Potassium and chloride conductances in rat Leydig cells: effects of gonadotrophins and cyclic adenosine monophosphate

Volume-activated chloride channels in mice Leydig cells

Production and secretion of testosterone in Leydig cells are mainly controlled by the luteinizing hormone (LH). Biochemical evidences suggest that the activity of Cl(-) ions can modulate the steroidogenic process, but the specific ion channels involved are not known. Here, we extend the characterization of Cl(-) channels in mice Leydig cells (50-60 days old) by describing volume-activated Cl(-) currents (I(Cl,swell)). The amplitude of I(Cl,swell) is dependent on the osmotic gradient across the cell membrane, with an apparent EC(50) of approximately 75 mOsm. These currents display the typical biophysical signature of volume-activated anion channels (VRAC): dependence on intracellular ATP, outward rectification, inactivation at positive potentials, and selectivity sequence (I(- )> Cl(- )> F(-)). Staurosporine (200 nM) did not block the activation of I(Cl,swell). The block induced by 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB; 128 microM), SITS (200 microM), ATP (500 microM), pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS; 100 miccroM), and Suramin (10 microM) were described by the permeant blocker model with apparent dissociation constant at 0 mV K(do) and fractional distance of the binding site (delta) of 334 microM and 47 %, 880 microM and 35 %, 2,100 microM and 49%, 188 microM and 27%, and 66.5 microM and 49%, respectively. These numbers were derived from the peak value of the currents. We conclude that I(Cl,swell) in Leydig cells are activated independently of purinergic stimulation, that Suramin and PPADS block these currents by a direct interaction with VRAC and that ATP is able to permeate this channel.

Hyperemesis gravidarum, a literature review

Hyperemesis gravidarum (HG) is a condition causing severe nausea and vomiting in early pregnancy often resulting in hospital admission. The incidence of HG is approximately 0.5% of live births, said to be higher in multiple pregnancies, hydatidiform mole and other conditions associated with increased pregnancy hormone levels. Both the aetiology and pathogenesis of HG remain unknown. We conducted a literature review (1966-now) to summarize the current evidence on the aetiology and pathogenesis of HG. The potential role of pregnancy-related hormones such as progesterone, estrogen and HCG has been widely studied; however, various other hormones such as leptin, placental growth hormone, prolactin, thyroid and adrenal cortical hormones have been implicated in the aetiology of HG. In addition to endocrinological hypotheses, the rationale and evidence considering infectious, immunological, psychological, metabolic and anatomical causes for HG have been analysed here. Many studies suffer from the low number of patients included, the variable definition used for HG and varying assay methodology used in studies of hormone measurement. This review highlights the need for more extensive studies addressing the pathogenesis and aetiology of HG.

Involvement of protein kinase C and cyclic adenosine 3',5'-monophosphate-dependent kinase in steroidogenic acute regulatory protein expression and steroid biosynthesis in Leydig cells

This study investigated the roles of the protein kinase C (PKC) and protein kinase A (PKA) pathways in regulating constitutive steroidogenesis and steroidogenic acute regulatory (STAR; herein designated by its common name, StAR) protein in R2C Leydig tumor cells. Inhibition of PKC and phospholipase C resulted in significant decreases in steroid production, phosphorylation of cAMP-responsive element binding (CREB) protein, and Star gene transcription under basal conditions in R2C cells. These observations were corroborated in MA-10 and mLTC-1 Leydig tumor cell lines, in which activation of PKC by phorbol-12-myristate-13-acetate (PMA, 10 nM) increased CREB phosphorylation and total StAR (tot-StAR) protein expression. However, induction of StAR protein by PMA did not result in the expected concomitant increase in steroids because PKC failed to phosphorylate StAR, the biologically active form of the protein. However, in conjunction with PMA, minor increases in PKA activity using submaximal doses of (Bu)2cAMP (0.05-0.1 mM; a concentration range insufficient for induction of StAR), were able to stimulate dramatic increases in both phospho-StAR (P-StAR) and steroid production. Human chorionic gonadotropin stimulation also resulted in a further enhancement in P-StAR and progesterone production when added to PMA-treated MA-10 cells. Similar results for tot-StAR and P-StAR expression were observed in primary cultures of immature rat Leydig cells treated with PMA and submaximal doses of (Bu)2cAMP. In summary, the present study demonstrates that basal activities of both PKC and PKA play important roles in the constitutive steroidogenic characteristics of R2C cells. This study also demonstrates for the first time a role for PMA-induced PKC in StAR protein regulation and the requirement for submaximal doses of cAMP to produce steroids in Leydig cells.

