Cell membrane stretch modulates the high-conductance Ca2+-activated K+ channel in bovine trabecular meshwork cells

PURPOSE

Anterior chamber structures are subjected to changes in intraocular pressure (IOP). Several studies have pointed out that trabecular meshwork (TM) cells are sensitive to mechanical stretch and that cell-signaling mechanisms are activated in response to elevated pressure. Because membrane stretch has been shown to be a modulator of several ionic conductances, this study was conducted to determine its effects on the high-conductance Ca(2+)-activated K(+) (BK(Ca)) channels present in TM cells.

METHODS

Primary cultures of TM cells from bovine eyes were used. Patch-clamp recordings were performed in the cell-attached, inside-out, and whole-cell configurations. To stretch the cell membrane, both suction to the rear end of the patch pipette and hypotonic shock were used. Intracellular calcium concentration ([Ca(2+)](i)) was measured in TM cells loaded with fura-2, using an epifluorescence microscope coupled to a charge-coupled device (CCD) camera.

RESULTS

Electrophysiological characterization of BK(Ca) channels was in agreement with previous studies. In cell-attached patches, the open probability of the BK(Ca) channel (i.e., the amount of time the channel is open) increased consistently when 14- to 45-mm Hg suctions were applied at a constant depolarized voltage. At a constant pressure (25 or 45 mm Hg), channel openings increased when depolarizing pulses were applied to the patch. Stretch activation of the BK(Ca) channel was not mediated by increases in [Ca(2+)](i), because it was present in inside-out patches maintained at a constant Ca(2+) concentration. Nevertheless, it cannot be ruled out that at low suction levels, a minimum Ca(2+) concentration is necessary for channel activation. Whole-cell currents carried by BK(Ca) channels increased when the isotonic solution in the bath was exchanged with a hypotonic solution and were selectively blocked by iberiotoxin. In our conditions, the hypotonic shock did not modify [Ca(2+)](i).

CONCLUSIONS

The data show that in TM cells, open probability of the BK(Ca) channel is enhanced by membrane stretching as well as by membrane depolarization and [Ca(2+)](i). Changes in membrane tension induced by cell volume increase also activated whole-cell BK(Ca) currents. Homeostatic mechanisms in TM cells may involve BK(Ca) channel activation in response either to changes in cell volume or changes in IOP.

Long-term alteration of S-type potassium current and passive membrane properties in aplysia sensory neurons following axotomy

In many neurons, axotomy triggers long-lasting alterations in excitability as well as regenerative growth. We have investigated mechanisms contributing to the expression of axotomy-induced, long-term hyperexcitability (LTH) of mechanosensory neurons in Aplysia californica. Electrophysiological tests were applied to pleural sensory neurons 5-10 days after unilateral crush of pedal nerves. Two-electrode current-clamp experiments revealed that compared with uninjured sensory neurons on the contralateral side of the body, axotomized sensory neurons consistently displayed alterations of passive membrane properties: notably, increases in input resistance (R(in)), membrane time constant (tau), and apparent input capacitance. In some cells, axotomy also depolarized the resting membrane potential (RMP). Axotomized sensory neurons showed a lower incidence of voltage relaxation ("sag") during prolonged hyperpolarizing pulses and greater depolarizations during long (2 s) but not brief (20 ms) pulses. In addition to a reduction in spike accommodation, axotomized sensory neurons displayed a dramatic decrease in current (rheobase) required to reach spike threshold during long depolarizations. The increase in tau was associated with prolongation of responses to brief current pulses and with a large increase in the latency to spike at rheobase. Two-electrode voltage-clamp revealed an axotomy-induced decrease in a current with two components: a leakage current component and a slowly activating, noninactivating outward current component. Neither component was blocked by agents known to block other K(+) currents in these neurons. In contrast to the instantaneous leakage current seen with hyperpolarizing and depolarizing steps, the late component of the axotomy-sensitive outward current showed a relatively steep voltage dependence with pulses to V(m) > -40 mV. These features match those of the S-type ("serotonin-sensitive") K(+) current, I(K,S). The close resemblance of I(K,S) to a background current mediated by TREK-1 (KCNK2) channels in mammals, raises interesting questions about alterations of this family of channels during axotomy-induced LTH in both Aplysia and mammals. The increase in apparent C(in) may be a consequence of the extensive sprouting that has been observed in axotomized sensory neurons near their somata, and the decrease in I(K,S) probably helps to compensate for the decrease in excitability that would otherwise occur as new growth causes both cell volume and C(in) to increase. In peripheral regions of the sensory neuron, a decrease in I(K,S) might enhance the safety factor for conduction across regenerating segments that are highly susceptible to conduction block.

