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

Human adrenal glomerulosa cells express K2P and GIRK potassium channels that are inhibited by ANG II and ACTH

In whole cell patch clamp recordings, it was discovered that normal human adrenal zona glomerulosa (AZG) cells express members of the three major families of K⁺ channels. Among these are a two-pore (K2P) leak-type and a G protein-coupled, inwardly rectifying (GIRK) channel, both inhibited by peptide hormones that stimulate aldosterone secretion. The K2P current displayed properties identifying it as TREK-1 (KCNK2). This outwardly rectifying current was activated by arachidonic acid and inhibited by angiotensin II (ANG II), adrenocorticotrophic hormone (ACTH), and forskolin. The activation and inhibition of TREK-1 was coupled to AZG cell hyperpolarization and depolarization, respectively. A second K2P channel, TASK-1 (KCNK3), was expressed at a lower density in AZG cells. Human AZG cells also express inwardly rectifying K⁺ current(s) (K_IR) that include quasi-instantaneous and time-dependent components. This is the first report demonstrating the presence of K_IR in whole cell recordings from AZG cells of any species. The time-dependent current was selectively inhibited by ANG II, and ACTH, identifying it as a G protein-coupled (GIRK) channel, most likely K_IR3.4 (KCNJ5). The quasi-instantaneous K_IR current was not inhibited by ANG II or ACTH and may be a separate non-GIRK current. Finally, AZG cells express a voltage-gated, rapidly inactivating K⁺ current whose properties identified as K_V1.4 (KCNA4), a conclusion confirmed by Northern blot. These findings demonstrate that human AZG cells express K2P and GIRK channels whose inhibition by ANG II and ACTH is likely coupled to depolarization-dependent secretion. They further demonstrate that human AZG K⁺ channels differ fundamentally from the widely adopted rodent models for human aldosterone secretion.

Adrenal fasciculata cells express T-type and rapidly and slowly activating L-type Ca2+ channels that regulate cortisol secretion

In whole cell patch-clamp recordings, we characterized the L-type Ca(2+) currents in bovine adrenal zona fasciculata (AZF) cells and explored their role, along with the role of T-type channels, in ACTH- and angiotensin II (ANG II)-stimulated cortisol secretion. Two distinct dihydropyridine-sensitive L-type currents were identified, both of which were activated at relatively hyperpolarized potentials. One activated with rapid kinetics and, in conjunction with Northern blotting and PCR, was determined to be Cav1.3. The other, expressed in approximately one-half of AZF cells, activated with extremely slow voltage-dependent kinetics and combined properties not previously reported for an L-type Ca(2+) channel. The T-type Ca(2+) channel antagonist 3,5-dichloro-N-[1-(2,2-dimethyl-tetrahydro-pyran-4-ylmethyl)-4-fluoro-piperidin-4-ylmethyl]-benzamide (TTA-P2) inhibited Cav3.2 current in these cells, as well as ACTH- and ANG II-stimulated cortisol secretion, at concentrations that did not affect L-type currents. In contrast, nifedipine specifically inhibited L-type currents and cortisol secretion, but less effectively than TTA-P2. Diphenylbutylpiperidine Ca(2+) antagonists, including pimozide, penfluridol, and fluspirilene, and the dihydropyridine niguldipine blocked Cav3.2 and L-type currents and inhibited ACTH-stimulated cortisol secretion with similar potency. This study shows that bovine AZF cells express three Ca(2+) channels, the voltage-dependent gating and kinetics of which could orchestrate complex mechanisms linking peptide hormone receptors to cortisol secretion through action potentials or sustained depolarization. The function of the novel, slowly activating L-type channel is of particular interest in this respect. Regardless, the well-correlated selective inhibition of T- and L-type currents and ACTH- and ANG II-stimulated cortisol secretion by TTA-P2 and nifedipine establish the critical importance of these channels in AZF cell physiology.

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.