A voltage-gated K+ current in renal inner medullary collecting duct cells

We studied the K+-selective conductances in primary cultures of rat renal inner medullary collecting duct (IMCD) using perforated-patch and conventional whole cell techniques. Depolarizations above –20 mV induced a time-dependent outward K+ current ( Ivto) similar to a delayed rectifier. Ivto showed a half-maximal activation around 5.6 mV with a slope factor of 6.8 mV. Its K+/Na+ selectivity ratio was 11.7. It was inhibited by tetraethylammonium, quinidine, 4-aminopyridine, and Ba2+ and was not Ca2+ dependent. The delayed rectifying characteristics of Ivto prompted us to screen the expression of Kv1 and Kv3 families by RT-PCR. Analysis of RNA isolated from cell cultures revealed the presence of three Kv α-subunits (Kv1.1, Kv1.3, and Kv1.6). Western blot analysis with Kv α-subunit antibodies for Kv1.1 and Kv1.3 showed labeling of ∼70-kDa proteins from inner medulla plasmatic and microsome membranes. Immunocytochemical analysis of cell culture and kidney inner medulla showed that Kv1.3 is colocalized with the Na+-K+-ATPase at the basolateral membrane, although it is also in the cytoplasm. This is the first evidence of recording, protein expression, and localization of a voltage-gated Kv1 in the kidney IMCD cells.

4-aminopyridine decreases progesterone production by porcine granulosa cells

BACKGROUND

Ion channels occur as large families of related genes with cell-specific expression patterns. Granulosa cells have been shown to express voltage-gated potassium channels from more than one family. The purpose of this study was to determine the effects of 4-aminopyridine (4-AP), an antagonist of KCNA but not KCNQ channels.

METHODS

Granulosa cells were isolated from pig follicles and cultured with 4-AP, alone or in combination with FSH, 8-CPT-cAMP, estradiol 17beta, and DIDS. Complimentary experiments determined the effects of 4-AP on the spontaneously established pig granulosa cell line PGC-2. Granulosa cell or PGC-2 function was assessed by radio-immunoassay of media progesterone accumulation. Cell viability was assessed by trypan blue exclusion. Drug-induced changes in cell membrane potential and intracellular potassium concentration were documented by spectrophotometric determination of DiBAC4(3) and PBFI fluorescence, respectively. Expression of proliferating cell nuclear antigen (PCNA) and steroidogenic acute regulatory protein (StAR) was assessed by immunoblotting. Flow cytometry was also used to examine granulosa cell viability and size.

RESULTS

4-AP (2 mM) decreased progesterone accumulation in the media of serum-supplemented and serum-free granulosa cultures, but inhibited cell proliferation only under serum-free conditions. 4-AP decreased the expression of StAR, the production of cAMP and the synthesis of estradiol by PGC-2. Addition of either 8-CPT-cAMP or estradiol 17beta to serum-supplemented primary cultures reduced the inhibitory effects of 4-AP. 4-AP treatment was also associated with increased cell size, increased intracellular potassium concentration, and hyperpolarization of resting membrane potential. The drug-induced hyperpolarization of resting membrane potential was prevented either by decreasing extracellular chloride or by adding DIDS to the media. DIDS also prevented 4-AP inhibition of progesterone production.

CONCLUSION

4-AP inhibits basal and FSH-stimulated progesterone production by pig granulosa cells via drug action at multiple interacting steps in the steroidogenic pathway. These inhibitory effects of 4-AP on steroidogenesis may reflect drug-induced changes in intracellular concentrations of K+and Cl- as well as granulosa cell resting membrane potential.

Luteinizing hormone (LH) drives diverse intracellular calcium second messenger signals in isolated porcine ovarian thecal cells: preferential recruitment of intracellular Ca2+ oscillatory cells by higher concentrations of LH

