Control of cytosolic free calcium by intracellular organelles in bovine adrenal glomerulosa cells. Effects of sodium and inositol 1,4,5-trisphosphate

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.

The putative imidazoline receptor agonist, harmane, promotes intracellular calcium mobilisation in pancreatic β-cells

beta-Carbolines (including harmane and pinoline) stimulate insulin secretion by a mechanism that may involve interaction with imidazoline I(3)-receptors but which also appears to be mediated by actions that are additional to imidazoline receptor agonism. Using the MIN6 beta-cell line, we now show that both the imidazoline I(3)-receptor agonist, efaroxan, and the beta-carboline, harmane, directly elevate cytosolic Ca(2+) and increase insulin secretion but that these responses display different characteristics. In the case of efaroxan, the increase in cytosolic Ca(2+) was readily reversible, whereas, with harmane, the effect persisted beyond removal of the agonist and resulted in the development of a repetitive train of Ca(2+)-oscillations whose frequency, but not amplitude, was concentration-dependent. Initiation of the Ca(2+)-oscillations by harmane was independent of extracellular calcium but was sensitive to both dantrolene and high levels (20 mM) of caffeine, suggesting the involvement of ryanodine receptor-gated Ca(2+)-release. The expression of ryanodine receptor-1 and ryanodine receptor-2 mRNA in MIN6 cells was confirmed using reverse transcription-polymerase chain reaction (RT-PCR) and, since low concentrations of caffeine (1 mM) or thimerosal (10 microM) stimulated increases in [Ca(2+)](i), we conclude that ryanodine receptors are functional in these cells. Furthermore, the increase in insulin secretion induced by harmane was attenuated by dantrolene, consistent with the involvement of ryanodine receptors in mediating this response. By contrast, the smaller insulin secretory response to efaroxan was unaffected by dantrolene. Harmane-evoked changes in cytosolic Ca(2+) were maintained by nifedipine-sensitive Ca(2+)-influx, suggesting the involvement of L-type voltage-gated Ca(2+)-channels. Taken together, these data imply that harmane may interact with ryanodine receptors to generate sustained Ca(2+)-oscillations in pancreatic beta-cells and that this effect contributes to the insulin secretory response.

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

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

Angiotensin II activates cholesterol ester hydrolase in bovine adrenal glomerulosa cells through phosphorylation mediated by p42/p44 mitogen-activated protein kinase

In adrenal glomerulosa cells, the stimulation of aldosterone biosynthesis by angiotensin II (Ang II) occurs via activation of the Ca2+ messenger system, increased expression of the steroidogenic acute regulatory protein, and enhanced transfer of cholesterol to the inner mitochondrial membrane. We examined here whether Ang II affects the activity of cholesterol ester hydrolase (CEH), also named hormone-sensitive lipase, the enzyme recruiting cholesterol from intracellular pools, in bovine adrenal glomerulosa cells. In bovine adrenal tissue, CEH activity was detected with characteristics similar to those reported in other tissues (Michaelis constant = 46.3 +/- 6.7 microM, n = 3; maximal velocity = 1 nmol/mg.min). This activity was significantly enhanced in isolated bovine glomerulosa cells challenged for 2 h with 10 nM Ang II (to 149 +/- 11% of controls, n = 3). Similarly, 25 microM forskolin raised CEH activity to 151 +/- 5% of controls (n = 3). This increase in activity of CEH was not due to an increase in the amount of enzyme protein but was associated with an increased phosphorylation of the enzyme to 337 +/- 33% of controls (n = 9, P < 0.0001). Potassium ion (K+) and forskolin also stimulated [32P]orthophosphate incorporation, although to a lesser extent (to 157 +/- 18% and 186 +/- 25% of controls, respectively). On SDS-PAGE, the majority of this radioactivity was associated with a species of 172 kDa, corresponding to a CEH dimer. Both Ang II-induced CEH phosphorylation and pregnenolone production were significantly reduced (to 47 +/- 6% and 50 +/- 8% of controls with Ang II alone, respectively) in the presence of PD098059, an inhibitor of p42/p44 MAPK. Indeed, Ang II challenge led to a rapid 32P incorporation into p42/p44 MAPK. These results demonstrate that, in addition to its known effects on intramitochondrial cholesterol transfer, Ang II also promotes aldosterone biosynthesis by rapidly increasing cholesterol supply to the outer mitochondrial membrane.

