K(V)LQT CHANNELS ARE INHIBITED BY THE K-ASTERISK( CHANNEL BLOCKER 293B)

Previous data have indicated that the chromanol 293B blocks a cAMP act ivated K+ conductance in the colonic crypt, a K+ conductance in pig ca rdiac myocytes and the K+ conductance induced by IsK protein expressio n in Xenopus oocytes. We have also shown that cAMP-activated cystic fi brosis transmembrane conductance regulator (CFTR) up-regulates, apart from the typical Cl- current, a 293B- inhibitable K+ current. Very rec ently it has been shown that the IsK protein interacts with K(v)LQT su bunits to produce a K+ channel. These data have prompted us to ask the following questions: Is the 293B-inhibitable current in oocytes expre ssing CFTR and activated by cAMP caused by an endogenous Xenopus K(v)L QT (XK(v)LQT), and is mouse K(v)LQT (mK(v)LQT) expressed in oocytes in hibited by 293B? Antisense and sense probes for XK(v)LQT were coinject ed with CFTR cRNA into oocytes. After 3-4 days the oocytes were examin ed by two electrode voltage clamp. It was found that in control oocyte s expressing CFTR and stimulated by isobutylmethylxanthine (IBMX, 1 mm ol/l) 293B (10 mu mol/l) reduced the conductance (G(m)). In oocytes co injected with the sense probe for XK(v)LQT and pretreated with IBMX 29 3B still reduced G(m), whilst the 293B-inhibitable G(m) was almost com pletely absent in oocytes coinjected with XK(v)LQT antisense. In anoth er series a full length clone for mK(v)LQT was generated by PCR techni ques and the cRNA was injected into oocytes. After several days these oocytes, unlike water injected ones, were found to be strongly hyperpo larized and their G(m) was increased significantly. The oocytes were d epolarized significantly and their G(m) was reduced reversibly by 10 m u mol/l 293B. These data indicate that CFTR activation by IBMX indeed co-activates an endogenous oocyte XK(v)LQT channel and that this chann el is inhibited by a new class of channel blockers, of which 293B is t he prototype.