The present study examines Ca2+ second messenger signaling driven by LH in isolated porcine thecal cells. To this end, we implemented semiquantitative fluorescent (fura-2) videomicroscopic imaging of single thecal cells in vitro. Stimulation of 388 cells with LH (5 microg/ml) elicited an intracellular Ca2+ ([Ca2+]i) signal in 85+/-5.3% of individual thecal cells (n = 11 experiments). Among 337 LH-responsive cells, we identified four predominant temporal modes of [Ca2+]i signaling: 1) [Ca2+]i oscillations with periodicities of 0.5 to 4.5 min(-1) (63+/-4.5%), 2) a [Ca2+]i spike followed by a sustained plateau (17+/-2.6%), 3) a [Ca2+]i spike only (5.8+/-2.6%); and 4) a [Ca2+]i plateau only (3.8+/-1.5%). The prevalence, but not the amplitude or frequency, of LH-induced [Ca2+]i oscillations in thecal cells was dependent on the agonist concentration. Reduced availability of extracellular Ca2+ induced by treatment with EGTA or cobaltous chloride did not block the initiation, but reversibly abolished ongoing [Ca2+]i oscillations (72% of cells) or increased the mean [Ca2+]i interspike periodicity from 1.09+/-0.16 to 0.59+/-0.07 min(-1) (P < 0.05). Putative phospholipase C inhibition with U-73122 (10 microM) also abolished or frequency-damped LH-driven [Ca2+]i oscillations in 95+/-4.7% of cells. [Ca2+]i oscillations in thecal cells were not abrogated by overnight pretreatment with pertussis toxin. We conclude that 1) thecal cells (unlike earlier findings in granulosa cells) manifest a diverse array of [Ca2+]i signaling responses to LH at the single cell level; 2) LH can dose dependently recruit an increasing number of individually [Ca2+]i oscillating thecal cells; 3) extracellular Ca2+ is required for LH to sustain (but not initiate) frequent and high amplitude [Ca2+] oscillations in thecal cells; and 4) these signaling actions of LH are mediated via phospholipase C, but not a pertussis-toxin sensitive mechanism. Accordingly, the present data extend the apparent complexity of LH-induced [Ca2+]i second messenger signaling and identify at the single cell level LH's dose-responsive drive of [Ca2+]i oscillations in gonadal cells.

Could growth retardation in cystic fibrosis be partly due to deficient steroid and thyroid hormonogenesis?

Cystic fibrosis (CF) mainly affects Caucasians of northwestern-European ancestry with severe morbidity. The individuals are malnourished and growth retarded. The latter is thought to be the consequence of delayed maturation of the hypothalamic-pituitary-gonadal axis due to malnourishment. However, there is evidence that steroid and thyroid hormone syntheses may be impaired in CF. Thyrotropin stimulates the uptake and efflux of iodide before the halide is incorporated into thyroid hormones and it is becoming apparent that gonadotropins likewise mobilise chloride ions in Leydig cells prior to steroidogenesis. Since the primary defect causing CF is the mutated cystic fibrosis transmembrane conductance regulator (CFTR)--a chloride channel residing on the apical membrane of wet epithelia, the growth retardation in CF may in part be due to deficient hormone syntheses. The latter may involve CFTR or may be the halide channel activated by glycoprotein hormones prior to hormonogenesis.

Human chorionic gonadotropin: a secretory hormone

There are several physiological and pathophysiological situations where there is an apparent fluid flux across plasma membranes at the time when human chorionic gonadotropin (hCG) levels are high. These fluxes may take the form of a fluid loss from gastrointestinal tract (e.g. emesis/hyperemesis gravidarum) or accumulations in enclosures (e.g. amniotic fluid or hydatidiform mole). What is not obvious though is whether hCG is the cause of these fluid fluxes. Although glycoprotein hormones like hCG are mainly hormonogenic, their action in the latter process involves the efflux or conductance of halide ions. Since the basis of fluid secretion is an active efflux of ions such as chloride stimulated by a humoral agent, accompanied by a passive diffusion of water across a cell wall, I hypothesize that hCG is also a secretory hormone and responsible for fluid fluxes in the above and other clinical situations.

Role of chloride and inhibitory action of inorganic nitrate on gonadotropin-stimulated steroidogenesis in mouse Leydig tumor cells

The involvement of adenylate cyclase-cyclic adenosine monophosphate (AC-cAMP) in gonadotropin-stimulated testicular steroidogenesis is well known. Little is known about the role of guanylate cyclase-cyclic guanosine monophosphate (GC-cGMP) or early chloride conductance stimulated by gonadotropins in steroidogenesis. Human chorionic gonadotropin (hCG) 1 IU/L caused significant androgen secretion without a discernible effect on cAMP production. Despite negligible intracellular cAMP, the protein kinase A inhibitor H89 blocked basal and hCG-stimulated steroidogenesis. The GC inhibitors methylene blue (MB) and LY83583 decreased androgen secretion, but hCG did not stimulate cGMP production and there was not a steroidogenic response to exogenous cGMP. A chloride-channel inhibitor, diphenylamine-2-carboxylate (DPC), at concentrations up to 0.6 mmol/L stimulated basal steroid secretion and hCG 10 IU/L stimulated cAMP production, but higher concentrations had an inhibitory effect. Substitution of chloride by gluconate enhanced basal steroid secretion, but nitrate completely abolished the effect of 1 IU/L hCG on androgen secretion, which could be partially overcome by increasing the gonadotropin concentration. In conclusion, chloride, perhaps by activating AC-cAMP, mediates the steroidogenic action of gonadotropins in mouse Leydig tumor cells (MLTC-1). Inorganic nitrate probably inhibited steroidogenesis via conversion to nitric oxide (NO) without involving the GC-cGMP pathway. Nevertheless, the results obtained with GC inhibitors suggest a role for the GC-cGMP pathway in Leydig cell steroidogenesis.