Control of calcium homeostasis by angiotensin II in adrenal glomerulosa cells through activation of p38 MAPK

Angiotensin II-induced activation of aldosterone secretion in adrenal glomerulosa cells is mediated by an increase of intracellular calcium. We describe here a new Ca2+-regulatory pathway involving the inhibition by angiotensin II of calcium extrusion through the Na+/Ca2+ exchanger. Caffeine reduced both the angiotensin II-induced calcium signal and aldosterone production in bovine glomerulosa cells. These effects were independent of cAMP or calcium release from intracellular stores. The calcium response to angiotensin II was more sensitive to caffeine than the response to potassium, suggesting that the drug interacts with a pathway specifically elicited by the hormone. In calcium-free medium, calcium returned more rapidly to basal levels after angiotensin II stimulation in the presence of caffeine. Thapsigargin had no effect on these kinetics, but diltiazem, which inhibits the Na+/Ca2+ exchanger, markedly reduced the rate of calcium decrease and abolished caffeine action. The involvement of this exchanger was supported by the effect of cell depolarization and of a reduction of extracellular sodium on the rate of calcium extrusion. We also determined the mechanism of angiotensin II action on the exchanger. Phorbol esters reduced the rate of calcium extrusion, which was increased by baicalein, an inhibitor of lipoxygenases, and by SB 203580, an inhibitor of the p38 MAPK. Finally, we showed that angiotensin II acutely activates, in a caffeine-sensitive manner, p38 MAPK in glomerulosa cells. In conclusion, in bovine glomerulosa cells, the Na+/Ca2+ exchanger plays a crucial role in extruding calcium, and, by reducing its activity, angiotensin II influences the amplitude of the calcium signal. The hormone exerts its action on the exchanger through a caffeine-sensitive pathway involving the p38 MAPK and lipoxygenase products.

Mechanisms involved in ATP-evoked Ca2+ oscillations in isolated human granulosa-luteal cells

Using single-cell microfluorimetry, we have shown that ATP evoked repetitive Ca2+ oscillations in intact Fura-2 loaded human granulosa-luteal cells (hG/LCs) in the absence of extracellular Ca2+. Sustained increases in [Ca2+]i required extracellular Ca2+ and ATP depleted stores were refreshed by brief (2 min) incubation with external Ca2+. Basal [Ca2+]i was unaffected by caffeine (1 mM), but 20 mM caffeine inhibited ATP-evoked Ca2+ release in the absence of external Ca2+. Thimerosal (10 microM) evoked repetitive Ca2+ spikes, under Ca(2+)-free conditions, which fused to form an elevated plateau when external Ca2+ was replaced. Thimerosal-induced changes in [Ca2+]i were reversibly inhibited by the thiol reducing agent dithiothreitol (1 mM). The periodicity and amplitude of the [Ca2+]i oscillations produced by thimerosal and ATP differ. ATP- or thimerosal-evoked changes in [Ca2+]i were unaffected by dantrolene sodium (10 microM). The Ca(2+)-ATPase inhibitor thapsigargin (1 microM) increased [Ca2+]i and attenuated subsequent ATP-evoked changes in [Ca2+]i. We conclude that ATP stimulates an oscillatory release of Ca2+ from InsP3-sensitive stores in hG/LCs.

Sources and sites of action of calcium in the regulation of aldosterone biosynthesis