Human myofibroblastic hepatic stellate cells express Ca(2+)-activated K(+) channels that modulate the effects of endothelin-1 and nitric oxide

BACKGROUND/AIMS

High-conductance Ca(2+)-activated K(+) (BK(Ca)) channels modulate the effects of vasoactive factors in contractile cells. It is unknown whether hepatic stellate cells (HSCs) contain BK(Ca) channels and what their role in the regulation of HSCs contractility is.

METHODS

The presence of BK(Ca) channels in HSCs was assessed by the patch-clamp technique. The functional role of BK(Ca) channels was investigated by measuring intracellular calcium concentration ([Ca(2+)](i)) and cell contraction in individual cells after stimulation with endothelin-1 in the presence or absence of specific modulators of BK(Ca) channels.

RESULTS

BK(Ca) channels were detected by patch-clamp in most of the activated HSCs studied. Incubation of cells with iberiotoxin, a BK(Ca) channel blocker, increased both the sustained phase of [Ca(2+)](i) elicited by endothelin-1 and the number of cells undergoing contraction, while the use of NS1619, a BK(Ca) channel opener, induced opposite effects. Stimulation of HSCs with S-nitroso-N-acetyl-penicillamine (SNAP), a nitric oxide (NO)-donor, increased the opening of BK(Ca) channels and reduced the effects of endothelin-1. Conversely, iberiotoxin abolished the inhibitory effect of SNAP on endothelin-induced [Ca(2+)](i) increase and cell contraction.

CONCLUSIONS

Activated human HSCs contain BK(Ca) channels that modulate the contractile effect of endothelin-1 and mediate the inhibitory action of NO.

Identification of Kir2.1 channel activity in cultured trabecular meshwork cells

PURPOSE

To study the presence of inwardly rectifying K(+) (Kir) channels in cultured bovine (BTM) and human (HTM) trabecular meshwork cells.

METHODS

Cultures of BTM and HTM cells were obtained by an extracellular matrix digestion technique. Whole-cell patch-clamp recordings of BTM cells were performed with the appropriate solutions to detect K(+) currents. Also, Western blot analysis of Kir2.1 protein expression was performed on both cultured BTM and HTM cells.

RESULTS

A strong inwardly rectifying current at negative potentials to the equilibrium potential for K(+) (E(K+)) and highly selective for K(+) was detected in 60% of cultured BTM cells. The slope conductance of the inward rectification was more pronounced when the extracellular [K(+)] was increased and was proportional to [K(+)](0.45). The current was blocked by Ba(2+) and Cs(+) in a voltage- and concentration-dependent manner, with K(d) at 0 mV, of 74.7 microM and 45.6 mM, respectively. Current amplitude was reduced by increasing extracellular [Ca(2+)]. The current was insensitive to 10 microM glibenclamide and 10 nM tertiapin. The application of 100 microM 8-Br-cAMP reduced the current by 50%. Kir2.1 channel expression was detected in confluent monolayers of BTM and HTM cells by Western blot analysis.

CONCLUSIONS

A population of cultured BTM cells expressed an inwardly rectifying K(+) current that illustrates the biophysical and pharmacologic characteristics of the detected Kir2.1 channel protein. Kir2.1 channels are also thought to be present in HTM cells. Kir2.1 channels could be related to TM physiology, because they are involved in contractile and cell volume regulatory responses, two mechanisms that modify TM permeability.