Signal transduction involving cyclic AMP-dependent and cyclic AMP-independent mechanisms in the control of steroidogenesis

The control of steroidogenesis via signal transduction mechanisms involving cAMP-dependent and cAMP-independent mechanisms is reviewed. Several structurally unrelated factors that are potent stimulators of steroidogenesis whose actions do not require cAMP and/or synthesis of proteins have been identified. These include various interleukins, a lipophilic factor from macrophages, a steroidogenic inducing protein from follicular fluid and an imidazole compound, calmidazolium. All of these factors are capable of inducing maximum steroidogenesis. Calcium is required for steroidogenesis in all steroidogenic cells. With the exception of the effects of angiotensin II, there is little evidence for a role of IP3 in the stimulation of the release of calcium from intracellular stores in steroidogenic cells under physiological conditions. There may however, be a cAMP-mediated activation of a plasma membrane calcium channel. Chloride channels that can be regulated by cAMP-dependent and -independent mechanisms, are present in steroidogenic cells. Chloride ions exert a negative effect on steroidogenesis because exclusion of chloride from the extracellular medium markedly enhances cAMP-stimulated steroidogenesis. Arachidonic acid and its lipoxygenase products are involved in the control of steroidogenesis via cAMP mediated processes. An arachidonic acid related thioesterase has been isolated that is activated by ACTH and which may be involved in the release of arachidonic acid. It is concluded that while cAMP is a second messenger for LH/ACTH in the control of steroidogenesis, other signalling systems exist which are potentially equally effective in controlling steroidogenesis. In addition, the action of cAMP requires other signalling pathways involving calcium and chloride ions, as well as arachidonic acid and its lipoxygenase products.

Functional assessment of the calcium messenger system in cultured mouse Leydig tumor cells: regulation of human chorionic gonadotropin-induced expression of the steroidogenic acute regulatory protein

The steroidogenic acute regulatory (StAR) protein, a 30-kDa mitochondrial factor, is a key regulator of steroid hormone biosynthesis, facilitating the transfer of cholesterol from the outer to the inner mitochondrial membrane. StAR protein expression is restricted to steroidogenic tissues, and it responds to hormonal stimulation through different second messenger pathways. The present study was designed to explore the mechanisms of extracellular calcium (Ca2+) involved in the hCG-stimulated expression of StAR protein and steroidogenesis in a mouse Leydig tumor cell line (mLTC-1). Extracellular Ca2+ (1.5 mmol/liter) enhanced the hCG (50 microg/liter)-induced increases in StAR messenger RNA (mRNA) and protein levels (1.7 +/- 0.3-fold; 4 h), as monitored by quantitative RT-PCR and immunoblotting. The potentiating effect of Ca2+ on the hCG-stimulated StAR response correlated with the acute progesterone (P) response. In accordance, omission of Ca2+ from the extracellular medium by specific Ca2+ chelators, EDTA or EGTA (4 mmol/liter each), markedly diminished the hCG-stimulated P production. The Ca2+ effect on hCG-induced StAR mRNA expression was dramatically suppressed by 10 micromol/liter verapamil, a Ca2+ channel blocker. The Ca2+-mobilizing agonist, potassium (K+; 4 mmol/liter), greatly increased the hCG responses of StAR expression and P production, which conversely were attenuated by Ca2+ antagonists, further supporting the involvement of intracellular free Ca2+ ([Ca2+]i) in these responses. The interaction of Ca2+ or K+ with hCG accounted for a clear increase in the StAR protein level (1.4-1.8-fold; 4 h) compared with that after hCG stimulation. Inhibition of protein synthesis by cycloheximide (CHX) drastically diminished the hCG-induced StAR protein content, indicating the requirement for on-going protein synthesis for hCG action. The transmembrane uptake of 45Ca2+ was increased by 26% with hCG and was strongly inhibited by verapamil. [Ca2+]i moderately augmented the response to hCG in fura-2/AM-loaded mLTC-1 cells within 30-40 sec, reaching a plateau within 1-3 min. Interestingly, the calcium ionophore (A23187) clearly increased (P < 0.01) StAR mRNA expression, in additive fashion with hCG. Northern hybridization analysis revealed four StAR transcripts at 3.4, 2.7, 1.6, and 1.4 kb, with the 1.6-kb band corresponding to the functional StAR protein; all of them were up-regulated 3- to 5-fold upon hCG stimulation, with a further increase in the presence of Ca2+. The mechanism of the Ca2+ effect on hCG-stimulated StAR expression and P production was evaluated by assessing the involvement of the nuclear orphan receptor, steroidogenic factor 1 (SF-1). Stimulation of hCG significantly elevated (2.1 +/- 0.3-fold) the SF-1 mRNA level, which was further augmented in the presence of Ca2+, whereas EGTA and verapamil completely abolished the increase caused by Ca2+. Cells expressing SF-1 marginally increased StAR expression, but coordinately elevated StAR mRNA levels in response to hCG and hCG plus Ca2+ compared with those in mock-transfected cells. On the other hand, overexpression of the nuclear receptor DAX-1 remarkably diminished (P < 0.0001) the endogenous SF-1 mRNA level as well as hCG-induced StAR mRNA expression. In summary, our results provide evidence that extracellular Ca2+ rapidly increases [Ca2+]i after hCG stimulation, presumably through opening of the transmembrane Ca2+ channel. Neither extracellular Ca2+ nor K+ alone has a noticeable effect on StAR expression and steroidogenesis, whereas they clearly potentiate hCG induction. The Ca2+-mediated increase in hCG involved in StAR expression and P production is well correlated to the levels of SF-1 expression. The stimulatory effect of hCG that rapidly increases [Ca2+]i is responsible at least in part for the regulation of SF-1-mediated StAR expression that consequently regulates steroidogenesis in mouse Leydig tumor cells.

