Activation of calcium current in voltage-clamped rat glomerulosa cells by potassium ions

Aldosterone-Regulated Sodium Transport and Blood Pressure

Aldosterone is a major mineralocorticoid steroid hormone secreted by glomerulosa cells in the adrenal cortex. It regulates a variety of physiological responses including those to oxidative stress, inflammation, fluid disruption, and abnormal blood pressure through its actions on various tissues including the kidney, heart, and the central nervous system. Aldosterone synthesis is primarily regulated by angiotensin II, K⁺ concentration, and adrenocorticotrophic hormone. Elevated serum aldosterone levels increase blood pressure largely by increasing Na⁺ re-absorption in the kidney through regulating transcription and activity of the epithelial sodium channel (ENaC). This review focuses on the signaling pathways involved in aldosterone synthesis and its effects on Na⁺ reabsorption through ENaC.

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.

T-Type Calcium Channel: A Privileged Gate for Calcium Entry and Control of Adrenal Steroidogenesis

Intracellular calcium plays a crucial role in modulating a variety of functions such as muscle contraction, hormone secretion, gene expression, or cell growth. Calcium signaling has been however shown to be more complex than initially thought. Indeed, it is confined within cell microdomains, and different calcium channels are associated with different functions, as shown by various channelopathies. Sporadic mutations on voltage-operated L-type calcium channels in adrenal glomerulosa cells have been shown recently to be the second most prevalent genetic abnormalities present in human aldosterone-producing adenoma. The observed modification of the threshold of activation of the mutated channels not only provides an explanation for this gain of function but also reminds us on the importance of maintaining adequate electrophysiological characteristics to make channels able to exert specific cellular functions. Indeed, the contribution to steroid production of the various calcium channels expressed in adrenocortical cells is not equal, and the reason has been investigated for a long time. Given the very negative resting potential of these cells, and the small membrane depolarization induced by their physiological agonists, low threshold T-type calcium channels are particularly well suited for responding under these conditions and conveying calcium into the cell, at the right place for controlling steroidogenesis. In contrast, high threshold L-type channels are normally activated by much stronger cell depolarizations. The fact that dihydropyridine calcium antagonists, specific for L-type channels, are poorly efficient for reducing aldosterone secretion either in vivo or in vitro, strongly supports the view that these two types of channels differently affect steroid biosynthesis. Whether a similar analysis is transposable to fasciculata cells and cortisol secretion is one of the questions addressed in the present review. No similar mutations on L-type or T-type channels have been described yet to affect cortisol secretion or to be linked to the development of Cushing syndrome, but several evidences suggest that the function of T channels is also crucial in fasciculata cells. Putative molecular mechanisms and cellular structural organization making T channels a privileged entry for the "steroidogenic calcium" are also discussed.

Dopaminergic modulation of GABAergic transmission in the entorhinal cortex: concerted roles of α1 adrenoreceptors, inward rectifier K⁺, and T-type Ca²⁺ channels

Whereas the entorhinal cortex (EC) receives profuse dopaminergic innervations from the midbrain, the effects of dopamine (DA) on γ-Aminobutyric acid (GABA)ergic interneurons in this brain region have not been determined. We probed the actions of DA on GABAA receptor-mediated synaptic transmission in the EC. Application of DA increased the frequency, not the amplitude, of spontaneous IPSCs (sIPSCs) and miniature IPSCs (mIPSCs) recorded from entorhinal principal neurons, but slightly reduced the amplitude of the evoked IPSCs. The effects of DA were unexpectedly found to be mediated by α1 adrenoreceptors, but not by DA receptors. DA endogenously released by the application of amphetamine also increased the frequency of sIPSCs. Ca(2+) influx via T-type Ca(2+) channels was required for DA-induced facilitation of sIPSCs and mIPSCs. DA depolarized and enhanced the firing frequency of action potentials of interneurons. DA-induced depolarization was independent of extracellular Na(+) and Ca(2+) and did not require the functions of hyperpolarization-activated (Ih) channels and T-type Ca(2+) channels. DA-generated currents showed a reversal potential close to the K(+) reversal potential and inward rectification, suggesting that DA inhibits the inward rectifier K(+) channels (Kirs). Our results demonstrate that DA facilitates GABA release by activating α1 adrenoreceptors to inhibit Kirs, which further depolarize interneurons resulting in secondary Ca(2+) influx via T-type Ca(+) channels.

