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W. M. Dismuke, C. Mbadugha, D. Z. Ellis; Involvement of the Maxi K Channel in the NO-Induced Increases in Outflow Facility and Decreases in TM Cell Volume. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3709.
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Nitric Oxide (NO) increases aqueous humor outflow facility in the TM but the mechanisms of regulation are not known. Cellular mechanisms thought to regulate outflow facility include cellular contractility and/or changes in cell volume of TM cells. Thus decreases in cell volume are associated with increases in outflow facility while increases in cell volume are associated with decreases in outflow facility. NO mediates its effects via activation of soluble guanylate cyclase (sGC), increases in cGMP and activation of protein kinase G. This study investigated the involvement of the Maxi K channel in regulating the NO-induced increases in outflow facility and NO-induced decreases in cell volume.
Outflow facility was measured using an anterior segment organ perfusion system. Human and porcine TM cells were isolated after collagenase digestion of TM explants according to the methods of Stamer et al., 1995. Cell volume was measured according to the modified protocols of Mitchell et al., 2002 and Bush et al., 2005. Low passage human TM cells were loaded with Calcein AM and visualized using a Leica confocal microscope. Images were taken of TM cells that were treated with or without various drugs including: NO donors; Maxi K channel inhibitor; sGC inhibitors; and cGMP analogs.
Exposure of TM cells to either the NO-donor, DETA-NO or the cGMP analog, 8-Br-cGMP, resulted in a time-dependent decrease in cell volume. The NO-induced decreases in TM cell volume were abolished by the sGC inhibitor, ODQ and the Maxi K channel inhibitor, IBTX. Perfusion of the anterior eye segment with DETA-NO resulted in increases in outflow facility which was abolished by IBTX. Additionally, the time-course for the NO-induced increases in outflow facility correlated with the time-course for the NO-induced decreases in cell volume.
The ability of DETA-NO to decrease TM cell volume with a time-course that correlated with the NO-induced increases in outflow facility suggest the involvement of cell volume changes as a regulator of outflow facility. The ODQ inhibition of NO-induced decreases in cell volume suggests the involvement of sGC and cGMP in the NO-induced response. Further evidence for the involvement of cGMP in mediating the NO-induced decreases in TM cell volume was obtained by using 8-Br-cGMP. The ability of IBTX to inhibit the NO-induced response suggests the involvement of the Maxi K channel in mediating the NO-induced decreases in TM cell volume and the NO-induced increases in outflow facility.
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