Stimulation of maxi-K channels in trabecular meshwork by tyrosine kinase inhibitors

PURPOSE. Muscarinic agonists contract and tyrosine kinase inhibitors relax precontracted trabecular meshwork, a smooth muscle-like tissue involved in the regulation of aqueous humor outflow. The effect of tyrosine kinase inhi bitors on membrane currents of cells stimulated by acetylcholine was examin ed. METHODS. Cells from bovine trabecular meshwork were studied using both the perforated patch-clamp technique with nystatin and the single-channel techn ique. RESULTS. Application of the tyrosine kinase inhibitor genistein (5 x 10(-5) M) on trabecular meshwork cells stimulated with acetylcholine resulted in a reversible increase in outward current to 578% +/- 154% (n = IG) of the i nitial current level. The effect of genistein was dose dependent. Reversal potential was hyperpolarized by 15 +/- 3 mV (n = 9). Tyrphostin 51, a synth etic inhibitor of tyrosine kinases, had the same effect (433% +/- 46%; n = 7). Daidzein, a nonactive structural analogue of genistein, had no effect ( n = 4). The stimulation of outward current by tyrosine kinase inhibitors wa s blocked by substitution of tetraethylammonium (TEA(+)) for potassium, whe reas the potassium channel blockers glibenclamide (K-ATP) and apamin (low-c onductance calcium-activated potassium channel) had no effect. Blockage of the high-conductance calcium-activated potassium channel (maxi-K) by charyb dotoxin or iberiotoxin (10(-7) M) suppressed 86% +/- 18% (n = 4) of the res ponse. Depleting the cells of calcium did not have an effect on the current stimulated by genistein. In the excised inside-out configuration, open pro bability increased to 417% +/- 39% (n = 3) after exposure to genistein. CONCLUSIONS. In trabecular meshwork, tyrosine kinase inhibitors activate ma xi-K (K-Ca) channels. Hyperpolarization caused by efflux of potassium could lead to the relaxation of trabecular meshwork by tyrosine kinase inhibitor s.