hCG: its pancreatic and duodenal receptors and in vivo electrolyte secretion in female rats

A state of fluid flux probably resulting from ion movement across the plasma membrane occurs during early pregnancy or trophoblastic disease, manifesting as emesis or hyperemesis gravidarum or hydatidiform moles. In emesis or hyperemesis gravidarum, excessive secretion induced by a humoral agent may trigger vomiting by distending and activating the gastrointestinal (GI) tract mechanoreceptors. This agent may be human chorionic gonadotropin (hCG). High-affinity hCG binding sites similar to those in the ovary were found in the duodenum and pancreas of female rats, with dissociation constant values of 0.11 +/- 0.02, 1.9 +/- 0.6, and 4.7 +/- 3.5 nM, respectively. The isoelectric point for duodenal and ovarian proteins was 5.5. With the use of two antisera directed against amino acid residues 24-33 and 239-249 of the lutropin receptor, positive immunohistochemical staining was seen in smooth muscle, Brunner's glands, parasympathetic ganglia, crypt cells, and blood vessels of the duodenum, in zymogen granules of acini, and in intralobular ducts and blood vessels of the pancreas. Under nonreducing conditions, 150- and 170-kDa proteins were seen, through Western blot analysis, in the pancreas, duodenum, and ovary. Administration of hCG to female rats in vivo caused a significant increase in HCO-3 and K+ secretion from the duodenum and pancreas. We hypothesize that during pregnancy hCG stimulates excessive secretion of electrolytes (and fluid) into the upper GI tract, which culminates in the vomiting during pregnancy.

Modulation of steroidogenesis by chloride ions in MA-10 mouse tumor Leydig cells: roles of calcium, protein synthesis, and the steroidogenic acute regulatory protein

It has previously been shown that omission of extracellular chloride ions during culture of rat Leydig cells markedly enhances LH-stimulated steroidogenesis. In the present study, the mechanisms of the effect of chloride omission on (Bu)2cAMP-stimulated steroidogenesis in MA-10 mouse Leydig tumor cells have been investigated. It was found that chloride omission enhanced progesterone production 2- and 4-fold in the absence and presence, respectively, of submaximally stimulating levels of (Bu)2cAMP (0.1 mM) during incubation for 2 h. This enhancement of stimulation increased continuously with time, because after 6 h, (Bu)2cAMP-stimulated progesterone production was 15-fold higher in the absence of chloride. These effects were not found in the presence of maximum stimulating levels of (Bu)2cAMP (1 mM). Omission of calcium from the incubation medium decreased (Bu)2cAMP-stimulated progesterone production by over 70% in the presence and absence of chloride. Progesterone production was still enhanced by the omission of chloride in the absence of calcium, but the effects were less marked than those in the presence of calcium. Addition of the protein synthesis inhibitor, cycloheximide, completely inhibited (Bu)2cAMP-stimulated, but not basal, steroidogenesis in the absence and presence of chloride ions during 2- and 6-h incubation. Total protein synthesis (measured by the incorporation of [3H]methionine) was 4-fold higher in cells incubated in chloride-free medium compared with that in cells incubated in chloride-replete medium in the presence of 0.1 mM (Bu)2cAMP. No effects were found on basal levels. Several proteins specific to the steroidogenic machinery were quantified in mitochondria isolated from cells incubated with and without chloride by Western blot analysis after separation by PAGE. Omission of chloride increased (4-fold) the level of the steroidogenic acute regulatory (StAR) protein in the cells incubated with (Bu)2cAMP (0.1 mM). There was no increase in either the levels or activities of cytochrome P450 cholesterol side-chain cleavage enzyme (cytP450scc) or 3beta-hydroxysteroid dehydrogenase. No effects were found on the basal level of any of the proteins measured. These results are consistent with a cAMP-dependent regulatory role of chloride ion efflux in the control of steroidogenesis, which requires protein synthesis. It is proposed that this occurs by increases in StAR protein synthesis via a general increase in cAMP-dependent protein synthesis and/or by enhancement of the steroidogenic effects of StAR.

