responses elicited by low concentrations of all three stimuli, which are believed to promote steroid secretion via discrete but interacting cellular signalling mechanisms. In addition, SRIF consistently lowered cellular cAMP levels in the presence of each of the three agents. However, SRIF caused a small and transient increase rather than a decrease in basal ([Ca2+]i), and had no effect on the subsequent elevation of ([Ca2+]i) by AII and K+. These data indicate that activation of a Gi-like protein by SRIF influences steroid responses to all three major regulators of glomerulosa-cell function, and suggest that basal levels of cAMP play a facilitatory or permissive role in the control of aldosterone production by predominantly calcium-mobilizing regulators of mineralocorticoid secretion.

Angiotensin II-stimulated changes in calcium metabolism in cultured glomerulosa cells

Studies were performed to evaluate the relationships between the effects of angiotensin II on calcium metabolism and cytosolic free calcium concentration in primary monolayer cultures of bovine adrenal glomerulosa cells. As noted previously (Kramer (1988) Am. J. Physiol. (in press], angiotensin II produced rapid dose-dependent increases in cytosolic calcium characterized by both an initial transient component and a secondary sustained component. In the absence of extracellular calcium, angiotensin II produced an initial increase in cytosolic calcium comparable to that produced in the presence of calcium, but failed to maintain a sustained calcium signal. The initial, angiotensin-stimulated increase in cytosolic calcium was inhibited by dantrolene and 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate (TMB-8) in a concentration-dependent fashion. The onset of the angiotensin-stimulated calcium signal was accompanied by a dose-dependent increase in the rate of calcium efflux that achieved a maximum within 2-3 min and then declined to a level 2.5-3 times that from control cells. The initial rate of calcium influx was also increased about 2.5-fold by angiotensin II, an effect that was only apparent in cells that had been treated with the peptide for at least 5 min. These results indicate that the calcium signal produced by angiotensin II is initiated by the rapid mobilization of calcium from an intracellular site(s) and sustained by the continued uptake of extracellular calcium. Moreover, the kinetics of the calcium signal as well as the final, sustained calcium concentration achieved reflect the balance between intracellular calcium release, calcium influx and calcium efflux.

Functional responses of aequorin-loaded human neutrophils. Comparison with fura-2-loaded cells

Aequorin-loaded human neutrophils in response to chemotactic peptides and ionomycin showed a sharp rise in their intracellular Ca2+ concentration which decayed within 2 min. Depletion of extracellular Ca2+ suppressed only the ionomycin-induced increase. Fura-2-loaded cells also showed a sharp rise in the intracellular Ca2+ concentration in response to each stimulator, while the decline was extremely slow in the ionomycin-induced Ca2+ increase. Depletion of extracellular Ca2+ reduced the duration of ionomycin-induced Ca2+ increase. Cytochalasin B almost equally potentiated the rise in the intracellular Ca2+ concentration induced by each stimulator. Aequorin-loaded cells showed impaired phagocytotic activity, while degranulation and oxygen radical production were not affected.

Involvement of Ca2+ influx in F(-)-stimulated pepsinogen release from guinea pig gastric chief cells

In isolated guinea pig gastric chief cells, pepsinogen release was stimulated by NaF in a dose-dependent manner. Cholecystokinin (CCK) and Ca2+ ionophore A23187 had no additional effect on NaF-stimulated pepsinogen release. CCK caused a rapid increase in intracellular free Ca2+ concentration ([Ca2+]i) monitored by Quin-2 and markedly stimulated inositol phosphate accumulation in chief cells. By contrast, NaF did not cause any change in [Ca2+]i. NaF, even at a maximal concentration for pepsinogen release, appeared to be relatively ineffective on inositol phosphate accumulation. On the other hand, NaF markedly stimulated Ca2+ influx into chief cells. These results suggest that F- stimulates pepsinogen release probably by increasing Ca2+ influx into chief cells. Since F- is a well known activator of guanine nucleotide regulatory proteins (G proteins), it is proposed that there may exist a G protein regulating the opening of Ca2+ channel in gastric chief cells.