Multiple T-type Ca2+ current subtypes in electrophysiologically characterized hamster dorsal horn neurons: possible role in spinal sensory integration

Whole cell patch-clamp recordings were used to investigate the contribution of transient, low-threshold calcium currents (I(T)) to firing properties of hamster spinal dorsal horn neurons. I(T) was widely, though not uniformly, expressed by cells in Rexed's laminae I-IV and correlated with the pattern of action potential discharge evoked under current-clamp conditions: I(T) in neurons responding to constant membrane depolarization with one or two action potentials was nearly threefold larger than I(T) in cells responding to the same activation with continuous firing. I(T) was evoked by depolarizing voltage ramps exceeding 46 mV/s and increased with ramp slope (240-2,400 mV/s). Bath application of 200 μM Ni(2+) depressed ramp-activated I(T). Phasic firing recorded in current clamp could only be activated by membrane depolarizations exceeding ∼43-46 mV/s and was blocked by Ni(2+) and mibefradil, suggesting I(T) as an underlying mechanism. Two components of I(T), "fast" and "slow," were isolated based on a difference in time constant of inactivation (12 ms and 177 ms, respectively). The amplitude of the fast subtype depended on the slope of membrane depolarization and was twice as great in burst-firing cells than in cells having a tonic discharge. Post hoc single-cell RT-PCR analyses suggested that the fast component is associated with the Ca(V)3.1 channel subtype. I(T) may enhance responses of phasic-firing dorsal horn neurons to rapid membrane depolarizations and contribute to an ability to discriminate between afferent sensory inputs that encode high- and low-frequency stimulus information.

Role of voltage-gated calcium channels in potassium-stimulated aldosterone secretion from rat adrenal zona glomerulosa cells

The mineralocorticoid aldosterone plays an important role in the regulation of plasma electrolyte homeostasis. Exposure of acutely isolated rat adrenal zona glomerulosa cells to elevated K(+) activates voltage-gated calcium channels and initiates a calcium-dependent increase in aldosterone synthesis. We developed a novel 96-well format aldosterone secretion assay to rapidly evaluate the effect of known T- and L-type calcium channel antagonists on K(+)-stimulated aldosterone secretion and better define the role of voltage-gated calcium channels in this process. Reported T-type antagonists, mibefradil and Ni(2+), and selected L-type antagonist dihydropyridines, inhibited K(+)-stimulated aldosterone synthesis. Dihydropyridine-mediated inhibition occurred at concentrations which had no effect on rat alpha1H T-type Ca(2+) currents. In contrast, below 10 microM, the L-type antagonists verapamil and diltiazem showed only minimal inhibitory effects. To examine the selectivity of the calcium channel antagonist-mediated inhibition, we established an aldosterone secretion assay in which 8Br-cAMP stimulates aldosterone secretion independent of extracellular calcium. Mibefradil remained inhibitory in this assay, while the dihydropyridines had only limited effects. Taken together, these data demonstrate a role for the L-type calcium channel in K(+)-stimulated aldosterone secretion. Further, they confirm the need for selective T-type calcium channel antagonists to better address the role of T-type channels in K(+)-stimulated aldosterone secretion.

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.

TASK (TWIK-related acid-sensitive K+ channel) is expressed in glomerulosa cells of rat adrenal cortex and inhibited by angiotensin II

The present study was conducted to explore the possible contribution of a recently described leak K+ channel, TASK (TWIK-related acid-sensitive K+ channel), to the high resting K+ conductance of adrenal glomerulosa cells. Northern blot analysis showed the strongest TASK message in adrenal glomerulosa (capsular) tissue among the examined tissues including heart and brain. Single-cell PCR demonstrated TASK expression in glomerulosa cells. In patch-clamp experiments performed on isolated glomerulosa cells the inward current at -100 mV in 30 mM [K+] (reflecting mainly potassium conductance) was pH sensitive (17+/-2% reduction when the pH changed from 7.4 to 6.7). In Xenopus oocytes injected with mRNA prepared from adrenal glomerulosa tissue the expressed K+ current at -100 mV was virtually insensitive to tetraethylammonium (3 mM) and 4-aminopyridine (3 mM). Ba2+ (300 microM) and Cs+ (3 mM) induced voltage-dependent block. Lidocaine (1 mM) and extracellular acidification from pH 7.5 to 6.7 inhibited the current (by 28% and 16%, respectively). This inhibitory profile is similar (although it is not identical) to that of TASK expressed by injecting its cRNA. In oocytes injected with adrenal glomerulosa mRNA, TASK antisense oligonucleotide reduced significantly the expression of K+ current at -100 mV, while the sense oligonucleotide failed to have inhibitory effect. Application of angiotensin II (10 nM) both in isolated glomerulosa cells and in oocytes injected with adrenal glomerulosa mRNA inhibited the K+ current at -100 mV. Similarly, in oocytes coexpressing TASK and ATla angiotensin II receptor, angiotensin II inhibited the TASK current. These data together indicate that TASK contributes to the generation of high resting potassium permeability of glomerulosa cells, and this background K+ channel may be a target of hormonal regulation.