The role of free calcium as a crucial intracellular messenger in the stimulation of aldosterone biosynthesis by various agonists is well established. Using electropermeabilized or Ca(2+)-clamped adrenal zona glomerulosa (ZG) cells, we have previously shown that Ca2+ entry into the mitochondrial matrix is required for the activation of steroidogenesis. We now describe the use of various strategies to answer the following questions: 1. Which pathway does Ca2+ follow before triggering steroidogenesis? 2. Which step of steroidogenesis is under the control of Ca2+? The first approach combined the patch-clamp method, in the perforated patch configuration, with microfluorimetry of Ca2+; in the second approach, ZG cells were transiently transfected with a chimeric cDNA encoding for the calcium-sensitive photoprotein aequorin linked to a mitochondrial targeting presequence; in a third approach, ZG mitochondria were isolated and fractionated into outer membranes, contact sites and inner membranes and the effect of prior exposure of the ZG cells to a physiologically elevated intracellular calcium concentration or to angiotensin II (Ang II) on cholesterol content was then examined in those three mitochondrial fractions. The results of these combined approaches allow us to propose the following scheme: The source of calcium which is predominantly responsible for mediating the steroidogenic effect of potassium appears to be funneled through the T-type calcium channels to close proximity of the mitochondria. This signal, as well as that triggered by Ang II, appears to be relayed within the mitochondrial matrix. This rise of mitochondrial calcium is associated with a transfer of free cholesterol from the outer to the inner mitochondrial membrane, via the contact sites. Thus the main role of the calcium messenger is to promote intramitochondrial cholesterol transfer and supply to the P450scc enzyme.

Calcium stimulates intramitochondrial cholesterol transfer in bovine adrenal glomerulosa cells

In adrenal glomerulosa cells, angiotensin II (Ang II) stimulates aldosterone synthesis through rises of cytosolic calcium ([Ca2+]c). The rate-limiting step in this process is the transfer of cholesterol to the inner mitochondrial membrane, where it is converted to pregnenolone by the P450 side chain cleavage enzyme. The aim of the present study was to examine the effect of changes in [Ca2+]c and of Ang II on intramitochondrial cholesterol distribution. Freshly prepared bovine zona glomerulosa cells were submitted to a cytosolic Ca2+ clamp (600 nM) or stimulated with Ang II (10 nM). Mitochondria were isolated and subfractionated into outer membranes (OM), inner membranes (IM), and contact sites (CS). Cholesterol content was determined by the cholesterol oxidase assay. Stimulation of intact cells with Ca2+ led to a marked decrease in cholesterol content of OM (to 54 +/- 24% of controls, n = 5) and to a concomitant increase of cholesterol in CS and IM (to 145 +/- 14%, n = 5). When glomerulosa cells were exposed to Ang II, a marked increase of cholesterol in CS occurred (to 172 +/- 16% of controls, n = 5). No significant changes were detected in OM cholesterol, suggesting a stimulation of cholesterol supply to the mitochondria in response to Ang II. Cycloheximide specifically and significantly reduced Ca2+-activated cholesterol transfer to CS and IM. In conclusion, our data indicate that one of the main functions of the Ca2+ messenger is to increase cholesterol supply to the P450 side chain cleavage enzyme by enhancing endogenous intermembrane cholesterol transfer to a mitochondrial site containing the enzymes responsible for the initial steps of the steroidogenic cascade.

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.

Comparative effects of a highly specific protein kinase C inhibitor, calphostin C and calmodulin inhibitors on angiotensin-stimulated aldosterone secretion

We have examined the relative roles of the calcium-calmodulin system and protein kinase C in angiotensin-mediated aldosterone secretion. We used a highly specific protein kinase C inhibitor, calphostin C and two well-accepted calmodulin inhibitors, W-7 and calmidazolium. Although both types of inhibitors affected angiotensin-induced aldosterone secretion, as judged by the inhibitory doses of these compounds, angiotensin-evoked aldosterone secretion was more sensitive to calmodulin inhibition than protein kinase C inhibition. Manipulation of OFFracellular calcium by dantrolene and thapsigargin also modified aldosterone secretion significantly.