Simultaneous determination of intracellular free calcium and aldosterone production in bovine adrenal zona glomerulosa

Angiotensin II (AII) induces an initial rapid but transient rise in [Ca2+]i detected with aequorin in bovine adrenal capsule strips. The rise in [Ca2+]i begins immediately after AII addition, reaches a peak in 30 seconds, and returns to near basal values within 5 minutes. The [Ca2+]i transient is receptor-mediated and its height is dose-dependent. The increase in [Ca2+]i is largely due to the release of Ca2+ from an intracellular pool. The uncorrected peak rise in [Ca2+]i after 1 X 10(-6) M beta-[asp1]-AII stimulation is approximately 3 fold, from 110 nM to 300 nM; the peak rise, corrected for diffusion and nonsynchronous cellular response, is from 110 nM to 1.2 microM. Perifusion of aequorin-loaded strips with beta-[asp1]-AII, an aminopeptidase-resistant analog of AII, allows the simultaneous measurement of [Ca2+]i and aldosterone production rate. Levels of agonist which generate a transient rise in [Ca2+]i also produce a sustained increase in aldosterone production rate, but the two events are temporally separated: the transient rise in [Ca2+]i precedes the increase in aldosterone production rate. However, there is a strong correlation, r = 0.94, between the amplitude of the initial [Ca2+]i transient and the magnitude of the sustained increase in steroid production rate.

A spatial-temporal model of cell activation

A spatial-temporal model of calcium messenger function is proposed to account for sustained cellular responses to sustained stimuli, as well as for the persistent enhancement of cell responsiveness after removal of a stimulus, that is, cellular memory. According to this model, spatial separation of calcium function contributes to temporal separation of distinct phases of the cellular response. At different cellular sites, within successive temporal domains, the calcium messenger is generated by different mechanisms and has distinct molecular targets. In particular, prolonged cell activation is brought about by the interaction of calcium with another spatially confined messenger, diacylglycerol, to cause the association of protein kinase C with the plasma membrane. Activity of the membrane-associated protein kinase C is controlled by the rate of calcium cycling across the plasma membrane. In some instances, a single stimulus induces both protein kinase C activation and calcium cycling and thus causes prolonged activation; but in others, a close temporal association of distinct stimuli brings about cell activation via interaction of these intracellular messengers. Persistent enhancement of cell responsiveness after removal of stimuli is suggested to be due to the continued association, or anchoring, of protein kinase C to the membrane.

A G protein directly regulates mammalian cardiac calcium channels

A possible direct effect of guanine nucleotide binding (G) proteins on calcium channels was examined in membrane patches excised from guinea pig cardiac myocytes and bovine cardiac sarcolemmal vesicles incorporated into planar lipid bilayers. The guanosine triphosphate analog, GTP gamma S, prolonged the survival of excised calcium channels independently of the presence of adenosine 3',5'-monophosphate (cAMP), adenosine triphosphate, cAMP-activated protein kinase, and the protein kinase C activator tetradecanoyl phorbol acetate. A specific G protein, activated Gs, or its alpha subunit, purified from the plasma membranes of human erythrocytes, prolonged the survival of excised channels and stimulated the activity of incorporated channels. Thus, in addition to regulating calcium channels indirectly through activation of cytoplasmic kinases, G proteins can regulate calcium channels directly. Since they also directly regulate a subset of potassium channels, G proteins are now known to directly gate two classes of membrane ion channels.

Calcium in the regulation of aldosterone secretion and vascular smooth muscle contraction

A model of angiotensin II action has been developed in which the flow of information from cell surface to cell interior proceeds by two temporally distinct branches: a calmodulin branch largely responsible for initiating the response; and a C-kinase branch for sustaining it. There are at least two initial events: a prompt and sustained increase in calcium influx rate, and prompt hydrolysis of phosphatidylinositol 4,5-bisphosphate. The latter leads to the generation of water-soluble inositol 1,4,5-trisphosphate and lipid soluble diacylglycerol. The rise in inositol 1,4,5-trisphosphate concentration causes the redistribution of intracellular calcium, a transient rise in the calcium concentration in the cytosol, and the activation of calmodulin-dependent enzymes, including protein kinase(s). As a result, several cellular proteins are rapidly phosphorylated and initiate the cellular response. The rise in calcium and these initial phosphorylation events are transient, however, so that an additional mechanism is necessary to sustain the response. The rise in diacylglycerol content, along with the transient rise in cytosolic calcium, leads to a shift of the C-kinase from a calcium-insensitive to a calcium-sensitive, plasma membrane-associated form. In this location, the activity of C-kinase is regulated by the rate of calcium flux across the plasma membrane. As a result of the activity of the C-kinase, a second set of cellular proteins becomes phosphorylated, and these control the sustained phase of the response.

