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

Hyperpolarizing large-conductance, Ca2+-activated K+ channels (BK) are impo rtant modulators of vascular smooth muscle and endothelial cell function. I n vascular smooth muscle cells, BK are composed of pore-forming cy subunits and modulatory beta subunits. However, expression, composition, and functi on of BK subunits in endothelium have not been studied so far. In patch-cla mp experiments we identified BK (283 pS) in intact endothelium of porcine a ortic tissue slices. The BK opener DHS-I (0.05-0.3 mu mol/l), stimulating B K activity only in the presence of beta subunits, had no effect on BK in en dothelium 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 exp ression was demonstrated. To investigate the functional role of beta subuni ts, 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 fr om 73.4 mV to 49.6 mV at 1 mu mol/l [Ca2+](i) and an decrease of the EC50 v alue 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 asso ciation to beta subunits. The lack of the beta subunit indicates a substant ially different channel regulation in endothelial cells compared to vascula r smooth muscle cells.