In vitro and in vivo activation of rat hepatic stellate cells results in de novo expression of L-type voltage-operated calcium channels

Following chronic liver injury, hepatic stellate cells (HSCs) transdifferentiate into myofibroblast-like cells, which develop contractile properties and contribute to increased resistance to blood flow. We investigated whether this phenotypic activation includes changes in the expression of L-type voltage-operated Ca2+ channels (VOCC), which mediate Ca2+ influx and regulate cell contraction in vascular cell types. Rat HSCs were studied in the quiescent phenotype and after their activation in vitro (cultured on plastic for 14 days) and in vivo (isolated from rats with CCl(4)-induced cirrhosis). Patch-clamp studies showed Ca2+ currents through L-type VOCC in HSCs activated both in vitro and in vivo, whereas no currents were detected in quiescent HSCs. Moreover, binding studies with (3)H-isradipine, a specific L-type VOCC antagonist, showed a large number of binding sites in activated HSCs, while no specific binding was found in quiescent HSCs. Finally, messenger RNA (mRNA) encoding L-type VOCC was not detected in quiescent HSCs as assessed by reverse transcription-polymerase chain reaction (RT-PCR) and Northern blot analysis, whereas it was present in activated HSCs. Stimulation of L-type VOCC with KCl resulted in a marked increase in [Ca2+](i) followed by cell contraction in HSCs activated both in vitro and in vivo, whereas no effects were observed in quiescent HSCs. We conclude that the activation of HSCs is associated with up-regulation of L-type VOCC that mediate Ca2+ influx and cell contraction. These results may be relevant to the pathogenesis of portal hypertension.

Angiotensin II induces contraction and proliferation of human hepatic stellate cells

BACKGROUND & AIMS

Circulating levels of angiotensin II (ANGII), a powerful vasoconstrictor factor, are frequently increased in chronic liver diseases. In these conditions, hepatic stellate cells (HSCs) proliferate and acquire contractile properties. This study investigated the presence of receptors for ANGII and the effects of ANGII in human HSCs activated in culture.

METHODS

The presence of ANGII receptors was assessed by binding studies. The effects of ANGII on intracellular calcium concentration ([Ca(2+)](i)), cell contraction, and cell proliferation were also assessed.

RESULTS

Binding studies showed the presence of ANGII receptors of the AT1 subtype. ANGII elicited a marked dose-dependent increase in [Ca(2+)](i) and cell contraction. Moreover, ANGII stimulated DNA synthesis and increased cell number. All these effects were totally blocked by losartan and reduced by nitric oxide donors or prostaglandin E(2). The effects of ANGII were barely detectable in quiescent cells (2 days in culture), suggesting that phenotypic transformation of HSCs is associated with a marked increase in the effects of ANGII.

CONCLUSIONS

ANGII induces contraction and is mitogenic for human-activated HSCs by acting through AT1 receptors. These results suggest that activated HSCs are targets of the vasoconstrictor action of ANGII in the intrahepatic circulation.

Facility changes mediated by cAMP in the bovine anterior segment in vitro

The aim of this study was to investigate the influence of substances that increase intracellular cAMP levels on the aqueous humor outflow facility (C) of isolated bovine anterior segments. Anterior segments were perfused in vitro at a constant pressure of 10 mmHg for 270 min with a general protocol as follows: 90 min control perfusion with DMEM, 90 min of experimental perfusion with DMEM containing the test drug(s), and 90 min of postdrug-perfusion with DMEM. C was calculated as the ratio between the rate of medium inflow (microliter/min) and the perfusion pressure (mmHg). Anterior segments can be perfused in vitro for up to 5 hr without significantly modifying their C. The addition of epinephrine, forskolin, dibutyryl-cAMP or isobutylmethylxanthine to the control perfusion medium elicited a significant increase of C. If, during isobutylmethylxanthine perfusion, forskolin or epinephrine was added, C increased significantly. Finally, perfusion with indomethacin prior to addition of epinephrine prevented the increase of C induced by epinephrine. Epinephrine, the adenylate cyclase activator forskolin, the cAMP analog dibutyryl-cAMP, and the phosphodiesterase inhibitor isobutylmethylxanthine all increase aqueous facility. It seems reasonable to suspect that the cAMP system is involved in epinephrine's effects on bovine trabecular meshwork cells. Moreover, the complete inhibition by indomethacin of the outflow facility increase induced by epinephrine suggests that prostaglandins may be involved in the outflow facility mechanisms related to adrenoreceptor stimulation of trabecular meshwork cells.