title: Stimulation of maxi-K channels in trabecular meshwork by tyrosine kinase inhibitors.
abstract: OBJECTIVE
  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 inhibitors on membrane currents of cells stimulated by acetylcholine was examined.
  
  
  METHODS
  Cells from bovine trabecular meshwork were studied using both the perforated patch-clamp technique with nystatin and the single-channel technique.
  
  
  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 = 16) of the initial current level. The effect of genistein was dose dependent. Reversal potential was hyperpolarized by 15+/-3 mV (n = 9). Tyrphostin 51, a synthetic 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 was blocked by substitution of tetraethylammonium (TEA+) for potassium, whereas the potassium channel blockers glibenclamide (K-ATP) and apamin (low-conductance calcium-activated potassium channel) had no effect. Blockage of the high-conductance calcium-activated potassium channel (maxi-K) by charybdotoxin or iberiotoxin (10(7) M) suppressed 86%+/-18% (n = 4) of the response. Depleting the cells of calcium did not have an effect on the current stimulated by genistein. In the excised inside-out configuration, open probability increased to 417%+/-39% (n = 3) after exposure to genistein.
  
  
  CONCLUSIONS
  In trabecular meshwork, tyrosine kinase inhibitors activate maxi-K (K(Ca)) channels. Hyperpolarization caused by efflux of potassium could lead to the relaxation of trabecular meshwork by tyrosine kinase inhibitors.
authors.affiliation: Institut für Klinische Physiologie, Universitätsklinikum Benjamin Franklin, Freie Universität Berlin, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Stumpff
authors.forename: Y
authors.initials: Y
authors.lastname: Que
authors.forename: M
authors.initials: M
authors.lastname: Boxberger
authors.forename: O
authors.initials: O
authors.lastname: Strauss
authors.forename: M
authors.initials: M
authors.lastname: Wiederholt
mesh_terms: D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002417:Cattle; D002478:Cells, Cultured; D018999:Charybdotoxin / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D008564:Membrane Potentials; D018408:Patch-Clamp Techniques; D010455:Peptides / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism*; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000378:metabolism*; D011505:Protein-Tyrosine Kinases / Q000037:antagonists & inhibitors*; D019789:Tetraethylammonium / Q000494:pharmacology; D014129:Trabecular Meshwork / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism
chemical_list: D004791:Enzyme Inhibitors; D010455:Peptides; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D018999:Charybdotoxin; D019789:Tetraethylammonium; C064719:iberiotoxin; D011505:Protein-Tyrosine Kinases; D000109:Acetylcholine; D011188:Potassium; D002118:Calcium