Ca2+ and K+ channels of normal human adrenal zona fasciculata cells: properties and modulation by ACTH and AngII

In whole cell patch clamp recordings, we found that normal human adrenal zona fasciculata (AZF) cells express voltage-gated, rapidly inactivating Ca²⁺ and K⁺ currents and a noninactivating, leak-type K⁺ current. Characterization of these currents with respect to voltage-dependent gating and kinetic properties, pharmacology, and modulation by the peptide hormones adrenocorticotropic hormone (ACTH) and AngII, in conjunction with Northern blot analysis, identified these channels as Ca_v3.2 (encoded by CACNA1H), Kv1.4 (KCNA4), and TREK-1 (KCNK2). In particular, the low voltage–activated, rapidly inactivating and slowly deactivating Ca²⁺ current (Ca_v3.2) was potently blocked by Ni²⁺ with an IC₅₀ of 3 µM. The voltage-gated, rapidly inactivating K⁺ current (Kv1.4) was robustly expressed in nearly every cell, with a current density of 95.0 ± 7.2 pA/pF (n = 64). The noninactivating, outwardly rectifying K⁺ current (TREK-1) grew to a stable maximum over a period of minutes when recording at a holding potential of −80 mV. This noninactivating K⁺ current was markedly activated by cinnamyl 1-3,4-dihydroxy-α-cyanocinnamate (CDC) and arachidonic acid (AA) and inhibited almost completely by forskolin, properties which are specific to TREK-1 among the K2P family of K⁺ channels. The activation of TREK-1 by AA and inhibition by forskolin were closely linked to membrane hyperpolarization and depolarization, respectively. ACTH and AngII selectively inhibited the noninactivating K⁺ current in human AZF cells at concentrations that stimulated cortisol secretion. Accordingly, mibefradil and CDC at concentrations that, respectively, blocked Ca_v3.2 and activated TREK-1, each inhibited both ACTH- and AngII-stimulated cortisol secretion. These results characterize the major Ca²⁺ and K⁺ channels expressed by normal human AZF cells and identify TREK-1 as the primary leak-type channel involved in establishing the membrane potential. These findings also suggest a model for cortisol secretion in human AZF cells wherein ACTH and AngII receptor activation is coupled to membrane depolarization and the activation of Ca_v3.2 channels through inhibition of hTREK-1.

Forskolin induced increase in spontaneous activity of auditory brainstem neurons is comparable to acoustic stimulus evoked responses

Contemporary proposals for the pathophysiology of tinnitus due to cochlear damage underscore increased spontaneous activity of auditory brainstem neurons. One of the several consequences of the cochlear injury is the activation of the ERK pathway, suppression of phosphodiestase E activity, and putatively setting a long-term increase in intracellular levels of cyclic AMP at central auditory neurons. Local application of forskolin also increases intracellular cyclic AMP and spontaneous neural activity. We measured the effects of locally applied forskolin on spontaneous firing rate of isolated neurons in the peri-olivary region of the superior olive complex in anesthetized adult Long Evan rats. Forskolin induced increase in spontaneous neural activity was comparable to supra-threshold tone evoke neural responses. These results are viewed in context of hyperexcitability as a correlate of tinnitus.

ACTH induces Cav3.2 current and mRNA by cAMP-dependent and cAMP-independent mechanisms

Bovine adrenal zona fasciculata (AZF) cells express Ca(v)3.2 T-type Ca(2+) channels that function pivotally in adrenocorticotropic hormone (ACTH)-stimulated cortisol secretion. The regulation of Ca(v)3.2 expression in AZF cells by ACTH, cAMP analogs, and their metabolites was studied using Northern blot and patch clamp recording. Exposing AZF cells to ACTH for 3-6 days markedly enhanced the expression of Ca(v)3.2 current. The increase in Ca(v)3.2 current was preceded by an increase in corresponding CACNA1H mRNA. O-Nitrophenyl,sulfenyl-adrenocorticotropin, which produces a minimal increase in cAMP, also enhanced Ca(v)3.2 current. cAMP analogs, including 8-bromoadenosine cAMP (600 mum) and 6-benzoyladenosine cAMP (300 mum) induced CACNA1H mRNA, but not Ca(v)3.2 current. In contrast, 8-(4-chlorophenylthio) (8CPT)-cAMP (10-50 mum) enhanced CACNA1H mRNA and Ca(v)3.2 current, whereas nonhydrolyzable Sp-8CPT-cAMP failed to increase either Ca(v)3.2 current or mRNA. Metabolites of 8CPT-cAMP, including 8CPT-adenosine and 8CPT-adenine, increased Ca(v)3.2 current and mRNA with a potency and effectiveness similar to the parent compound. The Epac activator 8CPT-2'-O-methyl-cAMP and its metabolites 8CPT-2'-OMe-5'-AMP and 8CPT-2'-O-methyl-adenosine increased CACNA1H mRNA and Ca(v)3.2 current; Sp-8CPT-2'-O-methyl-cAMP increased neither Ca(v)3.2 current nor mRNA. These results reveal an interesting dichotomy between ACTH and cAMP with regard to regulation of CACNA1H mRNA and Ca(2+) current. Specifically, ACTH induces expression of CACNA1H mRNA and Ca(v)3.2 current in AZF cells by mechanisms that depend at most only partly on cAMP. In contrast, cAMP enhances expression of CACNA1H mRNA but not the corresponding Ca(2+) current. Surprisingly, chlorophenylthio-cAMP analogs stimulate the expression of Ca(v)3.2 current indirectly through metabolites. ACTH and the metabolites may induce Ca(v)3.2 expression by the same, unidentified mechanism.