Effects of calcium channel blockers on rabbit corneal endothelial function

The effects of calcium channel antagonists and agents that alter intracellular Ca2+ mobilization on corneal endothelial function have been examined. All experiments, except where specifically designated, were performed in the continuous presence of extracellular Ca2+. Verapamil (at 50 microM) increased the swelling rate of corneas bathed in normal Ringer solution whereas nifedipine and diltiazem (both up to 100 microM) were without effect. The nifedipine analog nisoldipine caused corneal swelling at 10 microM and 50 microM but nimodipine was without effect. When briefly exposed to a Ca(2+)-free solution corneal swelling was enhanced after subsequent exposure to 50 microM verapamil in normal Ringer but not after 50 microM diltiazem in normal Ringer, indicating that Ca2+ entry from the bathing solution into the cell was important and was apparently impeded by verapamil. Cadmium (0.6 and 1 mM) but not nickel (up to 250 microM) caused swelling of corneas bathed in normal Ringer. A Ca2+ channel agonist, BAY-K-8644, alone did not influence corneal thickness but when presented to the endothelium with 50 microM verapamil the swelling rate was much reduced compared to verapamil alone. The agonist, therefore, presumably maintained some Ca2+ channels open in face of the Ca2+ channel blocker. An agent that inhibited the release of intracellular Ca2+ stores (TMB-8) caused an initial corneal swelling over the first 1.5 hr of perfusion but thereafter had no effect on corneal thickness. In the presence of continued extracellular Ca2+ one explanation for the results is that modulation of intracellular Ca2+ by agents that alter plasma membrane transfer of Ca2+ influences apical junction permeability.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of intercellular calcium signaling in glial cells studied with dantrolene and thapsigargin

Mechanical stimulation of a single cell in a primary mixed glial cell culture induced a wave of increased intracellular calcium concentration ([Ca2+]i) that was communicated to surrounding cells. Following propagation of the Ca2+ wave, many cells showed asynchronous oscillations in [Ca2+]i. Dantrolene sodium (10 microM) inhibited the increase in [Ca2+]i associated with this Ca2+ wave by 60-80%, and prevented subsequent Ca2+ oscillations. Despite the markedly decreased magnitude of the increase in [Ca2+]i, the rate of propagation and the extent of communication of the Ca2+ wave were similar to those prior to the addition of dantrolene. Thapsigargin (10 nM to 1 microM) induced an initial increase in [Ca2+]i ranging from 100 nM to 500 nM in all cells that was followed by a recovery of [Ca2+]i to near resting levels in most cells. Transient exposure to thapsigargin for 2 min irreversibly blocked communication of Ca2+ wave from the stimulated cell to adjacent cells. Glutamate (50 microM) induced an initial increase in [Ca2+]i in most cells that was followed by sustained oscillations in [Ca2+]i in some cells. Dantrolene (10 microM) inhibited this initial [Ca2+]i increase caused by glutamate by 65-90% and abolished subsequent oscillations. Thapsigargin (10 nM to 1 micron) abolished the response to glutamate in over 99% of cells. These results suggest that while both dantrolene and thapsigargin inhibit intracellular Ca2+ release, only thapsigargin affects the mechanism that mediates intercellular communication of Ca2+ waves. These findings are consistent with the hypothesis that inositol trisphosphate (IP3) mediates the propagation of Ca2+ waves whereas Ca(2+)-induced Ca2+ release amplifies Ca2+ waves and generates subsequent Ca2+ oscillations.

Greater importance of Ca(2+)-calmodulin in maintenance of ang II- and K(+)-mediated aldosterone secretion: lesser role of protein kinase C

In this study we have investigated various components of the stimulus-secretion coupling process leading to aldosterone secretion from the calf adrenal glomerulosa cells as evoked by angiotensin II (AII) and potassium (K+). The roles of Ca2+, calmodulin and protein kinase C in the sustained phase rather than initiation of aldosterone secretion were of special interest. Our investigations revealed that the reduction of extracellular Ca2+ by EGTA or interruption of Ca2+ influx by nitrendipine at various time points after stimulation with either AII or K+ markedly compromised aldosterone secretion. Calmodulin inhibitors, calmidazolium and W-7 reduced aldosterone secretion profoundly. Agonists of protein kinase C, phorbol ester or diacylglycerol analogues failed to stimulate aldosterone secretion while the protein kinase C inhibitor, H-7, only partially inhibited aldosterone secretion at a concentration which completely inhibited protein kinase C activity. Calmodulin inhibitors produced significantly greater inhibition of aldosterone secretion than inhibitors of protein kinase C.