Luteinizing hormone activates chloride currents in hen ovarian granulosa cells

Luteinizing hormone (LH) induces progesterone production in hen ovarian granulosa cells, and this induction is inhibited when chloride ions are removed from the culture medium. This suggests that chloride channels may be involved in the signal transduction pathway responsible for the LH-induced progesterone production. In this report, we examined effects of LH on plasma membrane ion currents in single granulosa cells isolated from the largest preovulatory follicle (Fl) of the hen (Gallus domesticus). Using the perforated patch whole cell voltage clamp technique, we found that addition of LH rapidly activated a chloride current in these cells. This chloride current was present at all voltages tested (-90 to +50 mV), showed outward rectification and showed no obvious time or voltage dependence. Its magnitude was 3.5-fold that of the total resting membrane current measured before LH treatment. LH is known to elevate cyclic AMP in these cells. We found that addition of the cAMP analog Sp-cAMPS mimicked LH in inducing chloride currents in these cells. We conclude that LH can activate a chloride conductance in granulosa cells, and that this action may be mediated by cAMP.

Inhibitory effect of internal sodium and Hepes on the voltage-dependent potassium conductance of rat Leydig cells

We used the whole-cell configuration of the patch-clamp technique to investigate modulation of the voltage-dependent K+ conductance of rat Leydig cells by intracellular pH, Hepes, and Na+. pH (range 6.8-7.4) has no effect on current. Removal of Na+ (from 10 to 0 mM) increases the amplitude of K+ current and abolishes the plateau effect on peak current. Reduction of Hepes (from 25 to 10 mM) increases the amplitude of K+ current and, simultaneously, shifts the threshold for current activation to the left, but the plateau of current is still present. Removal of Na+ cancels the potentiating effect of Hepes on frequency of occurrence of time-dependent inactivation of K+ current. We conclude that the blocking effect of Hepes and Na+ on K+ current may presumably involve two distinct mechanisms, but we can not exclude some cooperativity in their ability to inactivate the voltage-gated potassium conductance of rat Leydig cells.

Modulation of K+ conductances by Ca2+ and human chorionic gonadotrophin in Leydig cells from mature rat testis

1. Although the control of steroidogenic activity of the Leydig cell by the peptides luteinizing hormone (LH) and human chorionic gonadotrophin (hCG) is clearly mediated by cAMP, the extent to which Ca2+ controls the Leydig cell function is less well defined. In the present study, the whole-cell configuration of the patch-clamp technique was used to investigate the modulation of potassium conductances by calcium and hCG, in the Leydig cells from mature rat testis. 2. In symmetrical glutamate solutions, depolarizations elicited outwardly rectifying currents, which were mainly carried by potassium and were blocked by tetraethylammonium and 4-aminopyridine. For values of [Ca2+]i below 10(-8) M, transient currents of low amplitudes, insensitive to charybdotoxin (CTX) and iberiotoxin (IBTX), were activated above -40 mV. For [Ca2+]i values of 10(-7) M and above, noisy currents with slow activation kinetics were activated above 0 mV. These currents were sustained and were sensitive to CTX and IBTX. 3. Both current types were modulated by intracellular calcium. Ionomycin and a [Ca2+]i elevation in the range from 10(-9) to 10(-7) M, both inhibited the CTX-insensitive currents, whereas a rise in the calcium concentration above 10(-7) M increased the amplitude and shifted the threshold of activation of the CTX-sensitive currents to less positive levels. 4. hCG (1-50 i.u. ml-1), in conditions where the chloride currents were strongly inhibited by 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS), induced a partial inhibition of the CTX-insensitive currents but was unable to increase the CTX-sensitive currents. 5. No voltage-sensitive calcium current was recorded in control or hCG-stimulated cells. 6. The results indicate that hCG inhibits one kind of Ca(2+)-modulated channel, perhaps as a result of a moderate [Ca2+]i rise, but is unable to increase the intracellular Ca2+ concentration to the range in which large conductance Ca(2+)-dependent channels are activated.

