(-)-Isoproterenol modulation of maxi-K⁺ channel in nonpigmented ciliary epithelial cells through a G-protein gated pathway

Purpose. Adrenergic agents decrease intraocular pressure by reducing aqueous humor secretion from ciliary epithelial cells. Since the ionic concentration of aqueous humor contributes to intraocular pressure, we have investigated the effect of (-)-isoproterenol, a β-adrenergic agonist on the maxi-K⁺ channel in rabbit nonpigmented ciliary epithelial (NPE) cells. Methods. Single-channel currents were recorded from the basolateral surface of acutely isolated NPE cells using patch clamp techniques. Results. A calcium dependent maxi-K⁺ channel was identified in 31% of cell-attached patches. In the excised condition the channel was activated in presence of calcium. In symmetrical K⁺ solution a linear current-voltage relationship and unitary conductance of 158 ± 15 pS was observed. Replacing K⁺ with Na⁺ the current-voltage curve shifted to the right and approached a reversal potential for K⁺ (∼-80 mV). Barium (2 mM) from the intracellular side or iberiotoxin (50 nM) from the extracellular side blocked the channel activity. In cell-attached patches, the β-receptor agonist (-)-isoproterenol (2.5 μM) increased channel open probability (P_o) only when applied directly through the patch pipette. β₂-adrenoceptor antagonists (ICI-118, 551, l-timolol) blocked the channel activity more efficiently than the β₁-adrenoceptor antagonist betaxolol. In excised patches, (-)-isoproterenol increased baseline P_o 5-fold (0.5 ± 0.13) when GTP (100 μM) and GTPγS (100 μM) were present at the cytosolic surface of the pipette (control; P_o, 0.12 ± 0.006). GTP augmented baseline channel activity (0.1 ± 0.004) 7-fold (0.7 ± 0.03) when (-)-isoproterenol was included in patch pipette. Conclusions. Rabbit NPE cells expressed maxi-K⁺ channels on their basolateral surface. The adrenergic agonist (-)-isoproterenol activated these channels via a β₂-adrenoceptor that was modulated by a direct G-protein gated pathway.