GTP and Ca2+ modulate the inositol 1,4,5-trisphosphate-dependent Ca2+ release in streptolysin O-permeabilized bovine adrenal chromaffin cells

The inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release was studied using streptolysin O-permeabilized bovine adrenal chromaffin cells. The IP3-induced Ca2+ release was followed by Ca2+ reuptake into intracellular compartments. The IP3-induced Ca2+ release diminished after sequential applications of the same amount of IP3. Addition of 20 microM GTP fully restored the sensitivity to IP3. Guanosine 5'-O-(3-thio)triphosphate (GTP gamma S) could not replace GTP but prevented the action of GTP. The effects of GTP and GTP gamma S were reversible. Neither GTP nor GTP gamma S induced release of Ca2+ in the absence of IP3. The amount of Ca2+ whose release was induced by IP3 depended on the free Ca2+ concentration of the medium. At 0.3 microM free Ca2+, a half-maximal Ca2+ no Ca2+ release was observed with 0.1 microM IP3; at this Ca2+ concentration, higher concentrations of IP3 (0.25 microM) were required to evoke Ca2+ release. At 8 microM free Ca2+, even 0.25 microM IP3 failed to induce release of Ca2+ from the store. The IP3-induced Ca2+ release at constant low (0.2 microM) free Ca2+ concentrations correlated directly with the amount of stored Ca2+. depending on the filling state of the intracellular compartment, 1 mol of IP3 induced release of between 5 and 30 mol of Ca2+.

Two Ca2+ transport systems are distinguished on the basis of their Mg2+ dependency in a post-nuclear particulate fraction of bovine adrenal cortex

Inositol 1,4,5-trisphosphate (InsP3) is a second messenger responsible for Ca2+ release from an internal store whose nature and location remains undefined. To get more information on this intracellular Ca2+ store, a post-nuclear particulate fraction was prepared from bovine adrenal cortex and its Ca2+ uptake and release activities were monitored with the fluorescent indicator Fura-2. In the presence of Mg2+ (2 mM), the particulate preparation showed high ATP-dependent Ca2+ sequestering activity and decreased the ambient Ca2+ concentration to about 150 nM. In the absence of Mg2+, Ca2+ was still sequestered but less efficiently, reaching a level around 170 nM. In the presence of Mg2+, the Ca2+ released by a maximal dose of InsP3 (2 microM) was completely resequestered whereas in the absence of Mg2+, no resequestration occurred even after complete degradation of InsP3. The use of selective agents such as oligomycin, saponin, ionomycin and biliary salts indicated that Ca2+ was stored in three different pools which are distinct from the mitochondria and from inside-out membrane vesicles. Our data also indicate that InsP3 releases Ca2+ from a pool which is filled up by a Mg2(+) -dependent Ca2+ ATPase.

Effect of GTP and Ca2+ on inositol 1,4,5-trisphosphate induced Ca2+ release from permeabilized rat exocrine pancreatic acinar cells

The effects of Ca2+ and GTP on the release of Ca2+ from the inositol 1,4,5-trisphosphate (IP3) sensitive Ca2+ compartment were investigated with digitonin permeabilized rat pancreatic acinar cells. The amount of Ca2+ released due to IP3 directly correlated with the amount of stored Ca2+ and was found to be inversely proportional to the medium free Ca2+ concentration. Ca2+ release induced by 0.18 microM IP3 was half maximally inhibited at 0.5 microM free Ca2+, i.e. at concentrations observed in the cytosol of pancreatic acinar cells. GTP did not cause Ca2+ release on its own, but a single addition of GTP (20 microM) abolished the apparent desensitization of the Ca2+ release which was observed during repeated IP3 applications. This effect of GTP was reversible. GTP gamma S could not replace GTP. Desensitization still occurred when GTP gamma S was added prior to GTP. The reported data indicate that GTP, stored Ca2+ and cytosolic free Ca2+ modulate the IP3 induced Ca2+ release.

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.