Erythroid differentiation factor stimulates hydrolysis of polyphosphoinositide in Friend erythroleukemia cells

We examined an early action of erythroid differentiation factor (EDF), a polypeptide which induces differentiation of Friend murine erythroleukemia (MEL) cells. (Eto et al., Biochem. Biophys. Res. Commun. 142: 1095-1103, 1987). In MEL cells, EDF caused a rapid and transient increase in cytoplasmic concentration of free calcium, [Ca2+]c. EDF increased [Ca2+]c even in the absence of extracellular calcium. When [3H]inositol-labeled MEL cells were incubated with EDF, EDF rapidly increased radioactivity in inositol trisphosphate, bisphosphate and monophosphate. EDF also increased [3H] diacylglycerol in [3H]arachidonate-labeled MEL cells. These results indicate that EDF increases [Ca2+]c by stimulating hydrolysis of polyphosphoinositide.

BAY-K-8644, a calcium channel agonist, induces a rise in cytoplasmic free calcium and iodide discharge in thyroid cells

BAY-K-8644, a calcium channel agonist, induces a rise in cytoplasmic free calcium and iodide discharge in cultured porcine thyroid cells. The cytoplasmic free calcium concentration, [Ca2+]i, was measured using aequorin, a calcium-sensitive photoprotein. BAY-K-8644, a dihydropyridine derivative, acts as a Ca channel agonist and induces a rise in [Ca2+]i and iodide discharge; 0.5 nM BAY-K-8644 is a minimal dose to effect a rise in [Ca2+]i and iodide discharge and 50 nM BAY-K-8644 produces the maximal effect. The data indicate that BAY-K-8644-induced iodide discharge is mediated by a rise in [Ca2+]i.

Insulin-like growth factor II increases cytoplasmic free calcium in competent Balb/c 3T3 cells treated with epidermal growth factor

To determine the role of calcium in the action of insulin-like growth factor II (IGF-II), we have examined the effect of multiplication stimulating activity, the rat IGF-II, on cytoplasmic-free calcium concentration, [Ca2+]c, in aequorin-loaded Balb/c 3T3 cells. IGF-II does not cause any change in [Ca2+]c in quiescent cells. By contrast, IGF-II induces changes in [Ca2+]c in platelet-derived growth factor(PDGF) - pretreated competent cells: when competent cells are incubated with epidermal growth factor (EGF) for 10 min, subsequent IGF-II induces an immediate increase in [Ca2+]c. Without EGF treatment, IGF-II does not cause any increase in [Ca2+]c. The priming action of EGF is time dependent, requiring approximately 10 min for the maximum effect. The IGF-II-mediated increase in [Ca2+]c is totally dependent on extracellular calcium and is blocked by lanthanum. When DNA synthesis in PDGF-treated competent cells is assessed by measuring [3H]thymidine incorporation, IGF-II by itself has only a small effect. Likewise, a brief treatment with EGF results in only a small increase in [3H]thymidine incorporation. By contrast, in competent cells briefly treated with EGF, IGF-II causes a marked stimulation of [3H]thymidine incorporation. These results indicate that IGF-II increases [Ca2+]c in competent Balb/c 3T3 cells treated with EGF by stimulating calcium influx and that IGF-II-stimulated calcium influx may be related causally to its action on cell proliferation.

Direct activation of mammalian atrial muscarinic potassium channels by GTP regulatory protein Gk

The mammalian heart rate is regulated by the vagus nerve, which acts via muscarinic acetylcholine receptors to cause hyperpolarization of atrial pacemaker cells. The hyperpolarization is produced by the opening of potassium channels and involves an intermediary guanosine triphosphate-binding regulatory (G) protein. Potassium channels in isolated, inside-out patches of membranes from atrial cells now are shown to be activated by a purified pertussis toxin-sensitive G protein of subunit composition alpha beta gamma, with an alpha subunit of 40,000 daltons. Thus, mammalian atrial muscarinic potassium channels are activated directly by a G protein, not indirectly through a cascade of intermediary events. The G protein regulating these channels is identified as a potent Gk; it is active at 0.2 to 1 pM. Thus, proteins other than enzymes can be under control of receptor coupling G proteins.

Comparison of the changes in cytoplasmic free calcium concentration induced by phenylephrine, vasopressin and angiotensin II in hepatocytes

Effects of phenylephrine, vasopressin and angiotensin II on cytoplasmic free calcium concentration, [Ca2+]c, were examined by monitoring aequorin bioluminescence in isolated hepatocytes preloaded with aequorin. In the presence of 0.5 mM calcium in the medium, the pattern of changes in aequorin bioluminescence induced by phenylephrine was different from that induced by vasopressin or angiotensin II. When extracellular calcium concentration was reduced to 1 microM, however, these three agents induced identical changes in aequorin bioluminescence. These results suggest that the mode of action of phenylephrine on cytoplasmic free calcium concentration differs from that of either vasopressin or angiotensin II and that the difference in ability to increase calcium influx may account for the distinct patterns induced by these agents.