Effect of human chorionic gonadotrophin on chloride current in human syncytiotrophoblasts in culture

Human trophoblast differentiates in vivo and in vitro by the fusion of cytotrophoblastic cells to form syncytiotrophoblasts. A large amount of human chorionic gonadotrophin (hCG) is produced by the syncytiotrophoblasts, which express hCG luteinizing hormone (LH) receptors. Since recent investigations with electrophysiological techniques support the conclusion that hormonal effects can be mediated by modulations of the membrane ionic conductances of the cells, a perforated patch-clamp technique was used to investigate the possible presence of a chloride current evoked by hCG. The perifusion of hCG (500 mIU/ml) activated a time-independent current, which presents a linear current-voltage (I/V) relationship in symmetrical chloride concentrations. The reversal potential was -1.8 mV with 142 mM Cl- external solution and 134 mM cl- internal solution. This reversal potential shifted with changes in the transmembrane Cl- gradient. Moreover, this hCG-induced current was sensitive to 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) (50 microM), to diphenylalamine-2-carboxylic acid (DPC) (0.5 mM) and to 9-AC (1 mM), three known chloride channel blockers. These results confirm the autocrine action of hCG in the physiology of the trophoblast.

Endocrine, paracrine and autocrine regulation of testicular steroidogenesis

Testicular steroidogenesis takes place almost exclusively in Leydig cells. Some metabolism of the androgens produced by Leydig cells takes place in seminiferous tubules, especially in the immature animal (e.g. aromatization and 5 alpha- reduction). Luteinizing hormone (LH) is the main tropic regulator of Leydig cell function, without which quantitatively important androgen production is not possible. LH acts through a receptor that belongs to the seven times cell membrane spanning, G protein associated, receptor family, and cyclic AMP is the main second messenger of its signal transduction. Information about the involvement of other signal transduction systems in LH action has also emerged recently. The action of LH is under manyfold modulation by other hormones (e.g. prolactin, growth hormone and insulin), growth factors and bioactive peptides. In this modulation, various paracrine and autocrine mechanisms play an important role. Seminiferous tubules influence the development and function of adjacent Leydig cells through several growth factors. When germ cells are damaged, Leydig cells in the vicinity proliferate faster. Leydig cell morphology also depends on the germ cell composition in the neighbouring seminiferous tubules, and certain stages of the seminiferous epithelial cycle increase the Leydig cell capacity to produce testosterone. Also negative modulation of Leydig cells by Sertoli/germinal cell derived factors has been demonstrated. However, the physiological importance of the paracrine and modulatory influences of the different hormones and growth factors still remains obscure since almost all information has so far been obtained from in vitro studies. In the study of testicular steroidogenesis, the main switch of the function, LH action, is well known whereas the role of the "in house" circuits of paracrine and autocrine regulation remain to be elucidated.

Granulosa cells have calcium-dependent action potentials and a calcium-dependent chloride conductance

We have found chicken granulosa cells to be excitable. Experiments using the whole-cell patch-clamp technique showed that they had membrane resting potentials of -62 +/- 3 mV (n = 8) and generated action potentials, either in response to 10-ms depolarizing current pulses or, on occasion, spontaneously. The action potentials persisted in a Na(+)-free bath and were reversibly blocked by 4 mM Co2+. They lasted 0.9-3.0s with 64 mM Cl- in the pipette, were shortened 67 +/- 8% by the Cl- channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB; 20 microM), and lengthened to 8.7 +/- 2.2 when the Cl- equilibrium potential (Vcl) was changed from -20 mV to -2 mV by using 134 mM Cl- in the pipette. With conventional whole-cell voltage-clamp, slowly activating and inactivating currents, which reached maximum amplitude after 0.35-1.40 s, were evoked by depolarizing voltage steps. These slow currents activated between voltage steps of -60 mV and -50 mV and reached a maximum inward amplitude at about -40 mV. Changing the Cl- concentration in the pipette (VCl of -2MV or -20 mV) or bath (VCl of -2 mV or + 18 mV) shifted their reversal potential in a direction consistent with a Cl- electrode. They were inhibited by the Cl- channel antagonists 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS; 0.5 mM), NPPB (20 microM), and 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS; 0.5 mM). The slow currents were blocked by Ca2+ deprivation, or by CO2+ (4 mM), or by replacing external Ca2+ with Ba2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for two types of potassium current in rat choroid plexus epithelial cells

