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

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.

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.

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.

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)

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.