Functional alteration of dihydropyridine-sensitive Ca(2+) channels in the adrenal glomerulosa of pregnant rats

Our previous work on aldosterone secretion suggested that dihydropyridine-sensitive calcium channels, one type of voltage-dependent calcium channels (VDCC), are functionally impaired in adrenal capsule preparations from the pregnant rat. The aim of this study was to determine whether, during pregnancy, the density and/or activity of these channels is altered in the adrenal zona glomerulosa. These VDCC measured with [(3)H]nitrendipine binding were not different between membrane preparations of nonpregnant and pregnant rats. Western blots were performed using two different antibodies, a polyclonal (PcAb) directed against the alpha(1)-subunit of VDCC and a monoclonal (McAb) that recognizes an intracellular domain of that protein. McAb immunoreactivity showed a significant decrease in preparations from pregnant rats, whereas no difference was observed with PcAb. VDCC activity was estimated by (45)Ca(2+) uptake in isolated adrenal cortex and by intracellular calcium concentration ([Ca(2+)](i)) in adrenal glomerulosa cells with the Ca(2+) probe fura PE3. These measurements revealed that KCl stimulation produced greater Ca(2+) influx in nonpregnant than in pregnant rats. Nifedipine (a blocker of VDCC) inhibited this stimulation only in nonpregnant rats, whereas BAY K 8644 (an activator of VDCC) increased Ca(2+) influx in pregnant rats only. These data suggest that, during pregnancy, the altered regulation of calcium homeostasis in adrenal glomerulosa is linked to a conformational alteration of VDCC.

Selective inhibition of potassium-stimulated rat adrenal glomerulosa cells by ruthenium red

The effect of the cationic dye, ruthenium red (RR), on ionic fluxes, Ca2+ signal generation, and stimulation of aldosterone production was studied in isolated rat adrenal glomerulosa cells. In these cells, increased extracellular [K+] as well as angiotensin II (Ang II) elevate cytoplasmic Ca2+ concentration and thereupon activate steroidogenesis. However, the mode of action of the two stimuli are different: while a dihidropyridine-sensitive mechanism contributes to the response to both agonists, Ang II induces Ca2+ release from intracellular stores and causes capacitative Ca2+ influx, whereas K+ was recently shown to activate a plasma membrane Ca2+ current (Igl) independently of membrane depolarization. The difference is reflected in the sensitivity of the response of the cells to RR. The Ang II-induced Ca2+ signal and aldosterone production were not inhibited, but rather slightly potentiated by the dye. This potentiation was probably the consequence of the membrane-depolarizing effect of RR, due to the observed inhibition of the resting K+ conductance. Conversely, Ca2+ signal and aldosterone production were significantly reduced by RR when the cells were stimulated by moderately elevated [K+] (6-8 mM). Our patch clamp studies suggest that this effect was related to the inhibition of different voltage-dependent and -independent inward Ca2+ currents and indicates the functional importance of the latter in the signal transduction of the potassium-stimulated glomerulosa cell.