The whole-cell patch-clamp technique was applied to rat choroid plexus epithelial cells. The resting membrane potential was -53 mV. The whole-cell conductance was mainly K+ selective, and the K+ current observed appeared to contain two distinct components. Depolarizing voltage pulses (more positive than 0 mV) evoked time-dependent outward currents which resembled delayed-rectifying K+ currents in other tissues. The current exhibited time-dependent activation and, at potentials more positive than 40 mV, slower time-dependent inactivation. The reversal potential measured by tail current analysis showed a shift of 43 mV for a tenfold increase in extracellular K+ concentration ([K+]o). The current was reduced by extracellular 5 mM Ba2+, 5 mM tetraethylammonium (TEA+), 5 mM Cs+ and 1 mM 4-aminopyridine (4-AP). In contrast, hyperpolarizing voltage pulses evoked time-independent, inward-rectifying currents. The reversal potential measured by voltage-ramp commands showed a shift of 42 mV for a tenfold increase in [K+]o. The chord conductance did not appear to increase with increasing [K+]o. The current was reduced by extracellular 5 mM Ba2+ and 0.5 mM Cs+, but not by 5 mM TEA+ or 1 mM 4-AP. These data suggest that two populations of K+ channel contribute to the conductance of choroid plexus epithelial cells.

Calcium and potassium currents in porcine granulosa cells maintained in follicular or monolayer tissue culture

We studied membrane currents in granulosa cells (GC), immediately after collection or after variable culture time in the everted-follicle wall or in the monolayer. GC in both systems express an inward calcium current (ICa) with T-type kinetics and voltage dependence. GC in the everted-follicle culture express an outward potassium current (IK) kinetics, which remains unchanged during three days in culture. IK has delayed-rectifier kinetics, but is insensitive to TEA, 4-AP and apamine. GC in monolayer culture develop a new, inactivating delayed-rectifier potassium current (InK), which progressively dominates as cells advance from day one to day three in culture. A similar InK was recorded in large luteal cells. A possible link between luteinization and the appearance of InK is hypothesized.

Characterization and cyclic AMP-dependence of a hyperpolarization-activated chloride conductance in Leydig cells from mature rat testis

We recently described a cyclic AMP-activated current in the membrane of Leydig cells from mature rat testis by using the whole-cell configuration of the patch-clamp technique (Noulin & Joffre, 1992a). In the present study, further experiments were performed in symmetrical CsCl solutions. We show that this current corresponds to a hyperpolarization-activated chloride conductance. Voltage jumps to negative potentials, applied from a holding potential of +60 mV, activated a time-dependent inward current. In control cells, the curve of steady-state current activation typically ranged from +60 mV (0) to -120 mV (1) and had its midpoint at -40 mV. Deactivation at positive potential was characterized by an instantaneous outwardly rectifying current which decayed with time. The kinetics of activation and deactivation were described by a double and a single exponential, respectively. Cyclic AMP, added to the pipette solution, increased both the inward rectification and the amplitude of the steady-state current in the range of 0 to -60 mV. The activation threshold was unchanged, while the V0.5 of the activation curve was shifted by 24 mV to more positive potentials. Consequently, the activation curve was steeper. The two rate constants of activation were increased and were strongly voltage dependent. In parallel, the amplitude of the instantaneous outward current and the rate constant of deactivation were increased. The reversal potential of this current was close to ECl. It did not change with equimolar replacement of cesium by TEA, and shifted with the chloride concentration gradient. This current was inhibited by chloride channel blockers. These results indicate a hyperpolarization-activated chloride conductance in the membrane of Leydig cells which is modulated by cyclic AMP. This nucleotide acts by modifying the kinetics of inward current and both the kinetics and the amplitude of deactivating outward current.

Cyclic AMP- and calcium-activated chloride currents in Leydig cells isolated from mature rat testis

Using the whole-cell recording technique, we have investigated some properties of the membrane currents induced by depolarization of Leydig cells isolated from mature rat testis. Internal perfusions with cyclic AMP or calcium induce currents which reveal the properties of the cyclic AMP-activated and calcium-activated chloride conductances. Both currents are sensitive to EC1 displacement and are similarly affected by SITS and DIDS, two stilbene-derived chloride channel blockers. However, at the resting membrane potential, the cyclic AMP-dependent, not the calcium dependent conductance, is activated. We discuss the involvement of these results in the physiology of the Leydig cells.