Abstract
Purpose :
Large conductance Ca2+ activated K+ (BK) channels are key regulators of trabecular meshwork contractility and cell volume. Iberiotoxin (IBTX), a specific blocker of BK channel transmembrane pore forming α-subunits, inhibits the increase in outflow facility caused by nitric oxide. Auxiliary β-subunits (β1-4) modulate BK channel characteristics, and BK-β4 channels are resistant to IBTX. Here we investigate the effects of IBTX and martentoxin (MTX), a specific BK-β4 blocker, on murine outflow facility.
Methods :
Expression of BK-α and β subunits was examined by qPCR in homogenised C57BL/6J anterior segments (N=2) and in human trabecular meshwork (TM; N=3) and Schlemm’s canal (SC; N=2) cell strains. Localisation of BK-β4 in the outflow pathway was examined by confocal immunofluorescence in C57BL/6 mice (N=1). iPerfusion was used to measure outflow facility, C, in enucleated mouse eyes treated with IBTX (100 or 500nM, N=4 or 5 pairs) or MTX (100nM, N=12) versus vehicle-perfused contralateral eyes.
Results :
Similar levels of BK-α and BK-β4 mRNA were detected in murine anterior segments and in human TM and SC cells. BK-β4 labelling was observed along trabecular beams and in juxtacanalicular cells, co-localising to cell processes connecting sub-endothelial to SC inner wall cells (Fig.1). IBTX significantly reduced C by 16% [6, 25] (geometric mean [95% CI]; p=0.01, N=9), whereas MTX significantly decreased C by 35% [27, 42] (p<0.0001, N=12). The effect of MTX on C was significantly greater than that of IBTX (p=0.01).
Conclusions :
BK-β4 is expressed within the outflow pathway and contributes to the regulation of outflow facility. Localisation of BK-β4 to processes connecting juxtacanalicular and SC cells, which experience stretch in response to IOP elevation, suggests that BK-β4 channels may be involved in outflow mechanosensation and regulation of IOP.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.