The β(4)-Subunit of the Large-Conductance Potassium Ion Channel K(Ca)1.1 Regulates Outflow Facility in Mice

PURPOSE: The large-conductance calcium-activated potassium channel K(Ca)1.1 (BK(Ca), maxi-K) influences aqueous humor outflow facility, but the contribution of auxiliary β-subunits to K(Ca)1.1 activity in the outflow pathway is unknown. METHODS: Using quantitative polymerase chain reaction, we measured expression of β-subunit genes in anterior segments of C57BL/6J mice (Kcnmb1-4) and in cultured human trabecular meshwork (TM) and Schlemm's canal (SC) cells (KCNMB1-4). We also measured expression of Kcnma1/KCNMA1 that encodes the pore-forming α-subunit. Using confocal immunofluorescence, we visualized the distribution of β(4) in the conventional outflow pathway of mice. Using iPerfusion, we measured outflow facility in enucleated mouse eyes in response to 100 or 500 nM iberiotoxin (IbTX; N = 9) or 100 nM martentoxin (MarTX; N = 12). MarTX selectively blocks β(4)-containing K(Ca)1.1 channels, whereas IbTX blocks K(Ca)1.1 channels that lack β(4). RESULTS: Kcnmb4 was the most highly expressed β-subunit in mouse conventional outflow tissues, expressed at a level comparable to Kcnma1. β(4) was present within the juxtacanalicular TM, appearing to label cellular processes connecting to SC cells. Accordingly, KCNMB4 was the most highly expressed β-subunit in human TM cells, and the sole β-subunit in human SC cells. To dissect functional contribution, MarTX decreased outflow facility by 35% (27%, 42%; mean, 95% confidence interval) relative to vehicle-treated contralateral eyes, whereas IbTX reduced outflow facility by 16% (6%, 25%). CONCLUSIONS: The β(4)-subunit regulates K(Ca)1.1 activity in the conventional outflow pathway, significantly influencing outflow function. Targeting β(4)-containing K(Ca)1.1 channels may be a promising approach to lower intraocular pressure to treat glaucoma.