Calcium mobilization and influx during the biphasic cytosolic calcium and secretory responses in agonist-stimulated pituitary gonadotrophs

Stimulation of enriched pituitary gonadotrophs by gonadotropin-releasing hormone (GnRH) elicits dose-dependent biphasic elevations of cytosolic calcium ([Ca2+]i) and luteinizing hormone (LH) release, with rapid initial peaks followed by sustained plateaus during continued exposure to the agonist. A potent GnRH-antagonist, [N-acetyl-D-p-Cl-Phe1,2,D-Trp3,D-Lys6,D-Ala10]GnRH, prevented the biphasic [Ca2+]i and LH responses when added before GnRH, and rapidly abolished both responses to GnRH when added during the plateau phase. In low Ca2+ medium the LH peak responses to GnRH were reduced and the subsequent sustained responses were almost completely abolished; reduction of extracellular Ca2+ during exposure to GnRH caused a prompt decline of LH release. The initial [Ca2+]i peak is derived largely from intracellular calcium mobilization with a partial contribution from calcium influx, while the sustained phase is dependent on the entry of extracellular Ca2+ through both L-type and dihydropyridine-insensitive channels. The presence of L-type voltage-sensitive calcium channels (VSCC) in pituitary gonadotrophs was indicated by the ability of elevated extracellular [K+] to stimulate calcium influx and LH release, and the sensitivity of these responses to dihydropyridine agonist and antagonist analogs. In cells pretreated with high [K+], the peak [Ca2+]i response to GnRH was enhanced but the subsequent plateau phase was markedly attenuated. This divergent effect of sustained membrane depolarization on the biphasic [Ca2+]i response suggests that calcium entry through VSCC initially potentiates agonist-induced mobilization of Ca2+ from intracellular storage sites. However, established Ca2+ entry through depolarization-activated VSCC cannot be further increased by agonist stimulation because both processes operate through the same channels, probably by changes in their activation-inactivation kinetics. Finally, the reciprocal potentiation by the dihydropyridine agonist, BK 8644, and GnRH of [Ca2+]i and LH responses confirms that both compounds act on the same type of channels, i.e., L-type VSCC, that participate in agonist-mediated calcium influx and gonadotropin secretion.

Caffeine-sensitive calcium stores in presynaptic nerve endings: a physiological role?

Ca2+-sensitive minielectrodes and the fluorescent cytosolic calcium probes, quin2 and fura2, were used to study some aspects of calcium homeostasis in intact and permeabilized synaptosomes from whole rat brain. Experiments in permeabilized synaptosomes revealed the existence of a vesicular, non-mitochondrial, ATP-dependent calcium uptake system with a vanadate sensitivity similar to that of brain microsomes, or endoplasmic reticulum-type calcium sequestering organelles. By using the fluorescent probes it was possible to show that caffeine mobilizes calcium from these internal stores in intact synaptosomes. Our results indicate a role of the caffeine sensitive calcium stores in the buffering of calcium loads elicited by plasma membrane depolarization.

Cytosolic free calcium in single microdissected rat cortical collecting tubules

Cytosolic free Ca2+ ([Ca2+]i) was measured in single fragments of rat cortical collecting tubule (CCT) by using fura-2 and a tubule superfusion device. Under basal conditions, i.e. with 1 mM of external Ca2+ ([Ca2+]o), the average steady state [Ca2+]i was 179 +/- 16 nM (n = 44 tubules). Random alterations of [Ca2+]o between 0 mM and 4 mM led to corresponding variations in steady state [Ca2+]i levels, which were linearly correlated with [Ca2+]o (average slope 93 +/- 34 nM [Ca2+]i per 1 mM [Ca2+]o for six tubules). In contrast, [Ca2+]i was little affected by decreasing external Na+ concentration. Cell membrane depolarization with 100 mM of external K+ induced a sustained drop in [Ca2+]i (21% as an average). The data suggest that steady state [Ca2+]i in CCT cells resulted from a non-saturable passive entry of calcium ions across cell membranes balanced with an active extrusion by calcium ATPase (pump and leak mechanism). The passive component cannot be accounted for either by Na+/Ca2+ exchangers nor by voltage-dependent calcium channels; it is best explained by the presence of voltage-independent calcium channels in cell membranes.