Differential response of cortical plate and ventricular zone cells to GABA as a migration stimulus

A microdissection technique was used to separate differentiated cortical plate (cp) cells from immature ventricular zone cells (vz) in the rat embryonic cortex. The cp population contained >85% neurons (TUJ1(+)), whereas the vz population contained approximately 60% precursors (nestin+ only). The chemotropic response of each population was analyzed in vitro, using an established microchemotaxis assay. Micromolar GABA (1-5 microM) stimulated the motility of cp neurons expressing glutamic acid decarboxylase (GAD), the rate-limiting enzyme in GABA synthesis. In contrast, femtomolar GABA (500 fM) directed a subset of GAD- vz neurons to migrate. Thus, the two GABA concentrations evoked the motility of phenotypically distinct populations derived from different anatomical regions. Pertussis toxin (PTX) blocked GABA-induced migration, indicating that chemotropic signals involve G-protein activation. Depolarization by micromolar muscimol, elevated [K+]o, or micromolar glutamate arrested migration to GABA or GABA mimetics, indicating that migration is inhibited in the presence of excitatory stimuli. These results suggest that GABA, a single ligand, can promote motility via G-protein activation and arrest attractant-induced migration via GABAA receptor-mediated depolarization.

Differential response of cortical plate and ventricular zone cells to GABA as a migration stimulus

A microdissection technique was used to separate differentiated cortical plate (cp) cells from immature ventricular zone cells (vz) in the rat embryonic cortex. The cp population contained >85% neurons (TUJ1(+)), whereas the vz population contained approximately 60% precursors (nestin+ only). The chemotropic response of each population was analyzed in vitro, using an established microchemotaxis assay. Micromolar GABA (1-5 microM) stimulated the motility of cp neurons expressing glutamic acid decarboxylase (GAD), the rate-limiting enzyme in GABA synthesis. In contrast, femtomolar GABA (500 fM) directed a subset of GAD- vz neurons to migrate. Thus, the two GABA concentrations evoked the motility of phenotypically distinct populations derived from different anatomical regions. Pertussis toxin (PTX) blocked GABA-induced migration, indicating that chemotropic signals involve G-protein activation. Depolarization by micromolar muscimol, elevated [K+]o, or micromolar glutamate arrested migration to GABA or GABA mimetics, indicating that migration is inhibited in the presence of excitatory stimuli. These results suggest that GABA, a single ligand, can promote motility via G-protein activation and arrest attractant-induced migration via GABAA receptor-mediated depolarization.

Calcium current activated by potassium ions in voltage-clamped rat hippocampal pyramidal neurones

1. Neuronal activity results in local elevation of extracellular K+ concentration ([K+]o). 2. Using the patch-clamp technique in the whole-cell configuration, we investigated whether extracellular K+ activates non-voltage-operated Ca2+ channels in pyramidal cells cultured from rat embryonic hippocampi. 3. K+ (12 mM) reversibly activated a sustained inward current at a holding potential of -100 mV. Membrane conductance and variance of noise were significantly increased by K+. This current could be observed at membrane potentials negative to +60 mV. 4. Inhibitors of inward rectifier K+ channels and hyperpolarization-induced cation current reduced the current only at potentials negative to -50 mV. 5. The K+-induced current was activated in Na+-free but not in Ca2+-free medium, did not depend on cytosolic [Cl-], and was blocked by Cd2+ but not by organic channel inhibitors. 6. Half-maximal activation of the current (at -100 mV) was attained at [K+]o approximately 20 mM. 7. The current is similar to Igl, a K+-induced Ca2+ current described in glomerulosa cells. It was also present in pyramidal cells from prefrontal cortex but not in hippocampal bipolar and glial cells. 8. Activation of K+-induced Ca2+ current may elevate cytoplasmic [Ca2+] at [K+]o levels which are insufficent to activate voltage-dependent Ca2+ channels.

Extracellular K+ activates a K(+)- and H(+)-permeable conductance in frog taste receptor cells

