Elevated maxi-K(+) ion channel current in glaucomatous lamina cribrosa cells

The connective tissue plates of the lamina cribrosa (LC) region are continuously exposed to a mechanically dynamic environment. To study how the LC cells respond to these mechanical forces, we measured the mechano-sensitive calcium dependent maxi-K(+) ion channel current in the cell membrane of LC cells of glaucoma and normal subjects. Primary culture LC cells from 7 normal and 7 age matched glaucoma donors were studied. Perfusion of cells with hypotonic solution was used to stretch the cell membrane. Whole-cell patch-clamp technique was used to measure the basal (non stretched) and hypotonic stretch-induced changes in maxi-K(+) ion channel activity in normal and glaucoma LC cells. The role of membrane-type Ca(2+) entry channel inhibition (verapamil) and internal Ca(2+) store re-uptake blockade (2-APB) on maxi-K(+) activity was also examined. Basal and stretched-induced maxi-K(+) current were significantly elevated in the glaucoma LC cells compared to normal controls (p < 0.05). In normal LC cells hypotonic stretch elevated the mean maxi-K(+) current from 18.5 ± 5.7 pA/pF (at Vp = +100 mV) to 88.4 ± 12.4 pA/pF (P < 0.05), and from 39.5 ± 7.3 pA/pF to 133.1 ± 18.5 pA/pF in glaucoma LC cells (P < 0.02). Verapamil and 2-APB significantly reduced basal maxi-K(+) current in glaucoma LC cells (33.1 ± 8.2 pA/pF to 17.9 ± 5.6 pA/pF; and 32.2 ± 8.3 pA/pF to 17.3 ± 5.4 pA/pF, P < 0.05, respectively) but not in normal LC cells (P > 0.05). Following hypotonic stretch, verapamil and 2-APB significantly (P < 0.05) reduced the maxi-K(+) current in both normal and glaucoma LC cells. Baseline and hypotonic stretch induced Ca(2+)-dependent maxi-K(+) channel activity are elevated in LC cells of glaucoma patients, which may result from the abnormally high levels of intracellular calcium in glaucoma LC cells.