Adrenergic stimulation of bovine non-pigmented ciliary epithelial cells raises cAMP but has no effect on K+ or Cl- currents

A low conductance chloride channel in the basolateral membranes of the non-pigmented ciliary epithelium of the rabbit eye

PURPOSE

Chloride efflux plays an important role in aqueous humor production. Chloride currents have been described in bovine non-pigmented ciliary epithelium (NPE), in transformed cultured human NPE, and in bovine volume-activated chloride channels described in the latter. It is the basolateral membranes of the NPE of ocular ciliary processes that comprise the exit pathway for the process of aqueous secretion performed by the double syncytial layer of ciliary epithelium, however. Therefore, we studied both cell-attached, and, excised, inside-out patches from basolateral membranes of the NPE.

METHODS

Cell-attached and cell-free excised patches were formed from the basolateral membranes of NPE and single channel currents recorded with a Dagan 3900A patch clamp amplifier.

RESULTS

A low conductance channel of 14 pS was observed and recorded in 30% of cell attached patches and in 35% of excised inside out patches under symmetrical conditions (160 mM chloride). This channel displayed a nearly linear current-voltage relationship, with an open probability that was not voltage-dependent. The channel was chloride-selective: N-methyl-D-glucamine (NMDG) used as cation did not alter the current profile nor the reversal protein. Further, with inside-out patches, the reversal potential was close to zero (0.3 +/- 0.5 mV (10) in symmetrical (160 mV) chloride, but shifted to -32.3 +/- 0.5 mV (5) when the concentration of chloride in the bathing solution was reduced to 40 mM while the recording pipette was held at 160 mM. This value approaches the theoretical equilibrium potential of chloride for these conditions. The channel anion permeability sequence was: I- > NO3- > or = Br- > Cl- > gluconate- approximately equal to aspartate-. Three different chloride-channel blockers inhibited the channel activity.

CONCLUSION

A low conductance channel, selective for chloride, and, modulated by beta adrenergic and VIP stimulation, based on it sensitivity to exogenously added cAMP, ATP and the catalytic subunit of PKA, is present in the exit basolateral membranes of rabbit NPE and contributes to the process of aqueous humor formation.

Three different Cl- channels in the bovine ciliary epithelium activated by hypotonic stress

1. Hypotonic solution induced transient Cl- channel activity in both pigmented and non-pigmented ciliary epithelial cells in cell-attached patches. 2. The activation time constants for these currents were 63 and 97 s for non-pigmented and pigmented ciliary epithelial (NPCE and PCE) cells, respectively. The currents inactivated during the exposure to hypotonic solution with time constants of 59 and 304 s for NPCE and PCE cells, respectively. 3. Single channel analysis revealed the presence of a low-conductance Cl- channel in both the NPCE (7.3 +/- 0.4 pS) and PCE (8.6 +/- 0.9 pS) cells. In addition, an intermediate-conductance (18.8 pS) channel was found alone in the NPCE cells and a maxi Cl- channel (105 pS) was found in the PCE cells. The similarities between the 18.8 pS Cl- channel and the volume-activated organic osmolyte-anion channel (VSOAC) are discussed. 4. The hypotonic activation of all three Cl- channels was prevented by the inclusion of 100 microM 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) in the patch pipette. 5. Inclusion of tamoxifen in the patch pipette inhibited only the intermediate-conductance channel in the NPCE cells. This is consistent with the identification of the intermediate 18 pS Cl- channel, found only in NPCE cells, as VSOAC. 6. Negative pressure also evoked single Cl- channel activity but the activation kinetics were quite different, suggesting that, even if the channels are the same, the second messenger pathways involved are different. 7. The activation/inactivation kinetics of these Cl- channels correlate well with the time course of regulatory volume decrease (RVD) in these cells, suggesting that these channels may well participate in the process of RVD. 8. The asymmetrical distribution of Cl- channels in the ciliary epithelium may be significant for aqueous humour secretion. For instance, the presence of the VSOAC-like channel in the NPCE cells may allow one-way traffic of anions and organic osmolytes into the eye and, if these solutes were loaded by the PCE cells, then a vectorial flow from the blood into the eye would occur.

P-glycoprotein regulates a volume-activated chloride current in bovine non-pigmented ciliary epithelial cells

1. The whole-cell patch clamp technique was used to investigate the swelling-activated currents in bovine non-pigmented ciliary epithelial (NPCE) cells. 2. Exposure to hypotonic solution activated a current that was blocked by 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB). The I-V relationship was shifted in the direction expected for a Cl- current when the external Cl- was replaced by gluconate (permeability ratio P(gluconate)/PCl = 0.17). The inhibition of the current evoked by voltage clamp steps of +80 mV yielded an IC50 for NPPB of 13.4 microM. 3. The current was found to be dependent on ATP. With ATP in the patch pipette the current could be repeatedly activated by exposure to hypotonic solution but when ATP was omitted the current ran down with time. 4. The development of this current was associated with visible cell swelling and inhibitors of regulatory volume decrease in these cells, e.g. tamoxifen, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS) and 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS), also inhibited this current. 5. The volume-activated current was additionally blocked by NPPB, verapamil, quinidine and dideoxyforskolin. 6. The current was independent of external calcium and exhibited slight outward rectification and time-dependent inactivation at strong depolarizing potentials. 7. Disrupting the cytoskeleton and microtubules with cytochalasin B and colchicine had no effect on the activation of the Cl- current. 8. An antibody (C219) to the MDR1 gene product, P-glycoprotein, caused a functional block of the swelling-activated Cl- current when added to the patch pipette. 9. Immunofluorescence studies using the monoclonal antibodies C219 and JSB-1 demonstrated the presence of P-glycoprotein in the ciliary epithelial cells. The immunofluorescence was stronger on the non-pigmented than on the pigmented cells. 10. It is concluded that swelling in NPCE cells activates a Ca(2+)-independent, ATP-dependent Cl- current and that the activity of this current is associated with P-glycoprotein. 11. It is suggested that this Cl- current contributes to regulatory volume decrease and may participate in the secretory activity of these cells.

Role of cyclic AMP-induced Cl conductance in aqueous humour formation by the dog ciliary epithelium

1. The effects of isoprenaline, a forskolin derivative NKH-477, and dibutyryl cyclic AMP (db cyclic AMP) on the membrane potential, conductance and cell volume of the dog non-pigmented ciliary epithelium (NPE) were investigated by intracellular potential recording, nystatin-perforated patch clamp technique and videomicroscopic cytometry. 2. The resting membrane potential of NPE was about -70 mV in physiological saline and was depolarized by isoprenaline in a dose-dependent manner with an ED50 of about 3 nM. This depolarization was competitively antagonized by the beta-adrenoceptor antagonist, timolol (pA2 = ca. 9) and almost completely blocked by the Cl transport blocker, DIDS. 3. In single dissociated NPE cells, 10 microM isoprenaline induced an inward current and caused a concomitant decrease in cell volume. The reversal potential measurement indicated that this inward current was carried mainly by Cl ion. DIDS (10 microM) abolished both the current and cell volume decrease. 4. NKH-477 (10 microM) or db cyclic AMP (1 mM) also induced an inward current together with a cell volume decrease, the properties of which were similar to those caused by isoprenaline. 5. These results suggest that beta-adrenoceptor stimulation in NPE leads to an increased rate of aqueous humour production by increasing Cl- efflux via an elevation of cyclic AMP and this effect is efficiently blocked by timolol.