cAMP analogs and their metabolites enhance TREK-1 mRNA and K+ current expression in adrenocortical cells

bTREK-1 K(+) channels set the resting membrane potential of bovine adrenal zona fasciculata (AZF) cells and function pivotally in the physiology of cortisol secretion. Adrenocorticotropic hormone controls the function and expression of bTREK-1 channels through signaling mechanisms that may involve cAMP and downstream effectors including protein kinase A (PKA) and exchange protein 2 directly activated by cAMP (Epac2). Using patch-clamp and Northern blot analysis, we explored the regulation of bTREK-1 mRNA and K(+) current expression by cAMP analogs and several of their putative metabolites in bovine AZF cells. At concentrations sufficient to activate both PKA and Epac2, 8-bromoadenosine-cAMP enhanced the expression of both bTREK-1 mRNA and K(+) current. N(6)-Benzoyladenosine-cAMP, which activates PKA but not Epac, also enhanced the expression of bTREK-1 mRNA and K(+) current measured at times from 24 to 96 h. An Epac-selective cAMP analog, 8-(4-chlorophenylthio)-2'-O-methyl-cAMP (8CPT-2'-OMe-cAMP), potently stimulated bTREK-1 mRNA and K(+) current expression, whereas the nonhydrolyzable Epac activator 8-(4-chlorophenylthio)-2'-O-methyl-cAMP, Sp-isomer was ineffective. Metabolites of 8CPT-2'-OMe-cAMP, including 8-(4-chlorophenylthio)-2'-O-methyladenosine-5'-O-monophosphate and 8CPT-2'-OMe-adenosine, promoted the expression of bTREK-1 transcripts and ion current with a temporal pattern, potency, and effectiveness resembling that of the parent compound. Likewise, at low concentrations, 8-(4-chlorophenylthio)-cAMP (8CPT-cAMP; 30 microM) but not its nonhydrolyzable analog 8-(4-chlorophenylthio)-cAMP, Sp-isomer, enhanced the expression of bTREK-1 mRNA and current. 8CPT-cAMP metabolites, including 8CPT-adenosine and 8CPT-adenine, also increased bTREK-1 expression. These results indicate that cAMP increases the expression of bTREK-1 mRNA and K(+) current through a cAMP-dependent but Epac2-independent mechanism. They further demonstrate that one or more metabolites of 8-(4-chlorophenylthio)-cAMP analogs potently stimulate bTREK-1 expression by activation of a novel cAMP-independent mechanism. These findings raise significant questions regarding the specificity of 8-(4-chlorophenylthio)-cAMP analogs as cAMP mimetics.

ACTH inhibits bTREK-1 K+ channels through multiple cAMP-dependent signaling pathways