Inositol polyphosphate production and regulation of cytosolic calcium during the biphasic activation of adrenal glomerulosa cells by angiotensin II

Stimulation of aldosterone production by angiotensin II in the adrenal glomerulosa cell is mediated by increased phosphoinositide turnover and elevation of intracellular Ca2+ concentration. In cultured bovine glomerulosa cells, angiotensin II caused rapid increases in inositol-1,4,5-trisphosphate (Ins-1,4,5-P3) levels and cytosolic Ca2+ during the first minute of stimulation, when both responses peaked between 5 and 10 s and subsequently declined to above-baseline levels. In addition to this temporal correlation, the dose-response relationships of the angiotensin-induced peak increases in cytosolic Ca2+ concentrations and Ins-1,4,5-P3 levels measured at 10 s were closely similar. However, at later times (greater than 1 min) there was a secondary elevation of Ins-1,4,5-P3, paralleled by increased formation of inositol 1,3,4,5-tetrakisphosphate that was associated with cytosolic Ca2+ concentrations only slightly above the resting value. These results are consistent with the primary role of Ins-1,4,5-P3 in calcium mobilization during activation of the glomerulosa cell by angiotensin II. They also suggest that Ins-1,4,5-P3 participates in the later phase of the target-cell response, possibly by acting alone or in conjunction with its phosphorylated metabolites to promote calcium entry and elevation of cytosolic Ca2+ during the sustained phase of aldosterone secretion.

Calcium response of single adrenal glomerulosa cells to external potassium

The cytosolic calcium (Cai2+) response to external potassium (K+) was examined in single rat zona glomerulosa (ZG) cells by monitoring fura-2-fluorescence with microspectrofluorometry and digital imaging microscopy. The majority (68%) of morphologically identified ZG cells demonstrated an increase in Cai2+ during K+ stimulation. Cai2+ rose monotonically from a mean basal level of 232 +/- 15 to 285 +/- 37 nM at 5 mM and 680 +/- 60 nM at 10 mM K+ for responsive ZG cells. The Cai2+ response was largely (greater than 90%) inhibited by nominal zero calcium or 1 mM cadmium and substantially modified in the presence of 10(-5) M nifedipine. The response kinetics were characterized by a rising phase that depended on the size of the Cai2+ change, with larger increases associated with a faster onset. Cai2+ approached a plateau level that was sustained for the duration of K+ stimulation from 1 to 5 min. Cai2+ appeared to be more uniformly distributed across the cell under resting conditions than during stimulation. Assessment of the Cai2+ response in single ZG cells documents 1) a majority, but not all, of ZG cells respond to K+, 2) simple kinetics consisting of a rapid onset and sustained plateau Cai2+ level, 3) a dose-dependent Cai2+ increase in the physiological range of K+, and 4) inhibition by calcium channel blockers and sensitivity to small increases in K+ consistent with activation of low-threshold calcium channels.

Dantrolene-induced inhibition of insulin release. A mechanism independent of effects on calcium fluxes

Dantrolene is felt to inhibit the release of Ca2+ from vesicular stores but only in response to certain stimuli; the mechanism responsible for its effects is unclear. Since our recent studies implicated arachidonic acid and other polyunsaturated fatty acids in Ca2+ mobilization and insulin release from pancreatic islets, we have now examined the effect of dantrolene on fatty acid-induced 45Ca2+ efflux and insulin release. Dantrolene inhibited insulin secretion induced by exogenous unsaturated fatty acids as well as that caused by endogenous fatty acids (generated via the exogenous provision of pancreatic phospholipase A2, or by p-hydroxymercuribenzoic acid, which prevents the reacylation of free fatty acids). In contrast, the effects of 50 mM K+, 2 mM BaCl2, 1 mM isobutylmethylxanthine or lysophosphatidylcholine were not impaired, suggesting that dantrolene does not inhibit nonspecifically the influx, mobilization or cellular effects of Ca2+, or poison exocytosis in general. However, dantrolene did reduce insulin secretion triggered by 12-O-tetradecanoylphorbol-13-acetate, mezerein or exogenous phospholipase C, all of which can activate protein kinase C; this inhibition was not accompanied by alterations in 45Ca2+ efflux. Furthermore, the 45Ca2+ efflux induced by phospholipase A2 or p-hydroxymercuribenzoic acid was not reduced by dantrolene. We conclude that the insulin secretion stimulated by unsaturated fatty acids involves two effects (one on Ca2+ fluxes, and one independent of Ca2+ mobilization). Dantrolene, in turn, may selectively probe such fatty acid-dependent insulin release; its inhibitory effect is predominantly, if not totally, independent of effects on Ca2+ fluxes, and may involve the inhibition of the effects of protein kinase C on exocytosis.

