Flufenamic acid enhances current through maxi-K channels in the trabecular meshwork of the eye

Abstract

Purpose. Flufenamic acid relaxes trabecular meshwork, a smooth muscle-like tissue involved in the regulation of ocular outflow in the eye. In this study, we attempted to determine if ionic channels are involved in this response. Methods. Cultured human (HTM) and bovine (BTM) trabecular meshwork cells were investigated using the patch-clamp technique. Results. In trabecular meshwork, flufenamic acid (10-5 M) reversibly stimulated outward current to 406 ± 71% of initial outward current level in BTM (n = 10) and 294 ± 75% of initial current level in HTM (n = 12) in all cells investigated; no significant differences emerged. The response was dosage-dependent. Replacement of potassium in all solutions eliminated the response to flufenamic acid (n = 4, BTM). Blocking KATP channels with glibenclamide (10-5 M, n = 6) and small-conductance calcium-activated potassium channels with apamin (10-6 M, n = 5) had no effect. A direct effect on calcium channels could also not be detected. Blockage of the large-conductance calcium-activated potassium channel (maxi-K) by iberiotoxin (10-7 M) suppressed 87 ± 9% (n = 6; HTM) and 91 ± 10% (n = 6; BTM) of the response. Depleting the cells of calcium did not significantly alter the response to flufenamic acid. Conclusions. Flufenamic acid stimulates maxi-K channels in trabecular meshwork of both human and bovine origin. This should lead to hyperpolarization, closure of L-type channels and lowered cytosolic calcium levels, possibly explaining the relaxation observed in response to this substance.