Endothelial K+ channel lacks the Ca2+ sensitivity-regulating β subunit

Hyperpolarizing large-conductance, Ca2+-activated K+ channels (BK) are important modulators of vascular smooth muscle and endothelial cell function. In vascular smooth muscle cells, BK are composed of pore-forming cy subunits and modulatory beta subunits. However, expression, composition, and function of BK subunits in endothelium have not been studied so far. In patch-clamp experiments we identified BK (283 pS) in intact endothelium of porcine aortic tissue slices. The BK opener DHS-I (0.05-0.3 mu mol/l), stimulating BK activity only in the presence of beta subunits, had no effect on BK in endothelium whereas the alpha subunit selective BK opener NS1619 (20 mu mol/l) markedly increased channel activity. Correspondingly, mRNA expression of the beta subunit was undetectable in endothelium, whereas alpha subunit expression was demonstrated. To investigate the functional role of beta subunits, we transfected the beta subunit into a human endothelial cell line (EA.hy 926). beta subunit expression resulted in an increased Ca2+ sensitivity of BK activity: the potential of half-maximal activation (V-1/2) shifted from 73.4 mV to 49.6 mV at 1 mu mol/l [Ca2+](i) and an decrease of the EC50 value for [Ca2+], by 1 mu M at +60 mV was observed. This study demonstrates that BK channels in endothelium are composed of alpha subunits without association to beta subunits. The lack of the beta subunit indicates a substantially different channel regulation in endothelial cells compared to vascular smooth muscle cells.