Bovine adrenal zona fasciculata (AZF) cells express bTREK-1 K⁺ channels that set the resting membrane potential and function pivotally in the physiology of cortisol secretion. Inhibition of these K⁺ channels by adrenocorticotropic hormone (ACTH) or cAMP is coupled to depolarization and Ca²⁺ entry. The mechanism of ACTH and cAMP-mediated inhibition of bTREK-1 was explored in whole cell patch clamp recordings from AZF cells. Inhibition of bTREK-1 by ACTH and forskolin was not affected by the addition of both H-89 and PKI(6–22) amide to the pipette solution at concentrations that completely blocked activation of cAMP-dependent protein kinase (PKA) in these cells. The ACTH derivative, O-nitrophenyl, sulfenyl-adrenocorticotropin (NPS-ACTH), at concentrations that produced little or no activation of PKA, inhibited bTREK-1 by a Ca²⁺-independent mechanism. Northern blot analysis showed that bovine AZF cells robustly express mRNA for Epac2, a guanine nucleotide exchange protein activated by cAMP. The selective Epac activator, 8-pCPT-2′-O-Me-cAMP, applied intracellularly through the patch pipette, inhibited bTREK-1 (IC₅₀ = 0.63 μM) at concentrations that did not activate PKA. Inhibition by this agent was unaffected by PKA inhibitors, including RpcAMPS, but was eliminated in the absence of hydrolyzable ATP. Culturing AZF cells in the presence of ACTH markedly reduced the expression of Epac2 mRNA. 8-pCPT-2′-O-Me-cAMP failed to inhibit bTREK-1 current in AZF cells that had been treated with ACTH for 3–4 d while inhibition by 8-br-cAMP was not affected. 8-pCPT-2′-O-Me-cAMP failed to inhibit bTREK-1 expressed in HEK293 cells, which express little or no Epac2. These findings demonstrate that, in addition to the well-described PKA-dependent TREK-1 inhibition, ACTH, NPS-ACTH, forskolin, and 8-pCPT-2′-O-Me-cAMP also inhibit these K⁺ channels by a PKA-independent signaling pathway. The convergent inhibition of bTREK-1 through parallel PKA- and Epac-dependent mechanisms may provide for failsafe membrane depolarization by ACTH.

Curcumin potently blocks Kv1.4 potassium channels

Curcumin, a major constituent of the spice turmeric, is a nutriceutical compound reported to possess therapeutic properties against a variety of diseases ranging from cancer to cystic fibrosis. In whole-cell patch-clamp experiments on bovine adrenal zona fasciculata (AZF) cells, curcumin reversibly inhibited the Kv1.4K+ current with an IC50 of 4.4 microM and a Hill coefficient of 2.32. Inhibition by curcumin was significantly enhanced by repeated depolarization; however, this agent did not alter the voltage-dependence of steady-state inactivation. Kv1.4 is the first voltage-gated ion channel demonstrated to be inhibited by curcumin. Furthermore, these results identify curcumin as one of the most potent antagonists of these K+ channels identified thus far. It remains to be seen whether any of the therapeutic actions of curcumin might originate with its ability to inhibit Kv1.4 or other voltage-gated K+ channel.

Pituitary adenylate cyclase activating polypeptide reduces expression of Kv1.4 and Kv4.2 subunits underlying A-type K(+) current in adult mouse olfactory neuroepithelia

A-type K(+) currents (I(A)) in olfactory receptor neurons have been characterized electrophysiologically but the molecular identities of the underlying channel subunits have not been determined. Using RT-PCR, immunoblot and immunohistochemistry, we found that the two candidate channel families underlying I(A), shaker and shal, are expressed in olfactory epithelia of Swiss Webster mice. Specifically, Kv1.4, the only I(A) candidate from the shaker family, and Kv4.2 and Kv4.3 from the shal family were expressed, but Kv4.1 mRNA was not amplified from the olfactory epithelia. Immunoblot and immunohistochemical studies confirmed the existence of Kv1.4 and Kv4.2/3 subunits. Furthermore, quantitative RT-PCR showed that pituitary adenylate cyclase activating polypeptide (PACAP) reduced the expression of Kv1.4 and Kv4.2 but did not reduce the already low expression of Kv4.3. The PACAP-induced reduction of Kv4.1 and Kv4.2 expression was completely blocked by inhibiting the phospholipase C (PLC) pathway but was still significantly downregulated by PACAP when the cyclic AMP pathway was inhibited. In addition, downstream of the PLC pathway, calcium mediated the reduction of both Kv1.4 and Kv4.2 expression and I(A) current density. Phosphokinase C (PKC) activation did not affect Kv1.4 and Kv4.2 mRNA expression, even though PKC reduced I(A) current density. Together with our previous studies, our data suggest that A-type K(+) currents in olfactory receptor neurons are composed of multiple K(+) channel subunits, among which Kv1.4 and Kv4.2 are subject to transcriptional modulation by PACAP. We also found that PACAP predominately uses a PLC-calcium pathway to modulate Kv4.1 and Kv4.2 expression. Modulation of A-type K(+) current expression may contribute to the previously observed neuroprotective effects of PACAP on olfactory receptor neurons.

