Pharmacological activation of cloned intermediate- and small-conductance Ca2+-activated K+ channels

We previously characterized 1-ethyl-2-benzimidazolinone (1-EBIO), as well a s the clinically useful benzoxazoles, chlorzoxazone (CZ), and zoxazolamine (ZOX), as pharmacological activators of the intermediate-conductance Ca2+-a ctivated K+ channel, hIK1. The mechanism of activation of hIK1, as well as the highly homologous small-conductance, Ca2+-dependent K+ channel, rSK2, w as determined following heterologous expression in Xenopus oocytes using tw o-electrode voltage clamp (TEVC) and excised, inside-out patch-damp techniq ues. 1-EBIO, CZ, and ZOX activated both hIK1 and rSK2 in TEVC and excised i nside-out patch-clamp experiments. In excised, inside-out patches, 1-EBIO a nd CZ induced a concentration-dependent activation of hIK1, with half-maxim al (K-1/2) values of 84 mu M and 98 mu M, respectively. Similarly, CZ activ ated rSK2 with a K-1/2 of 87 mu M. In the absence of CZ, the Ca2+-dependent : activation of hIK1 was best fit with a K-1/2 of 700 nM and a Hill coeffic ient (n) of 2.0. rSK2 was activated by Ca2+ with a K-1/2 of 700 nM and an n of 2.5. Addition of CZ had no effect on either the K-1/2 or n for Ca2+-dep endent activation of either hIK1 or rSK2. Rather, CZ increased channel acti vity at all Ca2+ concentrations (V-max). Event-duration analysis revealed h IK1 was minimally described by two open and three closed times. Activation by 1-EBIO had no effect on tau(o1), tau(o2), or tau(c1), whereas tau(c2) an d tau(c3) were reduced from 9.0 and 92.6 ms to 5.0 and 44.1 ms, respectivel y. In conclusion, we define 1-EBIO, CZ, and ZOX as the first known activato rs of hIK1 and rSK2. Openers of IK and SK channels may be therapeutically b eneficial in cystic fibrosis and vascular diseases.