Inositol trisphosphate isomers in angiotensin II-stimulated adrenal glomerulosa cells

The stimulation of phosphoinositide metabolism by angiotensin II (Ang II) was studied in [3H]inositol-labelled bovine adrenal glomerulosa cells. After separation of the phosphoinositols by ion-exchange high-performance liquid chromatography, it was shown that the formation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and inositol 1,3,4-trisphosphate (Ins(1,3,4)P3) followed distinct kinetics. The first compound to increase upon stimulation with 10(-7) M Ang II was Ins(1,4,5)P3, which reached a maximum (250% of basal level) within 10 s. At lower concentrations of Ang II, this response was slower. The formation of Ins(1,4,5)P3 depended upon the concentration of Ang II, with an EC50 of 2.4 +/- 1.5 X 10(-9) M Ang II. The potency of Ang II in stimulating the turnover of phosphoinositides and in increasing the biosynthesis of aldosterone was very similar, whereas the peptide was ten times more potent in its ability to mobilize Ca2+. Ang II was also able to stimulate the production of Ins(1,4,5)P3 in permeabilized glomerulosa cells. This effect was mimicked by a non-hydrolysable analog of GTP (GTP gamma S), suggesting that a GTP binding protein is involved in the mechanism coupling the Ang II membrane receptor to phospholipase C. These results strengthen the view that Ins(1,4,5)P3 plays a key role as second messenger in the steroidogenic response to Ang II in adrenal glomerulosa cells.

Lysosomotropic amines modulate neutrophil calcium homeostasis

Lysosomotropic amines can raise the acidic internal pH of the neutrophil lysosome and inhibit neutrophil function. Because pH and calcium regulation are intimately connected in various types of excitable cells, we studied the effects of several lysosomotropic weak bases on neutrophil calcium homeostasis. Base-treated cells had normal to minimally elevated resting cytosol free calcium, but weak bases produced significant release of calcium from organelles when this release was directly measured in permeabilized cells, even after depletion of inositol-triphosphate-sensitive stores. Collapse of transmembrane pH gradients with monensin similarly released organelle calcium. The initial cytosol calcium response to f-met-leu-phe was enhanced by some of the lysosomotropic amines but the calcium rise was more transient in base-treated cells than in control samples. These findings suggest that existence of an acidic intracellular compartment, such as the lysosome, is important to normal calcium homeostasis in the neutrophil and that pH sensitivity and inositol triphosphate sensitivity may define two pools of releasable organelle calcium. The effect of pH perturbation on calcium homeostasis may partially account for the inhibition of neutrophil function by lysosomotropic amines.

Stimulus-secretion coupling in angiotensin-stimulated adrenal glomerulosa cells

Adrenal glomerulosa cell is a suitable model for a comparative study of signal transducing mechanisms since its secretory activity is regulated by at least three different mechanisms: the adenylate cyclase-cAMP system (for ACTH), the voltage-dependent Ca2+ channel (for K+ and perhaps for angiotensin II) and the inositol 1,4,5-trisphosphate-Ca2+ system (for angiotensin II and vasopressin). The role of inositol phosphates, extracellular Ca2+ and protein kinase C in the induction and sustaining of aldosterone production by cells exposed to angiotensin II is critically reviewed.

Linkage between neutrophil degranulation and calcium discharge

Calcium flux across organelle and plasma membranes is an important event in neutrophil activation. We measured calcium discharge into the media from neutrophils stimulated with formyl-methionyl-leucyl-phenylalanine after treatment with cytochalasin b. Cytochalasin markedly potentiated calcium efflux from stimulated neutrophils, and similarly promoted release of lysosomal enzymes into the media. Colchicine neither reproduced nor modified the cytochalasin effect. Neutrophil cytoplasts discharged very little calcium in response to stimulation, and discharge was not significantly altered by cytochalasin b. These findings indicate that neutrophil degranulation is accompanied by efflux of calcium into the media, and suggest that the neutrophil granules constitute a source of mobilizable calcium which could be used to modify the extracellular microenvironment.