1. The effect of extracellular K+ on membrane currents of bull frog (Rana catesbeiana) taste receptor cells (TRCs) was investigated by the patch clamp and fast perfusion techniques. Extracellular K+ (2.5-90 mM) increased a TRC resting conductance and enhanced both inward and outward whole-cell currents. 2. To isolate the inward current activated by external potassium (PA current), TRCs were dialysed with 110 mM NMGCl while extracellular NaCl was replaced with NMGCl. Under these conditions, the PA current displayed an S-shaped current-voltage (I-V) curve in the -100 to 100 mV range. Extracellular Rb+ and NH4+, but not Li+, Na+ or Cs+, evoked similar currents. 3. The PA current reversal potential (Vr) did not follow the equilibrium K+ potential under experimental conditions. Therefore, K+ ions were not the only current carriers. The influence of other ions on the PA current Vr indicated that the channels involved are permeable to K+ and H+ and much less so to Na+, Ca2+ and Mg2+. Relative permeabilities were estimated on the basis of the Goldman-Hodgkin-Katz equation as follows: PH:PK:PNa = 4000:1:0.04. 4. All I-V curves of the PA current were nearly linear at low negative potentials. The slope conductance at these voltages was used to characterize the dependence of the PA current on external K+ and H+. The slope conductance versus K+ concentration was fitted by the Hill equation. The data yielded a half-maximal concentration, K1/2 = 19 +/- 3 mM and a Hill coefficient, nH = 1.53 +/- 0.36 (means +/- S.E.M.). 5. The dependence of the mean PA current and the current variance on the K+ concentration indicated a rise in the open probability of the corresponding channels as extracellular K+ was increased. With 110 mM KCl in the bath, the single channel conductance was estimated at about 6 pS. Taken together, the data suggest that extracellular K+ may serve as a ligand to activate specific small-conductance cation channels (PA channels). The mean number of the PA channels per TRC was estimated as at least 2000. 6. Extracellular Ba2+, Cd2+, Co2+, Ni2+ and Cs+ blocked the PA current in a potential-dependent manner. The PA current was blocked by Cs+ as quickly as the blocker could be applied (approximately 15 ms). The time course of the divalent cation block was well fitted by a single exponential function. The time constants were estimated at 26.5 +/- 1.9, 41.7 +/- 3.1, 56.1 +/- 4.2 and 370 +/- 18 ms at 1 mM Cd2+, Co2+, Ni2+ and Ba2+, respectively. The blocker efficiency at negative voltages followed the sequence: Cs+ > Cd2+ > Ba2+ > Ni2+ > Co2+. 7. The data indicate that protons and divalent blockers act within the PA channel pore and that H+ and the divalent ions probably act via similar mechanisms to affect the PA current. These observations and the strong pH dependence of the PA current Vr suggest that H+ occupation of the PA channel pore leading to interruption of K+ flux is the main mechanism of the pH dependence of the PA current. 8. Extracellular K+ enhanced the sensitivity of isolated TRCs to bath solution acidification due to activation of the PA channels. With 10 mM K+ in the bath, half-maximal depolarization of the TRCs was observed at pH values of 6.4-6.8. The possible role of the PA channels in sour transduction is discussed.

Voltage dependent calcium channels in adrenal glomerulosa cells and in insulin producing cells

We have examined the structure and function of Ca2+ channels in excitable endocrine cell types, in rat adrenal glomerulosa cells and in two insulin producing cell types, the rat pancreatic beta cell and the INS-1 cell line. In previous studies on glomerulosa cells, we observed low (T-type) and high threshold (L-type) voltage dependent Ca2+ currents in addition to a K+ induced inward rectifying Ca2+ current (Igl). beta cells are known to exhibit T-, L- and N-type currents. We have now found that INS-1 cells also show low threshold (T-type) and high threshold Ca2+ currents. The latter was further resolved by organic inhibitors into L-type and P/Q-type currents and no Igl was detected. The expression of the pore-forming alpha 1 subunit of voltage dependent Ca2+ channels was studied by means of reverse transcription-polymerase chain reaction (RT-PCR), followed by restriction enzyme mapping and/or sequencing. Both in glomerulosa and pancreatic beta cells, the neuroendocrine (D) class of the alpha 1 subunit, known to be responsible for L-type current, represents the majority of the PCR product. Comparable amounts of the neuroendocrine (D) and the neuronal A-type alpha 1 subunits dominate the message in INS-1 cells. Different characteristics of Ca2+ currents in these cell types is discussed in view of the channel repertoire.

Cytoplasmic Ca2+ signalling and reduction of mitochondrial pyridine nucleotides in adrenal glomerulosa cells in response to K+, angiotensin II and vasopressin