Excitability of auditory brainstem neurons, in vivo, is increased by cyclic-AMP

Physiological control of auditory neural responses is critical for accurate representation of acoustic information, such as sound source localization and speech perception. Central auditory neural responses are almost certainly regulated by a range of mechanisms, including second messenger systems, such as the cAMP pathway. An increase in spontaneous neural discharge is known to accompany cochlear insults. Here we report that an increase in spontaneous as well as tone-evoked discharge can also be induced by pressure application of forskolin, a pharmacological agent that elevates intracellular cAMP level by activating adenyl cyclase. The forskolin induced increase in superior olivary complex (SOC) brainstem neurons is specific, dose-dependent, and reversible, whereas application of artificial cerebrospinal fluid (aCSF, the vehicle) does not alter activity. Forskolin-application also has a relatively greater effect on spontaneous activity compared to tone evoked responses. Blockade of the hyperpolarization-activated current, Ih, by ZD7288, consistently reversed the effects of forskolin. Based on these findings, we propose that the second messenger, cAMP, can significantly modulate neural excitability and spontaneous discharge in SOC neurons, principally by shifting the activation of Ih channels.

Angiotensin II inhibits bTREK-1 K+ channels in adrenocortical cells by separate Ca2+- and ATP hydrolysis-dependent mechanisms

Bovine adrenocortical cells express bTREK-1 K+ channels that set the resting membrane potential (V(m)) and couple angiotensin II (AngII) and adrenocorticotropic hormone (ACTH) receptors to membrane depolarization and corticosteroid secretion. In this study, it was discovered that AngII inhibits bTREK-1 by separate Ca2+- and ATP hydrolysis-dependent signaling pathways. When whole cell patch clamp recordings were made with pipette solutions that support activation of both Ca2+- and ATP-dependent pathways, AngII was significantly more potent and effective at inhibiting bTREK-1 and depolarizing adrenal zona fasciculata cells, than when either pathway is activated separately. External ATP also inhibited bTREK-1 through these two pathways, but ACTH displayed no Ca2+-dependent inhibition. AngII-mediated inhibition of bTREK-1 through the novel Ca2+-dependent pathway was blocked by the AT1 receptor antagonist losartan, or by including guanosine-5'-O-(2-thiodiphosphate) in the pipette solution. The Ca2+-dependent inhibition of bTREK-1 by AngII was blunted in the absence of external Ca2+ or by including the phospholipase C antagonist U73122, the inositol 1,4,5-trisphosphate receptor antagonist 2-amino-ethoxydiphenyl borate, or a calmodulin inhibitory peptide in the pipette solution. The activity of unitary bTREK-1 channels in inside-out patches from adrenal zona fasciculata cells was inhibited by application of Ca2+ (5 or 10 microM) to the cytoplasmic membrane surface. The Ca2+ ionophore ionomycin also inhibited bTREK-1 currents through channels expressed in CHO-K1 cells. These results demonstrate that AngII and selected paracrine factors that act through phospholipase C inhibit bTREK-1 in adrenocortical cells through simultaneous activation of separate Ca2+- and ATP hydrolysis-dependent signaling pathways, providing for efficient membrane depolarization. The novel Ca2+-dependent pathway is distinctive in its lack of ATP dependence, and is clearly different from the calmodulin kinase-dependent mechanism by which AngII modulates T-type Ca2+ channels in these cells.

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.

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.

Modulation of native TREK-1 and Kv1.4 K+ channels by polyunsaturated fatty acids and lysophospholipids

