Our results show cell swelling activates two ionic conductances: a BK
Ca and a Cl
swell. As described in several cell types, K
+ and Cl
− effluxes are accompanied by water to restore normal cell volume (RVD).
23 25 In particular, BK
Ca currents had an approximately threefold increase over the resting values in isotonic medium. Cell swelling activated Cl
swell as soon as 4 minutes after bath exchange to hypotonic medium and reached a maximum between 10 and 12 minutes. Cl
swell currents decreased when returning to isotonic medium
(Fig. 1B) showing that their activation is a reversible process. Also, the effects of well-known blockers (tamoxifen and DIDS) of Cl
swell currents and their ionic selectivity are in agreement with previous reports in TM cells
19 and other cell types.
35 Special mention should be made to the effects of extracellular ATP on Cl
swell currents. After activation by hypotonicity, addition of ATP to the recording bath partially blocked Cl
swell currents
(Fig. 3C) . This effect has also been previously described,
46 47 but its physiological relevance is unknown. We have also found, as previously reported (Cui M, et al.
IOVS 2001;42:ARVO Abstract 743),
40 that hypotonic stimuli trigger the release of ATP from TM cells. ATP, through activation of purinergic receptors, increased BK
Ca currents
(Fig. 7) , perhaps contributing to the RVD. Nevertheless, the increase of ATP concentration in the extracellular environment may be part of a physiological mechanism to prevent excessive activation of Cl
swell currents, since the blocking effect is only at high concentrations (in the millimolar range) but not at lower concentrations (micromolar) where the effect is to stimulate BK
Ca currents. It should be mentioned that in the absence of hypotonic stimulus, ATP (in micromolar concentrations) activates Cl
swell currents in TM cells (data not shown) as seen in other studies.
48 We should note that ATP can be released due to mechanical stimulation in TM cells (Cui M, et al.
IOVS 2001;42:ARVO Abstract 743)
36 or can reach the TM after being released in the AH by different ocular tissues and nerves (Fleiszig SMJ, et al.
IOVS 2001;42:ARVO Abstract 3146).
49 50 Also, we cannot exclude that ATP can be liberated on cell stimulation with several agonists, as seen in other cell types.
36 Therefore, it can be hypothesized that ATP acts in an autocrine/paracrine manner in the TM, regulating outflow facility by volume changes in TM cells. Moreover, Fleischhauer et al.
40 have recently shown that released ATP may be a source of adenosine and that adenosine agonists reduce TM cell volume.