We have examined the mitochondrial formation of NAD(P)H in rat adrenal glomerulosa cells. A short-term elevation of the K+ concentration from 3.6 to 8.4 mM induced a reversible increase in the formation of reduced pyridine nucleotides. Potassium applied after the addition of rotenone had no further effect, confirming that the redox signal was of mitochondrial origin. Inhibition of aldosterone synthesis by aminoglutethimide in K+-stimulated cells decreased the rate of decay of the NAD(P)H signal upon the termination of stimulation, indicating that the NADPH formed was consumed in aldosterone synthesis. When the NAD(P)H signal was measured simultaneously with the cytoplasmic free Ca2+ concentration ([Ca2+]i), elevation of the K+ concentration to 6.6 or 8.4 mM induced parallel increases in [Ca2+]i and NAD(P)H formation. The rates of increase and decrease of NAD(P)H were lower than for [Ca2+]i, confirming that the redox signal was secondary to the Ca2+ signal. Angiotensin II (100 pM-1 nM) induced an oscillatory NAD(P)H signal which usually returned to a lower baseline concentration, while a sustained signal with superimposed oscillations was observed at higher concentrations. Simultaneous measurements showed that NAD(P)H levels followed the [Ca2+]i pattern evoked by angiotensin II. Vasopressin (100 nM) also induced parallel oscillations of [Ca2+]i and NAD(P)H. A sustained rise in the extramitochondrial Ca2+ concentration to 1 microM induced a sustained elevation of the intramitochondrial Ca2+ concentration in permeabilized cells, as measured with rhod-2. A sustained rise in [Ca2+]i evoked by long-term stimulation with 8.4 mM K+ or 2.5 nM angiotensin II resulted in sustained NAD(P)H production. These Ca2+-dependent changes in the mitochondrial redox state support the biological response, i.e. aldosterone secretion by glomerulosa cells.

The role of voltage-dependent calcium channels in angiotensin-stimulated glomerulosa cells

The concept that voltage-dependent Ca2+ influx is essential in the aldosterone stimulating action of angiotensin II (AII) has been recently challenged by the demonstration of the dihydropyridine (DHP) insensitive 'capacitative' Ca2+ uptake mechanism. The DHP-sensitivity of AII-induced aldosterone secretion is still to be explained. In rat glomerulosa cells the lag phase of AII-induced depolarization is more than 30 s, and there is no enhanced Ca2+ influx within the first min of stimulation. Yet we observed that DHPs as well as diltiazem influenced also the peak of cytoplasmic Ca2+ signal, although the peak (approximately 12 s) is attributed to Ca2+ release alone. Nifedipine reduced the Ca2+ transient induced by AII even after complete inhibition of Ca2+ channel activity. Recalling the loose attachment of InsP3 receptors (IP3R) to the plasma membrane, and the homology between the cytosolic domain of IP3R and the Ca2+ release channel (ryanodine receptor) of skeletal muscle, we proposed that DHP-sensitive L-type Ca2+ channels (DHP receptors) influence InsP3-induced Ca2+ release rather than Ca2+ influx in AII-stimulated cells. Although the dominant isoform is the neuroendocrine (D) one, the skeletal muscle isoform of L-type voltage-dependent Ca2+ channel is also expressed in rat glomerulosa cells. This isoform may be a candidate for protein-protein interaction between DHPR and subplasmalemmal IP3R, similarly to that occurring between DHP receptors and ryanodine receptors in skeletal muscle.

Potent block of potassium currents in rat isolated sympathetic neurones by the uncharged form of amitriptyline and related tricyclic compounds

1. The block of K+ currents by amitriptyline and the related tricyclic compounds cyproheptadine and dizocilpine was studied in dissociated rat sympathetic neurones by whole-cell voltage-clamp recording. 2. Cyproheptadine (30 microM) inhibited the delayed-rectifier current (Kv) by 92% and the transient current (KA) by 43%. For inhibition of Kv, cyproheptaidine had a KD of 2.2 microM. Dizocilpine (30 microM) inhibited Kv by 26% and KA by 22%. The stereoisomers of dizocilpine were equally potent at blocking Kv and KA. 3. Amitriptyline, a weak base, was significantly more effective in blocking Kv at pH 9.4 (KD = 0.46 microM) where the ratio of charged to uncharged drug was 50:50 compared with pH 7.4 (KD = 11.9 microM) where the ratio was 99:1. 4. N-methylamitriptyline (10 microM), the permanently charged analogue of amitriptyline, inhibited Kv by only 2% whereas in the same cells amitriptyline (10 microM) inhibited Kv by 36%. 5. Neither amitriptyline nor N-methylamitriptyline had a detectable effect on Kv when added to the intracellular solution. 6. It is concluded that the uncharged form of amitriptyline is approximately one hundred times more potent in blocking Kv than the charged form. However, this does not seem to be due to uncharged amitriptyline having better access to an intracellular binding site.