The modulation of TREK-1 leak and Kv1.4 voltage-gated K+ channels by fatty acids and lysophospholipids was studied in bovine adrenal zona fasciculata (AZF) cells. In whole-cell patch-clamp recordings, arachidonic acid (AA) (1-20 microM) dramatically and reversibly increased the activity of bTREK-1, while inhibiting bKv1.4 current by mechanisms that occurred with distinctly different kinetics. bTREK-1 was also activated by the polyunsaturated cis fatty acid linoleic acid but not by the trans polyunsaturated fatty acid linolelaidic acid or saturated fatty acids. Eicosatetraynoic acid (ETYA), which blocks formation of active AA metabolites, failed to inhibit AA activation of bTREK-1, indicating that AA acts directly. Compared to activation of bTREK-1, inhibition of bKv1.4 by AA was rapid and accompanied by a pronounced acceleration of inactivation kinetics. Cis polyunsaturated fatty acids were much more effective than trans or saturated fatty acids at inhibiting bKv1.4. ETYA also effectively inhibited bKv1.4, but less potently than AA. bTREK-1 current was markedly increased by lysophospholipids including lysophosphatidyl choline (LPC) and lysophosphatidyl inositol (LPI). At concentrations from 1-5 microM, LPC produced a rapid, transient increase in bTREK-1 that peaked within one minute and then rapidly desensitized. The transient lysophospholipid-induced increases in bTREK-1 did not require the presence of ATP or GTP in the pipette solution. These results indicate that the activity of native leak and voltage-gated K+ channels are directly modulated in reciprocal fashion by AA and other cis unsaturated fatty acids. They also show that lysophospholipids enhance bTREK-1, but with a strikingly different temporal pattern. The modulation of native K+ channels by these agents differs from their effects on the same channels expressed in heterologous cells, highlighting the critical importance of auxiliary subunits and signaling. Finally, these results reveal that AZF cells express thousands of bTREK-1 K+ channels that lie dormant until activated by metabolites including phospholipase A2 (PLA2)-generated fatty acids and lysophospholipids. These metabolites may alter the electrical and secretory properties of AZF cells by modulating bTREK-1 and bKv1.4 K+ channels.

Corticotropin induces the expression of TREK-1 mRNA and K+ current in adrenocortical cells

Bovine adrenal zona fasciculata (AZF) cells express a two-pore/four-transmembrane segment bTREK-1 K+ channel that sets the resting potential and couples hormonal signals to depolarization-dependent Ca2+ entry and cortisol secretion. It was discovered that corticotropin (1-2000 pM) enhances the expression of bTREK-1 mRNA and membrane current in cultured AZF cells. Forskolin and 8-pcpt-cAMP mimicked corticotropin induction of bTREK-1 mRNA, but angiotensin II (AII) was ineffective. The induction of bTREK-1 mRNA by corticotropin was partially blocked by the A-kinase antagonist H-89. 8-(4-Chloro-phenylthio)-2-O-methyladenosine-3'-5'-cyclic monophosphate, a cAMP analog that activates cAMP-regulated guanine nucleotide exchange factors (Epac), failed to increase bTREK-1 mRNA. Corticotropin-stimulated increases in bTREK-1 mRNA were eliminated by inhibitors of protein synthesis or gene transcription. bTREK-1 current disappeared after 24 h in serum-supplemented medium, but in the presence of corticotropin, bTREK-1 expression was maintained for at least 48 h. The enhancement of bTREK-1 mRNA and ionic current contrasts with the corticotropin-induced down-regulation of the Kv1.4 voltage-gated K+ current and associated mRNA in AZF cells. These results demonstrate that corticotropin rapidly and potently induces the expression of bTREK-1 in AZF cells at the pretranslational level by a cAMP-dependent mechanism that is partially dependent on A-kinase but independent of Epac and Ca2+. They further indicate that prolonged stimulation of AZF cells by corticotropin, as occurs during long-term stress or disease, may produce pronounced changes in the expression of genes encoding ion channels, thereby reshaping the electrical properties of these cells to enhance or limit cortisol secretion.

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.

An ACTH- and ATP-regulated background K+ channel in adrenocortical cells is TREK-1

Bovine adrenal zona fasciculata (AZF) cells express a background K(+) channel (I(AC)) that sets the resting potential and acts pivotally in ACTH-stimulated cortisol secretion. We have cloned a bTREK-1 (KCNK2) tandem-pore K(+) channel cDNA from AZF cells with properties that identify it as the native I(AC). The bTREK-1 cDNA is expressed robustly in AZF cells and includes transcripts of 4.9, 3.6, and 2.8 kb. In patch clamp recordings made from transiently transfected cells, bTREK-1 displayed distinctive properties of I(AC) in AZF cells. Specifically, bTREK-1 currents were outwardly rectifying with a large instantaneous and smaller time-dependent component. Similar to I(AC), bTREK-1 increased spontaneously in amplitude over many minutes of whole cell recording and was blocked potently by Ca(2+) antagonists including penfluridol and mibefradil and by 8-(4-chlorophenylthio)-cAMP. Unitary TREK-1 and I(AC) currents were nearly identical in amplitude. The native I(AC) current, in turn, displayed properties that together are specific to TREK-1 K(+) channels. These include activation by intracellular acidification, enhancement by the neuroprotective agent riluzole, and outward rectification. bTREK-1 current differed from native K(+) current only in its lack of ATP dependence. In contrast to I(AC), the current density of bTREK-1 in human embryonic kidney-293 cells was not increased by raising pipette ATP from 0.1 to 5 mm. Further, the enhancement of I(AC) current in AZF cells by low pH and riluzole was facilitated by, and dependent on, ATP at millimolar concentrations in the pipette solution. Overall, these results establish the identity of I(AC) K(+) channels, demonstrate the expression of bTREK-1 in a specific endocrine cell, identify potent new TREK-1 antagonists, and assign a pivotal role for these tandem-pore channels in the physiology of cortisol secretion. The activation of I(AC) by ATP indicates that native bTREK-1 channels may function as sensors that couple the metabolic state of the cell to membrane potential, perhaps through an associated ATP-binding protein.

