Characterization of maxi-K-channels in bovine trabecular meshwork and their activation by cyclic guanosine monophosphate.

PURPOSE Electrophysiological characterization of trabecular meshwork cells and investigation of their response to elevation of cytosolic cyclic guanosine monophosphate (cGMP). METHODS Bovine trabecular meshwork cells were cultured according to established methods and were studied, using the whole-cell and single-channel configurations of the patch-clamp technique. RESULTS In single-channel experiments, cells expressed a channel with characteristics typical of maxi-K-channels. The channel was densely distributed in the membrane and had a high conductance of 326 +/- 4 pS (Pico Siemens) (symmetrical 150 mmol/l KCl; 37 degrees C) for potassium and negligible conductance for sodium (0.9 +/- 1 pS). The open probability could be elevated by depolarization, increasing cytosolic calcium, or adding adenosine triphosphate (1 mmol/ l). The channel could be blocked by external charybdotoxin (10(-8) mol/1), external TEA+ tetraethyl ammonium chloride (1 mmol/l) and by internal Ba2+ (10 mmol/l), whereas external Ba2+ and internal TEA+ (10 mmol/l) had no effect. In whole-cell experiments, trabecular meshwork cells displayed a strong outward conductance. Part of this conductance (35 +/- 5%) could be blocked by charybdotoxin and stimulated by ionomycin (10(-5) mol/1). Addition of 8-bromo-cGMP (10(-3) mol/1) stimulated the current to 290 +/- 57% (n = 4) of the original level, charybdotoxin led to a reduction of this current to 156 +/- 28% of the initial value. CONCLUSIONS Trabecular meshwork cells express maxi-K-channels. These channels can be stimulated by raising internal cGMP levels and are known for their importance in smooth muscle relaxation. The results in this study supply further evidence that trabecular meshwork displays smooth muscle-like properties and contributes to the clarification of the mechanism leading to the relaxation of trabecular meshwork by nitrate and nonnitrate vasodilatators.