Potent blocker of high-conductance Ca2+-activated K+ (BKCa) channels. Binds to the α-subunit of BKCa (Ki = 1.9 nM for block of currents in α-subunit-expressing oocytes) and enhances binding of charybdotoxin to BKCa channels in vascular smooth muscle. Also inhibits sarco/endoplasmic reticulum Ca2+-ATPase (IC50 = 5 - 50 μM).