Molecular diversity of vascular potassium channel isoforms

1. One essential role for potassium channels in vascular smooth muscle is to buffer cell excitation and counteract vasoconstrictive influences. Several molecular mechanisms regulate potassium channel function. The interaction of these mechanisms may be one method for fine-tuning potassium channel activity in response to various physiological and pathological challenges. 2. The most prevalent K+ channels in vascular smooth muscle are large-conductance calcium- and voltage-sensitive channels (maxi-K channels) and voltage-gated channels (Kv channels). Both channel types are complex molecular structures consisting of a pore-forming alpha-subunit and an ancillary beta-subunit. The maxi-K and Kv channel alpha-subunits assemble as tetramers and have S4 transmembrane domains that represent the putative voltage sensor. While most vascular smooth muscle cells identified to date contain both maxi-K and Kv channels, the expression of individual alpha-subunit isoforms and beta-subunit association occurs in a tissue-specific manner, thereby providing functional specificity. 3. The maxi-K channel alpha-subunit derives its molecular diversity by alternative splicing of a single-gene transcript to yield multiple isoforms that differ in their sensitivity to intracellular Ca2+ and voltage, cell surface expression and post- translational modification. The ability of this channel to assemble as a homo- or heterotetramer allows for fine-tuning control to intracellular regulators. Another level of diversity for this channel is in its association with accessory beta-subunits. Multiple beta-subunits have been identified that can arise either from separate genes or alternative splicing of a beta-subunit gene. The maxi-K channel beta-subunits modulate the channel's Ca2+ and voltage sensitivity and kinetic and pharmacological properties. 4. The Kv channel alpha-subunit derives its diverse nature by the expression of several genes. Similar to the maxi-K channel, this channel has been shown to assemble as a homo- and heterotetramer, which can significantly change the Kv current phenotype in a given cell type. Association with a number of the ancillary beta-subunits affects Kv channel function in several ways. Beta-subunits can induce inactivating properties and act as chaperones, thereby regulating channel cell-surface expression and current kinetics.

Reciprocal modulation of voltage-gated and background K(+) channels mediated by nucleotides and corticotropin

Bovine adrenal zona fasciculata (AZF) cells express two types of K(+)-selective ion channels including a rapidly inactivating bKv1.4 current (I(A)) and an ATP-dependent noninactivating background current (I(AC)) that sets the resting membrane potential. Whole-cell, patch-clamp recording from cultured AZF cells was used to demonstrate a novel reciprocal modulation of these two K(+) channels by intracellular nucleotides and corticotropin. Specifically, increases in I(AC) activity induced by intracellular ATP, as well as GTP and 5'-adenylyl-imidodiphosphate (AMP-PNP), were accompanied by a corresponding decrease in the amplitude of the voltage-gated I(A) current. The reduction in I(A) current was observed only when patch pipettes contained ATP or other nucleotides at concentrations sufficient to support activation of I(AC). Conversely, the nearly complete inhibition of I(AC) by corticotropin was accompanied by the coincident reappearance of functional I(A) channels. In the absence of I(AC) current, corticotropin failed to alter I(A). The reciprocal modulation of AZF cell K(+) channels by nucleotides and corticotropin was independent of membrane voltage. These results demonstrate a new form of channel modulation in which the activity of two different K(+) channels is reciprocally modulated in tandem through hormonal and metabolic signaling pathways. They further suggest that I(A) and I(AC) K(+) channels may be functionally coupled in a dynamic equilibrium driven by intracellular ATP and G-protein-coupled receptors. This may represent a unique mechanism for transducing biochemical signals to ionic events involved in cortisol secretion.